Formation of copper aluminate spinel and cuprous aluminate delafossite to thermally stabilize simulated copper-laden sludge

The study reported herein indicated the stabilization mechanisms at work when copper-laden sludge is thermally treated with γ-alumina and kaolinite precursors, and evaluated the prolonged leachability of their product phases. Four copper-containing phases - copper oxide (CuO), cuprous oxide (Cu2O), copper aluminate spinel (CuAl2O4), and cuprous aluminate delafossite (CuAlO2) - were found in the thermal reactions of the investigated systems. These phases were independently synthesized for leaching by 0.1M HCl aqueous solution, and the relative leachabilities were found to be CuAl2O4<CuAlO2≪≪Cu2O<CuO. The sintering condition and formation mechanism employed to stabilize copper into CuAl2O4 and CuAlO2 are extensively discussed here. With a 3h of short sintering, it was found that CuAl2O4 could be effectively formed between 850 and 950°C by the γ-alumina precursor. Although kaolinite had a lower incorporation capability than γ-alumina, it was found to transform a considerable amount of copper into CuAl2O4 between 950 and 1000°C. At higher temperatures, CuAlO2 was produced only in the γ-alumina system as the occurrence of Cu2O-cristobalite solution in the kaolinite system precluded the production of CuAlO2. The hypothesis that the spinel formation mechanism has two stages was supported by the results of the changing Cu/Al mole ratio in the system, and the rate-limiting step was identified as the diffusion process in the second stage.