Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/66

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ACID

AND

ALKA: -I MANUFACTURE 2NaCl + S02 + O + H20 = Na2S04 + 2HC1, without the sulphur is recovered in a much more valuable form than intervention of lead chambers. This process, which re- it originally possessed in the shape of pyrites, and the quires complicated and costly plant, has been most remaining 15 per cent, as well as the solid residue of the ingeniously elaborated by the inventors, and has been operation are at least rendered innocuous. made a perfect success for the manufacture of very pure In spite of these great improvements the Leblanc sulphate of soda, also for the complete condensation of the process cannot economically compete with the ammoniaHC1, but the latter is less easily obtained in the strong soda process, principally for two reasons. The sodium in state possible with muffle furnaces, owing to the dilution the latter costs next to nothing, being obtained from of the gas with nitrogen, &c. (comp. Uncy. Brit. vol. xxii. natural or artificial brine in which the NaCI possesses an p. 242). extremely slight value. The fuel required is less than The manufacture of sodium sulphate and hydrochloric half the amount used in the Leblanc process. Moreover, acid has been put on an entirely different economical the process has been gradually elaborated into a very basis through several great changes in the alkali industry, complicated and costly, but perfectly regularly working which have checked the further expansion of the Leblanc scheme, in which the cost of labour and the loss of process. This process, which formerly absorbed most of ammonia are reduced to a minimum. The only way in the salt-cake produced, is giving place more and more to which the Leblanc process could still hold its own was by the ammonia - soda process, which starts directly from being turned in the direction of making caustic soda, to sodium chloride. Outside Great Britain the Leblanc which it lends itself more easily than the ammonia-soda process now plays a very subordinate part, and in Great process; but the latter has recently invaded even this Britain itself it will certainly not spread any farther field. One advantage, however, still remained to the and will probably ultimately succumb, although this may Leblanc process. All endeavours to obtain either hydrobe in a somewhat remote future. The only other con- chloric acid or free chlorine in the ammonia-soda process siderable outlet for salt-cake is the manufacture of glass, turned out economical failures, all the chlorine of the which naturally cannot be indefinitely extended. These sodium chloride being ultimately lost in the shape of circumstances, by restricting both the quantity and still worthless calcium chloride. The Leblanc process thus more the value of the sodium sulphate, necessarily con- remained the sole purveyor of chlorine in its active shapes, ferred a greater value upon the hydrochloric acid manu- and in this way the fact is accounted for that, at least in factured along with it and as this acid was until recently Great Britain, the Leblanc process still furnishes nearly indispensable for the production of chlorine, the enhanced half of all the alkali made, while in other countries its price of bleaching-powder and chlorate of potash for a time proportional share is very much less. The profit made compensated the manufacturer for his loss on the salt-cake, upon the chlorine produced has to make up for the loss and thus again reacted favourably on the economical on the alkali. aspect of the Leblanc process. Much more attention is But the position still held by the Leblanc process is now devoted than formerly to the preparation of the seriously threatened by the development of the electrolytic hydrochloric acid in a strong state, and to processes decomposition of the alkaline chlorides, which has taken intended to make it yield a greater amount of chlorine. place principally since 1890. These processes possess some The recent development of the electrolysis of the alkaline of the advantages of both the old processes. They start chlorides necessarily again decreases the value of the with the cheapest form of NaCl, viz., brine; and they at hydrochloric acid obtained in the manufacture of salt-cake, once furnish the most valuable form of alkali, viz., caustic and must in the end lead to a further restriction in the soda, and at the same time either free chlorine, or, directly, production of the latter. bleaching-liquor of chlorates. Up to a certain stage they The Manufacture of Alkali.—The term “alkali” is require no fuel, except for the production of energy, which usually applied to several sodium compounds, viz., the may be furnished by the cheapest kind of motors,—best of hydrate (caustic soda), the carbonate (soda-ash), and the all, by water-power. Only for the conversion of the strong bicarbonate. The corresponding potassium compounds solution of alkali obtained in the electrolytic cell into a are almost exclusively made in Germany, from Stassfurt solid form is some fuel indispensable. The great diffisalts, and in France, from the “ vinasse ” of beet-root culties at first experienced with electrodes and diaphragms manufactures. The alkali manufacture in this sense is do not seem to exist any more ; and so far as the producintimately connected with the chlorine industry, and both tion of chlorine compounds is concerned, there seems to be have for a long time formed the most important of British nothing to stop the triumph of electrolysis over the old chemical industries. In vol. xxii. p. 242 a description processes, which were described in vol. v. p. 678. We has been given of the two principal forms of the soda must not omit to notice that the electrolysis of sodium manufacture, viz., the Leblanc process and the ammonia- chloride and the manufacture of alkali in a commercial soda process. The Leblanc process was for a long time shape are decidedly more difficult than the corresponding that by which nearly all alkali was made. During the operations in the case of potassium chloride. But last decade many improvements have been effected in all to the inroads of electrolysis into the domain of alkali its stages, principally by the development of mechanical manufacture, there is one natural limitation brought about furnaces and by attention to details j in this connexion by the fact that this process produces for each 40 parts Dr Hurter s name should be especially mentioned. Its of NaOH, equivalent to 53 parts of Na C0, 35'5 parts of 3 greatest drawback, viz., the loss of the sulphur originally chlorine, which yield about 100 parts of2 bleaching-powder, introduced into the process, and formerly only imperfectly whereas the present consumption of chlorine products is and at not sufficiently small cost recovered from the in the proportion of about one part of bleaching-powder “tank-waste” by the processes of Mond and Schaffner, (or its equivalent of chlorates) to five parts of alkali. has been overcome by the Claus-Chance sulphur recovery Manifestly only that quantity of alkali can be made which process. This consists in decomposing the calcium sulphide corresponds to the sale of the chlorine products necessarily of the tank-waste, which had been previously reduced to made at the same time, and this means that about ninea thin watery paste, by means of impure carbonic acid in tenths of the alkali consumed must be made in other the shape of lime-kiln gas, and converting the resulting ways. As far as we can see, the ammonia-soda process H.2S either into sulphuric acid, or, preferably, into a very will supply the principal portion of the great remainder, pure form of free sulphur. Thus 85 per cent, of the I but for some time the Leblanc process will still contribute