THE RECOVERY OF CYANOGEN 557
an ordinary dry purifier, access to which is obtained. through portions of the top whicli are made removable on the lines of a Green’s purifier. The inlet gas entering at the base of the vessel travels in the first instance through a tar extractor operated on the bubbling principle, the gas passing through the cyanide liquor whicli forms the seal. Above the tar extractor is arranged, on suitable brackets, a tier of ordinary wooden purifier grids which support a layer of sperrt oxide, evenly spread over the whole cross-section of the vessel, and about 24 inch.es in depth. The sperrt oxide used should preferably be of good quality, containing not less than 50 per cent, of sulphur. It is kept in a moist condition by a simple arrangement of sprays fitted around the four sides of the vessel at a point well above the oxide level. . These sprays may bo scen in Fig. 347, wlucli shows <i complctc section of th.e box witli th.6 two inlets and outlets for the gas. A short period of spraying every day with water or a w e a k cyanide liquor is all that is necessary. In. this way the polysulphide solution is formed within the interstices of tlie oxide bed, and is immediately converted into ammonium sulphocyanide by the incoming gas. The cyanide liquor, after trickling through the oxide mass, drops to the base of the vessel, from
which it overflows to the storage tank by way of a seal. With regard to the-actual formation of the sulphocyanide it would seem that a straiglitforward. reaction on the following lines takes place—
(NH4)2S2 (ammonium polysulphide) + HCN = NH4SCN + NH4.SH.
There is, however, some probability that a portion of the sulphocyamde is formed in two stages, with. the Intermediate production of thiocyanic acid. Tlms ammonium polysulphide gives ammonium sulphide and thiocyanic acid as follows
(1) (NH4)2S2 + HCN == (NH4)2S + HSCN (thiocyanic acid).
The last-named. then combines with. more ammonia, in the form of the hydrate,, to give the sulphocyanide—
(2) HSCN + NH40H = NH4SCN + H2O.
Probably the most surprising feature of the polysulphide process lies m tlie faet that although sulphur is inevitably required for the completion of the reaction, the sperrt oxide, after working for some period, shows no loss in sulphur content, and occasionally some inerease is found. It would seem, then, that there is a simul-taneous absorption of ammonia, sulphmetted hydrogen, and oxygen, and that