Abbildungen der Seite
PDF
EPUB

into which the wood is thrown, in such a manner that the side of the log comes into contact with the face of the stone, against which the wood is forced by the hydraulic cylinders attached to the pockets. In some machines the stone revolves in a horizontal position, while in others the stone rotates in a vertical position, the former appliances being used chiefly in Scandinavia, the latter both in Scandinavia and in America. (Fig. 42.)

The fibres of the wood are torn away from the logs by the friction of the stone, and the constant stream of flowing water carries the pulp into a pit below the grinder. When a considerable quantity of water is used in this way, the

[graphic][merged small][merged small]

pulp is kept at a normal temperature, but by limiting the proportion of water the pulp acquires a thick consistency and its temperature is raised to such an extent that it is impossible to keep the hand immersed in it any length of time. This class of pulp is made on the vertical stone grinders and is known as "hotground" pulp.

The pieces of wood used vary in diameter from 4 inches to 24 inches. In the latter case the pieces are split into sections by means of a log-splitter. (Fig. 43.)

Screening. The pulp flows from the grinders over a coarse sieve which retains any large splinters or chips, and it is then carefully screened. The screens used for this purpose are shallow iron boxes, the bottoms of which are brass plates grooved all over with very narrow slits. These plates are kept in a state of violent agitation by suitable mechanism so that the fine pulp passes through the slits and the coarser pulp remains on the surface of the plates. This

[ocr errors][subsumed]

coarse pulp is sometimes converted into finer pulp by means of a Jordan engine and screened over again in order to prevent waste. Another method of screening is the use of what is known as a "shaking sieve." The pulp flows on to a long brass plate in which the usual holes or slits have been cut. The box is

[graphic]

placed in a sloping position with the lower end open, so that the stream of fine pulp and water falls through the slits, leaving the coarse pulp on the surface of the plate, and this gradually works down towards the open end of the box, where it falls into suitable receptacles. There are other varieties of apparatus and numerous differences in the practical methods employed which it is unnecessary to give in detail. (Fig. 44.)

[ocr errors]

Fig. 42.-Grinder.

[graphic][subsumed][merged small][subsumed][ocr errors][subsumed][merged small][graphic]

Pressing. The proportion of water to pulp at this stage in the manufacture is so large that it is necessary to pass the mixture through a machine which removes the water and produces the mechanical wood pulp in a more solid form for marketable purposes. This is effected by means of a wet press machine, shown in Fig. 45.

[graphic][merged small][graphic][subsumed][ocr errors][subsumed][subsumed][merged small][merged small][merged small][merged small]

The

The mixture of pulp and water flows into a large wooden tank in which slowly rotates a hollow cylindrical drum, covered with fine wire-cloth. water passes through the wire, but the pulp adheres to the surface of the drum in the form of a thin skin. The water is regularly discharged from the central axis of the drum as fast as it gets through the wire, and the skin of the pulp is carried up by the revolving drum above the surface of the mixture in the tank, where it comes into contact with an endless travelling felt or blanket to which it adheres. The drum continually revolves and produces an endless skin of pulp, which is constantly transferred to the felt. The pulp carried by the felt is passed between two rollers, which squeeze out more water, and at the same time

[graphic][merged small]

the pulp adheres to the upper roller, leaving the clean felt to travel back over the lower roller to the hollow drum. The pulp is wound round the upper roll until a sufficiently thick sheet is produced, and then it is cut off. The amount of water in the pulp at this stage is about 70 per cent.

The thick sheets are folded up into a convenient size, usually 24 inches long by 18 inches wide, and piled up into a small stack, which is at once submitted to great pressure in order that the quantity of water may be still further reduced. By this means a pulp is obtained consisting of 50 per cent. air-dry fibre and 50 per cent. water. (Fig. 47.) After packing, the pulp is ready for export in the form of a bale of moist sheets. Mechanical wood pulp intended for the manufacture of paper is seldom prepared in the dry state, as there are many practical disadvantages in the subsequent manipulation of the pulp.

1

« ZurückWeiter »