A kinetic study on the mechanisms of metal leaching from the top surface layer of copper aluminates and copper ferrites.

Recent studies have reported the potential copper immobilization in aluminates (CuAl2O4 and CuAlO2) and ferrites (tetragonal CuFe2O4 and cubic CuFe2O4) and suggested a reliable method to stabilize metals in reusable ceramic products. In this study, copper immobilization effect was further analyzed in the leaching solutions with pH close to environmental conditions. The results from the chemical equilibrium model Visual MINTEQ illustrated that almost all copper, aluminum, and iron formed complexes with CH3COO- ions in the leachates. The dissolution behavior on sample surface was further explicated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The weight percentage of leached copper was lower than 0.1% even after 22-day leaching, indicating the successful copper stabilization in aluminates and ferrites. The results showed the highest copper concentrations in CuAlO2 leachates and the smallest leached copper amount from tetragonal CuFe2O4, respectively. The incongruent dissolution with Al-O or Fe-O bonds still remaining on the solid surface may be beneficial for preventing further leaching of copper. Furthermore, the modeling of reaction kinetics found that copper leaching from the CuAl2O4 and CuAlO2 obeyed the second-order reaction with correlation coefficients higher than 0.99. Moreover, the shrinking core model was chosen to analyze the leaching mechanisms of both CuFe2O4 ferrites, and the diffusion through product layer model acted as the rate-controlling step in their leaching process.