Biermann Ch. Handbook of Pulping and Papermaking

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124 5. PULP BLEACHING

tive stage or else the oxidant will undo what the

reductive compound accomplished.

Dithionite, hydrosulfite bleaching

Hydrosulfite (the conmion name in the indus-

try, but dithionite is the preferred name) bleaching

is carried out with 0.5-1.0% dithionite on wood.

Previously, zinc dithionite was used because it is

very stable. However, zinc is toxic to fish, there-

fore,

the sodium form has replaced the zinc form.

Zinc dithionite was prepared in the pulp mill from

zinc and sulfur dioxide as follows: Zn -f- 2SO2 -*

ZnS204. Bleaching is carried out at pH 5-6 with

chelating agents such as ethylenediaminetetraacetic

acid (EDTA) or sodium tripolyphosphate (0.1-

0.2% on pulp) to prevent metal ions such as

iron(III) from coloring the pulp. Bleaching is

often carried out in the refiners. The reaction

time is on the order of 10-30 minutes. The bright-

ness gain is only 5-8%.

Dithionite reacts with oxygen, so bleaching

with it is carried out at 4% consistency; consisten-

cy below this is unnecessarily dilute, so reaction

with dissolved oxygen consumes dithionite. Con-

sistency above 4% leads to entrained air that

consumes dithionite. [The solubility of oxygen in

the atmosphere is only a few milliliters of gas per

liter of water at 25°C (77°F), and decreases with

increasing temperature; thus, entrained oxygen is

more significant than dissolved oxygen at high

consistencies for dithionite degradation.] Using

temperatures as high as 70°C (158°F) reduces the

oxygen solubility in water. Fig. 11-1 shows the

solubility of oxygen (from air) in water as a

function of temperature.

Sometimes dithionite is added between the

refiner plates where temperatures above 100°C

(212°F) cause steam to displace air. Dithionite

ion reduces lignin and is itself oxidized to sulfite

ion. If hydrogen peroxide and dithionite are used

in a two-stage process, the dithionite must be the

second stage or hydrogen peroxide will reoxidize

those moieties reduced by the dithionite. The

reaction of dithionite is shown below.

SA'" + 2H2O -> 2HS03- + 2H^ -h 2 e-

Peroxide bleaching

Some metal ions, such as Fe^"*",

Mrf"*",

and

Ctf^, catalytically decompose hydrogen peroxide,

so peroxide bleaching is carried out with agents

that deactivate these metal ions.

Fe^^

H,0

2^2

H2O + 1/2 O,

Chelating agents, such as EDTA, have the added

gain of preventing pulp discoloration by binding

with ferric ion that would otherwise form a col-

ored complex with the phenolic lignin structure.

Sodium silicate (5% on wood) is also used

(usually after the addition of magnesium

ion).

The

mechanism for inactivating the ions by sodium

silicate is not clear; it may precipitate the ions,

but, strictly speaking, it is not a chelating agent.

Buffering action is required to keep the pH high

even as organic acids are produced as a result of

some carbohydrate degradation. Sodium silicate

acts as a buffering agent.

Bleaching conditions are 0.5-3% peroxide

and 0.05% magnesium ion (to mitigate carbo-

hydrate degradation by oxygen under alkaline

conditions) on

pulp,

temperature of 40-60°C (104-

140°F) (about 20°C lower than with chemical

pulps since lignin removal is not the goal), pH of

10.5-11,

consistency of 10-20%, 1-3 hour reten-

tion

time,

with a brightness gain of 6-20%.

Hydrogen peroxide with sodium hydroxide

and/or sodium peroxide (NaOOH) is used to pro-

duce the high pH that is necessary to produce the

active perhydroxyl ion, HOO". The formation of

the perhydroxyl ion is as follows:

HA ^ HOO- H- H+ pJ^, = 11.65 @ 25°C

Some carbohydrate degradation occurs and is

responsible for about half of the peroxide con-

sumed. Pine and fir are difficult to brighten.

Color reduction occurs by altering chromophoric

groups such as orthoquinones. The pulp is some-

times subsequently treated with SO2 to neutralize

OH" and reduce any residual peroxide.

5.3 MEASUREMENT OF LIGNIN CONTENT

General

considerations

The measurement of lignin in chemical pulps

is a vital tool to monitor the degree of cook (ex-

tent of delignification during pulping) or to mea-

sure residual lignin before bleaching and between

BLEACHING MECHANICAL PULPS 125

various stages of bleaching to monitor the process,

although pulp brightness between bleaching stages

is often used to control the bleaching operation.

Since most of the lignin remains in mechanical

pulps and bleached mechanical pulps, it is not

measured in these pulps.

Lignin is easily measured indirectly by mea-

suring the amount of an oxidant (such as chlorine

or potassium permanganate) consumed by lignin in

a sample of pulp of known mass. Fig. 5-1 is a

comparison of several cooking degree nimibers.

Methods based on consumption of potassium

permanganate (kappa and K numbers) are the most

conmion ones used. The idea (Fig. 5-2) is to treat

the pulp with an excess of permanganate ion (or

chlorine for Roe or C number) for a specified

period of time to oxidize the lignin. After the

specified reaction time, the residual permanganate

(or chlorine) is determined by indirect titration;

i.e., the excess permanganate converts iodide to

iodine, and the iodine is titrated with thiosulfate

(Section 14.5).

Other methods of lignin measurement are

included here since they appear in some of the

older literature or have laboratory uses.

Kappa number

The kappa test is an indirect method for

determining lignin by the consumption of perman-

ganate ion by lignin. The kappa number is the

number of milliliters of 0.1 KMn04 consumed by

one gram of pulp in 0.5 N sulftiric acid after a ten

minute reaction time at 25°C (77°F) under condi-

tions such that one-half of the permanganate

remains unreacted. The 50% residual perman-

ganate is titrated to determine the exact consump-

tion. Experimentally, 30-70% excess is common

with factors to convert this to 50%. TAPPI

Standard T236 is based on this procedure. The

kappa number test can be used on bleached pulps,

unbleached pulps, and high yield chemical pulps

by use of a single scale with bleached pulps

always giving low numbers and unbleached pulps

giving high numbers, unlike the K number. A

variety of lignin contents are accommodated by

varying the amount of pulp used in the test, but

keeping the amount of KMn04 constant.

It is useful to know how these tests corre-

spond to actual lignin content. This is achieved by

measuring the lignin content of pulps gravimetri-

cally and relating this to the kappa number. The

Sulphite Pulps

Kappa

SCAN

Sulphate Pulps

TAPPI-2Km BjSrkm. Oslr. Johnsen Enso Tingle Sieber Roe Holse Roe Sieber Tingle Enso Johnsen

6SIP.

Bj6rknf«. TAPPI-2Km SCAN

«OmI 25ml ^—- '• " -. -. _. _. ........ „ . .. .

Sdderq.

- Noll

Orig.

Oslo

20^

'2-1

e-1

26-1

24-

22-

20-^

IB-

le^

H-

12-

10-3

Grig.

Oslo - Noil S6derq.

•30 g.2 I

•20

E"3 E

'^l.S E.2.5

E-3

E.2.5

•20

F-lOO

F-90

15 |eo

E- E-70

P-10 E

t-50 tr

~rQ

t,J,m

BLEACHING MECHANICAL PULPS 127

Klason lignin prcx:edure is described below.

Klason lignin is considered to be essentially the

same as the actual lignin content. Alander,

Palenius, and Kyrklund (1963) give the following

relationship for sulfite and kraft chemical hard-

wood pulps:

Klason lignin, % = 0.15 x kappa number

This relationship is approximately correct for

softwoods as well. Chiang et al (1987) found the

coefficient of 0.159 for Douglas-fir and 0.168 for

western hemlock. The authors related kappa

number to the sum of Klason lignin and acid solu-

ble lignin (which are defined below).

Permanganate

number,

number

The permanganate (or K) number, is really

four different tests. A constant amount of

pulp

used with either 25 ml (for bleached pulp), 40 ml

(Fig. 5-2), 75 ml, or 100 ml (for high yield pulps)

of permanganate. Results of

the

100 ml K number

test are not easily compared to the results of the

75 ml (or any other) K number test, so there is no

continuum or results for all types of

pulps

as with

the kappa test. Guillory (1982) gives the relation-

ship in the equation:

log (kappa no.) =

0.837

+ 0.0323 (40 ml K no.)

Therefore, a 40 ml K number of

corresponds to

a kappa no. of 14.5, 20 (40 ml K number) corre-

sponds to 30.4 kappa number, and 30 (40 ml K

number) corresponds to 64.1 kappa number.

Roe

number

The Roe number is a measure of lignin

content by the number of grams of gaseous Clj

consumed by 100 grams dry pulp at 25 °C (77 °F)

in 15 minutes. TAPPI Standard 202 (now with-

drawn) was one method. Alander, Palenius, and

Kyrklund (1963) give the following relationship

for hardwood pulps:

Roe number = 0.158 x kappa - 0.2 (kraft)

Roe number = 0.199 x kappa + 0.1 (sulfite)

Chlorine

number,

C, hypo number

The chlorine number is a test method similar

to that of

Roe,

except the CIO2 is generated

situ

by acidification of sodium hypochlorite. TAPPI

Standard T 253 uses this method to determine a

hypo number. The following empirical equation

relates the chlorine number to the Roe number.

Chlorine number = 0.90 x Roe number

Klason

lignin,

acid

insoluble lignin

Klason lignin is the residue obtained after

total acid hydrolysis of

the

carbohydrate portion of

wood. It is a gravimetric method for determining

lignin directly in woody materials, for example, by

TAPPI Standard T 222. This method is not used

for routine quality control in the mill, but has uses

in the laboratory. Wood meal or pulp is treated

with 72% sulfiiric acid at 20°C (68°F) for 2.0

hours followed by dilution to

sulfiiric acid and

refluxing for 4 hours. The lignin is filtered in a

tared crucible, washed, dried, and weighed. The

isolated lignin in this manner is degraded consid-

erably; nevertheless, it corresponds (by weight)

closely to the original amount of lignin in the

sample. Some of the lignin, especially in sulfite

or hardwood pulps, remains soluble (called acid

soluble lignin) and can be estimated spectrophoto-

metrically in the UV region.

In the wood chemistry literature many modi-

fications of

the

hydrolysis conditions exist, but the

difference in the amount of lignin isolated is

probably not appreciable. For example, the

primary hydrolysis may be carried out for 1.5 h at

25^C (77^F) or

h at 30°C (86°F); the secondary

hydrolysis is often carried out at 4 or 6% H2SO4

for 3 to 4 hours, but TAPPI Standards seldom

offer such flexibility.

5.4 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 poly-

merization 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 proper-

ties of

papers

derived from these pulps. Cellulose

viscosity is measured by dissolving the pulp in

128

5. PULP BLEACHING

LiJ

STAGE 3

/ STAGE 2

/ STAGE 1

CHEMICAL ON PULP

Fig. 5-3. Hypothetical increased brightness vs.

chemical usage for three stages of bleaching.

cupriethylenediamine solution and measuring the

viscosity of the solution (TAPPI Standards T 254

and T 230). See Section 6.3 for more detail.

Full

chemical

pulp bleaching

Chemical pulp bleaching

accomplished with

various compounds containing chlorine or oxygen

and alkali extractions in several stages. Fig. 5-3

indicates why several stages with different com-

pounds are used. 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.

Since lignin is a complex molecule with different

types of linkages, the use of different chemicals

will break various types of bonds. For example,

a large increase in brightness is achieved by using

relatively small amounts of ClOj in a later stage

that could only be achieved using massive amounts

of additional Clj in stage 1; use of large amounts

of chlorine in stage 1 would also cause much

carbohydrate degradation. Plate 20 shows

handsheets made from pulp at various stages of the

bleaching process. Plate 21 shows several wash-

ers of a four stage bleach plant.

Since chemical pulps are dark to begin with,

bleaching increases brightness up to 70% with a

maximum brightness of about 92%. Unlike

bleached mechanical pulps, the high brightness is

stable since the lignin is removed. Bleached

chemical pulps are insensitive to color reversion,

but high temperatures may induce some color

reversion. Lignin removal is accompanied by

significant losses of pulp yield and strength of the

individual fibers. However, the strength of fiber-

fiber bonding increases after bleaching.

Bleaching of chemical pulps involves the use

of chemicals which are more specific to lignin

removal than to carbohydrate degradation com-

pared with the chemicals used in pulping. If this

were not true, one could simply continue the pulp-

ing process in order to remove more lignin. On

the other hand, bleaching is much more expensive

than pulping for a given amount of lignin removal.

Within the range of useable bleaching chemicals,

some are very specific to lignin removal while

others are much less specific and cause apprecia-

ble 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. Residual lignin is removed in later stages

with expensive, but highly selective bleaching

agents like chlorine dioxide, hypochlorite, and

hydrogen peroxide.

Table 5-1 is a summary of conditions used in

various bleaching stages. Bleaching letters are

explained below. Typical sequences are CEH to

a brightness of 84-86% or CEHD or CEHHD to

a brightness of

92%.

Fig. 5-4 shows the layout of

a CEHDP sequence. Each stage consists of a

pump to mix the chemical with the pulp, a reten-

tion tower to provide time for the bleaching

chemical to react with the pulp (Plate 22), and a

washer (Fig. 5-5) to remove the bleaching chemi-

cals and solubilized pulp components. Diffusion

washers may also be used (but are much less

common) and are similar to those discussed in

Chapter 3 in regards to pulp washing.

C stage,

Chlorine (Greek

chloros,

greenish yellow)

Normally, chlorine is the first bleaching

stage, where unbleached pulp is treated with

elemental chlorine, CI2, that is either gaseous or in

solution, at a pH of 0.5-1.5. Bleaching is carried

out with a pulp consistency of 3-4%, ambient

BLEACHING CHEMICAL PULPS 129

Table 5-1. Summary of conditions used in various bleaching stages of chemical pulps.

General condi-

tions

Chemical addi-

tion (on pulp)

Pulp consisten-

|pH

Temp.

Time, hours

C stage

3-8%

3-4%

1 0,5-1.5

1 20-30"

1 0,3-1.5

E, stage

2-3%

10-18%

11-12

50-95°

0.75-1.5

MM^mm

iiiiiiiiii

iiilillii

mmmmmi

iiiiiiiiii

llliilill

iiii»iii:i

D stage

0.4-

0.8%

10-12%

3.5-6

60-80°

3-5

P stage

1-2%

NajOj;

Mg^+;

silicate

10%

8-10

60-70°

2-4

O stage

2-3%

0.4-.8

MPa

60-120 psi

Mg2+ 1

20-30%

10-12%

10-12

90-110°

0.3-1.0

temperature since the reaction is quick, and a

retention time of

0.3-1.0

hour. Pressurized,

upflow reactors are used since the solubiUty of

chlorine in water is low (4 g/L at STP). Chlorine

application is 6-8% on softwood kraft pulps or 3-

on sulfite or hardwood kraft pulps.

Elemental chlorine was the first agent used to

chemically bleach cellulose fibers (cotton, not

wood) and has been used since shortly after its

discovery in 1774 by Scheele. Its large-scale use

commercially for pulp bleaching had to wait until

stainless steel was available in the 1930s.

Chlorine is manufactured concomitantly with

sodium hydroxide by electrolysis of sodium chlo-

ride;

since these two chemicals are produced to-

gether, one often speaks of the chlor-alkali indus-

try. One method of producing these chemicals is

shown in Fig. 5-6. The production of chlorine is

summarized as follows:

2NaCl + 2H,0 + elect. -> CL + 2NaOH + H,

Unbleached Pulp

]!M-< A H-

Chlorination

Stage

Time, hr: 1 hr

Temp.,

'F: ambient

Cons.:

3.0%

pH: 1.5

Bright., %: 20-25%

Washer

Mixer

Steam -

NaOH

IZf^

Hypo-

chlorite

Caustic

^RTIE

.J:^

:2^

I Steam

Caustic

Extraction

Stage

1-2

160*

12%

10.5

30-35%

Hypo-

chlorite

Stage

2-4 hr

90"

12%

9.0

65-75

Thick

Stock

Pump

Mixer

Steam

Peroxide

Silicate

Epsom

Salt

'TZJ-o

Chlorine

Dioxide

Stage

5hr

160»

12%

3.0

85-90%

Peroxide

Stage

4hr

160'

12%

10.5

90-92%

Storage

Fig. 5-4. CEHDP sequence used to bleach pulp to 90-92% brightness. Reprinted from Making

Pulp and

Paper,

130 5. PULP BLEACHING

Fig. 5-5. An open, bleach washer of the vabuum drum type. Modem washers are enclosed.

Chlorine is not overly specific to lignin, and

much carbohydrate degradation occurs through its

use.

The chlorine reacts with lignin by substitu-

tion of hydrogen atoms for chlorine atoms (partic-

ularly on the aromatic ring), oxidation of lignin

moieties to carboxylic acid groups, and, to a small

extent, addition of chlorine across carbon-carbon

double bonds (Fig. 5-7). The substitution reac-

tions (Fig. 5-8) are probably the most important in

the ultimate lignin removal.

It has been known for a long time that chlo-

rine is first rapidly depleted by the pulp and then

Direct ^

Currentc;,«/)>jCl2

2^^*2H20

Asbestos Paper

Diaphragm >

Perforated Iron .

Cathode Plate X

Glass ^' '^ ^

Fig. 5-6. A method of NaOH and Clj manufac-

ture.

Redrawn from J. Ainsworth, Papermaking,

®1957

Thilmany Paper Co., with permission.

is depleted much more slowly. This indicates

several types of reactions are occurring. Fig. 5-9

shows how chlorine reacts with pulp as a function

of time. The substitution and addition reactions

are much faster than the oxidation reactions and do

not leave CI" in solution, whereas oxidation reac-

tions do. This is the basis for determining the

type of reaction occurring.

Oxidation includes reactions with both lignin

and carbohydrates. Oxidation of the carbohydrates

leads to a decreased cellulose viscosity and de-

creased pulp strength. The species HCIO (Fig.

17.1 shows its formation versus pH) especially

contributes to oxidative degradation of the carbo-

hydrates; therefore, chlorination is carried out

above pH 0.5 (to avoid acid hydrolysis of cellu-

lose) and below pH 1.5 (to avoid carbohydrate

degradation by oxidation). Also sodium hypochlo-

rite bleaching is carried out with excess NaOH to

avoid carbohydrate oxidation by HCIO. Accord-

ing to Giertz (1951) the actual species causing

oxidation may be a complex of hypochlorous acid

and hypochlorite ion as the pH of maximum

degradation is 6.5 to 7.5 and not 4-5, where

hypochlorous is at a maximum (Fig. 17-1).

suBsmunoN

R-H

CI,

BLEACHING CHEMICAL PULPS 131

> R-CI + HCI

ADOmON

+ CI,

H H

R—C—(

OXIDATION

R—

+ CI2 + H2O

-OH + HCI

Fig. 5-7. Examples of Clj reactions with lignin. Oxidation reactions occur with both lignin and

carbohydrates. In the case of the latter, significant decreases in pulp strength result.

In the past, the amount of chlorine added to

pulp was controlled by measuring the residual

chlorine after the chlorination stage. In fact it is

better to avoid adding excess chlorine, particularly

at elevated chlorination temperatures, so that the

substitution reactions occur to completion (rapid-

ly),

but oxidation reactions are controlled by not

allowing residual chlorine to be present. As Fig.

5-9 shows, the oxidation reactions do not increase

the amount of lignin removed in the alkali extrac-

tion of sulfite pulp; however, this is less true for

kraft pulps because of their highly condensed

nature (O'Neil, et al, 1962). For the same reason

kraft pulps require longer chlorination periods than

sulfite pulps, 60-90 minutes versus 15-45 minutes,

respectively. The practice of substituting about

10%

CI2

with CIO2

(CD)

has been practiced for

a long time since it results in a stronger pulp by

R" = H for CHLOROCATECHOLS

R" = CH, for CHLOROGUAIACOLS

avoiding over-chlorination.

Lignin is not removed to a large degree in

this stage, and the pulp actually gets darker (with

a characteristic orange color). The pulp is diluted

to 1% consistency and washed to remove acid

which would otherwise consume alkali in the next

stage. The lignin removal and brightness increase

actually occur in the alkali extraction stage that

invariably 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 bleach-

ing (e.g., O and CD bleaching stages). Dioxin

and bleaching is discussed in Chapter 11. Many

mills have already replaced up to 50% of the CI2

with CIO2. Chlorine-free bleaching sequences are

used commercially for sulfite pulps at a few mills.

Fig. 5-8. Example reactions of bleaching agents

with lignin.

Time, hours

Fig. 5-9. Reaction of excess

CI2

and sulfite pulp

with time. The top 3 curves use the left scale.

The bottom curve (right scale) is residual lignin

after NaOH extraction. After Giertz (1951).

132 5. PULP BLEACHING

Papers made from these pulps are promoted as

"environmentally

friendly"

or words to that effect.

It is difficult to make bright grades of kraft pulps

without the use of chlorine, but it is possible if a

system is designed specifically for this purpose.

The future will bring many changes in this area.

CD stage

The CD or Co stage is a modification of C

stage bleaching, where some of the chlorine is

replaced with

ClOj.

CIO2 acts as a scavenger of

chlorine radicals in this stage. Substitution of

10%

of the chlorine with chlorine dioxide is used

to prevent over chlorination. Substitution of 50%

or more of chlorine with chlorine dioxide at many

mills is becoming common to reduce production of

dioxins and other chlorinated organic chemicals.

One problem is that since CI2 and NaOH are both

derived from electrolysis of NaCl, a slackening in

the CI2 market will mean much higher NaOH

prices. Other industries have taken advantage of

excess CI2 supplies, such as the polymer industry

in the manufacture of polyvinyl chloride and other

chlorinated polymers, which has mitigated this

effect.

E stage

The E stage is

extraction

of degraded lignin

compounds, which would otherwise increase the

chemical usage in subsequent bleaching stages,

with caustic (NaOH) solution. It follows the C

stage and sometimes other stages of bleaching.

When it follows the C stage (Ei), it is used at

2-3

pulp,

often with a downflow tower due to

the high consistency of pulp (10-18%), with a

temperature of 50-95°C (120-200°F), and a reac-

tion time of 0.75 to 1.5 hours. High temperatures

and alkali loading up to 5% are used to remove

hemicelluloses for dissolving pulps or absorbent

pulps.

In later E stages, alkali is used at less than

on pulp. The alkali displaces chlorine and

makes the lignin soluble by reactions such as:

Lignin-Cl + NaOH

Lignin-OH + NaCl

The lignin in the Ej effluent gives a dark color

that is ultimately responsible for much of

the

color

the

final

mill effluent. Recently oxygen gas has

been incorporated into this stage

(0.5%

on pulp)

many mills and the term

applies.

H stage

The H stage consists of bleaching with hypo-

chlorite solution, usually as the sodium salt

NaClO; this is the same chemical found in house-

hold liquid bleach. This stage is carried out at

4-18% consistency, 35-45°C (95-113°F), 1-5

hours, and pH 10. The process is often controlled

by measuring the oxidation-reduction

potential.

is important to maintain the pH above 8 because

below this pH hypochlorite is in equilibrium with

significant amounts of hypochlorous acid (Fig. 17-

1),

which is a powerful oxidant of carbohydrates,

with a E° of 1.63 V. Since the pH is high, lignin

is continuously extracted as it is depolymerized.

Hypochlorite reacts principally by oxidation.

About

1 %

on wood (based on chlorine) is used.

This chemical is more selective than elemental

chlorine, but less selective than chlorine dioxide;

consequently, the use of hypochlorite has de-

creased since the advent of chlorine dioxide.

Cellulose is oxidized along its chain, forming

aldehyde groups and making random cleavage

more likely. Calcium hypochlorite was discovered

in 1798 by dissolving CI2 in an aqueous slurry of

calcium hydroxide

and

was the principal bleaching

agent of the industry for a century. Sodium

hypochlorite, which is now used since it leads to

less scaling, is made from chlorine as follows:

CI2 + 2NaOH -> NaOCl + NaCl + H2O

D stage

The D stage involves bleaching with chlorine

dioxide. Chlorine dioxide (first studied for pulp

bleaching in 1921 by Schmidt and used for com-

mercial pulp bleaching in the mid 1940s) is rela-

tively expensive, but highly selective for lignin.

This makes it very useful for the latter bleaching

stages where lignin is present in very low concen-

trations. It is explosive at concentrations above 10

kPa (1.5 psi, or 0.1 atm); hence, it cannot be

transported and must be manufactured on site. Its

solubility is 6 g/L at 25 °C with a partial pressure

of 70 mm Hg. It is used at consistencies of

10-12%,

60-80°C (140-176°F), for 3-5 hours at a

pH of 3.5-6. It is used at 0.4-0.8% on pulp.

Downflow towers are used to decrease the risk of

gas accumulation. The D stage is useful for

reducing shive contents.

BLEACHING CHEMICAL PULPS

133

CIO2 -* y2Cl2 4- O2 (undesirable breakdown

above 100 torr pressure)

2CIO2 + H2O ->

HCIO3 + HCIO2

(slow, undesired de-

composition in soln.)

Chlorine dioxide may react in two steps. In

the first step C102' is formed; this then reacts

under acidic conditions to form CI". Thus, CIO2

ClOj"

-> CI". It reacts by oxidation; one reac-

tion is shown in Fig. 5-8.

P stage

Bleaching with hydrogen peroxide, H2O2, is

not common for chemical pulps. (But this is

changing somewhat as mills look for chlorine-free

systems.) It is usually used for brightening me-

chanical pulps, but when it is used to bleach

chemical pulps it appears as the last stage of a

sequence such as C-E-H-P or C-E-H-D-P. It is

used at 10% consistency, 60-70°C (140-160°F),

pH of 8-10, for 2-4 hours. It is an expensive

bleaching agent, but may be used more frequently

as the use of elemental chlorine decreases. Perox-

ide oxidizes carbonyl groups of carbohydrates

(produced by oxidants such as hypochlorite) to

carboxylic acid groups. Its use with chemical

pulps is fairly similar to that with mechanical

pulps,

except for a higher temperature, as de-

scribed in Section 5.2.

O stage,

oxygen pulping

and

bleaching

Oxygen bleaching or pulping is the delignifi-

cation of pulp using oxygen under pressure (550-

700 kPa or 80-100 psi) and NaOH (3-4% on

pulp).

Oxygen bleaching has been used commer-

cially since the late 1960s. This is an odorless,

relatively pollution-free process used prior to

chlorination at high consistencies (20-30%) or

medium consistencies (10-15%). Delignification

is carried out at 90-130°C (195-266°F) for 20-60

minutes. The key to the use of

delignification

was the discovery that small amounts of magne-

sium ion (0.05-0.1

on pulp) must be present to

protect the carbohydrates from extensive degrada-

tion.

This is the most inexpensive bleaching

chemical to use, but also the least specific for

lignin removal. A considerable decrease in cellu-

lose viscosity accompanies this process.

Bleaching may be thought of as extended

delignification, that is, an extension of

the

pulping

process. Many mills are considering an oxygen

delignification step after pulping but before the

traditional chlorination first step of bleaching.

Some call it oxygen delignification; others call it

oxygen bleaching. Many mills have added oxygen

to the first alkali extract step, designated as the

E^,

step,

because it is not a major change to the

process and saves bleaching chemicals in subse-

quent stages. Oxygen is even less specific at

lignin removal than chlorination so it is used to

decrease the softwood pulp kappa number from

30-35 after pulping (20-24 for hardwoods) to 14-

18 after the oxygen bleaching stage. Bleaching to

kappa numbers below this leads to unacceptable

losses of the cellulose viscosity.

The effluent of oxygen delignification can be

used in the brown stock washers or otherwise

ultimately

sent to

the recovery boiler because there

is no chloride ion present that would lead to high

dead loads and corrosion in the recovery boiler.

Reducing the amount of lignin removed by the

chlorination step by half reduces by half

the

color

and BOD from the bleach mill water discharge

after secondary treatment. Environmental consid-

erations are the main reason for using oxygen

bleaching. The bleach plant produces mush of the

color of the final mill discharge.

There are two main methods of oxygen

bleaching: medium and high consistency. The

high consistency

process

30%

consistency and

90-110°C (195-230°F) in a pressurized reactor.

In the

medium consistency process

pulp

10-15 %

consistency (the normal consistency

it comes off

the drum washers) is used. While high consisten-

cy oxygen bleaching is more common, there are

some difficulties with it. The gas in the reactor

contains high concentrations of oxygen

and

volatile

organic chemicals such as methanol, ethanol, and

acetone that are potentially explosive; it is diffi-

cult to evenly distribute NaOH and O2; and the

pulp is saturated with oxygen which

tends

to cause

foaming on the pulp washers.

Studies indicate that the temperature of

oxygen bleaching is important to the selectivity of

the process with better selectivity at 100°C

(212°F) than 130°(266^F). Partial pressure of

oxygen above 200 kPa (30 psig) leads to little

increase in the rate of delignification.