Leaching and heat treatment of chrome shavings: Stability of chromium (III)

Abstract

This study investigated chromium (III) stability in chromium wet blue shavings (WBCS) during the leaching with water at different pH conditions and after they were heat treated. Raw WBCS were gathered from 10 local tanneries and stored between 0 and 140 days. Samples were characterized by Fourier Transform Infrared Spectroscopy, Temperature Programmed Reduction with hydrogen gas flow, Thermogravimetric-differential thermal analysis, scanning electron microscope, X-ray diffraction and leached with water at pH 3, 7, and 9 for 2h, 350 rpm at 25 °C. Released chromium in the leachates was quantified by UV–vis spectroscopy. WBCS were also heat treated up to 500 °C before leaching with water at pH 7. The results showed that chromium III in the shavings requires a minimum storage time of 14 days to finish its stabilization between the collagen fibers. Chromium VI was not detected in WBCS without heat treatment, and chromium leached was best released at pH 3. In heat-treated WBCS, the extraction of chromium III in water increases with the treatment temperature; however, at 300 °C, chromium stabilizes strongly. Between 400 and 500 °C, chromium III oxidizes to chromium VI, obtaining Cr6+ in higher proportion in the leachates than Cr3+. Thus, it is essential that WBCS pre-treat them with basifying agents in the tanning process or treatment at 300 °C to use in the circular economy.

Keywords

• Tannery
• Collagen
• Wet blue
• Circular economy
• Heat treatment