Biermann Ch. Handbook of Pulping and Papermaking

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374 16.

PULPING

CALCULATIONS

19-,

IH-

% 6-

20'

40 60

Bisulfite ion, %

80 100 80

100 80 60 40

Sulfurous acid, %

20 40 60 80 100

Sulfite ion, %

Fig. 16-8. Titration curve of 0.1 M

SO2

with two moles of NaOH.

EXAMPLE 9. Case 1. A cooking liquor consists of 6% free SOj. What is the form of

SO2?

Answer.

All of the SO2 is in the form of

H2SO3

and the pH is about 1.2 at 25°C. This is only a

hypothetical cooking liquor.

Case 2. A cooking liquor consists of 4.2% free SO2 and 1.8% combined SO2. What is

the form of the SO2? Answer. The limiting chemical is combined SO2, so 1.8%

combined SO2 reacts with 1.8% free SO2 (leaving 2.4% free SO2 as H2SO3) to produce

3.6% MHSO3, pH « 1.85.

Case 3. A cooking liquor contains 2.5% free SO2 and 2.5% combined SO2. What is the

form of the SO2? Answer. All of the free reacts with all of the combined SO2 to give

MHSO3. The pH is 4.3, but there is little buffering capacity. This is a square

liquor.

Case 4. A cooking liquor contains 1% free SO2 and 5% combined SOj. What is its

actual composition? Answer. 1% free SO2 reacts with 1% combined SO2 (leaving 4%

M2SO3) to produce 2% MHSO3. The pH is about 5.8.

Case 5. NSSC is carried out with sulfite at pH 8-9. Fig. 16-8 shows that all of the SO2

is in the combined form. Notice also there is no buffer capacity in this region. NSSC

cooking is carried out with 10-15% carbonate to supply some buffering action.

SULFITE LIQUOR CALCULATIONS

375

EXAMPLE 10. A solution is

NajSOa (as SO2) and

NaHSOj (as SO2). What is the free and

combined SO2? What is the actual concentration of NaHSQj?

SOLUTION. 2% NaHSOj -* 1% H2SO3 + l%Na2S03. Therefore, this corresponds to 4%

combined SO2 and

1 %

free

SO2. From Table 16-4 the gravimetric factor of 1.624

is obtained to convert

SO2

NaHSOj.

2% NaHSOj

(SO2

basis) x 1.624 = 3.25%

NaHSOa = 32.5 g/L NaHSOj.

EXAMPLE 11. Given: A digest charge with 1 kg of dry wood, a calcium liquor containing 5%

free

SO2

and

1 %

combined

SOj

and a liquorrwood ratio of

4:1

(including the water

in the wood).

Calculate: the concentrations and amounts of

the

actual chemical species. Assume

the specific gravity of the liquor is 1.00.

SOLUTION. The total SO2 is equal to the concentration in the liquor times the liquor to wood

ratio,

24%

on wood, which is 240 grams

SO2

for the charge. The

1 %

combined

reacts with

1 %

free to give 2% (60 g on wood) as bisulfite leaving 4% (160 g on

wood) as sulfiirous acid. From Table 16-4 80 grams bisulfite (SO2 basis)

corresponds to 80 x 1.578 = 126.4 g Ca(HS03)2 on wood or 31.6 g/L. In a

similar fashion 160 g free

SO2

corresponds to 205.2 g

H2SO3

on wood or 51.3 g/L.

PROBLEM. For this problem, what would the concentrations be for Mg based cooking?

Answer: 29.1 g/L Mg(HS03)2.

16.8 SULFITE LIQUOR ANALYSIS

Sulfite pulping liquors could be titrated with

NaOH to each endpoint to determine the free SO2

and combined SO2. However the first endpoint is

not very sharp, and even the second endpoint may

not be very sharp in sulfite liquors. Palmrose

(1935) developed a method where, under acidic

conditions, all of the SO2 is converted to

804"^

periodate ion as shown in the following two

chemical equations. Periodate thus measures the

total

SO2.

KIO3 + 3H2SO3 ^ KI + 3H2SO4

2KIO3 + 3(HS03)2 -* 2KI + 3H2SO4 + 35042-

I of KIO3 is reduced from +5 to -1 while each

sulfur of

SOs^'

is oxidized from +4 to +6. The

equivalent weight of KIO3 is 1/6 the molecular

weight, and the equivalent weight of

H2SO3

is one-

half the molecular weight. These reactions are

actually fairly slow and the endpoint would easily

be overrun. Small amounts of KI (from the

indicator solution), however, allow the following

two reactions, which are rapid, to occur:

IO3-

+ 3H2SO3 +

51-

-* 3804^- + 3I2 + 3H2O

3SO42-

+ 3I2 + 3H2O +

3H2SO3

61-

+ 6H2SO4

The titration is carried out with 0.1

potas-

sium iodate to blue endpoint using Kl/starch

indicator. The excess

at the end of

the

reaction

reacts with starch to give a characteristic blue

color, a well known reaction used as an indicator

for starch. The slight excess of

KIO3

at the end of

the reaction is reacted with thiosulfate.

The liberated acid is then titrated with 0.1 M

NaOH to a methyl red endpoint and represents the

free SO2, Notice that

the

latter titration is a strong

376

16.

PULPING CALCULATIONS

acid-Strong base titration with a well-resolved

endpoint. When this method was first developed

calcium was the only base used for sulfite pulping

and the liquors were necessarily acidic. With

other bases where the cooking liquor is above pH

of 4-5, a known amount of sulfuric acid is added

before the iodate titration. The liberated acid is

then titrated but the additional amount of sulfuric

acid added is subtracted from the free SO2 value

obtained in the second titration. This procedure is

described in TAPPI Standard T 604.

EXAMPLE 12. A sulfite liquor was diluted 1:10.

A 10.00 ml aliquot of the diluted solution

was titrated by the Palmrose method. Calcu-

late the total and combined SO2 in the origi-

nal solution based on the following amounts

of titrants:

10.78 ml of 0.0946 iVNaOH

Cyclooctasulfur

Catenasulfur

12.18 ml of 0.2060 NKIO^ and

SOLUTION: The total SO2 'mN= 10 x (12.18

ml X 0.2060 N)nO ml = 2.51 N,

The total SO2 = 2.51 N x 32 g/eq SO2 =

80.32 g/L total SO2 or 8.03% total SO2.

The free SO2 = 10 x (10.78 ml x 0.0946

iV)/10ml = 1.020 JV.

The free SO2 = 1.02

x 32 g/eq SO2 =

32.64 g/L free SO2 or 3.26% free SOj.

16.9 THE CHEMISTRY OF SULFUR

Elemental sulfur

Elemental sulfur occurs in many complex

forms.

The chemistry of elemental sulfur present-

ed here is a simplification of

its

complex chemistry

but will be useful to explain its properties. Crys-

talline sulfur contains sulfur rings with 6 to 20

sulfur atoms or chains of sulfur atoms called

catenasulfur, S«,. The most common form is

cyclooctasulfur, Sg, which has two common allo-

tropes: orthorhombic sulfur, S„, and monoclinic

sulfur, S^. S„ is, thermodynamically, the most

stable of the Sg forms. The structures are:

Above 95 °C S^ slowly converts to S^. With

rapid heating, the melting point of S« (113°C) is

obtained. S^ melts at 119°C. S^ crystallizes

from sulfur melts and over the course of months

converts to S„. Liquid Sg sulfur becomes increas-

ing viscous above 160°C as it is converted to

catenasulfur. Above 200°C the viscosity of the

catenasulfur decreases as its maximum degree of

polymerization is at 200°, where the formula

weight is above 200,000. The boiling point of

sulfur is 445

°C.

If catenasulfur of high viscosity

is quenched by pouring into ice water, a plastic

solid results. The solid catenasulfur slowly reverts

to Sg over time. Sg is soluble in CS2, S« is not

soluble.

Sulfur is recovered in large amounts from

H2S in natural gas by the reaction:

2H2S + SO2 -^ 3S 4- H2O

In Europe large amoimts of sulfur are used

from iron pyrite, FeS (the substance called fool's

gold because of its similarity to real gold), which

is a solid. Sulfur is also a by-product of copper

production from CuS. Sulfur is also mined in

large amounts in its elemental form from volcanic

deposits by the Frasch process, where steam is

injected into the ground to heat the sulfur and the

molten sulfur is pumped from the ground.

Sulfur

combustion to

produce

SO2

Elemental sulfur is burned to produce sulfur

dioxide.

S +

^ SO2 (gas)

Above 1000°C (1830°F) no sulfur trioxide is

produced; however, some might be produced in

the process of cooling the gases. Sulfur trioxide,

which produces sulfuric acid upon reaction with

water, is very undesirable in pulping reactions and

is removed during the cooling/scrubbing process

(by counter current flow of water and sulfur

dioxide).

THE

CHEMISTRY

OF SULFUR

377

SO3 + H2O -^ H2SO4

The SO2 forms H2SO3 in water which in turn

is reacted with metal bases to produce sulfite pulp-

ing liquors, as described in other sections. Be-

cause H2SO3 is much more acidic than H2CO3,

salts of carbonate can be used to form salts of

sulfite. For example, wet SO2 is traditionally

formed into calcium bisulfite by calcium carbonate

(limestone) by the following equation:

CaC03 + 2H2SO3 -* Ca(HS03)2 + CO2 + H2O

Total sulfur by gravimetric analysis

Often the total content of organically bound

and inorganic sulfur is desired. This is accom-

plished by treating the sample with a strong

oxidant under alkaline conditions to convert the

sulfiir to sulfate. For example, when sodium

peroxide is used, sodium sulfate is formed and the

organic chemicals are largely converted to carbon

dioxide and water. After the sample is acidified

with HCl, the SO/" is precipitated with excess

BaCl2.

The precipitate is washed with small

amoxmts of water and then dried in a muffle

furnace. The precipitate is weighed and converted

to a sulfiir equivalent with the appropriate gravi-

metric factor.

16.10 CALCINING EQUATIONS

Two equations are used to characterize

calcining of lime mud to produce fresh lime. The

specific energy consumption is an indication of

how much fiiel is required to process the lime mud

and is often reported as Btu per ton of lime.

16.10 ANNOTATED BIBLIOGRAPHY

H'factor and process control equations

Edwards, L. and S.-E. Norberg, Alkaline

delignification kinetics, A general model

applied to oxygen bleaching and kraft pulp-

ing,

TappiJ.

56(11):

108-111(1973).

Kubes, G.J., B.I. Fleming, J.M. MacLeod,

and H.I. Bolker, Viscosities of unbleached

alkaline pulps. 11. The G-factor, /. Wood

Chem,

Tech, 3(3):313-333(1983).

Hatton, J.V., Development of yield predic-

tion equations in kraft pulping, Tappi J.

56(7):97-100(1973).

Hatton, J.V., The potential of process control

in kraft pulping of hardwoods relative to

softwoods, Tappi /. 59(8):48-50(1973).

Lin, C.P., W.Y. Mao, and Jane, C.Y.,

Development of a kappa number predictive

equation in kraft pulping for all types of

hardwood, Tappi /. 61(2)72(1978).

6. Paulonis, M.A. and A. Krishnagopalan,

Adaptive inferential control of kraft batch

digesters as based on pulping liquor analysis,

TappiJ.

74(6):

169-175(1991).

Tasman, J.E., Kraft delignification models,

TappiJ.

64(3)175-176(1981).

specific energy consumption =

fuel to kiln

CaO output

The lime availability is an indication of the

purity of the lime in terms of available CaO

divided by the amount of lime product.

lime availability

CaO

lime

8. Vroom, K.E., The "H" factor: A means of

expressing cooking times and temperatures as

a single variable. Pulp Paper Mag. Can.

58(3):228-231(1957).

Sulfite liquor analysis

9. Palmrose, G.V., A mill test for the exact

determination of combined sulphur dioxide.

Tech.

Assn. Papers, XVin:309-310(1935);

the same article is reprinted as

ibid.

Paper

Trade

J. 100(3)38-39(1935).

Chemistry

of sulfur

10.

Cotton, F.A. and G. Wilkinson, Advanced

as mass ratio Inorganic Chemistry, 4th ed., Wiley and

Sons,

New York, 1980. 1396 p.

378 16. PULPING CALCULATIONS

EXERCISES

Kraft

liquor-chemical calculations

Derive the gravimetric conversion factor to

express NajCOj as NajO.

White liquor has the following composition

of the actual chemical species. Calculate

TTA, AA, EA, TA, causticizing efficiency

and sulfidity.

NaOH = 145 g/L Na^S = 55 g/L

Na^COs = 17 g/L Na2S04 = 14 g/L

Kraft

liquor-chemical analysis

The effective alkali concentration of a white

liquor is 93.7 g NasO/L. The sulfidity is

37.5%

and the causticizing efficiency is

77.9%.

Calculate the following.

a) The A, B, C values in ml for a 10 ml

aliquot of liquor titrated with 1.017 N

HCl.

b) The molar concentrations of the chemi-

cal species NaOH, NajS, and Na2C03 in

this liquor.

c) The active and effective alkali charges

on wood (as a percent) if 200 ml of this

liquor is used to pulp 200 g Douglas-fir

chips at

50%

MCgr

(with some addition-

al dilution water).

H-factor

What is the H-factor for a cook which is held

at 165 °C for L43 hours? If this H-factor is

satisfactory, how long should one cook if the

temperature is increased to 170°C for a

dif-

ferent cook?

Sulfite

liquor

calculations

5. By comparing sulfite cooking chemicals all

on a

SO2

basis, one is really comparing them

on an equal (molar or weight) basis. Circle

the correct choice.

6. Analysis of a sulfite cooking liquor indicates

a total SO2 of

and

free SO2.

The combined SO-, is %.

The SO2 as sulfiirous acid is

The SO2 in the form of the bisulfite ion

(HSO3) is %, and the SO2 in the form

of sulfite ion (SOj^) is %.

7. Analysis of a sulfite cooking liquor indicates

a total SO2 of 8%, and a combined SO2 of

2%.

Solve for unknown free SOj, as well as

the concentration of

the

sulfiirous acid, bisul-

fite

ion,

and sulfite ion species (all as

SO2

which equals g/100 ml), and the pH of the

liquor.

8. A solution is made by weighing 1.1 grams

sulfiirous acid (as sulfiirous acid) and 10.3

grams sodium sulfite (as sodium sulfite) to

make 0.193 liters of solution. Solve for

unknown free SO2, combined SO2, total SO2,

as well as the concentration of the sulfiirous

acid, bisulfite ion, and sulfite ion species (all

as % SO2 which equals g/100 ml), and the

pH of the liquor.

Sulfite liquor analysis

9. Analysis of a pulping liquor with iodometric

titration gives a

SO2

value of 10%. Titration

with NaOH gives 6.5% SOj. Solve for un-

known

free

SO2, combined SO2, total

SO2,

well as the concentration of the sulfiirous

acid, bisulfite ion, and sulfite ion species (all

as % SO2 which equals g/100 ml), and the

pH of

the

liquor. If sodium is the base, how

many grams of sulfiirous acid, sodium bisul-

fite, and/or sodium sulfite are needed to

make one liter of cooking liquor?

BLEACHING AND PULP PROPERTIES CALCULATIONS

17.1 DILUTION WATER CALCULATIONS

The water per ton of pulp ratio (V) is solved

from the consistency (c) as V = (100-c)/c.

EXAMPLE 1. After the brown stock washers,

the consistency of unbleached pulp is 11.2%.

Calculate the volume of water (in m^/t oven-

dry pulp) required to dilute the slurry to 3%

consistency for chlorination.

SOLUTION. At 11% consistency there are 89 t

water/(ll t pulp) = 8.09 t water/t pulp. At

consistency there are 97 t water/(3 t

pulp) = 32.33 t/t. Therefore 32.33 - 8.09 =

24.24 t water/t pulp to be added. Since 1 t

= 1000 kg = 1 m^ of water, this is 24.24 m^

of water per metric ton of pulp.

17.2 CHLORINE BLEACHING

The pH is of utmost importance in chlorine

bleaching for several reasons. First the form of

chlorine in solution is dependent on the pH ac-

cording to the following equilibrium reactions.

CI2 + H2O •=i H^ + CI + HOCl

K^ = 3.9 X 10-^at25°C (17-1)

HOCl

4=^

H^ + ocr

K^ = 3.5 X 10-^ at 18°C

(17-2)

From these equilibria. Fig. 17-1 can be construct-

ed in the manner of Giertz (1951). This is very

similar to other diprotic acids plotted in Chapter

16,

except the three species are plotted together.

Determining the concentration of the actual species

at low pH is tricky because chlorine is a moderate-

ly strong acid (and so its ionization alters the pH

significantly) and three species are formed by the

ionization instead of two.

First, the concentration of chlorine in water

at 25°C is 0.091 M at saturation (i.e., one atmo-

sphere pressure of CI2). (Like the solubility of

CO2,

the solubility of

CI2

is highly pH dependent.)

The pH of saturated chlorine water is calculated as

follows:

ir,

[Hoci][Hi[cr]/[cy

[H1

(0.091 -[HI)

Solving for [H+] gives [H"'] = 0.029 M. There-

fore,

a saturated chlorine solution in water, with-

out addition of acid or base, has a pH of -log

0.029 or 1.54, an actual chlorine concentration,

[Cy, of 0.062 M, a hydrochloric acid concentra-

tion of 0.029 M, and a hypochlorous acid concen-

tration of 0.029 Af.

The concentration of chlorine species will be

considered using 0.05 M CI2 initially, since this

will not exceed the saturated pressure of chlorine

at any pH, as a fimction of pH. Acid or base can

be hypothetically added without dilution to achieve

the pH's shown in Fig. 17-1. Initially [Cy =

0.028, [H+] = 0.022, and the pH is 1.66.

The ratio of

CI2

to HOCl as a function of pH

(where the source of the acid is immaterial) is

solved in the following manner, where x =

[HOCl] = [CI]:

K, = [HT A:2/(0.05 - x)

[H-^]

•\-

K,X' 0.05 ^, = 0

This equation is easily solved at various pH's by

use of the quadratic equation (see page 322). (At

pH = 0, X = 0.00423 M. At pH 1.66, x =

0.0222 in agreement with the calculation above.)

When 0.05 M NaOH is added, all of the CI2

is converted to HOCl and the pH is of the weak

acid system

HOCl.

HOCl is easily handled as any

other weak acid. The pH is then calculated from

Eq. 13-5 as4og(^, x Q'^ = 4.38. When 0.10 M

379

380 17. BLEACHING AND PULP PROPERTIES CALCULATIONS

100

90-

80-

3 60-

•g

70-

50-

40-

30-

20-

""""''

Cl-C!

>•"

«.i....,

HOCI

'**"*»

^^\

1 /

/ \

NaOCI

10 11

Fig. 17-1. Chlorine species as a function of pH in saturated chlorine water (0.05 M

CI2)

at 25°C.

Obviously the total solubility is pH dependent with NaOCl being very soluble at the higher pH's.

NaOH is present the chemical is essentially all

OCl' and the pH is given as:

(14-(^b X [OCl])''') = 10.08.

The pH at intermediate amounts of base is given

in the form of

the

Henderson-Hasselbalch equation

(see Eq. 14-2 for this equation); consequently, the

ratio of HOCI to OCl' is not concentration depen-

dent (except for the effect of activity coefficients)

from pH 5 to 9.

pH = p^, + log(C;,./Q)

= 7.46 + log ([OCl-]/[HOCl]

Because the actual concentration of chlorine

species is pH dependent, pulp bleaching with

chlorine is also pH dependent. The desired

reaction of substitution occurs most rapidly at the

lowest pH values; however, acid hydrolysis of

cellulose and hemicellulose also increases with

decreasing pH. (Acid hydrolysis leads to a de-

crease in the cellulose viscosity.) The hydrolysis

of chlorine is also temperature dependent.

Rydholm in Pulping Processes (p. 920) gives the

hydrolysis constants of CI2 as a function of tem-

perature; they are 1.45 x

10"^

at 0°C and 9.75 x

10-^

at 60°C.

CHLORINE DIOXIDE 381

17.3 CHLORINE DIOXroE

The action of

CIO2

has been studied in detail

by Schmidt (1923). He found CIO2 to be highly

reactive with aromatics and phenolics but not

reactive with carbohydrates. Today CIO2 is used

in the later bleaching stages since it is very selec-

tive for lignin removal. It is also being used in

early stages for environmental reasons. Chlorine

dioxide can be produced from chlorite by oxida-

tion with hypochlorite or reduction with suitable

reducing agents (Giertz, 1951). Chlorine dioxide

formation by reduction of sodium chlorate in

acidic aqueous solutions occurs with different

reducing agents that give rise to many processes as

shown in Table 17-1. Chlorine is reduced from a

valence of +5 in chlorate to a valence of +4 in

chlorine dioxide. Chlorate is produced by the

disproportionation reaction as follows:

3C10-

2 CI- +

CIO3-

-1 +5

The overall reaction is:

NaCl -h 3H2O + electricity-* NaClOs + 3H2

Chlorine dioxide is also produced by the

Lurgi process from

CI2,

H2O, and electricity. The

chlorine dioxide is produced by the same reaction

as the Rapson process. The NaCl and CI2 prod-

ucts of

this

reaction are used to generate additional

chlorine dioxide. Also NaClOj is generated by

electrolysis. Fig. 17-2 shows the process. The

process is electrical intensive and uses

8-9,000

kWh/tofC102.

17.4 CHEMICAL ANALYSIS OF

BLEACHING LIQUORS AND

CHLORINE EQUIVALENCY

The determination of active bleaching agents

is performed by iodometric titrations. The bleach-

ing solution is added to an aqueous solution con-

taining excess potassium iodide, KI. The KI is

oxidized to iodine while the bleaching agent is

reduced. The amount of iodine liberated is mea-

sured by titration with thiosulfate using a starch

indicator to observe the final disappearance of

iodine. In the case of chlorine the reactions are as

follows:

CI2 +

21-

2C1-

+ I2 (17-3)

I2 + 2Na2S203 -> 2NaI + Na2SA (17-4)

Bleaching chemicals are normally expressed as

amount of "available chlorine." This allows

bleaching chemicals to be considered in terms of

their equivalent weights. Table 17-2 shows the

conversions for several bleaching agents. Keep in

mind that in Eq. 17-3 one mole of KI reacts with

one mole of electrons; therefore, the equivalent

Table 17-1. Formation of CIO2 by reduction of NaClOj.

Name of

Process

Mathieson

Solvay

Rapson

R-2,

R-3,

1 SVP

Reducing Agent

Species

SO2

CH3OH

HCl

NaCl

Change of

Valence

S: +4-+6

C: -2 - +4

CI:

-1-0

CI:

-1-0

Chemical Equation

2NaC103 + SO2 + H2SO4 - 2CIO2

-1-

2NaHS04 |

eNaClOj + CH3OH + 6H2SO4 - 6CIO2 + CO2

+ 6NaHS04

-1-

SHP |

2NaC103 +

4HC1

- 2C10j

-1-

2NaCl + Clj+H^ol

NaC103

-1-

NaCl + H2SO4 - CIO2 + ^ACk +

Na;S04 + H;0

382 17. BLEACHING

AND

PULP PROPERTIES CALCULATIONS

INPUTS

H2O

HCl

GENERATION

"7^

ELECTRICITY

OUTPUT

CHLORATE

GENERATION

N0CIO3

NoCl

HCl

CHLORINE DI

I DEI

GENERATION

CIO2

Fig. 17-2. Chlorine dioxide generation from CI2, H2O, and electricity by the Lurgi process.

weight of KI is equal to its formula weight. On weight of

is one-half of its formula weight. It

the other hand, one mole of I2 reacts with two should be clear that the equivalent weight of

CI2

moles of electrons in Eq. 15-4, and the equivalent one-half of its formula weight.

Table 17-2. Equivalent weights of bleaching chemicals.

Bleaching

agent

Chlorine

Hypochlorite

Hypochlor-

ous acid

Chlorine

dioxide

Chlorine

monoxide

Sodium

chlorate

Oxygen

Iodide

(sodium)

Chemical equation

CI2 + 2e -*

2C1-

NaOCl 4- H2O + 2e -* NaCl + 20H-

HOCl + H+ +e ^

ViClj

+ H2O;

then CI2 rxn. as above

CIO2 + 4H+ + 5e -* Cr + 2H2O

CI2O + 4e -* Cl-

NaClOj + 6e -* CI"

O2 + 2H2O + 4e ^ 40H-

I2 + 2Na-' + 2e -» 2NaI

Equivalent weight

as formu-

la weight

FW/2

FW/5

FW/4

FW/6

FW/4

g/equiv

35.5

37.25

26.25

13.5

21.75

17.75

8.00

149.89

Gravimetric

factors

to CI2

1.000

0.953

1.352

2.630

1.632

2.029

4.438

0.237

from

CI2

1.000

1.049

0.739

0.380

0.613

0.493

0.225

4.222

E\W

1.36

0.89

1.63

1.36

1.27

0.41^

0.620

•inliVNaOH.

CHEMICAL ANALYSIS OF BLEACHING LIQUORS 383

EXAMPLE 2. Write the balanced reaction of chlorine dioxide with iodide.

SOLUTION: 2CIO2 + 10 T + 8 H^ -*

201"

+ 5I2 4- 4H2O

EXAMPLE 3. Suppose a chemical loading of

NaClO on pulp (as chlorine) is desired. What is the

actual amount of NaClO that should be used?

SOLUTION: 2% as CI2 x (37.25 g NaC10)/(35.5 g Cy = 2.1% NaClO as NaClO.

PROBLEM: Suppose 2% CIO2 as chlorine is desired. What should the actual concentration be?

Answer: 0.76% CIO2.

EXAMPLE 4. Given the data in the following table, fill in the blank spaces.

1 Stage

1 Chemical input on pulp,

as CI2

1 Chemical input on pulp,

true

1 Yield per stage, %

1 Cumulative yield, %

1 Pulp input, kg

1 Pulp output, kg

Chemical input, kg

1000

SOLUTION: Chlorine and caustic are expressed

"as

is", so the chlorine input would be

the

pulp

charged and the caustic input would be 3% of the charge. Hypochlorite and chlorine

dioxide charges are given in terms of equivalent chlorine and must be converted as

explained.

Hypochlorite: 3% (as Cy X 1.049 = 3.15% (as NaClO) based on pulp.

Chlorine dioxide: 2% (as Cy x

0.380

= 0.76% (as CIO2) based on pulp.

The yield per stage is the ratio of output per stage to the input per stage, and the

cumulative yield (for the entire bleaching sequence) is the ratio of the final output to the

input of the first stage of bleaching. It is also the sequential product of the yields per

stage:

Cumulative yield stage 1: 100% x 0.99 = 99%