Chromium is a potentially toxic metal occurring in water and groundwater as a result of natural and anthropogenic sources. Microbial interaction with mafic and ultramafic rocks together with geogenic processes release Cr (VI) in natural environment by chromite oxidation. Moreover, Cr (VI) pollution is largely related to several Cr (VI) industrial applications in the field of energy production, manufacturing of metals and chemicals, and subsequent waste and wastewater management. Chromium discharge in European Union (EU) waters is subjected to nationwide recommendations, which vary depending on the type of industry and receiving water body. Once in water, chromium mainly occurs in two oxidation states Cr (III) and Cr (VI) and related ion forms depending on pH values, redox potential, and presence of natural reducing agents. Public concerns with chromium are primarily related to hexavalent compounds owing to their toxic effects on humans, animals, plants, and microorganisms. Risks for human health range from skin irritation to DNA damages and cancer development, depending on dose, exposure level, and duration. Remediation strategies commonly used for Cr (VI) removal include physico-chemical and biological methods. This work critically presents their advantages and disadvantages, suggesting a site-specific and accurate evaluation for choosing the best available recovering technology.

https://www.mdpi.com/1660-4601/17/15/5438/pdf

Chromium Pollution in European Water, Sources, Health Risk, and Remediation Strategies: An Overview.

The extraction of vanadium from titanomagnetites and other sources

Dealkalization processes of bauxite residue: A comprehensive review

Innovative methodology for comprehensive and harmless utilization of waste copper slag via selective reduction-magnetic separation process

Bauxite residue (red mud) with a high alumina content requires large amounts of fluxes during smelting, which increases the overall process cost, the energy consumption during smelting, and the acid consumption during slag leaching and decreases the throughput of bauxite residue in the smelting phase. Therefore, alumina was removed (or recovered) from bauxite residue by alkali roasting prior to smelting. The sample after alumina removal could be smelted at 1500 °C without the need of adding any flux to obtain a clear slag–metal separation. After smelting, the slag was leached with acids (HCl, H2SO4, or HNO3). The recovery yields of the rare-earth elements (REEs), except scandium, drastically decreased in the low-alumina slags, compared to the original bauxite residue. These poorer recovery yields are due to the formation of a perovskite (CaTiO3) phase, which binds REEs except scandium. CaTiO3 is a stable phase and does not dissolve in acids under normal conditions. Therefore, slag was quenched to suppress the CaTiO3 formation. The REEs and titanium could also be dissolved in acidic solutions at 25 °C by quenching the molten slag. Removal of alumina from bauxite residue by alkali roasting with sodium carbonate requires high temperatures (>900 °C). Therefore, sodium carbonate was replaced with sodium hydroxide to decrease the energy consumption by decreasing the roasting temperature (<500 °C).

Recovery of Rare Earths and Major Metals from Bauxite Residue (Red Mud) by Alkali Roasting, Smelting, and Leaching

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

https://pubs.rsc.org/en/Content/ArticlePDF/2015/GC/C4GC02360A

Reclamation of reactive metal oxides from complex minerals using alkali roasting and leaching – an improved approach to process engineering

Mineral sulphide (MS)-lime (CaO) ion exchange reactions (MS + CaO = MO + CaS) and the effect of CaO/C mole ratio during carbothermic reduction (MS + CaO + C = M + CaS + CO(g)) were investigated for complex froth flotation mineral sulphide concentrates. Phases in the partially and fully reacted samples were characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The primary phases during mineral sulphide-lime ion exchange reactions are Fe3O4, CaSO4 Cu2S, and CaS. A complex liquid phase of Ca2CuFeO3S forms during mineral sulphide-lime exchange reactions above 1173 K. The formation mechanisms of Ca2CuFeO3S liquid phase are determined by characterising the partially reacted samples. The reduction rate and extent of mineral sulphides in the presence of CaO and C increase with the increase in CaO/C ratio. The metallic phases are surrounded by the CaS rich phase at CaO/C > 1, but the metallic phases and CaS are found as separate phases at CaO/C 1 and the reacted samples are excessively sintered.

Mineral sulphide-lime reactions and effect of CaO/C mole ratio during carbothermic reduction of complex mineral sulphides

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: MS+CaO+C = M+CaS+CO(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 exten...

Carbothermic reduction of Zambian sulphide concentrates in presence of lime

The reaction mechanisms for the carbothermic reduction of complex mineral sulfide concentrates in the presence of lime were studied between 1073 K and 1323 K. The reaction mechanisms were studied by stopping the reduction experiments at different times and analyzing the reaction products by x-ray diffraction and scanning electron microscopy techniques. Magnetite (Fe3O4) and digenite (Cu1.8S) were the initial phases formed during reduction of CuFeS2 and Cu5FeS4 mineral particles, such that metallization of iron occurred before copper above 1173 K and at an equal stoichiometric ratio of CaO and C. The metallization of iron was found to take place via reduction of intermediate oxide phase (Fe3O4/FeO), whereas metallization of copper occurred via diffusion of S2− ions away from the mineral particles or via formation of Cu-O-S liquid phase. Metallic iron and cobalt were embedded in the copper matrix due to a preferential reduction of iron and cobalt from the Cu-Fe-S and Cu-Co-S type of mineral particles. The effects of CaO/C ratio were analyzed and the rate of reactions was increasing with an increase in the CaO/C ratio. The formation of liquid phase has been discussed. The experimental results were found to be in good agreement with the thermodynamic predictions.

Yotamu Stephen Rainford Hara

Papers

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

Reclamation of reactive metal oxides from complex minerals using alkali roasting and leaching – an improved approach to process engineering

Mineral sulphide-lime reactions and effect of CaO/C mole ratio during carbothermic reduction of complex mineral sulphides

Carbothermic reduction of Zambian sulphide concentrates in presence of lime

Study of Reaction Mechanisms for Copper-Cobalt-Iron Sulfide Concentrates in the Presence of Lime and Carbon