Modern Gasworks Practice

THE RECOVERY OF CYANOGEN 563 In a still more recent method, he shortens the series of reactions by introducing the iron already in combination with the cyanogen. Thus :— 3 Fe(CN)2 + 6 Ca(OH)2 + 12 HCN = 3 Ca2Fe(CN)6 + 12 H2O. In order to prepare pure cyanides, Feld first wash.es coal gas with hot magnesia solution to remove carbonic acid, then removes sulphuretted hydrogen by means of a solution of lead, iron, or manganese and then wash.es the gas with a cold solution of magnesia. This solution takes up all the hydrocyanic acid and gives up the whole of it when boiled. The hydrocyanic acid thus obtained is pure, and can be used for the direct preparation of cyanides. Ä recent process which has been commer-cially taken up in Germany by the firm of Otto, Dahlhausen, but which has not yet had time to prove its practical results, is that of von der Horst. He uses an alkali or, preferably, ammonia to which a suitable small quantity of a solution of copper is added. Complex compounds, such as (NH4)3CuCy4, are then formed; and these, on being treated with dilute acids, give off pure hydrocyanic acid. A precipitate of sulphur and sulphocyanide of copper is also fornied on boiling with dilute acid. EXTRACTION FROM SPENT OXIDE It has already been pointed out that when no special means is provided for the recovei y of cyanogen from the gas a large proportion of it is arrested by the oxide of iron in the dry purifiers. Owing to the possibility of revivifying the iron sulphide, and, accordingly, the utilization of the mass over and over again, considerable quantities of cyanogen may have been collected by the time the material is finally spent. There are numerous processes for the recovery of cyanogen from oxide, but, in general, they may be classified into two distinet groups, namely— (a) Those in which the free sulphur is first extracted by means of some solvent such as carbon disulphide. (b) Those in which the spent material is straightway treated for the recovery of the cyanogen. The processes in the latter group are those chiefly in use, and consist, in the main, of the lixiviation of the material for the removal of the soluble salts, such as ammonia and the sulphocyanides. The insoluble cyanide salts then undergo tieatment to convert them into soluble compounds, and finally the whole of the cyanogen is recovered as potassium ferrocyanide. Lixiviation of the material is carried. out in tanks in which., by any suitable arrangement, the oxide is thoroughly washed with water. The liquor thus obtained is treated in the ordinary manner with lime for the recovery of ammonia, a certain amount of calcium sulphocyanide being formed by the action of the lime. The next stage is the recovery of the insoluble ferrocyanid.es by treating them with lime or other alkali. This process is now frequently carried out in a mechanical stirring apparatus. The material is led, together with the dilute liquid obtained in the first stage of the process, into a boiler, and the mass is heated with steam and thoroughly intermixed by means of the stirring apparatus. The sludge obtained is then caused to flow to a filter-press in which the cyanide solution is separated.