Inhibition of Cr(III) oxidation during thermal treatment of simulated tannery sludge: The role of phosphate

• Cr(III) oxidation was remarkably inhibited by adding (NH4)2HPO4.
• Cr(VI) was reduced due to the presence of phosphate.
• Converting Ca into phosphate minerals eliminated the potential of Ca to trigger Cr(III) oxidation.
• Cr(III) reoxidation was prevented because the P-hosting phases also captured CaO.

The presence of Ca compounds may be an important factor contributing to oxidation of Cr(III) to Cr(VI) during the incineration of tannery sludge. In this study, Cr(III) oxidation was inhibited by preventing Ca participating in Cr(III) oxidation reactions via incorporating it in phosphate minerals. Cr(III) oxidation was remarkably suppressed through immobilizing Ca in phosphate minerals. Around 16% of Cr(III) would be oxidized in samples without phosphate heated at 900 °C for 1 h, which could be reduced substantially to 0.25% with the addition of phosphate at n(P)/n(Ca) = 2. Moreover, phosphate not only captured free CaO, inhibiting Cr(III) oxidation, but seized Ca from CaCrO4, resulting in formed Cr(VI) being reduced. Inhibition of Cr(III) oxidation performed resistant to elevating temperature over 1000 °C and prolonged retention time, due to the irreversible incorporation of Ca into phosphate mineral species. Moreover, incorporating Ca into phosphate minerals thoroughly eliminated the potential of Ca to trigger Cr(III) oxidation. Additionally, the P-hosting phases produced (e.g., Ca(PO3)3 and Cr(PO3)3) were also able to capture free CaO and inhibit Cr(III) oxidation, so Cr(III) oxidation continued to be inhibited during flue gas cooling and fly ash reheated process. A case study, in which tannery sludge was heated, confirmed that adding phosphate effectively inhibited Cr(III) oxidation. Adding phosphate could therefore be an effective way of inhibiting Cr(III) oxidation when solid waste containing Cr(III) is heated.