BLEACHING POWDER. (FR., Chlorure ; GER., Chlorkalk, Bleichpulver.)—Synonym, chloride of lime ; formula, CaC1202 CaC12.
The exact nature of bleaching powder remains still a matter of doubt. It is sometimes regarded as a simple compound of chlorine with lime—whence its name—sometimes as an oxychloride of calcium, sometimes as a mechanical mixture, or as an absolute compound of oxychloride and hypochlorite of lime. For the various discussions upon this part of the subject, the scientific reader is referred to the writings of Millon, Fresenius, Kolb, Crace-Calvert, Schor lemmer, and a host of others.
Bleaching powder, as ordinarily manufactured, is a dull white •powdery substance, often agglomerated into small round lumps readily friable between the fingers. It always contains a certain amount of free chlorine, which imparts a, strong pungent odour, rather agreeable than otherwise, unless too powerful. A small percentage of moisture usually present keeps it feathery, and it readily absorbs a further amount of water when exposed to the influence of the atmosphere, finally turning into a dirty white paste. Under the influence of strong light—especially sunshine —decomposition takes place, with the formation of clilorate of calcium. The bleaching property of the compound is owing to tho presence of chlorine, the most powerful bleaching agent known. The available amount of this gas contained in a sample of ordinary bleaching powder is about 36 per cent. Beyond this about 4 per cent. goes to form chloride and chlorate of calcium. Of the 36 per cent., probably 4 per cent. as a rule is free and disengaged during the various manipula Hone preceding the actual bleaching process. For this reason, it used to be eustomary to sell a 32 per cent. bleaching powder, and probably an article of this constitution would be as good for the consumer, and in every way Letter for the manufacturer, than the bleaching powder usually put upon the market. In tbe endeavour to secure a high strength, both loss and damage is caused, by the disengagement of free chlorine gas and the prolonged process of manufacture.
The art of bleaching is of very great antiquity, and until a comparatively recent date consisted of alternate treatments of the substance operated upon by various alkaline washings and exposure to the action of the sun and air. In this way, the oxygen of the air formed some combination with the colouring matter which could be extracted by water or an alkaline liquor. The use of chlorine was first suggested by Berthollet in 1785, and within a few years afterwards the process was worked upon a considerable scale in Scotland. The gas was generated in a glass or wooden appa ratus by heating a mixture of salt, peroxide of manganese, and sulphuric acid, and passed into water. When a saturated solution was obtained, it was removed, and the goods bleached by being immersed in it and thoroughly boiled. The glass or wooden vessels speedily gave way to an apparatus constructed of strong sheet lead, encased in a metal jacket, with an agitator to effect a perfect mixture of the ingredients. Heat was applied by an underneath fire, or by a steam pipe introduced between the lead and the outer shell. This improved apparatus was used extensively for something like fifty 'years, an ordinary charge consisting of 120 parts of manganese;to 150 of salt, and 185 of strong sulphuric acid. The chlorine gas was taken off by lutes and pipes in a manner very similar to that to be presently described.
The addition of an alkali to the water—at first caustic potash—was made about 1792, and, in 1798, Charles Tennant, of Glasgow, patented the use of lime, to bo employed as lime water. In this way, the article known as " bleaching liquor," still manufactured occasionally for paper makers, was obtained. In 1799. the absorption of chlorine by dry
lime was patented, and the commercial article now known aa " bleaching powder " introduced.
The process of manufacture, as at present carried on, is as follows :—Instead of mixing together the salt, manganeae, and sulphurio acid, the liquid hydrochloric acid obtained by condens ing the gases from the sulphate of soda process, in the manner fully set forth in a previous article (see Soda), is employed, and run upon a known weight of peroxide of manganese contained in a " still." The best form of still is shown in Figs. 316, 317, and 318, and will be readily understood. The buttom consists of one slab of stone (good free-stone, or, better still, " Yorkshire flag "), not less than 10 in. in thickness. Into it are sot sides of similar material, about 6 in. thick. The groovcs into which the aides are let are about 1 in. in depth, aud a very little wider than the side, so ae to allow of " stemming.' At the four corners, a, b, d, Fig. 317, a diamond is cut and thoroughly stemmed with dry fire clay, fireclay just moistened with tar and heated, red lead and glass, or any other suitable com pound. Sometimes the indiarubher rods described when speaking of hydrochloric acid condensers are used, but this construction is apt to be faulty. The whole still is securely bound together with 1 in. iron rods, which pass through the ends, and are acrewed up against upright pieces of wood, Fig. 317. The cover of the still is formed of three stones, bedded in a mixture of tar and china clay. ln it are various openinge—a square manhole in the centre, through which the charge of man ganese is also introduced, and round holes, to admit of steam column, acid, gas, and " dip" lutes. A false bottom—technically " tables "—is formed about 9 in. from the bottom stone, with stout alabs, about 12 in. wide and 5 in. thick, roughly dressed and resting upon stone or brick supports, running along the sides of the still, Fig. 316. Steam is introduced through a small iron pipe, protected from the action of the acid and gas by the stoneware column shown in Figs. 319, 320, and 321. This is aet at the back of the still, and has an opening only underneath the tables. The hydrochloric acid is run in through the lute shown in Fig. 322, let into the cover near the steam column, while the " dip lute," shown in Fig. 323, likewise set in the cover, alkwa the workman from time to time to gauge the amount of acid he is running in. The stills are built in a range, and incline about 4 in. towards a gutter along the front, which conveys away the waste liquors, &c. A good size of still is 9 ft. by 6 ft. by 4 ft. 6 io. deep, the cost, complete, being about NU A charge of manganese, about 6 cwt., is thrown into the still, and roughly spread upon the tables. The door is then replaced, and made tight with any convenient method of plastering, and the still is connected by the gas lute with the bleaching-powder chamber. Hydrochloric acid, of not less than 18° Twaddell, is then run in from the stock cistern, until the manganese is just covered. After being allowed to stand for a short time, good "strong " steam is introduced, at a boiler pressure of 45 lh., and kept at full blast for a quarter of an hour. A rapid disengagement of chlorine takes place, the gas passing away to the chambers, and a mixture of free hydrochloric acid, chloride of iron, and chloride of manganese is left in the still. Successive blasts of steam are pressed into this mixture from time to time until the operation is complete. The " bend " of the gas lute is then removed, and the contents of the still raked out, through a small opening in front, into the gutter. The reaction occurring is represented by the following equation :— MuO, 4HC1 = MnCl, 2H20 +012.