GB494281A - Process and apparatus for the purification of gas obtained from coal, lignite, and the like - Google Patents

Abstract

In the purification of coal, lignite, coke oven &c. gas, from hydrogen sulphide, carbon bisulphide, cyanogen compounds and carbon dioxide by washing with caustic alkali solution, the foul liquor consisting mainly of unchanged caustic alkali, with the alkali sulphide, thiocyanate and carbonate, is regenerated by means of calcium hydroxide in the presence of alkali carbonate added to the liquor in a quantity corresponding to or greater than the equivalent of alkali fixed by the acids in the gas other than carbon dioxide. The presence of carbonate during the regeneration inhibits the formation of soluble calcium salts, calcium carbonate being precipitated. When sodium compounds are used, the regenerated liquor therefore consists of NaOH, Na2CO3, Na2S and NaCNS (the ammonia in the gas having been previously removed or not) and this liquor is recirculated to the primary washing tower. Bye-products may be recovered from the regenerated liquor. Thus by controlled evaporation crystals of Na2S may be obtained and a concentrated solution of NaCNS. The latter liquor or the liquor before crystallizing out Na2S may be treated with p zinc sulphate solution to give white zinc sulphide containing traces of zinc carbonate and hydroxide and a liquor comprising sodium sulphate and thiocyanate. The latter liquor may be concentrated to give Na2SO4, 10H2O crystals; the residual liquor rich in NaCNS may be dried and calcined to give anhydrous Na2SO4. This residual liquor, however, may be treated with CuSO4 and SO2 to give a white precipitate of cuprous thiocyanate and the final liquor neutralized with Na2CO3 before crystallizing out Na2SO4, 10H2O therefrom.