Multi-stage membrane integrated system to achieving low-mixed-salt-discharge of high-salinity mining wastewater: System design and experimental validation

Minimizing the release of highly saline mine wastewater is crucial in order to meet the zero liquid discharge requirement of the coal chemical industry. However, the inorganic salts produced, such as NaCl and Na2SO4, have low value and are difficult to consume locally, particularly in western China. Therefore, this study proposes the multi-stage membrane integrated system with the aim of achieving low discharge of mixed salts in high-salinity mining wastewater. Two different combinations of reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), and bipolar membrane electrodialysis (BMED) are proposed for the integrated system, specifically designed for treating Bulianta coal mining wastewater ZLD projects in Inner Mongolia, western China. To compare the effectiveness of these combinations, experimental tests were conducted using ED to concentrate pure solutions of NaCl and Na2SO4 separately. The results indicated that, in terms of energy consumption for transferring the same charge across the membrane, Na2SO4 solution was more suitable to be concentrated than NaCl solution under low feed concentration (cation concentration<1.0 M) in ED, and the corresponding energy saving to transfer on unit charge significantly decreased from 6.15 × 10−3 kWh/mol charge to 1.60 × 10−4 kWh/mol when cation concentration ranged from 0.2 M to 1.0 M. Then, the production of acid and base in BMED was experimentally investigated using Na2SO4 solution and a mixed solution of NaCl and Na2SO4. Results revealed that the energy consumption decreased from 150.62 Wh/[H++OH-]mol to 97.88 Wh/[H++OH-]mol when the Na2SO4 solution increased from 0.2 M to 1.4 M, meanwhile the energy consumption for mixed solution of NaCl and Na2SO4 is strongly related to the mole ratio of Na2SO4 to NaCl. Thus, the utilization of NF membranes with high selectivity for Na2SO4 over NaCl in the NF stage, combined with high feed concentrations in the ED stage, is crucial for achieving lower energy consumption in BMED. Based on experimental results and analyses, the NF concentrate is more effectively treated in ED and BMED when dealing with mining wastewater with a high mole ratio of Na2SO4 to NaCl (>1).