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

In nature, the commonly occurring reactive metal oxides of titanium, chromium, aluminium, and vanadium often chemically combine with the transition metal oxides such as iron oxides and form complex minerals. Physico-chemical separation of transition metal oxides from the remaining reactive metal oxides is therefore an important step in the purification of reactive oxide constituents. Each purification step has quite a high energy requirement at present. Current practice in industry yields sulphate and neutralized chloride waste from titanium dioxide enrichment, red mud from bauxite refining, slag and leach residues from vanadium extraction and chromite ore process residue (COPR) from chromate processes. In this review article, a novel alkali-based oxidative roasting and aqueous leaching for the extraction of mineral oxides is explained in the context of the original work of Le Chatelier in 1850, which was unsuccessful in the industrialization of bauxite processing for alumina extraction. However, much later in the 19th century the alkali-based oxidative mineral roasting was successfully developed for industrial scale manufacturing of chromate chemicals, which yields COPR. The crystal chemistry of mineral oxides, namely alumina, titanium dioxide, and chromium oxide in naturally occurring minerals is briefly reviewed in the context of chemical extraction, which is then developed as a model for developing thermodynamic chemical equilibrium principles for analyzing the physical separation and enrichment of such reactive metal oxides by forming water-soluble and water-insoluble alkali complexes. The involvement of the alkali roasting chemistry of non-magnetic titaniferous mineral waste is also reported in the initial separation of rare-earth oxide mixtures for subsequent separation of individual oxides. The paper concludes with a generic approach to process chemistry which minimizes waste generation and therefore helps in reducing the overall process and energy costs. Examples of recovering alkali from high pH solution using carbon dioxide are also demonstrated

Chemical Characterization of Transition Metal (V, Zr, Nb) Impurities in Rutile

The dissolution and chemical properties of vanadium, zirconium, and niobium in rutile lattice are analyzed by comparing the octahedral site preference energies, with respect to Ti4+ ion in solvent phase. The oxides are often present in rutile lattice and interfere in achieving the quality of pigment grade TiO2 due to favourable Gibbs energy for dissolution in the solvent rutile matrix. Binary mixtures of V2O5 – TiO2; Nb2O5 – TiO2 and ZrO2 – TiO2 with compositions of 80, 85, 90, 95 and 97 wt% TiO2 were made, pelletized and sintered at 1100 oC for 24 hours. The solid-solution mixtures were then cooled and leached in 2M NaOH for 3 hours at 60 oC. Phase and lattice parameter characterizations, and microstructural and compositional analyses were then carried out using X-ray diffraction and SEM/EDX, respectively. The leachates were analysed by Atomic Absorption Spectroscopy, using which the solute dissolution model for oxides were investigated

A Kinetic Analysis of Acid Leaching of Niobium and Zirconium from Titania Waste Residue Stream: An Energy Efficient Methodology for the Reclamation of Metal Values

Residues from rutile chlorination plants often contain relatively high concentrations of critical metals, essential for energy devices. Ironically, the conventional method of extraction of niobium and zirconium is quite energy demanding and detrimental to the environment. In this investigation, the kinetics of dissolution of niobium and zirconium from titania waste in hydrochloric acid solutions are investigated. The thermodynamic stability of minerals demands the use of chlorination, carbothermic reduction or alkaline fusion for breakdown of their mineral concentrate and upgrading before leaching in acid, usually hydrofluoric acid. Reclamation of niobium and zirconium values from titania waste presents an opportunity for a low energy process to be utilised. The effects of parameters leaching temperature (25 - 90 oC), acid concentration (0.5 - 2.5M), stirring speed and solid-to-liquid ratio were determined in the experiments. Leachates were analysed by ICP OES, using which the models for the leaching processes and activation energies were determined

Kinetics of hydrochloric acid leaching of niobium from TiO2 residues

Production of TiO₂ generates waste containing significant quantities of valuable metals which if recovered, could have a positive impact on the economics of TiO₂ production and waste management. In this investigation, the kinetics of HCl leaching of niobium from TiO₂ residues are studied. The complex mineralisation of niobium in its primary ores makes economic recovery very difficult, often demanding the use of chlorination, carbochlorination or fusion with alkali fluxes for breakdown of its mineral concentrates and upgrading before leaching in acid, usually hydrofluoric acid. The effects of parameters leaching temperature (25–90 °C), HCl concentration (0.5–4 M), stirring speed (100–500 rpm) and solid–liquid ratio were determined in the experiments. A maximum niobium extraction rate > 90% was achieved within 60 min of leaching the residues in 4 M HCl at 70 °C. The kinetics analysis showed that the dissolution of niobium in HCl is governed by pore diffusion of the random pore model, with an activation energy of 16.8 ± 1.2 kJ mol−¹ Nb