Application of bipolar electrodialysis to the recovery of acids and bases from water solutions

The studies of acid purification and base purification via bipolar electrodialysis are presented. The process of acid recovery involved spent solutions of acid (HCl or H2SO4) and iron salt obtained in the course of conventional electrodialysis. Bipolar electrodialysis yielded an acid solution concentrated 51-fold (in the case of hydrochloric acid) and 63-fold (in the case of sulphuric acid) as compared to the feeding solution for conventional electrodialysis. The solution of the recovered acid was only slightly contaminated with iron salt (0.12 and 0.13%). The process of base purification yielded an NaOH solution concentrated nine-fold as compared to the feeding solution for the bipolar electrodialysis process. The contamination of the recovered base varied from 1.75 to 2.50%.

1. Bipolar electrodialysis enables the recovery and further concentration of hydrochloric and sulphuric acids from the acid and iron salt concentrates produced in the course of conventional electrodialysis. Thus, bipolar electrodialysis, in combination with a two-stage monopolar electrodialysis process, yields the recovery of two valuable components of the after etching rinse: high-quality water and acid. 2. The separation properties of Neosepta BP-1 membranes towards iron ions can be regarded as very good. The calculated mean molar flow of the acid in the acid purification process was found to be approximately 800 times the calculated mean molar flow of the salt. The process yielded an acid solution concentrated 51-fold to 63-fold as compared to the feeding solution for a conventional electrodialysis process. The contamination of the recovered acid solution with iron salt was very low, ranging between 0.12 and 0.13%. Unit energy consumption in the acid purification process varied from 1.39 to 1.68 kWh/kg of the recovered acid. 3. The separation properties of the bipolar membranes depend both on salt type and on salt concentration. Specifically higher contamination values were obtained with the increasing sodium nitrate concentration in the feed. In the process of sodium base recovery from the NaOH and NaNO3 solution, the calculated mean molar flow of the base was 85 to 125 times the calculated mean molar flow of the salt.