Modern Gasworks Practice

THE RECOVERY OF CYANOGEN 555 British Cyanides Company’s Process The process of the British. Cyanides Company, of Oldbury, recovers the hydrocyanic acid in the form of a sulphocyanide. The metliod consists essentially of bh e preparation of a solution contaming ammonium polysulphide by the addition of coarsely powdered sulphur to ordinary ammoniacal liquor. A rotary washer-scrubber is employed for bringing gas and liquor in thorough contact, and the top of each bay is provided with a cup-shaped receiver having a closely fitting lid. These cups are periodically filled with the powdered sulphur, and the ammonium sulphide in the liquor combines with tlie free sulphur added to give ammonium polysulphides, (NH4)2S2 to (XII 4)285. The process is dependent upon the faet that whereas hydro-cyanic acid will combine with ammonium polysulphide, it will not react with ammonium sulphide. Ammonium sulphocyanide is formed. in the washer, the liquor flowing out with a strength varying from 15° to 20° Twaddel. With the disulpliide the reaction, occurring is as follows:— (NH4)2S2 + HCN = NH4SCN + NH4HS. Whilst the plant is in operation it is usual to test the liquor in the various bays of the washer-serubber from time to time in order to ensure that it consists of ammonium polysulphide and not the sulphide. This may be done by adding a small quantity of acid to the liquor, whén a decided precipitate of sulphur will be given if the polysulphide is present, whilst the sulphide will give practically 110 precipitate at all. In this system the hydrocyanic acid is extracted before the ammonia has been. removed from the gas, as in. this way the proportion of ammonium sulphide is inereased and the efficiency of extraction rendered higlier. To this end the plant is usually placed. immediately after the exhausters, with some ef&cient form of tar extractor coupled up to its inlet. It is found that by the British Cyanides process practically the whole of the hydrocyanic acid is removed from the gas, and the liquor may be readily worked up to a strength. of from 3 to 4 Ib. of ammonium sulphocyanide per gallon. As regards ammonia, the strength. of the liquor may be as high as 40 oz., but 90 per cent, of the ammonia is present in the “ fixed ” state. In the case of some works the liquor is passed on to the sulphate plant and treated for the recovery of ammonia in the ordinary way, by the addition of lime. The lime then takes the place of the ammonia and gives rise to calcium sulphocyanide, which. may be sold direct to the cyanide works, by whom it is converted into sodium cyanide. The conversion. of ammonia into calcium sulphocyanide takes place as follows :■— 2 (NH4SCN) + Ca(OH) 2 = Ca(SCN) 2 + 2 NH40H. It has been found that in. addition to absorbing hydrocyanic acid some reduc-tion of the carbon disulpliide in the gas may take place as the gas travels through, the plant. But whilst in some cases so mueh as 20 to 25 grains of CS2 per 100' cubic feet was absorbed, at other times a reduction of only 1 to 2 grains was shown. No reliance can be placed upon the apparatus for this purpose. Colman, however, in a series of experiments, found that in no case was the CS2 inereased during the