Bleaching Powder. By the action of chlorine gas upon hydrate of lime, a compound is produced which is known by the common name of chloride of lime. By the calico printers, and others who make use of it for its bleaching properties, it is called bleaching powder. It is also known as hypochlorite of lime, chlorinated lime, etc. The compound was first prepared by Mr. Tennant of Glasgow, in experimenting upon the best applications of chlorine to bleaching purposes. He first made it in the form of the saturated liquid solution; and in 1799 he took out a patent for impregnating dry quickline with chlorine. By the suggestion of one of his partners, slacked lime, or the hydrate, was substituted for the quicklime, having the property of absorbing large quantities of the gas, which the quicklime has not. In preparing it, a pure quality of lime is required, free from iron, clay, and magnesia, the presence of which would seriously affect the bleaching process. It should also be well and freshly burned, and freed from all carbonic acid. Enough water is then to be added to it to cause it to fall into a fine white powder, which is the hydrate of lime. Chlorine is prepared by several different processes.
One of these, still common, though becoming superseded by other methods and by modifications, consists in decomposing hydrochloric acid by heating it in contact with coarsely pulverized black oxide of manganese. This substance furnishes a large amount of oxygen gas, which in mutual decomposition unites with the hydrogen of the hydrochloric acid to form water, setting free the chlorine, an atom of which takes the place of the oxygen, forming chloride of manganese, and another atom escapes. These changes are represented by the following formula, the first part of the equation being the materials employed, and the second the products obtained: 4HC1 + Mn2O2 =2H2O + 2MnCl + 2Cl. Another process consists in mixing the manganese oxide with common salt and adding sulphuric acid. The changes which are then effected are represented as follows: 2NaCl + 2H2S-O4+ Mn2O2=Na2SO4 + Mn2SO4+ 2H2O +2C1. It is important that the manganese ore should be of the purest quality, in order to obtain from it the largest quantity of oxygen gas. Black oxide of manganese when pure gives up at a white heat 33.1 per cent, of its weight of oxygen, and passes into the red oxide.
Chlorine gas is thus prepared in large alembics or stills, which are made of cast iron, where exposed to strong heat, and in part of strong sheet lead; or sometimes of stones closely fitted and cemented to each other. The lower portion is sometimes made double for the purpose of introducing steam to heat the mixture in the inner vessel. The materials introduced are in the following proportions, rated as if pure, but varying with their impurities: binoxide of manganese, 100 parts; common salt, 150 parts; and sulphuric acid, of specific gravity 1.6, about 185 parts. The temperature is kept at about 180° F., and the materials are kept in agitation by a stirrer, which is made to revolve in the lower part of the vessel. As the gas is evolved, it passes by a lead pipe to the purifier, and into the top of the chamber in which the hydrate of lime is deposited in trays, which are placed upon shelves. Heat is generated by the chemical combination; but it should not be allowed to exceed 62° F., the supply of chlorine being checked to keep the temperature down.
For two days the process goes on, when it is stopped, that the workmen may enter with half a set of trays of fresh hydrate of lime to replace an equal quantity which has been exposed four days to the action of the gas, and to stir over that which has been in two days. Half a charge is thus taken out every two days. When well made, it should be a uniform white powder, without lumps, smelling of chlorine, dissolving with little residue in 20 parts of water with alkaline reaction, and attracting moisture very slowly from the air. When prepared in a liquid state, the gas is passed into lime water, till this is saturated with it. The solution, for the quantity of lime it contains, is stronger than the dry powder, but it is not so permanent in character, the chlorine sooner escaping from it. - Mr. Tennant of Glasgow employs a method devised by Mr. C. T. Dunlop for liberating chlorine from common salt with nitrate of soda and sulphuric acid. If one equivalent of nitrate of soda and three of chloride of sodium are decomposed by sulphuric acid, nitrous acid, hydrochloric acid, and chlorine are generated. The acids are separated by passing all three of the gases successively through sulphuric acid and water.
The chlorine, not being absorbed by either the acid or the water, may be passed on into the lime chamber. The process of Mr. Weldon consists in neutralizing the residual liquor containing manganese chloride, which is produced in the ordinary process, with hydrochloric acid and manganese oxide, with finely divided carbonate of lime. This produces a neutral mixed solution of chloride of manganese and chloride of calcium, holding in suspension considerable sulphate of lime and small quantities of oxide of iron and alumina. The mixture is then pumped into settling tanks, where these substances subside, leaving the liquor clear, which is then run off into a vessel called the oxidizer. Air is forced through it and milk of lime added until the manganese in the liquor is principally converted into peroxide. This process is now extensively employed. Deacon's process, designed to obviate the use of manganese oxide, is founded on the fact that if a mixture of hydrochloric acid and oxygen is heated in the presence of certain substances, a catalytic force causes the decomposition of the hydrochloric acid, the hydrogen combining with the oxygen, while the chlorine is set free.
The gases are passed through a reverberatory furnace heated to 700° or 750° F. over pieces of brick which have been saturated with a solution of sulphate of copper, and dried. - The precise chemical constitution of chloride of lime has always been a subject of controversy, which can hardly be held as settled at the present time. Dr. Ure considered the commercial article as a mixture, in no definite proportions, of chlorine and hydrate of lime, and believed that the more definite compound prepared with dry calcium hydrate contained chlorine in direct combination with the hydrate. Fresenius regards it as a mixture of calcium chloride, CaCl, and calcium hypochlorite, CaOCl or CaC1O2; and this is the view taken by Wagner and others. These opinions, it must be borne in mind, relate to the pure, dry article, and not to the commercial one. The subject has lately been carefully investigated by Kolb (Juhresbericht, 1867), who finds that the most concentrated preparation which can be produced by saturating dry calcium hydrate with chlorine contains 38.5 per cent, of chlorine, 45.8 of lime, and 24.7 of water, in which the water and the whole of the lime are essential constituents. Commercial bleaching powder contains more water as well as free lime.
Dry chloride of lime is decomposed by water with separation of calcium hydrate and the formation of a solution containing chloride and hypochlorite of calcium. Kolb, reasoning from the fact that dry bleaching powder and the solution comport themselves differently under the influence of free chlorine and heat, thinks that the first does not contain a ready-formed hypochlorite, but is a compound which may be represented by the formula Ca3H6O6Cl4. Dry chloride of lime, moreover, is completely decomposed by carbonic acid with evolution of chlorine, while only half the lime is precipitated from the solution by this agent, with separation of hypochlorous acid, which does not act upon the remaining chloride. Solid chloride of lime in moist air behaves in the same way, from which it appears that bleaching powder, on exposure without the addition of an acid, yields hypochlorous acid and not free chlorine. For the determination of the available amount of chlorine in a given quantity of bleaching powder, see Chlorimetry.