Utilization of waste sodium sulphate to produce potassium sulphate

The process of manufacturing potassium sulphate with an application as a fertilizer was developed. Post-chromium sodium sulphate containing sodium dichromate was utilized as a raw material. The conditions used during conducting individual stages of the process were as follows: i) separation of chromium from the sodium sulphate solution (reduction of Cr(VI) to Cr(III)) - pH 1.5 - 2.0; the mole ratio of sodium sulfite to chromium (VI) - (1.4 - 1.6): 1.0 pH of Cr2O3źnH2O precipitation - 8.5), ii) a two-stage conversion of sodium sulphate with potassium chloride (I stage - molar ratio KCl:Na2S04:NaCl = 1.5:1.0:0.6, total concentration of salts - 35 wt.%; II stage - molar ratio KCl:K3Na(S04)2 = 4:1, total concentration of salts - 30 wt.%) and iii) a two-stage crystallization of salts from the mixture of mother solutions (I stage - evaporation to 60 wt.%, crystallization at 2°C; II stage - evaporation to 60 wt.%, crystallization at 110°C). Raw potassium sulphate (98 wt.% K2S04, 1 wt.% Cl, 6 ppm Cr(III)) for the production of low-chloride fertilizers and raw sodium chloride (by-product) were obtained. The proposed technology allowed closing the mass fluxes. The mixture of K3Na(S04)2 and KCl obtained after the first stage of the mother liquor concentration was recycled to the point of potassium sulphate crystallization. On the other hand, the post-crystallization liquor remained after the second stage of concentration was recycled to the process of glaserite crystallization.