Zn stabilization mechanism during the gasification-melting of sewage sludge

[Display omitted] •Sewage sludge gasification-melting process was in-situ characterized by a visual furnace.•Significant Zn release occurred both in solid sintering and liquid sintering stage.•Hematite readily combined with Zn to form ZnFe2O4, enhancing Zn immobilization.•Kaolin stabilized Zn by forming spinel crystal and molten encapsulation. The Zn stabilization mechanism in the gasification-melting process of sewage sludge (SS) was investigated using an in-situ visual furnace and the species distribution of Zn. The results demonstrated that Zn release mainly occurred in the solid sintering stage (800–900 °C), and the liquid sintering stage (>900 °C) during the gasification-melting of SS. In the solid sintering stage, the combination of Fe2O3 and Zn resulted in the formation of stable zinc ferrite spinel, which achieved the efficient enrichment and stabilization of Zn in the residual. The mineral melting of SS above 900 °C caused the decomposition and volatilization of Zn-containing phases, leading to the release of Zn. The Zn release at high temperatures can be effectively reduced by the addition of kaolin. On the one hand, the kaolin addition promoted the formation of zinc-aluminum spinel and aluminum silicate which stabilized by Zn in the crystal structure. On the other hand, the addition of kaolin facilitated the formation of the liquid phase, stabilizing Zn through the coating effect. This study demonstrated the key roles of Zn-Fe minerals and slag in the Zn stabilization during SS gasification-melting.