THE PRELIMINARY PURIFICATION OF COAL GAS 543
Process 2.—The iron sulphide liquor is treated with SO2, whicli is obtained by the burning of sulphur. Free sulphur is precipitated, and soluble ferrous thio-sulphate, thionate, and sulphate are formed—2 FeS +3 S02= 2 FeS2O3 + S.
Process 3.—This liquor (FeS2O3) is again used for treating the gas—2FeS2O3 + 4 NH3 + 2 H2S — 2 FeS 4-2 (NH4)2,S2O3. The reaction this time is somewhat different. The ammonia and H2S in the gas are again absorbed, whilst iron sul-phide, together with some free sulphur, is precipitated—sulphate, thiosulphate, and thionate of ammonia being formed. The liquor is alternately treated with crude gas and SO2 until it contains from 30 to 45 per cent, of ammonia salts.
Liquor with
Remainder of NH3 and SH2 Absorbed by Excess of —
FeSO4, FeSjOj. etc.
FeSO. Saturated by NH3 and SH2 into FeS and (NH4),SO4
Centrifugal
— Dryer
Fig. 342.— Line Diagram showing pkinciple of Fbld’s Original Pbocess.
Process 4.—The liquor undergoes oxidation, by whicli process the thiosulphate and th ionat cs are transformed into sulphate. To this end the hot liquor is treated with. SO 2, and an excess of air. The ferrous sulphide is dissolved, and the ammonia all converted into sulphate, whilst free sulphur is precipitated—2 FeS + 2’(NH4)2,S2O3 + 3SO2 + 2O2 = 2(NH4)2,SO4 + 2FeSO4 + 5S.
Process 5.—The free sulphur is separate»! from the liquor by filtration.
Process 6.—The liquor coming from the filter-press, and containing the sulphate of ammonia, also sulphate of iron, is treated with the crude gas (containing H2S and NH3), by the action, of which. the ferrous sulphate is decomposed, forming again sulphate of ammonia and. iron sulphide (see under Process 1). The iron is separated by filtration, and the liquor containing the sulphate is evaporated, leaving the salt behind—this being dried in a centrifugal machine.