Ion exchange membranes for acid recovery: Diffusion Dialysis (DD) or Selective Electrodialysis (SED)?

Diffusion dialysis (DD) is a unique concentration-gradient membrane process for acid recovery by taking advantage of the perm-selectivity of anion-exchange membrane for protons against metal ions. Selective electrodialysis (SED) is a promising technology for acid recovery via a potential difference as the driving force. The difference of membrane stack structure and driving force makes the two membrane processes have their own advantages for acid recovery. This study systematically compared the acid recovery performances between DD and SED from the aspects of acid recovery ratio, metal leakages ratio, membrane flux as well as process cost in the HCl/FeCl2 waste acid system. The acid recovery rate in DD increased from 20.17% to 84.74% and the Fe2+ leakage rate decreased from 2.35% to 1.47% when the flow rate ratio (water/waste acid) changes from 0.25:1 to 2:1. In contrast, a satisfied acid recovery ratio of 90.75% and Fe2+ leakage of 1.57% was obtained in SED at the current density of 25 mA·cm−2. By balancing the capital cost and operating cost, the coupling of DD and SED is profitable to strengthen the superiority of each technology for process intensification.