It is impossible to give anything like exact figures for the amount of oaustic soda necessary to properly boil esparto, so much de pending upon the quality of the grass, the style of boiler employed, and the class of paper proposed to be manu factured. The same effects may sometimes be produced by boiling with a small quantity of caustic soda for a long time, or at a high pressure ; or for a shorter time, or at a lower pressure, with a larger proportion.
The proportion given by Routledge, to whom the development of the esparto process is due, is 10 per cent, of pure caustic soda. In respect to the boilers, it is said that Roeckner's form is the most economical, as, other things being equal, a smaller quantity of soda is sufficient. The time required is also subject to much variation, some mills boiling for four and others for eight hours.
Washing.—The boiling being completed, the steam is turned off, the lid is removed, and the almost black liquid, highly charged with matter (resin, silica, &c.) extracted from the grass, is run away to a large store-well. Water is now run in, the lid is fastened on, and steam is turned on again for a short time. The liquor from this washing is also run into the store-well. The grass is next removed by the bottom door H, and is carried away in trucks to the " wet-picking " house, where it is again overhauled, and any unhoiled portions are removed. In some mills, where a great degree of purity is not required in the paper, this sorting process is dispensed with.
It is now taken to the breaking- or washing-engine, where it is reduced to pulp, and washed free from the liquor remaining after the partial washing in the boiler. The construction of the engine will be readily understood by reference to Fig. 1050. It consists essentially of a large rectangular vessel, with oval ends, in the centre of which is a partition B, technically called the "mid-feather." The roll A, carrying the 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 so as to bring the pulp well under the roll, as shown by the dotted line D. Immediately under the roll, is what is celled the "bed-plate," the eud of which is seen at I, extending up to the mid-feather, and fitted with knives, similar to those in the roll A. The distance between the roll and the bed-plate can be altered by means of the handle E. After passing between the roll and the bed-plate, the pulp flows down the peculiar elevation known as the "back-fall," shown by the dotted line D', and finds its way round to the other side of the mid-feather. On the inclined part of the floor, and just in front of the bed Plate I, is placed a grating for the purpose of catching small pieces of stone, and other heavy sub stances that may have found their way into the pulp. There is generally a similar grating, but
with rather finer openings, on the other side of the mid-feather. The whole engine is usually made entirely of iron in one casting, though in some cases, the sides are of iron and the floor is of wood. The drum 0 serves to run off the wash-water. It is divided into four or five compartments by the partitions shown by the dotted lines c. The centre of the drum is a conical tube, the narrow end of which is towards the mid-feather ; this causes the water to flow into the spout L, and down the mid-feather, which is made hollow at this part for the purpose. Or the water, as in the older forms, may be conducted along a trough placed across the engine. The sides of the drum are commonly made of mahogany, as this is found to stand the action of the soda better than any other wood. The circumference is covered with fine copper gauze, hacked with a very much coarser material. The drum can be raised or lowered by the small wheel F, and it is driven by a belt from the shaft that bears the roll. The pulp is discharged through a valve in the bottom. The tap K may be used for the purpose of cleaning the engine. The breaker is first half-filled with water, and then the charge of grass is put in. In about 20 minutes, it is sufficiently broken up, and the water is then run off by the washing-drum, fresh water at the same time running in, in order to ensure a thorough washing. At this stage of the manufacture, the pulp goes by the name of "half-stuff." Bleaching.—The next step in the process is to bleach the pulp. For this purpose, it is run from the breaker into a " poacher," placed at a slightly lower level. The poacher very closely resembles the breaker, except that it is somewhat larger, and contains, instead of knives, a number of cast-iron Paddles on the circumference of a hollow drum, with which to thoroughly stir the pulp. Bleaching liquor, usually made in another part of the mill, by dissolving bleaching-powder in water, is now run in, and the whole is thoroughly mixed up by means of the paddles. It is the practice in some mills to assist the bleaching operation by the addition of a small quantity of an acid, either hydrochloric or sulphuric. Other manufacturers attain the same end by heating the mass with a jet of steam to about 32° (90° F.) ; others, again, put in a small quantity of bicarbonate of soda, a portion of the carbonic acid of which has the same effect as hydrochloric or sulphuric acids.