A Text-book of Paper-making

by C. F. Cross; E. J. Bevan

FIG. 18.

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.

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.