Combination of Thermal, Hydrometallurgical and Electrochemical Tannery Waste Treatment for Cr(III) Recovery

A combination of thermal (500–750 °C in air) and hydrometallurgical (acidic) treatments have been applied to dried tannery sludge, resulting in the initial conversion of Cr(III) to Cr(VI) and its subsequent leaching as wastewater with high Cr(VI) concentration content (3000–6000 mg/L), presenting an extraction efficiency over 90%. The optimal electrochemical conditions for the subsequent Cr(VI) reduction with respect to acid concentration and acid kind were established by applying appropriate rotating disc electrode (RDE) experiments, using a glassy carbon (GC) electrode, and found to be equal or higher than 0.5 M H2SO4 (for the respective Cr(III) concentration range studied). The result from leaching Cr(VI) wastewater was further treated in small electrochemical bench-scale reactor for its conversion back to Cr(III) form, potentially reusable in the tanning industry. Ti-based anodes and a reticulated vitreous carbon (RVC) cathode were used to treat small (350–800 mL) samples in batch, as well as in batch-recirculation prototype electrochemical reactors, under the application of constant current or appropriately applied potential to achieve Cr(VI) conversion/reduction efficiency over 95%.