Modern Gasworks Practice

THE PRELIMINARY PURIFICATION OF COAL GAS 545 into thiosulphate, sulphur separating out. The effluent liquor is run into a tank, from which it is pumped into the acidifier, traversing, on its way, a collector in which a portion of the sulphur is deposited. In the acidifier the thiosulphate liquor is treated with. sulphur dioxide, prepared by roasting sulphur or pyrites in the sulphur furnace, and which. has been freed from dust in an electrical purifier, and cooled. The sulphur dioxide re-converts the thiosulphate into polythionate, which is returned to tlie washer. This cycle of operations results in. the progressive enrichment of the polythionate; and when the concentration has reached a certain stage, a portion of the liquor is pumped from the acidifier into the boiler, wliere the polythionate is decomposed by heat into sulph.ate. sulphur, and sulphur dioxide. In order to make certain. that the liquor to be decomposed no longer contains any thiosulphate, sulphur dioxide may also be introduced into the boiler. The sulphate of ammonia liquor flows through a collector into the evaporator, where it is worked up into solid salt, this being discharged by an. ejector into tlie whizzer. The sulphur retained in the collector is also put through a whizzer, the sulphate liquor thus recovered being run into the evaporator. The various stages of the process, and the reactions which occur, are as follows :—• The cru.de coal gas is washed with a solution of ammonium tetrathionate, the reactions occurring being on the following lines :— (a) 6NH4OH + 4(NH4)2S4O6=5(NH4)2S2O3 +2(NH4)2S3O6 + 3H,O. (6) 3H2S + (NH4)2S4O6= (NH4)2S2Os + 5 S + 3H2O. The tetrathionate lye which has been employed for washing is then treated with a current of sulphur dioxide. In this way the ammonium thiosulphate and trithionate formed are reconverted into tetrathionate— (c) 2 (NHä)2S2O3 + (NH4)2S3O6 + 2S + 3SO2= 3(NH4)2S406. When the tetrathionate lye becomes sufficiently concentrated it is heated, decomposition taking place with the formation of ammonium sulphate, free sulphur, and sulphur dioxide— (d) (NH4)2S4O6= (NH4)2SO4 + S + SO2. The sulphur dioxide and a portion of the free sulphur are then used again in the earlier stages of the process. With German gas coal it has been found that sulphur exists in excess of that required for the formation of the sulphate of am-monia ; thus some considerable quantity of free sulphur also results as a by-product. In the first instance tlie tetrathionate solution is prepared by the action of sulphur dioxide on gas- liquor as follows :— 2 (NH4)2S + 6 SO2 = 2 (NH4)2S4O6. Cobb’s Process One of the most promising processes for the direct recovery of ammonia is that introduced recently by J. W. Cobb. From the point of view of simplicity, the method has many advantages in comparison with. its competitors. In this N N