Treatment of Straw.—Tho preparation of paper-pulp from straw has been the subject of numerous patents, chiefly in France and America. They do not, however, differ from each other to any great extent in principle.
Cutting.—The first process to which straw is subjected is that of cutting. This is done principally for the purpose of rendering it more convenient to pack into the boilers ; it also makes it more easily cleaned. The cutter is similar to that ordinarily used for 'cutting hay for stable uses. The out straw has to be cleaned and freed from the dust and dirt that invariably accompany it. This operation may be performed in various ways. One very suitable method is to blow the straw by means of a violent blast of air along a wooden tube or shaft, into a chamber, whose sides are made of coarse wire gauze, through which the dirt escapee. The straw thus purified is taken to the boilers.
Boiling.—The boilers are usually rotary, and closely similar to those used for rags. The heat may be applied in several ways : directly ; by means of a steam-jet opening into the boiler ; or by means of a coil of steam-pipe. The direct method is but seldom used, as it sometimes entails damage to the pulp, through overheating due to imperfect circulation. This cannot happen with the two latter arrangements. The objection to heating by means of an open steam-pipe is that the liquors are unduly diluted by the condensed steam, and, for this reason, the last-mentioned plan is preferred by some. A solution of caustic soda is the agent invariably used, the strength necessary depending very largely on circumstances, such as the pressure, and time of boiling. A higher pressure than that required for esparto and rags is necessary, 60-80 lb. a sq. in. being usual. The time may vary from 6 to 12 hours, and the amount of caustic soda from 15 to 25 lb. a cwt. Some manufacturers recommend a preliminary boiling in water; this has the effect of removing a large quantity of matter from the straw, and of rendering the subsequent boiling process easier. The washing may be performed in large vats provided with perforated bottoms, or in an ordinary washing-engine, in which latter case, the roll should carry only blunt knives, as the fibres are generally sufficiently disintegrated by the boiling process. When properly washed, the pulp may
be bleached by one of the ordinary methods. When this is accomplished, antichlor is put in to remove the excess of bleach, and the pulp, which is much too finely divided to allow of its being pressed, is made into a coarse thick web of paper ; by this means, most of the lime and other substances from the bleach are got rid of. The machine on which this is done corresponds exactly to a paper-making machine proper, without the drying-cylindere and calenders. This method may also be adopted before bleaching, in order to get rid of the last traces of the liquor from the boiling operation, and free the pulp from knots, sand, &c., the machine being provided for this purpose with sand-tables. Thy pulp, in the form of rolls or webs of thick paper, is ready to be taken to the beaters, where its subsequent treatment is similar to that of other fibres. Owing to the fact just mentioned, that the straw is to a considerable extent disintegrated by the boiling process, very little working in the beaters is sufficient. In other establishments, the straw is drained in large stone chests, similarly to esparto and rags.
Treatment of Wood.—The manufacture of paper-pulp from wood is confined almost exclusively to Sweden and America. This appears to be due to the fact that, in these countries, abundance of suitable wood is found, and can be very cheaply .obtained. Many attempts have been made in this country, but they have been nearly all abandoned. In America, poplar is the wood generally employed, on account of the ease with which it can be disintegrated.
The method of treating it is as follows. The wood is brought to the mills in cords of about 5 ft. long. The bark is removed, and the blocks are out into thin slices across the grain, in a machine constructed somewhat on the principle of an ordinary chaff-cutter. The wood is passed slowly and steadily, by means of an advancing screw, along a trough, at the end of which it comes against very strong steel knives, firmly fixed on a rapidly-revolving cast-iron disc.