Electrodialytic desalination and concentration of coal-mine brine

Electrodialysis (ED) of coal-mine brine was investigated as the first stage of an ED-evaporation-crystallization system. Univalent ion permeable ACS and CMS membranes (Tokuyama) were applied in the first step while normal-grade CMV and AMV Asahi Glass membranes and electrodialysis reversal (EDR) were used in the second step (for the first step diluate treating). Electrodialysis in the countercurrent flow mode and electrodialysers with small intermembrane distances were applied to diminish problems that result from major differences in ED concentrate and diluate concentrations. A single-pass low residence operating time mode was applied to avoid gypsum crystallization in the EDR concentrate. A coal-mine brine with 32.8 g/LCl^- content was desalinated and concentrated at a current density of 344-688 A/m² in the first step and 300 A/m² in the second. First step diluate was treated by EDR yielding 85% recovery. The total energy consumption found was equal to 7.8-14.4 kWh/m³ inlet brine depending on the first step current density and the first step ED concentrate composition (TDS equal to 136-290 g/L). It was found that ED concentrate met requirements concerning low Ca²⁺ and SO^2-₄ concentrations to avoid gypsum crystallization during further evaporation. The comparison of RCC vapour compression evaporation and proposed electrodialytic concentration showed an advantage of ED treatment.