Studies were made on the preparation of high purities of sodium sulfate and hydrogen sulfide.Material for testing was smelt (17.5% of Na2S ; 52.5% of Na2CO3 ; 3.12% of Na2S2O3 ; 14.7% of Na2SO4 ; 3.52% of Na2SO3 ;0.05% of Fe2O3 and Al2O3; 0.63% of SiO2, etc.) obtained from the combustion of a NSSC black liquor in a KP type recovery furnace. Several effective methods were found out, but Process D was the simplest of all.Process DThe smelt was dissolved by water to prepare an aqueous solution (a green liquor). The green liquor was treated with 6N-H2SO4 to prepare an acid solution while generating hydrogen sulfide (83.4% of H2S ; 13.6% of CO2; <1% of SO2 and 2% of H2O). The neutralized solution was evaporated to such dryness, that a high purity of sodium sulfate (98.6% Na2SO4 ; Na2S2O3 nil; Na2S nil; 0.2% of Na2SO3 ; 4ppm of Fe) was attained.Sodium sulfate of higher purity (99.0% of Na2SO4 ; Na2S2O3 nil; Na2S nil; Na2SO3 nil) containing no reductive material was obtained by adding an oxidizing agent (H2O2 or Na2S2O8) before neutralizing the acid solution with a pH 2.5, in Process D with NaOH or by Process E or G.
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