Biermann Ch. Handbook of Pulping and Papermaking

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114 4. KRAFT SPENT LIQUOR RECOVERY

r^vvvv^^v-NpH^v'^^yv^^v-^^

FEED

PIPE

LIFTING DEVICE

DRIVE GEAR

TTT

LIQUOR STORAGE

^^»

_n OVERFLOW

PIPE

-RAKE ARMS

-RAKE BLADES

Fig. 4-13. A single compartment gravity sedi-

mentation (settling) clarifier and storage unit

suitable for green or white liquor.

smelt and greatly increase the amount of dregs.

The metal salts arise from non-process elements

(impurities) of the wood and corrosion products of

the processing equipment, particularly from the re-

covery boiler. Improvements in the design of the

recovery boiler have decreased the dregs yield to

less than 4 kg/t (8 lb/ton) pulp. The dregs are

washed in a dregs washer, often a drum filter or

sedimentation washer (Fig. 4-14) where about 90-

95%

of the sodium chemicals are removed, of

which there is about 1-4 kg (2-9 lb) (NajO basis)

per ton of pulp in the dregs before the washers.

Slaker

The slaker is a chemical reactor in which

lime is mixed with green liquor (Fig. 4-15). The

reaction temperature is 99-105°C (210-220°F).

Using a high temperature and lime directly from

the kiln gives a lime mud that settles well. The

lime,

CaO, forms slaked

lime,

Ca(0H)2, and much

of the causticizing reaction occurs here where the

retention time is 10-15 minutes. These two chemi-

cal equations are shown below.

slaking reaction:

CaO + H2O -> Ca(0H)2

causticizing reaction:

Ca(0H)2 + Na2C03 ^ 2NaOH + CaC03(5)

Grits (large, unreactive lime particles and

insoluble impurities corresponding to about 0.5-

of the lime feed) are removed here by the

classifier, which uses a raking action. The grits

are often sent to landfills, although a few mills

will send them through a ball mill and use them to

neutralize the acid effluent of the bleach plant.

The extent of the causticizing reaction (and

therefore the causticizing efficiency in the white

liquor) depends on the concentration of the initial

NajCOj and the amount of lime used. With con-

centrations below 16% of actual chemical, the

theoretical conversion is over 90%. At concen-

trations above this, the theoretical conversion

drops off quickly. It is desired to have about

1 %

excess lime. Much more than this increases the

FRESH

WATER

MIXER

DREGS FROM

THE GREEN -

LIQUOR

CLARIFIER

r>^vvv><>o^>d4-^r^.o^vy^?^

DILUTE^

DREGS

BAFFLES

DRIVE GEAR

|W^

yjL

WEAK

WASH

LIQUOR

-RAKE ARMS

-RAKE BLADES

WASHED

DREGS

Fig. 4-14. Sedimentation type green liquor dregs washer.

COOKING LIQUOR REGENERATION US

Fig. 4-15. The top (top) and side (bottom) views of a slaker.

turbidity of the white liquor and decreases the

filtration rates of the lime mud. Commercially,

about 75-85% of the ultimate level of conversion

is achieved in the agitator section of the slaker.

The causticity efficiency should be 3-4% below

the equilibrium value to avoid excess liming.

Causticizers

The causticizers are two to four continuous

flow, stirred reactors that are used to complete the

causticizing reaction

(Fig.

4-16). The contents are

stirred with a pitched blade turbine at 70-80 rpm.

The liquor/lime slurry flows through them in

116 4. KRAFT SPENT LIQUOR RECOVERY

Fig. 4-16. The causticizers. The left msert

shows a sample drawn for quality analysis. The

right insert shows proper sample settling.

series with a total retention time of

1.5-2.5

hours.

The internal surfaces must be stainless steel or

another corrosion resistant material.

White liquor clarifier

The white liquor clarifiers are settling tanks

(gravity sedimentation) used to remove the lime

mud (CaCOj) from the white liquor. The clarified

liquor should have a turbidity below 100 ppm.

The lime mud leaves with a solids content above

35%

in order to minimize entrained soda that is

removed in the lime mud washer. Like the green

liquor clarifiers, white liquor clarifiers with single

compartments with at least 12 hours storage (see

Fig. 4-13) are now much more common than the

multi-compartment clarifiers.

Poor lime settling may be a result of excess

lime to the slaker (more than 1% excess), a low

lime availability (below 80-85% that is indicative

of a high level of contaminants due to inadequate

removal of dregs and/or grits or incomplete

slaking due to overloading the lime kiln), a high

percentage of low reactivity unburned fresh lime

(i.e.,

purchased lime that has not been through a

lime kiln or is aged), or inadequate white liquor

clarification or clarifier operation. Lime burned at

too low a temperature gives much mud of high

viscosity; lime burned at too high a temperature

gives a slow causticizing reaction and a slow

settling lime mud with entrained alkali. Polymeric

additives are often used to help white liquor

clarification.

White liquor pressure filters

Recently, some mills have been installing

white liquor pressure filters after the white liquor

clarifiers to supply additional clarification. Typi-

cally the filters have polypropylene filter tubes

through which the white liquor flows under a

pressure of 140-210 kPa (20-30 psig) to trap

further lime mud. A reverse, flushing cycle re-

moves the embedded lime mud that is added to the

bulk of the lime mud (from the white liquor

clarifiers) prior to the lime mud washers.

Lime mud washer

The lime mud washer removes most of the

15-20%

entrained alkali (Na20 basis) from the

lime mud, usually by sedimentation

washing.

It is

typically a settling tank (or two in series) where

fresh (make-up) water is used to wash the lime

mud. If it were not removed, the NajS would

cause slagging in the kiln and reduced sulfur

compounds would be released as HjS. About

1 %

alkali on lime mud remains after washing. The

liquor (weak wash or weak white liquor) is then

used to dissolve smelt from the recovery boiler.

It is important to maintain a proper water balance

during lime mud washing to avoid

formation

e^ess

weak wash. Excess weak wash mZ i

Tl'lT "T" ""'*

^°''

^^1"^^1«

chemical

that

creates

disposal problem.

Lime mud

filter

•^M

^^?

^^^ "^"^ ^^'^

'*°'^^8^

is diluted

25-

30%

solids (as measured by an X-ray detector)

before

going to the lime mud

filter.

The lime mud

filter

a rotary drum vacuum filter washer used

fo final hme washing and thickening to 60-70%

sohds before the lime

enters the

kiln.

Centrifuges

have been

installed

instead

drum vacuum

filter

thicken

imie,

but

they

have lower water

ren^v!

nLr^fl'" '"^'^^^ ^""'•Sy '^osts and lower

lime bin throughput.

COOKING

LIQUOR

REGENERATION

II7

S;T?5'

ft^'T ''^'' «P^-«°- «f

a lime kiln

tnat

15 ft in diameter, 440 feet lona O,.H

«=^

TOO

u„.,day of lii^ ;^"aL°'£,Sf S

about 2 to 5 from horizontal to propel the lime

along as fte kilt, rotate, at ab J

I'tiT

o«e..^or4?B=f,™-fr-£"'

specific

energy consumption =

3ESLtoldln

CaO output

Lime kiln

The lime kiln is a chemical reactor in which

hme mud (CaC03)

dried,

heated,

and

convmeS

u'AP'"^^''

'™°'^"

""^""'"S

(Plate 19)

described by

the chemical

equation

below.

calcining reaction:

CaCOj

t CaO +

CO,(g)

fh.

'T^'°''

^™^

''' *^

"^In

should be on

*^3i^ /wlnnT- ^^O-required at abom

200-300 kg/t (400-600 lb/ton) for pulp, with the

^wer

amounts

for

high yield pulps

L^iinerbo^:

and the

higher amount for low yield

pulps such

bleachable

grades.

Normally

a rotary kiln is used

but some mills use a fluidized bed calcintr

(^tn3")in'?^'^^^-'-^''^^)-2.4to4m

400 mi

'V*^^^" ^ 30 to 120 m (100 to

400 ft) long

and produce

40 to 400

t/day

CaO

However, in mid 1991, the Union Cai^p ISun

Fuel oil or natural gas combustion (or occa-

reS Thf

''"r^^"'

^^'^°C'

2200°F)

Sde th^ f ?!• ^'^- ^-^^ *°^s the steps

imide the hme kiln. The rotary kiln requires

about

7.8-10

GJ/t CaO (7-9 x i? BtiJtrcS

^ver 300 t/d) and long length (100 m or 330 ft)

wntVr'ir""'

"^'°^'^'

^Wl^

low

capaci-

or 200

ft)

kilns use

the

higher amount.

The purity of the lime is given by the

lime

avazZaW/fy

that

is defined

as:

y ^ne ftwe

lime availability

^^Q

lime

(as mass ratio)

Iverlv w^ °V' *' '"^^""S li"^^ ™'d is

occur to

the

extent that is should

Some mills with short lune kilns or where

thehmekilnisthebottleneckandhiSri:^-

AIR

FUEL

V ’^^’^ LIME MUD (CQCO^

5-95X

CQD)

60-70X SOLIDS ^

LIME (85-95;i

CQD)

fig.

4-17. Function and operation of the lime kiln.

VENTURI

SCRUBBER

SMOKESTACK

118 4. KRAFT SPENT LIQUOR RECOVERY

put is required, use flash driers to dry the lime

mud to less than 1% moisture content before it

enters the kiln. In flash drying, the exit gases of

the kiln (much hotter than 200^*0 since dry mud is

being fed into it) are used to dry the lime mud.

The lime mud is combined with the gases in a

mixer. The flue gases and water vapor are then

separated from the lime mud in a cyclone.

Heat is recovered from the flue gases and hot

burned lime of the lime kiln. A development of

the early 1980s is the discharging of the hot,

burned lime product through about 8-10 integral

tube coolers forming a ring around the lime kiln

on the outer side of the discharge end. Secondary

air flows through these tubes in the opposite

direction as the lime so that it is preheated, there-

by increasing the combustion temperature or lime

throughput. However, the lime must remain

above 320°C (600°F) so it will slake well.

Sulfur oxides are not emitted to a large extent

because the CaO is a good scavenger for the

highly acidic forms of these compounds and

CaSOj and CaS04 are formed. Emissions of

nitrogen oxides are high due to the excess air and

high combustion temperatures. See Section 11.6

for more information. Pure oxygen gas has been

used as a partial substitute for air in order to

decrease HjS generation from the lime mud and

increase lime mud throughput rates.

Ring formation within the lime kiln is part of

the territory. If it becomes a severe problem,

expensive shutdowns may result. Ring formation

can result (Tran and Barham, 1991) when non-

condensable gases are introduced to the lime kiln

and the ring occurs from the formation of CaS04.

The use of a TRS scrubber column for the NCG

prior to their introduction to the lime kiln can

drastically decrease ring formation.

The exhaust gases must be treated in order to

remove particulate matter. Usually a venturi

scrubber (Fig. 4-18) is used. The flue gases flow

through a constriction on the order of 1 ft^ with

the introduction of a water spray to trap the

particulates. In the mid 1970s, a few mills started

using electrostatic precipitators (ESP, Fig. 4-11) to

treat the lime kiln exhaust gases. ESP have many

advantages over the venturi scrubbers, including

cleaner exhaust

gases,

easier control of the system,

and low exhaust fan power requirements since the

exhaust is not forced through a narrow constric-

tion. Now most new systems use electrostatic

precipitators to capture the particulates.

Salt cake (Na2S0J

Salt cake is the make-up chemical for the

kraft process that is used to replace chemicals lost

(mostly through the pulp) in pulping. About 50-

100 lb/ton pulp is normally dissolved in the con-

centrated black liquor just before spraying into the

recovery furnace. The name "sulfate" process was

derived from the use of this salt as a make-up

chemical; however, sodium sulfate is not involved

in the actual pulping process. The make-up chem-

ical can be other chemicals as well.

Cross recovery

Cross recovery is the collection of waste

NSSC liquor and burning it in a nearby kraft mill.

Kraft mills recover the sodium and sulfur as Na2S

and NajCOj after the recovery furnace. The kraft

mill pays the NSSC mill for Na2S04, which allows

the NSSC mill to purchase fresh cooking chemi-

cals (sulfur and either NaOH or Na2C03).

4.6 ANNOTATED BIBLIOGRAPHY

U.S. Environmental Protection Agency,

Environmental pollution control. Pulp and

Paper Industry, Part I, air, EPA-625/7-76-

001 (1976).

Brown stock

washing

Crotogino, R.H., N.A. Poirier, and D.T.

Trinh, The principles of pulp washing, Tappi

J. 70(6):95-103(1987). This is an excellent

overview of pulp washing from the theoreti-

cal and equipment perspectives.

Perkins, J.K., Ed.,

1982

Brown Stock Wash-

ing, Short Course Notes, TAPPI Press,

Atlanta, GA, 1982, 49 p.

This is a very useftil introduction to brown

stock washing. Descriptions of Chapters 1-9

follow. Perkins, J.K and K.M. Jackson, The

washing of brown stock, pp 1-6 (chap. 1),

gives a historical overview of the basic wash-

ing methods; Perkins, J.K., Rotary filter

washing equipment, pp 7-9 (chap. 2) has 10

ANNOTATED BIBLIOGRAPHY 119

'' \

•II1\

Fig. 4-18. The top view shows the lime kihi duct carrying the flue gases to the venturi scrubber

(bottom), and from the scrubber to the chimney.

120 4. KRAFT SPENT LIQUOR RECOVERY

figures with descriptions of the title equip-

ment; Hough, G.W., Auxiliary components

and design for vacuxmi filter washing sys-

tems,

pp 11-16 (chap. 3) addresses stock

flow, drop legs, seal chambers, and seal

tanks;

McSweeney, J.M., Control and

instrumentation, pp 17-20 (chap. 4); Smyth,

Jr.,

J.A., The mathematics and variables of

counter-current brown stock washing, pp 21-

26;

Guillory, A.L., Sorbed soda and its

determination, pp 27-31 (chap. 6); Guillory,

A.L.,

Brown stock washer efficiency and

physical testing, pp 33-37 (chap. 7);

Perkins, J.K., Analyzing and troubleshooting

a washing system, pp 39-45 (chap. 8); and

Perkins, J.K., Newly introduced methods of

brown stock washing, pp 47-48 (chap. 9).

Phillips, J.R., and J. Nelson, Diffusion

washing system performance (plus a simpli-

fied method for estimating saltcake loss from

any equipment combination.

Pulp

Paper

Can.

78(6):T123-T127(73-76)(1977). This article

includes a brief review of the equipment.

Norden, H.V., V.J. Pohjola, and R.

Seppanen, Statistical analysis of

pulp

washing

on an industrial rotary drum. Pulp Paper

Mag. Can. 74(10):T229-T337(83-91)(1973).

Regression analysis showed the dependence

of the underflow leaving the drum upon

various factors.

heat, viscosity, chemical composition, and

thermal conductivity.

8. Harvin, R.L. and W.F. Brown, Tappi /.,

36(6):270-274(1953). The specific heat of

black liquor is fairly linear from 1 cal/g at

0% solids to 0.70 at 60% solids.

Black liquor

evaporation

9. Gudmundson, C, H. Alsholm, and B.

Hedstrom, Heat transfer in industrial black

liquor evaporator plants, Svensk Papper-

stidning. Part I, 75(19):773-783(1972); Part

II,

75(22):901-908(1972). The overall heat

transfer coefficient in climbing film evapora-

tors is dependent on the heat flux, boiling

point of the liquid, and the liquid feed rate,

temperature, viscosity, and surface tension.

Foaming flow produces remarkable heat

transfer, especially at low heat flux, com-

pared to non-foaming flow. The error be-

tween calculated and measured data was less

than 15% for

95%

of the data.

Falling

film

evaporators

10.

Shalansky, G., D. Burton, and B. Lefebvre,

Northwood

pulp &

timber's experience using

the falling film plate type evaporator for high

solids concentration of kraft black liquor,

Pulp Paper

Can.

93(1):51-56(1992). This is

a very useful article on the subject with some

nice figures to demonstrate the concepts.

Black liquor properties

6. Lowendahl, L., G. Petersson, and O. Samu-

el-son, Formation of carboxylic acids by

degradation of carbohydrates during kraft

cooking of pine, Tappi J.

59(9):

118-

120(1976). The amounts of 26 organic acids

in black liquor are given.

11.

Burris, B. and J.F. Howe, Operational expe-

riences of the

first

eight-effect

tubular

falling-

film evaporator train, Tappi J., 70(8):87-

91,94(1987). This claims a steam economy

of 8.4 when evaporating black liquor

firom

to 50% solids for a system that handles 1

million pounds per hour.

7. Venkatesh, V and X.N. Nguyen, 3. Evapo-

ration and concentration of black liquor, in

Hough, G, Ed., Chemical Recovery in the

Alkaline Pulping

Process, TAPPI Press, At-

lanta, 1985, pp 15-85. A general present-

ation of black liquor properties is given,

including specific gravity as a function of

solids content, boiling point rise, specific

Recovery boiler

12.

Taylor, S.S., Good firing practices unprove

boiler safety, Am. Papermaker, 52(4, misla-

beled as 3 in the front):39-42(1989).

13.

Koncel, J.A., Preventive maintenance can

end smelt-water explosions, American

Papermaker,

54:(8):26-27(1991). The cause

ANNOTATED BIBLIOGRAPHY 121

of 125 BLRB explosions are itemized along

with a six-step emergency shutdown proce-

dure.

Useful guidelines for preventing smelt-

water explosions are found here. "Industry

statistics show that smelt-water explosions

take place in one of every 72 operating

BLRBs each year."

14.

Bauer, D.G., and W.B.A. Sharp, The in-

spection of recovery boilers to detect factors

that cause critical leaks, Tappi J. 74(9):92-

100(1991). This is a technical article along

these lines that should be in the library of

anyone responsible for recovery boiler safety.

Casticizing

15.

Mehra, N.K, C.F. Cornell, and G.W.

Hough, Chap. 5. Cooking liquor prepara-

tion, in Hough, G, Ed., Chemical Recovery

in the Alkaline Pulping Process, TAPPI

Press,

Atlanta, 1985, pp 191-256. A good

discussion of the theoretical causticizing

efficiency as well as material and energy

balances for liquor and lime cycles is pre-

sented here.

16.

Daily, CM. and J.M. Genco, Thermody-

namic model of the kraft causticizing reac-

tion, J. Pulp Paper Sci, 18(1):J1-J10(1992).

The thermodynamics of the causticizing

reaction has been modeled based on K.S.

Pitzer's method to predict the ratio of the

activity coefficients of hydroxyl and car-

bonate anions in strong electrolyte solutions.

overflow decreased from 1.35% to 0.26%,

while the slurry solids concentration of the

underflow increased from 32% to 44% using

a 100% recycle rate [Tappi J.

75(3):20(1992)].

Cogeneration

18.

Price, K.R. and W.A. Anderson, New

cogeneration plant provides steam for Oxnard

papermaking facility, Tappi J. 74(7):52-

55(1991). Here cogeneration using the GE

LM2500, which is a modification of the GE

CF6-6 aircraft engine used in DC-10 jet

aircraft, is described.

Lime kilns

19.

Amer. Papermaker, 55{\y3A-35{l992), This

article describes the use of ESP in a new

lime kiln.

20.

Tran, H.N. and D. Barham, An overview of

ring formation in lime kilns, Tappi /.

74(1):

131-136(1991).

Miscellaneous considerations

21.

Sell, N. and J. C. Norman, Reductive burning

of high-yield pulp by the addition of pulver-

ized coal, Tappi J.

75(10):

152-156.

Pulver-

ized coal was added to pulp liquor (from a

mill that makes 78% yield sodium bisulfite

hardwood pulp) to help recover the sulfur as

sulfide (77%) and sulfite (20%) instead of

sulfate, which is the typical recovery product

for sulfur at these mills.

17.

Lewko, L.A. and B. Blackwell, Lime mud

recycling improves the performance of kraft

recausticizing, Tappi J.

74(10):

123-

127(1991). This claims to improve the

causticizing process by recycling some of the

lime mud to the slaker. This might be im-

portant at a mill that is limited by the lime

kiln or lime mud washing. It also will de-

crease the dead load slightly in the recovery

boiler and lead to a small increase in boiler

capacity. In Table 11 the "common over-

flow" units should be mg/L not g/L. Reports

after this paper tell of a mill whose free lime

mud content in the white liquor clarifier

Although not discussed in this paper, it

would seem that high sulfiir-content coals

might actually be burned in a recovery boiler

with sulfiir recovery in the smelt.

EXERCISES

General

What are the three main objectives in the

chemical recovery cycle?

Write the chemical equations as indicated.

a) Reaction of NaOH in the recovery boil-

er:

122 4. KRAFT SPENT LIQUOR RECOVERY

b) The formation of white liquor from

green liquor:

c) The regeneration of calcium so it can be

used in reaction b):

d) Concentration of black liquor from

15 %

to 50% solids:

Where is turpentine recovered during kraft

pulping? Where is tall oil collected?

What is cross recovery?

Pulp washing

Indicate the effect of a high dilution factor in

pulp washing on the following:

Increase Decrease

a) sodium loss

b) evaporation costs __

c) bleaching cost

6. The steam economy of the multiple effect

evaporators is 4.24; suppose the dilution

factor in the brown stock washers goes from

3 tons water per ton pulp, to 4 tons water per

ton pulp. How much more steam is required

to process a ton of pulp?

Liquor concentration

What are the two types of bodies used in

multiple-effect evaporators?

8. Black liquor is first concentrated to about 45-

50%

solids using the multiple-effect evapora-

tors.

Final concentration is achieved by

direct contact evaporators or by concen-

trators. Since the late 1960s, direct contact

evaporators have not been installed in pulp

and paper mills. Why is this the case?

9. Why is the concentration of black liquor

usually limited to about 65-75% solids prior

to combustion in the recovery boiler? Why

would a higher solids content be desirable?

Recovery boiler

10.

Describe the location of the oxidation and

reduction zones in the recovery boiler. Give

some examples of chemical reactions that

occur in each of these zones.

11.

What is the purpose of the electrostatic

precipitator after the recovery boiler?

12.

What would happen to the sodium sulfide if

it was oxidized in the bottom of the recovery

boiler? Is this beneficial?

Lime cycle

13.

What are the three zones in the lime kiln in

terms of what is happening to the lime?

14.

What are the consequences of not removing

the dregs from the green liquor?

Causticizing

15.

Of what is the casticizing efficiency of white

liquor a measure?

Miscellaneous

16.

It has been a long practice at many mills to

add some black liquor to the white liquor and

chips before pulping. What are some possi-

ble advantages of adding black liquor to the

digester as part of the digester charge?

PULP BLEACHING

5.1 INTRODUCTION

Bleaching

Bleaching is the treatment of wood (and other

lignocellulosic) pulps with chemical agents to

increase their brightness. The percentage of

bleached pulp to total pulp production in the U.S.

has risen from 32% in 1950 to 40% in 1990.

Bleaching of chemical pulps involves a much

different strategy than bleaching mechanical pulps.

Bleaching of chemical pulps is achieved by lignin

removal. Lignin removal in chemical pulps leads

to greater fiber-fiber bonding strength in paper,

but the strong chemical used in bleaching chemical

pulps decreases the length of cellulose molecules,

resulting in weaker fibers.

Bleaching mechanical pulps is achieved by

chemically altering the portions of

the

lignin mole-

cule that absorb light (i.e., have color). Obvious-

ly, lignin removal in mechanical pulps is counter-

productive; bleaching mechanical pulps is referred

to as lignin-preserying. Sometimes bleaching

mechanical pulps is called "brightening" to distin-

guish it from bleaching of chemical pulps.

Brightness

Brightness is a term used to describe the

whiteness of pulp or paper, on a scale from 0%

(absolute black) to 100% (relative to a MgO stan-

dard, which has an absolute brightness of about

96%) by the reflectance of blue light (457 nm)

from the paper. See Section 7.8 for more details

on these tests. The approximate brightness levels

of some pulps are as follows:

Unbleached kraft

Unbleached sulfite

Newsprint

Groundwood

White tablet paper

High grade bond

Dissolving pulp

20%

35%

60%

65%

75%

85%

90%

Color reversion

Color reversion is the yellowing of pulps on

exposure to air, light, heat, certain metallic ions.

and fungi due to modification of residual lignin

forming chromophores. Mechanical pulps are

particularly susceptible to color reversion, though

chemical pulps may experience this when exposed

to high temperatures.

Consistency

Bleaching stages are carried out at consisten-

cies from 3-20 %. Higher consistencies of 10-20 %

are used with chemicals such as oxygen, peroxide,

and hypochlorite, which react with the lignin

slowly. By using high consistencies, higher con-

centrations of the bleaching agent are realized for

a given chemical loading, which increases the

reaction rate.

5.2 BLEACHING MECHANICAL PULPS

Mechanical

pulps

are bleached with chemicals

designed to alter many of the chromophores.

Chromophores are most often conjugated double

bond systems arising in the lignin of

pulps.

Other

chromophores such as sap-stain induced by micro-

organisms, dirt, metal ions such as ferric that

complex with lignin, and water impurities may

impart color to paper. Bleaching of mechanical

pulps involves masking the lignin that is present,

instead of removing the lignin as is the case for

bleaching chemical pulps. To emphasize this

distinction, bleaching mechanical pulps is often

referred to as brightening.

Brightening mechanical pulps is accomplished

with reducing agents, such as dithionite, or oxi-

dants,

such as hydrogen peroxide, often in a single

stage process. Fairly limited brightness improve-

ments are realized (6-12% typically) with a maxi-

mum brightness of

60-70%

in a single stage, or up

to 75% in a two-stage process. The pulp bright-

ness also dependents on the wood species from

which the pulp was derived. Brightened mechani-

cal pulps are subject to color reversion. Since the

lignin is largely decolored, but not removed, there

is only a small loss of yield. If two stages are

used, the oxidative stage is used before the reduc-

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