This review highlights the alkali roasting of aluminium, chromium and titanium minerals as an alternative to mitigate the environmental pressure associated with their mineral processing.

Extraction of titanium and chromium oxides may be achieved via roasting the respective minerals with alkali at high temperatures, followed by water and organic acid leaching. In this study, sodium and potassium hydroxides are used as alkali for roasting of chromite ores and ilmenite mineral concentrates. The thermodynamic analysis of the roasting process is discussed in terms of designing the process. Samples of chromite and titaniferous minerals were roasted with NaOH and KOH in a temperature range of 400˚C -1000˚C in an oxidizing atmosphere. The roasted chromite and ilmenite samples were further processed in order to extract water-soluble Na 2 CrO 4 from the reacted chromite and purify titanium dioxide from titaniferous minerals, respectively. The TiO 2 purity obtained after roasting at 400 o C with NaOH and double leaching was 49.2 wt.% , whereas when using KOH the purity was 54.5 wt.%. The highest TiO 2 purity obtained after roasting at 1000 o C for 2 hours and double leaching with water and organic acids was 84 wt.%. At low temperature (400 o C) the recovery of chromium was higher for chromite roasted with KOH than for chromite roasted with NaOH. However, at high temperatures (700 o C and 1000 o C) chromium recoveries were similar when roasting with both hydroxides. Around 95% chromium extraction yield was achieved when chromite was roasted with sodium and potassium hydroxides at 1000 o C for 2 hours and water leached.

Highly polycrystalline and pure delafossite phase CuAlO 2 powder has been synthesised within a short annealing period, shorter than most conventional processes. This is an improvement over the conventional synthesis procedures. Conventional synthesis procedure has seen CuAlO 2 only formed at high annealing temperatures ≥ 1100°C over long annealing time, some as long as 96 hours. In the current process, a pure phase devoid of impurities has been obtained at reduced calcination time of 1.5 hours in an argon atmosphere at a temperature of 1150°C. This was confirmed by XRD and SEM/EDX. High temperature DC/AC electrical measurements show a change in conduction mechanism from mixed conductivity (ionic + p-type) in the temperature range of 375 ≥ T ≥ 25°C to intrinsic type behavior above 375°C. The activation energies for these two regimes are 0.27 eV and 0.08 eV respectively. This change from mixed to DC conductivity is confirmed by spectral analysis too. Spectral analysis using the power law also revealed that conduction is of long range hopping. Use of platinum as a contact electrode at elevated temperatures has a detrimental effect on the electrical properties since it encourages the formation of CuAl 2 O 4 at the interface due to the formation of more stable Cu−Pt alloy by virtue of the chemical reaction Pt + 2CuAlO 2 − → CuAl 2 O 4 + Pt Cu .

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The investigation focuses on an alternative route to metallic phase extraction from complex Zambian copper-cobalt sulphide mineral concentrates, derived from the Copperbelt region. In the context of developing a novel process route for metal extraction without sulphur dioxide emission and slag waste generation, we have studied the reduction of copper-cobalt sulphide concentrates via carbothermic reduction in the presence of lime by following the equilibrium: MSzCaOzC5MzCaSzCO(g), where M represents the metallic copper, cobalt and iron. The effects of stoichiometric ratios of MS/CaO and MS/C were analysed over a range of temperature between 1073 and 1273 K. The reaction mechanism was analysed by identifying the phases formed, which were characterised by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. The metallic phase analysis confirmed the presence of 85-97 wt-% alloy, which formed as a result of carbothermic reduction. The extent of metallization was analysed by plotting percentage reduction (%R) for each reaction against time (t), from which three different regions of overall reaction were determined. Based on this approach, the overall reaction mechanism is discussed.