The alkali employed may be either caustic lime, caustic {83} soda, sodium carbonate, or a mixture of the latter and lime, which is of course equivalent to using caustic soda. The proportion of alkali depends upon so many considerations that it is quite impossible to give exact information on this point. In the case of caustic soda, a quantity equal to from 5–10 per cent. on the rags may be taken to be a fair average. If lime be used it should be slaked with water, made into a thin milk and carefully filtered through fine wire cloth to keep back the particles of sand, coal, &c., which lime is always liable to contain. From 5–10 per cent. may be used. The amount of lime actually dissolved in the water is relatively small (1·3 grms. per litre); the portion in solution however, rapidly combines with the grease, dirt, and colouring matter of the rags and forms with them insoluble compounds, a fresh portion of lime being at the same time dissolved. This formation of insoluble compounds constitutes an important objection to the use of lime, as they are liable {84} to remain to some extent fixed in the rags and are with great difficulty removed by washing. For this reason, therefore, the more soluble alkali is to be preferred. Moreover, the lime sometimes tends to exert a hardening effect upon the cellulose. Notwithstanding these objections, lime is used by some paper-makers in preference to soda. In making choice of the chemical for boiling, much depends on the quality of the rags and the nature of the paper for which they are intended, so that no definite rules can be given.
The time of boiling varies from 2 to 6 hours, according to the quality of rags, the chemical employed, and the pressure. The use of very high pressures should be avoided as far as possible, as there is a danger, owing to the correspondingly high temperature, of fixing the dirt and colouring matters instead of dissolving them.
The quantity of water should be kept as low as may be, in order to have as strong a solution of alkali as possible; this effects a saving both in the time of boiling, and in the alkali. This is of great importance where it is necessary to evaporate the whole of the waste liquors. It should also be remembered that a certain amount of water is always formed by condensation in all boilers in which live steam is used. On the other hand, if too little water be added, the rags are liable to become “burned” and the fibre therefore weakened. During the operation the boiler should be made to revolve slowly, in order to produce thorough circulation of the liquor.
The boiling being completed, the pressure is allowed to fall, either by cooling or by blowing off from a cock usually provided for that purpose, and the liquor allowed to collect at the bottom of the boiler. It is then run off by the cock J (Fig. 18), and the rags drained as much as possible. Water is then run in to give the rags a preliminary washing. If time permits the steam may be turned in and the operation assisted. After again draining, the rags are withdrawn from the boiler into any suitable receptacle. A convenient form is that of a rectangular iron box on wheels, which can be readily transferred from one part of the mill to another. {85}
The next process is that of washing. This is usually performed in a washer or breaker, the construction of which is shown in Fig. 19.
It consists essentially of a rectangular vessel with rounded ends, in the centre of which, but not extending the whole {86} way, is a partition B, known as the “mid-feather.” The roll A, which is furnished with a number of steel knives G, and driven from the wheel H, revolves in one of the compartments formed by the mid-feather. In this compartment the floor is inclined in such a way as to bring the pulp well under the roll, as shown by the dotted line D. Immediately under the roll is what is called the “bed-plate,” the end of which is seen at I, extending up to the mid-feather, and fitted with knives similar to those in the roll A. The arrangement of the knives, both in the bed-plate and the roll, is similar to that given in Figs. 33 and 34. The distance between the roll and the bed-plate can be varied at will by means of the handle E, which is so arranged as to raise both ends of the roll simultaneously. In those breakers of an older pattern, one end only of the roll was raised, and thus the knives became worn unequally.
After passing between the roll and the bed-plate, the pulp flows down the “back-fall” D′, and finds its way round to the other side of the mid-feather. On the inclined part of the floor, and immediately in front of the bed-plate I, a small depression is made, covered with an iron grating, for the purpose of catching buttons, small pieces of stone, and other heavy substances that may have found their way into the rags. There is generally a similar grating with rather finer openings on the other side of the mid-feather. The engine is constructed of iron, generally made in one casting.
The dirty water from the rags is removed by the “drum-washer” C. It is divided into compartments by the partitions shown by the dotted line c. The centre of the drum is formed of a conical tube, the narrow end of which is towards the mid-feather. The ends of the drum are generally made of mahogany, as this is found to stand the action of alkali better than any other wood. The periphery is covered with fine copper or brass wire-cloth, laid on to a backing of a much coarser material. An improved form of backing has been introduced lately, which is much more durable than wire. It is formed of brass cut into the form shown in {87} Fig. 20. The drum can be raised or lowered by the small wheel F, and it is driven by a belt on the shaft that bears the roll.
The wash-water passes through the wire cloth into the compartments formed by the partitions c, and finding its way down to the narrow end of the inner conical tube, flows out through the side of the drum into a trough which is placed across the washer to receive it. Or it may, as shown in the drawing, be conducted through the mid-feather itself, which is made hollow at this part for the purpose.
Another form of drum-washer, called the siphon-washer, is sometimes used. Its construction will be understood by reference to Figs. 21 and 22.
The drum is simply a hollow cylinder of wire cloth, the ends of which are formed of wood. Inside the cylinder is the siphon tube A, into which the water passing through the wire cloth flows. The continuation of the siphon tube B (Fig. 22) is made of flexible tubing. The action of the siphon is commenced by filling it with water through the cock C (Fig. 22). The water then flows in the direction indicated by the arrows. The tube A is fixed, and passes through a hollow journal fitted on to the end of the drum. {88} The other end is connected with the rod E, on which the driving pulley D is placed.
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.