bleaching of pulp

8/7/2019 Bleaching of Pulp

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BLEACHING STAGES AND SEQUENCES

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PAPER

BASIS WEIGHT PAPER DENSITY CHEMICALBLEACHING

SEQUENCES

PAPER ISO SIZES WOOD PROPERTIES PULP PROPERTIES PAPER PROPERTIES

LENGTH OF PAPER IN

ROLLPAPER DEFECTS

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Unbleached pulps exhibit a wide range of brightness depending on the pulping process and

fibrous raw material used. The sulfite process produces the brightest pulp, up to 65% whereasKraft, soda and semi-chemical may produced pulp of only 15% brightness.

Holocellulose (Cellulose + hemi-cellulose) are inherently white and do not contribute to color. It

is the chromophoric group in lignin which are responsible for color.

Two approaches are used in the chemical bleaching of pulps. One approach utilizes selective

chemicals that destroy some of the chromophoric group but do not attack lignin. The other

approach is to almost totally remove all lignin. The first approach provide around 70% brightness

and retain high pulp yield, while second approach provide 90% + brightness but reduces pulp

yield.

BLEACHING STAGES AND SEQUENCES

SYMBOLS STAGE AMOUNTTIME pH CONSISTENCYTEMPERATURE DESCRIPTION

A Acid WashTo remove metal

element from pulp

hing of Pulp http://www.paperonweb.com/b

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B

Boron

Hydride

NaBH4

C Chlorination5 -7% on

pulp

40

min 2 3 - 4% 20 - 25 0C

Elemental chlorine (Cl2)

is an effective

de-lignifying agent. As it

breaks lignin bonds, it

adds chlorine atoms to

the lignin degradation

products, thus

producing significant

amounts of chlorinated

organic material.

DChlorine

Dioxide

0.6-1.0%

on pulp

180

min

3.5 -

4.010 - 12% 60 - 80

0C

Chlorine dioxide (ClO2)

is a highly selective

chemical that can both

de-lignify and brighten

pulp. It oxidizes lignin,

but does not add

chlorine atoms onto

lignin fragments;

however, small amounts

of elemental chlorine

and other chlorine

compounds formed

during the chlorine

dioxide bleaching

process react with

degraded lignin to form

chlorinated organic

compounds.

CD or C/D

or D/C

Chlorine and chlorine

dioxide added together

EAlkaline

Extraction

3 -4% on

pulp

120

min12 10 - 20% 45 - 95

0C

To remove colored

components from

partially bleached pulps

that have been rendered

soluble in dilute warm

alkali solution.

EOAlkali extraction

reinforced with oxygen

EP

Alkali extraction

reinforced with

hydrogen peroxide

EOP

Alkali extraction

reinforced with oxygen

and hydrogen peroxide

FFormamidine

Sulfinic Acid


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HSodium

Hypo-chlorite

1% on

pulp11-11.5 3 - 15% 30 - 60

0C

Sodium hypochlorite

(NaOCl) is an

inexpensive

de-lignifying agent

formed by mixing

elemental chlorine with

alkali at the mill.

M

Chlorine

Monoxide orHypochlorous

NNitrogen

Compounds

O Oxygen1-5 - 2.0%

on pulp

60

min>7 10 - 15% 85 - 95

0C

Oxygen removes lignin

and modify other

coloring components. In

the oxygen

delignification/bleaching

stage the pulp is treated

with oxygen in apressurized vessel at

elevated temperature in

an alkaline

environment.

P Peroxide

2 -3 % on

pulp

1 -2

hr 10 - 30% 65 - 800

C

Hydrogen peroxide

(H2O2) is mainly used

to brighten pulps in the

final bleaching stages.

Peroxide is often used

at the end of a

conventional bleachingsequence to prevent the

pulp from losing

brightness over time.

Preferred for

mechanical and

recycled fiber.

Paa

Peracetic

Acid,

CH3COOOH

Q Chelatin

To control the

brightness restrictingand reversion effects of

iron salts and other

heavy metals in the

pulp.

W Wash

Pulp is washed almost

every bleaching stage to

remove reactants of

preceding stage.


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X Xylanase

Xylanase-based

enzymatic pretreatment,

in a TCF (Totally

Chlorine Free)

sequence, results in

easier bleaching and

delignification of the

pulp, causing a bleach-

boosting effect.

YSodium

Hydrosulfite

0.5 - 1.2%

on pulp

35-

60

min

5.5 5 - 8% 60 - 750C

Reductive bleaching.

Good for recycled

fibers.

Z Ozone0.1 - 1.0%

on pulp2.5 5 - 15% 90 brightness.

2.

Classical Bleaching Sequences

H

CEH

CEHH

CEHEH

CEHHD

CEHDH

CEDED


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bleaching of pulp

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