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.

Chromium has a wide range of applications including metals and alloys manufacturing, pigments, corrosion resistance coatings and leather tanning. The production of chromium chemicals is based on the oxidative alkali roasting of chromite ores, which leads to the formation of water-soluble alkali chromates. Previous investigations reported that when chromite is roasted with soda-ash, a molten salt containing chromium, which is mainly composed of sodium carbonate and sodium chromate (Na2CO3-Na2CrO4 binary mixture), forms under typical roasting conditions. The physical properties of the liquid phase, which are dependent on the temperature, charge and gangue compositions, play an important role on the oxidation reaction and may limit the chromate recovery by hindering the oxygen transport to the reaction interface.This investigation focuses on the alkali roasting of chromite ore at 1000 o C using NaOH and KOH, followed by water leaching. The influence of the alkali ratio on the chromium extraction yield is analysed, and the results obtained with both hydroxides are compared. Sample characterisation and thermodynamic analysis, including phase diagrams, equilibrium calculations and computation of liquidus curves, are combined with the purpose of studying the formation of the molten salt phase under different roasting conditions and its effect on the final chromium recovery.

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Oxidative alkali roasting of chromite is the state-of-the-art process for manufacturing chromium-containing chemicals, which involves dealing with serious environmental problems arising from handling Cr 6+ -containing wastes generated in this process. In this article a new method for the extraction of Cr2O3 from chromite ores is explained, based on the carbothermic reduction of concentrates in the presence of alkali investigated in the temperature range of 950-1050 o C. Under these conditions, the iron oxides present in the ore body are reduced to metallic iron and the resulting separation of chromium complex occurs by forming sodium chromite (NaCrO2). The reduced samples are magnetically separated for the recovery of an iron-rich fraction, and a non-magnetic fraction containing NaCrO2, MgO and other impurities. The further treatment of the non-magnetic fraction by leaching yields a Cr2O3-rich product of approximately 85% purity, with remaining alumina, alkali and magnesia. The main advantage of the process is that, under reducing and subsequent leaching conditions, the oxidation of Cr 3+ to Cr 6+ is completely avoided; thereby decreasing the risk of land, air and water pollution.