556 MODERN GASWORKS PRACTICE
passage of the gas through the washer, and an average reduction of 10 grains resulted from tests extending over some period. The exact männer in which the CSa is absorbed is not perfectly clear, but it may unite with the ammonium sulphide to give ammonium thiocarbonate (see page 506) as follows :—
(NH4)2S+CS2=(NH4)2CS3.
The carbon dioxide in the gas is prejudicial to the absorption of CS2, for it reacts with thiocarbonate and drives the CS2 out of combination. When the CO2 content in the gas is low the efflciency of CS2 absorption should, therefore, be in-■creased. The ammonium thiocarbonate gradually becomes converted into ammonium sulphocyanide in the washer—
(NH4)2CS3 = NH4SCN + 2 H2S, .so that in this way the carbon disulphide removed is turned to useful account.
The Williams Polysulphide Process
The British C'yanides Company have largely given up the process described .above and have substituted for it a more efficient and economical method, known
;as the polysulphide process. In the newer process the hydrocyanic acid is recovered "in the form of ammonium sulphocyanide, but the necessity of introducing a special supply of free sulphur is obviated by making üse of the sulphur in spent oxide for the production of the polysulphides. A diagrammatic sketch of the apparatus is shown in Fig. 346. From this it will be seen that the vessel employed resembles