The Manufacture of Paper / With Illustrations, and a Bibliography of Works Relating to Cellulose and Paper-Making

by R. W. Sindall

The wood at this stage can be used either for the manufacture of mechanical or chemical pulp. As a general rule the pieces are taken indiscriminately for either process, but sometimes the wood is sorted out, the clean stuff free from knots and blemishes being reserved for high quality chemical pulp.

Grinding.—The main feature of the grinding process is the attrition of the wood when held against the surface of a rapidly revolving grindstone, the fibres as they are rubbed off being instantly carried away from the stone by a current of water. A complete description of the machines used and the modifications of the process practised by manufacturers is impossible in this book, but the following points will be sufficient.

The machine consists of a large grindstone about 54 inches in diameter, and 27 inches thick. It rotates in a vertical or in a horizontal position at a high speed. The stone revolves inside a casing which is provided with a number of pockets, so called, into which the pieces of wood are thrown at regular intervals, as fast as the wood is ground by the friction of the stone.

A continual stream of water playing upon the surface of the stone washes away the pulp into a tank or pit below the machine.

The quality of the pulp may be varied by the conditions under which it is made. By limiting the proportion of water so that the wood remains in contact with the stone for a longer time the temperature of the mass in the pockets rises. Such hot ground pulp, as it is termed, is tough and strong.

When the fibres are washed away from the stone as fast as they are produced the temperature does not rise, and cold ground pulp is made, which is not characterised by the somewhat leathery feel of the pulp made at the higher temperature.

The surface of the stone plays an important part also. If the stone is smooth the wood is rubbed away slowly, but if the surface has been roughened and grooved by means of a special tool the fibres are torn away quickly. In the first case the pulp comes from the stone in a finely-ground state and in a uniform condition, while in the second the pulp is coarse and chippy.

The output of the machine is, however, much increased by the use of sharp stones and by the application of considerable pressure to the blocks of wood.