In bleaching it is considered to be more advantageous to employ moderately strong liquors rather than weaker ones, inasmuch as the object is effected in less time than when weaker liquors are employed. An extreme in the opposite direction, however, must be avoided, since a very strong bleach will inevitably cause injury to the fibre. Sometimes the potchers are fitted with steam-pipes, in order that the diluted bleaching liquor may be heated, if required, to facilitate the operation. If the temperature be raised too high, however, the effect upon the fibre will be at least as injurious as if too strong a bleach were employed. It must also be borne in mind that in either case, after the pulp has been bleached and the liquor allowed to run off, the mass has to remain some time—even if pressed to remove as much of the liquor as possible—in direct contact with the products resulting from the decomposition, and probably some undecomposed hypochlorite also, which will continue their chemical action upon the fibre until removed by washing, or neutralised by one or other of the agents employed for the purpose.
Sour Bleaching.—When the bleaching liquor, after acting upon the half-stuff for some time, has become partially exhausted, dilute sulphuric acid—about one part acid to fifteen parts of water—is added, which, by liberating hypochlorous acid, hastens the bleaching considerably, and when the chemical action resulting from this treatment is nearly complete, the spent liquor is allowed to drain away, and fresh bleaching liquor is introduced, the strength being regulated by the progress made in the first case, which will depend upon the character of the fibre treated. In the second application of the bleach no acid is used. When sulphuric acid is added to the bleaching liquor, as above, the process is termed sour bleaching. Sometimes hydrochloric acid is used for this purpose, but in either case it is necessary to avoid employing the acid in too concentrated a state, or in too great a quantity, otherwise free chlorine will be liberated, which, besides being injurious to the health of the workmen and the surrounding machinery, also involves loss, while the colour and strength of the fibre itself will also be impaired. In some mills the bleaching is effected in the beating-engine, the bleaching liquor being pumped in while the machine is in motion.
Respecting the time which the bleaching operation should occupy, there appears to be some difference of opinion, or, at all events, the practice seems to vary in different mills, but there is, no doubt, an advantage, so far as ultimate yield is concerned, in moderately slow bleaching at a moderate temperature, inasmuch as there is less risk of chemical action upon the cellulose itself than when strong liquors are used, at a higher temperature, with a view to hasten the operation and economise the bleaching powder.
Fig. 20.
Bleaching with Chloride of Lime (Preparation of the Bleaching Liquor).—Chloride of lime, or hypochlorite of lime, commonly called bleaching powder, when well prepared, contains from 32 to 35 per cent. of active chlorine. Being readily decomposed by the air, and also by heat, this substance should always be stored in a cool and dry place until required for use. A solution of bleaching powder is generally prepared in large tanks lined with lead, which are provided with agitators or stirrers, so that the powder, when added to the water, may be freely diffused, and its active material dissolved in the liquid. A machine, or "bleach-mixer," manufactured by Messrs. Bryan Donkin and Co., of Bermondsey, is shown in Fig. 20, which is so constructed that the strong bleach liquor does not destroy it. The device for agitating the contents of the tank explains the principle of the machine. To prepare the bleaching liquor, about ½ lb. of chloride of lime to each gallon of water is used, which yields a liquor at about 6° T. When the required quantity of bleaching powder and water have been introduced into the mixer and sufficiently agitated, the vessel is allowed to rest until the residue, which chiefly consists of free lime and its carbonate, has subsided, when the clear liquor may be run off for use. When all the clear liquor has been drawn off the residue should be washed with water, and after again settling, the washing water run off, and fresh water added, these washings being repeated as often as necessary to remove the last traces of the "bleach," as it is technically called. The washing waters may be used in lieu of water in the preparation of fresh bleaching liquors. In some mills the bleaching powder is mixed with from 2 to 3 times its weight of water; the mixture is then well agitated and the residue afterwards allowed to settle, the clear solution being afterwards drawn off and the residue then washed as before. In either case the residual matter is afterwards well drained and then cast aside. The bleaching liquor is stored in large tanks ready for use, from which it is withdrawn as required by means of a syphon or otherwise.
Bleaching with Chlorine Gas (Glaser's Process).—This method of bleaching is not so much adopted in England as formerly, but has found much favour in Germany; indeed, within the past few years, namely, in March 3rd, 1880, a process was introduced by Mr. F. Carl Glaser for treating straw, in which, after boiling with caustic soda as usual, the pulp is bleached by the action of chlorine gas. The straw, after being separated from weeds by a slight or superficial picking, is cut into pieces of from ⅓ to ⅔ of an inch in length. The cut straw is then placed in a rotary boiler for about four hours, at a pressure of about 4 to 4½ atmospheres, in a solution composed of 29 lbs. of caustic soda at 71°, and 48 lbs. of calcined soda at 90°, rendered caustic, for every 220 lbs. of straw. After boiling, the dirty ley is drawn off, and the boiled straw subjected to two washings with water. It is then conveyed to the washing-engine, where it is washed for an hour; the drum of the machine should have a sieve or sifter, the apertures of which are about 60 to the square inch. The washed straw is next dried by centrifugal force in a hydro-extractor, until it contains about 70 per cent. of water, which is necessary for the action of the chlorine gas. To effect this, so as to obtain not very solid or close cakes of straw, the holes of the wire of the hydro-extractor should not be more than 50 to the square inch. The cakes of straw thus formed are then exposed to the action of chlorine in leaden chambers of the ordinary kind, in which they are placed in layers upon hurdles, or upon shelves. If the chlorine is produced by hydrochloric acid, for every 220 lbs. of unboiled straw, 51½ lbs. of the acid at 20° B., and a corresponding quantity of 70 per cent. peroxide of manganese are used. After the bleaching operation, the acid formed is removed by washing in a washing-engine. If a complete reduction of the fibres has not been effected by the bleaching, this may be completed by the aid of well-known machines, and either before or subsequent to the after-bleaching there is used for 220 lbs. of straw about 4½ lbs. of chloride of lime, at 35° [per cent.?] The patentee then gives the following explanation:—"As pine wood or fir is chemically freed from its colouring principle and transformed into fibres as well as cellulose, the object of the intense action of the chlorine is to destroy the mucilage of the straw, as well as the incrusting matters which have not been destroyed by the boiling with caustic soda, and consequently to strip or expose and open the fibres." It will be readily seen that this process bears a close resemblance to Mr. C. Watt's wood-pulp process.
Electrolytic Bleaching (C. Watt, jun.'s, Process).—At the present time, when the means of obtaining the electric current for practical purposes in the arts have so far exceeded that which would have been deemed probable some forty years since, we find that many ingenious processes, which were found to be unpractical at that time from the want of cheap electrical power, have since reappeared in the form of patented inventions, which would seem to possess every merit—but originality.
So long ago as September 25th, 1851, the author's brother, Mr. Charles Watt, obtained a patent for, amongst other claims, decomposing chlorides of sodium and potassium, and of the metals of the alkaline earths into hypochlorites by electricity. It may be well to make a few extracts here from his specification in order that some of the subsequent patents, to which we shall refer, may be traced to what may, perhaps, be considered their true origin. In the specification in question, the inventor says:—"The third part of my invention consists of a mode of converting chlorides of potassium and sodium, and of the metals of the alkaline earths, into hypochlorites and chlorates, by means of a succession of decompositions in the solution of the salt operated upon, when induced by the agency of electricity.... Electricity first decomposes the chloride, the chlorine being eliminated at one of the electrodes, and the alkaline or earthy metallic base at the other electrode.... The liberated chlorine will, when it is set free, combine with a portion of alkali or alkaline earth in the solution, and a hypochlorite will be formed. The hypochlorite thus formed will, by the continued action of heat, be resolved partly into a chlorate of the alkali or alkaline earth, and partly into a chloride of the metallic base, and the chloride will again be subjected to decomposition, and a hypochlorite formed.... If I desire to produce a hypochlorite of the alkali or earth, I merely keep the vessel warm ... and continue the process until as much of the saline matter has been converted into a hypochlorite as may be required for the purpose to which the solution is to be applied. This mode of forming a hypochlorite of the alkalies and alkaline earths may be used for preparing a bath for the purpose of bleaching various kinds of goods, and the bath may be strengthened [recuperated] from time to time by the action of the electric current."