Pulping

1. Any process of pulp preparation begins with barking. Bark is removed in big steel cylinders called barking drums. Logs tumble against each other and as a result bark is stripped. An­other common method is using powerful jets of water.

2. If bark is not removed we get darker and dirtier pulp. The bleaching of such a pulp is very difficult, but this disadvantage is overcome by the greater yield of the pulp. The technology of utilizing the whole tree is already known and is used at many mills. But at the majority of pulp and paper mills bark is removed.

3. Mechanical pulp (groundwood) consists of wood obtained by grinding wood into a fibrous condition. The process by which mechanical pulp is produced is very simple: it involves the wet grinding of logs. The composition of the mechanical pulp is practically identical with that of the wood. Groundwood made from fresh wood is of light yellow colour.

4. The pulp produced by the mechanical process is inferior to that produced by the chemical processes: it is used in making those kinds of paper where the highest quality is not demanded and the price is principal consideration, for example, in newsprint. Such paper turns yellow owing to the action of sunlight and the atmosphere on the lignin contained in the wood pulp.

5. Mechanical pulp is much cheaper to make than any other form of paper pulp. In the first place only about 2 per cent of raw material is lost, as compared to 50 per cent in the chemical processes. Secondly, there are no chemicals required. Finally, the equipment necessary for making mechanical pulp is much cheaper.

6. Another way of making wood pulp which combines mechanical and chemical methods is called the semi-chemical process. It was developed particularly for the pulping of hardwoods and has many variations. The pulpwood chips are cooked in a mild chemical solution, and then the fibers are separated mechanically. Semi-chemical pulps are used in making corrugated paper boards, е. g. cartons and many other items.

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Where is the mechanical pulp used?

Дата добавления: 2015-07-11