Modern Gasworks Practice

566 MODERN GASWORKS PRACTICE THE LIME METHOD A second successful method 1 is based on lines somewhat similar to those of Bürgevin, and is particularly applicable in cases where hydrocyanic acid is recovered as ammonium sulphocyanide, and finally as calcium sulphocyanide, as is the case in the polysulphide cyanide process. The calcium sulphocyanide produced is first evaporated to dryness, and is then mixed with an excess of slaked lime, and sub-jected to a moderate heat of not less than 540° C. Under the influence of this temperature the cyanides are broken up in the presence of steam, and yield ammonia and carbon oxysulphide. The ammonia may be drawn off to an absorption vessel containing sulphuric acid, while the carbon oxysulphide will be mostly removed by the excess of lime. The männer in which the lime and cyanide are admixed is of uo little importance for the effective working of the process. With regard to the actual reactions taking place, these appear to be on lines such as the following:— (a) Ca(SCN)2 + Ca(0H)2 + 2 H20 = 2 N'IIti + COS + CaCO3 + CaS. The precaution is taken of removing the carbon oxysulphide evolved by arranging for a layer of lime 2 or 3 inches deep to be placed over the mixture in the Hearing chamber. The carbon oxysulphide is then removed as follows :— (&) COS + 2 Ca(0H)2= CaS + CaCO3 + 2 H2O. The removal of the carbon oxysulphide in this way is essential; for ammonia and this compound may combine, with the possible formation of urea, according to the following equation :— COS +2 NH3= H2S + C0(NH2)2. So faf as the respective merits of the two more modern processes are concerncd, there is little to choose between them on the ground of efficiency—both giving a conversion of about 98 per cent. For a works having its own sulphuric acid plant, however, or in cases where supplies of acid can be readily obtained, the sulphuric acid method is undoubtedly to be preferred. In the lime method, use miglit con-ceivably be made of the waste h.eat from the retort bench.es, or one or two retorts could be set apart for the special cyanide purpose. Furthermore, if it was not desired to recover the evolved ammonia there and then, it could be passed into the foul main along with the gas, when, by absorption of sulphuretted hydrogen and carbonic acid, it would lighten the work of the ordinary purifying vessels. From the financial standpoint there is nothing particularly alhiring about the conversion of cyanides into ammonia. It must be remembered, however, that the indirect advantages accruing from the employment of a cyanogen plant are the chief recommendation of the process. As before stated, the universal extraction of cyanogen is scarcely feasible at the present time, owing to the restricted market for this residual and the growing tendency to produce it by other means. 1 Williams, Eng. Pat. 2841, 1914.