Modern Gasworks Practice

552 MODERN GASWORKS PRACTICE sulphocyanide (NH4SCN) or sodium ferrocyanide (prussiate of soda, Na4FeC6N6). Ultimately the sulphocyanide or prussiate is converted into sodium cyanide for gold working processes; and, so far as this operation is concerned, recovery as prussiate is to be preferred. Per se, the extraction of cyanogen as a distinct hy-product will, in normal times, only prove profitable in the case of the larger gasworks. Indirectly, however, the removal of the compound from the gas presents many advantages, and if it could be generally extracted and turned to profitable account the task of the gas engineer would certainly be made easier. Apart from the fouling of gasholder water, and. the corrosion of gasholder plates and other apparatus, the removal of the cyanogen is strongly to be urged, owing to its eSect on the efficiency of the present-day purifying material. It is beyond all question that oxide of iron, whether natural or artificial, will remain in an active condition for a much greater length of time when the cyanogen is absent. Both Bueb and Guillet have shown that in eome cases the efficiency of the “ dry ” purifiers may be increased. by so much. as 50 per cent, if the compound, is extracted. Perthuis has pointed out tliat in certain circumstances cyanogen. is responsible for a reaction in the purifiers which may cause the material in them actually to increase the sulphur impurities in the gas. On works where 110 recovery plant for cyanogen is in use the bulle of the substance will be removed by the oxide or lime purifiers, although some slight reduction will take place during the passage of the gas througli the wet purification plant. No definite statement can be made as to th.e amount of cyanogen which will pass through, the oxide boxes. In many works the gas at the outlet of the purifiers shows only a trace, wliile in other cases as much. as 8 to 10 grains of hydro-cyanic acid per 100 cubic feet of gas will be found. The cyanogen, by combining with the iron salts, may be absorbed in the purifiers as ferrocyanides. In this way a portion of the active material is rendered inert so far as the absorption of sulphiiretted hydrogen is concerned. Very much depends, however, upon whether the prevailing conditions are acid or alkaline. If formed, Prussian blue, by coating the granules of the oxide, exerts an effect far greater in proportion than the actual extent in which it is present. Sperrt oxide with a content of 25 per cent, of sulphur has been found to contain as much as 121 per cent, of Prussian blue, although in normal cases the amount of the latter present will not exceed per cent. As regards the formation of Prussian blue in the purifying boxes the reactions taking place are by no means simple ; but, when conditions per mit, there seems little doubt that the ferric-ferrocyanide is the result of oxidation of a ferrous cyanide which. is formed in the first instance. Thus the hydrocyanic acid reacts with the oxide of iron :— Fe2O3 + 4 HCN = 2 Fe(CN)2 (ferrous cyanide) + 2 H2O + O, and by oxidation gives 18Fe(CN)2 + 3 O2— 2Fe2O3 + 2 Fe4,3 Fe(CN)6 (Prussian blue). Many different opinions have been expressed as to the probable reactions account-ing for the Prussian blue, and although the above seems reliable, it is open to question. Again, there is a likelihood tliat the hydrocyanic acid does not react with the active