FIG. 22.
The water passing through the wire cloth on the drum carries with it a certain amount of fibre which, unless special precautions are taken (see “Save-all,” p. [154]) is lost. The amount is considerable in the case of weak rags, and care should therefore be taken that the washing is not prolonged more than is consistent with proper cleansing, and that the pulp should not be too much broken up at this stage.
The washed and broken pulp goes by the name of “half-stuff.” {89}
The following is the usual plan of treating rags in the washer:—It is first half-filled with water, and the rags from the boiler put in gradually until nearly filled. Water is now allowed to flow in at the opposite end to where the drum-washer is placed. This, by the action of the roll, mixes thoroughly with the pulp and extracts all the soluble matter, and also carries with it fine insoluble impurities.
The action of the knives in the roll on the rags passing between it and the knives in the bed-plate serves to break them up and thoroughly disintegrate them. The dirty water then passes away through the drum-washer, the stream of pure water being regulated so as to keep the level constant. This is continued until only pure water passes away. The supply is then stopped, the washer still being kept in action. As the level falls the drum is lowered by means of the handle F. When sufficiently drained the pulp is discharged through a valve in the bottom of the engine. It is now ready to be bleached. This may be done in separate engines called “potchers,” somewhat resembling the breaker or washer already described, or it may be done in the breaker itself. The process of bleaching will be described in Chapter VII.
Occasionally, the bleaching process is conducted in the “beater” itself, but this is not to be recommended.
Esparto.
FIG. 23.
The first treatment that the esparto undergoes is that of “picking.” The bands of the bales in which esparto is packed, generally by hydraulic pressure, are cut, and the grass is spread out on tables by women, who carefully remove such impurities as weeds, root-ends, &c., which from their nature are with great difficulty boiled and bleached, and which if not removed would be liable to appear in the finished paper as dark-coloured specks, technically known as “sheave.” This treatment is called “dry-picking,” in contradistinction to a subsequent process, known as “wet-picking.” A portion of the table is covered with coarse wire gauze, through which when the grass is spread over it loose particles of sand, dirt, &c., escape. The cleaning of esparto can be much better accomplished by means of machinery. Fig. 23 is an elevation of a machine manufactured for the purpose by Messrs. Masson, Scott and Bertram. The grass, which may be in the form of sheaves, as taken from the bales, is put in by the hopper A. It passes thence to a conical {91} drum made of steel bars placed very close together, driven rapidly by the shaft B. It is provided with five rows of teeth. Fixed to the sides of the willow there is also a row of stationary teeth. The grass is thoroughly broken up and dusted by the action of the teeth, and is transferred to the wide end of the willow at C, where it is carried forward on the travelling tables D. During its passage along the tables it can be examined and picked by women standing on the platforms E. The dust and dirt passing through the steel bars of the drum are drawn away by a fan through the pipe F.