In electrodialysis stacks there exists the problem of membrane deterioration, and especially of the membranes close to the electrodes. The electrodes themselves have a limited period of life as they are corroded and thus special materials must generally be used which are corrosion resistant. The above problems are the result of the formation of chlorine which tends to attack the membranes and also the electrodes, and also of acid and base formation. For every Faraday of current passed through an electrolysis stack one equivalent of acid and one of base have to be rinsed away. Generally acid is added to the cathode compartment to prevent scale formation and the acid from the anode compartment is generally discarded as it is contaminated by chlorine. If chlorine formation can be eliminated, it ought to be possible to continuously neutralize the acid and the base which is formed by circulating the anolyte and the catholyte. Generally this is not feasible as a large buffering capacity is required.
It is known to use certain redox systems for decreasing the above defined problems. Thus, certain quinones or similar compounds may be oxidized and reduced. Certain cations such as iron or the like can be used for the same purpose. These additives are quite problematical as it is imperative that even traces of these should be prevented from passing through the membranes; they must be non-toxic, not too expensive, stable, and not precipitate on the membranes. Results obtained with conventional redox systems have not been satisfactory.
USPTO Backgrounds
USPTO Backgrounds