Effect of incineration temperature on chromium speciation in real chromium-rich tannery sludge under air atmosphere

As a hazardous waste, the disposal of chromium enriched tannery sludge has attracted increasing public concern due to its potential adverse risks towards the environment. And incineration is considered to be an effective method to stabilize heavy metals, like Cr, in solid phase during tannery sludge treatment. In this study, real chromium enriched tannery sludge without pre-treatment was incinerated at 300°C-1200 °C under air atmosphere to investigate the transformation of chromium speciation. Here detailed thermal behavior, phase transformation and chromium speciation were characterized by TG-DSC, XRD and XPS, respectively. Experimental results show that content of Cr(VI) increases gradually with the increase of temperature from 300 °C to 500 °C and reaches a maximal level of 46% total Cr at 500 °C, with different Cr(VI) species of CaCrO₄, MgCrO₄ and Cr₅O₁₂. However, the content of Cr(VI) decreases gradually with the further increase of temperature, with only about 5% Cr(VI) at high temperature of 1200 °C, due to formation of Cr(III) species of Cr₂O₃ crystallite and MgCr₂O₄ spinel. Besides, a growing number of hexagonally shaped flake-like crystallite Cr₂O₃ can be discovered from characterization results of XRD and SEM. Finally, the reduction of CaCrO₄ to Cr₂O₃ in the presence of SiO₂ is thermodynamically feasible over 700 °C, indicating possible transformation of Cr(VI) to Cr(III) through controlled incineration temperature.