Jute, Manilla, Adansonia, &c.
A cheaper method, and one which is largely adopted in mills where only a coarse dark-coloured paper is required, consists in boiling the jute or other fibre in lime. Such a treatment is, however, not so effective as one in which caustic soda is used, and the fibre produced is harder and coarser in every way. The yield of pulp, however, owing to the fact that the fibre is imperfectly resolved, is greater. In many cases, from questions of economy, lime may be advantageously employed.
Jute can only be employed successfully when it is imperfectly resolved, that is to say, when it is not reduced to the state of pure cellulose. In such a state it forms a comparatively weak fibre, owing to the fact that it is resolved into its individual fibres. If the filaments are not completely resolved, then a strong fibre is the result, but in this case, the pulp cannot be obtained in a pure white state. Owing to this fact, jute has had up to the present only a limited consumption, except for the manufacture of coarse papers. {104} Jute and Adansonia are largely used for papers where strength is of more importance than appearance, such, for example, as paper for telegram forms, strong wrapping paper, &c.
Wood.
When an alkaline treatment is adopted, the alkali employed is, at least in this country, caustic soda. In Germany the sulphide of sodium processes have been very largely adopted. The alkali method is, however, being superseded by the more perfect and more economical method of treatment with sulphurous acid or acid sulphites.
“Broke” Paper.
FIG. 27.
Mechanical Wood Pulp.
The wood is first cut up into blocks, the size of which is determined by the width of the stones used for grinding; any knots present are cut out with an axe. The stones are made of sandstone, and are covered over three quadrants with an iron casing, the remaining quadrant being exposed. The surfaces of the stones are made rough by the pressure of a steel roll studded with points, and which is pressed against it while revolving. In addition to this, channels about 1⁄4 in. deep are cut into the stone at distances of 2–3 in. They are made in two sets, crossing each other in the centre of the stone, and serve to carry off the pulp to the sides of the stone, in addition to giving increased grinding-surface. The pressure of the blocks of wood against the stones is steadily maintained by screws worked by suitable gearing; this is necessary in order to obtain a pulp of uniform character. A stream of water is kept constantly playing on the stone; by this means, the pulp as fast as it is formed can be conveniently carried away. It is first passed {106} through a rake, which retains small pieces of wood that have escaped grinding. The stream of pulp then passes through the sorters, the object of which is to keep back such portions of the wood as have not been sufficiently disintegrated. These consist of cylinders about 3 ft. long and 2 ft. in diameter, covered with a coarse wire-cloth. The fibres that are retained by this wire fall into the refiners, which consist of a couple of horizontal cylinders of sandstone, the upper one only of which revolves. Here they are further {107} disintegrated, and are again passed through the wire-cloth; this is repeated until all the fibres have passed through. The pulp, after passing through the first sorter, may be conducted through a series of gradually increasing fineness, and, by this means, be separated into different qualities. Though pulp so prepared cannot compete with chemically-prepared stuff, as the fibres are extremely short, and have comparatively little felting-power, it may be used with advantage as a sort of filling-material.