Modern Gasworks Practice

THE RECOVERY OF CYANOGEN 551 The process, however, is not so simple as might be supposed from the above brief outline, and many modifications have been introduced. from time to time with. the obj eet of rendering the work more economical and of inereasing the efficiency of recovery. In addition to being employed for the treatment of gold-bearing ores, cyanides find a limited use in the manufacture of dyes and paints, and in electro-plating. CYANOGEN IN COAL GAS The cyanogen in coal gas is almost wholly in the form of hydrocyanic acid, although some authorities say that it is also present as free cyanogen and ammonium cyanide. Hydrocyanic acid. is produced by the combination of hydrogen, carbon, and nitrogen, at high, temperatures in the retort, and makes its appearance among the products at temperatures considerably higher than those at wliicli primary ammonia is at a maximum (540° C.). It is probable that a portion of the ammonia itself is split up into its elements, hydrogen and nitrogen, these re-combining, with. the addition of the carbon atom, to form hydrocyanic acid (HCN). As regards the original nitrogen in coal, it has been shown that less than 2 per cent, of this goes to form cyanogen, the quantity of the latter considerably inereasing when high and extreme temperatures of distillation are employed (see page 386). The most favour-able temperature in the retorts for the formation of cyanogen is said to be 1,200° C., and the maximum amount recoverable from a ton of English coal is about 1| Ib. of hydrocyanic acid, or about 10,000 grains. Butterfield1 states that German gasworks alone have yielded in recent years. approximately 10,000 tons of ferrocyanide per a.nnum, while the output of the British gasworks has not exceeded 5,000 tons. Were cyanogen recovery to be ex-tended to all gasworks in this country, the annual output would amount to 24,000 tons of ferrocyanide or the equivalent in other cyanide products. Apart from any profit attached to the process (and th.erc is no margin for extravagance in working expenses), it may be said that the removal of the hydrocyanic acid from the gas is folio wed by many indirect ad vantages not actually to be measured in terms of money, for the product is responsible for derangements in dry purification, and. for the corrosion of steelwork, more particularly gasholder sheeting. Cyanogen compounds may be found:— (a) In the gas, chiefly as hydrocyanic acid. (b) In the ammoniaeal liquor, mainly as ammonium cyanide, ferrocyanides, and sulphocyanide. (c) In spent oxide, chiefly as “ Prussian blue ” (ferric-ferrocyanide). (<Z ) In spent lime, as calcium sulphocyanide, and to some extent as calcium ferrocyanide. (e) In spent liquor from the sulpliate plant, as ferrocyanide compounds. The chief processes nowadays, in this country at any rate, are tliose by means of which the cyanogen is extracted from the gas and recovered as eith.er (a) ammonium. 1 Gas World, 1918, 62, 463.