The purpose of bleaching
The colour of pulpA bleach plant layout
The aim of bleaching
Residual lignin colors the pulpA fiberlineA bleach plantNumber of stages effects on bleaching
Terms
Terminology
Abbreviations
Bleaching chemicals
History
Share of delignification due to cooking and bleaching at different times
Theory
The colour of pulp

 

 

 

 

 

 

 

 

 

 

The purpose of bleaching

The objective of bleaching is to improve the brightness and cleanliness of pulp. This occurs either by removing or brightening the colored substances in the pulp. Residual lignin is a major contributing factor in color, so it must be removed or brightened The colour of pulp. Depending on the purpose we speak of bleaching to remove lignin and bleaching to preserve lignin.

Chemical pulps are typically bleached using lignin-removing bleaching and mechanical pulps by lignin-preserving bleaching. Brightness with lignin removal bleaching lasts a lot better i.e. there is a lot less yellowing.

In bleaching the cleanliness of pulp improves when the fibers of the fiber bundles, or shives, are released as the last of the residual lignin is removed from the pulp and any bark debris dissolves. The chemicals used in bleaching also effectively dissolve extractives contained in the pulp (e.g. resin).

Some or all of the objectives below are usually set for bleaching:

The most important of these is the increase in brightness.

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The aim of bleaching Residual lignin colors the pulpA fiberlineA bleach plantNumber of stages effects on bleaching

The aim of bleaching pulp is to continue delignification and, using bleaching chemicals, to remove any lignin, known as residual lignin, that remains after the cooking and oxygen stages, which could not be broken down and dissolved in the cooking and oxygen stages, without sacrificing pulp yield or fiber properties Residual lignin colors the pulp. Chemicals used for this are more selective than the chemicals in cooking and in oxygen stage. In other words, these chemicals can break down the residual lignin into small water- or alkali-soluble parts with the minimum effect on carbohydrates (i.e. on yield and strength).

An essential part of bleaching is washing dissolved lignin out of the pulp on the washer following the bleaching stage A fiberline A bleach plant.

The target brightness cannot be achieved in only one bleaching step without sacrificing pulp strength. Therefore pulp is bleached in several steps, and the pulp is washed between them. Multi-stage bleaching gives the best results regarding both quality and economy Number of stages effects on bleaching. There are alkaline and acidic bleaching stages. With only alkaline or acidic stages the target brightness would not be attained, so both are always used in bleaching.

The fiber properties are changed during bleaching in the desired direction:

 

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TermsTerminology

Bleaching can be divided into Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF).

ECF

Elemental Chlorine Free, bleaching without chlorine gas or hypochlorite, but with chlorine dioxide (ClO2) used in one or more stages.

TCF

Total Chlorine Free , , bleaching without chlorine chemicals, where oxygen-containing chemicals such as oxygen, hydrogen peroxide and ozone are used.

The environmental discharges in bleaching are usually measured in terms of AOX and COD.

AOX

Adsorbable organic halogens

A method of measurement, which gives the total of organically bound chlorine of all sizes of molecules, or TOCl.

TOCl

Total Organic Chlorine compounds of all sizes

Small molecule chlorine compounds known to be harmful to the waterways (e.g. chlorophenols).

COD

Chemical Oxygen Demand

Oxygen consumption caused by substances in wastewater as the substances oxidize as a result of effective chemical oxidants.

BODx

Biological Oxygen Demand

Oxygen consumed as bacteria decompose the substances in the wastewater. The measurement is made in standard conditions ( 20 °C and x days) letting the bacteria “eat” the substances in aqueous solution and measuring the oxygen consumption.

The following terms are used in bleaching when dosing chemicals and evaluating pulp:

Charges of chlorine-containing chemicals are generally expressed in kilos of ”active chlorine” per tonne of pulp. This is due to the fact that different bleaching agents have a bleaching effect of different magnitude. For instance the oxidizing capability of a kilo of chlorine dioxide is 2.63 times that of a kilo of chlorine, i.e. less is needed to achieve the same bleaching effect.

Likewise, the bleaching effect of oxygen chemicals can also be compared with that of chlorine.

The chlorine dioxide dose of D0 is usually expressed using the Kappa factor. The more lignin is removed in the stage, the greater the Kappa factor used. The Kappa factor is generally 1-2.

= (Active chlorine, kg/tonne of pulp) / (Kappa number of pulp entering bleaching)

The brightness of pulp is measured as the ability of a pulp sheet to reflect light directed onto it. This is affected by both the light absorption and light scattering of the pulp. The wavelength of 457 nm is used in measurement.

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AbbreviationsBleaching chemicals

Here are some general abbreviations used for bleaching chemicals:

A

Acid hydrolysis (e.g. the removal of hexenuronic acid groups) .

B

Boron hydride, NaBH4

C

Chlorine, Cl2

Caa

Caro's acid, peroxomonosulfuric acid, H2SO5

D

Chlorine Dioxide, ClO2

E

Alkaline Extraction, NaOH

  • pressurized EOP
  • non-pressurized EOP

H

Hypochlorate, NaOCl, Ca(OCl)2

mP

Activated acid peroxide (i.e. molybdate peroxide)

O

Oxygen, O2

P

Hydrogen Peroxide, H2O2 (under alkaline conditions)

Paa

Peracetic acid, CH3COOOH

Q

Chelation, EDTA, DTPA

X

Enzyme treatment (usually Xylan)

Z

Ozone, O3

 

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History Share of delignification due to cooking and bleaching at different times

In general, full brightness cannot be achieved in one bleaching stage, instead several consecutive stages must be used. Traditionally, bleaching has been done with chlorine-containing chemicals: with (elemental or gaseous) chlorine (C), hypochlorite (H) or with chlorine dioxide (D). Between stages, the dissolved lignin has been extracted with alkali. Typical traditional bleaching sequences were CEHDED and CEDED.

The principle was that the vast majority of the residual lignin was removed with the cheapest chemical i.e. chlorine, and only the final vestiges of lignin were removed with expensive chlorine dioxide.

When the transition was made to recycle bleach plant filtrates in order to reduce bleach plant wastewater effluent, the temperature of the chlorine stage began to rise, which had a detrimental effect on pulp strength. To prevent this, chlorine dioxide was added to the chlorine stage, i.e. the sequence used became (DC)EDED.

The pressurized reactor (EO) or pre-reactor (EO) have made it possible to mix small amounts of oxygen gas into the pulp in the alkali stage, where the oxygen improves delignification. Small amounts of hydrogen peroxide may also be used in the alkali stage to improve delignification. Peroxide does not require pressurized reactors.

Conventional bleaching including an elemental chlorine stage was the dominant method for a long time. Even as recently as 1990 approx. 94% of bleached pulp was produced by chlorine bleaching. Since then however, the situation has changed, mainly for environmental reasons, as the AOX and dioxine discharges in wastewaters were reduced. ECF bleaching, where chlorine dioxide is used but no gaseous chlorine, quickly became common. Nordic countries abandoned the use of chlorine gas completely in pulp bleaching in 1994, and the dominant method since then has been ECF bleaching.

Pulp can also be bleached totally without chlorine chemicals. This kind of oxygen chemical bleaching is usually known by the abbreviation TCF. Bleaching chemicals in TCF bleaching are oxygen-containing chemicals such as oxygen, hydrogen peroxide and ozone. The latest chemicals to be used are the peracids. These are also oxygen-containing chemicals.

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Theory Pulp colour

The majority of lignin (over 90%) is dissolved in the cooking of sulfate pulp. Alkaline cooking chemicals however, react with lignin, dying it a dark brown color. Thus the brightness of the wood material is considerably reduced at the start of a sulfate cook, even though the lignin dissolves. At the end of a sulfate cook, the color of the wood material decreases significantly as the lignin dissolves. After cooking softwood pulp contains 3 – 4.5% lignin and hardwood 2 - 3% lignin. Most of the color of the pulp originates from this lignin.

The pulp is brightened by removing this residual lignin. Determining the amount of lignin is however comparatively problematic, so in practice the kappa number is used as a measure of the amount of lignin. The kappa number describes the ability of pulp to consume permanganate, and correlates with the amount of lignin.

The effectiveness and chemical nature of the bleaching chemicals must be known for successful bleaching and in order to achieve the desired result, as must their reactivity with lignin and carbohydrates.

The reaction between pulp and bleaching chemicals is often chemically complex, because there are a lot of different types of reactive groups in pulp, which may take part in the reaction.

Often only a certain type of chemical reaction is necessary and thus desirable regarding the bleaching result. Alongside this, several secondary reactions may occur, which needlessly consume the bleaching chemicals or are even detrimental to the desired bleaching result. When selecting bleaching conditions of course the aim is to favor bleaching reactions and restrict harmful secondary reactions.

In final bleaching, when the lignin content of the pulp is already low, bleaching chemicals should, besides removing residual lignin, oxidize colored groups and decolorize them.

Bleaching chemicals can be divided into three groups according to their function:

1 Group

The chlorine (Cl2), ozone (O3) and peroxide acid (Paa and Caa)

reacts with all aromatic lignin units

2 Group

The chlorine dioxide(ClO2) and oxygen (O2)

reacts in general with lignin structures that have free phenolic hydroxyl groups

3 Group

The hypochlorite (H) and hydrogen peroxide (H2O2)

reacts only with certain functional groups

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