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6 Bleaching of Mechanical Pulp

6.3

Technology of Mechanical Pulp Bleaching

Reductive bleaching is the most simple option for bleaching, not only because

just one chemical has to be added to the pulp, but also because the available low

or medium consistency after screening and latency chest can be applied. Other

than using an effective mixer and a tower or tube, no additional equipment is required.

Peroxide bleaching is more complicated, however. High-consistency dewatering

Is an essential requirement for bleaching effectiveness. Modern twin-wire presses

easily reach a consistency above 40%, and today – even when large amounts of

chemical must be added – consistency in the bleaching tower is well above 30%.

Four different chemicals must be mixed with the pulp. The chelant can be added

as a pre-treatment prior to the latency chest. Caustic soda, sodium silicate, and

hydrogen peroxide are either premixed with additional dilution water in a descending

cascade to a concentration of less than 10% H2O2 in the mixture. Alternatively,

these chemicals can be added directly, using a very effective mechanical

mixer, and mixed immediately with the pulp. Special attention must be paid to

the design of the chemical addition. A backflow of liquor into the storage tanks is

a serious safety threat, and may occur during a production halt. However, such a

hazard can be avoided by installing back-pressure valves, or even more effectively

by a system using the free flow of liquor into a small intermediate tank.

An example of a high-consistency peroxide bleach plant is shown in Fig. 6.12.

The retention time in the tower can be adjusted by controlling the filling height.

Mechanical discharge is essential for correct control, and older bleach plants operating

at a lower consistency (<25%) can be discharged by injecting water at the

base of the tower. An agitator (propeller) provides sufficient shear force to mix

water and fiber bundles to a uniform discharge consistency below 5%. At very

high consistency, simple stirring does not provide sufficient mixing, and the fiber

bundles and water remain separate. Discharge at a higher consistency requires

less volume of dilution water and allows post-bleaching with dithionite at medium

consistency after destruction of the peroxide excess with bisulfite. The second

twin-wire press is not required in the case of post-bleaching with dithionite. It is,

however, a technological advantage because it separates the water loop of the

bleach plant from the paper machine, and this may be important for retention

control. After an intense peroxide treatment the amount of anionic compounds

(“trash”) in the water circuit is high, and may cause problems.

Somewhat more complicated is a bleach plant with two peroxide stages

(Fig. 6.13). Three presses are required to allow recycling of the peroxide excess.

The pulp is dewatered to a high consistency, and excess liquor from the main

bleaching step is then added. This dilutes the consistency to a level between 10%

and 12%. In order to initiate a reaction, an addition of caustic soda to the peroxide

content might be required. Following this MC-stage, the next press generates the

high consistency required in the main bleaching stage. The effluent of this press

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