4_sulfate_process_2.pdf

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4. SULFATE PROCESS

Table of content

PRINCIPLE OF THE SULFATE PROCESS .......................................................................................................................1KRAFT COOKING LIQUORS........................................................................................................................................1

Black liquor........................................................................................................................................................3Green liquor .......................................................................................................................................................3White liquor........................................................................................................................................................3

KRAFT COOKING CHEMISTRY ...................................................................................................................................4Sodium sulfide (Na2S).........................................................................................................................................4

Alkali ..................................................................................................................................................................5Polysulfide and anthraquinone...........................................................................................................................7

COOKING PROCESS ...................................................................................................................................................8HFACTOR................................................................................................................................................................9QUESTIONS ............................................................................................................................................................ 10

Kaj HenricsonProfessor Pulping Technology

Lappeenranta University of Technology

August 2004

Educational course material and only for internal and personal use duringthe course: An introduction to chemical pulping technology.


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Principle of the sulfate process

Picture 1 shows the basics of a fiber line of the sulfate process also called the kraft process. The

fiber line is a fiber line for producing bleached hardwood or softwood pulp to be used for

instance in the manufacturing of writing paper. The objective of the kraft cooking process is toseparate the fibers in wood chemically and to dissolve most of the lignin contained in the fiber

walls as gently as possible. The key operating variables during kraft cooking are the charge of

cooking chemicals, time and temperature. After cooking, kraft pulp is washed to recover theused cooking chemicals and dissolved organic material. The pulp is then screened to removedebris and bleached to the target brightness. The resulting sulfate or kraft pulp is dried and baled

or taken directly to a paper mill.

COOKING WASHING SCREENING BLEACHINGSULPHATE

PULP

PAPER MILL

DRYING

TREECROSS

CUTTINGSTORAGECHIPPINGBARKING SCREENING


PULP

PAPER MILL

DRYING


PULP

PAPER MILL

DRYING

TREECROSS

CUTTINGSTORAGECHIPPINGBARKING SCREENINGTREE

CROSS

CUTTINGSTORAGECHIPPINGBARKING SCREENING

Picture 1. The fiber line operation of the sulfate process for producing bleached pulp

Picture 2 shows the cooking chemical

regeneration cycle of the sulfate process.

After cooking, the spent liquor is separated

from the fibers by washing. The resultingweak black liquor is evaporated to remove

excess water, and the strong black liquor from

evaporation is taken to the recovery boiler.The spent liquor is burned in the recovery

boiler to produce heat and a smelt consistingof the inorganic sodium-containing cookingchemicals. The smelt is dissolved in water to

produce green liquor that is recausticized to

form the white liquor that is the cooking

liquor needed in kraft cooking. In therecausticizing process, lime is used to convert

sodium carbonate into sodium hydroxide.

Picture 2. Sulfate pulp mill, chemicalregeneration cycle

Kraft cooking liquors

The kraft cooking liquor is a mixture of white liquor, water in chips, condensed steam and weak

black liquor used to adjust the liquor-to-wood ratio. The white liquor is a strongly alkalinesolution (pH~14), in which the main active compounds are sodium hydroxide (NaOH) and

sodium sulfide (Na2S). White liquor also contains small amounts of sodium carbonate (Na2CO3),sodium sulfate (Na2SO4), sodium thiosulfate (Na2S2O3), sodium chlorine (NaCl) and calsium

carbonate (CaCO3) plus other accumulated salts and non-process elements. These additional

compounds can be considered inert from a cooking point of view. They enter the white liquoreither as contaminants with the used raw materials or as a result of inefficiencies in the chemical

regeneration cycle.

PMS

Taskinen


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Hydrolyzed NaOH and Na2S are the active species in kraft pulping, and Na2CO3 is that to alesser extent. The main reaction variables in alkaline cooking are wood species (i.e. their main

chemical components), chip dimensions, temperature, time, and the concentration of cooking

chemicals (OH- and HS- ions).

OHOHNaOHNaOH 22 +++ +

OHSNaOHSNa 22

22 2 +++ +

++ OHHSOHS 22

During kraft cooking, about 45% to 65% of the wood material will result in pulp and the rest of

the wood is dissolved to form the organic material in the black liquor. The pulp yield varies

depending on the pulp type produced. The yield is lower in pulp to be bleached for white boardand paper grades and higher in pulp for brown board and paper grades.

CELLULOSE

830 596

HEMICELLULOSE

596

LIGNIN

106

EXTRACTIVES ECT222425

LIGNIN

HEMICELLULOSE

730CELLULOSE

WOOD

PULP 1000

DISSOLVED WOOD0,53*2128 = 1128

TURPENTINE,

METHANOL,

NON-

CONDENSABLE

25

ORGANIC DRY SOLIDSIN BLACK LIQUOR

1103

1000

0.47=2128

CELLULOSE

830 596

HEMICELLULOSE

596

LIGNIN

106

EXTRACTIVES ECT222425

LIGNIN

HEMICELLULOSE

730CELLULOSE

WOOD

PULP 1000

DISSOLVED WOOD0,53*2128 = 1128

TURPENTINE,

METHANOL,

NON-

CONDENSABLE

25

ORGANIC DRY SOLIDSIN BLACK LIQUOR

1103

1000

0.47=2128

Picture 3. Wood material during kraft cooking

Picture 4. Schematic diagram relating to the terminology used in describing the composition of kraftcooking liquors

In Scandinavia, the chemical properties and composition of kraft cooking liquors are usually

defined as:

Total alkali, g/l = all Na- compounds (NaOH, Na2S, Na2CO3, Na2SO4, Na2S2O3,

Na2SO3)

Active alkali, g/l = NaOH + Na2S

Fapet: p. A43

PV, Modified by KH


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Effective alkali, g/l = SNaNaOH 22

1+

Sulfidity, % = 1002 + SNaNaOH

SNa

s

Causticity, % = 10032

+ CONaNaOH

NaOH

Reduction, % = 100422

2

+ SONaSNa

SNa

The amounts of the compounds are calculated as equivalents of NaOH or Na2O and can be

converted to other equivalents based on their sodium contents as expressed in Table 1. The

conversion of the compounds into other sodium compounds present in white liquor takes placeby means of equivalent molecular weights. BA = MA/MB, where BA is a coefficient through

which compound B can be converted into A, MA is the equivalent molecular weight of

compound A, MBis the equivalent molecular weight of compound B.

Table 1. Conversion factors for kraft cooking chemicals

Compound

as is

Multiply to get

Na2O

From Na2O

to compound

Multiply to get

NaOH

From NaOH

to compound

NaOH 0.775 1.290 1.000 1.000

Na2O 1.000 1.000 1.290 0.775

Na2S 0.795 1.258 1.025 0.975

NaHS 0.554 1.807 0.714 1.400

Na2CO3 0.585 1.710 0.775 1.325

Na2SO4 0.437 2.290 0.563 1.775

Black liquorBlack liquor is formed during cooking, and it contains the used cooking chemicals and thedissolved organic material. The water content in weak black liquor is high and the liquor

contains only 15%-20% of dry solids, the rest being water. Weak black liquor is evaporated to

form strong black liquor having a dry solids content of 65%-85%.

Green liquor

Green liquor is formed when the smelt from the recovery boiler is dissolved in water. The main

components of green liquor are Na2CO3and Na2S. Green liquor also contains inactive sodium

compounds, accumulated salts and non-process elements. Non-process particles include

unburned carbon, solids from the furnace construction and some calcium compounds. Green

liquor needs to be clarified or filtered before used for cooking chemical production.

White liquor

White liquor contains the active cooking chemicals. It is an aqueous solution where the active

chemicals are NaOH and Na2S. Mill white liquor also contains some inactive dead-loadchemicals. The primary one is Na2CO3. White liquor is formed when green liquor is treated with

lime in order to form NaOH from Na2CO3.

Fapet 6A: p.A43


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Table 2. Typical white liquor composition

Concentration, g/lCompound Molar

weight /mNaOH Na2O Actual compound

NaOH 40 95 73.6 95

Na2S 78 40 31.8 39

Na2CO3 106 23 17.8 30.5Na2SO4 142 4 3.1 7.1

Na2S2O3 158 2 1.6 4.0

Na2SO3 126 0.5 0.4 0.8

Other - - - 3.0

Na2O 62 - - -

Effective alkali 135 105.4

Active alkali 115 89.5

Total alkali 164.5 128.3 179.4

Sulfidity 29.6%

Causticity 80.5%

Reduction rate 80.0% (all S-compounds are taken account in calculations)

Kraft cooking chemistry

Sodium sulfide (Na2S)

Sulfide increases the selectivity of the sulfate cooking process and increases the rate of

delignification. There is a catalytic effect caused by the hydrosulfide ion on the cooking process.

The sulfide ion is only, to some extent, bound to the organic material dissolved during cooking.The sulfide ion leads to the formation of odorous gases that cause the characteristic smell at

sulfate pulp mills.

Increasing sulfidity up to 30% has a strong impact on the cooking process. Further increase in

sulfidity improves the process only marginally. Sulfidity is 35%-45% in most Scandinavianmills and in North American mills 25%-35%. Generally, sulfidity tends to increase when the

losses of mill liquor decrease.

When sulfidity increases from 0% to 31% as shown in Picture 5, the time needed to delignify the

pulp is reduced in the temperature interval 150C to 170C. When cooking time can be reducedthe yield will increase and the pulp strength will improve as the fibers as subjected to strong

alkaline conditions a shorter time.

At the same degree of delignification, yield will increase when sulfidity is increased as shown in

Picture 6. For hardwood the effect can be seen up to a sulfidity of 20-30 % after which the effectis small. For softwood the benefits of a higher sulfidity continues up to a sulfidity over 50%.

Modified by KH;PV: p.383


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Picture 5. Effect of cooking time, temperature and sulfidity on the kraft cooking of spruce

Picture 5 shows how the lignin content of wood decreases during cooking as a function of time,temperature and sulfidity. The cooking has been done using the same charge of effective

chemical to the cooks.

SULFIDITY

T

OTALYIELD

%(OFWOOD)

YIELD PINE,

KAPPA 55

YIELD BIRCH,

KAPPA 55

SULFIDITY

T

OTALYIELD

%(OFWOOD)

YIELD PINE,

KAPPA 55

YIELD BIRCH,

KAPPA 55

Picture 6. Effect of sulfidity on the total yield in

the kraft cooking of spruce and birch

Picture 7. Effect of white liquor sulfidity on thefiber length distribution of beatenpulp

The pulp properties improve when sulfidity increases. This is illustrated in Picture 7, which

shows fiber length distribution for softwood fibers after refining as a function of sulfidity. The

fiber length as such is not influenced by sulfidity but the fibers become stronger and are moreresistant to being cut during refining.

Alkali

A major portion (30%-40%) of the total alkali charged will be consumed during the

impregnation phase or the extraction phase although lignin dissolution is slight. The alkali goes

Modified by KH

Fapet 6A: p.A59

Fapet 6A: p.A49

Modified by KH;

PV: p.322


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to the neutralization of the acid groups formed, the end-wise degradation of polysaccharides andthe splitting of acetyl groups from the hemicellulose chains. Sulfide takes part only in minor

amount in the dissolution of the polysaccharides.

Most of the alkali consumption takes place during the extraction stage also called impregnation

and initial cooking. Only a limited amount of alkali is consumed during the bulk delignificationphase, but the consumption increases again during residual delignification. Delignification itself

does not require much alkali, but the conditions need to be strongly alkaline for lignin todissolve. There needs to be a certain amount of alkali left at the end of the cook to maintain a pHhigh enough also at the end of the cook.

Sulfide and hydrosulfide ions are not consumed or transformed much during cooking. The

balance between hydroxyl ions and hydrosulfide ions changes during cooking due to the

consumption of alkali.

Picture 8. Consumption of effective alkali as afunction of lignin yield in the kraftcooking of pine and birch

H FACTOR

PINE BIRCH

EFFECTIVE ALKALI = 24%SULFIDITY = 25%

RESIDUALLIGNIN,%(O

FWOOD)

BULK DELIGNIFICATION

RESIDUALDELIGNIFICATION

EXTRACTION PHASE

H FACTOR

PINE BIRCH

EFFECTIVE ALKALI = 24%SULFIDITY = 25%

RESIDUALLIGNIN,%(O

FWOOD)

BULK DELIGNIFICATION

RESIDUALDELIGNIFICATION

EXTRACTION PHASE

Picture 9. Rate of dissolution of pine and birchlignin as a function of H factor

EA in chipsEA in chips

Picture 10. Effective alkali in the liquid phase and inside the chips during kraft cooking. The time it

takes for the alkali to impregnate the chips at the beginning of the cook is about 30minutes. Much of the alkali is consumed before actual delignification starts.

Fapet 6A: p.A47

AP: p.215

Modified by KH;

PV: p.304


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Polysulfide and anthraquinone

Polysulfide stabilizes hemicelluloses during cooking and reduces carbohydrate dissolution in

cooking of hardwoods and softwoods. Proper application of polysulfide will lead to substantial yield

improvements as shown in the picture. These yield increases in pulping sustain in bleaching.Polysulfide is used at many mills as a way to increase the yield during kraft pulping. There will be a

slight change in pulp properties when polysulfide is used.

Picture 11. Yield increase for spruce as a function of polysulfide sulfur addition

Unmodified anthraquinone can stabilize carbohydrates against alkaline degradation by slowing

down peeling reactions on hardwoods and softwoods. The way anthraquinone functions in kraft

pulping can be explained as shown in Picture 12. Anthraquinone (AQ) reacts with reducing endgroups of carbohydrates, stabilizing them against alkaline peeling and producing reduced

anthrahydroquinone (AHQ) which is alkali soluble. AHQ reduces lignin, which becomes more

reactive; AQ is formed again and reacts with carbohydrates. This semi-catalytic redoxmechanism explains why very small additions of anthraquinone are effective.

Anthraquinone is used as an additive at many kraft mill to increase pulp yield. The cost of the

amount of anthraquinone used must be balanced against the savings in wood costs.

Anthraquinone is also used in sulfur-free cooking to speed up delignification and increase yield.

Especially on hardwood, soda-anthraquinone cooking can be used and the results are almostsimilar to kraft cooking. There are a few mills using soda-anthraquinone cooking.

Picture 12. The oxidation and reduction cycle in anthraquinone-enhanced kraft pulping

Fapet 6A: p.A52

Fapet 6A: p.A53


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Picture 13. Effect of anthraquinone on mixed southern hardwood soda pulping

Cooking process

Before the chips are impregnated with the cooking liquor, they are usually presteamed. There

are two reasons for presteaming chips: (1) to drive off the air inside the chips and replace it withsteam; and (2) to heat the chips. Air inside the chips prevents the cooking liquor from

penetrating and diffusing into the chips. Steam can, contrary to air, condensate, and steam makes

it possible for the cooking liquor to penetrate and diffuse into the chips.

After presteaming, the cooking liquor is added to the chips, the suspension is pressurized andimpregnation begins. The objective of impregnation is to distribute the cooking liquor uniformly

into the chips. Impregnation consists of two different processes: penetration of void parts of the

chip and diffusion. As early as during impregnation, the chemical reactions start, the wood

material is dissolved and alkali is consumed.

After or during impregnation, the temperature is raised, and as the temperature rises,

delignification starts. Most of the delignification takes place after reaching a temperature above

140C. Normal final cooking temperatures are in the range of 150C-170C. Most new digesters

have been designed for rather low cooking temperatures. During cooking, the alkalinity of thecooking liquor can be controlled by adding alkali and by exchanging cooking liquids during

cooking.

The chips retain their structure during cooking and are about the same size after cooking as

before cooking. After cooking, the chips are soft and can be disintegrated into fibers by lightagitation if the kappa number has been lowered sufficiently. Some high-yield pulp grades

require that the chips are disintegrated by refining after cooking in order to produce a pulpsuspension consisting of individual fibers.

The variables affecting the cooking process can be divided into three categories: chip quality,white liquor properties and cooking control variables. The mixture of chip grades, available at

the chip storage, determines chip quality. Mill chemical balances and the operation of the

chemical recycling process determine the white liquor properties. Cooking control variables arecontrolled during cooking. The main cooking control variables are time and temperature (H

factor), alkali charge and liquor-to-wood ratio.

Fapet 6A: p.A54


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The charge of alkali is dependent of type of wood used. Usually softwoods require a largercharge of alkali as percentage on wood than hardwoods. Too small a charge of alkali will lead to

high amounts of reject after cooking due to uncooked parts of chips remaining after cooking.

Too large a charge will lead to increased operational costs, lower yield and corrosion problemsespecially in the black liquor recovery cycle.

The goal of the kraft cook is to get a high yield of cellulose with a high degree of polymerization

and a low yield of lignin. The cooking conditions are chosen to make the cook as selective aspossible.

A high yield of hemicelluloses increases the yield on wood but also influences the fiberproperties. To some extent, the hemicellulose yield can be influenced by controlling the cooking

conditions.

Extractives cause problems during pulp washing, bleaching and papermaking. The extractives

are removed as thoroughly as possible during cooking together with the black liquor and bydegassing during cooking. Hardwood extractives are especially harmful and the amount of

hardwood extractives can be reduced by adding black liquor or tall oil from softwood cooking to

the hardwood cooking process. Some additive chemicals to cooking can also be used to lower

the amount of extractives in the pulp after cooking.

H factor

As mentioned earlier, time and temperature are very important factors in cooking. They can be

represented by a single numerical value, the H factor. As long as the H factor is constant, the yieldand lignin content of cooking will be the same if the other conditions like raw material and alkali

concentration are the same.

The contribution of the heating time to the H factor is very small compared to the contribution of the

cooking time at higher temperatures (Picture 14). Modern digester control systems automaticallycompute and accumulate the H factor during the cook to compensate for deviations from the

intended cooking cycle.

Picture 14. Temperature, reaction rate and accumulated H factor during kraft cooking. The H factor isrepresented by the integral of reaction rate and cooking time. The reaction rate is stronglydependent on temperature.

AP: p.54


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Questions

1. Factors influencing yield during normal kraft cooking. / Massasaantoon vaikuttavat tekijttavanomaisessa sulfaattikeitossa.

2. Polysulfide and anthraquinone in alkaline cooking. / Polysulfidin ja antrakinonin kyttalkalisessa keitossa.

3. The stages of delignification and the consumption of alkali during kraft cooking. / Ligniinin

liukenemisen eri vaiheet sulfaattikeitossa ja alkalin kulutus keiton aikana.4. The effect of sulfide on delignification and the consumption of sulfide during cooking. /

Sulfidin vaikutus keittoon ja sulfidin kulutus keiton aikana.5. The use of the H factor in controlling cooking. / H-tekij keiton ohjauksessa.6. Main cooking control variables. / Keiton ohjauksen trkeimmt parametrit.7. Give the definition for the following pulping terms: a)total alkali, b)active alkali, c)effective

alkali, d)sulfidity, e)causticity, f)reduction. / Mrittele seuraavat termit: a)kokonaisalkali,b)aktiivialkali, c)tehollinen alkali, d)sulfiditeetti, e)kaustisointiaste, f)reduktioaste.

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