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

Evgenios Kokkinos,Aikaterini Touni,Anastasios Zouboulis,Sotiris Sotiropoulos,Ioanna Mintsouli,Aggeliki Banti

doi:10.3390/app11020532

Abstract

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%.

Highlights

Tanneries can transform raw skin into leather, by using physico-chemical and mechanical techniques
According to the X-ray diffraction (XRD) spectra (Figure 2), in all thermally treated samples, the peaks related to the presence of chromium were attributed to the hexavalent samples, the peaks related to the presence of chromium were attributed to the hexavalent form CaCrO4 (Joint Center for Powder Diffraction Studies—JCPDS card No 08-0458), form CaCrO4 (Joint Center for Powder Diffraction Studies—JCPDS card No 08-0458), except for the samples treated at 750 ◦ C, where the observed peaks were attributed to except for the samples treated at 750 °C, where the observed peaks were attributed to the the trivalent form MgCr2 O4 (JCPDS card No 10-0351)
It should be clarified that the trivalent form MgCr2O4 (JCPDS card No 10-0351)

Summary

Introduction

Tanneries can transform raw skin into leather, by using physico-chemical and mechanical techniques. The main chemical reagents, used to produce leather with desired commercial properties, are based on Cr(III) salts. Due to the low effectiveness of the aforementioned tanning processes, tanneries produce large quantities of hazardous wastewaters [1]. In order to treat this waste, several physico-chemical and biological methods can be applied, resulting in sludge production with high chromium and organic matter content. A specific case, generating great interest and showing certain prospects, is the application of thermal treatment, whether it concerns energy recovery [3], or as an intermediate stage of a more complex process [4]. The main feature that makes thermal treatment attractive is the great reduction in sludge volume, due to the combustion of organic matter content [5]

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