Abstract
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.
Highlights
Sodium chlorate used for preparing chlorine dioxide and a variety of chlorate salts is an important inorganic salt widely used in many areas, such as papermaking bleach, drinking water and wastewater treatment, printing and dyeing, medicine, pesticide, tanning, mineral processing (Bai, 2012)
The Na2Cr2O7 concentrations were all lower than 0.05 g/L, Na2SO4 concentrations were lower than 1 g/L in the nanofiltration permeate, and the permeate could be recycled for the brine refining stage
Na2Cr2O7 in salt sludge was less than 8.5 ppm, lower than the United States environmental protection agency of hexavalent chromium content in the sludge discharge standard 10 ppm
Summary
Sodium chlorate used for preparing chlorine dioxide and a variety of chlorate salts is an important inorganic salt widely used in many areas, such as papermaking bleach, drinking water and wastewater treatment, printing and dyeing, medicine, pesticide, tanning, mineral processing (Bai, 2012). We put forward the combination separation technology of nanofiltration and freezing crystallization for treating the crystallization mother liquor of sodium chlorate production in order to achieve the recycling utilization of sodium dichromate. It reduces costs, makes the wastewater and sludge produced in the production of sodium chlorate no longer contain chromium, and reaches safe discharge standards, so as to realize clean production. Makes the wastewater and sludge produced in the production of sodium chlorate no longer contain chromium, and reaches safe discharge standards, so as to realize clean production It achieves the purpose of separation sulfate ion from the system, and improves the efficiency of electrolysis
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