Thermal separation of zinc and other valuable elements from municipal solid waste incineration fly ash

Abstract

Municipal solid waste incineration fly ashes can contain high concentrations of Zn and other valuable elements including Cu, Pb, Sn, and Sb. These elements can potentially be separated from fly ashes by selectively volatilising and condensing them in thermal processes. This study presents a thermal process for production of zinc concentrates from fly ashes and assesses the impact of the reducing gas atmosphere used in the process on the volatility of Zn, Pb, and Cu. Concentrates were produced by heating samples of municipal solid waste incineration fly ash in a reducing atmosphere, consisting of 10% H2 and 90% N2, and selectively condensing elements which volatilised from the ash at 900 and 1100 °C. Thermodynamic equilibrium calculations were used to predict the volatile behaviour of Zn, Pb, and Cu in the reducing atmosphere and in an oxidising atmosphere (air). The extent of volatilisation of these elements was also determined experimentally for both the reducing atmosphere and the oxidising atmosphere by means of mass balances based on elemental analyses of untreated and heat-treated ashes. The concentrate produced at 900 °C contained high concentrations of Zn (55.9 ± 12.5% w/w) and Pb (4.7 ± 1.1% w/w). These metals and other higher value elements, specifically, Sn, Sb, and Bi, were greatly enriched in the concentrates when compared with the untreated fly ash, showing a high potential for the developed process to separate multiple valuable elements from ashes. However, several deleterious impurities, particularly K, Na, and Cd, also reported to the concentrates in significant concentrations. Further refinements to the process are needed to separate these elements from Zn. The reducing gas atmosphere substantially increased the volatility of Zn, but suppressed the volatility of Cu and, to a lesser extent, the volatility of Pb. The equilibrium calculations overestimated the release of Cu in oxidising conditions and the release of Zn and Pb in both oxidising and reducing conditions.