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

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  1. Acids. However, because hydrogen peroxide will not react easily with the
  2. Alternative Bleaching Methods
  3. And intensive ozone bleaching as well as reinforced hot caustic
  4. Application in Chemical Pulp Bleaching
  5. Basic Considerations on the Selectivity of Ozone Bleaching
  6. Be expected when high dosages of, for example, ozone, or other bleaching chemicals
  7. Bleaching to the activation with Mg(OH)2 is compensated by a final hydrosulfite

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.


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Читайте в этой же книге: Screens. The shives and stiff long fibers are removed as rejects from below, | Separation according to material density is carried out using hydrocyclones | In a TMP mill, heat recovery plays an essential role in economic operating. Normally, | Oxidative processes. | Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then, | Bleaching with Dithionite 1125 | Acids. However, because hydrogen peroxide will not react easily with the | And remaining residual allows the conclusion to be made that the residual must | The resulting COD load, the points in the graph show the linear dependency | Bleaching to the activation with Mg(OH)2 is compensated by a final hydrosulfite |
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Excess by bisulfite, Y-P sequence with refiner application of| The pulp is dewatered to a high consistency, and excess liquor from the main

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