Boiling Waste Paper.—This is sometimes effected in iron vats, about 8 feet deep and 8 feet in diameter at the bottom, and about 6 inches wider at the top. At the bottom of each vat is a false bottom, closely perforated with small holes. Steam is introduced by a pipe below the false bottom, which passes through the perforations and thus becomes uniformly distributed to all parts of the vat. To facilitate the emptying of the vats, the false bottoms have connected to them three or four iron rods, to the tops of which iron chains are hooked, and by this means the false bottom, carrying the mass of boiled paper can be raised by a steam hoisting engine or crane and deposited where desired. When the boiling is commenced, the vat should first be about one-fourth filled with a solution of soda-ash, and the steam then turned on. When the liquor boils, the papers having been previously dusted, are introduced gradually, and well distributed through the liquor; if they are thrown into the vat in large quantities at a time, and especially if they are in a compact state, the portions in contact may not be reached by the liquor, and an imperfect boiling will be the result. To ensure a uniform distribution of the boiling liquor over the surface of the material, an iron pipe extends from the centre of the false bottom to nearly the top of the vat, and this pipe is covered with a hood, which causes the soda liquor to be evenly spread over the whole mass. The vats are either cased with wood or coated with asbestos to prevent the escape of heat, and the vessel is covered with a flat iron cover, which is generally in two halves. The steam enters the tubs at the side, below the false bottom, and the exhausted liquor is drawn off through a valve connected to the bottom of the vat. In some mills the liquor is not drawn off after each boiling, but the boiled paper is hoisted from the vat as before described, and the liquor strengthened by the addition of from 10 to 20 lbs. of soda-ash for each 100 lbs. of the paper to be next boiled. Paper that is thickly coated with printing ink requires an extra dose of soda-ash. The boiling is continued for twelve to twenty-four hours according to the nature and condition of the waste paper under treatment.
Waste papers are frequently boiled, after dusting, in revolving boilers, in a solution of soda-ash or caustic soda, but it not unfrequently happens that some portions of the material become so agglomerated or half pulped during the boiling that the alkali fails to reach all the ink, and as this cannot be removed by the after processes of washing and breaking, it remains in the body of the pulp and necessarily forms a constituent part of the paper to be produced from it. The mass, when discharged from the boiler and drained is then conveyed to the washing-engine, in which it becomes broken and freed from alkali and so much of the ink as may have been dissolved or loosened, and it is afterwards treated in the beater and mixed with varying portions of other paper stock, according to the quality of paper to be produced. In some mills the boiled waste paper is disintegrated after boiling, by means of the edge-runner (Fig. 19).
Ryan's Process for Treating Waste Paper.—The following process for treating waste paper so as to produce a "first-class clean paper" therefrom, was patented by Mr. J. T. Ryan, of Ohio. The waste paper is first passed through a duster in the usual way, all thick old books being previously torn apart to separate the leaves. The papers are then boiled in a hot alkaline liquor without pulping them, whereby the alkali acts on the surfaces of the papers, and dissolves off, carrying away all the ink into the liquor. The papers, which are still in sheet form, are then drained as free as convenient from the alkaline liquor, and are next washed in the washing-engine, which leaves the material perfectly clean. It is then pulped in the beating-engine; and it is claimed that it can be formed into first-class paper without the addition of any new or expensive paper stock. The details of the process are thus given by the patentee: "Into a bucking-keir put a soda-ash solution having a density of 5° B., at 160° F., put in the stock, and shower for eight hours at a temperature of 160° F., without pulping the paper, then lift and drain, and cleanse well in the washing-engine; then pulp and form into paper. As the draining will always be imperfect, each charge removed will carry away some of the soda-ash solution, and leave the remainder of impaired strength. After each drainage add water to make up for loss in quantity of the solution, and add enough soda-ash solution at a density of 13° B., to bring all the liquor up to 5° B. at 160° F. In about eighteen working days the liquor will have accumulated considerable ink and other matter. Then blow one half of the liquor, and restore the quantity for proper working. None of the soda-ash solution is wasted, except such as falls to drain and what is blown out as last mentioned." In carrying out this process every care must be taken to guard against pulping before the alkali is washed out.
[CHAPTER IX.]
BLEACHING.
Bleaching Operation.—Sour Bleaching.—Bleaching with Chloride of Lime.—Donkin's Bleach Mixer.—Bleaching with Chlorine Gas (Glaser's Process).—Electrolytic Bleaching (C. Watt's Process).—Hermite's Process.—Andreoli's Process.—Thompson's Process.—Lunge's Process.—Zinc Bleach Liquor.—Alum Bleach Liquor.—New Method of Bleaching.
Bleaching Operation.—The half-stuff treated in the breaking-engine is run into the potcher, and the water it contains is lifted out as far as practicable by the washer; the spent liquor from the presses or drainers is then run in in lieu of water, and as much fresh bleaching liquor as may be required is then measured in, and in from two to six hours the pulp becomes perfectly white. "However well managed a mill may be," says Mr. Arnot, "it is scarcely possible to avoid having a small residue of unused chlorine in the liquid which drains from the bleaching stuff." The rule, therefore, is to use this liquor in the way above indicated, by which the unexhausted chlorine, operating upon fresh half-stuff, becomes available, and is, therefore, not wasted. "That as little of this residual chlorine as possible may remain in the stuff," Mr. Arnot further observes, "when put into the beating-engine, powerful hydraulic presses are employed to compress the stuff and squeeze out the liquid. These presses should be large enough to contain easily the whole contents of a poaching-engine, and of unexceptional workmanship. The perforated lining especially should be carefully prepared and properly secured. I have seen much trouble from negligent workmanship in this respect. Recently I examined a number of samples of press drainings, and found the unexhausted chlorine to vary very much—from a few grains of bleaching powder per gallon to about one ounce."
Sometimes it is the practice to partly fill the potcher with water, and the engine being set in motion, the half-stuff is gradually introduced until the full charge has been given, and the stuff is then washed for some time, after which the drum-washer is raised, and the bleaching liquor then run in, care being taken that the necessary quantity is not exceeded, otherwise the fibre will suffer injury from the chemical action of the bleaching agent. When vitriol is employed to liberate the hypochlorous acid, the vitriol, previously diluted with water, should be placed in a small lead-lined tank in such a position that the acid liquor may slowly trickle into the engine at the rate of 1 lb. of sulphuric acid in twenty minutes. As soon as the bleaching is complete the stuff is emptied into large stone chests, each of which will hold the contents of two engines. On the bottom of these chests are perforated zinc drainers, while a similar drainer runs up the back of each chest. The bleached stuff is allowed to remain as long as may be convenient in these chests, after which it is removed to the beating or refining engines. In some mills the bleaching is effected in the breaking-engine, while at other mills the operation is performed in the beating-engine.