Recycling sodium dichromate in sodium chlorate crystallization mother liquor by nanofiltration membrane

In the process of sodium chlorate electrolysis production, chemical precipitation is often used to remove the sulfate radical in the crystallization mother liquor which leads to the dichromate deposit to salt sludge together with sulfate, and finally causes chromium pollution. The nanofiltration membrane and freezing crystallization integrated technology was presented to treat crystallization mother liquor. The research conducted in this work shows that, Na2Cr2O7 concentration was lower than 0.05 g/L, Na2SO4 concentration was lower than 1 g/L in the nanofiltration permeate and it could be recycled for brine refining stage avoiding the chromium pollution caused by subsequent treatment. The content of Na2Cr2O7 in salt sludge was less than 8.5 ppm below the 10 ppm discharge standards. Then, the nanofiltration concentrate was frozen, Na2SO4 crystallized and precipitated. The supernatant mainly containing sodium dichromate could recycle to the electrolytic system. This method not only could recycle sodium dichromate, avoiding chrome sludge generated, but also could remove sulfate radicals.   Key words: Sodium chlorate, chrome sludge, nanofiltration, freezing crystallization, sodium dichromate recycling, sodium sulfate.