Modern Gasworks Practice

THE RECOVERY OF CYANOGEN 567 If a coal yields 5 Ib. of calcium sulphocyanide per ton, this quantity of sulpho-cyanide would give about Ib. of sulphate of ammonia, thus increasing the total yield of sulphate of ammonia from an average of 27 Ib. per ton of coal to 31| lb., or by about 15 per cent. __ __ . Attention may be drawn liere to a process in connection with cyanides which has been introduced by Devaucelle.1 Briefly, it consists in heating a mixture of carbon and a cyanized. mass in a retort through which a gas rich. in nitrogen is passed, while at a later period steam is blown through the retort for the purpose of producing ammonia. H. Baker 2 is responsible for a process whereby the sulphocyanides extracted from coal gas are worked up into insoluble sulphocyanides by th.e addition of metallic compounds. The insoluble cyanides are roasted with or withoui; air, when without the presence of air—ammonia is obtained. Th.e sulphide of the metallic compounds remains in the retort, and this is again roasted in a current of air or oxygen so as to decompose the sulphide into oxide, with the evolution, of sulphur dioxide. On the other hånd, if the sulphocyanides are roasted in the first place in the presence of air, ammonia and sulphur dioxide are obtained direct, and the oxide remaining may be used again for precipitating the cyanides. Baker states that on decom-posing the cyanides in the presence of air the ammonia is evolved first and the sulphur dioxide at a later stage. Therefore, it is possible to recover the two gases separately. The process refers principally to the recovery of cyanides from spent oxide of iron and the effluent liquor from sulphate of ammonia plants. __ After the ammonia has been evolved from the liquor soda ash is added to it, and it is boiled to concentration. The liquid is then of a blood-red colour owing to the presence of undecomposed. sulphocyanides, and precipitation. of the latter is effected by the addition of metallic compounds as stated above. TESTING COAL GAS FOR CYANOGEN The gas to be tested for cyanogen is passed through two wash-bottles of about 250 to 300 c.c. capacity, each containing 100 c.c. of ammonium polysulphide solution (made by shaking up a 10 per cent, solution of ammonium sulphide with pow-dered sulphur). A little (say 10 grains) of powdered sulphur is also put into each wash-bottle, to ensure the presence of an excess. The gas should be passed through the wash-bottles at the rate of about 2 cubic feet per hour, and the test should, pre-ferably, be continued for sixteen hours or more. The amount of gas passed should be determined by a small meter. At the conclusion of the test, the contents of the wash-bottles are transferred to a litre flask, and, with the washings made up to 1,000 c.c., 50 c.c. (or such aliquot portion as represents from 1 to 2 cubic feet of gas) are trans-ferred to a small porcelain basin, and evaporated on the water-batli down, to about 10 c.c. Some hot water and a little moist carbonate of lead are added (to remove all sulphides), and the whole filtered. The filtrate is slightly acidified with. dilute sulphuric acid, a solution of sulphurous acid added until the liquor smells strongly o 2 Eng. Pat. 12946, 1915. 3 Eng. Pat. 16879, 1917.