The introduction of sulfates to suppress Cr(III) oxidation during incineration of tannery sludge and reduce leachability toxicity of incineration residue

Incineration is an alternative approach to disposal of tannery sludge due to the significant volume reduction. But the Cr(III) oxidation caused by Ca hydrates or carbonates in the incineration process could lead to incineration residue exceeding the required standard in the leaching toxicity of Cr(VI) and being listed in hazardous waste. A novel method of suppressing Cr(III) oxidation by introducing sulfates during the incineration of tannery sludge is proposed in this study. Cr(III) oxidation can be effectively inhibited with the introduction of NH2SO3H, NH4HSO4 and NaHSO4 at n(S)/n(Ca) over 1.2. The combination reaction of sulfates and CaO prevents the Cr(III) oxidation. The inhibition of Cr(III) oxidation by sulfates is the most effective at 700–900 °C because almost all Ca is incorporated into CaSO4 phase. NH2SO3H and NH4HSO4 perform better in inhibiting Cr(III) oxidation than NaHSO4. The intermediate product of Cr2(SO4)3, which is formed below 500 °C, would decompose into Cr2O3 with the temperature of over 600 °C. However, part of Cr2O3 is still oxidized into CaCrO4 aroused by CaSO4 at a temperature of over 900 °C. Kinetic study showed Cr(III) oxidation could be effectively suppressed even if the heating time extends to 2 h at 800 °C. Toxicity Characteristic Leaching Procedure showed that the leaching concentrations of Cr(VI) from incineration residues with the introduction of NH2SO3H and NH4HSO4 at n(S)/n(Ca) of over 1.2 satisfied the requirement, and resultant residues could not be classified as hazardous waste. Due to inhibiting Cr(III) oxidation with high efficiency at 700–900 °C, the introduction of sulfates can be considered as a potential approach to prevent Cr(VI) formation during the incineration of tannery sludge by fluidized bed incinerator.