Application of ZnAlFeO4 spinel and residual ash generated from pyrometallurgical magnetic separation processes: Catalysis and heavy metals immobilization

The recycling of electroplating sludge (ES), a byproduct rich in heavy metals (HMs), is receiving increasing attention in recent years, especially in the context of HMs recovery and recycling. This study investigated the application of ZnAlFeO₄ spinel and residual ash generated from pyrometallurgical magnetic separation processes, focusing on the catalytic performance and solidification of HMs. The photocatalytic degradation of methylene blue and tetracycline using ZnAlFeO₄ spinel was conducted, with degradation rates of 74.9 % and 83.6 %, respectively. The residue ash, rich in Zn and Si/Al impurities, was stabilized through co-sintering with fly ash, forming calcium silicate glass, which immobilized HMs effectively. The optimal solidification of HMs was achieved at 1400 °C with 50 % fly ash, reducing Zn leaching to 3.35 mg/L and Cr below detectable levels. The mechanism of HMs solidification involves the formation of vitreous liquid slag, providing fixed sites for the immobilization of HMs within the silicate structure and effectively restraining their release. However, an excess of fly ash resulted in the formation of CaAl₂Si₂O₈ crystals, disrupting the silicate glass framework and releasing HMs. The study provides insights into the transformation of ES into functional materials, contributing to waste management and environmental protection through the development of sustainable technologies.