Oxygen Delignification & Bleaching

Pulp from the Sulphate process is brown in colour as a result of the chemical treatment which dissolves the lignin. The bleaching plant transforms the colour of the pulp by treating it in several stages with various chemicals including oxygen, chlorine dioxide and peroxide. With sulphite pulp, the colour is significantly lighter and it can be used for newsprint and other applications without needing to being bleached.

The number and order of stages used may vary according to the intended use of the pulp as well as with the type of wood that may be used. A bleaching sequence or a connection of bleaching stages, is normally described using internationally accepted terminology, where each stage or bleaching tower has a letter that indicates chemical contained in it. Some of the most currently used bleaching stages are:

D = Chlorine dioxide stage

E = Extraction stage where chlorinated lignin is dissolved in alkali

O = Oxygen stage

P = Peroxide stage

Each bleaching stage basically consists of three parts: a mixer, a bleaching tower and a washer .

Oxygen delignification usually takes place before the bleaching plant. This stage is generally placed between the screen room and the washing plant.

Most bleaching chemicals are bought ready to use from specialised chemical companies and delivered by road or rail. Chlorine dioxide, however, needs to be produced in the pulp mill, as it is too dangerous to transport. Chlorine Dioxide is extremely poisonous and unstable and can explode at high concentrations.

Many bleaching chemicals are highly corrosive. Pipes, pumps and valves are therefore normally fully lined, acid-proof high alloy steel or titanium.

The bleaching process. Click on the image to enlarge.

Some examples of NAF valves successfully used in this process

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