Modern Gasworks Practice

THE RECOVERY OF CYANOGEN 565 compounds are caused to react at red heat. The reaction is eutirely completed at this temperature, and the gaseous mixture liberated will consist chiefly of free ammonia. If care is taken to see that the mixture is homogeneous, it is said that at least 95 per cent, of the original nitrogen in the cyanide compounds can be recovered in the form of ammonia. The approximate quantity of lime required for effecting the transformation is ten times (by weight) of the amount of nitrogen contained in the original material to be treated; and it is suggested that the ammonia should be converted into ammonium sulphate by passing the gas through a seal of sulphuric acid. The method also allows for the partial conversion into ammonia of the small proportion of nitrogen compounds, other than cyanides, which. is to be fotind in purifying Materials. It is well known that the action of sulphuric acid upon potassium sulpho-cyanide gives tliiocyanic acid— KCNS + H2SO4 = HCNS + KHSO4, the latter then being decomposed by water, in the presence of an excess of sulphuric acid, into carbon oxysulphide and ammonia— HCNS + H2O = NH3 + COS, the ammonia, at the same time, combining with the sulphuric acid. So far as the hydrolysis of ammonium sulphocyanide is concerned the South Metropolitan Gas Company and E. V. Evans 1 have introduced a process which consists essentially of treating the sulphocyanide solution with. sulphuric acid of ■definite strength and vohime at an elevated. temperature. The reactions involved are as follows :— («) 2 NH4SCN + H2SO4 = 2 HSCN + (NH4)2SO4. (&) 2 HSCN + 2 H2O = 2 COS + 2 NH3. The ammonia produced combines with the sulphuric acid, which is in large excess, and the acid liquor obtained may be taken to a sulphate of ammonia satur-ator. The carbon oxysulphide evolved is dealt with. in a Claus kiln, and the sulphur from it is thus recovered. The process would appear to be som.ewh.at delicate, o wing to the faet that, if the working temperature is carried too high, the cyanogen is likely to be volatilized, and lost in the form of thiocyanic acid. Accordingly, temperature regulation plays an important part; but, working under good conditions, a conversion into ammonia of 98 per cent, of the total nitrogen should be possible. In addition to carbon oxysulphide being evolved, it seems probable' that some sulphuretted hydrogen results from the reaction. This is most likely, owing to the faet that carbon oxy-sulphide is freely absorbed by ammonia, and, on evaporation, yields sulphuretted hydrogen, or possibly urea. It is feasible, too, that a portion of the carbon oxysulphide may split up in the presence of water and steam, and yield carbon dioxide and sulphuretted hydrogen, according to the equation— COS + H2O= CO2 4-H2S. 1 Eng. Pat. 29047, 1913.