Modern Gasworks Practice

558 MODERN GAS WORKS PRACTICE as the oxide takes up sulphuretted hydrogen, which. is eventually oxidized to sulphur—a continuous supply of free sulphur is automatically provided. The pro-cess would, in faet, be continuous but that the physical condition of the spent oxide necessitates periodical removal from the vessel and the substitution of a fresli layer. The period for which the material will remain in action varies under normal con-ditions between four and eight months. Six vessels now in operation at one gasworks have shown themselves capable of extracting practioally the whole of the hydro-cyanic acid over a period of six months without requiring remaking. The cyanide boxes are usually made either 20 or 25 feet square by 6 feet deep. The former size is capable of dealing efiectively with lj million cubic feet of gas per diem, whilst the latter will take up to 2 million cubic feet. For larger volumes of gas the boxes are operated in parallel, although, when possible, it is preferable to arrange for two purifiers to be worked in series, so that the gas travels upwards through one and downwards through, the second. Figuros obtained in actual practice show that one ton of spent oxide is capable of removing the whole of the cyanide from 350 tons of coal. One of the chief recommendations of the method is the männer in which. the duties of washers. scrubbers, and dry purifiers are lightened. Of the ammonia contained in the crude gas entering the plant an average of from 30 to 35 per cent, is absorbed, the sulphuretted hydrogen shows a reduction of 35 per cent., whilst carbon dioxide falls off by from 5 to 12 per cent. No claim can be made for the absorption of carbon disulphide. It is essential to remember th.at tlicro must be a small proportion of oxygen m the inlet gas. In general, this proportion varies between 0-2 to 0-6 per cent. ; and by oxidizing the iron sulphides in the purifier—gives rise to the free sulphur which is vital to the efEective working of the process. The cyanide liquor flowing away from the plant varies in strength from 2J to 3 Ib. of ammonium sulphocyanide per gallon, and (after conversion to calcium sulphocyanide) it may be concentrated to any desired strength. In some instances the ammonium sulphocyanide is treated direct with. soda ash, in which case the product obtained is sodium sulphocyanide, having a strength of about 4 1b. of the latter compound per gallon of liquor. The Davis-Neill Prussiate Process The Davis-Neill process was first introduced at the Linacre works of the Liverpool Gas Company. The hydrocyanic acid is extracted from the gas by washing with a solution of ferrous carbonate and soda. The first step in the process is the preparation of the ferrous carbonate, which. is made by mixing copperas (sulphate of iron) with ordinary soda ash (carbonate of soda)— FeS04 + Na2CO3 = FeCO3 + Na2SO4. The mixture of ferrous carbonate and sodium sulphate is then passed through th.e first filter-press, in which the ferrous carbonate is retained as cakes. After thorough. washing with clean water the cakes are removed to a second mixing tank or agitator, where thfey are intermixed with a further quantity of soda ash. The