Co-combustion of Zn/Cd-hyperaccumulator and textile dyeing sludge: Heavy metal immobilizations, gas-to-ash behaviors, and their temperature and atmosphere dependencies

This study quantified and revealed the temperature and atmosphere dependencies of the enrichment rates and speciation distributions of Zn and Cd and the behaviors of Cl and S for the co-combustion of a hyperaccumulator (SAH) of Zn and Cd and textile dyeing sludge (TDS) at a blend ratio of 3:1 (ST31). The addition of Al-rich TDS to SAH provided the chemisorption sites for Zn and Cd and generated stable Al/Si structures for their stabilization in the ST31 ash. The rising temperature and the atmosphere change from N2/O2 to CO2/O2 transformed Zn and Cd into their oxidizable and residual fractions. Cl promoted the volatilizations of the heavy metals, with its content in the ST31 ash falling from 86.28% at 650 °C to 17.98% at 950 °C. The S content (31.08–33.86%) of the ST31 ash existed mainly as CaSO4 and was slightly higher in the CO2/O2 than N2/O2 atmosphere (29.45%) since the high CO2 concentration adversely influenced the decomposition of CaCO3, while S indirectly affected the migrations of Zn and Cd. The combined results of the experiments, thermodynamic simulations, and multi-objective optimization pointed to 850 °C in the oxy-fuel atmosphere with 30% O2 concentration as the optimal settings in order to stabilize Zn and Cd with an acceptable risk. The possible reaction pathways and immobilization mechanisms were also derived considering the interactions among minerals, Zn, Cd, Cl, and S.