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._—This fibre, on account of its high percentage of non-cellulose constituents, requires a large amount of soda to resolve it; on the other hand, being of the nature of a pecto-cellulose, the process of disintegration may be conducted at a low pressure; in fact, it is the practice at some mills to boil in open vessels, in which case, however, a larger amount of soda is required. As has been already stated, it may be taken as a general rule for all fibres, that within certain limits the higher the pressure employed the less soda is required. The quantity necessary also varies with the district from which the grass comes. Caustic soda is the chemical invariably employed; lime, on account of its forming insoluble compounds with the { 90} non-cellulose portion of the grass, is inapplicable. Not only does the amount of soda depend upon the pressure, but it also depends to a considerable extent on the form of boiler employed. The use of rotary boilers is objectionable, as the esparto tends to collect together into compact masses, which are with difficulty penetrated by the liquor. It is therefore the almost universal practice to employ stationary boilers.