Modern Gasworks Practice

546 MODERN GASWORKS PRACTICE case the gas is washed with a solution, of zinc sulphate which, after performing its function, is regenerated for further use. Put briefly, the various stages of thc process are as follows :— (1) Grude gas is treated in a centrifugal washer with a solution of zinc sulphate— ZnS04 + H2S + 2 NH3 = (NH4)2SO4 + ZnS. (2) The sulphate of ammonia is filtered out and recovered by evaporation. (3) The precipitated zinc sulphide is roasted, yielding zinc oxide and sulphur dioxide— 2 ZnS + 3 O2 = 2 ZnO + 2 SO2. (4) The roaster gases containmg sulphur dioxide, are then blown through water containing in suspension the zinc oxide from a previous roasting, zinc sulphate resulting from the reaction— 2 SO2 + O2 + 2 H2O + 2 ZnO = 2 ZnS04 + 2 H2O. One of the merits of the process is that tarry matter in the criide gas does not interfere, as it is destroyed during the roasting process. Hemingway1 has introchiced a process for the elimination of hydrogen sulphide from gases by treating th.ern in purifiers with iron carbonate, and he extends the process to the treatment of ammoniacal liquors. The process is carried out in such a männer that the ferrous carbonate is recovered for further use. When applied to the desulphurization of coal gas, the gas is passed through, scrubbers in the usual männer, and ammoniacal liquor derived tlierefrom is conveyed to a tank and there treated with. ferrous carbonate. The liquor having thus become “ sweetened, and the iron salt having subsided. in the form of sulphide, the sweet liquor (ammoni-acal liquor deprived of its hydrogen sulphide) is rim off to a still, or returned to the scrubbers ; while the iron sulphide precipitate is converted into ferrous sulphate for use in the next stage of the process. The sweet liquor is conveyed to a second tank, and is there treated with the ferrous sulphate betöre referred to. The ferrous salt, absorbing carbonic aoid, subsides as ferrous carbonate; and the sweet liquor (now containmg a little extra ammonium sulphate) is run off to a still, while the ferrous carbonate precipitate is withdrawn for re-use in the first stage of the process. The ammoniacal liquor, draining from the precipitate, is led to the ammonia still as before. By the third stage of the process, ferrous carbonate is reproduced for use in the first stage, and the cycle of operations may be repeated without the introduction. of further supplies of the ferrous salt, except so far as may be necessary for making good trifling losses in working. Semi-Direct Processes These processes, although, scientifically, not so perfect as those discussed above, in. view of the necessity for introducing sulphuric aoid, possess the advantage that they are comparatively simple in operation so long as certain necessary pre- 1 Eng. Pat. 117387.