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flow charts for Phosphoric Acid

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Page 1: flow charts for Phosphoric Acid
Page 2: flow charts for Phosphoric Acid

PHYSICAL PROPERTIES OF PHOSPHORIC ACID

MOLECULAR FORMULA : H3PO4

MOLECULAR MASS : 98g

CHEMICAL NAME : orthophosphoric acid

COMMON NAME : phosphoric acid

BOILING POINT : 133°c

MELTING POINT : -17°33

DENSITY : 1.83g/cc

VAPOUR PRESSURE : 267 Pa at 20°C

SOLUBILITY :miscible in water

ODOUR : slight acid odour

APPEARANCE : bronish/greenish viscous liquid

Page 3: flow charts for Phosphoric Acid

STRUCTURE OF PHOSPHORIC ACID

P

H

H

H

O

O

O O

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Page 5: flow charts for Phosphoric Acid

THERMAL PROCESS

ELECTRIC FURNACE PROCESS

BLAST FURNACE PROCESS

WET PROCESS(SULFURIC ACID)

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•The raw material used is elemental phosphorus .•This process has been abandoned because large amount of energy is required.

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REACTIONS INVOLED :

Ca3(PO4)2 + 3H2SO4 +6H2O (ROCK PHOSPHATE)

2H3PO4 + 3(CaSO4.2H2O) (GYPSUM)

SIDE REACTIONS:

CaF2 + H2SO4 + 2H2O 2HF + CaSO4.2H2O

6HF + SiO2 H2SiF4 +2H2O

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40% H3PO4

TRAVELLING PAN FILTER

HOT WASH WATER

SLURRY TO LAGOON

SLUDGE

GROUND PHOSPHATE ROCK

SINGLE EFFECT EVAPORATOR

REACTOR

WASHED GYPSUM

TO GYPSUM PLANT

WEAK ACID WASH

FUMESCRUBBER

COOLING AIR

93-98% H2SO4

RECYCLE H3PO4

75% H3PO4

CL

AR

IFIE

R

VENT GAS

SLURRY

GYPSUM

STEAM

H3PO4

HF(g)

HF(l)

Ca3(PO4)2 + 3H2SO4 +2H2O

2H3PO4+3(CaSO4.2H2O)

CaF2+H2SO4+2H2O 2HF+CaSO4.2H2O

SIDE REACTION

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• Igneous found in Kola, South africa,Brazil • sedimentery rocks found in Jordan ,Algeria,Morocco,etc

• Flourapatite (found in igneous)• Francolite(found in sedimetary)

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DESCRIPTION OF PRODUCTION PROCESSES Raw Materials for Phosphoric Acid ProductionBones used to be the principal natural source of phosphorus but phosphoric acid today is produced from phosphatic ores mined in various parts of the world.Phosphate ores are of two major geological origins:-– Igneous as found in Kola, South Africa, Brazil, etc.– Sedimentary as found in Morocco, Algeria, Jordan U.S.A., etc.The phosphate minerals in both types of ore are of the apatite group, of which the mostcommonly encountered variants are:-– Fluorapatite Ca10(PO4)6(FOH)2

– Francolite Ca10(PO4)6–x(CO3)x(FOH)2+x Fluorapatite predominates in igneous phosphate rocks and francolite predominates in sedimentary phosphate rocks.

Page 12: flow charts for Phosphoric Acid
Page 13: flow charts for Phosphoric Acid

The fluorine is liberated as hydrogen fluoride during the acidulation of phosphate rock.In the presence of silica this reacts readily to form fluosilicic acid via silicon tetrafluoride. CaF2 + 2H+ 2HF + Ca++

4HF + SiO2 SiF4 + 2H2O

3SiF4 + 2H2O 2H2SiF6 + SiO2

The fluosilicic acid may decompose under the influence of heat to give volatile silicontetrafluoride and hydrogen fluoride.

H2SiF6 SiF4 +2HF

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GAS SCRUBBING SYSTEMSGAS SCRUBBING SYSTEMS

A number of different scrubbing systems have been used for removing fluoride. These can vary both in the scrubbing liquor and in the type of scrubber used. The most widely used scrubber is the void spray tower operating at atmospheric pressure but others, such as packed bed, cross-flow venturi and cyclonic column scrubbers have been Extensively employed.A product containing up to 22% fluosilicic acid is recovered in the fluoride recoverysystem at atmospheric pressure and the removal efficiency is better than 99% (90% with one absorber). Silica is removed from the acid by filtration. Fresh water, recycled pond water, sea water and dilute fluosilicic acid have all been used as scrubbing liquor.Gas from the evaporator flash chamber is first fed through an entrainment separator if asystem operating under vacuum is used. Essentially, this removes any P2O5 values from the gas. Only one scrubbing stage is generally used and 17-23% fluosilicic acid is obtained with a recovery efficiency of about 83-86%.

Page 15: flow charts for Phosphoric Acid
Page 16: flow charts for Phosphoric Acid

Let us see the manufacture of phosphoric acid by wet process Involving sulfuric acid leaching of dihydrate process.

• Bones used to be principal natural source of phosphoric acid .• Nowadays phosphoric acid is produced from phosphatic ores mined in different parts of the world.

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DIHYDRATE PROCESS

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PROCESS DESCRIPTION :

Grinding :

Some grades of commercial rock do not need grinding ,their particle size distribution being acceptable for dihydrate reaction section (60_70% <150) most other phosphate rock need particle size reduction ,Generally using ball or rod mills. Other mills operate with wet or dry rock.

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This stage separates the phosphoric acid from the calcium sulphate dihydrate. Five tonnesof gypsum are generated for every tonne (P2O5) of product acid produced. The filter mediummust move in sequence through the various stages for continuous operation. The initialseparation must be followed by at least two stages of washing, to ensure a satisfactoryrecovery of soluble P2O5. It is only possible to achieve the desired degree of separation ata reasonable rate if the filtration is either pressure or vacuum assisted and in practice vacuumis always used. The remaining liquid must be removed from the filter cake as far aspossible at the end of the washing sequence. The cake must then be discharged and thecloth efficiently washed to clear it of any remaining solids which might otherwise build upand impair filtration in subsequent cycles. The vacuum must be released during the dischargeof the cake and it is beneficial to blow air through in the reverse direction at thispoint to help dislodge the solids.The filtrate and washings must be kept separate from one another and have to be separatedfrom air under vacuum conditions and then delivered under atmospheric pressure, asproduct, or for return to the process. The pressure difference is usually maintained bydelivering the filtrates below the surface of the liquid in barometric tanks placed at a levelsufficiently below the separators for the head of liquid to balance the vacuum.The most common filtration equipment is of three basic types: tilting pan, rotary table ortravelling belt

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The tri calcium phosphate is converted by reaction with concentrated sulphuric acid into phosphoric acid and insoluble calcium sulphate. The reactor maintains an agitated reaction volume in circulation. The reaction system consists of a series of separate agitated reactors. The operating conditions for di hydrate precipitation are 26-32% P2O5 and 70-80°C . This temperature is controlled by passing the slurry through a flash cooler, which also de-gasses the slurry and makes it easier to pump. The temperature can also be controlled by using an air circulating cooler.

REACTION :

But in the interests of economy of materials and space, the multi-vessel reaction system is replaced by a single tank in some processes. Some of these single tanks may be divided into compartments which are virtually separate reactors.The operating conditions for dihydrate precipitation are 26-32% P2O5 and 70-80°C .This temparature is controlled by Passing it through flash coolers which also de_gasses the slurry and makes it easier to pump. The temparature also controlled by using circulating cooler.

2Ca3(PO4)2+3H2SO4+6H2O2H3PO4+3CaSO4.H20

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There is a long history of direct contact concentrators, in which evaporation is effected bybringing the acid into intimate contact with hot combustion gas from a burner, enablingequipment walls to be made of materials and in thicknesses which are suitable for efficientindirect heat transfer. Various patterns of direct-fired concentrator have been devised.Currently, almost all evaporators that are being built today for this service are of theforced circulation design.

CONCENTRATION :

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• This is the most diffused process• There is no phosphate rock quality limitation• On-line time is high• Operating temperatures are low• Start-up and shut-down are easy• Wet rock can be used (saving drying costs)

ADVANTAGES OF DIHYDRATE PROCESS :

DISADVANTAGES OF DIHYDRATE PROCESS :

• Relatively weak product acid • High energy consumption in the concentration stage• 4%-6% phosphorus tri oxide losses,most of them co crystallizes with ca-sulfate.

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HEMIHYDRATE PROCESS

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The forced circulation evaporator consists of a heat exchanger, vapour or flash chamber, condenser, vacuum pump, acid circulating pump and circulation piping. A fluosilicicacid scrubber is usually included in the forced circulation evaporator system. All the evaporatorsin this service are generally of the single-effect design because of the corrosive nature of phosphoric acid and the very high boiling point elevation. The heat exchangersare fabricated from graphite or stainless steel with the rest of the equipment made fromrubber-lined steel. All equipment designs will be made using the best practices of engineering available. More than one evaporator may be used, with the acid passing in sequence through each, depending on the degree of concentration required.

PROCESS DESCRIPTION :

Operating conditions are selected in this process so that the calcium sulphate is precipitatedin the hemihydrate form. It is possible to produce 40-52% P2O5 acid directly, with consequentvaluable savings in energy requirements. Figure 5 shows a simplified flow diagramof a HH process. The stages are similar to those of the dihydrate process but grindingmay be unnecessary.

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CAPITAL SAVINGS.

Purer acid:Acid from the HH process tends to contain substantially less free sulphate and suspendedsolids and lower levels of aluminium and fluorine than evaporated dihydrate process acidof the same strength.

LOWER ROCK GRINDING REQUIREMENTS. A satisfactory rate of reaction can be achieved from much coarser rock than in the dihydrateprocess, because of the more severe reaction conditions in the HH process.The disadvantages of HH systems are:-

Filtration rate. Hemihydrate crystals tend to be small and less well formed than dihydrate crystals andthus hemihydrate slurries tend to be more difficult to filter than dihydrate slurries unlesscrystal habit modifiers are used to suppress excessive nucleation. With a good HHprocess however, there is no need to use crystal habit modifiers. There are examples ofphosphate rocks that produce hemihydrate crystals achieving higher filtration rates thanobtained with dihydrate crystals.

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– Residual acidity (P2O5)– Fluorine compounds– (These are only harmful if disposal is into fresh water because disposal into sea– water results in the formation of insoluble calcium fluoride.)– Undesirable trace elements– Radioactivity

HARMFUL IMPURITIES PRESENT IN GYPSUM

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GYPSUM DISPOSAL

Around 5 tonnes of gypsum are generated per tonne of P2O5 produced as phosphoric acid.This represents a serious disposal problem with the individual phosphoric acid productionunits of over 1,000t.d-1 capacity now being built.Two methods can be used to dispose of gypsum:-– Disposal to land– Disposal into water

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Disposal to water

The gypsum can be pumped through an outfall into the sea at coastal sites and estuaries.Disposal into rivers is no longer practised, as it is not a good environmental option.Disposal of gypsum into the sea has the advantage that gypsum is more soluble in seawater than in fresh water. However, some of the impurities in the gypsum should be controlled.Clean gypsum itself (CaSO4) is soluble and is not harmful to the environment.A phosphoric acid plant with high efficiency is essential for this method of disposal andonly “clean” phosphates can be used in the plant if the pollution is to be kept within localenvironmental quality standards.

Disposal on land Disposal on land, under proper conditions, is the best environmental option although it isnot possible everywhere because it requires space and certain soil qualities where the gypsum stack is situated.Dry gypsum from the filter in some plants is transported by belt conveyors to the gypsumstorage pile. The pile area is completely surrounded by a ditch which collects the runoffwater including any rain water

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To man Phosphoric acid is corrosive to all parts of the body.

Contact with the skin can cause redness and burns.

Splashes in the eyes cause irritation and burns.

Acid mists may cause throat and lung irritation.

To environment

Phosphoric acid is harmful to aquatic life.

HAZARDOUS EFFECTS OF PHOSPHORIC ACID

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Process Single-stage filtration.Produces strong acid directly 40-48% P2O5.No intermediate storage if acid is produced at user's strength.Uses coarse rock.Ease of operation.

Limited number of rocks processed industrially.Large filter area required for 48% P2O5 acid.High lattice loss, low P2O5 efficiency (90-94%).Produces impure hemihydrate. Tight water balance. Requires higher grade alloys.Care required in design and shut-down.

ADVANTAGES OF HEMIHYDRATE PROCESS

DISADVANTAGES

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Commonly used methods for the determination of fluoride in solutions from gas samplingsystem are colorimetric and ion selective electrode methods.Colorimetric methods include the zirconium SPADNS (sulpho phenyl azo dihydroxynaphthalene disulphonic acid) method as the most widely used. Fluoride reacts withzirconium lake dye to produce a colourless complex for spectrophotometric determination.A fluoride selective electrode using a lanthanum fluoride membrane may be used.

EMISSIONS INTO AIR

ON LINE METHODS

A volumetric method may be used which relieson the titration of fluoride ion against lanthanum nitrates to an end point determined bycoloration of an indicator dye asuch as Alizarin Red S or Eriochrome Cyanine R.

MANUAL METHODS

SEPARATION OF FLOURINE

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• ~800 Million to 1 Billion Tons in Stacks in Florida

• ~30 Million Tons Being Added Each Year

PHOSPHOGYPSUM PROBLEM

Florida Institute of Phosphate Research

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CONCENTRATES MANUFACTURING

Sulfur

Phos. Rock

Anhydrous Ammonia

Sulfuric Acid Plant

Air

SulfuricAcid

HeatHeat

Phosphate Rock Storage

PhosphoricAcid Plant

Phosphoric AcidPhosphoric Acid

Merchant GradeMerchant GradePhosphoric AcidPhosphoric Acid

GypsumStack

GranulationPlant

NH3 Storage

GranularGranularCrop NutrientsCrop Nutrients

Animal FeedIngredientsPlant

Defluorinated Feed Phosphates

ExportedElectricPower

Cogen-erationPlant

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CONCENTRATES MANUFACTURING

Sulfuric Acid Plant

Phosphoric Acid Plant

Granulation Plant

Product Warehouses

Gypsum Stack

Animal FeedIngredientsPlant

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Page 36: flow charts for Phosphoric Acid

GYPSUM PRODUCTION, 1997

Production(Mt/yr)

RelativeAmount

PG in Florida 30 1

Gypsum in US 20 0.67

Gypsum in World 114 3.80

Florida Institute of Phosphate Research

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PROCESS WATER PROBLEM

Each stack has 1 to 3 billion gallons of process water

pH is about 1 to 2 Dilute mixture of

Phosphoric, sulfuric, fluorsilicic acids Saturated with calcium sulfate Contains numerous other ions and

ammonia

Florida Institute of Phosphate Research

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PROCESS WATER PROBLEM

Piney Point Other Gypsacks

Florida Institute of Phosphate Research

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PINEY POINT PROBLEM

Approximately 1 billion gallons of low pH, high conductivity water

Water near the top of the stack Threatening to spill into Bishop’s Harbor

Florida Institute of Phosphate Research

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PINEY POINT WATER INVENTORY REDUCTION

Trucking Lime treatment and removal Reverse osmosis with no

pretreatment (US Filter) Ocean Dumping Pretreatment/reverse osmosis

project (IMC/FIPR)-in negotiation

Florida Institute of Phosphate Research

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PROBLEMS WITH OTHER STACKS

There are approximately 20 other stacks which will eventually have to be closed

The water in these stacks has a pH of 1 to 2

The conductivity of the water is greater than at Piney Point

Florida Institute of Phosphate Research

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POSSIBLE SOLUTIONS

Reduce the accumulation of phosphogypsum

Reduce the amount of water on the stacks

Improve the quality of the water on the stacks

Florida Institute of Phosphate Research

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POTENTIAL USES FOR PHOSPHOGYPSUM

Road building Agriculture Landfills Oyster Culch Roofing Tile

Florida Institute of Phosphate Research

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RADIOACTIVITY OF PHOSPHOGYPSUM

Phosphogypsum pCi/g

Northern Florida 5 to 10

West Central Florida 20 to 35

Florida Institute of Phosphate Research

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BARRIERS TO PHOSPHOGYPSUM USE

Regulatory agencies Public fear of the word

radioactivity

Florida Institute of Phosphate Research

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POSSIBLE SOLUTIONS

• Reduce the accumulation of phosphogypsum

• Reduce the amount of water on the stacks

• Improve the quality of the water on the stacks

Florida Institute of Phosphate Research

Page 47: flow charts for Phosphoric Acid

• Monitoring of emissions plays an important part in environmental management. It can bebeneficial in some instances to perform continuous monitoring. This can lead to rapiddetection and recognition of irregular conditions and can give the operating staff the possibilityto correct and restore the optimum standard operating conditions as quickly as possible.Emission monitoring by regular spot checking in other cases will suffice to survey thestatus and performance of equipment and to record the emission level.In general, the frequency of monitoring depends on the type of process and the processequipment installed, the stability of the process and the reliability of the analytical method.The frequency will need to be balanced with a reasonable cost of monitoring.

EMISSION MONITORING IN PHOSPHORIC ACID PLANT

INTRODUCTION :

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PHOSPHORIC ACID PLANT

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