CHAPTER-4

PULP AND PAPER INDUSTRY AND ITS ENVIRONMENTAL ASPECTS

Pulp and paper mills make paper from both virgin raw material and wastepaper. In the mills analyzed, virgin raw material is transformed into pulp in a Kraft pulp mill, which is comprised of a pulp plant, a bleach plant and chemical recovery plant, and wastepaper is transformed into pulp in a hydrapulping and secondary fiber processing plant. Utilities and services (such as, water, steam, power, etc.) required by the core processes of the mill are supplied by the utilities and services section of the mill, which is mainly comprised of a boiler house or a co-generation plant, and one or more water treatment plants.

On the basis of the environmental analysis carried out for five large-scale pulp and paper mills, process maps were developed for the processes of pulp and paper industry (Kraft pulping, Kraft pulp bleaching, hydrapulping and secondary fiber processing, paper machine, and utilities and services). These maps included the following two components:

Process and material flow diagrams

Verbal description of the operations and activities (representing the normal operating conditions, abnormal operating conditions and realistic and potential emergency operating conditions) of each of the processes from the environmental angle

The process maps developed are given in the first five sections of Chapter-4.

Through critically examining the developed process maps, in the light of applicable legal environmental requirements, environmental aspects of the pulp and paper industry were identified and the identified aspects are given in Section-6 of this chapter.

CHAPTER – 4.1

PROCESS MAPS OF KRAFT PULPING PROCESS

4.1.1 OverviewKraft pulping process involves cooking of prepared raw materials (wood chips, grasses, wheat straw, bagasse, etc.) with cooking chemicals (pulping chemicals), and processing (blowing, washing, screening and cleaning) the cooked material into unbleached pulp. Blowing of cooked material involves release of hot vapours. Heat is recovered from these vapours through using a blow heat recovery system. Washing of the cooked material (blown pulp) generates black liquor, which contains spent cooking chemicals and solubilized constituents (mostly non-cellulosic organic constituents) of the raw material being cooked. Cooking chemicals and energy are recovered through concentrating and burning the liquor and causticizing resultant smelt (green liquor), in a chemical recovery plant. Recovered cooked chemicals are used in pulping in the form of white liquor.

Storage, handling and preparation of raw materials, despite being same for different pulping processes, has also been included under the Kraft pulping process for convenience. Overall Kraft pulping process, including storage, handling and preparation of raw materials, and Kraft liquor cycle, is schematically shown in Figure-4.1.1.

Three of the five mills analyzed (Alpha Pulp Mill, Beta Pulp and Paper Mill and Gamma Pulp and Paper Mill) are employing the Kraft pulping process. All the three mills have chemical recovery plants. Kraft pulping process of the Alpha Pulp Mill includes a pre-hydrolysis step for facilitating manufacture of rayon grade pulp.

4.1.2 Storage, Handling and Preparation of Raw materials for PulpingOperations and activities associated with the storage, handling and preparation of raw materials for pulping are schematically shown in Figure-4.1.2.

Receipt, handling, storage and preparation of raw materials for pulping may include the following operations/activities: Transportation and handling Storage Debarking Washing and winnowing Chipping or crushing and cutting Screening of chips

4.1.2.1 Transportation and handlingRaw materials are transported to the mill site by road, rail, or waterways. Within the mill premises, fork lifters, tractors, trolleys, or trippers and conveyors may be used for handling

the raw materials. These operations may involve use of fuel oils and electrical energy, and may be associated with air pollution and noise pollution problems. Spillage of material may also be a problem.

4.1.2.2 StorageRaw material is usually stored in open space. Ground barrier of concrete or asphalt is often provided underneath the stored raw material for reducing dirt contamination and/or inhibiting attack by termites and other ground organisms. Storage is associated with fire hazard. About 1% of the wood stored is usually lost in storage per month to respiration, thermal degradation, chemical reactions and microbial activity. For minimizing the storage losses, chemical sprays (chip preservative treatment) are often used. Depending on the raw material stored and chemical sprays used, drainage water from the raw-material storage yard may require treatment. Bulky raw materials, such as bagasse (which contain residual sugars), are often stored under water. Such storage may generate wastewater, which may require treatment prior to disposal. Raw materials (specially wood chips) are often stored in enclosed or built areas, and storage bins and silos.

4.1.2.3 DebarkingDebarking is required when wood is used as raw material. Either a wet operation, or a dry operation, may be employed for wood debarking. This operation produces bark as solid waste (4 to 5% of the wood debarked). Wet debarking involves use of water and generation of wastewater. Mechanical debarking involves use of electrical energy. Dry debarking can cause air pollution and noise pollution problems. Debarking operation may not always be carried out within the mill premises.

4.1.2.4 Washing and winnowingWashing is a wet process, while winnowing is a dry process. These are meant for the removal of sand, soil and debris from the raw materials. These processes are also often used for removing pith from the raw materials like bagasse and grasses. Washing involves use of water and generation of wastewater, which is rich in grit. Winnowing is associated with air pollution and noise pollution problems. In case of wood based refiner pulping process, even wood chips may require washing for the removal of sand, grit and other foreign materials prior to loading to the digester.

4.1.2.5 Chipping or crushing and cuttingConversion of woody raw material into chips is an electrical energy intensive process, and it is associated with both air pollution and noise pollution problems. Chipping operations are also associated with fire hazards. Blunt and broken blades of chippers etc., may contribute to the scrap generated by the mill. Crushing and cutting operations may be needed for the raw materials like bamboo.

4.1.2.6 Screening of chipsChipping leads to the generation of chips of various sizes. Only the right size chips are to be loaded to the digester. Through screening (on vibratory screens), oversize and under-size chips are segregated. Over-size chips are re-chipped and recycled, while the undersize chips are discarded as wood dust. Screening is associated with air pollution and noise pollution

problems.

4.1.3 Kraft Pulping and Pulp ProcessingKraft pulping process can be considered to include the following activities/operations: Digester loading Pre-hydrolysis Cooking Blowing Blow heat recovery

Processing of the Kraft pulp can be considered to include the following activities/ operations: Screening Washing Cleaning Thickening

4.1.3.1 PulpingKraft pulping, in addition to the above-mentioned activities and operations, also includes a blow heat recovery system, which produces hot water from the blow tank vapours. All the three mills analyzed are employing batch Kraft pulping process. Except in the old pulp unit of the Beta Pulp and Paper Mill, pulping is done through indirect heating of the digester contents in an external pre-heater with saturated medium pressure steam. Direct steam injection pulping is practiced only in the old pulp unit of the Beta Pulp and Paper Mill.

4.1.3.1.1Digester loadingLoading refers to the packing of the digester with the prepared raw material (often with the help of steam), while purging out air and other non-condensable gases, the addition of cooking chemicals (sodium hydroxide and sodium sulfide) in the form of white liquor, and the adjustment of wood to liquor ratio in the digester (usually with weak black liquor, WBL).

If cooking involves a pre-hydrolysis step, then loading of white liquor is delayed till pre-hydrolysis is over. Loading of the digester for pre-hydrolysis may in fact include loading of the prepared raw material and hot water or hot foul condensate.

4.1.3.1.2Pre-hydrolysisPre-hydrolysis is an optional operation. It is usually employed when rayon grade pulp is produced and/or when pentasons removal is desired. Pre-hydrolysis involves cooking of raw material in water (cooking medium) at elevated temperature (155 to 170C) and draining out the cooking medium as pre-hydrolysate liquor (PH liquor). After loading, the digester contents are heated (by circulating the cooking medium through an external heat exchanger, pre-heater, wherein it is indirectly heated by MP steam) to gradually increase temperature to desired level and maintained at that temperature for a fixed duration. Steam condensate is generated at the pre-heater and it can still be used for low temperature heating purposes. Further, this condensate, if not contaminated, can be recycled and reused as boiler feed water. For avoiding build up of false pressure, non-condensable gases accumulated in the digester are vented into the atmosphere continuously and/or intermittently. These

vented gases may be rich in volatile organic compounds.

For draining out the cooking medium, supply of MP steam to the pre-heater and circulation of the medium through the pre-heater are stopped, and the digester vent is opened for allowing escape of relief gases and drop of digester content’s pressure to a desired value. After this, the medium is drained out as PH liquor through a screen plate. Frequently, pre-hydrolysed material of the digester is washed in hot water and the wash water is drained out as pre-hydrolysate wash liquor (PH wash liquor). PH liquor is acidic and hot, and it is rich in organic matter. Characteristics of PH liquor sampled from Alpha Pulp Mill are given in Table-4.1.1. PH liquor has a tendency to clog pipelines through crust formation.

4.1.3.1.3 Cooking

If the loaded raw-material is pre-hydrolyzed first, after draining out the PH liquor, the digester contents are added with white liquor and wood to liquor ratio is suitably adjusted, usually, with weak black liquor (WBL), which is obtained from brown stock washing of the pulp processing section. Cooking involves raising the temperature of digester contents to 155-170C by circulating the cooking medium through pre-heater and indirectly heating it with MP steam. Steam condensate generated at the pre-heater has recycling and reuse potential both as a low temperature heating medium and as boiler feed water. During cooking, in order to avoid development of false pressure, the accumulated non-condensable gases are vented out continuously and/or intermittently. Such vented gases may be rich in reduced organic sulfur compounds. Anthraquinone is often added to the digester during cooking, may be for catalyzing or accelerating fragmentation of lignin and for rendering it vulnerable to attack and dissolution by cooking chemicals.

In the Beta mill, instead of using a pre-heater, direct steam injection cooking is practiced.

4.1.3.1.4BlowingPreparation for blowing includes stopping circulation of the cooking medium through the pre-heater and brining down digester pressure to the desired level through venting. Once pressure is dropped to the desired level, digester contents (cooked material along with the cooking medium) are blown under pressure into a blow tank. Blowing leads to drop in the temperature of the blown material to under 100ºC. Excess heat of the blown material is lost in the form of blow vapours from the blow tank. Sometimes blow may not be complete and some of the cooked material may still be left in the digester. In such cases, the digester is added with the black liquor (obtained from pulp washing), pressurized with steam and then the digester contents are re-blown. Heat is recovered from the blow vapours through a blow heat recovery system.

In one of the pulping units of Beta Pulp and Paper Mill, where grasses, straw and bagasse are used as raw-material, cooking liquor is blown separate from the cooked material. After blowing the liquor, the cooked material is drained out from the digester as slurry with the help of water jets. Generalized batch Kraft pulping process is schematically shown in Figures-4.1.3 & 4.1.4.

4.1.3.1.5Blow heat recovery During blowing, temperature of the blown material is dropped to about 105C. This drop results in the release of excess heat in the form of blow vapours from the blow tank. These vapours are hot (>100C) and odoriferous (due to the presence of reduced organic sulfur

compounds). Blow heat recovery system is used for recovering heat from these vapours and for containing the foul smelling gases.

After cleaning in a cyclone separator (for removing fibrous matter), blow vapours are made to flow through two (direct contact) condensers, connected in series, and brought in contact with cold condensate (which is pumped from the cold condensate section of an accumulator tank). Hot condensate, generated in the condensers from the condensation of vapours and from the heat transfer to the cold condensate, is collected into the hot condensate section of the accumulator tank. Accumulator tank has a top hot condensate section and a bottom cold condensate section, both separated by a moving interface. Non-condensable fraction of the blow vapours are allowed to escape into the atmosphere through a vent provided on the second condenser.

Hot condensate of the accumulator tank is circulated through a fiber filter and a heat exchanger and collected as cold condensate into the cold condensate section of the tank. In the heat exchanger, heat content of the hot condensate is transferred and used for producing hot water from the process water. Excess condensate of the accumulator tank is allowed to overflow into sewer from the cold condensate section of the tank. In the Gamma Pulp and Paper Mill, part of the hot condensate is used in the brown stock washing.

Process flow diagram of the heat recovery system of the Gamma Pulp and Paper Mill is shown in Figure-4.1.4.

4.1.3.2 Pulp ProcessingProcessing of Kraft pulp includes the following operations/activities.

4.1.3.2.1ScreeningPulp from the blow tank is usually screened in a pressure screen or a vibratory screen or a gravity centrifugal screen (and sometimes defiberized in a disc refiner), prior to its washing. Because of high rejection rates, rejects of pressure screens are usually further screened in a secondary pressure screen and/or a vibratory screen for recovering useful fiber from the rejects. Pressure screens may also separate tramp material from the pulp. Use of vibratory screens is associated with foaming and liquor spatter problem. Foaming problem is relatively less (when compared with vibratory screens) in case of gravity and centrifugal screens. Rejects generated from screening (known as knots) are either discarded as waste (through burning, land filling, etc.) or loaded back to the digester either along with fresh chips or as a separate batch. Dripping of cooking liquor is common in the knots handling area. See Figure-4.1.5 for a flow diagram of screening in a pulp processing unit of the Beta Pulp and Paper Mill.

In the old pulp unit of Beta Pulp and Paper Mill, where cooking medium is blown separate from the cooked material, the blown medium is screened separately, and the fibrous mass recovered from such screening is added to the pulp slurry prior to its screening, cleaning and washing.

4.1.3.2.2Pulp washing (Brown Stock Washing)Multistage counter current washing is practiced for minimizing water consumption and obtaining relatively higher strength weak black liquor (WBL). Three or more rotary vacuum drum washers, connected in series, are used. For ensuring better washing, hot water is used. Gamma Pulp and Paper Mill is using hot foul condensate, obtained from the

condensation of blow vapours to partially replace hot water in the pulp washing. Perforated screen plate of the drum is cleaned by hot water or by the vapour condensate of the blow tank. Rotary drum washers are usually enclosed in hoods and ventilated by active ventilation systems for avoiding foul smells and vapours problem.

A typical drum washer includes a vat, a rotating drum, a repulper, a seal tank connected to the drum by a barometric leg (for creating vacuum in the rotating drum and for conveying the collected filtrate from the drum to the seal pit), and pumps & piping for pumping and conveying the seal pit overflows to the drum washer for reuse in the displacement showers and repulper of the upstream washing unit. Wash water is also reused in the vat for pulp dilution in the same washing unit. In order to avoid foaming problems, seal tanks are ventilated into a foam tank, and this tank in turn is provided with foam breaking and venting arrangements.

Rotating vacuum drum picks up pulp from the vat in the form of a mat and liquor is removed from it when the drum is outside the vat. Application of wash liquor through displacement showers leads to further washing of the pulp. Washed pulp mat is removed with the help of a doctor blade from the drum, repulped in the repulper and sent to the next washer. Once the pulp mat is removed, prior to submergence in the vat pulp, water sprays clean perforated screen plate of the drum. Figure-4.1.6 gives the details on rotary vacuum drum washer.

WBL generated from the multistage counter current brown stock washing and collected into the seal tank of the first brown stock washer, is, ultimately, pumped to the chemical recovery plant for producing white liquor. However, a part of this WBL is reused in the blow tank for zone dilution of pulp, in the screening operations for elutriation, and in the digesters for adjusting wood to liquor ratio and for facilitating reblows.

In the Alpha Pulp Mill, a rotary pressure washer (also known as DD Washer) is used for pulp washing. In this case, water from the pulp mat is removed or displaced by applying pressure. A single rotary pressure washer may have as many as four liquor displacement stages of washing. Foaming and odor problems are almost eliminated in this type of washing.

Rotary pressure washer can be considered to include the following zones:a) Pulp feed zone: The pulp to be washed is pumped into this zone at high pressure and

formation of a pulp mat over the perforated plate of the washer drum is facilitated.b) Washing zone: Four-stage counter-current displacement washing of the pulp is carried

out in this zone. Hot water under high pressure is used as washing medium in the last stage of washing. Circulating filtrate pumps, located between the successive washing stages, pressurize the washing medium and supply it to the subsequent stage of washing.

c) Washed pulp discharge zone: In this zone, the pulp mat is first exposed to suction for sucking out the washing medium, and then to short pressurized air impulse, from inside, for loosening the pulp mat. Loosened pulp mat is discharged and conveyed by a screw conveyor to next process step.

d) Perforated screen plate washing zone: Here, very high pressure wash water is used in the wash water pipe showers for washing the plate.

4.1.3.2.3Pulp cleaning Pulp cleaning is meant for the removal of sand, dirt and other dense particles from the pulp.

High density cleaners (HD cleaners) and/or multi-stage centri-cleaners are used for cleaning the pulp. In the Beta Pulp and Paper Mill, a reiffler, wherein heavier particles are allowed to settle under gravity, is used for pulp cleaning.

HD cleaners are employed for cleaning the pulp prior to brown stock washing. These can tolerate higher pulp consistencies (3-4%), and are appropriate for removing relatively larger size heavier particles. These cleaners have stock savers attached to them for re-suspending the rejects and recovering good fiber. This re-suspension and recovery of fiber involves use of elutriation fluid.

Centri-cleaners are usually used for cleaning the washed pulp of around 1% consistency. These cleaners prove very appropriate for the removal of heavier but small size particles like grit and sand. Usually multistage centri-cleaners (three or more) are used. This cleaning process involves separation of the input pulp stream into an “accepts” stream and a “rejects” stream. Rejects stream of one stage of centri-cleaning is cleaned in the next stage of centri-cleaning in order to recover good fiber and concentrate the rejects. Accepts stream of a centri-cleaner stage is passed through the earlier centri-cleaner stage as input pulp. A fiber-miser is used for the final processing of rejects and recovery of good fiber. Typical flow sequence of a multi stage centri-cleaning process is shown in Figure-4.1.7.

4.1.3.2.4Decker washing and thickeningRotary vacuum drum washer, similar to the one used in brown stock washing, is used for thickening the processed pulp and for further wash-removal of residual spent cooking chemicals. Water is used in the displacement showers of the decker and in the showers meant for cleaning the perforated plate of the drum washer. Filtrate or wash water generated by this operation (known as decker backwater) is mostly reused in the centri-cleaning operation for adjusting the pulp consistency. In the new pulp unit of the Beta Pulp and Paper Mill, washed pulp is not subjected to any cleaning, and no decker is used for thickening of the pulp. Instead, output of the fourth brown stock washer is taken into a high density unbleached pulp tower. In the Gamma Pulp and Paper Mill, part of the washed pulp is fractionated into long and short fiber fractions. The long fiber fraction is dewatered in a screw press and transported to a nearby paper mill for use as stock.

Overall process flow diagram for the pulp processing section of the Alpha Pulp Mill is shown in Figure-4.1.8.

4.1.4 Chemical Recovery PlantCooking chemicals/soda recovery involves the following aspects: a) Concentrating the WBL generated from the brown stock washing to over 60% solids

level so that the black liquor solids can be burnt in the recovery boiler furnace – WBL, depending on the brown stock washing practices, may have 8 to 18% solids. Concentrated black liquor is usually known as thick black liquor (TBL)

b) Burning of TBL in a recovery boiler furnace and dissolving the resultant smelt in water for obtaining green liquor

c) Recausticizing the green liquor for obtaining white liquor, which can be directly used as cooking chemical in the pulping process

4.1.4.1 Concentrating WBL into TBLThis can be considered to include the following operations/activities:

a) Concentration of WBL into semi-thick black liquor (STBL) in a Multiple Effect Evaporator (MEE)

b) Concentration of STBL into TBL in one or more Forced Circulation Evaporators (FCEs)

c) Frequent cleaning of effects, evaporators, tanks, pumps and piping and other machinery and equipment

4.1.4.1.1 Concentration of WBL in Multiple Effect Evaporators (MEE)

Two or more effects, connected in series, are used in the MEE for concentrating WBL into STBL. An Effect can be considered to include a heater section and a hot liquor flashing section. Heater section of the effect is comparable to a shell and tube type heat exchanger. Black liquor is pumped through the tubes of this section, while heating medium is introduced on the shell side. Low pressure saturated steam (LP steam), or flashed vapours of some other effect or a FCE, is used as heating medium. Heating medium, introduced on the shell side, condenses on the tubes and the heat released is transferred to the liquor flowing inside the tubes. Condensate generated on the shell side is collected into a condensate pot through a condensate leg, and disposed off as foul condensate. In the effects, where LP steam is used as heating medium, the condensate collected is not considered as foul condensate. It is mostly reused as boiler feed water. Flashed Vapours and Non-condensable Gases Handling System (FVNG Handling System) is used for handling the non-condensable gases accumulating on the shell side of the condenser.

Super heated black liquor is allowed to flow from the heater section into the hot liquor flashing section. Here, the liquor is allowed to flash and loose its excess heat in the form of flashed vapours. The generated flash vapours are separated and sent either to the heater section of some other effect, which is operating at relatively lesser temperature, for use as a heating medium, or to the FVNG handling system. Process flow diagram of a typical effect is shown in Figure-4.1.9.

In the Gamma Pulp and Paper Mill, effects with the heater section divided into two or more parts are used. In the Beta Pulp and Paper Mill, pre-heaters are used between successive effects. In the Alpha Pulp Mill, pre-heaters are used upstream to the MEE. In the latter case, flash vapours obtained from flashing of the steam condensate generated by the digester pre-heaters of the pulp unit are used as heating medium. Just as the heater section of an effect, pre-heaters also produce condensate just as the effects.

4.1.4.1.2FVNG handling SystemFVNG handling system usually includes a surface condenser, two steam ejectors, an inter-cooler and an after-cooler. Schematic process flow diagram of a typical FVNG handing system is shown in Figure-4.1.10. Non-condensable gases of different effects and flashed vapours of the first effect (which receives WBL) are connected to the shell side of the surface condenser. Cooling water flowing through the tubes of the surface condenser ensures condensation of the vapours. The accumulating non-condensable gases are sucked and ejected out from the shell side of the condenser by a steam ejector, which works on venturi principle. Out put of this ejector is cooled and condensed by water sprays in an inter-cooler. Non-condensable gases accumulating in this inter-cooler are sucked out by another ejector. Output of this ejector is in turn cooled and condensed in an after-cooler again by water sprays. Non-condensable gases accumulating in the after-cooler are vented out into the atmosphere. Condensate generated in the surface condenser is conveyed to a

seal pit through a barometric leg. Overflows of the seal pit are disposed off as foul condensate. Cooling waters generated by inter-cooler and after-cooler are sewered. A separate cooling tower is provided for supplying cooling water to the surface condenser, inter-cooler and after-cooler of the FVNG handling system.

In some of the FVNG handling systems, a pre-cooler is used for cooling non-condensable gases and first effect vapours with water sprays prior to their entry into the surface condenser. In the Beta Pulp and Paper Mill, a separate steam ejector has been provided for handling non-condensable gases from the shell side of different effects. However, output of this ejector is taken into the surface condenser, which is handling flashed vapours of the first effect.

4.1.4.1.3Concentration of Black Liquor in Forced Circulation Evaporator (FCE)STBL generated by the MEE is concentrated further into TBL in one or more FCEs, connected in series. An FCE includes a heater (which is similar to shell and tube type heat exchanger) and a vapour separator. Black liquor is pumped through the tubes of the heater and heated by LP steam taken on the shell side. Steam condensate generated is mostly reused as boiler feed water. Hot black liquor coming out from the heater is flashed in the vapour separator. Vapours separated are used as heating medium in the effects of MEE. Flashing leads to the concentration of the black liquor. There may be two or more FCEs for concentrating the liquor to the desired level of solids. Process flow scheme of a forced circulation evaporation system of the Gamma Pulp and Paper Mill is shown in Figure-4.1.11. This includes three forced circulation evaporators connected in series. The system is used for concentrating the semi-concentrated black liquor of 28 to 45% solids into TBL of >60% solids.

4.1.4.1.4Cleaning of Effects, Evaporators, Tanks, Pumps & piping, and other Machinery and Equipment

Procedure followed for the cleaning, in the Gamma Pulp and Paper Mill, involves the following steps:a) Isolating the effect, evaporator, tank or other machinery that has to be cleaned b) Draining out the black liquor present in it and taking it back into WBL storage tank c) Flushing the isolated effect, evaporator, tank or other machinery by process water and

taking the flushings into the WBL storage tank d) Circulating hot water or hot caustic solution, or sulfuric acid or mixture of sulfuric acid,

and sodium sulfate solution through the isolated effect, evaporator, tank or other machinery. After circulation for a specified period, taking the circulating solution to the WBL storage tank

e) Opening the effect, evaporator or tank, for cooling; after cooling, cleaning the interior with high pressure water jets; and draining out the wash water into sewer

f) Closing the effect, evaporator or tank and filling with process water and keeping it ready for taking back into line (at the time of taking into line, this water is apparently drained into sewer)

4.1.4.2 Burning Black Liquor Solids in the Recovery Boiler Recovery boiler essentially performs the following functions:

Further concentrating and drying of TBL, and burning of the black liquor solids Recovering spent cooking chemicals as molten smelt, and generating green liquor from

the smelt Reducing oxidized sulfur compounds (present in the black liquor as well as added to the

furnace as make up chemicals) into sulfides

Burning of black liquor solids and generation of green liquor essentially includes the following operations or activities: Loading of black liquor solids to the furnace Supply of combustion air Supply of boiler feed water Superheated steam generation Flue gases treatment and disposal Green liquor production from smelt

In addition to the above, burning of black liquor solids can also be considered to include the following important auxiliary operations/activities: Soot blowing Boiler start-up and shut down operations Handling emergency situations

Process and material flow diagram for one of the recovery boilers of the Gamma Pulp and Paper Mill is shown in Figure-4.1.12.

4.1.4.2.1Loading of black liquor solidsTBL, supplied from the evaporators section, is first added with make-up chemicals (salt cake, Na2SO4), ESP (electrostatic precipitator) ash (solids separated from the flue gases by ESP), and ash collected in the hoppers of the boiler tube bank and economizer. Then, prior to loading to the furnace, the liquor is heated to about 120C, usually with MP steam, in one or two pre-heaters. Steam condensate generated at the pre-heaters has the potential for reuse as source of heat and as boiler feed water. Hot black liquor is then circulated through a ring header, and from there loaded to the recovery boiler furnace, through fire guns. In the Gamma Pulp and Paper Mill, hopper ash of the boiler tube bank and economizer is not directly mixed with TBL, instead it is slurried with weak white liquor (WWL) or process water and taken into the WBL storage tanks, apparently for avoiding emergency situations. In the Alpha Pulp Mill, salt cake, instead of mixing with black liquor, is directly loaded into the furnace.

4.1.4.2.2Supply of combustion airCombustion air is supplied at three different levels into the furnace as primary, secondary and tertiary air. Usually, as much as 50 to 65% of the air is supplied as primary air. But, in the Gamma Pulp and Paper Mill, primary air, secondary air and tertiary air constituted 34.5%, 52.7% and 12.8% respectively. Forced draft fans (FD fans) are used for supplying the combustion air.

Combustion air, specially primary and secondary air, is preheated by MP steam in one or two pre-heaters. In the Gamma Pulp and Paper Mill, primary and secondary air are preheated to 171C and 144C respectively. Hot flue gases are sometimes used for the air pre-heating. One of the recovery boilers of the Gamma Pulp and Paper Mill has two economizers (primary and secondary economizers). Boiler feed water coming out from the

secondary economizer is used for preheating the combustion air. Steam condensate generated at the air pre-heater has the potential for reuse as source of heat and as boiler feed water.

4.1.4.2.3Supply of feed water De-aerated boiler feed water is fed to the stream drum of the boiler through economizer of the boiler. From stream drum, this water flows, through the boiler down comer tubes, into mud drum. From there it flows back to the stream drum through the furnace wall tubes and riser tubes of the boiler tube bank. Water of the stream drum is also circulated through the screen tubes of the furnace. Boiler water is blown down from the stream drum and mud drum (continuously as well as intermittently) in order to prevent salt accumulation and subsequent scaling of water tubes. Usually, the feed water consumed is 10% more than the steam generated. This additional 10% feed water compensates for the losses incurred in the form of boiler blow-down, steam consumption in soot blowing, etc.

4.1.4.2.4Super heated steam generationSteam is separated in the steam drum, and super heated in one or two super-heaters (primary and secondary). A part of the super-heated steam is tapped for soot blowing purpose. One of the recovery boilers of the Gamma Pulp and Paper Mill was found to produce about 3 tons of steam at 37.5 kg/cm2 pressure and 425C temperature for every ton of black liquor solids burnt. Soot blowing consumed about 5% of the steam generated. Superheated steam produced by the recovery boilers is sent to the turbine generator for power generation. During start-up and shut-down operations, the steam produced may not be fit for use in the turbine generator. Such steam is usually vented out as exhaust.

4.1.4.2.5Flue gases treatment and disposalFlue gases of the furnace flow through the boiler and economizer sections and come out at about 160-180C temperature. These gases are then passed through an electrostatic precipitator (ESP), in order to remove suspended particulate matter, prior to disposing through a stack into the atmosphere. ESP ash, generated from such treatment of flue gases, contains residual cooking chemicals, and hence, it is mixed with the TBL prior to the latter’s burning in the recovery boiler furnace. Induced draft fan (ID fan) located down-stream to the ESP is used for drawing the flue gases and maintaining slight negative pressure in the furnace. A part of the solids, entrained in the flue gases, may get separated and collected into hoppers of boiler tube bank and economizer. One of the recovery boilers of the Gamma Pulp and Paper Mill has a coarse separator prior to the economizer for the removal of heavier and larger entrained solid particles. The ash collected into these hoppers is also mixed with the TBL.

4.1.4.2.6Green liquor production from smelt Bottom smelt of the recovery boiler furnace is made to flow out through one or more water cooled spouts and fall into a smelt dissolving tank. In this tank, the smelt is dissolved in WWL (from the causticizing section) and/or process water for obtaining green liquor. For avoiding formation of explosive mixture of smelt and water, the smelt stream falling into the dissolving tank is shuttered by a steam jet. Spraying of the dissolving tank contents with the help of a high volume recirculation pump, the heat generated from the mixing of smelt in water is rapidly dissipated and accidents are avoided. Further, for ensuring safety, the dissolving tank is provided with an oversized vent (for quick relief of pressure developed in case of any explosion) and with emergency doors.

4.1.4.2.7Soot blowing Solids entrained in the flue gases usually settle on the water tubes (of the furnace, super heater, boiler tube bank and economizer sections) and adversely affect the heat exchange process. Soot blowing is practiced, specially for cleaning the tubes free of soot and ensuring efficient heat exchange. High pressure superheated steam, extracted from the steam header of the recovery boiler, prior to its connection to the turbine header, is used for blowing the soot. The blown soot partially gets collected into the bottom hopers and, partially, gets carried along with the flue gases. Soot blowing usually increases load on ESP and release of SPM in the exhaust.

4.1.4.2.8Start-up and shut down operations Shut-down involves gradual reduction in the rate of firing of black liquor solids, and shift to the firing of furnace oil and finally running the boiler only on the furnace oil. Running of the boiler on furnace oil is continued until the porous charred bed of the furnace disappears and comes out as smelt. Once the furnace bed of smelt disappears, firing of furnace oil is gradually reduced and ultimately stopped. The movement pressure and temperature specifications become unacceptable, sending of steam to the turbine generator is stopped and the generated steam is vented. As and when flue gas temperature drops below a specified value, ESP is bypassed and the flue gases are discharged directly through stack into the atmosphere.

Once firing of fuel is stopped, cleaning of the recovery boiler interior, through hand lancing and with high pressure hot water jets first from outside and then from inside is started. This cleaning is a time and water consuming process. Gamma Pulp and Paper Mill takes about 16 hours time, while consuming about 20 m3/hour of water for the cleaning. Wash water generated from the furnace and super heater sections from such cleaning is recovered and treated as green liquor. However, the wash water generated in the boiler and economizer sections is recovered and mixed with WBL for minimizing the chemical loss. ESP is cleaned less frequently (may be annually) with air jets rather than with hot high pressure water jets.

Start-up is initiated through firing wood or some other solid fuel for about 5 to 6 hours, and then, firing of oil is started. Oil firing is gradually increased to its maximum level, while wood burning is stopped, and continued till the generated steam attains specified pressure and temperature (35 kg/cm2 pressure and 380C temperature for one of the recovery boilers of the Gamma Pulp and Paper Mill). Then, firing of black liquor is started and gradually increased to the normal firing rate, while gradually reducing and ultimately stopping firing of oil. Steam generated during the start-up is vented out into the atmosphere until it attains the pressure and temperature required for connecting it to the turbine main. ESP is taken into line only when temperature of the flue gases crosses threshold temperature for the ESP.

One shut down and subsequent start-up operation, in the Gamma Pulp and Paper Mill having 325 tons/day black liquor solids firing capacity, consumes about 8 m3 of furnace oil and 3 to 4 tons of wood.

4.1.4.2.9Handling emergency situationsEmergency situations associated with a recovery boiler are of two types – boiler tube leakages, and development of explosive situation in the dissolving tank.

Water wall tube leakage can have very serious consequences. When a water wall tube leaks, water can reach the smelt bed and result in explosive mixture of smelt and water. When a tube leakage is detected, active air supply to the furnace (FD fan) is stopped, but induced draft of flue gases (ID fan) is continued. However, loading of black liquor is continued in order to ensure quenching of the smelt bed. When the tube leakage is heavy, drastic steps like drainage of water wall tubes are taken.

Water tube leakage in the boiler tube bank, economizer, or super heater, is not that serious. Still, leakages in the tube bank can dilute TBL. Leaked water, through the hopper ash route can reach the TBL. Diluted TBL can result in the loading of furnace with excess water. This in turn can lead to the formation of explosive mixtures of smelt and water. For avoiding this, ash collected in the boiler and economize hoppers is not directly mixed with the TBL. Instead, it is slurried and taken into the WBL.

4.1.4.3 Causticizing Green Liquor and Preparation of White Liquor Green liquor obtained from burning of black liquor solids in the recovery boiler is stored and converted into white liquor through causticizing and clarification operations. The white liquor, thus produced, is stored and supplied to the pulping unit for reuse as cooking liquor. Causticizing green liquor and preparation of white liquor is associated with the generation of lime mud, which requires proper handling and disposal. In the Alpha pulp mill, this lime mud is calcinated for producing quick lime. This lime is used in the green liquor

causticizing process. Causticizing section of the chemical recovery plant can be considered to include the following operations/activities:a) Green liquor processingb) Lime slacking and green liquor causticizingc) Processing and supply of white liquor to the pulping process d) Dregs washing and wash water recausticizing e) Lime mud washing f) Lime mud dewatering g) Calcination of lime mud

Figure-4.1.13 gives schematic of the causticizing unit of the Alpha Paper Mill.

4.1.4.3.1Green liquor processing Green liquor processing involves screening, clarification, storage and preheating operations. Through screening and clarification operations insoluble and suspended impurities are separated as dregs from the green liquor. For ensuring higher causticizing efficiencies, prior to loading to drum slacker, pre-heating of green liquor is practiced. Usually LP steam is used for the indirect pre-heating of green liquor. In the Beta Pulp and Paper Mill, the screenings separated are burnt in the recovery boiler. Efforts are made to avoid dilution of green liquor while handling.

4.1.4.3.2Lime slacking and green liquor causticizing Preheated green liquor is added with quick lime (calcium oxide) and fed to the slacker. Slacked slurry is first screened in a trommel screen, for removing stones and other larger size impurities, and then sent to a grit classifier. Grit separated from the slurry in the grit classifier is washed with hot water prior to disposal. Wash water generated from such washing is mixed with the slacked slurry. Slacked slurry is then taken into two or more atmospheric causticizing tanks (having vents) connected in series. Through injecting steam, and in certain cases through pressurizing the causticizing tank contents, high temperature is maintained for achieving better causticizing efficiencies.

Loading of lime to the slackers is associated with suspension and spillage of lime. Lime slacker is a source of noise pollution.

4.1.4.3.3Processing and supply of white liquor to pulping Causticized slurry is clarified in white liquor clarifier tanks and clarifier overflows are supplied as clear white liquor to the pulping unit. For improving the clarity, in certain cases, the clarified white liquor is passed through a polishing filter prior to sending to the pulping unit. White liquor, in the Alpha Pulp Mill, is maintained at elevated temperature through indirect heating with the hot water generated by the blow heat recovery system, and loaded to digesters as hot white liquor. Sludge settling at the bottom of the white liquor clarifier tanks is taken out as underflow into a recausticizer tank for recausticizing dregs wash water.

4.1.4.3.4Dregs washing Green liquor clarification leads to the generation of dregs. These dregs are mixed with hot water in a dregs mixer tank and washed in a dregs wash tank. Wash water generated from such washing is recausticized in a recausticizer tank through adding underflows of white liquor clarifier and hot water. Recausticized slurry is taken to the lime mud washers. Washed dregs, which are rich in carbon and inorganic impurities, such as calcium and iron compounds, are disposed as solid waste. In the Alpha Pulp Mill, where limekiln is used for the calcination of lime mud, scrubbing wastewater generated from the limekiln exhaust is also used for recausticizing the dregs wash water.

4.1.4.3.5Lime mud washing Recausticized contents of the recausticizer tank are washed with hot water and clarified in lime mud washers (LMWs). The lime mud is subjected to counter current washing in two or more lime mud washers connected in series. That is, underflow of the preceding LMW (LMW-1) is mixed with clarified overflow of the following LMW (LMW-3) and taken to the LMW in question (LMW-2) for clarification. Wash water generated from such counter current washing is collected and stored as weak white liquor (WWL) within the premises. The stored WWL is supplied to smelt dissolving tank of the recovery boiler for the preparation of green liquor. WWL is also reused at a few other places in both evaporators and recovery boiler sections. Hot water demands of causticizing section, in general, and lime mud washing, in particular, are mostly met from the reuse of foul condensate obtained from the evaporators section. Direct injection of steam is often practiced for maintaining temperature of the hot water. Lime mud washers require frequent cleaning in order to avoid clogging problems, and such cleaning generates large quantities of wastewater with heavy

load of suspended solids, which may settle in the sewer lines carrying the wastewater.

4.1.4.3.6Lime mud dewateringWashed lime mud (underflow of the last LMW) is sent to the lime mud dewatering unit. Rotary vacuum drum filter is usually used for dewatering the lime mud. Process flow scheme of the rotary vacuum drum filter dewatering system is shown in Figure-4.1.14. Vacuum pump is used for creating vacuum inside the rotary drum. Hot water is used in the displacement washing showers and for cleaning the perforated screen plate of the rotary drum. Frequently, compressed air is also used for the cleaning of the perforated screen plate. Process water is consumed by the vacuum pump for gland sealing and cooling purposes. Lime mud dewatering generates two types of wastewaters – water separated from the lime mud, and gland cooling and sealing water of the vacuum pump. Both the wastewater streams have recycling and reuse potential (water separated from the lime mud is partially recycled to the recausticizing tank). Lime mud dewatering unit is source of noise pollution.

Dewatered lime mud is usually disposed off as solid waste. In the Alpha Pulp Mill, the dewatered sludge is added with make up sea shell and calcinated in a limekiln for regenerating reburnt lime, which is used in the slacker for causticizing the green liquor. In the other mills, lime for causticizing is obtained from outside and lime mud generated from the causticizing is disposed off as non-hazardous solid waste.

4.1.4.3.7Calcination of lime mud Most of the mills, mainly because of quality constraints, dispose off the dewatered lime mud as non-hazardous solid waste. Further, some of the mills procure lime stone and produce the burnt lime required through calcinating it in a limekiln. Gamma Pulp and Paper Mill is disposing of the dewatered lime mud as non-hazardous solid waste, and at the same time calcinating lime stone for on-site generation of lime. Of the three mills, only Alpha Pulp Mill is calcinating the lime mud it generates in an inclined rotating limekiln, and reusing the resultant reburnt lime for causticizing the green liquor. This mill uses sea shell as make up lime.

A limekiln can be considered to have a flame end and an exhaust end. Flame end is maintained at 1150 to 1250C temperature through firing oil or gas. Furnace oil is pre-heated either electrically or with steam and atomized with air or steam and then introduced into the furnace through burners for combustion. Hot gases of the kiln move from the hot end to the cold end, and escape out into atmosphere through a chimney. FD fan and ID fan are used to supply combustion air to the kiln, and draw exhaust gases through an air pollution control device (provided for the removal of suspended lime dust) and discharging into atmosphere through a stack. Dewatered lime mud is introduced at the exhaust end and made to move counter current to the hot gases. During such movement, the lime mud is first dried and then calcinated. Calcinated or reburnt lime is discharged from the flame end of the limekiln at about 950C. In some of the limekilns, excess heat of the reburnt lime is used for pre-heating the combustion air entering the kiln.

Exhaust gases of the kiln are dust laden, and hence are treated either in a scrubber, or in an electrostatic precipitator (fabric filters may also prove appropriate), prior to their discharge into the atmosphere. Dust separated from the exhaust gases of the limekiln (in case of an ESP) or wasted scrubbing solution (in case of wet scrubbers), because of the presence of lime, have reuse potential. Limekilns are associated with dust pollution and noise pollution

problems.

Overall process flow scheme of the limekiln of the Alpha Pulp Mill is shown in Figure-4.1.15.

Figure-4.1.1: Kraft Pulping – schematic overview diagram

Figure-4.1.2: Process flow diagram of a wood based Pulping unit of the Beta Pulp and Paper Mill

Figure-4.1.3: Schematic of Kraft pulping process (excludes blowing and blow heat recovery operations)

Figure-4.1.4: Pulp Blowing and Blow Vapour Heat Recovery System of the Gamma Pulp and Paper Mill

Figure-4.1.5: Process flow diagram for pulp screening in a pulp processing unit of the Beta Pulp and Paper Mill

Figure-4.1.6: Schematic process flow diagram for multistage counter-current brown stock washing

Figure-4.1.7: Typical process flow diagram for the centri-cleaning operation

Figure-4.1.8: Process flow diagram for pulp processing section of the Alpha Pulp Mill

Figure-4.1.9: Process flow scheme for a typical effect of a multiple effect evaporation system

Figure-4.1.10: Schematic of flashed vapours and non-condensable gases handling system (FVNG Handling System)

Figure-4.1.11: Process flow scheme of forced circulation evaporation system of the Gamma Pulp and Paper Mill

Figure-4.1.12: Process flow diagram for one of the recovery boilers of Gamma Pulp and Paper Mill

Figure-4.1.14: Process flow scheme of lime mud dewatering system

Figure-4.1.13: Process flow scheme for causticizing unit of the Alpha Pulp Mill

Figure-4.1.15: Process flow scheme for the limekiln of Alpha Pulp Mill

Table-4.1.1: Composition of PH liquor sampled from Alpha Pulp Mill

Parameter PH liquor

pH 3.6

Acidity (mg/l as CaCO3) 9,310

Temperature 98C

Total solids (mg/l) 50,122

Total dissolved solids (mg/l) 44,926

Total volatile solids 97.5% of Total Solids

COD (mg/l) 92,000

Sulfates (mg/l) 270

CHAPTER – 4.2

PROCESS MAPS OF KRAFT PULP BLEACHING PROCESS

4.2.1 Bleaching stages Bleaching stages employed by the three mills analyzed (Alpha Pulp Mill, Beta Pulp and Paper Mill and Gamma Pulp and Paper Mill) are Chlorine bleaching Chlorine-chlorine dioxide bleaching Alkali extraction Alkali-oxygen extraction Chlorine dioxide bleaching Hypo bleaching

Sequence of bleach stages employed by the mills is indicated by numerals (starting from 1) in Table-4.2.1.

4.2.1.1 Chlorine bleaching and chlorine-chlorine dioxide bleachingChlorine bleaching is employed as the first stage of bleaching in a conventional bleach plant. This bleaching is carried out usually at 2.5 to 3.5% of pulp consistency. Chlorination of unbleached pulp is an exothermic reaction. Hence, use of backwater generated from chlorine bleaching, for diluting high consistency unbleached pulp, leads to high temperature chlorine bleaching. But, high temperature bleaching is associated with severe cellulose degradation, specially if any residual chlorine is left in the pulp coming out from the chlorination tower. For chlorine bleaching, pH below 2 is most appropriate. Higher pH can be destructive to cellulose. Use of the backwater in the stock dilution can make it easier and cheaper to maintain of pH below 2.

Chlorine is first dissolved in dilution water and then dosed, at the desired rate, to the diluted stock in a medium consistency mixer (MC mixer). Rate of chlorine application is actually 75 to 80% of the pulp’s total chlorine demand. Pulp mixed with chlorine is retained in an up-flow chlorine bleach tower for 45 to 90 minutes. Pulp coming out from the tower is washed in a rotary vacuum drum washer.

In all the three pulp mills analyzed (Alpha, Beta and Gamma mills) chlorine is partly substituted by chlorine dioxide (ClO2). ClO2 is dosed usually prior to the dosing of chlorine, may be for ensuring quick reaction and avoiding carryover of ClO2 to the washer (which can cause corrosion problems). In the new pulp unit of Beta Pulp and Paper Mill, fumes emanating from both the bleaching tower and the rotary vacuum drum washer are collected and scrubbed by sulfurous acid solution prior to venting into the atmosphere (Figure-4.2.1).

Chlorine bleaching contributes organic halides (dioxins, benzofurans etc.) to the environment. About 10% of the chlorine applied is reported to end up as adsorbable organic

halides (AOX). Chlorine reactions with lignin are mainly responsible for the formation of chlorinated organics. ClO2 forms relatively less of chlorinated organics. It is 2.63 times more powerful than Cl2 as a bleaching agent on weight basis and 2.5 times on moles basis. On molar basis, atomic chlorine present in ClO2 is just 50% to that present in the molecular chlorine. Hence, use of ClO2 as bleaching agent reduces generation of organo chlorines by about 80%. ClO2 bleaching is also reported to reduce colour discharge from bleaching. Chloroform emission from bleach plant vents, which is primarily due to chlorine bleaching, is also reduced by ClO2 substitution. At the level of 15% substitution, chloroform emission is reduced from 0.35 kg/ton of pulp to 0.01 kg/ton of pulp.

Chlorine gas is sourced from outside, stored on-site and used in the bleaching process. ClO2, on the other hand, is generated on site principally from sodium chlorate, by reducing the latter in the presence of reducing agents such as methanol, sulfur dioxide etc., and used in the form of aqueous solution. ClO2 is extremely unstable and corrosive substance. It is potentially explosive. Corrosivity of ClO2 impairs mill’s efforts in the direction of chemical recovery, and recycling and reuse of water.

4.2.1.2 Alkali extraction and oxidative alkali extractionPurpose of alkali extraction is to remove the chlorinated and oxidized lignin from the pulp by solubilization. Alkali extraction stage is used after the chlorine bleaching stage. Certain bleaching sequences include more than one alkali extraction stages. In the Alpha Pulp Mill, a second extraction stage is employed after a hypo bleaching stage. In most of the bleach sequences, oxygen is also dosed to the heated pulp, which is already dosed with alkali, prior to its retention in the extraction tower. Such dosing of oxygen reduces active chlorine demand and colour discharge from bleaching. This extraction, where oxygen is dosed, is known as oxidative extraction. Except in the old pulp unit of the Beta Pulp and Paper Mill, all the pulp units analyzed are using oxidative alkali extraction. Certain bleach sequences involve use of small amount of peroxide (about 0.25% by weight of the pulp) along with oxygen for achieving further reduction in the bleaching chemicals consumption.

Alkali extraction is carried out on pulp of medium consistency (12 to 15%) chlorine bleached pulp at 60 to 80C temperature. The pulp is first added with caustic (at a dose of about 60% to the chlorine dose) in the repulper of the rotary vacuum drum washer of the chlorine bleach stage, and then heated through direct injection of low pressure saturated steam (LP steam) in a heater mixer for raising its temperature to the desired level. After heating, the pulp is transferred into a down-flow extraction tower and retained there for about 2 hours. From the extraction tower, pulp is taken out and washed in a rotary vacuum drum washer prior to sending to the next stage of bleaching. pH of the pulp coming out from the extraction tower is maintained above 10.8.

In case of oxidative alkali extraction, about 4 to 6 kg of oxygen per ton of pulp is dosed to the alkali dosed and heated pulp, and mixed in a high intensity mixer prior to transferring to the extraction tower. The pulp is first passed through an up-flow tube (of 5 to 10 minutes retention), also known as pre-retention tube, for oxygenation to occur, and then transferred into the extraction tower. Pressurization of the pre-retention tube has been reported to result in a saving of 11 kg of active chlorine per ton of pulp bleached, and to reduce AOX generation by 0.5 kg per ton of pulp. Application of one kg of oxygen is reported to result in a saving of 2 kg of active chlorine. Additionally, oxidative extraction has been reported to increase pulp delignification by 25% over that achieved through chlorination and caustic extraction.

4.2.1.3 Chlorine dioxide bleaching Pulp of 11-14% consistency pulp is heated to 70 to 75C in a heater mixer, through direct injection of steam. Then the pulp is mixed with ClO2 in another mixer and then transferred into an up-flow tower, or a down-flow tower (which is having an initial up-flow leg). In cases where up-flow towers are used, pulp coming out from the tower is first flashed, for removing ClO2 vapors and other off gases, and then washed in a rotary vacuum drum washer. In cases where down-flow towers, with initial up-flow legs, are used, residual ClO 2

of the pulp is neutralized through using sulfur dioxide gas and sodium hydroxide solution prior to transferring the pulp to the rotary vacuum drum washer for washing.

Certain bleaching sequences include more than one stages of ClO2 bleaching. For the first stage bleaching pH recommended at the outlet of the bleach tower is 3.5 to 4.0, and in the subsequent stages, pH of the pulp can be in the range of 5.5 to 6.0. Sulfuric acid is used for maintaining the pulp pH within desired range. New pulp unit of the Beta Pulp and Paper Mill is employing two stages of chlorine dioxide bleaching.

4.2.1.4 Hypo chlorite bleaching Hypochlorite bleaching brightens the pulp while preserving its lignin content through preferentially destroying certain chromophoric groups of lignin. This bleaching involves attack on cellulose and chloroform emission. Further, handling of calcium hypo, the bleaching chemical, is troublesome. Problems and disadvantages associated with hypo bleaching are forcing mills to replace hypochlorite bleaching with ClO2 bleaching.

Hypochlorite bleaching is carried out at 35 to 40C and 9.0 pH. For maintaining the pH at the desired level, caustic is added to the pulp. In the old pulp unit of the Beta Pulp and Paper Mill, sulfamic acid is also used, along with the hypo and caustic, in the hypo bleaching. Bleaching sequence of this pulp unit, in fact, involves two hypo-bleaching stages. Bleaching chemicals are dosed to the pulp in the repulper of the preceding bleaching stage. Pulp, after adjusting pH and dosing with hypo, is retained in the hypo tower for 1 to 2 hours. Pulp taken out from the hypo tower is washed in a rotary vacuum drum washer prior to sending to the next stage of pulp processing.

4.2.2 Infrastructure and equipment of a generic bleaching stageProcess and material flow scheme for a generic bleaching stage is shown in Figure-4.2.2. Facilities and equipment associated with a generic bleaching stage include: Heater mixer for raising the pulp temperature to the desired level by direct injection of

steam An MC mixer used mostly for mixing bleaching chemicals with pulp Stand pipe, pump and piping for lifting medium to high consistency pulps to bleach

reaction towers A bleach reaction tower, either an up-flow or a down-flow type, or a down-flow type

with an initial leg of up-flow tube A pulp washer, for washing the bleached pulp (it is usually a rotary vacuum drum

washer)

Rotary vacuum drum washer includes a vat, a rotary vacuum drum, a repulper, a drop barometric leg sealed in a seal pit, and pumps and piping for lifting and reusing seal pit overflows (backwater). This backwater can be reused in: repulper of the previous bleaching stage

outlet of the bleach tower of the same bleach stage for diluting and facilitating pumping of pulp

vat of the same bleach stage for stock dilution displacement shower banks of the rotary vacuum drum washers of some other bleach

stage

Efforts are also made by mills for reusing the backwater at all other feasible points. Left out backwater is allowed to drain out as effluent into the sewer. In certain cases, where the generated backwater is insufficient for meeting the recycling and reuse demands, makeup water is added to the seal pit. In the displacement showers, for washing the pulp, process water, or warm water, or hot water, or backwater of bleach decker or of some other bleaching stage, or even foul condensate produced from blow vapor condensation, or a combination of two or more of these are used. Pulp mat formed over the drum is separated with the help of a doctor blade, after the displacement washing, and collected into the repulper of the washer. In the repulper, pulp is diluted and bleaching chemicals of the next bleach stage are, in some cases, added to the pulp. From repulper, washed pulp is lifted, usually with the help of a standpipe and high consistency pump, and conveyed to the next stage of bleaching. After removing the pulp mat, perforated screen plate of the rotary vacuum drum is cleaned by a water shower (or compressed air) prior to its resubmergence in the pulp of the vat.

Rotary vacuum drum washers are usually enclosed by hoods, which in turn are connected to an active ventilation system. Vent fumes collected by this system are usually treated in an appropriate air pollution control device (APCD) prior to discharging into the atmosphere. Similarly, depending on the need, bleaching reaction tower and seal pit are also connected to active ventilation systems. The vent gases are treated in an appropriate APCD prior to discharging into the atmosphere. Schematic diagram of the ventilation system (including the APCDs) employed in the bleach plant of the new pulp unit of the Beta Pulp and Paper Mill (which is using CDEODD bleach sequence) is shown in Figure-4.2.1

4.2.3 Wood pulp bleach plant of the Beta Pulp and Paper MillSchematic process and material flow diagram of the new bleach plant of Beta Pulp and Paper Mill is shown in Figures-4.2.3a & 4.2.3b. Various operations and activities of this plant include: Chlorine-chlorine dioxide bleaching (Cl2-ClO2 bleach stage) Oxidative alkali extraction stage Chlorine dioxide bleaching –1 (ClO2-1 bleach stage) Chlorine dioxide bleaching –2 (ClO2-2 bleaching) Centri-cleaning Thickening and washing Handling of fumes, vent gases and foam

4.2.3.1 Chlorine - chlorine dioxide bleachingConsistency of the washed brown stock (which is to be bleached) received from the HD tower of the unbleached pulp processing unit is adjusted to the desired level mainly through adding the backwater of Cl2-ClO2 stage of bleaching. Depending on the need even process water is used for this purpose. After consistency adjustment, bleaching chemicals Cl2 and ClO2 are added to the pulp and mixed in the MC mixer. Cl2-ClO2 bleaching is carried out at normal temperature, and hence no heater mixer is included in this bleach stage. Pulp dosed

with the bleach chemicals is retained in an upflow tower (known as C/D tower) for desired duration. After this, the pulp is washed on a rotary vacuum drum washer and sent to the next stage of bleaching.

For washing, in the displacement showers of the washer, warm water, backwater of ClO2-1 bleaching and backwater of oxidative extraction stage are used. For cleaning the perforated screen plate of the rotary drum, hot water is used. Excess backwater generated by the washer, which could not be reused, is drained out as effluent. Quantity of the backwater drained out depends on the quantity of water used in the displacement showers and in the showers meant for cleaning of the screen plate, and on the consistency difference between unbleached pulp of the HD tower and the pulp falling into the repulper).

4.2.3.2 Oxidative alkali extractionWashed pulp is dosed with caustic and consistency is adjusted with the extraction stage backwater in the repulper of the Cl2-ClO2 bleach stage. Then the pulp’s temperature is raised to the desired level by heating in a heater mixer through direct injection of steam. Oxygen is dosed and mixed with the hot alkaline pulp in the MC mixer. Pulp, mixed with the extraction chemicals, is, then, passed through an upflow leg (known as E/O pre-tube) and retained in a reaction tower (known as E/O tower) for the desired duration. After this, it is washed and sent to the next stage of bleaching.

All water requirements of oxidative extraction, excepting those of the displacement showers and screen plate cleaning showers, are satisfied through reuse of the extraction stage backwater. Further part of the backwater is reused in the displacement showers of the washer of Cl2-ClO2 bleach stage. Hot water and backwater of ClO2-1 bleach stage are used in the displacement showers, and air and process water are used in the showers meant for cleaning the screen plate. Depending on the need, warm water is added to the seal pit of the washer, and excess backwater is allowed to drain out as effluent. The amount of wastewater discharged depends on many factors, such as, water consumed in the displacement showers and in the showers meant for cleaning the screen plate, water added to the seal pit as makeup water, backwater supplied to the Cl2-ClO2 bleach stage for reuse in the displacement showers.

4.2.3.3 Chlorine dioxide bleaching –1 (ClO2-1 bleach stage)Washed pulp of the oxidative extraction stage is added with alkali and its consistency is adjusted to the desired level in the repulper of the extraction stage itself. The pulp is then heated to the desired temperature in a heater mixer, through direct injection of steam. Pulp is then dosed with ClO2 and mixed in an MC mixer and then passed through an up flow reaction tower (known as D1 tower). Pulp coming out from the D1 tower is washed and then supplied to the next stage of bleaching.

Backwater of the ClO2-1 bleach stage is reused within the same bleaching stage for meeting most of the water demands excepting that of displacement showers and of the showers meant for cleaning perforated screen plate of the rotary drum. Further, this backwater is also used in the displacement showers of Cl2-ClO2 bleach stage and oxidative alkali extraction stage. Most of the backwater is first neutralized with SO2 prior to supplying for reuse. Hot water and backwater of ClO2-2 bleaching are used in the displacement showers, and screen plate of the rotary drum is cleaned by compressed air. Almost no backwater is drained out as wastewater by ClO2-1 bleaching.

4.2.3.4 Chlorine dioxide bleaching –2 (ClO2-2 bleach stage)This stage of bleaching is very similar to the ClO2-1 bleaching. Alkali is dosed at the repulper of ClO2-1 bleaching. Pulp is heated in the heater mixer and ClO2 is dosed in the MC mixer. Pulp dosed with the bleaching chemicals is passed through an upflow tower (known as D2 tower). Washed pulp of the ClO2-2 bleach stage is taken into a stock chest after adjusting its consistency to the desired level through adding bleach decker backwater. Process water is used in the displacement showers. Backwater of the ClO2-2 bleach stage, in addition to the internal reuses, is reused in the displacement showers of ClO2-1 bleaching. Backwater is mostly reused after neutralization with SO2. There is provision for the addition of hot water as makeup water in the seal pit of the ClO2-2 bleaching. Almost no backwater is drained out as effluent.

4.2.3.5 Centri-cleaning of bleached pulpBleached pulp stored in the stock chest is diluted to the desired consistency with the bleach decker backwater and passed through a set of primary centri-cleaners. Accepts of these centri-cleaners are sent to the bleach decker for thickening and washing. Rejects stream of these centri-cleaners is further processed in a series of three more sets of centri-cleaners (secondary, tertiary and quaternary centri-cleaners) for recovering good and useful fiber. Rejects stream of the final stage of centri-cleaning is drained out as waste. Water requirements of the centri-cleaning system are totally met from the reuse of bleached decker backwater.

4.2.3.6 Thickening and washing of bleached pulpBleached and centri-cleaned pulp is washed and thickened in a bleach decker. Washed and thickened pulp is stored in the HD towers as bleached pulp and from there supplied to the paper machines for the manufacture of paper. Process water is used in the displacement showers for washing the pulp and in the showers meant for cleaning the perforated screen plate of the rotary drum of the bleach decker. Bleach decker backwater is mostly reused for adjusting consistency specially during centri-cleaning. For facilitating use, backwater is stored in an overhead tank and from there supplied to the places of use. Process water is also used as makeup water in the seal pit. Excess bleach decker backwater is allowed to drain into sewer as effluent.

4.2.3.7 Handling of fumes, vent gases and foam The system used for the collection, handling and disposal of fumes, vent gases and foam includes hoods for all the 5 rotary vacuum drum washers of the bleach plant piping for conveying the fumes, vent gases and foam foam tanks, cyclone separators and scrubbers for breaking down foam into vent gas and

liquid streams, for separating entrained water of vent gases and for scrubbing the vent gases.

Backwater of the extraction stage is used in the form of spray for foam breaking. SO 2

solution is used in the scrubbing tower for scrubbing ClO2 contaminated fumes and vent gases. Schematic diagram of this system is shown in Figure-4.2.2.

ClO2 contaminated foam and fumes from the seal pits of Cl2-ClO2 bleach stage, ClO2-1 bleach stage and ClO2-2 bleach stage are collected into the foam tank for breaking them

down into vent gases and liquid stream. The liquid separated is drained out as effluent. The vent gases are sent to the scrubber for neutralization with SO2. Fumes collected from the towers and hoods (of the rotary vacuum drum washers) of Cl2-ClO2 bleach stage, ClO2-1 bleach stage and ClO2-2 bleach stage, which are contaminated with ClO2, are conveyed separately to the scrubber for neutralization with SO2. Water with dissolved SO2 is used here as scrubbing solution and the scrubbing solution is kept in circulation and continuously replenished by injecting SO2.

A blower is provided for collecting and conveying the fumes, vent gases and foam, through the system, and discharging neutralized vent gases into the atmosphere. Vent gases, fumes and foam of washers and seal pits of the extraction stage and bleach decker are not subjected to any treatment. Instead these are connected to the ventilation system just before the blower for facilitating their proper collection and disposal into the atmosphere.

4.2.4 Bleach Chemicals and their productionBleach plants analyzed in the study are using the following bleach chemicals: Chlorine Chlorine dioxide Caustic Sodium or calcium hypochlorite Sulfur dioxide

Of these, the following are prepared on site and made available for bleaching: Chlorine dioxide (it is produced on site from sodium chlorate and supplied to the bleach

plant in the form of a solution prepared in chilled water) Sodium or calcium hypochlorite solution (it is prepared from lime or caustic and

chlorine gas and supplied at the desired strength to the bleach plant) Sulfur dioxide (in the Alpha Pulp Mill, sulfurous acid solution is prepared on site from

elemental sulfur and made available to the bleaching plant) Oxygen (it is produced from atmospheric air through using molecular sieves and

supplied to the bleach plant)

4.2.4.1 Chlorine dioxideChlorine dioxide is unstable and potentially explosive in pure form. Since it cannot be shipped in pure form and even as a concentrated solution, this bleach chemical is produced on site as a gas through chemical reduction of sodium chlorate in highly acidic medium. The produced gas is absorbed in chilled water for producing about 7-gpl solution and supplying to the bleach plant. Methanol or sulfur dioxide or sodium chloride is used as reducing agent for reacting with the sodium chlorite. Production of chlorine dioxide is associated with the production of sulfuric acid, sodium sulfate etc., and byproducts. Sulfuric acid is used usually to provide acidic medium for the reaction to occur.

4.2.4.1.1Chlorine dioxide production unit of Beta Pulp and Paper MillBeta pulp and paper mill is using Solvay process for the on-site production of chlorine dioxide. Process and material flow diagram of the ClO2 production unit is shown in Figure-4.2.4. Filtered solution of hot sodium chlorite, methanol and sulfuric acid are supplied into a generator-crystallizer for the reduction of chlorite and production of ClO2 gas. For

separating by-products formed (sodium sulfate crystals), contents of the generator-crystallizer are continuously circulated through a hydroclone. Sodium sulfate crystal rich stream separated by this hydroclone is passed through a filter for separating the sulfate crystal slurry and sending to the chemical recovery unit for use as a makeup chemical. The filtrate generated is recycled back to the generator-crystallizer. For maintaining desired temperature, contents of the generator-crystallizer are circulated through a reboiler where they are heated by steam.

Off gases of the generator-crystallizer (rich in ClO2) are cooled indirectly by process water and then absorbed in chilled water in an absorption tower. The mill is having a lithium bromide based heat pump for producing and supplying the chilled water required. The resultant ClO2 solution is stored in a tank and supplied to the bleach plant. Unabsorbed gases, of the ClO2 absorption tower, are collected and vented off with the help of a steam jet air ejector system, which uses MP steam. A two stage scrubbing system is employed for handling the off gases of ClO2 solution storage tank. In the first stage of scrubbing chilled water is used and the resultant scrubbing solution is supplied to the ClO2 absorption tower for absorbing ClO2. In the second stage, white liquor of the causticizing section of chemical recovery plant is used and the resultant scrubbing solution is sent to the chemical recovery unit for reuse.

ClO2 production unit involves handling of hazardous substances covered under the hazardous chemicals rules, 1989. For taking care of the emergency situations that may arise, the following provisions are there in the ClO2 unit: Emergency process water supply to the generator-crystallizer Generator dump tank for dumping contents of the generator-crystallizer Containments for the key chemical tanks like chlorite and acid tanks Closed sewers with barometric legs of different ejector systems directly opening

into them

4.2.4.2 HypochloriteHypochlorite is unstable and looses its strength on standing. At higher temperatures (>50ºC) or in the absence of excess alkali, it decomposes and forms chlorate. Hence, hypochlorite is produced on-site through reacting chlorine with 5% caustic solution or milk of lime.

In the Beta Pulp and Paper Mill, calcium hypo is produced through reacting sniff gases and chlorine line purgings of the on-site chlor-alkali plant with milk of lime. Process and material flow diagram of the hypo unit is shown in Figure-4.2.5a. Milk of lime of desired strength is prepared through slaking lime in a lime slacker in hot water and cleaning the resultant slurry in a classifier and a grit-settling tank. Two hypo towers connected in series are used for counter-current scrubbing of the sniff gases and chlorine line purgings with milk of lime. During such scrubbing, chlorine of the sniff gases and purgings reacts with lime and produces hypo. Once strength of the produced hypo (scrubbing solution) reaches the desired level, it is drained out as hypo solution, stored in a hypo storage tank and supplied to the bleach plant.

In the Alpha Pulp Mill, which produces rayon grade pulp, sodium hypo is used in the bleach plant. This hypo is produced through reacting 5% caustic solution with chlorine. Process and material flow diagram of the hypochlorite unit is shown in Figure-4.2.5b. In this unit liquid chlorine supplied in bullets is evaporated in chlorine evaporator and used in the hypo tower for reacting with caustic. Heat energy required for such evaporation is

supplied through jacket heating by steam. 50% strength caustic is diluted to 5% strength with process water in an alkali tank and used in the hypo tower. Sodium hypochlorite produced is stored in a hypo storage tank and supplied to the bleach plant.

4.2.4.3 Sulfur dioxideIn the Alpha Pulp Mill, sulfur dioxide is produced on-site. Process and material flow diagram of this sulfur dioxide unit is shown in Figure-4.2.6. Elemental sulfur, which is stored in open space, is first melted in a melting tank by LP steam and liquid sulfur is injected into the furnace for combustion in excess air. Rate of burning is controlled in the furnace by regulating the furnace temperature through jacket cooling by process water. Required air is supplied with the help of a blower. During the start-up, diesel oil is burnt in the furnace for raising the temperature to the desired level prior to the starting the burning of liquid sulfur. Exhaust gases of the furnace, which are rich in SO2, are cooled by process water and then absorbed in process water in an absorption tower for producing sulfur dioxide solution (sulfurous acid). This solution is stored in a tank and supplied to the bleach plant.

4.2.4.4 OxygenAbsorption based separation systems are used for on-site production of oxygen from atmospheric air. It is an energy intensive dry process. It requires almost no material inputs except atmospheric air. And, it has no significant air pollution impacts. Process flow diagram of an on-site oxygen production unit of Beta Pulp and Paper Mill is shown in Figure-4.2.7. Atmospheric air is first compressed and treated for the removal of water. There may be inter-cooler and after-cooler for ensuring the water removal. Then dry air is passed through a molecular sieve adsorbent at high pressure. At this pressure, nitrogen and other impurities of the air are picked up by the adsorbent, while oxygen comes out unaffected from the adsorption system. This oxygen is further pressurized, stored and supplied to the bleach plant. Once saturated with nitrogen and other impurities, the molecular sieve is regenerated through lowering the pressure, where the adsorbed nitrogen escapes into the atmosphere.

Figure-4.2.2: Schematic diagram of a typical bleach stage

Figure-4.2.1: Schematic diagram of the active ventilation system of the new bleach plant of Beta Pulp and Paper Mill

* Continued in Figure 4.2.3b

Figure-4.2.3a: Schematic process and material flow diagram for the new bleach plant of the Beta Pulp and paper Mill

* Continued from Figure 4.2.3a

Figure-4.2.3b: Schematic process and material flow diagram for the new bleach plant of the Beta Pulp and paper Mill

Figure-4.2.4: On-site chlorine dioxide production unit of Beta Pulp and Paper Mill

Figure-4.2.5: Process and material flow diagrams of Hypochlorite units of Beta Pulp and Paper Mill and Alpha Pulp

Mill

Figure-4.2.6: Process and material flow diagram of sulfur dioxide production unit of Alpha Pulp Mill

Figure-4.2.7: Process flow diagram of non-cryogenic oxygen production unit of the Beta Pulp and Paper Mill

Table 4.2.1: Sequence of bleaching stages adopted by the various mills.

Bleaching stage New bleach plant of Alpha mill

New bleach plant of

Beta mill

Old bleach plant of

Beta mill

Bleach plant of Gamma

mill

Oxygen delignification

Ozone delignification

Enzymatic treatment of pulp

Chlorine bleaching 1

Cl2 – ClO2

bleaching1 1 1

Extraction 2

Oxidative extraction

Oxidative peroxide extraction

2 2 2

ClO2 bleaching 4 3, 4 5

Hypo chlorite bleaching

3 3, 4 3, 4

Peroxide bleaching 5

In this table the order of bleach stages is shown by natural numbers starting from 1 (for first stage) onwards.Here, nth number indicates nth stage in the bleaching sequence.

CHAPTER – 4.3

PROCESS MAPS OF WASTE PAPER PULPING AND

PROCESSING OF SECONDARY FIBER

4.3.1 OverviewPulping is concerned with the disintegration of waste paper and/or paper board into fiber mass or pulp stock. Processing of the pulp stock is concerned with removal of contaminants and print, and with bleaching and colour stripping. Processing results in the production of furnish that is used in the stock preparation for paper making. Processes involved in the pulping and pulp processing are 1. Hydrapulping2. Contaminants removal3. Deinking 4. Bleaching and colour stripping

Process flow schemes employed by the two secondary fiber based paper mills analysed (Epsilon paper Mill and Delta Paper Mill) are shown in Figure-4.3.1. Examination of these process flow schemes indicate that the operations/activities listed below are typically employed in the pulping and processing of secondary fiber:

a. Hydrapulpingb. Screeningc. Cleaningd. Fractionatione. Dispersion and kneading

f. Refiningg. Flotationh. Washing and thickeningi. Bleaching and colour stripping

4.3.2 HydrapulpingHydrapulping is meant for processing the raw-material (waste paper) into defibered stock and supplying to the subsequent operations. Further, pulping is supposed to facilitate separation/removal of contaminants, such as adhesives, inks, laminated materials, etc., from the stock in the subsequent operations. High consistency (>12%) batch pulpers are usually preferred for hydrapulping. In both the waste paper based mills analysed, high consistency batch pulpers are used. Lower level of contaminant degradation, better contaminant removal, lower energy requirements and saving of chemicals (such as deinking chemicals) are the main advantages reported with these pulpers.

The waste paper in the hydra-pulper is slushed and the slushed fiber mass or pulp stock is extracted through an extraction screen plate. For facilitating discharge through this extraction screen plate, the stock is diluted to 3 to 5% consistency, prior to extraction. Unslushed material and contaminants that can not pass through the extraction screen plate are left behind as rejects in the pulper. In certain cases, slushed material is transferred in

total into a detrashing screen, rather than extracting through the screen plate. This screen allows only accepts to pass through to the subsequent operations, while accumulating rejects within. Rejects accumulated in the pulper or in the detrashing screen are removed at regular intervals and discharged, as rejects, after washing for recovering the residual useful fiber.

Water is consumed in the hydrapulping process for the following purposes: diluting the stock and adjusting its consistency to the desired level (backwater of

pulp processing, rather than process water, is mostly used for this purpose) washing the rejects and recovering useful fiber in the screens rotor gland cooling and sealing purposes (in fact, usually, circulating oil cooling

system is used for this purposes, and process water is used for cooling this circulating oil)

Higher pH and temperature substantially reduce pulping time (result in better defiberization) and energy requirements of pulping. Further, they significantly affect separation and dispersion of ink and other contaminants. For ensuring high pulping temperatures hot water is used and direct steam injection into the pulper is practiced. For raising pH to the desired level, caustic is usually added into the pulper. The caustic, in addition to ensuring better defibering, limits attrition of contaminants. Hydrogen peroxide is often added to the pulper along with sodium silicate and other chemicals, usually, for preventing alkali darkening of fiber and/or facilitating print separation.

Surfactants are often added to the pulper for facilitating dispersion removal of wax, which is a common additive in the corrugated grades of waste paper. Low temperature pulping (>50C) is often preferred over high temperature pulping, specially, for preventing melting and dispersal of contaminants like wax that cause stickies problem, and facilitating their easy removal during cleaning operations. When deinking is desired, pulper is added with deinking chemicals, such as hydrogen peroxide, peroxide stabilizing chemicals, etc. If the waste paper contains wet strength resins, appropriate chemicals are added to the pulper for breaking these resins.

Filtrate generated from the screening of rejects is rich in useful fiber, and mostly reused in the hydra-pulper for stock consistency adjustment.

4.3.2.1 Tridyne Pulper of the Epsilon Paper MillTridyne pulper is a high consistency (12%) batch pulper. It is used in the mill for hydrapulping old corrugated cuttings, cement sack broke and used cement sacks. Waste paper bales are opened, manually screened for removing undesirable materials and loaded into the pulper with the help of a conveyor. Backwater of the pulp mill is mostly used for slushing and adjusting consistency of the stock. Slushing is carried out at elevated temperature and pH. Steam is directly injected into the pulper for maintaining the slushing temperature and caustic flakes are added for raising the pH. After slushing, the pulp is diluted to 5 to 6% consistency and neutralized with hydrochloric acid, and then extracted out through an extraction screen plate.

Once the slushed pulp is extracted out, the material that could not pass through the extraction screen plate and accumulated in the pulper is drained out as rejects stream into a trommel screen, prior to the start of next batch of slushing. This rejects stream contains residual fiber, plastics, metallic pins, etc., and trommel screen is used for recovering water and residual fibre from these rejects in the form of trommel screen filtrate. This filtrate is

reused in the tridyne pulper for consistency adjustments. Rejects of the screen are conveyed out and disposed off as trommel screen rejects. Some fraction of the trommel screen rejects are manually sorted for recovering plastics & ployethylene, metal pieces and unslushed fibrous material separately.

Circulating oil cooling system is used for the gland cooling of the rotor of tridyne pulper. Process water is used for cooling this circulating oil and the cooling water generated is drained into sewer as effluent.

Schematic diagram of the tridyne pulper and the associated units of the Epsilon Paper Mill is shown in Figure-4.3.2.

4.3.3 Processing of waste paper stockProcessing of waste paper stock, obtained through hydrapulping, involves conversion of the heterogeneous raw material into a homogeneous stock through removal of non-fibrous material and contaminants. This processing can be considered to include the following three processes:a) Removal of contaminantsb) Deinkingc) Bleaching and colour stripping

4.3.3.1 Removal of contaminantsContaminants are removed from the pulp stock through cleaning, screening, washing, flotation, etc., operations. Usual practice is to remove larger size contaminants first and then smaller size contaminants. Further, the stock is processed first at higher consistency and then at lower consistency for contaminants removal.

Pulping, which is meant for the break down of inter-fiber bonds and fiber separation (defiberization), leads to separation of contaminants, such as, additives, adhesives, inks, laminated materials, etc., from the fiber and bringing them into suspension. The suspended contaminants are then removed from the stock through screening, cleaning, flotation and washing operations. pH of the alkaline stock is, sometimes, adjusted to 5-6 to facilitate agglomeration of certain types of colloidal contaminants and their removal through fine screening and cleaning operations. First, alkaline flotation, and then, acidic washing at 5 to 6 pH is sometimes practiced for the efficient removal of contaminants, specially ink particles and stickies. Use of solvents (water free solvent wash) for dissolving waxes, stickies and other contaminants has also reported. But recovering the used solvent is problematic and hence scarcely used.

Dispersion/kneading is often used, after pulping, for dispersing contaminants like wax, bitumen, etc., or for rolling up the contaminants like foil and polyethylene. The dispersed, or rolled up, contaminants are then removed from the stock by washing and/or flotation operations, or by screening and cleaning operations. High consistency and high temperature dispersion (>80C), using high speed dispergers, is often practiced for reducing the contaminants size and disguising their presence. Talc is often added to the pulper, or to the flotation cell (for reducing foam levels) or even to the deinked stock, for pacifying or tackling stickies.

4.3.3.2 Deinking

Deinking is concerned with the removal of print without loosing useful fiber. For deinking to occur, ink particles should not be bound to fibers or trapped in the fibrillar areas. Further, these should not be present in the bound water layers of individual fibers.

Hydrapulping leads to the breakdown of print into ink particles of a wide size range. Dispersion/kneading also helps in the deinking process through breaking down larger ink particles into smaller ones, which in turn can be removed by wash deinking. Either flotation deinking, or wash deinking, or both, are used for removing ink particles from the repulped stock. The latter is effective in removing smaller size ink particles (1 to 10 micron size particles), while the former for removal of larger ink particles (10 to 150 micron size particles).

Use of caustic during pulping helps in breaking down the print, through weakening bonds between the fibers and the print, and the ester bonds of the print vehicle network (the network that holds ink particles together). However, use of caustic can darken the recycled fiber, specially mechanical fiber. For preventing this, hydrogen peroxide is used along with caustic in the pulping process. For preventing decomposition of the added hydrogen peroxide, sodium silicate and chelating agents, such as diethylene triamine penta acetic acid (DTPA), are also used in the pulping process. Sodium silicate, in addition protecting the hydrogen peroxide, assists deinking process through – buffering the system and supporting the peroxide activity, acting as a dispersing agent for the print particles, and reacting with calcium and forming precipitate that helps in the agglomeration of print particles. Sodium silicate is also reported to reduce loss of fiber during flotation deinking.

Soaps, surfactants, talc, etc. are also used in the deinking process. Soaps are added either to the pulper or to the flotation cell. These assist in the flotation removal of ink particles, by enhancing their hydrophobic nature, and agglomerating them. Efficient performance of soaps require presence of sufficient concentration of calcium ions. Hence, frequently, calcium salts, such as, calcium chloride and calcium hydroxide, are also added to the flotation deinking systems. In some cases, instead of soaps, fatty acids are added to the pulper. These acids in turn create soap in situ. Displectors (fatty acid alkoxylates), which assist both in detaching print from the fiber and collecting the detached print particles into agglomerates with hydrophobic surface, are also sometimes used in the deinking systems. Talc is also added to the flotation deinking systems mainly for pacifying the stickies and reducing foam levels.

Proprietary mixtures of surfactants and solvents (usually known as ink collector chemicals) are used in the wash deinking systems. These chemicals do not require calcium ions. Here, the surfactant molecules keep ink particles in suspension, till they are removed by wash deinking systems, through adhering to the latter and making them hydrophilic. Further, the surfactants are believed to wet the print and allow the applied deinking chemicals to penetrate into its micro cracks. Use of cyclohexyle pyrrolidone, as two phase mixture with water for removing the print, is also reported.

For efficient removal of UV cured inks, which are used in printing and photocopying, solvents (such as paraffin, terpentine) and surfactants, or solid thermoplastics, are added to the pulper, and pulping is carried out at elevated temperature. High temperature pulping results in the flowing of print into spheres. As the stock cools, these spheres get hardened, and such hardened spheres are then removed either through screening (in slotted pressure screens) or through centri-cleaning. Enzymes are also commonly used for deinking. The added enzymes detach print from fiber through degrading cellulose and hydrolyzing ester bonds of the print vehicle network. Enzyme deinking can eliminate use of sodium

hydroxide, sodium silicate, hydrogen peroxide, etc., in the deinking systems.

Use of surfactants in wash deinking can affect coagulation, flocculation and separation treatment of backwater. Similarly, use of flocculants in the treatment of backwater, when the latter is reused in the deinking process, can affect performance of the surfactants in the wash deinking process. Deinking is associated with the loss of fiber, fines and fillers from the stock.

Delta Paper Mill is using a two stage flotation deinking system. Schematic diagram of this is shown in Figure-4.3.3.

4.3.3.3 Bleaching and color stripping Oxidative bleaching agents, like hydrogen peroxide and sodium hypochlorite, and/or reductive bleaching agents, like sodium hydrosulfite and formamidine sulphinic acid, are usually used for the bleaching and color stripping of recycled fiber. Commonly used bleaching processes include:a) Hydrogen peroxide bleachingb) Sodium hydrosulfite bleachingc) Hypochlorite bleaching

4.3.3.3.1Hydrogen peroxide bleaching Wastepaper composed mainly of mechanical fiber is, usually, bleached with hydrogen peroxide.. Bleaching chemicals (hydrogen peroxide, and sodium hydroxide, sodium silicate and chelating agents) are applied to the stock at any of the following three places: in the hydra-pulper; in the bleach tower, and in the dispersion unit. High temperature, high consistency and longer retention time are considered important for ensuring effective bleaching. Further, pH around 10.5 is considered optimal for the peroxide bleaching.

Bleaching in the pulper is not very effective (specially due to low temperature). Still, split bleaching, wherein part of the bleaching dose is applied to the stock in the pulper, is practiced (may be to prevent alkali darkening of the stock rather than for bleaching). Bleaching, in a bleach tower, is, usually, carried out on the deinked stock. This bleaching, which is carried out at higher temperature (70C) and higher consistency of stock, is more effective than that in a pulper. For protecting the stock from alkali darkening, bleach tower output is ensured to contain residual peroxide. Dispersion unit, where high pulp consistency (around 25%) and high temperature (>70C) conditions are maintained for achieving the desired dispersion, is considered as the most appropriate place for peroxide bleaching. For providing necessary retention time, dispersed stock is held in a retention tower prior to sending for further processing. Dispersion bleaching is usually followed by flotation.

Sodium silicate is known to adversely affect drainage properties of the pulp. Use of chelating agents, such as DTPA sodium salt of DTPMP (diethylene triamino penta methylene phosphoric acid) is reported to reduce the dose of sodium silicate required. Recycled backwater may have high catalase activity, and its use can lead to the catalytic decomposition of the peroxide added.

4.3.3.3.2Sodium hydrosulfite bleaching This bleaching is effective for the stock that consisted mainly of chemical fiber. It is carried out on deinked stock at relatively lower consistency (3 to 5% consistency), pH (6.2 to 7.2), and temperature (around 60C) conditions through retaining in the bleach tower for 1 to 2

hours. Some mills follow two stage bleaching: sodium hydrosulfite bleaching and hydrogen peroxide bleaching. After hydrosulfite bleaching, prior to peroxide bleaching, the bleached stock should be sufficiently washed to ensure that no residual hydrosulfite or its byproducts, such as, sodium bisulfite and sodium thiosulfite are left in the stock. Hydrosulfite, and its byproducts can exert additional peroxide and caustic demand during peroxide bleaching.

4.3.3.3.3Hypochlorite bleaching Sodium hypochlorite is a color stripping chemical. It discharges dyes from recycled fiber rather than bleach the latter. Hypochlorite is preferred specially for discharging red and yellow colors, for which hydrosulfite and hydrogen peroxide are ineffective. If the fraction of high yield pulp is greater than 10 to 15% in the stock, hypo bleaching may prove ineffective. Hypo bleaching can impart yellow color, specially, to the mechanical fiber of the stock. Optimal conditions for the hypo bleaching are – pulp consistencyof 10 to 15%, pH around 10, temperature about 40C, and retention time upto 2 hours. Stock coming out from the hypo bleaching tower is usually ensured to contain residual hypochlorite in order to prevent coloring of the stock. Hypochlorite bleaching generates chlorinated derivatives (chloroform in the emissions and chlorinated organic compounds in the effluent).

4.3.3.3.4Bleaching of pulp stock in the Delta Paper MillIn the Delta Paper Mill, the pulp stock is first subjected to oxidative bleaching with hydrogen peroxide, and then, to reductive bleaching with sodium hydrosulfite and bisulfite. Screened, cleaned, deinked and dewatered stock is heated through direct steam injection in a screw heater mixer and then passed through the disperger. At the disperger, oxidative bleach chemicals are dosed to the stock. After dispersion, the stock is transferred into a bleach tower and retained there for about 90 minutes. Then, the stock is passed first through a flotation cell and then through a disc filter for thickening. Thickened stock is again heated through direct steam injection and dosed with the reductive bleach chemicals. After this, the stock is passed through an upflow tube (bleach tower). Stock takes about one hour for passing through the upflow tube. Pulp stock coming out from the upflow tube is stored in a processed stock chest and used in the stock preparation. A schematic diagram of the two stage bleaching practiced in the Delta Paper Mill is shown in Figure-4.3.3.

4.3.4 Operations and activities associated with secondary fiber processing

Processing of the waste paper stock can be considered to include the following operations/ activities:a) Cleaningb) Screeningc) Flotationd) Washing, thickening and dewateringe) Kneading and dispersionf) Refining

4.3.4.1 Cleaning of the pulp stockCleaning is mainly concerned with the removal of high density impurities from the stock.

High density centrifugal cleaners (HD cleaners) and diluted stock centrifugal cleaners (centri-cleaners) are usually used for this purpose.

4.3.4.1.1HD cleanersHD cleaners are used for removing high density materials, such as, staples, grit, fragments of bale wire, nuts, bolts, etc., from relatively high consistency (2.5% or more) stock. These cleaners are very much similar to centri-cleaners, but have an electrically powered impeller for imparting centrifugal force to the pulp. High density materials are removed from the stock and get collected into a dirt tank provided at the rejects discharge end of the cleaner. Useful fiber present in this dirt tank (along with the rejects) is recovered through use of elutrition water. This water is introduced into the dirt tank for maintaining certain minimum upflow velocity, which is sufficient for re-suspending the fiber and carrying it back into the body of the cleaner. Rejects accumulated in the dirt tank are drained out at regular intervals. In the Epsilon Paper Mill, these rejects are screened over a side hill screen for draining out water prior to disposal.

4.3.4.1.2Centri-cleanersCentrifugal cleaners are used for the removal of heavier particles (sand, grit, clay, etc.), abrasive contaminants and lighter particles (waxes, polyethylene film, polystyrene, etc. provided they are not stably dispersed) from the stock. Three types of centri-cleaners – forward flow cleaner, reverse flow cleaners and thru flow cleaner – are, usually, used. Forward flow centri-cleaners are meant for the removal of heavier particles and the other two types are for the removal of lighter particles. Stock is diluted to 0.7 to 1.2% consistency and then passed through the centri-cleaners. Centri-cleaning leads to the division of the feed stock into “lights” stream and “heavies” stream, or into “accepts” stream and “rejects” stream.

Rejects stream is usually about 10% of the feedstock. This stream still has high concentration of useful fiber. For recovering this fiber, the rejects stream is further cleaned in a series of centri-cleaners. In the Epsilon Paper Mill, a three stage centri-cleaning system of forward flow centri-cleaners is used. In the Delta Paper Mill, a five stage centri-cleaning system is used. Rejects stream of one stage of centri-cleaning is fed as input to the next stage of centri-cleaning. Accepts stream of one stage of centri-cleaning is fed to the previous stage of centri-cleaning through mixing with its input stream. Consistency of rejects stream is higher than that of input stream and accepts stream. Hence, before feeding to the next stage of centri-cleaning, consistency of the rejects stream is reduced to the desired level through adding water.

Schematic diagram of a three stage centri-cleaning system employed in the Epsilon Paper Mill is shown in Figure-4.3.4. Rejects from the last stage of centri-cleaning are passed through a side hill screen for draining out water prior to disposal.

4.3.4.2 Screening of pulp stockScreening is concerned with the removal of relatively larger size contaminants (like plastics, fiber flakes, stickies, etc.) from the pulp stock. Trommel screens, perforated or slotted pressure screens, screw screens, vibratory screens, side hill screens, etc., are usually used for screening the pulp stock.

Trommel screens are rotating drum screens and these are mostly used for the removal of

plastics from high consistency (around 5%) pulp stocks extracted from the hydra-pulper. Perforated pressure screens are used for coarse screening of the stock at relatively high consistency (operated at 2.5% consistency of pulp in the Epsilon Paper Mill). Fine slotted pressure screens are operated at relatively low consistency (operated at 1.0% consistency in the Epsilon Paper Mill) for fine screening the pulp stock. Vibratory screens and screw screens are usually used as rejects sorters for recovering useful fiber from the rejects streams of perforated or slotted pressure screens. Side hill screens are mostly used for draining out water from the rejects stream prior to the disposal of the latter.

Perforated pressure screens are good for removing fiber flakes from the pulp stock. These have tramp metal traps for accumulating heavier coarse impurities of the stock. In other words, these screens divide the fed stock into an “accepts” stream, a “lighter rejects” stream and a “heavier rejects” stream. Lighter rejects stream is usually deflaked first and then subjected to the next stage of screening for recovering useful fiber from rejects. In the Epsilon Paper Mill, a separate type of screen known as Belcor screen, where both deflaking and screening occur simultaneously, are used for recovering useful fiber from the rejects stream. Rejects stream of the Belcor screen is further screened over a vibratory screen for recovering useful fiber. Fine slotted pressure screens are efficient in removing stickies. These are usually located after the centri-cleaners. However, in the Epsilon Paper Mill, these are located upstream to centri-cleaners. Rejects stream of these screens is first centri-cleaned and then passed through a secondary fine slotted pressure screen for recovering useful fiber. Rejects stream of this secondary screen is further screened over a vibratory screen for recovering useful fiber. Schematic diagram of pulp screening systems employed in the Epsilon Paper Mill is shown in Figure-4.3.5.

Water is required for adjusting consistency of the pulp stock prior to loading to the pressure screen and of the rejects prior to loading to the subsequent stages of screening. Water is also consumed in the pressure screens both for diluting the rejects and the screen plate to achieve uniform consistency within the screening compartment. Water is also required on the vibratory screens for washing down useful fiber from the rejects.

4.3.4.3 Flotation Flotation has two applications, namely, deinking flotation and dissolved air flotation. Deinking flotation is used for removing ink particles and other smaller size contaminants (samller in size than the fines and fiber) from the stock. Dissolved air flotation is used for the recovery of fines and fiber from backwater.

For the flotation removal, contaminant particles should be hydrophobic and their size should be in the range of 10 to 150 microns. Further, the particles should be in suspension. Flotation is mostly carried out on cleaned and screened pulp stocks of 0.6 to 1.2% consistency, under alkaline conditions at 40 to 45C. Flotation may involve use of soap and deinking agents (ink collector chemicals), talc, etc. Salts like calcium chloride are also added to the stock for ensuring requisite hardness. Hydraulic retention time for flotation deinking units is usually around 20 minutes.

Flotation is an energy intensive operation. Usually 4:1 or 10:1 air to stock ratio is employed. Air bubble size is maintained in the range of 0.3 to 0.5 mm. Smaller bubble size increases loss of fiber, while larger bubble size reduces deinking efficiency. Flotation generates waste in the form of froth or scum. This froth is usually removed as overflow, or through a vacuum system. The froth removed usually contains useful fiber and for recovering this fiber, the froth is defoamed and again subjected to flotation (secondary

flotation). Flotation requires dilution water and elutrition water, for adjusting and maintaining stock consistencies, and generates significant quantities of effluent.

Delta Paper Mill uses altogether three flotation cells (primary flotation cell; post-flotation cell and secondary flotation cell) for flotation deinking purpose. Process and material flow diagram of flotation cells of the Delta Paper Mill is shown in Figure-4.3.3. This mill uses the primary flotation cell for contaminants removal from screened stock at 1.2% consistency. Post flotation cell is used for contaminants removal after oxidative bleaching of the stock, again at 1.2% consistency. Froth generated by both primary and post-flotation cells is collected into a foam tank, defoamed and then processed in the third flotation cell (secondary flotation cell).

Dissolved air flotation systems are usually employed for efficiently recovering fines and fiber from (machine) backwater. In these systems, part of the clarified output of the flotation unit is super-saturated with compressed air in a pressurized mixing chamber. This super-saturated water is then introduced, along with the backwater, from which fines and fiber are to be recovered, into a non-pressurized flotation unit. In this unit, additional air of the water is released in the form of very fine microscopic air bubbles. These bubbles get attached to the fines and fiber of the backwater, reducing their density, and causes them to float to the surface. Through removing this floating layer, the fines and fiber are recovered and the backwater is clarified. Epsilon Paper Mill is using dissolved air flotation system for recovering fiber and fines from the machine backwater. Schematic diagram of the system employed by this mill is shown in Figure-4.3.6.

4.3.4.4 Pulp washing, thickening and dewatering Screening, cleaning and flotation of the pulp stock is carried out at low consistency. Kneading/dispersion, bleaching, refining, etc. operations are carried out on the stock usually after screening and cleaning operations, and these operations are usually performed at high consistencies. This necessitates introduction of thickening and/or dewatering operations in between the two groups of operations. Washing of stock is needed as an operation for the removal of dissolved, colloidal and dispersed contaminants, such as ash, fillers, fines, ink particles etc. Subsequently, processed stock is finally thickened prior to sending to the storage chest for use in the stock preparation.

For increasing washing efficiency, temperature and pH of the stock are often increased, and synthetic surfactants are often used. Similarly, surfactants and phosphates are often added to the stock for improving the fillers removal efficiency. Occasionally, acid wash is practiced for removing sticky particles. Some hydrophilic organic materials dissolve or become suspended during the alkaline pulping and, on the paper machine, on acidification, these materials are destabilized and precipitated as sticky particles.

Gravity thickeners, rotary vacuum drum washers/thickeners, disk filters, belt washers, screw presses, twin roll dewatering presses, side hill screens, etc., are used for the washing and/or thickening and dewatering of pulp stock. Thickening, specially dewatering, is usually carried out in two stages. In the Epsilon Paper Mill, the pulp is thickened first in a gravity thickener to 5-6% consistency and then in a twin role dewatering press (TRDP) to 28% consistency. In the Delta Paper Mill, a disc filter and a screw press are used for thickening the pulp first to 10% consistency, and then to 30% consistency, respectively. Schematic diagrams of the thickening and dewatering systems employed in the Epsilon Paper Mill and in the Delta paper mill are shown Figures-4.3.7 & 4.3.3. Multi-stage washing is often practiced for maximizing the washing or ash removal efficiency.

Washing, thickening and dewatering generate backwater. This water is mostly reused in the pulp processing, usually after certain level of pretreatment, specially, for removal of the colloidal and dispersed contaminants. However, reuse of backwater is usually associated with problems like corrosion, deposition, scaling, etc. Further, the recycled water may have higher biological activity and thus can affect the effectiveness of hydrogen peroxide, which is added to the stock for deinking, bleaching, etc., purposes.

Different equipments used for pulp washing, thickening, and dewatering operations, are discussed below.

Gravity thickener: It is a rotary drum filter and it thickens the stock through draining out water. The rotary drum is partially submerged in a vat containing dilute stock. Because of water level difference between contents of the vat and filtrate of the drum, fiber mat is formed over the drum and water is drained out under gravity from this mat. For achieving further thickening of the stock and to couch off the thickened stock from the drum surface, a rubber couch roll is often used. Perforated screen plate of the drum is maintained clean through application of a water shower. Gravity thickeners can thicken the stock from about 1% consistency to above 3.5% consistency. Poor sealing between the vat and the drum can increase fiber loss into the filtrate.

Rotary vacuum drum washer/thickeners: These are very similar to brown stock washers. This washer/thickener requires water both in displacement showers and in the showers meant for cleaning the perforated screen plate of the drum. Using these washers/ thickeners, the stock can be thickened to 10 - 16% consistency or even more. In these units, relatively higher efficiencies of pulp washing can be achieved. These units are also commonly used for washing the pulp after each stage of bleaching and for brown stock washing, and also for thickening the washed and bleached pulps.

Disk filters: These are mostly used as thickeners for thickening the stock from very low consistency to as high as 30% consistency. In the Delta paper mill, disk filters are used for thickening the stock from around 1% consistency to around 10% consistency (see Figure-4.3.8 for the schematic diagram of the disc filter system used). Vacuum and compressed air assisted disk filters are in fact used in this mill for thickening the screened, cleaned and flotated stock. Disc filters are also used for recovering fiber from backwaters. Water is required for cleaning the perforated screen plate of the disc. Filtrate generated by the disc filter is collected as two separate streams – cloudy filtrate stream and clear filtrate stream. Cloudy filtrate stream is reused for diluting the stock during secondary fiber processing, while the clear filtrate stream is drained out as wastewater.

Belt washer/thickener: These are very efficient, and have made rotary vacuum drum washers and gravity thickeners less popular. These washers resemble Fourdrinier of a paper machine. On this washer, the stock is dewatered in the nip formed by large diameter rollers and wire, or between a grooved large diameter rollers and wire. Thickened/washed pulp mat of the wire is doctored off and the wire is cleaned prior to its return to the head box, where the stock is loaded. Cleaning of wire requires water. Stock is fed to the belt washers at 0.7 to 3% consistency and thickened to above 6% consistency. As high as 80% ash removal efficiency can be achieved in these washers. A belt washer is used in the Delta Paper Mill, not for secondary fiber processing, but for thickening the sludge recovered from the settling of backwaters.

Side hill screens: These can also be used for the washing and thickening of pulp. But use of these is associated with certain disadvantages, such as, very low capacity, high fiber loss, etc. In the Epsilon Paper Mill, these screens are extensively used for draining out water

from the final rejects generated from both pulp screening and cleaning operations.

Screw presses: These are used as both washing units and thickeners. These can thicken 2 to 10% consistency feed stock into as high as 35% consistency stock. These presses are very efficient in removing ash and ink from the stock, and the efficiency of removal increases with the decreasing consistency of feed stock. Backwater generated by screw presses is relatively rich in fiber and fines, and hence, mostly reused. In the Gamma Pulp and Paper Mill these are used for dewatering the fractionated brown stock.

Twin roll dewatering press (TRDP): Thickened pulp is squeezed by forcing it to pass between two rotating rollers, while pressing one roll against the other by a hydraulic system. Process water is used for hydraulically holding the rollers closer and for cleaning the rollers and doctor blades. Water squeezed out from the stock, water used in the hydraulic system for holding the rollers closer, and water used for cleaning the rollers and doctor blades come out as wastewaters which have reuse potential.

4.3.4.5 Kneading and dispersion Dispersion is an operation meant for any of the following purposes: Dispersion of ink particles and avoiding specky appearance without removal - specks of

size > 40 microns are visible; specks of smaller size (<40 microns) affect pulp brightness; but, specks of size < 1/10th of the wavelength of visible light have no effect on brightness

Dispersion of stickies Release of print from the recycled fiber Size reduction of the ink particles and other contaminants Mixing of bleach chemicals with the pulp stock

Dispersion is carried out at relatively high temperature (in the range of 50 to 125C) on thickened stock of consistency >15%. Dispersion of asphalt like contaminants may require a temperature >150C. Higher temperature dispersion uses pressurized systems. Dispersion is an energy intensive operation, and generates no rejects or waste streams. It reduces pulp refining requirements, but leads to the generation of fines.

Either kneaders (low speed dispersion units) or dispergers (high speed dispersion units) are used in the dispersion operations. Kneaders are preferred when the objective is to separate contaminants from the fiber and to break them into smaller size particles. Dispergers are preferred for dispersing speckies and ink particles and preventing specky appearance. Since dispersion is carried out at high temperature on high consistency stock, disperger is considered as an appropriate place for the addition of bleach chemicals, such as, NaOH, H2O2, sodium silicate, etc.

Dispersion units, specially kneaders, are frequently followed by washing or flotation units for the removal of dispersed contaminants. Chemicals, such as, caustic, hydrogen peroxide and sodium silicate are usually added in the pre-flotation kneading. Further, the pre-flotation kneading may be followed by a soaking tower. In some installations pulping is followed by kneading for rolling up the contaminants, such as, foil and polyethylene. Screening and/or cleaning operations are usually used for removing the rolled up contaminants. Dispergers are usually installed at the end of the wastepaper processing street for stickies dispersion, for speck size reduction, or for the elimination or reduction of pulp refining needs.

4.3.4.6 RefiningRefining is used less frequently in the secondary fiber processing. When used high consistency refiners are employed. Purpose of such refining is for imparting certain special properties, like, stretchability. Refining is an energy intensive activity and refiners are associated with noise and vibration problems. Circulating oil cooling systems are usually associated with these refiners, and temperature of the circulating oil regulated through cooling with water. Epsilon paper Mill, which produces industrial paper for the manufacture of cement sacks, is employing a double disc refiner for refining its dewatered stock at 28% consistency.

Figure-4.3.1: Process flow schemes for waste paper pulping and processing

Figure-4.3.2: Schematic diagram of the tridyne pulper and the associated units of the Epsilon Paper Mill

* Continued in Figure-4.3.3b

Figure-4.3.3a: Schematic process and material flow diagram for waste paper pulping and processing unit of Delta Paper Mill

* Continued from Figure-4.3.3a** Continued in Figure-4.3.3c

Figure-4.3.3b: Schematic process and material flow diagram for waste paper pulping and processing unit of Delta Paper Mill

(contd..)

* Continued from Figure-4.3.3b** Continued in Figure-4.3.3d

Figure-4.3.3c: Schematic process and material flow diagram for waste paper pulping and processing unit of Delta Paper Mill

(contd..)

* Continued from Figure-4.3.3c

Figure-4.3.3d: Schematic process and material flow diagram for waste paper pulping and processing unit of Delta Paper Mill

(contd..)

Figure-4.3.4: Schematic diagram of a three stage centri-cleaning system employed in the Epsilon Paper Mill

Figure-4.3.5: Schematic diagram of pulp screening systems employed in Epsilon Paper Mill

Figure-4.3.6: Dissolved air flotation system (Krofta unit) employed for fiber and fines recovery from secondary fiber in the Epsilon

Paper Mill

Figure-4.3.7: Schematic diagram of the thickening and dewatering system employed in the Epsilon Paper Mill

CHAPTER – 4.4

PROCESS MAPS OF PAPER MAKING ON PAPER MACHINE

4.4.1 OverviewPaper making involves processing of fiber mass or pulp for improving its paper formation properties, and adding different additives and process control chemicals to it for preparing paper furnish. Diluted paper furnish is transformed into a pulp mat or paper web by spreading it as a thin layer and dewatering on a paper former. Dewatered paper web is dried to reduce moisture content to acceptable level through energy intensive drying operations. Dried paper is further processed for obtaining the final paper for shipment. Papermaking can be considered to include the following activities:a) Stock preparationb) Approach system to the paper machinec) Wet-end operations and activitiesd) Dry-end operations and activitiese) Off-machine operations and activities

4.4.2 Stock PreparationStock preparation is an interface between the pulp mill and the paper machine. Fiber mass or pulp obtained from the pulp mill is used here for preparing furnish, which can be run on a paper machine to produce paper or paperboard. Pulp used in the stock preparation may include any of the following or a combination of two or more of the following: a) High consistency bleached or unbleached pulp (in case of integrated pulp and paper

mills)b) Pulp obtained from the processing of secondary fiber (in case of waste paper based

mills)c) Broke pulp (wet and dry broke from the paper machine and fiber recovered from the

paper machine backwater)d) Dewatered or dried pulp (bleached or unbleached) and sheet pulp obtained from other

pulp mills for use as input raw-material

A mill may have one or more streets for stock preparation. Each of these streets will be processing a different type of pulp or preparing a different type of stock or furnish. Different streets of a mill may be interconnected through operations like fractionation, blending, etc.

4.4.2.1 Operations and activities of stock preparationStock preparation may include the following operations/activities: Hydrapulping Fractionation Beating and refining Cleaning and screening

Fiber recovery and thickening Blending of different pulps and different additives in desired proportions

Stock preparation described hereunder excludes hydrapulping and processing of waste paper (secondary fiber).

4.4.2.1.1HydrapulpingHydrapulping is an important operation in the mills that use pulps (dewatered pulps and dry pulp sheets) sourced from other mills. It is also an important operation in the broke system of a paper mill (broke system is meant for collecting, pulping, processing, and reusing paper machine broke). However, in case of an integrated mill, since pulp is supplied as high-density stock by the mill’s pulp unit(s), hydrapulping is not an important operation. Hydrapulping is concerned with adding water and slushing the received pulps into a stock of desired consistency. Mostly, backwater is used in the hydrapulping.

4.4.2.1.2Fractionation Fractionation is an enrichment process involving division of a low consistency fed stock into two or more classes on the basis of fiber length. Multistage pressurized fractionators with either holes or slots are used for this purpose. Beta Pulp and Paper mill is fractionating unbleached virgin bamboo pulp in a 3-stage fractionation unit for separating long fiber fraction. The separated long fiber fraction is dewatered in a screw press and supplied to a paper mill, which is manufacturing cement sacks, for blending with the secondary fiber. Fractionation contributes no rejects. However, water is consumed for consistency adjustments and backwater is generated from the dewatering of fractionated pulp.

4.4.2.1.3Beating and refiningBeating/refining of pulp is meant for imparting optimum paper making properties to the pulp. Recycled fiber, being preformed, is not usually subjected to this operation. Beating is an electrical energy intensive batch operation. Hollander Beater is usually used for beating the pulp.

Refining is energy intensive continuous process. Conical refiners or disc refiners or conical disc refiners are used for refining the pulp. Generally refining is practiced in two stages. Objective of the first stage refining is to optimize strength development. This stage of refining is performed on relatively high consistency pulp. Recycled fiber, and high yield mechanical and chemi-mechanical pulps usually do not require this stage of refining. Objective of the second stage refining (which is also known as machine refining) is to achieve control over freeness of pulp. Blended pulps of machine chest, having about 3% consistency, are subjected to the second stage refining prior to being sent to the head box.

Refining produces no rejects. However, this activity is associated with noise and vibration problems. Water is used in the refiners for gland cooling and sealing. Some refiners (specially high consistency refiners) have circulating oil-cooling system and water is consumed in these systems for cooling the circulating oil. Refining may result in shortening of fiber and formation of fines, and reduce drainability of the pulp.

4.4.2.1.4Blending Blending is concerned with mixing of different stocks (mechanical fiber, chemical fiber,

recycled fiber, broke, etc.) in desired proportions and addition of different non-fibrous components (additives and process control chemicals) to the blended stock.

Additives may include the following: Wet-end sizing chemicals such as modified rosin, IVAX (commercial name for

dispersed rosin formulation for internal sizing), wax emulsions, synthetic sizing agents, etc.

Dry strength additives such as natural and modified starches, gums and non-synthetic polymers, and polyacrylamides – starches and gums are added as cooked low concentration solutions.

Wet strength additives such as urea-formaldehyde, melamine-formaldehyde, polyamide resins, etc.

Fillers such as clay (kaolin, bentonite), calcium carbonate, titanium oxide, talc (magnesium silicate), etc.

Dyes, pigments and optical brighteners

Process control chemicals added may include Alum or polyaluminum chloride Drainage and retention aids Pitch dispersants like talc Defoamers and slimicides Silicates Acids and alkalies (for adjusting pH) Corrosion inhibitorsDepending on the fiber stock used, and on the product manufactured, chemicals and additives used may vary. Stuff boxes are usually used for metering and accurate proportioning of different pulps. Accurate dosing of additives and chemicals is achieved through preparing solutions or suspensions of known concentration and regulating their flow rates.

Blending is performed in a chest. But, for simple paper machine furnish, an independent blending chest may not be used. Instead blending is performed within the machine chest. Some of the additives and process control chemicals may not be added to furnish in the blending chest. Instead, they may be added to the furnish, prior to loading into the head box, or to the white water, which is used for diluting the furnish, prior to its loading into the head box. In the Epsilon Paper Mill, while drainage and retention aids are added to the furnish prior to its loading to the head box, defoamers and slimicides are added to the white-water.

4.4.2.2 System for the collection, processing and reuse of brokeThis system is meant for the collection, processing, and recycling of paper machine broke. Fiber recovered from the clarification or filtration of machine backwater (and/or of backwater from the stock preparation area) is also handled by this system.

Two categories of broke, wet broke and dry broke, is generated by a paper machine. Broke generated by wet operations of the machine (pulp mat formation, dewatering and press dewatering of the formed pulp mat) is known as wet broke. Broke generated by dry operations of the machine (cylinder drying, calendaring, sizing, coating, sheeting, reeling, winding, etc.) and by various off the machine operations (such as, sheeting, coating, rewinding, finishing or packaging, etc.) is known as dry broke.

Collection, processing and recycling of broke and recovered fiber usually involves the following operations: Pulping of the wet broke in couch pit Pulping of the dry broke in under the machine pulper (UTM pulper) Clarification or filtration or thickening Cleaning, deflaking and screening Broke storage and supply for reuse in the stock preparationMost of the wet broke is collected into the couch pit. During normal machine running, only trims of the pulp mat are slushed and washed by low volume showers into the couch pit. Whenever the pulp web or paper mat breaks, total pulp mat is slushed and washed by high volume showers into the couch pit. From the couch pit, slushed broke is continuously pumped into a broke storage chest. Pumping rates of broke are maintained high during breakage of the pulp mat or paper web for promptly removing the broke accumulated in the couch pit and avoiding pulp wastage in the form of couch pit overflows.

Dry broke mainly includes the following: Broke generated during threading Edge cuttings and paper rejects at rewinders and sheet cutters Paper reel rejects due to the lack of properties or quality

Most of the dry broke generated is loaded to a low consistency UTM pulper, either continuously or in batches, for slushing. This slushing involves consumption of water and electrical energy, and even steam. Slushed pulp of the UTM pulper is pumped into the broke storage chest, usually after cleaning, screening and deflaking, and if needed, after thickening to the desired consistency. Concentration of contaminants and flakes in this pulp is relatively higher, and hence, this pulp usually requires cleaning and screening.

Broke generation rates are highly variable, and UTM pulper capacity frequently proves insufficient for handling all the dry broke generated. Beta Pulp and Paper Mill has installed a few additional hydrapulpers for taking care of the additional broke. In the Epsilon Paper Mill, this additional broke, which could not be handled by UTM pulper, is repulped along with wastepaper.

Fiber recovered from the backwater, with the help of save all units (Krofta unit, disc filter, backwater clarifier, etc.), is recycled and reused through pumping into the broke storage chest and supplying it to the blending unit.

Capacity of the broke storage chest may, frequently (specially during long periods of upset operation of the paper machine), prove insufficient. Broke storage capacity in certain cases (in the Epsilon Paper Mill) is enhanced through thickening the broke and storing as relatively high consistency pulp in the broke storage chest.

Broke of the storage chest is subjected to zone dilution to the desired consistency and then pumped to the blending chest for reuse, usually after passing through cleaning, deflaking and screening units. In the Epsilon Paper Mill, instead of subjecting the total broke to cleaning, deflaking and screening operations, only dry broke, after pulping in the UTM pulper, is subjected to cleaning and deflaking operations. Process flow scheme of the broke system employed by Epsilon Paper Mill is shown in Figure-4.4.1.

4.4.2.3 Stock preparation in the Epsilon Paper Mill Epsilon Paper Mill uses secondary fiber, virgin bamboo long fiber fraction and imported virgin sheet pulp for preparing the stock. Operations and activities involved in the stock

preparation are: Hydrapulping HD cleaning Centri-cleaning Screening Refining Thickening and dewatering Blending

Stock preparation in the Epsilon Paper Mill is schematically shown in Figure-4.4.2.

4.4.3 Approach System to the Paper Machine An approach system can be considered to include the operations and activities, between blending chest and head box of a paper machine, to which blended furnish is subjected to. Typical operations and activities that constitute an approach system are: Machine refining Metering of furnish Dilution of furnish Centri-cleaning Fine slot pressure screening Dosing of furnish with process control chemicals and additives

Blended furnish is taken into a machine chest and from there pumped into the head box for loading the paper machine. Furnish pumped from the machine chest is first subjected to machine refining for controlling its drainage properties. Then it is metered (with the help of a stuff box) and diluted with rich white-water (which is supplied from a white-water silo), in the fan pump to, desired consistency (usually <1%). Diluted furnish may be passed through a set of centri-cleaners and/or a fine-slotted pressure screen and then taken into the machine head box for loading the machine. Some of the additives and process control chemicals (specially drainage and retention aids) may be added, to furnish, in the machine chest, or in between the stuff box and the head box. For avoiding foaming problems, the stock is often de-aerated through spraying it into a vacuum compartment prior to taking it into the head box. Process flow sequence for approach system of paper machine of the Epsilon Paper Mill is shown in Figure-4.4.3.

Rich white-water generated by the wet-end of the machine is collected in a white-water silo and used for diluting furnish to desired consistency level. In the silo, steam is injected for maintaining white-water temperature at desired level.

Centri-cleaner rejects are further processed in secondary and tertiary centri-cleaners for recovering fiber from it. After tertiary cleaning, the rejects are often further processed in a fiber-miser prior to finally disposing it as waste. Dilution of rejects prior to centri-cleaning consumes dilution water at each stage of centri-cleaning. Similarly, rejects stream of the pressure screen is also processed further in secondary and tertiary screens prior to disposing as waste. Screening may also involve use of water specially for washing the final rejects.

In the Epsilon Paper Mill, alum solution, dye solutions, and IVAX (a wet-end sizing chemical) are added to furnish in the blending chest. Starch solution, and drainage and retention aids solution, in this mill, are added to the furnish between the stuff box and the head box. Figure-4.4.3 gives the details.

Preparation of solutions/suspensions of different additives and process control chemicals

involves use of process water, and even, steam. The systems used for the preparation, storage and dosing of different additives and process control chemicals require frequent cleaning, and such cleaning generates effluent. In case of the system used for dye solutions, frequency of cleaning is higher (required whenever dye composition is changed). Prior to each such cleaning, draining out of residual dye solution from the system is required. Drained out dye solutions add colour to the effluent.

4.4.4 Wet end operationsWet-end operations and activities are concerned with the formation of paper web, and with the gravitational, vacuum assisted and press dewatering of the web. The mills analyzed in the study are using two types of formers, namely, Fourdrinier formers and Cylinder formers. In both the cases, there are facilities and procedures for collecting and, mostly, reusing the backwater generated from the web dewatering. Vacuum system that is being employed for dewatering purposes is very important from the angles of water consumption, effluent generation, and noise and vibration problems.

4.4.4.1 Fourdrinier Former Wire part is central in the formation and dewatering of paper web in the Fourdrinier Former. This wire is continuous, traveling from breast roll to couch roll and returning back to the breast roll through a wire turning roll and a series of wire return rolls. Schematic process and material flow diagram of Fourdrinier Former of the Epsilon Paper Mill is shown in Figure-4.4.4. A head box, which is located at the breast roll end above the wire, is used for loading the Fourdrinier wire with furnish at the breast roll end. As the wire moves towards the couch roll, the web is dewatered and delivered at the couch roll end to the press dewatering section. Wire part of the Fourdrinier is divisible into the following four zones:a) Web formation and free draining zone b) Vacuum assisted web dewatering zone c) Web transfer zone d) Fourdrinier wire return zone

4.4.4.1.1Web formation and free draining zone This zone can be considered to include the wire part starting at Breast Roll and ending at vacuum assisted wet boxes. Important parts of this zone include Web Formation Board, Hydrofoils, Rotating Table Rolls, Sheraton Rolls and Stationary Deflectors.

Furnish is received on the wire part from the head box through a sluice opening of flow spreader. For ensuring width wise uniform distribution, this spreader is usually provided with a stock recirculation facility. This recirculation line of the spreader is connected to the suction of the fan pump for ensuring recycling and reuse of the furnish. Small shower jets, which are positioned close to the wire just behind the sluice jet/opening, are often used for generating turbulence and preventing premature flocking of fiber (a cause for poor web formation). Sluice jet delivered on the wire tends to fan out at its extremes and affect formation of web edge. For better control of this edge formation, edges of the sluice jet are bled off. Further, for containing the delivered furnish, rubber deckles are used along the wire edges during initial web formation stage.

Web formation mostly occurs over a forming board. For facilitating better web formation,

initial drainage and washout of fines and fillers is retarded in this formation board area. Hydrofoils and rotating table rollers are supposed to assist in draining water from the web, while the stationary deflectors are to help removal of water, which is pulled beneath the wire surface.

Wet boxes are vacuum assisted hydrofoils. These have full width slot at the bottom (working as a barometric leg) submerged in a water trough. Vacuum is maintained in these boxes through connecting them to a vacuum system. Water drained from the web, in the free draining zone and in the vacuum assisted wet boxes, is rich in fines, fillers and other chemicals (its consistency is usually above 0.01 to 0.02%). This water, which is also known as white-water, is collected into a tray (white-water tray) and conveyed to a white-water silo usually through shallow open channels. For suppressing foaming, defoamers are added to the white-water trays and water sprays are applied on the white-water channels. Temperature of the white-water present in the silo is maintained high, usually through direct injection of steam, for facilitating better drainage.

4.4.4.1.2Vacuum assisted web-dewatering zone Wire part that includes vacuum dry boxes can be considered as vacuum assisted web-dewatering zone. These dry boxes are connected to the vacuum system of the paper machine – for ensuring suction dewatering of the web – through cyclone separators. Cyclone separators remove entrained water from the suction air. Barometric legs provided to these separators help in conveying the removed water to a seal pit (seal pit-1). Water separated from the suction air is conveyed into the first compartment of the machine backwater tank.

A Dandy Roll is mounted above the wire in this zone. Water (in the form of high pressure shower) is used in the dandy roll for cleaning its mesh fabric. Upstream to this roll, a collection pan is provided for collecting the water held in the mesh fabric and flung out, and also the water used in the high-pressure shower. Water collected in the pan is conveyed into the wire pit.

4.4.4.1.3Web transfer zone This zone can be considered to include the couch roll and the wire turning roll, and even the pickup roll (which is in fact part of the first press). Some of the mills may also have a lump breaker roll over the couch roll. Couch roll is a hollow perforated shell and contains 1 or 2 stationary high vacuum suction boxes. These boxes are connected to the vacuum system and help in dewatering the web. High-pressure water showers are used in the couch roll for keeping perforated shell clean.

Dewatered web is transferred from the couch roll of the Fourdrinier to the pick up roll of the press section. Pick up roll is very similar to the couch roll. It also has stationary high vacuum suction boxes connected to the vacuum system. High-pressure water showers are used here, also, for keeping perforated shell of the pickup roll clean. Paper web transferred to pickup roll, actually, moves over a felt, which supports the web while it is moving through the first press. For facilitating hot press, steam showers are often applied on the web upstream to the couch roll.

A narrow strip on either side of the web is trimmed off, as the latter leaves the couch roll, in order to reduce the frequency of web breaks. High-pressure water jets, known as squirts, are used for such trimming. Trims separated from the web, and moving with the returning wire,

are slushed and washed into a couch pit (which is located immediately below the couch roll) by low volume water showers. When web breaks, full width web, instead of getting transferred to the pick up roll, moves with the returning wire. This web is slushed and washed into the couch pit through high volume water showers. Slushed pulp getting collected in the couch pit is known as wet broke. This broke is handled by the broke system described earlier and recycled to the blending chest for reuse.

After transferring the web to pick up roll, Fourdrinier wire turns back over a wire turning roll (located below the couch roll) and returns to the breast roll over a multitude of wire returning rolls (located under the white-water tray of the web formation and free draining zone).

4.4.4.1.4Fourdrinier wire return zoneFourdrinier wire return zone starts with wire turning roll and ends with the breast roll. This zone includes a multitude of wire return rolls (and also stretch rolls and guide rolls). Wire turning rolls are provided with doctor blades and debris collection trays. Further, these are provided with wetting showers for keeping them and the associated doctor blades clean. A series of showers are used for keeping the wire clean and free from buildups. Some of these are high-pressure needle showers. In some mills, flood showers are used for washing off stickies and flushing out fillers and fines from the wire. Prior to the breast roll, chemical showers, and even steam showers are usually applied on the wire for conditioning.

Wash water generated, from the cleaning of wire, is collected into a wire pit, located immediately below the wire, and from there conveyed to the second compartment of a backwater tank. First compartment of the backwater tank receives overflows from seal pit-1 and from white-water silo. From the first compartment, the backwater is pumped to the white-water silo for maintaining constant level in it. Excess backwater of the first compartment is allowed to overflow into the second compartment. Backwater of the second compartment is usually pumped through save all units (Krofta unit, disc filter, clarifiers, etc.) for recovering fiber. Clarified wastewater of the save all units is mostly reused at different places on the machine in place of process water. Excess backwater of the second compartment, which cannot be pumped through the save all units, is sewered as effluent. Similarly, the clarified effluent, which is not reused is also sewered.

4.4.4.2 Cylinder FormerCylinder formers are mainly used for the manufacture of package grade paper, especially, multiply webs. Cylinder former is identical to a gravity thickener. The furnish is delivered into vat of the former. Rotating drum screen of the former, due to hydraulic pressure gradient, picks up the furnish and hence a fiber mat is formed on it. A soft rubber couch roll provided over the drum screen help in picking up and transferring the web to the moving felt. In case of high-speed formers, a suction roll replaces the rubber couch roll.

Various alternative configurations include;a) Dry vat type cylinder formers, wherein the formation area is limited to a small part of

the vat b) Suction formers, wherein vacuum boxes are provided inside the rotating drum for

further reduction in the web formation area and for better dewatering of the fiber web c) Pressure formers, wherein pressure is applied on the furnish side (contrary to providing

suction in the rotating drum) for reducing the formation area required and for better dewatering.

On-top formers, also a type of cylinder formers, are under-the felt formers. On-top formers are one category of cylinder formers, where the fiber web is formed on the top of the felt. In these formers, the fiber mat is formed on a wire moving over the rotating suction drum. There is another category of on-top formers, known as ultra formers, wherein the fiber mat formed is sandwiched between the perforated drum screen and a felt, or between a wire (moving over the rotating suction drum) and a felt. In some of the modified ultra formers, partial formation of the fiber/paper mat is achieved on a Fourdrinier like former prior to the entry of the mat into the cylinder former over the rotating suction drum sandwiched between the wire and a felt.

4.4.4.3 Press dewatering of fiber/paper web Press dewatering of the paper web is carried out in a press section, which usually has two or three presses. Press is a mechanical dewatering device and includes the following: Press rolls – these may be suction rolls, very similar to couch rolls, having a

perforated shell rotating around one or two stationary high vacuum suction boxes connected to a vacuum system. Or, these rolls may be grooved, or having blind receptacles for holding the water removed from the paper web during pressing. Press rolls may have facilities, such as, sprays and doctor blades for cleaning their surface. Two press rolls usually form a press.

Press felt – this acts as a receptor for the water squeezed out from the web and it also supports the web. A press may have either one or two felts. In certain presses, a non-compressible fabric belt is used to support the felt. This belt also acts as a water receptor.

Facilities for removing water, and for cleaning and conditioning the felt and the belt – these facilities may include high-pressure showers, chemical showers, steam showers, suction boxes (known as Uhle boxes), etc.

Schematic diagram of the press section of Epsilon Paper Mill is shown in Figure-4.4.5. Web supported over a felt is passed through the press nip for press dewatering. A press nip may be single felted or double felted and it may be single vented or double vented (nip is formed by grooved or blind drilled rolls). First press acts also as a pick up roll, and hence, a suction roll is used for forming the first press nip. Use of non-compressible fabric belt is also common with the first press.

Water squeezed out from the web during pressing is temporarily held in the felt, in the non-compressible fabric belt (if used), and in the grooves and receptacles of the press rolls. If stationary suction boxes are provided in the press rolls, then part of the water is removed by the suction created by the vacuum system. In case of suction rolls, for keeping the perforated screen plate clean, high pressure water showers are used. For collecting the wash water generated from the cleaning of the roll and the water present in the grooves and receptacles of the rolls, a collection pan is usually provided on the upstream side of the suction roll.

Immediately after passing through the press nip, felt (and also the belt if used) is separated from the paper web and cleaned with high-pressure water showers. For better cleaning, these water showers are, at times, added with detergents and other chemicals. Water used in the form of showers for loosening the entangled material, is collected into a collection pan and sewered. After cleaning, the felt is dewatered by passing over one or more suction boxes (known as Uhle boxes). Further, before returning back to the press nip, felt is subjected to a variety of mechanical and/or chemical conditioning treatments. Suction

boxes of the rolls and Uhle boxes are connected to the vacuum system through cyclone separators. These separators remove entrained water from the suction air and convey the water to a seal pit (known as seal pit-2) through barometric legs. Overflow of the seal pit-2, which is rich in stickies, are sewered.

For transferring the web from couch roll to press, and from one press to the next, air blast, or suction, or some other mechanism is used. In certain cases, for avoiding multiple draws of paper web between presses, two or more press nips are provided on a single large smooth roll. For increasing paper mat consolidation, and for better dewatering, hot pressing (pressing at 60 to 90C) is preferred. In such cases, for raising the web temperature, steam showers are applied on one side of the web, while suction is applied on the other.

4.4.4.4 Vacuum system Stationary suction boxes of couch roll of Fourdrinier and suction rolls of presses, Uhle boxes of presses, and vacuum assisted wet and dry boxes of Fourdrinier are connected to a common vacuum system. This system includes a vacuum header, vacuum pumps, vacuum pump discharge trench, and a vacuum plume sump. Liquid ring vacuum pumps are commonly used for creating the required suction in the system. Vacuum plume sump is provided with a vent for the escape of the air sucked in. Schematic diagram of a typical vacuum system is shown in Figure-4.4.6.

Vacuum pumps consume electrical energy and are associated with noise and vibration problems. Most of the energy used is converted into waste heat. Water used for gland cooling and sealing by vacuum pumps drives out this waste heat. Hence, temperature of vacuum plume water is relatively higher than of the water used for gland cooling and sealing. Since the system is used for suction dewatering of the web and of the press felts, vacuum plume water gets contaminated with water and other particulate matter entrained in the suction air. Cyclone separators, which are provided in between the suction points of the system and the vacuum header, limit such contamination of vacuum plume water.

4.4.5 Dry-end Operations of a Paper Machine Dry-end operations of a paper machine may include the following:a) Drying of the paper webb) Surface sizing c) On-machine surface coating d) On-machine calendaring e) Reelingf) Sheeting and baling

4.4.5.1 Drying of dewatered paper web Drying is concerned with reducing moisture content of the dewatered paper web (which is coming out from the press section) to the desired level (usually to <10% level) through application of heat and evaporation of water from the web. Excepting Alpha Pulp Mill, all the five mills analyzed are using cylinder dryers for this purpose. Alpha Pulp Mill, which manufactures sheet pulp, on the other hand, is using an air float dryer for this purpose.

4.4.5.1.1Cylinder Drying Drying section comprises of a series of large diameter, rotating cylinders filled with steam.

Paper web, coming out from the press section, is passed over a series of cylinder dryers for evaporation dewatering. A synthetic permeable fabric (known as dryer felt) is used for tightly holding the web against cylinder dryer surface and facilitating better heat transfer. A dryer section may have 3 to 5 pairs of dryer felts. Each of these pairs includes a top felt and a bottom felt. In certain cases, where sheet fluttering and subsequent breaking is a problem, instead of using paired dryer felts, serpentine felts are preferred for continuously supporting the moving web. For tightly holding the web against cylinder dryer, these felts are controlled by tensioning and positioning rolls.

Saturated steam is injected, into each of the cylinder dryers, as a source of heat. Condensation of the injected steam on the inner surface of the cylinder releases heat, which is picked by the web, which is in contact with the cylinder on the outer side. Absorbed heat is used for the evaporation removal of water while the web is transferred from one cylinder to the next. Steam condensate formed inside the dryer cylinder is continuously siphoned out with the help of a specially designed siphon assembly. For ensuring proper siphoning, pressure differential is maintained between the steam inlet line and the condensate outlet line of the cylinder. For avoiding accumulation of non-condensable gases, while siphoning out the condensate, 15 to 20% of the steam applied is also siphoned out as entrained steam.

For better control over the pressure of the applied steam, dryer cylinders, of a dryer section, are usually divided into 3 to 5 groups, and each group is provided with independent steam pressure control and application system. Condensate, along with the entrained steam, drawn out from one group of dryer cylinders is taken into a separator tank for separating the entrained steam. Entrained steam thus separated is recycled and reused in either of the following two ways:a) Directly using it as input steam in some other group of dryers, which require relatively

lower pressure steamb) Passing the steam through a thermo-compressor, where it is mixed with high pressure

super heated steam, and reusing the resultant steam in the same group of dryers as input steam

Condensate of the separator tanks of all the groups of dryer cylinders is collected and pumped to the boiler house for reuse as boiler feed water, provided it satisfies quality requirements. Otherwise the condensate is drained out as effluent.

Entrained steam from that group of cylinders, which is operated at the lowest steam pressure, is separated and passed through a surface condenser, or through a direct contact condenser, for condensation. Non-condensable gases accumulating in this condenser are sucked out and vented out with the help of a blower. Schematic diagram of steam/condensate flow sequence for the dryer section of the Epsilon Paper Mill is shown in Figure-4.4.7. The Mill uses a direct contact condenser. Condensate, together with the process water used for condensing, is collected here into a warm water tank for reuse. Warm water that cannot be reused is drained out as effluent.

Auxiliary operations/activities associated with the cylinder drying

Auxiliary operations/activities associated with the cylinder drying of the Epsilon Paper Mill include the following: System for supplying hot dry air to facilitate evaporation dewatering of the paper web Ventilation system for driving out hot humid air from the cylinder drying area Condenser for handling exhaust steam and non-condensable gases of cylinder dryers Centralized circulating oil lubrication & cooling system for the rollers of cylinder

dryers Machine drives and their air cooling system Under the machine pulper (UTM Pulper) for handling dry broke

Auxiliary operations/activities of cylinder drying section of Epsilon Paper Mill are schematically shown in Figure-4.4.8.

Pockets formed between the top and bottom dryer cylinders, make escape of humid air (loaded with evaporated water) very difficult. To facilitate removal of this humid air, or to ensure better pocket ventilation, dry (hot) air is injected into these pockets at one end for driving out humid air through the other end. Frequently, drier felt rolls are also used for introducing dry air into these pockets. Humid air pushed out from the pockets is then removed by an active exhaust/ventilation system.

Adequate pocket ventilation is supposed to improve steam economy (kilojoules of energy or kilograms of steam consumed for evaporating one kilogram of water from the paper web) of the cylinder drying section. Steam economy can be significantly improved through heat recovery from the exhaust hot humid air. This can be recovered by air-to-air heat exchangers, wherein the incoming ventilation air is heated by outgoing hot humid exhaust air. But, heat recovery efficiency of these exchangers is usually low (10 to 15%). However, through using this hot humid exhaust air for process water heating, the recovery efficiency can be raised to as high as 80%.

Conventional mechanical drives were used for running the paper machine. The drives included a shaft running parallel to the machine. On this shaft, wherever required, in-drives were provided. Now-a-days sectional electrical drives are preferred over mechanical drives. The trend is to move from adjustable voltage, direct-current drives with analog speed regulators, to adjustable frequency alternating current drives with digital regulators. Rollers of the sectional electrical drives are air-cooled by a system comprising of a blower and air supply piping.

Rollers of the cylinder dryers are lubricated and cooled by circulating lubrication/cooling oil. Epsilon Paper Mill has a centralized circulating lubrication and oil cooling system for this purpose. This system includes a cooled lubricating oil storage tank, an oil cooler, and pumps and pipings, for circulating the cool oil through the rollers of the cylinder dryers. Oil cooler uses process water for cooling the oil.

4.4.5.1.2Air float dryerIn the Alpha Pulp Mill, sheet pulp is dried to 90 to 95% air-dry weight or 81 to 86% oven dry weight level, through an air float dryer. Pulp sheet coming out from the press section is introduced into the air float dryer chamber at the top from behind. Within the chamber, the pulp sheet is made to make a number of passes (back to front and front to back) for drying with hot air (impinged on both sides of the sheet) and then taken out from dryer chamber at the bottom from the front.

Fresh air, preheated by the outgoing hot humid exhaust air, is made to enter the dryer chamber at the bottom. After circulation for several times between passes of the pulp sheet, the air is eventually allowed to come out from the chamber at the top through air-to-air heat exchanger. On each pass/circulation within the dryer chamber, air is heated in a steam coil heater and pumped through horizontal blow boxes, which issue air jets through the openings on both top and bottom sides of the moving pulp sheet with the help of blowers. Schematic diagram of the air float dryer is shown in Figure-4.4.9.

4.4.5.2 Surface sizing Surface sizing is meant for providing resistance against penetration by aqueous solutions, for providing surface characteristics, and for improving physical properties like surface strength.

Cooked, enzyme modified, or oxidized, starch is used for surface sizing. Wax emulsions, or special resins, are often added to this starch solution. Starch is cooked either in a batch system or in a continuous system and supplied to the sizing unit as low viscosity solution of 4 to 10% solids. In case of a batch system, mixture of water and starch is heated and maintained at 88 to 93C for 20 to 30 minutes, either by direct steam injection or by circulating it through a heat exchanger. While heating by direct injection of steam, care is taken to avoid localized boiling of the starch slurry and subsequent foaming and spatter problems.

Surface sizing is applied at the size press, which is located usually between the last two groups of cylinder dryers. In case of heavier papers and boards, sizing is applied at the machine calendar stack. Size solution is taken in a water box, and with the help of a rubber lip it is applied on the calendar roll. Solution present on the calendar roll gets applied on the sheet at the calendar nip. Sizing may be limited to one side or applied on both sides of the paper web.

In the size press, the paper web entry nip is flooded with the starch solution. Overflows of this size press nip pond are collected in a pan located below the press and recycled to the size nip pond for reuse. Sizing involves adsorption of water by the paper web, and hence sized web requires drying at the expense of additional steam. Sizing is also associated with frequent machine shuts.

On one of the machines of the Beta Pulp and Paper Mill, surface size is applied at a size press, which is located between the third and last group of cylinder dryers. Schematic diagram of this sizing process is shown in Figure-4.4.10. The system used for surface sizing requires frequent cleaning and such cleaning involves use of process water and generation of wastewater. Splashing of size solution from the size press nip pond is a common problem.

4.4.5.3 On-machine surface coating Surface coating is categorized into pigment coating and functional coating. Functional coating is similar to sizing and usually involves application of lacquer, varnish, waxes, resins etc., as a coat on the sheet. Pigment coating on the other hand involves application of fillers mixed with adhesives and other components.

Coatings used in the pigment coating are aqueous dispersions of 50 to 79% solids. 80 to 90% of these solids are mineral pigments, such as, china clay, barium sulfate, calcium carbonate, synthetic silicates, titanium oxide, satin white (prepared from slaked lime and alum), etc., and plastic pigments like polystyrene. Rest 10 to 20% of the solids are binders and additives. Coating binders used include starches, proteins and synthetics.

Coating dispersions are prepared in the coating kitchen, and the preparation may involve the following steps: Preparation of dispersions of individual coating components and storing them

separately

Metering the individual dispersions, from storage into a high viscosity mixer in a preset order, and mixing

Extracting the mixed coat into agitated holding tanks and from there sending to the place of coat application.

Through staining, at almost every stage, foreign materials and oversize solids are prevented in the coat.

Usually a sized base sheet is subjected to surface coating. Coating is mostly an off-machine operation. Beta and Gama Pulp and Paper Mills are practicing off-machine surface coat application. On-machine surface coating is practiced when light to moderate coat application is desired and quality requirements of the coating are not very stringent. Two separate coating stations are required for coating both the sides of a base sheet. After coating on one side, the sheet is dried prior to coating on the other side. A wide variety of coaters are used for applying the coat. Commonly used categories of coaters include: roll coaters, air knife coaters, rod coaters, blade coaters, etc.

On one of the machines of the Delta Paper Mill on-machine surface coating is practiced.

After coating, the sheet is dried and calendared. For avoiding disturbance to the coating film, instead of conventional steam cylinder drying, air impingement drying or infrared drying is often followed. Tunnel dryers are also commonly used for drying the coated sheets. However, in case of one-sided coating, for drying the sheet high velocity convective hoods placed over the conventional cylinder dryers can be used. In case of infrared dryers, the sheet is dried without bringing it in contact with anything. However, dry air is used for driving out the moisture separated. In these dryers, coated sheet is passed over rollers, foils or air cushion through a tunnel for drying.

4.4.5.4 Calendaring Calendaring involves pressing the paper web in one or more roll nips. Objectives behind such pressing are to obtain a smooth surface, to achieve compaction of the web, to improve cross-direction uniformity, etc. Calendaring is performed on a dried paper web (at 3 to 4% moisture level) or sometimes on partially dried paper. Prior to calendaring, for ensuring enough moisture, the web is passed over a cooled sweat roll (having condensed moisture), or a steam shower, or moist shower is applied over the web, or moisture may be added to the web surface by means of water boxes.

Calendaring is done in a calendar stack. Within this stack, there can be one or two heat transfer rolls. Through these rolls hot water is usually circulated. Calendaring can be either an on-machine activity or an off-machine activity. On-machine calendaring is performed on the paper web usually after drying. However, in certain mills, it is carried out in the calendar nips provided between the last two groups of cylinder dryers.

4.4.5.5 Reeling Paper product is collected on a drum reel (also known as collecting roll). A reel, known as pope reel, is provided ahead of the drum reel but after the calendar stack, for applying adequate tension on the sheet being collected. Cooling water is usually circulated through the pope reel for cooling the paper web prior to its reeling.

4.4.5.6 Sheeting and Baling

In the Alpha Pulp Mill, dried pulp web coming out from the air float drier is slit and cut, and the product is obtained in the form of stacks of pulp sheets. Each of the stacks is conveyed to a hydraulic bale press, weighed, hydraulically pressed and packaged into bales through wrapping in brown paper and tying with metal straps. Bale press uses process water for cooling purposes. Packaging involves consumption of brown packaging paper and metal straps.

4.4.6 Off-machine operations and activitiesImportant among the off-machine operations/activities area) Tub sizingb) Calendaring and super-calendaring c) Rewinding d) Sheeting and balinge) Roll finishing and packaging

4.4.6.1 Tub sizingOff-machine tub sizing is practiced when better quality sizing is desired. Here the paper web is run through a bath containing sizing solution for the size application. Then with the help of a light nip excess size solution is removed from the paper web surface. Initial drying of the sized sheet is mostly accomplished by hot air impingement.

4.4.6.2 Calendaring and super calendaringOff-machine calendaring is relatively less practiced. Super calendaring is almost always an off-machine activity and it is preferred for producing highest quality printing papers. This activity involves use of calendar nips formed by an iron roll and compressed fiber roll (soft roll). Soft roll of the super-calendar is usually cooled by water.

4.4.6.3 Rewinding Purpose of rewinding is to cut and wind the full width large diameter paper reel into suitable size rolls. Winding involves trimming off of the two edges of the roll. Trims generated are pneumatically conveyed to the UTM pulper as dry broke. This activity involves consumption of paper board cores.

4.4.6.4 Sheeting and packagingHere, the paper reel is slit and cut into desired size sheet. The generated trimmings are handled as dry broke. The sheets are stacked together and packaged after wrapping in packaging paper. Packaging usually involves use of wrapping paper, polyethylene sheet, glue, etc.

4.4.6.5 Roll finishing or packagingRoll finishing may involve sealing, wrapping, crimping (folding over the wrapper overlap), heading (gluing a circular piece over the crimped overlap), and labeling. This activity also involves use of paper board cores, wrapping paper, polyethylene sheet, glue, etc.

Figure-4.4.1: Process and material flow diagram of Broke System of the Epsilon Paper Mill

Figure-4.4.2: Process flow scheme for stock preparation in the Epsilon Paper Mill

Figure-4.4.3: Process and material flow scheme for approach system of the Epsilon Paper Mill

Figure-4.4.4: Schematic process and material flow diagram of Fourdrinier former of Epsilon Paper Mill

Figure-4.4.5: Schematic diagram of press section of Epsilon Paper Mill

Figure-4.4.6: Schematic diagram of a typical vacuum system

Figure-4.4.7: Schematic diagram of steam/condensate flow sequence for the Dryers section of the Epsilon Paper Mill

Figure-4.4.8: Schematic diagram showing auxiliary operations and activities of cylinder dryers of the Epsilon Paper Mill

Figure-4.4.9: Schematic diagram of air float dryer used in the Alpha Pulp Mill

Figure-4.4.10: Schematic diagram of sizing employed on one of the machines of the Beta Pulp and Paper Mill

CHAPTER – 4.5

PROCESS MAPS OF UTILITIES AND SERVICES

OF PULP AND PAPER MILL

Utilities and services of a pulp and paper mill is considered to include the following:

I. Extraction, treatment and supply of waterII. Generation and supply of steam

III. Generation/extraction and supply of electrical energy`IV. Production and supply of instrumental air and plant airV. Maintenance activities of the mill

VI. Other utilities and services

4.5.1 Extraction, treatment and supply of water

4.5.1.1 OverviewThe pulp and paper mills analyzed have been using the following types of waters for meeting their water demands:a) Process waterb) Soft waterc) Demineralized waterd) Recycled steam condensatee) Chilled waterf) Warm water and Hot waterg) Foul condensateh) Backwater

Raw water, obtained from either a surface source or an underground source is treated and used by the mills both as process water and fire-water. The mills are using a fraction of the supplied process water in the production of soft water, demineralized water, chilled water, warm water and hot water for meeting their requirements. In addition to these, the mills are collecting and using the steam condensates and the foul condensates generated within. The mills are collecting the wastewaters generated within, rather than treating and disposing as effluent, and extensively reusing as backwaters, either directly or after some level of pre-treatment.

4.5.1.2 Process waterThree of the five mills studied are dependent on nearby rivers as source of water, while the other two are depending on the groundwater sources. The mills, which depend on groundwater, are directly using the extracted groundwater, without any treatment, as process water. But, the mills, which are using river water, are treating the extracted water mainly for the removal of suspended and colloidal solids prior to its use as process water.

Water treatment systems employed by these mills include the following units: Intake well Flash mixing unit (for mixing the coagulation chemicals) Clari-flocculation unit Filtration unit (only Alpha Pulp Mill is using this unit in the production of process

water; two other mills, Gamma Pulp and Paper Mill and Epsilon Paper Mill, are using this unit only for producing drinking water)

Chlorination unit (employed only for producing drinking water or water for domestic consumption)

Schematic diagram of a water treatment plant (WTP) employed in Alpha Pulp Mill is shown in Figure-4.5.1. This plant is using the following chemicals in the water treatment: Alum (as coagulation-flocculation agent) Lime (for adjusting pH) Chlorine (as disinfectant)

Alum and lime solutions are prepared in process water and dosed in a flash-mixing tank. Preparation of lime solution is associated with generation of grit. Dosing of lime and alum solutions is controlled on the basis of laboratory experimentation on coagulation-flocculation-settling removal of suspended and colloidal solids. Clari-flocculator is used for flocculating the destabilized colloidal solids, and for gravity settling and removal of both suspended solids and flocs formed through flocculation. Settled matter in the clari-flocculator is removed at regular intervals, as an underflow, through valve of the underflow drain. Each time that the valve is opened, it is kept open till clear water starts flowing out. Clarified water of the clari-flocculator is filtered in rapid sand filters for obtaining process water. Backwashing of these filters generates significant quantities of backwash water.

Filtered water is stored in an underground service reservoir and supplied to the mill as process water. Process water supply to the mill is regulated on the basis of water pressure in the main water supply line. A small fraction of the process water is chlorinated and supplied as drinking water. In the Gama and Epsilon mills, output of the clari-flocculator is directly supplied to the mill as process water. These mills have provisions for supplying the treated water also as fire-water. In the Epsilon Paper Mill, a separate reservoir, fire-water reservoir (which is partially separated from the service reservoir for process water), is used for storing treated water and supplying as fire-water. In the Alpha Pulp Mill, even the clari-flocculator is connected to the fire-water system. Loss of significant quantities of water through seepage is a common problem with water storage reservoirs and even with filtration units. The mills have separate fire-water system which is inclusive of fire-water pumps, fire-water distribution system, and fire hydrants. Each of the fire hydrants is supposed to have the minimum facilities required for effectively fire fighting in the event of any occurrence of fire.

4.5.1.3 Soft waterOf the five mills analyzed, soft water is extensively used in two mills – Alpha Pulp Mill (on the paper machine and in the approach system of the machine, beyond bleach decker) and Delta Paper Mill (as makeup water in the cooling tower of the steam turbine).

In both the mills, cation exchange resin beds are being used as water softeners for producing soft water. Exhausted ion exchange resin beds are regenerated by 5 or 10% solution of common salt. Regeneration of the softener involves the following steps: Backwashing the resin beds with process water

Dosing of beds with salt solution (process water is used for preparing the salt solution of desired strength)

Slow rinsing of the bed with process water Rapid rinsing of the bed with process water

Schematic process and material flow diagram of the soft-water plant of Delta Paper Mill is shown in Figure-4.5.2. Wastewater generated during regeneration of the softner is sewered as effluent.

4.5.1.4 Demineralized WaterThe mills analyzed are producing demineralized water (DM water) from process water mainly for utilization as boiler feed water. DM water plants include granular pressure filters, ion exchange resin beds, and degasification units. Backwashing of the pressure filters generate backwash water.

Ion-exchange resin beds are usually arranged in the sequence of cation exchange bed, degasifier and anion exchange bed. In the Alpha Pulp Mill, two anion exchange resin beds (weak base anion exchange bed, WBA, and strong base anion exchange bed, SBA) are in use. Here, degasifier is located between WBA and SBA, rather than after the cation exchange bed.

When water passes through a cation exchange bed, all its cations (Na+, Ca+2, Mg2+, etc.) are exchanged with the hydrogen ions at the ion exchange sites of the resin. Similarly, when water passes through the anion exchange bed, all its anions (SO4

-2, Cl-, etc.) are exchanged with the hydroxyl ions of the ion exchange sites of the resin. With time, all the hydrogen ions and hydroxyl ions (present at the ion-exchange sites of the cation or anion exchange resins respectively) get replaced by the cations and anions of the water being demineralized. As a consequence the beds get exhausted.

An exhausted bed requires regeneration for continued use in the ion-exchange process. This regeneration usually involves the following three steps:a) Backwashing of the resin bed b) Running of regeneration chemical solutions (5% HCl solution in case of cationic beds

and 5% NaOH solution in case of anionic beds) through the resin beds for regenerationc) First slow and then rapid rinsing of the regenerated beds for driving out residual

regeneration chemical solutions

Regeneration of the ion-exchange resin beds involves use of process water, degasified water and even DM water. Also, regeneration produces regeneration wastewater.

Degasifier includes a sump, a tower and a blower. Water, which is to be degasified, is sprayed in the tower and allowed to come in contact with the air blown inside at the bottom. When water comes in contact with air, carbonic acid present in it gets dissociated into H2O and CO2, and the latter is stripped out with the air.

DM water produced by passing through the cation exchange bed, degasifier and anion exchange bed may still have traces of ions. For removing these, the DM water, especially when it is used for producing high-pressure steam, is passed through a mixed ion-exchange resin bed. In the Alpha Pulp Mill, which produces high-pressure steam for running the steam turbine, mixed ion exchange resin bed is used for polishing the DM water. This mixed bed also requires regeneration.

Schematic process and material flow diagram of the DM water plant of the Delta Paper Mill

is shown in Figure-4.5.3. This mill uses the DM water produced as boiler feed water in the boilers which produce high-pressure super-heated steam (66 kg/cm2 pressure).

4.5.1.5 Recycled steam condensate Steam condensate is generated at all such places in the pulp and paper mills where MP steam or LP steam is used as heating medium for indirect contact heating. If there is no danger of contamination, this condensate is collected and returned to the boiler house for reuse as boiler feed water. In a typical mill, over 50% of the steam consumed is used for indirect heating. Purity and/or quality determine suitability of the condensate, so generated, for reuse as boiler feed water. Conductivity measurements made at all such points of generation where there is a danger of contamination are used as a basis for deciding whether the condensate should be rejected or reused as boiler feed water.

The mills make every effort to use excess heat of the steam condensate prior to its return to the boiler house for reuse as boiler feed water. Alpha Pulp Mill is using excess heat of the steam condensate generated at the pre-heaters of its batch digesters, in the form of flashed vapours, in the pre-heaters of multiple effect evaporator. Epsilon Paper mill is using excess heat of the steam condensate, which is siphoned out from the cylinder driers, through mixing with relatively higher pressure saturated steam in a thermo compressor for producing LP steam, which is reused in the cylinder driers as heating medium. Delta Paper Mill is using excess heat of the collected steam condensate for heating DM water, in a plate heat exchanger, prior to loading the latter to the boiler as boiler feed water. Prior cooling of the collected steam condensate is required for polishing the same in the mixed ion-exchange resin bed.

Condensate received at the boiler house is usually polished in an activated carbon column (for removing organic impurities) and/or in a mixed ion-exchange resin bed (for removing the ionic contaminants) prior to its reuse as boiler feed water. For ensuring continued availability of enough adsorption capacity, exhausted activated carbon of the column is replaced at regular intervals by fresh activated carbon. Further, since the activated carbon column also acts as a filter, it is backwashed at regular intervals. Just as the cation and anion exchange resin beds, mixed ion exchange resin bed also requires regeneration. Such regeneration involves use of HCl and NaOH solutions as regeneration chemicals, and generation of regeneration wastewater.

Polished steam condensate is stored and used as boiler feed water in place of DM water. Schematic process and material flow diagram of the system used, for the recycling and reuse of steam condensate, in the Delta Paper Mill is given in Figure-4.5.3.

4.5.1.6 Chilled waterMills require chilled water for absorbing the generated ClO2 gas in their on-site ClO2 plants. Ammonia or some other refrigerant based refrigeration systems or absorption heat pumps are used for producing the required chilled water from process water. In the old chilled water plant of the Alpha Pulp Mill, ammonia is used as refrigerant. But, now pulp mills consider use of refrigeration for chilled water production as a luxury. Availability of large quantities of steam is prompting the mills to use absorption heat pumps (in place of vapour compression based refrigeration systems) for producing chilled water. Beta Pulp and Paper Mill is having a lithium bromide based absorption heat pump in its ClO2 plant for producing the needed chilled water. Schematic process flow scheme of this chilled water unit is shown in Figure-4.5.4.

In this unit, water is used as a refrigerant. It is made to evaporate under vacuum in a mixing tank on the shell side. Such evaporation absorbs heat from process water (which is flowing on the tube side) and converts the latter into chilled water. Dehydrated lithium bromide is made to absorb the generated vapour, and the resultant hydrated lithium bromide is dehydrated through heating with MP steam and flashing out water vapour. Flashed out vapours are condensed through cooling and sent back into the mixing tank on the shell side for evaporation. Hot dehydrated lithium bromide is also cooled and then reused for absorbing the water vapour coming out from the mixing tank.

4.5.1.7 Warm water and hot waterWarm water or hot water is actually a preheated process water. In Alpha Pulp Mill, cooling water coming out from the surface condenser of the FVNG handling system of the multiple-effect evaporator, is partially used as warm water. This mill is using this warm water for producing hot water in the blow heat recovery system through using the waste heat recovered from the blow vapours. This warm water/hot water is mostly used in the brown stock washing and bleaching units of pulp mill in place of process water. Warm water is also produced at the direct contact condenser associated with the non-condensable gases handling system of cylinder driers. In the Epsilon Paper Mill, this warm water is collected into a warm water tank and mostly reused as shower water on the paper machine.

Causticizing unit also uses hot water for lime slacking, lime mud washing, and lime mud dewatering. But this hot water is mostly produced through directly injecting steam into both the process water and the foul condensate.

4.5.1.8 Foul condensateFoul condensate is generated at the following points in a Kraft pulp mill: At the blow heat recovery system through condensation of the vapours emanating from

the blow tank At the surface condenser, and at different evaporators and pre-heaters of the WBL

concentrating section of the chemical recovery plant, where flashed vapours of the black liquor are used as heating medium

This water is relatively clean and has volatile organic compounds and mercaptans as contaminants. This condensate is hot and can be substituted for hot water at all those points where organic contamination and mercaptans contamination are tolerable. Most of the foul condensate generated is sewered as effluent, and only a small fraction of it is reused at the following points: In the causticizing section for lime slacking (along with green liquor) and for washing

and dewatering of lime mud In the displacement showers of the last brown stock washer of the Kraft pulp processing

unit in the place of hot process water

4.5.1.9 BackwatersLimits prescribed for water consumption and wastewater generation have been mostly responsible for the collection and extensive reuse of backwater in place of the process water. Major sources of backwater in a pulp and paper mill include the following: Backwater generated at unbleached decker from thickening of washed brown stock Backwaters generated during washing of pulp at different bleach stages and at bleach

decker Backwater generated at the washing/thickening and/or dewatering operations of

secondary fiber processing and stock preparation Machine backwater getting collected in the second compartment of the machine

backwater tank

In addition to the above, backwater is generated at the flotation cells, screens, and cleaning units as “rejects” streams.

All the mills analyzed are reusing these backwaters to different degrees. Backwaters of unbleached decker and bleached decker are, mostly, reused for upstream pulp consistency adjustments (required for the pulp cleaning and screening). Backwater of one stage of bleaching is reused in another (provided that the reuse is compatible) in the displacement showers of the associated rotary vacuum drum washers. Backwater generated from the washing/thickening and dewatering is used in the hydrapulping and in the stock dilution (required for stock cleaning, screening, deinking, etc., purposes) on the upstream side. Machine backwater is reused, both directly, and after clarification, in the save all units on the paper machine in different showers, in the couch pit and UTM pulper. None of the mills analyzed is reusing backwater from flotation cells, cleaning and screening units.

4.5.2 Generation and supply of steam

4.5.2.1 Sources of energy for pulp and paper millsPulp and paper mills are energy intensive and may depend on the following sources of energy: Internally generated black liquor, bark and wood dust, and biogas produced from the

anaerobic digestion of prehydrolysate liquor External energy resources like coal, oil and other fuels like rice husk, bagasse, etc. Grid electricity supplied by state electricity boards (SEBs)

Coal and concentrated black liquor are burnt as fuels in the coal fired boilers and chemical recovery boilers, respectively. Oil is usually used as fuel in the DG sets and limekilns. It is also used in the boilers for start-up, and in the vehicles (meant for internal movement of materials) for transportation. In the Alpha Pulp Mill, biogas generated from anaerobic digestion of prehydrolysate liquor, along with oil, is used as fuel in the limekiln. Mills have low-pressure boilers for burning bark, wood dust, trash, etc.

4.5.2.2 OverviewBoiler house is one of the important utilities of a pulp & paper mill. It supports the mill through supplying medium-pressure (MP) and low-pressure (LP) saturated steam for utilization as heating medium. Four of the five mills analyzed have cogeneration units (cogeneration of heat energy and electrical power). In these mills, boiler house is concerned with the supply of high-pressure super-heated steam to the turbine generator. Extraction type turbines are used in these mills. MP and LP steam are extracted from these turbines and supplied to the mill for use as heating medium. Exhaust steam of the turbine is condensed, in a turbine condenser, and supplied back to the boiler as boiler feed water. Epsilon Paper Mill is not having a cogeneration unit. In case of Delta Paper Mill only one type of steam is extracted from the turbine.

Boiler houses of three of the five mills analyzed (Alpha Pulp Mill; Beta Pulp and Paper

Mill; and Gamma Pulp and Paper Mill) include mainly two types of boilers: coal fired boilers and chemical recovery boilers (use heavy black liquor, HBL, as fuel). The other two mills depend, totally on coal-fired boilers. Four of the mills analyzed have one or more of other types of boilers, which use bark, wood dust, ETP sludge, rejects generated from the secondary fiber processing, etc., as fuels.

Water required for steam generation (boiler feed water) is obtained from the following three sources: a) Steam condensate generated at the condenser of the turbine b) Steam condensate collected and returned from different points of the mill to the boiler

house for reuse as boiler feed waterc) Demineralized water (usually for use as make up boiler feed water)

Epsilon Paper Mill, which is not having a turbine generators does not produce high pressure (HP) steam, and hence, the boiler is not having super heaters.

Water tube/drum type boilers are mostly used for generating steam. In the Delta Paper Mill there is one fire tube/submerged tube boiler. The mills are having fluidized bed boiler furnace for burning pulverized coal as fuel. Delta Paper Mill is meeting its fuel requirement partially through burning petroleum coal (which is rich in sulfur, but has higher calorific value and is cheaper). The boilers used are having either a single drum (steam drum) or a two drum (steam drum and mud drum) natural circulation steam generators. These have bed tubes, water walls, super heaters, boiler tube bank, economizer, etc.

Activities and operations associated with steam generation and supply are:a) Reception, storage, preparation, conveyance and loading of coal to the boiler furnaceb) Pre-heating and supply of combustion air to the boiler furnace c) Handling and disposal of flue gasesd) Handling and disposal of bottom ash and flyashe) Conditioning, pre-heating and loading of boiler feed water f) Fate of boiler feed water in the boiler g) Handling and management of steamh) Distribution of steam and collection of steam condensate

A schematic diagram showing flow of fuel, combustion air and of flue gases in the boiler of the Delta Paper Mill is given in Figure-4.5.5. Figure-4.5.6 shows movement of boiler feed water, steam and cooling water in the cogeneration unit of the Delta Paper Mill.

4.5.2.3 Reception, storage, preparation, conveyance and loading of coal to the furnace

Coal received by road or rail is unloaded and stored in coal yards, which may be concreted and/or covered. Facilities are provided for transporting the stored coal to the roller conveyor feeder. Larger blocks of coal are hammered and reduced to acceptable size prior to loading on the conveyor. Prior to feeding to the crusher/pulverizer, coal is passed through a magnetic separator (for removing metallic impurities), and wetted with water (for controlling dust problems and minimizing fire hazards). Further, coal moving on the conveyor is manually screened for the removal of stones. Crushed/pulverized coal is screened/classified and then conveyed into a storage bin. From there, it is pushed into the boiler furnace through burners at desired rate with the help of pre-heated primary air. Oversize coal is recycled back to the pulverizer.

Coal handling is associated with the following two problems:

Pulverization and screening are associated with dust problems (water sprays are usually employed for controlling this problem), and accumulated coal dust has fire hazards

Storage and handling of coal are associated with fire hazards (fire hydrants are provided in the areas, where coal is being handled, to facilitate fire fighting)

Noise and vibration problems associated with pulverization and screening

4.5.2.4 Pre-heating and supply of combustion air to the furnaceForced draught fan (FD Fan) is used to take in combustion air. In most of the cases, this air is first pre-heated in the air pre-heater of the boiler, and then divided into two streams, namely, primary air and secondary air. Primary air is further pressurized by primary air fan (PA fan) and used to push pulverized coal into the furnace of the boiler through burners. Secondary air is supplied directly into wind box, and from there, the air enters the furnace through nozzles while fluidizing the bed (in fluidized bed boilers).

Air pre-heating is makes the combustion process more efficient. Further, through increasing the flame temperature, it intensifies the radiant heat transfer to water walls and super-heater, and ensures raised temperature all through the flue gas path. Proper control of air-fuel ratio is considered very important. Increasing airflow rate is often used as a strategy for raising the steam temperature. Rich mixture of fuel-air (high fuel-air ratio) can lead to incomplete and smoky combustion.

4.5.2.5 Handling and disposal of flue gasesFlue gases are generated in the furnace from combustion of the loaded fuel. These gases are hot and loaded with fly ash, and also have gaseous pollutants such as oxides of nitrogen and sulfur. Increasing combustion temperature increases concentration of nitrogen oxides in the flue gases. Fluidized bed technology is supposed to minimize NOX formation through lowering combustion temperature. Burning of low-sulfur coals and use of limestone as bed material in the fluidized bed boilers is supposed to minimize SO2 concentration in flue gases. Boiler of the Delta Paper Mill has a provision for mixing lime with pulverized coal and then loading it to the furnace.

Heat energy of the flue gases is recovered through heat transfer in Super-heaters Boiler tube bank Economizer Air pre-heater

In these heat transfer units, part of the dust/ash present in the flue gases gets separated from the flue gas, through gravity settling and impaction removal, and collected into the hoppers of the respective units. Temperature of the flue gases coming out from the last heat transfer unit (air pre-heater) may be in the range of 135 to 180C. Efforts are made to keep this temperature above but closer to the dew point temperature (> 120C) and to avoid condensation of water vapor. Condensation of water vapour, if allowed to occur, can form acids (with SO2 and SO3) and corrode metal structures in its contact. Elevated flue gas temperatures are useful for providing sufficient plume rise.

Flue gases are treated for the removal of particulate matter, prior to emission into atmosphere through tall stacks (SO2 emission rates are prescribed to be used as basis for deciding stack heights). Flue gases are treated for the removal of suspended particulate matter mostly by any of the following techniques:

Passing the gases first through one or more cyclone separators connected parallely, or through a multi-clone, and then through a fabric filter bag house

Passing the flue gases through an electrostatic precipitator

Flue gases are drafted from the furnace through the series of heat transfer units and air pollution control device, and pushed through the stack for emission into atmosphere with the help of an induced draught fan (ID Fan). Slight negative pressure is usually maintained in the furnace (in order to avoid backfiring and associated hazards), and this results in the infiltration of 5 to 10% of additional air into the furnace.

Boiler emissions attract application of prescribed emission standards.

4.5.2.6 Handling and disposal of bottom ash and flyashBottom ash may amount to 20 to 40% of the total ash generated. It is drained out from the bottom of the furnace. This ash is usually quenched and wetted with water, prior to loading in trucks and transporting for disposal.

In the mills employing fluidized bed technology, very little bottom ash is actually generated. Here, bed material of the furnace is partially drained out at regular intervals and replaced by fresh bed material. The drained out bed material is processed, through screening, and then added back to the furnace as fresh bed material. Oversize material separated during processing of drained out bed material is disposed off as non-hazardous waste. Occasionally, due to operational defects, the bed material gets transformed into large size lumps. In such a situation, total replacement of the bed material is practiced (one incident, needing for such replacement, has arised in the Delta Paper Mill during one of the study visits to the mill). Pulverized coal feeder pipes are frequently cleaned and this cleaning also generates waste.

Particulate matter separated from the flue gases in different heat transfer units (through which the gases are passed), and in the air pollution control device(s) is known as flyash. The heat transfer units and the air pollution control devices have hoppers for collecting and temporarily holding the separated flyash. Flyash accumulating in these hoppers is usually conveyed for disposal either through sluice conveyors or through pneumatic conveyors.

In case of pneumatic conveyors (which are used in Epsilon Paper Mill and Delta Paper Mill), flyash is first conveyed to a flyash bin and from there it is loaded to trucks for transportation and disposal. Usually a cyclone separator is used for separating air from the flyash being conveyed. Wetting with water is usually practiced for avoiding suspension of flyash in air during handling (loading to trucks).

In case of sluice conveyors, flyash is conveyed in water to a dewatering bin, and from there, after dewatering, loaded to trucks for transporting to the ash disposal site. Water recovered during the ash dewatering can be reused in the sluice conveyor for carrying the ash.

In Alpha Pulp Mill and Epsilon Paper Mill, some of the flyash generated is reused in brick making, concrete lining of surfaces and in road construction.

4.5.2.7 Conditioning, pre-heating and loading of boiler feed water Mills use the following three types of water as boiler feed water (the first two as boiler feed water and the last as make up boiler feed water): a) Condensate generated at the surface condenser of the turbine generator

b) Condensate returned from different parts of the mill (usually requires polishing prior to reuse)

c) DM water produced by the DM water plant

Presence of dissolved oxygen in the boiler feed water is associated with corrosion problem. Hence, boiler feed water is first deaerated in an open direct contact heater (known as deaerator) through steam stripping. Deaerators usually include a deaeration tower and a deaerated water holding tank below the tower. Direct injection stripping with steam raises temperature of the feed water and strips off dissolved gases. Elevated temperature reduces solubility of dissolved gases (especially O2 and CO2).

Feed water pumps are used for pumping the deaerated feed water from the holding tank of the deaerator to the economizer of the boiler at the desired rate and pressure. In between the holding tank and the economizer, there can be one or more high-pressure water pre-heaters (shell-and-tube type) for raising the feed water temperature to 170 - 280C. MP steam is used on the shell side of these pre-heaters for pre-heating the feed water which is flowing on the tube side.

Alkaline pH of feed water can cause scaling problems and acidic pH can cause corrosion problems. For avoiding corrosion problems, pH of the boiler water is maintained at around 10.5. At this pH, magnesium hydroxide can precipitate and cause scaling. For preventing such scaling, some form of sodium phosphate (tri, bi, or mono sodium phosphate) is added to the feed water. The phosphate added reacts with calcium and magnesium ions of the boiler water and form flocculent precipitate which remain in suspension. For preventing adherence of this precipitate to the boiler surfaces, modified lignin, tannin, starch, synthetic polymers, etc., are often added to the feed water.

For controlling corrosion from carbonic acid of the condensate, neutralizing or filming amines, like, Ellow Guard (a commercial name) are dosed to the feed water. For avoiding water entrainment, and reducing solids carry over into the steam, defoaming chemicals are often added to the feed water. Chemical oxygen scavengers, such as, sodium sulfite or hydrazine, and chelating agents (for the chelation removal of metals) are often added to the feed water. In certain cases, some of the above chemicals are dosed directly to the steam drum.

4.5.2.8 Fate of boiler feed water within the boilerFeed water is loaded to the economizer and temperature of the feed water is raised to almost saturation temperature. From there the water is allowed to flow into the steam drum. From the steam drum, water flows to the headers of water walls and bed tubes through down comers. In case of two-drum boilers, water first flows to the mud drum and then to the water wall headers. Water from the water wall headers and mud drum raises up in riser water tubes (which are forming the furnace water walls and the boiler tube bank) and returns to the steam drum as super heated water through natural circulation.

Steam is generated in the steam drum through flashing of the super heated water. The generated steam is separated and sent through primary and secondary super-heaters for further heating and generation of superheated steam. Steam drum internals avoid entrainment of water droplets and suspended solids and ensure that the separated steam is free from moisture. In case of the Epsilon Paper Mill, the boiler is not having super-heaters and steam separated from the steam drum is directly supplied to the mill as saturated medium pressure steam. Pressure and temperature of the super heated steam are regulated

with the help of a de-super-heater, which is located in between the two super heaters, through injecting deaerated boiler feed water.

Boiler water accumulates the impurities present in the feed water in traces and also many of the conditioning chemicals dosed. Excessive accumulation of these can lead to scaling of the heat transfer surfaces of the boiler. For avoiding this, continuous and intermittent blow down of some fraction of the boiler water (from steam drum, mud drum and also from the headers) is practiced. This boiler blow down water is a potential source for heat energy, but contaminants level in this water may be quite high.

4.5.2.9 Handling and management of steamSuper heated steam (also known as HP super heated steam) coming out from the secondary super-heater is conveyed to the turbine generator through HP steam header. A small fraction of the HP steam is usually tapped from this header for soot blowing purposes. Heat transfer units of the boiler (super heater, boiler tube bank, economizer, etc.) have soot blowers for the blowing off the soot deposited on the heat transfer surfaces with HP super-heated steam. Usually a pressure-reducing device (PRD) is provided on the HP steam header prior to the turbine generator. This device is supposed to serve the following two purposes: Adjusting pressure and temperature of the super heated steam Facilitating transformation of HP super heated steam into saturated MP steam and LP

steam, and supplying it directly to the mill whenever needed

Deaerated feed water is used in the PRD for adjusting pressure and temperature of the steam.

4.5.2.10 Steam distribution and condensate collection systemPulp and paper mills use steam mainly for the following purposes: Heating

Direct contact heating which generates no steam condensate (this use is considered wasteful and less preferred because of the inability to recover condensate)

Indirect contact heating which generates steam condensate – the generated condensate is returned to the boiler house for reuse as boiler feed water. Some of the cases of indirect heating are associated with the danger of condensate contamination – contaminated condensate is sewered rather than returned to the boiler house. This is needed because carry over of contaminants can damage automatic valves, traps, turbines and other equipment.

For process reactions and operations

Quantities and qualities (temperature and pressure) of steam required markedly vary from function to function within the mill. Because of better heat transfer efficiencies, saturated steam is preferred for heating purposes. Steam is distributed at somewhat higher pressure than required (either as MP steam, or as MP steam and LP steam), and its qualities are frequently adjusted as desired through using pressure reducing devices prior to making it available for the designated function. Some of the steam demand of the mill is met by the LP steam generated from flashing relatively higher-pressure condensate (condensate with 30 psig pressure or more) in flash tanks. In the Alpha Pulp Mill, condensate generated by the pre-heaters of the digesters is flashed and the resultant LP steam is used in the pre-heaters of the multiple effect evaporator. Mills can also use thermo-compressors for transforming of steam condensate into LP steam through mixing it with relatively higher-

pressure steam. In the Epsilon Paper Mill, thermo-compressors are used for transforming the siphoned out condensate of some of the cylinder driers into LP steam by mixing with medium pressure steam.

A steam distribution system can be considered to include steam distribution lines, pressure reducing valves, thermo-compressors, suitable traps for avoiding pockets in the lines, etc. These are mostly laid overhead and adequately insulated usually by calcium silicate insulation and aluminum jacketing on the outer side. Further, sufficient flexibility is introduced into the steam distribution lines for absorbing thermal expansions.

Condensate return systems are of two types: Pressurized condensate return systems – these include the condensate lines designed for

two-phase flow. These lines usually convey condensate (usually at 30 psig or more pressure) to flash tanks from the generation points. This conveyance does not require pumps and controls, steam pressure is infact used as driving force for this.

Unpressurized condensate return systems – traps of the heat exchangers discharge the condensate into condensate receiver tanks. Steam flashing out from these tanks is vented into the atmosphere, and the accumulating condensate is pumped to the condensate tank of the boiler house through the condensate lines (which are not sized for 2-phase flow) with the help of pumps and controls.

Condensate return lines are insulated and provided with traps and drains for avoiding pockets.

At certain points of steam consumption, where there is a chance for the contamination of condensate, the condensate generated is continuously monitored, and depending on the level of contamination detected, the condensate is either drained into sewer or returned to the boiler house for reuse. Alpha Pulp Mill is practicing this approach for draining or returning the steam condensate generated at the pre-heaters of the digesters.

4.5.3 Generation/extraction and supply of electrical energy Pulp and paper mills depend on the following three sources of electrical energy: Captive cogeneration units Grid power of the State Electricity Boards (SEBs)

Pulp and paper mills mostly depend on cogeneration units for power supply. All the mills excepting Epsilon Paper Mill are having cogeneration units. The mills rely on the SEB Grids only for the additional power supply required and depend least on the diesel generators for power mainly because of its high generation cost (mostly limited to peak hours during which grid power is costlier).

4.5.3.1 Captive cogeneration unitBoilers (both chemical recovery boilers and coal fired boilers including those that burn other fuels as well) produce high-pressure super heated steam. This steam is fed to extraction type partial condenser steam turbine generators through a PRD for power generation. The power generated is supplied to the mills for powering various drives and for various other purposes. Desired quantities of MP steam and LP steam are extracted from the turbine and supplied to the mill for heating purposes, and for process reactions and operations. Residual exhaust steam of the turbine is condensed in a surface condenser and the condensate is supplied back to the boiler as boiler feed water.

When turbine generator is not in operation, super heated steam is directly desuperheated in PRD and supplied as MP steam and LP steam to the mills. When steam demand of the mill is nil, no steam is extracted from the turbine. Circulating oil-cooling system is used for cooling the turbine bearings, gearbox, alternator bearings, servo controllers, etc. Circulating cooling water system is used for picking up heat from the surface condenser and cooler of the circulating oil, and dissipating into the atmosphere.

A cogeneration unit can be considered to include the following: Boiler producing high-pressure super-heated steam Pressure reducing device (PRD) Extraction type partial condenser steam turbine generators Condenser and non-condensable gases handling system Circulating cooling water system Circulating oil cooling system

4.5.3.1.1Brief process descriptionPressure and temperature of the high-pressure superheated steam supplied by the boilers is first adjusted in the PRD. This device consumes deaerated feed water for the pressure and temperature adjustment. When turbine is not in operation, the incoming super heated steam is de-superheated into saturated MP steam and LP steam and supplied to the mill for heating purposes.

Superheated HP steam is fed to the turbine generator. The rate of feeding is controlled with the help of a governor for regulating rpm of the turbine rotor. Saturated or slightly super heated MP steam and LP steam are extracted from the turbine at the desired rate for meeting the mill’s steam demands. Residual steam comes out of the turbine as exhaust steam. This steam is condensed in a surface condenser. Heat generated from such condensation is transferred to the cooling water circulating between a cooling tower and the surface condenser. Condensate generated at the surface condenser is stored in a condensate tank and supplied to the boiler as boiler feed water. Turbine shaft is connected to the rotor of an alternator through a gearbox. Rotation of this rotor produces alternate current. The governor controls rotational speed of the turbine rotor, in such a way that frequency of the generated electricity matches with that of the grid.

Surface condenser is a shell-and-tube type heat exchanger. Exhaust steam condenses on the shell side, while cooling water flows on the tube side. Warm cooling water, coming out from the condenser, is cooled in a cooling tower (a mechanical draught cooling tower) and circulated back to the condenser. Non-condensable gases accumulating on the shell side of the surface condenser are removed with the help of two or more steam jet air ejectors (SJAE) connected in series. MP steam (motive) is forced to flow through the SJAE for sucking out the accumulated non-condensable gases. The steam, along with the non-condensable gases, coming out from the first SJAE is condensed in an inter-cooler (which can be either a spray tower type or a shell-and-tube type). Non-condensable gases accumulating in the inter-cooler are in turn removed by a second SJAE, which also uses MP steam (motive) for the purpose. Output of the second SJAE is condensed in an after-cooler, and the accumulating non-condensable gases of the after cooler are vented into the atmosphere.

Cooling water is used in both inter-cooler and after-cooler, and warm cooling water coming out from these coolers is conveyed to the cooling towers. In cases, where the inter-coolers or after-coolers used are shell-and-tube type, pure steam condensate is generated. This

condensate is collected into the condensates tank and supplied to the boiler as boiler feed water.

Process and material flow (specially of water, steam and condensate) diagram of a typical cogeneration unit is shown in Figure-4.5.6.

4.5.3.1.2Circulating oil cooling system of the turbine generatorThis system includes a lube oil tank, an oil cooler, pumps and piping for circulating the cooling oil, and facilities for drying, cleaning and filtering the circulating cooling oil. Schematic process flow diagram of a circulating oil cooling system is shown in Figure-4.5.7. Bearings, gearbox and servo systems, associated with the turbine generator, are lubricated and cooled by the circulating cooling oil. Warm oil coming out from these is collected into a lube oil tank. From here, the oil is circulated through oil coolers and filters, back to the bearings, gearbox and servo systems.

Oil stored in the lube oil tank is also circulated through certain devices for the removal of moisture through evaporation and particulate matter through centrifugal separation. Circulating cooling water is used in the oil coolers for cooling the circulating oil. Leaks may contribute to the loss of oil and this may require addition of fresh oil for makeup. With time the circulating oil looses its properties and this necessitates occasional replacement of the oil in circulation with fresh oil.

4.5.3.1.3Circulating cooling water systemCirculating cooling water system includes a cooling tower, a cooling water sump, pumps and piping for circulating the cooling water, and facilities for producing and/or conditioning, and adding makeup water to the circulating cooling water. This system usually supplies cooling water to the following: Surface condenser of the turbine Inter-cooler and after-cooler associated with SJAEs Oil cooler of the circulating cooling oil system

Water is lost from the circulation cooling water system through evaporation and drifting. Further, for controlling buildup of salts, a part of the circulating cooling water is blown down, at regular intervals, as wastewater. For making up these losses, process water is added to the cooling tower sump. Cooling tower water is also, usually, dosed with cooling water-conditioning chemicals.

When the process water is hard water, it is softened in a soft water plant and, then, used as makeup water. Regeneration of water softening units requires common salt (as regeneration chemical) and process water. Regeneration of the softeners and cleaning of the associated sand filters and/or activated carbon filters consume process water and produce regeneration wastewater. Delta Paper Mill is using soft water as makeup water in the circulating cooling water system. For details see Figure-4.5.2.

4.5.3.2 Power from SEB grid and electrical energy system of the millBecause of the cost factor, and other reasons, pulp and paper mills make efforts to depend least on the SEB grid power. Mills have to pay for the power extracted from the grid, and power tariff may even vary within a day, with time (tariffs may be higher during peak hours

of the day). Grid power is available as AC current at higher voltage (as high as 115 kV), while voltage requirements of the motors used in the pulp and paper mills vary widely (from 13 kV to below 550 V). Power generated by the captive power generation units has relatively low voltage. Some of the devices/machinery (such as electrostatic precipitators) used in the mills require DC current. Extraction and use of grid power, and use of electrical energy generated on-site internally, thus necessitate creation and maintenance of a variety of facilities, specially for metering, for stepping-up or stepping-down voltage, and for converting AC to DC. Further, facilities are also required for ensuring protection and safety, and for integrating internally generated electrical energy with that extracted from the grid. A mill’s electrical energy system, in addition to the captive power generation units (cogeneration unit and diesel generation sets), includes transformers, rectifiers, electrical meters and monitoring devices, relay and circuit breakers, etc., as integral parts.

4.5.3.2.1TransformersTransformer core, along with the insulated coils, is submerged in transformer oil. Due to multitude of reasons, some fraction of the electrical energy flowing through is lost and emitted as waste heat in the transformers. Transformer oil picks up this heat and gets heated up. This heat is then pumped out and dissipated into the surrounding atmosphere through the following mechanisms: Natural circulation of the transformer oil through external air-cooled tubes Communication of the air present, in contact with the transformer oil, in the transformer

with the external atmosphere through the air breather

When the transformer is loaded, due to elevated temperatures, inside air expands and leaves the transformer through the breather. Insulation leaks may lead to generation of hot gases and these gases also find their way into the atmosphere through the breather. When the transformer is not loaded, due to cooling of the interior, cool atmospheric air moves in through the breather. This air can bring in moisture and contaminate the transformer oil. For avoiding this problem, moisture adsorbing substances, like silica gel, are used in the breather.

At regular intervals, the transformer oil is tested for a number of parameters and, depending on the requirements, the oil is conditioned (once a year or two) usually through filtration and vacuum dehydration. Aging can lead to loss of properties and make the transformer oil unfit for use. This necessitates replacement of old oil with fresh transformer oil. Discarded transformer oil is disposed off as hazardous waste. Due to leaks, transformer oil may be lost from the transformer, and for maintaining the oil at the desired level, fresh transformer oil is added at regular intervals as make-up oil.

Overloading of transformers, contamination of transformer oil (specially with water), obstructions to the natural circulation, and cooling of the warm oil, etc., can lead to accidents that involve pressure build-up and explosion of the transformer, spillage of oil, and fire. Transformer oil contains paraphenic, naphthanic, and aromatic compounds (like polychlorinated biphenyls), and, hence, it is highly toxic. Explosions and fire are usually associated with emission of toxic fumes and smoke.

4.5.3.2.2RectifiersRectifiers are mostly associated with the electrostatic precipitators (ESP) used for the treatment of flue gases from boilers. These are used for converting AC to high voltage DC and supplying it to the ESP. All components of the rectifier are built into a sturdy metal

frame and the frame in turn is immersed in a transformer oil tank. Energy losses occurring in the rectifier heat up the oil, and thermal energy of the hot oil is dissipated to the surrounding air.

Similar to that of transformers, transformer oil of the rectifiers requires regular conditioning and aged transformer oil requires replacement by fresh oil. Rectifiers also have accident potential.

4.5.4 Production and supply of Instrument air and plant airSupply of instrument air (pressurized and conditioned air) is required throughout the mill for the pneumatic control of instruments. All the five mills analyzed have independent instrument air systems for generating and supplying the instrument air. These systems through two-stage compression (by oil-free centrifugal compressor) to pressurize intake air to >100 psig pressure and condition it by reducing water content through condensation-separation (first in an inter-cooler, after the first stage of compression, and then in an after-cooler, after the second stage of compression) and by drying in an adsorption dryer (usually with duel-column configuration). Water content of the air is usually reduced to a level, which is equivalent to the dew point (of anticipated minimum ambient temperature minus 10ºF). Conditioning of the air also includes removal of suspended particulate matter (by passing it through filters) and oil (by passing it through an oil separator). Schematic process flow diagram of the instrumental air system employed in Epsilon Paper Mill is shown in Figure-4.5.8.

Ambient air is taken in through a filter and subjected to first stage compression. The air is then cooled in an inter-cooler (by circulating cooling water) and further compressed through second stage compression to >100 psig pressure. This compressed air is again cooled in an after-cooler (by circulating cooling water) and stored in a receiver tank as saturated, compressed and cooled air. Water gets removed, in both inter-cooler and after-cooler, through condensation-separation. From the receiver tank, the air is first passed through a pre-filter and an oil separator, and then dried in one of the two adsorption dryer columns. Dried air is then passed through an after-filter and supplied to the mill as instrument air. Pressure in the instrument air distribution header is maintained >40 psig, and the air is made available to instruments at about 20-psig pressure. For ensuring only conditioned air is supplied to instruments, instrument air distribution system often includes additional small dryers and filters.

While one of the two dryer columns is in use, the other one is regenerated by heat conduction technique. Electrical heaters placed in the adsorbent bed supply the heat required for regeneration, and a small flow of dried air (2-3% of the total flow) is used for purging the column during regeneration. Epsilon Paper Mill is having a separate cooling tower for ensuring cooling water circulation through the inter-cooler and the after-cooler.

Epsilon Paper Mill is not having a separate plant air system. It is tapping part of the saturated compressed and cooled air from the instrument air system at the receiver tank, storing in separate tanks and supplying as plant air or compressed air to the mill.

4.5.5 Maintenance activities of the millMaintenance activities involve repairs, replacements, minor modifications and maintenance of machinery, equipment and other infrastructure of the mill. These can also be considered to include housekeeping and lubrication, etc. Maintenance works can broadly be

categorized into civil, mechanical and electrical works. These works are executed either on-site, or in the maintenance workshops (mechanical and electrical workshops) of the mill. At times, maintenance works may be got outsourced through some other organization or unit. Execution of maintenance works may also involve outside organizations and contractors. Maintenance requires purchase and inventorying of the inputs required in the stores of the mill. Maintenance generates wastes and scrap which may have resale value, or which can be recycled and reused within the mill. Hence, much of the wastes and scrap generated is shifted to the scrap yard of the mill and stored there till it is disposed off. Maintenance activities may require partial or even total shutdown of mill operations. Further, maintenance may be associated with dumping and discarding of materials and cleaning of machinery, equipment, tanks, etc. All these can also generate wastes.

Maintenance activities can be considered to include the following:a) Civil worksb) On-site and off-site maintenance (in workshops of the mill) c) Lubricationd) Housekeepinge) Storesf) Scrap yard

4.5.5.1 Civil worksThese involve consumption of water, cement, sand and other building materials, electricity, etc. Frequently, these may involve use of contractor or contract labour. Depending on the size, civil works are associated with noise and dust problems and with the generation of construction waste. These works may cause disruptions, obstructions and inconveniences to the routine production operations/activities. Maintenance of tanks may require dumping of the contents and cleaning.

4.5.5.2 On-site and off-site (in the mill’s workshops) maintenance activities

On-site maintenance of machinery, equipment and other facilities is usually preferred over off-site maintenance. In certain cases, machinery, equipment and other facilities may not be compatible for off-site maintenance. Maintenance, usually, requires shutting down the associated operations and isolation of the machine or equipment or facility that require maintenance. Actual maintenance usually involves consumption of a variety of inputs (including oil & grease, and cotton) and use of a variety of tools obtained from the stores, and generates wastes/scrap. Depending on the resale value and in-plant recyclability and reusability, the generated waste/scrap may be either shifted to the scrap yard or disposed off directly. Once the maintenance job is over, the maintained machine or equipment or facility is inducted back into line. This again can disrupt operations of the mill.

Pulp and paper mills usually have three types of workshops, namely, mechanical workshop, electrical workshop and automobile workshop. Off-site maintenance in these workshops is not much different from the on-site maintenance, except for shifting of the machine/equipment to the workshop for maintenance and shifting of the maintained machine/equipment back to the site. Automobiles used for conveyance of materials within the premises of the mill are maintained in the automobile workshop. Maintenance workshops have the basic facilities, routine inputs and tools required in most of the maintenance jobs, and also the facilities like water closets, sinks, etc.

4.5.5.3 Lubrication Within a pulp and paper mill, many machines require lubrication. A few machines require cooling by circulating cooling oil systems. Mills employ specialized oilmen for ensuring lubrication (and maintenance of oil levels through lubrication and addition of fresh oils) of all such machines according to a pre-decided schedule. Further, at regular intervals, but relatively less frequently, lubricating oils and circulating cooling oils of different machines are examined for their properties, and in all such cases where properties are lost, aged oils, which are in use, are discarded and replaced by fresh oils. Discarded lube oils and cooling oils are hazardous. These are collected and temporarily stored (in stores) till their disposal (through sale to outside parties). For ensuring availability, inventory of lubricating oils and cooling oils is maintained in the mill’s stores and issued as and when required for use.

4.5.5.4 HousekeepingAll those operations and activities that can be improved through simple, practicable and common sense measures can be considered as subjects for housekeeping. A good housekeeping is supposed to minimize loss of materials, conserve water and energy (electricity), improve working conditions and worker safety, minimize environmental impacts, improve productivity, reduce production costs, etc. Bad housekeeping in the pulp and paper mills analyzed can be exemplified by the following: Leakages in pipes and equipment, spillages, and unintended overflows of tanks/chests Wasteful and unnecessary use of water or running of taps, and not properly metering

water Unnecessary running of pumps, and other equipment and machinery Hot and cold pipes with poor or damaged insulation Excessive dependence on corrective maintenance and not having documented

procedures or manuals for maintenance Unorganized stores and scrap yards, and disposal of empty cans, packaging material,

discarded insulation material, scrap and other wastes in unintended areas Unsorted wastes affecting their proper handling and disposal (including their recycling

and reuse, and resource recovery)

4.5.5.5 Stores Stores procure materials required by the mill, stock them within and issue for use as and when required. If the received materials are not properly inspected, the mill may end up receiving damaged materials, materials with missing components and materials not satisfying the specifications and quality requirements. If the received materials are not stocked as per the stocking conditions recommended by suppliers, if due consideration is not given to the non-compatibilities and dangerous nature of materials, and if safety measures and appropriate equipment are not used while handling the materials (receiving, transferring, stocking and issuing), the material may be damaged, accidents may occur and workers may be exposed to dangerous materials. Overstocking of materials, non-verification of expiration dates, and non-application of first-in first-out principle for managing the stock, makes wastage of materials imminent.

4.5.5.6 Scrap yardAll the mills analyzed have defined scrap yards. But, they are least managed and are not sufficiently secured. The mills have very little idea about what scrap/wastes they generate,

what is supposed to reach the scrap yard, and what scrap/waste (and, also, how much of it) is actually reaching the scrap yard. Much of the scrap/waste is in fact stored at many unintended places rather than in the defined scrap yards. Storage within the scrap yard is not sufficiently organized and conditions of storage required are not satisfied. Different types of scrap/waste is not sufficiently segregated and compatibility for storing different types of scrap/waste together is often not given due consideration. As a consequence, the materials held in the scrap yard are deteriorate and loose their resale value as well as their potential or suitability for in-plant recycling and reuse. Further, the stored material may spill over and lost to the surroundings through storm water, wind, etc., and contaminate the surrounding environment.

4.5.6 Other utilities and servicesThese may include: R&D facilities, and testing and analysis laboratories Water closets and sanitation facilities Maintenance of roads and mill premises Canteen Administrative and other office

4.5.6.1 R&D facilities and testing and analysis laboratoriesEach of the mills analyzed has one central laboratory and one or more smaller laboratories (the latter for meeting testing and analysis needs of the units like, water treatment plant, effluent treatment plant, DM water plant, etc.). Most of the R&D work and most of the testing and analysis work associated with the control of mill operations is carried out in the central laboratory. The other laboratories are used for routine testing and analysis jobs, that too, of specified units of the mill. Infrastructure of these laboratories includes a wide variety of instruments and laboratory scale units for R&D work. These laboratories are provided with, electricity and other utilities like plant air. These laboratories consume a wide variety of chemicals and glassware.

Samples collected from different parts of the mill are sent to these laboratories for testing and analysis. In the process of performing the testing and analysis work and carrying out R&D work, these laboratories generate both solid and liquid wastes. Main constituents of the solid wastes include discarded samples, packaging materials, broken glass, paper, etc. Because of contamination with a wide variety of chemicals, products of chemical reactions, and residual samples dumped, wastewater generated by these laboratories is hazardous. Additionally, these laboratories may also generate hazardous fumes, which are vent into the atmosphere.

4.5.6.2 Water closets and sanitation facilitiesMills have urinals, toilets, washbasins, and drinking water closets provided at all such places where worker density is high. Mills also have a few emergency water showers. These facilities are mostly connected to the drinking water supply system of the mill. Water is needed in these facilities for facilitating their effective use and for keeping the facilities clean and usable. Soaps/detergents and acid are also used for keeping these facilities clean and ensuring sanitary conditions. Despite these, unhygienic conditions and wasteful use of water are quite common in these facilities. These facilities generate significant quantities of wastewater (also known as domestic sewage). This wastewater is either partially or fully

allowed to mix with trade effluents of the mill, or collected separately (for treatment in the ETP along with the trade effluents or for separate treatment and disposal). Use of septic tank –soak pit systems for the treatment and disposal of this wastewater has also been observed in the mills analyzed.

The water taps provided in the mills are mostly used for cleaning the shop floors, for settling dust or wetting of roads, raw materials and coal, and for irrigating lawns and tree saplings. Even fire hydrants have been found effectively used in certain areas of the mills specially for settling dust and wetting roads and for cleaning vehicles. Very little of the water used in these (excepting that used for shop floor washing) is collected as wastewater. Even this little wastewater collected mostly flows into storm water drains of the mill.

4.5.6.3 Maintenance of roads and mill premisesActivities included under this head can include: Clearing of premises from wild vegetation Filling, leveling and landscaping Raising and maintaining lawns, plantations and horticultural plants Lighting of roads and mill premises Cleaning of storm drains and sewers Sweeping of roads and mill premises

These activities involve consumption of electricity and water and also manure and inorganic fertilizers, and generation of wastes (from sweeping, clearing of vegetation, grass cutting and pruning of trees and horticultural plants).

4.5.6.4 CanteenAll the mills analyzed have canteens within the premises for preparing and serving food to the mill workers. Canteens are supplied with groceries, cooking fuel and drinking water. Preparation and serving of food is associated with the generation of wastewater (which is usually rich in vegetable oils) and solid waste (which is rich in putrefiable organic matter). The wastewater generated is usually mixed with the other domestic sewage of the mill for treatment and disposal.

4.5.6.5 Administrative and other officesOffices of the mills have furniture, office appliances (like, computers, printers, photocopiers, air conditioners, water coolers, lights, fans, fridges, etc.) and sanitation facilities. Electricity, water and stationary are important inputs for the offices. Electricity is required for lighting, ventilation, air cooling/conditioning and powering various office appliances. Water is consumed in the sanitation facilities and in air-cooling. Offices generate both domestic sewage and office waste (which contain paper as one of the important constituent). The sewage generated is mixed with other domestic sewage of the mill for further treatment and disposal.

Figure-4.5.1: Schematic diagram of water treatment plant of the Alpha Pulp Mill

Figure-4.5.2: Schematic diagram of the soft water plant of Delta Paper Mill

Figure-4.5.3: Process and material flow diagram for the DM water plant and for the steam condensate recycling and

reuse system

Figure-4.5.4: Schematic process flow diagram of chilled water unit of Beta Pulp & Paper Mill

Figure-4.5.5: Material flow diagram for combustion air, fuel and combustion products for the boiler of Delta Paper Mill

Figure-4.5.6: Boiler and Turbine Generator - flow scheme for the boiler feed water and steam

Figure-4.5.7: Schematic diagram of the circulating oil cooling system of the Turbine of Delta Paper Mill

4.5.8: Schematic process flow-diagram of instrument air system of Epsilon Paper Mill