Chromic acid recovery by electro-electrodialysis: I. Evaluation of anion-exchange membrane

Electro-electrodialysis (EED) is a promising technology for economic recovery of chromic acid in plating shops. It could potentially achieve chromic acid recovery, removal of metallic impurities and purification of static rinses in one step. There are however, process limitations. These are mainly, the poor stability of the applied anion-exchange membrane (AEM) against the oxidative chromic acid solution and the increase of the AEM resistance, especially at the starting phase of the process, due to the formation of polychromates in the membrane. In this work, various AEM are investigated for chromic acid recovery. The membrane fumasep® FAP (FuMA-Tech GmbH) seems to be the most efficient. Its current efficiency is much higher than those previously reported in the literature. Furthermore, the fumasep® FAP is used for process optimisation experiments. The process performance depends on the concentration gradient between product (anolyte) and central compartment (exhausted rinse water) and the temperature. Our results show that batch processing is recommended and the chromate transfer rates through the AEM could be significantly increased when increasing the temperature up to 50 °C. Finally, low initial current density (10–20 mA cm−2), feed flow rate higher than 7 cm s−1 could help overcoming the process limitations attributed to high AEM resistance.