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