of the preparation of chlorine from hydrochloric acid by chemical processes; the electrolytic processes will be treated hereafter. It is clear that free chlorine must be prepared from hydrochloric acid by oxidizing the hydrogen. This can be done most easily by “active” oxygen, such as is present in the peroxides, in chromic or permanganic acid. Practically the only agent employed in this way, and that already by C. W. Scheele, the discoverer of chlorine, in 1774, is the peroxide of manganese (manganese dioxide), found in considerable quantities in nature as “manganese ore” (the purest of which is called pyrolusite), and also artificially regenerated from the waste liquors of a former operation. Even now, where chlorine is required for immediate use in some other chemical operations on a comparatively small scale, it is obtained by the action of hydrochloric acid on native manganese dioxide, according to the equation: MnCl2+4HCl＝MnCl2+Cl2+2H2O. This action must be promoted by heating the mixture, but even then nothing like all of the hydrochloric acid employed is made to act as above, because the attack on the manganese ore requires a certain minimum concentration of the acid. Formerly, instead of free hydrochloric acid a mixture of common salt and sulphuric acid was sometimes employed, but this is never done on a manufacturing scale now. Owing to the impossibility of employing any metal in contact with the acid, the “chlorine stills,” where the above reaction is carried out, must be made of acid-proof stones or “chemical” stoneware. This process is very costly, as much of the acid and all of the manganese is wasted. Moreover it is of a most disagreeable kind, as the waste “still-liquor,” containing very much free hydrochloric acid and even some free chlorine, forms a most deleterious impurity when finding its way into drains or water-courses, apart from the intolerable nuisance caused by the escapes of chlorine from the stills and otherwise, which cannot be at all times avoided.
Many endeavours were made to avoid the loss of the manganese in this operation, but with only partial or no success. The difficulty was only overcome by the Weldon process, being the inventions of Walter Weldon from 1866 onwards, and his process up to this day furnishes the greater proportion of chlorine manufactured in the world. It begins with “still-liquor,” obtained in the old way from native manganese ore and hydrochloric acid. This liquor is first treated with carbonate of lime (ground chalk or limestone) in a “neutralizing-well,” made of acid-proof material and provided with wooden stirring-gear. Here the free hydrochloric acid is converted into calcium chloride, and at the same time any ferric chloride present is converted into insoluble ferric hydroxide: 2FeCl3+3CaCO3+3H2O＝2Fe(OH)3+3CaCl2+3CO2. The sulphuric acid present is mostly precipitated as calcium sulphate. The mud thus formed is settled out, and the clear liquor, which is now quite neutral and contains both manganese and calcium chlorides, is mixed with cream of lime and treated by a strong current of air, produced by a blowing-engine. This is done in a tall iron cylinder, say 9 ft. wide and 30 ft. high, called the “oxidizer.” The air-pipe goes right to the bottom of the cylinder and there branches out into perforated side-pipes, so that the mass is thoroughly stirred up all the time. The first action of the lime is to convert the manganese chloride into manganous hydrate (Mn(OH)2) and calcium chloride; then more lime is added which greatly promotes and hastens the oxidizing process. The object of the latter is to convert the manganous hydroxide by the atmospheric oxygen into manganese dioxide, but this would take place much too slowly if there was not an excess of lime present ready to combine with the manganese dioxide to form a calcium manganite. Only so much lime is used that an acid manganite is formed corresponding to one molecule of calcium oxide to two of manganous oxide. This additional lime, which is called the “basis,” certainly takes up hydrochloric acid in the next stage of the process, but that causes no more waste of acid than the incomplete action on native manganese ore, mentioned before. The product obtained, called “Weldon mud,” is of such fine texture that it acts immediately with hydrochloric acid when mixed with it in the “Weldon stills” (fig. 4), and that this acid can be almost entirely neutralized thereby. The new still-liquor formed in this manner is treated as above, so that the manganese does its work over and over again. There is only a slight mechanical loss, which is reduced in the best managed works to about 2 parts of manganese dioxide to 100 of bleaching-powder. There are also other advantages of this process which explain its wide extension, in spite of the fact that only from 30 to 35 parts of the hydrochloric acid employed is converted into chlorine, the remainder ultimately leaving the factory in the shape of a harmless but useless solution of calcium chloride.
Weldon’s later attempts at superseding his classical process by other inventions which utilize a larger proportion of the chlorine, introduced as hydrochloric acid, have not been successful in the long run, although some of them were aided by the great technical skill of A. R. Pechiney. But the Deacon process, the invention of Henry Deacon (who was greatly aided by his chemist Dr Ferdinand Hurter), carried out since 1868, has attained to better, although nothing like complete, success in that direction.
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| Fig. 4.—Weldon Chlorine Still. (Sectional Elevation.) C, Stone steam column resting in stone socket K. |
The Deacon process, like the Weldon process, effects its object by the oxidizing action of atmospheric air, but in a very different manner. Weldon retained the principle of the Scheele process by employing the active oxygen of manganese dioxide to convert hydrochloric acid into free chlorine, and he employed the atmospheric oxygen only indirectly, for the recovery of manganese dioxide from the manganese chloride formed. But Deacon worked on the direct reaction: 2HCl+O＝H2O+Cl2. This reaction in ordinary circumstances is so slow as to be practically useless. If, however, a “contact-substance” is employed and that at the proper temperature, the process goes on at an immensely quickened rate and can even be carried out as a continuous operation. The only substance which possesses sufficiently strong catalytic properties for the reaction is cupric chloride. If pieces of porous clay are soaked in a solution of this salt and dried and kept at a temperature of 450° C. (in practice it is necessary to go to a rather higher temperature), it is possible continuously to convert a united stream of hydrochloric acid and atmospheric air, passed through the contact-substance in a “decomposer” (fig. 5), to a larger extent into chlorine and water, of course mixed with the excess of oxygen and all the nitrogen of the air. On a small scale it is possible to push the decomposition as far as 90% of the hydrochloric acid, but on the large scale only at most 60% is reached. The mixture of hydrochloric acid and air is taken directly from the “decomposing-pan” of an ordinary salt-cake furnace, is first cooled down in pipes sufficiently to condense most of the moisture present
