returns to its original position, fresh wood is put into the trough, and the operation repeated. In this way many tons of wood can be chopped up by one of these cutters in a day. The sliced wood is conveyed by trucks to an elevator by which it is hoisted up two storeys to a floor from which the boilers are filled. The boilers are upright cylinders, about 5 feet in diameter and 16 feet high, with semi-spherical ends, provided inside with straight perforated diaphragms, between which the chips from one cord of wood are confined. A solution of caustic soda, at 12° B., is introduced with the chips, and fires are started in a furnace underneath. At other works the boilers are heated by steam circulating through a jacket which covers the bottom and sides of the boiler.
The boiling is continued for about six hours, when the digestion is complete, and the contents of the boilers are emptied with violence, under the pressure of at least 65 lbs. of steam, which had been maintained inside. A large slide valve is attached to the side of each boiler for this purpose close to the perforated diaphragm, and connected by a capacious pipe with a sheet-iron cylinder of about 12 feet diameter and 10 feet high, which receives the contents—pulp, liquor, and steam. The object of these large chambers—one of which serves for two boilers—is to break the force of the discharging mass. The steam passes through a pipe on the top of each, and from thence through a water reservoir, while the liquid containing the pulp flows through a side opening and short pipe into movable boxes, or drainers, mounted on wheels, and each capable of holding the contents of one boiler; these boxes are pushed along a tramway up to the collecting chambers, where the pulp is received. In a building 132 feet long and 75 feet wide, ten digesting boilers are arranged in one straight line, and parallel with the boilers runs the main line of rails, side tracks extending from it to each of the chambers, and a turn-table is supplied at every junction. By this arrangement the drainer waggons can be pushed from the side tracks on to the main line, which leads to the washing-engines in an adjoining room. A system of drainage is established below the tramways, by which all the liquid which drains from the waggons is carried away and collected for treatment by evaporation; these carriers remain on the side tracks until the pulp is ready for the washing-engine.
When the greater portion of the liquor has drained off, warm water is sprinkled over the pulp from a hose for the purpose of extracting all the liquid which is sufficiently concentrated to repay the cost of evaporation—the most advantageous method of recovering the soda. The contents of the waggons—from the same number of boilers—are then placed in two washing-engines, each capable of holding 1,000 lbs. of pulp. After being sufficiently worked in these engines the pulp is transferred to two stuff-chests, and from thence conveyed by pumps to two wet-machines. The screens (strainers) of the wet-machines retain all impurities derived from knots, bark, and other sources, and the pulp, or half-stuff, obtained is perfectly clean and of a light grey colour. The pulp is bleached with solution of bleaching powder like rags, then emptied into drainers and allowed to remain therein with the liquid for twenty-four to forty-eight hours, or long enough to render the use of vitriol in the bleaching unnecessary. The portion of the white pulp which is to be worked up into paper in the adjoining mill is taken from the drainers into boxes running on tramways in the moist state, but all the pulp which has to be shipped to a distance is made into rolls on a large cylinder paper-machine with many dryers. The object being merely to dry the pulp, a very heavy web can be obtained, since the water leaves this pulp very freely. The wood pulp thus obtained is perfectly clean, of a soft, white spongy fibre, and a greater portion of it is mixed with a small proportion of rag pulp and worked into book and fine printing papers. Sometimes the wood pulp is used alone or mixed with white paper shavings for book paper. The fibres are rather deficient in strength, but as a material for blotting paper they are said to be unsurpassed, while the wood paper is much liked by printers.
The wood from poplar, which is generally preferred, furnishes a very white fibre, and is easily digested, but since the fibres are short it is sometimes found advantageous to mix them with longer fibres, as those of the spruce or pine, although the latter wood requires a much more severe treatment in boiling with alkali than the former. In reference to this process the following remarks appeared in The Chemist,[16] 1855:—"The process occupies only a few hours; in fact, a piece of wood may be converted into paper and printed upon within twenty-four hours." An interesting verification of this was published a few years since in an American paper, the Southern Trade Gazette, of Kentucky, which runs as follows:—"At a wood-pulp mill at Augusta, Ga., a tree was cut down in the forest at six o'clock A.M., was made into pulp, and then into paper, at six o'clock in the evening, and distributed amongst the people as a newspaper by six o'clock the next morning. From a tree to a newspaper, being read by thousands, in the brief round of twenty-four hours!" The wood-paper process referred to has given rise to many subsequent modifications, some of which we will briefly describe.
Sinclair's Process.—The wood is first cut into pieces about 1 inch broad,⅛th inch thick, and from 2 to 3 inches long. It is then placed in a boiler and a solution of caustic soda, in the proportions of 600 gallons to 10 cwts. of dry wood, is poured over it. The boiler having been securely closed, the heat is raised till a pressure of 180 to 200 lbs. on the square inch is obtained, when the fire is withdrawn and the boiler allowed to cool, after which the ley is blown off, the top door removed, and the contents scalded. The discharge door is now opened and the pulp transferred to a poaching-engine to be washed with pure water, when the resin, &c., are easily removed and the clean fibres obtained, which, it is said, are longer and firmer than those obtained by other methods.
Keegan's Process.—By this method soft deal or pine is sawn up into pieces from 6 to 12 inches long and ½ inch thick, it being preferable that all the pieces should be of an equal size, but the smaller they are the more rapid, of course, will be the operation. The pieces of timber are placed in a cylindrical boiler, turning upon a horizontal axis while the digestion is progressing. In a second boiler is prepared a solution of caustic soda of about 20° B. (specific gravity 1·161), which is introduced through a pipe into the first boiler, this being afterwards hermetically closed, and the soda is forced into the pores of the wood by means of a pump. When the wood is not more than half an inch in thickness a pressure of 50 lbs. on the square inch is sufficient, and the injection of the caustic soda solution is completed in half an hour. The superabundant liquor is pumped back into the second boiler for the next operation. The excess of liquor having been removed from the wood as stated, steam is introduced between the double sides of the first boiler, and the temperature of the wood raised from 150° to 190° C. (334° to 438° F.). The wood is next washed in the usual way until the liquor runs off perfectly limpid, and the half-stuff thus produced may be converted into pulp either before or after bleaching, according to the quality and colour of the paper to be produced.
American Wood-Pulp System.—Another method of carrying out the wood-pulp process has recently been described by Mr. E. A. Congdon, Ph.B.,[17] from which we extract the following:—"Poplar, pine, spruce, and occasionally birch, are used in the manufacture of chemical fibre. Pine and spruce give a longer and tougher fibre than poplar and birch, but are somewhat harder to treat, requiring more soda and bleach. Sticks of poplar, freed from bark, and cleansed from incrusting matter and dirt, are reduced to chips by a special machine having a heavy iron revolving disc set with knives, and are then blown by means of a Sturtevant blower into large stove chambers after passing over a set of sieves having 1¼-inch for the coarse and 1⅛-inch mesh for the fine sieves, from whence they pass to the digesters, which are upright boilers 7 by 27 feet, with a manhole at the top for charging the chips and liquor, and a blow-valve at the bottom for the exit of the boiled wood. A steam-pipe enters at the bottom, beneath a perforated diaphragm, and keeps the liquor in perfect circulation during the boiling of the wood by means of a steam-ejector of special construction."
Boiling.—The average charge of wood for each digester is 4·33 cords,[18] giving an average yield of 4,140 lbs. of finished fibre per digester. A charge of 3,400 gallons of caustic soda solution of 11° B. is given to each digester charged with chips, and the manhead is then placed in position and steam turned on. Charging the digester occupies from thirty to forty-five minutes, and steam is introduced until the gauge indicates a pressure
