Conventional versus microwave-assisted roasting of sulfidic tailings: Mineralogical transformation and metal leaching behavior

Abstract

Roasting is often required to convert sulfidic minerals into their sulfate and/or oxide forms with the aim to increase the extractability of targeted metals. In this study, sulfidic tailings of Neves Corvo Cu-Zn-Pb-mine (Portugal) underwent conventional roasting (CR) and microwave-assisted roasting (MR) to compare the effect of both heating technologies on the roasting behavior. Upon roasting in air, transformations of mineral phases in the tailings were studied by different techniques using quantitative X-ray diffraction, mineral liberation analysis, Raman spectroscopy and thermogravimetry-differential scanning calorimetry coupled to mass spectrometry. Also, the leaching behavior of elements from the roasted tailings was assessed in water. It was shown that CR and MR induce different reaction pathways for mineral transformations during roasting. While CR led to indirect pyrite oxidation through intermediate sulfate formation, direct pyrite oxidation was the main transformation pathway during MR. This change in reaction pathway can be attributed to the mineral-selective heating induced by microwaves, which in particular accelerates the oxidation rate of pyrite. In agreement with the proposed roasting mechanism, the leaching behavior in water showed that CR resulted in a higher extraction of Cu, Zn, Pb, As and Fe from the tailings – since their sulfate phases are more abundant – compared to MR at the same roasting temperature. Overall, tailings treated by CR and MR within the temperature ranges of 500 °C to 550 °C for 1 h gave optimal leaching efficiencies of the studied metals (water leaching at room temperature for 30 min; stirring speed of 600 rpm; solid-to-liquid ratio of 10 mL · g−1). Furthermore, thermodynamic equilibrium calculations showed that hydrolysis of soluble Fe(III) species strongly controls the pH of the leachates. In comparison to MR tailings, more acidic leachates were generated for CR tailings because of a more pronounced formation of ferric sulfate.