Mechanisms of zinc incorporation in aluminosilicate crystalline structures and the leaching behaviour of product phases

This study quantitatively evaluates a waste-to-resource strategy of blending zinc-laden sludge and clay material for low-cost ceramic products. Using ZnO as the simulated zinc-laden sludge to sinter with kaolinite, both zinc aluminate spinel (ZnAlO) and willemite (ZnSiO) phases were formed during the sintering process. To analyse the details of zinc incorporation reactions, γ-AlO and quartz were further used as precursors to observe ZnAlO and ZnSiO formations. By firing the ZnO mixtures and their corresponding precursors at 750-1350°C for 3h, the efficiency of zinc transformation was determined through Rietveld refinement analyses of X-ray diffraction data. The results also show different incorporation behaviour for kaolinite and mullite precursors during the formation of ZnAlO and ZnSiO in the system. In addition, with a competitive formation between ZnAlO and ZnSiO, the ZnAlO spinel phase is predominant at temperatures higher than 1050°C. This study used a prolonged leaching test modified from the US Environmental Protection Agency's toxicity characteristic leaching procedure to evaluate ZnO, ZnAlO, and ZnSiO product phases. The zinc concentrations in ZnO and ZnSiO leachates were about two orders of magnitude higher than that of ZnAlO leachate at the end of the experiment, indicating that ZnAlO formation is the preferred stabilization mechanism for incorporating zinc in ceramic products.