Modern Gasworks Practice

THE RECOVERY OF CYANOGEN m material (Fe2O3), but with the iron sulphide, and the experiments of Leybold would appear to corroborate this, the mode of reaction suggested being as follows :— FeS + 2 HCN= H2S + Fe(CN)2, the ferrous cyanide being finally oxidized to ferric-ferrocyanide as previously shown. The chicf point to notice is that moisture or the admission of steam to the boxes will aid the formation of Prussian blue whilst free ammonia will prevent it. If, therefore, a small quantity of ammonia (about 1 grain per 100 cubic feet) is allowed to travel forward in the gas from the serubber the formation of Prussian blue in any quantity will very seldom follow. The same effeet, however, is not seen. when an alkali such as lime is mixed in small proportion with the oxide in the purifiers, owing to the lime being quickly carbonated by the CO2 coming forward with the gas. W. S. Curphey1 states that hydrocyanic acid is undoubtedly prejudicial to the efficient working of purifiers; apart from any direct action it may have on sulphides of iron, liberating sulphuretted hydrogen, it reacts with oxide of iron in the presen.ee of ammonia to form insoluble iron ferrocyanides and double ferro-cyanides of iron and ammonium as well as ammonium sulphocyanide. If such reactions occur the oxide of iron is rendered inert, the reactive residue is coated over with insoluble bodies, and is rendered less porous. Moreover, th.e ammonia needed to neutralize the ferrous sulphate and to maintain the condition of mild alkalinity essential for efficient purification is fixed and prevented from performing its real funetion. Curphey concludes that excess of moisture is the factor which favoiirs the interaction of hydrocyanic acid and ammonia to form ferrocyanide and sulphocyanide. Thus, in the presence of moisture,* ammonia and hydrocyanic acid are dissolved and brought into intimate contact with. iron oxide and iron sulphide on the one hånd, and with sulphuretted hydrogen and free sulphur on the other, and at a slightly raised temperature. Accordingly, Curphey suggests that the following reactions occur :— 3. -—- LO _ o> æ MM + o ro t-o cc æ 1+ M + to o æ tz; qJL 1 II M W to izj æ hj ® ro SS o A a B + W to N> '-v Cß Jzj K to THE RECOVERY OF CYANOGEN FROM THE GAS As already stated, the cyanogen recovered as a by-product in the gasworks of this country is, without exception, removed from the gas alone. On the Continent the spent oxide is more frequently treated for this purpose ; but, in general, the dass of coal employed gives higher yields of cyanogen than do Englisli coals. Of the more important processes which have been adopted in this country the following 1 Alkali Works Report, 1915, p. 26.