Modern Gasworks Practice

THE DRY PURIFICATION OF CO AL GAS 573 over, and exposed to the action of the oxygen in the atmosphere. By this means the iron sulphides are oxidized and assume tlieir original character with. the deposition, of free sulphur ?— (a) 2 Fe2S3 + 3 02 + 2 H2O = 2 Fe2O3,H2O + 3 S2. (b) 4 FeS + 3 O2 + 2 H20 = 2 Fe2O3)H2O + 2 S2. The material, together with the free sulphur, is then returned to the purifier, when precisely the same reactions recur. Revivification may then be carried out again, the oxide being returned some three or four times to the purifiers until it contains upwards of 55 to 60 per cent, of sulphur, beyond which figure it will not be found economical to work. The dry purification of gas in this way may really be considered as a delayed catalytic action, the oxide of iron assuming the role of a catalyst. For instance, the final result of the process may be represented as follows 2 H2S + O2 = 2 H2O + S2. As regards the generally accepted reactions quoted above, it has been pointed out by G. Weyman1 that ferric sulphide Fe2S3 is the primary product, but the ferne hydrate reformed by oxidation is certainly not the compound Fe2O3,3H2O, and probably contains less water than is represented. by the formula Fe2O3,H2O. AV ater, therefore, is liberated which is not going to be recombined, and unless rem o ved will accumulate until the efficiency of the material is impaired. Weyman also draws attention to the faet that sulphur is slightly volatile in the presence of water vapour, and in this case can decompose steam, with the eventual formation of sulphur dioxide and sulphuretted hydrogen— 3S+2H2O = 2H2S+SO2. Such liberation of sulphur dioxide enormously inereases the complexity of the process, for it is capable of giving rise to compounds by reacting with. a large number of other substances present. Sulphur acids are formed, and under the acid conditions thus set up, the ferric slilphicle passes into ferrous sulphide. Artificial and Other Materials So far as the other mediums employed for the removal of sulphuretted hydrogen, are concerned, Weldon mud, a by-product of the bleaching industay, owes its activity to the presence of manganese dioxide. The reactions occurring are very similar to those appertaining to oxide of iron, a manganese sulphide capable of regeneration resulting. In many respects Weldon mud is superior to oxide of iron for piirification purposes. It cannot, however, be obtained now owing to the abandonment of the old Weldon chlorine process. The chief advantages which may be claimed for the material are inereased absorptive activity as compared with. oxide, whilst owing to the possibility of revivifying it in silu it is possible to work up the sperrt material to a high, sulphur percentage betöre removing it from the purifier. Owing to its sensitive nature Weldon mud was, in the days of universal lime purification, particu-larly suited for use in “ catch boxes.” 1North of England Gas Managers’ Assoen., May, 1920. See Gas World, LXXII, p. 458.