Thus it will be seen that this specification clearly described a process by which the chlorides of sodium and potassium, and of the metals of the alkaline earths (chloride of magnesium, for example), may be converted into hypochlorites by electrolysis, and the hypochlorite solution obtained used for the purposes of bleaching. It would appear difficult to conceive how any subsequent patent for accomplishing the same thing, and using essentially the same means, can claim originality in the face of such "prior publication" as was effected by the usual "Blue-book," which any person can buy for eightpence.
Hermite's Process.—The following description of this process has been furnished by the engineers engaged in connection with the process to the Paper Trade Review:—"Briefly described, the Hermite process consists in manufacturing a solution of high bleaching power by electrolysing an aqueous solution of magnesium chloride. The salt is decomposed by the current at the same time as the water. The nascent chlorine, liberated from the magnesium chloride, and the nascent oxygen, liberated from the water, unite at the positive pole, and produce an unstable oxygen compound of chlorine of very high bleaching power. The hydrogen and magnesium go to the negative pole; this last decomposes the water and forms magnesium oxide, whilst the hydrogen is disengaged. If in this liquid coloured vegetable fibre is introduced, the oxygen compound acts on the colouring matter, oxidising it. Chlorine combines with the hydrogen to form hydrochloric acid, which finding itself in the presence of magnesium in the liquid combines with it, and forms the initial chloride of magnesium."
Andreoli's Process.—This process consists, avowedly, in bleaching pulps "by means of hypochlorite of sodium, produced by electrolytical decomposition of a solution of chloride of sodium." In carrying out his process, M. Andreoli uses as an electrolyte "concentrated or non-concentrated sea-water, or a solution of chloride of sodium, the specific gravity of which varies according to the quality and nature of the materials to be treated. Generally the solution to be electrolysed works better with a density of 8° to 12° B., but although salt is cheap, and the solution when exhausted may be regenerated by passing an electric current, I always endeavour to have when possible (sic) a weak solution, and with some kinds of pulp an electrolyte having the density of sea-water (3° B.) is sufficiently strong to bleach."
The foregoing are the only electrolytic processes for bleaching fibres that need recording, and we fancy there will be little difficulty in tracing the resemblance between the two latter and the process of Mr. C. Watt.
Thompson's Process.—This process, for which a patent was obtained on February 3rd, 1883, may be thus briefly described:—In bleaching linen fabrics the material is boiled for about three hours in a solution of cyanide of potassium or sodium—about half an ounce of the salt to each gallon of water—to remove the resinous matter from the fibre, so that the cellulose may be exposed to the action of the bleach. The fabric is then washed, and again boiled for three hours more in a similar solution, and after being again washed is ready for bleaching. With cotton the preliminary boiling is not necessary, unless the material is greasy, in which case a solution of half the strength and two hours' boiling is sufficient. In ordinary cases cotton is not boiled at all, but is simply washed in cold water and squeezed. In bleaching, all vegetable fibres are treated in the same way, the only difference being in point of time. The cotton or linen, after being treated as described, is then piled somewhat loosely in an air-tight vessel, 9 lbs. of cloth to the cubic foot of space being considered sufficient. The vessel is then filled with a weak solution of bleaching liquor, consisting of about one ounce of dry bleaching powder to each gallon of water. "After the vessel has been filled, the liquor is immediately run out, and is replaced by an atmosphere of carbonic acid, which quickly liberates the chlorine on the fibre, and thus decomposes the water, uniting with the hydrogen and liberating the oxygen, the result of which, is to bleach the fibre or fabric. In about an hour the whole of the bleaching liquor in the fibre will have been thus decomposed, and this operation must be repeated until the material is of the proper whiteness to be withdrawn from the action of the chlorine. The material is then washed and squeezed. Chlorine, however, always leaves these materials of a yellowish white." To remove this tint, the material is passed through a solution of oxalic acid—about 2 oz. to the gallon—squeezed as it passes out of this solution, and then passed through another solution made by dissolving ¼ grain of triethyl rose aniline to the gallon of water, or 20 grains of indigo, as may be preferred. To this solution oxalic acid is added until it becomes of an opaque but bright turquoise blue. The material, after washing, is then white.
The patent describes and illustrates the apparatus to be used in conjunction with certain parts of existing apparatus used in bleaching.
Lunge's Process.—In this process acetic acid is used in place of hydrochloric or sulphuric acids, etc., to set free the chlorine or hypochlorous acid, in the ordinary method of bleaching with hypochlorite of lime, or bleaching powder, which, the inventor says, "combines all the advantages of the materials formerly employed, without any of their drawbacks.... The price is no impediment, for a minimal quantity is sufficient, the same being regenerated over and over again. At first acetic acid and chloride of lime decompose into calcium acetate and free hypochlorous acid. In the bleaching process the latter yields its oxygen, hydrochloric acid being formed. The latter instantly acts upon the calcium acetate; calcium chloride is formed and acetic acid is regenerated, which decomposes a fresh quantity of chloride of lime, and so forth. Consequently the smallest quantity of acetic acid suffices for splitting up any amount of chloride of lime.... The hydrochloric acid formed is never present in the free state, as it instantly acts upon the calcium acetate. This is very important, since hydrochloric acid weakens the fibre by prolonged contact, whilst acetic acid is quite harmless. Since there are no insoluble calcium salts present, the operation of 'souring' after bleaching is quite unnecessary; this not merely saves the expense of acid, and of the subsequent washing of the fabrics, but it also avoids the danger, especially present in the case of stout fabrics, of leaving some of the acid in the stuff, which concentrates on drying and weakens the fibre; it may also prove injurious in subsequent dyeing operations. But in the new process no free acid is present except acetic acid, which has no action upon fibre, even in its concentrated state and at a high temperature."
The acetic acid may be employed in various ways, including the following:—A small quantity of the acid may be added from the first to the bleaching liquor; or the fabric, after being treated in the ordinary way with a solution of the bleaching powder, may be steeped, without previous washing, in water containing a little acetic acid; or the fabric may be steeped in water acidulated with acetic acid, and bleaching liquor afterwards run in slowly and gradually, with continuous agitation in the usual way. In the case of hard water, or of impure bleaching liquors, a good deal of the acetic acid would be consumed in neutralising the lime; in this case, some hydrochloric or sulphuric acid may be added, but only sufficient for the purpose, so that no acid but hypochlorous or acetic acid exists in the free state. The process is applicable to the bleaching of vegetable fibres, whether spun or in the unspun state, and for bleaching paper pulp made from rags, wood, straw, esparto, etc. Besides acetic acid, any other weak organic acid of an analogous nature may be used.
Zinc Bleach Liquor.—Strong acids are often objectionable for liberating chlorine from bleaching powder, and especially in bleaching some classes of paper pulp. If a solution of sulphate of zinc be added to one of bleaching powder, sulphate of lime is precipitated, and the zinc hypochlorite formed at once splits up into zinc oxide and a solution of free hypochlorous acid. Chloride of zinc acts similarly; for a saturated solution of zinc in hydrochloric acid decomposes as much bleaching powder as half its weight of concentrated oil of vitriol.—Varrentrapp. Consequently zinc salts can be employed in place of sulphuric acid, and thus bleach the paper pulp very quickly. When this mixture is employed in bleaching pulp, the precipitated sulphate of lime resulting from the reaction and also the oxide of zinc formed, remain in the pulp, and serve as loading materials.
Alum Bleach Liquor.—Orioli[19] recommended for use, in paper-mills especially, a bleach liquor made by decomposing equivalent quantities of a solution of chloride of lime and sulphate of alumina, formerly known as Wilson's Bleach Liquor. Sulphate of lime is precipitated, and hypochlorite of aluminium remains in solution; this being a very unstable salt can be applied for bleaching without the addition of an acid, splitting up into aluminium chloride and active oxygen. Consequently the liquid always remains neutral, and the difficulty caused by the obstinate retention of free acid in the fibre, by which it is strongly acted upon in drying, in this case does not exist. The aluminium chloride also acts as an antiseptic, so that the paper stock may be kept for many months without undergoing fermentation or other decomposition. The solution is allowed to act for about ten minutes in the engine.—Lunge.