Modern Gasworks Practice

554 MODERN GASWORKS PRACTICE may be mentioned, altliough some have been discarded in favour of more modern meth ods:— (a) The Foulis process (prussiate of soda). (b) Wilton’s process (prussiate of soda). (c) The British Cyanides Company’s process (sulphocyanide). (cZ) The Davis-Neill process (prussiate of soda). (e) The polysulphide process (sulphocyanide). Other processes of interest are those of Ciselet and Deguide, and of the South. Metropolitan. Gas Company in collaboration with the Thann Chemical Manufacturing Company, of Alsace, and that of the Bartlett-Hayward. Company of America. The Foulis Process The Foulis process was patented in 1892 and first put into operation at the Glasgow gasworks. Chloride of iron is made by dissolving scrap iron in liydro-chloric acid in large vats, preferably made of slate. The iron. cliloride is tlien mixed with the requisite quantity of sodium carbonate so that iron carbonate results— FeCl, + Na2CO3 = FeC03 + 2 NaCl. The iron carbonate is precipitated and the sodium chloride is run to waste. The iron carbonate is then removed to a further tank, where it is mixed with more sodium carbonate in the necessary theoretical quantities. The tank is provided with. a stirrer and has an outlet leading into the first bay of a suitable washer-scrubber. The mixture is run direct into the washer and, combining with. the hydrocyanic acid in the gas, is converted into sodium ferrocyanide, which. is run away from the final bay as quickly as it is produced— FeCO3 + 2 NaaCO3 + 6HCN — Na4Fe(CN)fi (sodium ferrocyanide) + 3 C02 + 3 H2O. The crude ferrocyanide is evaporated nearly to dryness in paus, and when cool—and, consequently, solid—it is wheeled to the stores. In this condition it contains about 75 per cent, of sodium ferrocyanide. Oxidation of the FeCCE can be prevented by mixing in one bay of the washer-scrubber and by adding the whole of the sodium carbonate at one time. Wilton’s Process In Wilton’s process cyanogen is arrested in the process of scrubbing by adding to the rotary washer-scrubber an iron salt. It is more or less essential that the liquor in the washer-scrubber should be strong, i.e. ab out 16 oz., otherwise the whole of the hydrocyanic acid will not be arrested. Ammonium ferrocyanide is formed as foliows :— FeS04 + 6 HON + 6 NH40H = (NH4)4Fe(CN)6 + (NH4)2SO4 + 6 H2O, and is afterwards converted into Prussian blue by the addition of a fu.rth.er quantity of iron.— 3 (NH4)4Fe(CN)6 + 2 Fe2(SO4)3= Fe4,3 Fe(CN)6 + 6 (NH4)3804.