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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 %&$.
• 'nlike bleached mechanical pulps! the high brightness is stable since
the lignin is removed.
• (leached chemical pulps are insensitive to color reversion! but high
temperatures may induce some color reversion.
•
)ignin removal is accompanied by significant losses of pulp yieldand strength of the individual fibers.
• *owever! the strength of fiber fiber bonding increases after bleaching.
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• (leaching of chemical pulps involves the use of chemicals
– which are more specific to lignin removal than to carbohydrate degradation compared with
the chemicals used in pulping.
• +f this is not true! continue the pulping process in order to remove more lignin.
• bleaching is much more expensive than pulping for a given amount of lignin
removal.
• ,ithin the range of useable bleaching chemicals! some are very specific to ligninremoval while others are much less specific and cause appreciable carbohydrate
degradation and diminished yield.
• For example! oxygen and chlorine are relatively inexpensive! but not particularly
selective for lignin removal.
• These chemicals are used in the early stages of bleaching to remove most of the
lignin.
•
-esidual lignin is removed in later stages with expensive! but highly selective bleaching agents like chlorine dioxide! hypochlorite! and hydrogen peroxide. 4
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Summary of conditions used in various bleachingstages of chemical pulps.
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C stage, Chlorine
• ormally! chlorine is the first bleaching stage! where unbleached pulp is treated
with elemental chlorine! Cl&! that is either gaseous or in solution! at a p* of #./01./.
• (leaching is carried out with a pulp consistency of 203$! 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.
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Example reactions ofbleaching agentswith lignin.
Reaction of excess CI2 andsulte pulpwith time.
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• 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
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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
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• 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
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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 =#$> or medium consistencies =1#01/$>.
• @elignification is carried out at %#012#BC for # 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