Study on physicochemical characteristics, solidification and utilisation of tannery sludge gasification ash

Gasification is an attractive method for tannery sludge (TS) disposal because of its advantages: volume reduction, stabilisation, harmlessness, and energy recovery. TS reduction ash (A_R) and TS oxidation ash (A_O), simulated from a downdraft fixed bed gasifier (DFBG) and an updraft fixed bed gasifier (UFBG), were investigated on their physicochemical characteristics, solidification behaviour, and value-added utilisation. Results showed that the main mineral matters in A_R and A_O consisted of Fe-oxids and Fe-Cr compounds, and the DFBG was more suitable for TS gasification than the UFBG because of the lower content of Cr(Ⅵ) in A_R. With the addition of waste glass bottles (W_GB), the ash fusion temperatures (AFTs) and leaching concentrations of heavy metals in A_R and A_O decreased significantly, and the heavy metals in A_R and A_O were successfully immobilised by the wrapping effect of the molten W_GB. Moreover, gasification ash, as an auxiliary material for rock wool, reduced the AFTs and viscosity coefficient of the main chemical compositions in rock wool. With the addition of A_R, the occurrence of Fe-containing compounds and the extremely low risk of leaching toxicity of heavy metals were observed. The maximum addition proportion of gasification ash was dependent on the maximum content of Fe₂O₃ allowed in the raw materials of rock wool, and its addition ratio must be below 15%.