Reduction by Hydrogen of the Waste Rock Minerals Constituents of a Vanadium Ore

Abstract

A sample of vanadium ore has been characterized by X-ray diffractometry (XRD), scanning electron microscopy
coupled with dispersive energy spectrometry (SEM-EDS) and thermogravimetric analysis (TG). The mineral
composition was determined by the Rietveld method and the results showed that the major mineral constituents are maghemite (52.32%) and magnetite (15.73%). The SEM-EDS images showed that the vanadium is diffused over the crystal structure of almost all the minerals. Hydrogen reduction was assessed by thermogravimetry (TG), where three thermal events has been identified in the TG curves. The first event was probably related to two chemical reactions: (i) the oxidation of the mineral ulvospinel by different vanadium oxides, followed by the reduction of the products of this redox reaction by the hydrogen, and (ii) the reduction of the mineral magnesioferrite by the hydrogen. The other two events were attributed to the dehydroxylation of the phyllosilicates (greenalite, cronstedtite and sudoite) and the reduction of maghemite, magnetite, titanium oxides and vanadium oxides. Finally, the integration isoconversional method was used to determine the apparent activation energy (Ea), which ranged from 44.28kJ.mol-1 to 82.47 kJ.mol-1 for the Kissinger-Akahira-Sunose method (KAS), and from 33.44kJ.mol-1 to 65.54kJ.mol-1 for the Ozawa-Flynn-Wall method (OFW). These values imply that the reduction is controlled by mixed process for low conversion values (diffusion and chemical reaction are the controlling steps) and by chemical reaction for high conversion values.