Green utilization of the concentrated brine from two-stage membranes in coal chemical industry using selectrodialysis with bipolar membrane

Abstract Mixed salt from the concentrated brine that contains NaCl and Na2SO4 has been classified as hazardous waste since 2016 in China. This fact created an urgent and tremendous demand to treat these brine promptly. In this paper, we reported a method of using selectrodialysis with the bipolar membrane (BMSED) to reutilize the concentrated brine to produce high-purity HCl, NaOH, and Na2SO4, which fulfilled the demand above. The selective PPy(Cl) composite membrane, which was able to separate NaCl and Na2SO4 in the concentrated brine, was prepared. The 3.5 μm PPy(Cl) film significantly enhanced the selectivity of the anion exchange membrane (AEM) to chloride ions while sulfate ions were blocked. The BMSED stack with our multilayer PPy(Cl) composite AEM produced 1.23 mol/L HCl with a purity higher than 98.2%. More importantly, the prepared PPy(Cl) composite AEM was not only suitable for brine with a high Cl-/SO42- of 19:1, which was the typical ratio in concentrated brine from the coal chemical industry, but also able to treat the brine with a Cl-/SO42- as low as 1:1, showing superior capability. The effluent of the BMSED we developed, which contained 0.02 mol/L Na2SO4 with the Cl-/SO42- equaling to 1:1, could be further concentrated by reverse osmosis for sea water (RO-SW) and freeze crystallization to obtain Na2SO4 with a high purity accordingly. This reutilization route of concentrated brine was very green because it not only produced no hazardous waste, but also recycled valuable products. Therefore, it was believed a promising application in the coal chemical industry.