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 on the square inch is obtained, when the fire is withdrawn and the boiler allowed to cool, after which the ley is blown off, the top door removed, and the contents scalded.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 of 110 lbs., which occupies about three hours. This pressure is kept up for seven hours, when it is reduced by allowing the steam to escape into a large iron tank which acts as a separating chamber for the spent liquor it carries, the steam entering in at one end and passing out at the other through a large pipe, the liquor remaining in the tank. The steam is allowed to escape until the pressure is reduced to 45 lbs., when the digester is blown. The blow-cap being removed, the blow-valve is raised and the contents of the digester are discharged into a pan of iron covered with a suitable hood. The contents strike against a dash-plate placed midway in the pan, which thoroughly separates the fibres of the wood. The time occupied in the foregoing operations is from eleven to eleven and a half hours. It takes from nine to ten hours to free the pans from alkali, when they are removed to washing-tanks with perforated metal bottoms, where the material receives a final washing before being bleached.
Washing.—Each of the three digesters has a pan into which its contents are discharged, and there are also four iron tanks used for holding the liquors of various strengths obtained from the cleansing of the pulp and a fifth tank is kept as the separating-tank before mentioned. When the digester is blown, the pulp is levelled down with a shovel, and the liquor from the separating-tank is allowed to flow into it. The contents of the next strongest pan are pumped upon it, while at the same time the strongest store tank flows into this pan. This flowing from the tank to the pan, pumping from here to the pan just blown, and from there to the evaporators, is kept up until the liquor is not weaker than 6° B. hot (130° F.). The second pan is now down to 4° B. hot, and the process of "pumping back" is commenced. The two weakest tanks are put upon this pan and pumped out of the bottom of it into the two tanks in which are kept the strongest liquors. The two weak tanks have been filled in the process of completing the cleansing of the third pan (the weakest) on which water was pumped until the last weak tank stood at only ½° B. This pan, now cleaned, is hosed and pumped over to the washing tanks. A fresh blow is now made in this pan, and the same treatment kept up as with the first pan.
The foregoing system is thus illustrated by Mr. Congdon:—
| Pan | A.— | Just blown. | ||
| " | B.— | Partly cleaned. | ||
| " | C.— | Almost cleaned. | ||
| Tank | 1.— | 3½° | B. | hot. |
| " | 2.— | 2° | " | " |
| " | 3.— | 1° | " | " |
| " | 4.— | ½° | " | " |
| Separating tank, strong. | ||||
A is levelled down; contents of separating-tank allowed to flow upon it; B is pumped on to A; at the same time liquor from the two strong store tanks is put on it (B), and this continued to be sent from A to the evaporator until it is now weaker than 4° B. hot; the process of "pumping back" is then commenced. The two weakest are allowed in succession to flow on to it, and the liquor purified from the bottom of B into the two strong tanks, filling No. 1, the stronger, before No. 2. The weakest are filled in the process of completing the cleansing of C, on which water is pumped until the last tank from it tests only ½° B. C is now hosed and pumped over to the washing tanks. A fresh digester is blown in C, and the process repeated as with A.