Comparative study of alkali roasting and leaching of chromite ores and titaniferous minerals

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 oxidising atmosphere. The roasted chromite and ilmenite samples were further processed in order to extract water-soluble Na2CrO4 from the reacted chromite and purify titanium dioxide from titaniferous minerals, respectively. The TiO2 purity obtained after roasting at 400 °C with NaOH and double leaching was 49.2 wt.%, whereas when using KOH the purity was 54.5 wt.%. The highest TiO2 purity obtained after roasting at 1000 °C for 2 h and double leaching with water and organic acids was 84 wt.%. At low temperature (400 °C) the recovery of chromium was higher for chromite roasted with KOH than for chromite roasted with NaOH. However, at high temperatures (700 °C and 1000 °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 °C for 2 h and water leached

Towards sustainable processing of columbite group minerals: elucidating the relation between dielectric properties and physico-chemical transformations in the mineral phase

Current methodologies for the extraction of tantalum and niobium pose a serious threat to human beings and the environment due to the use of hydrofluoric acid (HF). Niobium and tantalum metal powders and pentoxides are widely used for energy efficient devices and components. However, the current processing methods for niobium and tantalum metals and oxides are energy inefficient. This dichotomy between materials use for energy applications and their inefficient processing is the main motivation for exploring a new methodology for the extraction of these two oxides, investigating the microwave absorption properties of the reaction products formed during the alkali roasting of niobium-tantalum bearing minerals with sodium bicarbonate. The experimental findings from dielectric measurement at elevated temperatures demonstrate an exponential increase in the values of the dielectric properties as a result of the formation of NaNbO3-NaTaO3solid solutions at temperatures above 700 °C. The investigation of the evolution of the dielectric properties during the roasting reaction is a key feature in underpinning the mechanism for designing a new microwave assisted high-temperature process for the selective separation of niobium and tantalum oxides from the remainder mineral crystalline lattice

An investigation on the formation of molten salt containing chromium oxide during roasting of chromite ore with sodium and postassium hydroxides

The extraction of chromium from chromite ore is based on the oxidative alkali roasting of the mineral forming water-soluble alkali chromates. Previous investigations reported the formation of a molten Na2CO3-Na2CrO4 binary mixture during roasting of chromite with sodium carbonate. The physical properties of the Na2CO3-Na2CrO4 liquid phase, which are dependent on temperature, charge and gangue composition, play an important role in the oxidation reaction and may limit the chromium recovery by hindering the oxygen transport to the reaction interface. This investigation focuses on the alkali roasting of chromite ore at 1000oC using NaOH and KOH, and subsequent water leaching. The influence of the alkali ratio on the chromium extraction yield is analysed, and the results obtained with both hydroxides are compared. The formation of molten salt phase under different roasting conditions and its effect on chromium recovery is studied by means of sample characterization and phase diagram analysis

A Cr⁶⁺⁻ Free Extraction of Chromium Oxide from Chromite Ores Using Carbothermic Reduction in the Presence of Alkali

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 Cr6+-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 °C. Under these conditions, the iron oxides present in the ore body are reduced to metallic iron and the resulting separation of chromium 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 atmosphere and subsequent leaching conditions, the oxidation of Cr3+ to Cr6+ is completely avoided; thereby decreasing the risk of land, air and water pollution

An Investigation on the Kinetics and Mechanism of Alkali Reduction of Mine Waste Containing Titaniferous Minerals for the Recovery of Metals

In a world where declining ore grades are increasingly common, it has become necessary to process low-grade feedstock. Carbothermic reduction in the presence of alkali (Na2CO3) has been adapted to beneficiate waste that contains titaniferous minerals (TiO2 ca. 12 wt%), in order to recover valuable constituents such as TiO2, Fe and V2O5. The waste from vanadium metal processing has environmental legacy as it leaves nearly 1 wt% V2O5 process waste, which is environmentally problematic due to V5+ ions in contact with water and soil. This investigation focuses on the kinetics and mechanism for alkali reduction of mineral waste bearing 10–12 wt% TiO2, which we studied in the 1073–1323 K range. The thermogravimetric analysis (TGA) technique was used to record weight loss data. Two distinct regimes demonstrated mixed-control kinetics: (1) at low temperatures the activation energy was found to be 199 kJ mol−1, which corresponds to the outward diffusion of O2− ions; and (2) at high temperatures the calculated value was 130 kJ mol−1, which is consistent with the activation energy for the outward diffusion of Fe2+ ions. The metallic iron, sodium titanate and sodium aluminosilicate phases that formed were characterised using X-ray powder diffraction (XPRD) and scanning electron microscopy (SEM) techniques, and their significance for metal recovery is explained