5.3bleaching Chemical Pulps

7/23/2019 5.3bleaching Chemical Pulps

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BLEACHING CHEMICAL PULPS

Cellulose viscosity• The cellulose degree of polymerization in low yield pulps and

bleached pulps is very important

– since these processes lower the degree of polymerization of cellulose

to the point where the paper strength properties are adversely affected.

• Cellulose viscosity of mechanical pulps and high yield pulps are

not measured

– since it is usually quite high and not a factor in the strength properties

of papers derived from these pulps.

• Cellulose viscosity is measured by dissolving the pulp in cupri

ethylene di amine solution.

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Full chemical pulp bleaching

• Chemical pulp bleaching is accomplished with

various compounds containing chlorine oroxygen and alkali extractions in several stages.

• The use of three to seven stages increases the

efficiency of bleaching by reducing the amount

of chemical required.

• This is due to the complex nature of lignin;

each bleaching chemical is going to react

differently with lignin.

• ince lignin is a complex molecule with

different types of linkages! the use of different

chemicals will break various types of bonds.

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• ince chemical pulps are dark!

– bleaching increases brightness up to "#$ with a maximum brightness of

about %&$.

• 'nlike bleached mechanical pulps! the high brightness is stable since

the lignin is removed.

• (leached chemical pulps are insensitive to color reversion! but high

temperatures may induce some color reversion.

•

)ignin removal is accompanied by significant losses of pulp yieldand strength of the individual fibers.

• *owever! the strength of fiber fiber bonding increases after bleaching.

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• (leaching of chemical pulps involves the use of chemicals

– which are more specific to lignin removal than to carbohydrate degradation compared with

the chemicals used in pulping.

• +f this is not true! continue the pulping process in order to remove more lignin.

• bleaching is much more expensive than pulping for a given amount of lignin

removal.

• ,ithin the range of useable bleaching chemicals! some are very specific to ligninremoval while others are much less specific and cause appreciable carbohydrate

degradation and diminished yield.

• For example! oxygen and chlorine are relatively inexpensive! but not particularly

selective for lignin removal.

• These chemicals are used in the early stages of bleaching to remove most of the

lignin.

•

-esidual lignin is removed in later stages with expensive! but highly selective bleaching agents like chlorine dioxide! hypochlorite! and hydrogen peroxide. 4



5

Summary of conditions used in various bleachingstages of chemical pulps.



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C stage, Chlorine

• ormally! chlorine is the first bleaching stage! where unbleached pulp is treated

with elemental chlorine! Cl&! that is either gaseous or in solution! at a p* of #./01./.

• (leaching is carried out with a pulp consistency of 203$! ambient temperature

since the reaction is quick! and a retention time of #.201.# hour.

• 4ressurized! up flow reactors are used.

• Chlorine application is 506$ on softwood 7raft pulps or 203$ on sulfite or

hardwood 7raft pulps.

• 8lemental chlorine was the first agent used to chemically bleach cellulose fibers.

• +ts large0scale use commercially for pulp bleaching.

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• The production of chlorine is summarized as follows9

&aCl : &*& : elect. < Cl& : &a* : *&

• Chlorine is not overly specific to lignin! and much

carbohydrate degradation occurs through its use.

• The chlorine reacts with lignin by substitution of

hydrogen atoms for chlorine atoms.

• The substitution reactions are probably the most

important in the ultimate lignin removal.

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• +t has been known for a long time that chlorine is first

rapidly depleted by the pulp and then is depleted much

more slowly.

• This indicates several types of reactions are occurring.

• The substitution and addition reactions are much faster

than the oxidation reactions and do not leave Cl in

solution! whereas oxidation reactions do.

• This is the basis for determining the type of reaction

occurring.

9



10

Example reactions ofbleaching agentswith lignin.

Reaction of excess CI2 andsulte pulpwith time.



• xidation includes reactions with both lignin and carbohydrates.

• xidation of the carbohydrates leads to a decreased cellulose

viscosity and decreased pulp strength.

• The species *Cl especially contributes to oxidative

degradation of the carbohydrates – chlorination is carried out above p* #./ =to avoid acid hydrolysis of

cellulose>

– and below p* 1./ =to avoid carbohydrate degradation by oxidation>.

• ?lso sodium hypochlorite bleaching is carried out with excess

a* to avoid carbohydrate oxidation by *Cl.

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• The amount of chlorine added to pulp was controlled by measuring the

residual chlorine after the chlorination stage.

• +n fact it is better to avoid adding excess chlorine! particularly at elevated

chlorination temperatures

•

The oxidation reactions do not increase the amount of lignin removed inthe alkali extraction of sulfite pulp

• For the same reason 7raft pulps require longer chlorination periods than

sulfite pulps! 5#0%# minutes versus 1/03/ minutes! respectively.

• The practice of substituting about 1#$ of Cl& with Cl& =C@> has been

practiced for a long time since it results in a stronger pulp by avoiding

over0chlorination.

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• )ignin is not removed to a large degree in this stage! and the pulp actually gets

darker.

• The pulp is diluted to 1$ consistency and washed to remove acid which wouldotherwise consume alkali in the next stage.

• The lignin removal and brightness increase actually occur in the alkali

extraction stage that follows the chlorination stage.

• Chlorination produces chlorinated organic materials including a very small

amount of dioxin

– which has led to the use of other chemicals to replace a part or all of the chlorine use in

bleaching

• Aany mills have already replaced up to /#$ of the Cl& with Cl&.

• Chlorine0free bleaching sequences are used commercially for sulfite pulps at a

few mills.

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C@ stage

• The C@ or C@ stage is a modification of C stage bleaching!

where some of the chlorine is replaced with Cl&.

• ubstitution of 1#$ of the chlorine with chlorine dioxide is

used to prevent over chlorination.

• ubstitution of /#$ or more of chlorine with chlorine dioxide

at many mills is becoming common to reduce production of

dioxins and other chlorinated organic chemicals.

• ne problem is that since Cl& and a* are both derived from

electrolysis of aCl! a slackening in the Cl& market will mean

much higher a* prices.14



E stage

• The 8 stage is extraction of degraded lignin compounds

– which would otherwise increase the chemical usage in

subsequent bleaching stages! with caustic =a*> solution.

•

,hen it follows the C stage =81> – it is used at &02 $ on pulp!

– consistency of pulp =1#016$>!

– with a temperature of /#0%/BC

–

and a reaction time of #."/ to 1./ hours.

• *igh temperatures and alkali loading up to /$ are used.

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• +n later 8 stages! alkali is used at less than 1$ on pulp.

• The alkali displaces chlorine and makes the lignin soluble by

reactions such as9

)ignin0Cl : a* < )ignin0* : aCl

• The lignin in the 81 effluent gives a dark color that is ultimately

responsible for much of the color of the final mill effluent.

• -ecently oxygen gas has been incorporated into this stage

=#./$ on pulp> at many mills and the term 8 applies.

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H stage

• The * stage consists of bleaching with hypochlorite solution

• This stage is carried out

– at 3016$ consistency!

– 2/03/BC !

– 10/ hours!

– and p* 1#.

• The process is often controlled by measuring the oxidation0

reduction potential.

• +t is important to maintain the p* above 6.

• *ypochlorite reacts principally by oxidation.17



• This chemical is more selective than elemental chlorine!

but less selective than chlorine dioxide.

• consequently! the use of hypochlorite has decreased since

the advent of chlorine dioxide.

• Cellulose is oxidized along its chain! forming aldehyde

groups and making random cleavage more likely.

• odium hypochlorite! which is now used since it leads to

less scaling! is made from chlorine as follows9

Cl& : &a* < aCl : aCl : *&

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D stage

• The @ stage involves bleaching with chlorine dioxide.

• This makes it very useful for the latter bleaching stages where

lignin is present in very low concentrations.

• +t is used at consistencies of 1#01&$! 5#06#BC ! for 20/ hours at

a p* of 2./05.

•

+t is used at #.30#.6$ on pulp.

• Cl& <#./ Cl 2 +O2

•&Cl& : *& → *Cl2 : *Cl& 19



P stage

• (leaching with hydrogen peroxide.

• +t is usually used for brightening mechanical pulps

– when it is used to bleach chemical pulps it appears as the last stage of a sequence such

as C080*04 or C080*0@04.

• +t is used

– at 1#$ consistency

– 5#0"#BC =13#015#BF>!

– p* of 601#! for &03 hours.

– +t is an expensive bleaching agent! but may be used more frequently as the use ofelemental chlorine decreases.

• 4eroxide oxidizes carbonyl groups of carbohydrates to carboxylic acid groups.

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O stage, oxygen pulping and bleaching

• xygen bleaching or pulping is the delignification of pulp using oxygen under pressure =//#0 "## k4a > and a* =203$ on pulp>.

• This is an odorless! relatively pollution0free process used prior to chlorination

at high consistencies =&#02#$> or medium consistencies =1#01/$>.

• @elignification is carried out at %#012#BC for &#05# minutes.

• The key to the use of & delignification was the discovery that small amounts

of magnesium ion =#.#/0#.1 $ on pulp> must be present to protect the

carbohydrates from extensive degradation.

• This is the most inexpensive bleaching chemical to use! but also the least

specific for lignin removal.

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CE!IC"# REC$%ER&

Chemical recovery is the process in which the inorganicchemicals used in pulping are recovered and regenerated orreuse!

"his process results in

1# recovery o the inorganic coo$ing chemicals

2# generation o large amounts o heat energy %y %urning theorganic materials derived rom the wood

3# reduction in air and water pollution %y converting the wasteproducts into useul materials

4# regeneration o the inorganic chemicals into pulpingchemicals!

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1. Concentration of black liquor by evaporation.

&. Combustion of strong black liquor to give the recovered inorganic

chemicals in the form of smelt. The smelt! a& and a&C2!

dissolved in water gives green liquor.

2. 4reparation of the white cooking liquor from green liquor. This is

done by converting the a&C2 to a* using Ca=*>&! which is

recovered as CaC2.

3. -ecovery of byproducts such as tall oil! energy! and turpentine.

/. -egeneration of calcium carbonate! CaC2! to calcium hydroxide!

Ca=*>&.

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the recovery process or &ratpulping

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5.3bleaching Chemical Pulps