Electrodialysis is a process for the separation of ions in saline solutions and is therefore suited to converting these into their corresponding acids and bases. The necessary separation of ions is effected by an electric field applied via an anode and a cathode as well as by consecutive ion exchange membranes.
In many types of industrial production it is necessary to add acids and bases to aqueous processes. However, this increases the salt concentration and/or the conductivity, which is why the recovery rate of the water in the cycle is limited as a result of the accumulation or the increase. Additionally, the acids and bases are themselves lost for the process. On the one hand, these can now be recovered by means of the electrodialysis and on the other the water, freed of the salt load, can also be recirculated.
Operation principle of a bipolar membrane stack applied in electrodialysis processes
The membrane stack consists of cation exchange, anion exchange and bipolar membranes. Here the sodium cations and sulfate anions of the salt are transferred by the electric field to base or acid chambers, where they form caustic soda and sulfuric acid with hydroxide ions (OH–) or protons (H+) formed heterolytically on bipolar membranes. Thus the acid and base concentrations increase during the process, whereas those of the saline solution decrease.
R&D focus and applications
The focus of the present research is on the reduction of the energy consumption of this process, in order to make it attractive for industrial scale use as well. Other possible applications for electrodialysis are in the food industry (pH modification, demineralization) and in the production of organic acids.