the bleaching liquor is stored. The supply tank is furnished inside with a gauge, divided into inches—each inch representing so many gallons of liquor—by means of which the workman is enabled to regulate the quantity of bleaching liquor he is instructed by the manager or foreman to introduce into the engine. About half an hour after the bleach has become well incorporated with the fibre, sulphuric acid in the proportion of six ounces of the acid (which must be well diluted with water) to each hundredweight of the fibre. The dilute acid should be added gradually, and the proportions given must not be exceeded. The bleaching being completed, the half-stuff is next treated in a machine termed the presse-pâte, which not only cleanses the material from sand and dirt, but also separates all knots and other imperfections from the fibre in a most effectual and economical manner. Indeed, we were much struck with the excellent working of this machine at Messrs. Spalding and Hodge's mill, at South Darenth, and the remarkably fine quality of the finished pulp obtained through its agency. The presse-pâte was formerly used in the preparation of pulp from straw, but its advantages in the treatment of esparto are now fully recognised. The apparatus and method of working it may be thus briefly described:—
The machine is on the principle of the wet end of a paper machine, and consists of several stone chests for holding the bleached half-stuff, in which are fitted agitators to keep the stuff in suitable condition. From these chests the stuff is pumped into a mixing box, and from thence over a series of sand traps made of wood, and with slips of wood fixed in the bottom, in which any sand present is retained. The stuff then passes into a series of strainers, which, while allowing the clean fibre to pass through, retain all impurities, such as knots, &c., and the clean stuff is allowed to flow on to the wire-cloth in such a quantity as to form a thick web of pulp. A greater portion of the water escapes through the wire-cloth, but a further portion is removed by the passage of the pulp across two vacuum boxes, connected with four powerful vacuum pumps, which renders the half-stuff sufficiently dry to handle; but to render it still more so, it now passes between couch rolls, and is either run into webs, or, as is sometimes the case, it is discharged into boxes, the web of pulp thus treated being about an inch in thickness.
CHAPTER VI.
TREATMENT OF WOOD.
Chemical Processes.—Watt and Burgess's Process.—Sinclair's Process.—Keegan's Process.—American Wood Pulp System.—Aussedat's Process.—Acid Treatment of Wood.—Pictet and Brélaz's Process.—Barre and Blondel's Process.—Poncharac's Process.—Young and Pettigrew's Process.—Fridet and Matussière's Process.
The advantages of wood fibre as a paper material have been fully recognised in the United States and in many Continental countries, but more especially in Norway, Sweden, and Germany, from whence large quantities of wood pulp are imported into this country. There is no doubt that our home manufacturers have recently paid much attention to this material, and it is highly probable that wood, as an inexhaustible source of useful fibre, will at no distant date hold a foremost rank. Indeed, the very numerous processes which have been patented since the Watt process was first made known, indicate that from this unlimited source of fibre the requirements of the paper-maker may be to a large extent satisfied, provided, of course, that the processes for reducing the various suitable woods to the condition of pulp can be economically and satisfactorily effected. The great attention which this material has received at the hands of the experimentalist and chemist—the terms not being always synonymous—shows that the field is considered an important one, as indeed it is, and if successfully explored will, it is to be hoped, yield commensurate advantages both to inventors and the trade.
The object of the numerous inventors who have devised processes for the disintegration of wood fibre—that is, the separation of cellulose from the intercellular matters in which the fibres are enveloped—has necessarily been to dissolve out the latter without injury to the cellulose itself, but it may be said that as yet the object has not been fully attained by either of the processes which have been introduced. To remove the cellular matter from the true fibre or cellulose, without degrading or sacrificing a portion of the latter, is by no means easy of accomplishment when practised on an extensive scale, and many processes which present apparent advantages in one direction are often found to exhibit contrary results in another. The field, however, is still an open one, and human ingenuity may yet discover methods of separating wood fibre from its surrounding tissues in a still more perfect manner than hitherto.
The various processes for treating wood for the extraction of its fibre have been classified into: (1) chemical processes; and (2) mechanical processes. We will give precedence to the former in describing the various wood pulp processes, since the pulp produced by the latter, although extensively used, is chiefly employed, in combination with other pulps, for common kinds of paper. In reference to this part of our subject Davis says:—"Experience has dictated certain improvements in some of the details of those earlier methods, by which so-called 'chemical wood pulp' is manufactured very largely on the Continent of Europe. … It is possible to obtain a pulp of good quality, suitable for some classes of paper, by boiling the chipped wood in caustic soda, but when it is desired to use the pulp so prepared for papers having a perfectly white colour it has been demonstrated in practice that the action of the caustic soda solution at the high temperature which is required develops results to a certain degree in weakening and browning the fibres, and during the past five years much labour has been expended in the endeavour to overcome the objections named. The outcome of these efforts has been a number of patents, having for their object to prevent oxidation and subsequent weakening of the fibres." In several of these patents, to which we shall refer hereafter, bisulphite of lime is employed as the agent to prevent oxidation and consequent degradation of the fibres, and in other processes bisulphite of magnesia has been used for the same purpose. Davis further remarks: "Although a common principle runs through all these methods of preparing cellulose from wood, they differ in detail, as in the construction of the digesters employed, methods of treating the wood stock before boiling it in the sulphurous acid solution, and also as regards pressure, blowing off the sulphurous acid gas, etc., but all these processes present a striking similarity to the method patented by Tilghmann in 1867." There can be no doubt that the action of caustic soda, under high pressures, is highly injurious both to the colour and strength of the fibres, and any process that will check this destructive action in a thoroughly practical way will effect an important desideratum.
I. Chemical Processes: Watt and Burgess's Process.—This process, which, with some modifications, is extensively worked in America, consists in boiling wood shavings, or other similar vegetable matter, in caustic soda ley, and then washing to remove the alkali; the wood is next treated with chlorine gas, or an oxygeneous compound of chlorine, in a suitable vessel, and it is afterwards washed to free it from the hydrochloric acid formed. It is now treated with a small quantity of caustic soda in solution, which instantly converts it into pulp, which only requires to be washed and bleached, and beaten for an hour and a half in the beating engine, when the pulp is ready for the machine. The wood-paper process as carried out in America has been described by Hofmann, from whose work[15] we have abridged the following:—
The wood, mostly poplar, is brought to the works in 5-feet lengths. The bark having been stripped off by hand, it is cut into ½-inch slices by a cutter which consists of four steel knives, from 8 to 10 inches wide by 12 to 15 inches long, which are fastened in a slightly inclined position to a solid cast-iron disc of about 5 to 7 feet diameter, which revolves at a high speed, chopping the wood—which is fed to the blades through a trough—into thin slices across the grain. The trough must be large enough to receive the logs, usually 10 or 12 inches thick, and it is set at such an angle that the logs may slide down towards the revolving cutters; this slanting position only assists the movement of the logs, while a piston, which is propelled by a rack, pushes them steadily forward until they are entirely
