Modern Gasworks Practice

564 MODERN GASWORKS PRACTICE The cakes from the press are afterwards thoroughly pulverized and sifted, mixed with lime, and finally treated again in a stilling tank. The resultant liquor con-tains chiefly calcium ferrocyanide and some calcium sulphocyanide. The next step is to precipitate the ferrocyanides from the solution, so that they may be recovered. This may be carried out in several ways, that is by iron, ammonia, or potassium salts. When utilizing ammonia the cyanide solution is treated with. hydrochloric acid, the acid being added until the mixture is acid. In this way a double salt, calcium-ammonium ferrocyanide, is obtained— Ca2Fe(CN)6 + 2 NH4C1= Ca(NH4)2Fe(CN)6 + CaCl2. The double cyanide may then be admixed with a further quantity of lime, when a pure calcium ferrocyanide results— Ca(NH4)2Fe(CN)6 + Ca(OH)2 = Ca2Fe(CN)6 + 2NH3 + 2H2O. Finally, the calcium ferrocyanide may be converted into potassium ferrocyanide, When potassium Chloride is used for precipitation a double salt is again formed, in the first instance— Ca2Fe(CN)6 + 2 KCl = CaK2Fe(CN)6 + CaCl2. This salt is tlien separated by filtration, washed, and treated with potassium carbonate, when potassium ferrocyanide results— CaK2Fe(CN)6 + K2CO3= CaCO3 + K4Fe(CN)6. CONVERSION OF CYANIDES INTO AMMONIA Although the hydrocyanic acid in coal gas may now be recovered by com-paratively simple means, universal extraction is not altogether possible, owing to the limited demand for the product. For this reason renewed attempts have been made to overcome the market difficulty by Converting the cyanogen into ammonia, for which there is always a steady demand. The idea is by no means novel, for mention is made of the process in scientific literature published scme seventy-five years ago, whilst a patent specification for another means was filed in 1882. As regards more recent investigation into the possibility of converting cyanides into ammonia, mention must first be made of the work of Henri Burgevin and Burk-heiser. The former conducted his experiments with the cyanide compounds present in spent purifying material. Ammonia can be extracted from these by dry dis-tillation, or by treatment with steam under pressure, followed by the use of alkalies, or by employing a mixture of soda and lime. Such processes are, however, unsatis-factory on a manufacturing scale, and Burgevin has overcome the drawbacks of using a costly reagent, soda; whilst by his method the incomplete nature of the reaction, is avoided. The employment of lime alone is sufficient, while a recovery of practically the whole of the nitrogen is claimed. According to the patent specifi-cation, when cyanide compounds are recovered. by means of raw purifying material, the latter must flrst be treated with. some suitable solvent (such as carbon disul-phide) in order to ensure complete absence of free sulpliur. The residue is then thoroughly intermixed with. slaked lime in the correct proportions, and the two