Bleaching
Bleaching objectives
The aim of bleaching
History
Theory
Terms
Abbreviations

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 . 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.

Delignifying bleaching is performed in a series of bleaching stages usually with intermediate washing between stages . A multistage process is necessary because it is not reasonable to achieve sufficient discoloration of pulp in a single stage.

Bleaching objectives

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

• Increased brightness
• Improving brightness stability
• Improved cleanliness
• Reducing pitch content

The most important of these is the increase in brightness.

Lignin in intact wood, as carbohydrates and extractives, is only slightly colored, but after alkaline pulping and as a result of the pulping reactions, the residual lignin fragments of pulp are highly colored. Thus, bleaching should be performed by removing the residual lignin. However, the obvious prerequisite is that no significant losses in pulp strength occur. In general, hardwood pulps can be discolored more easily than softwood pulps.

The effectiveness of the final washing stage is the most important factor for obtaining good brightness stability. The final peroxide stage removes reducing groups derived from carbohydrates and lignins such as oxidized cellulose and xylans, and o- and p-quinoid structures generated and incompletely removed in the chlorine dioxide stage. These groups can interact with HexA's thus enhancing reversion.

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).

The aim of bleaching

The aim of bleaching pulp is to continue delignification and using bleaching chemicals to remove any lignin known as residual lignin. Residual lignin remains in the pulp after the cooking and oxygen stages and can not be broken down and dissolved without sacrificing pulp yield or fiber properties . Chemicals used for delignification in bleaching 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 .

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 . 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:

• The length of the fiber is not changed in bleaching, but due to yield loss the linear density of the fiber is reduced. Thus there are more fibers in the unit weight of bleached pulp. This has a favorable effect on tear strength for instance. Thanks to the removal of lignin the fiber swells more easily and the pliability and flexibility of the fiber are increased. This facilitates the binding of the fiber network. Too great a loss of hemicellulose can on the other hand worsen the adhesive capacity of the points of contact, so that the end result may also be weakened pulp strength.
• The strength of the fibers is reduced as the carbohydrate chains are broken. However, a moderate weakening of viscosity is not considered a deterioration of pulp strength properties as a result of the contradictory factors mentioned above.
• Refinability generally improves in bleaching.
• In various ways there are great differences in bleached pulp between the brightness stability and durability of absorptivity. Pulp brightness diminishes during storage and refining as well as finished paper under the effects of heat, moisture and light.

Theory

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.

In order to achieve the desired result, the effectiveness and chemical nature of the bleaching chemicals must be known for successful bleaching as well as 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 :

History

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.

The bleaching lines started after year 2000 have mainly been ECF lines.

Terms

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

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

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

• Kappa number

Measure of how much 1 g of pulp use certain potassium permanganate solution in certain circumstances. This has been found to correlate to some degree with the lignin and hexenuronic acid content of pulp. Roughly it is 6.7 times lignin content of pulp in percents.

• K number

is 2/3 of kappa number

• Active chlorine

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.

• Kappa factor

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)

Kappa factor is the same as active chlorine multiple and equivalent chlorine multiple.

• ISO Brightness

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.

• Color

Measured as ClPtCl6 equivalent. Generated mainly in extraction stage. Roughly proportional to kappa number of pulp being bleached.

• Selectivity

The ability of bleaching chemicals to react with lignin and not react with carbohydrates. In other words chemicals with good selectivity 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).

Abbreviations

Here are some general abbreviations used for bleaching chemicals: