Alkali roasting of bomar ilmenite: rare earths recovery and physico-chemical changes.

In this work, the alkali roasting of ilmenite (FeTiO3) is presented as a process route for integrated beneficiation of the mineral for rutile-rich phase and rare earth oxides; the latter is released as a consequence of physical changes in the ilmenite matrix, during the water leaching after roasting. The oxidative alkali roasting transforms ilmenite mineral into water-insoluble alkali titanate and water-soluble ferrite. After roasting the insoluble alkali titanate is separated from rare-earth oxide mixture in colloidal form and water-soluble ferrite. Further leaching of alkali titanate is carried out with oxalic (0.3M) and ascorbic (0.01M) acid solution which removes the remaining Fe2+ ions into the leachate and allows precipitation of high-purity synthetic rutile containing more than 95% TiO2. Iron is removed as iron oxalate. The physico-chemical changes occurred during the roasting and leaching processes are reported by comparing the role of alkali on the roasting process and product morphologies formed

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

A comparison of methods for the estimation of the enthalpy of formation of rare earth compounds

Rare earth elements are helping drive the global transition towards a greener economy. However, the way in which they are produced is far from being considered green. One of the major obstacles to developing greener production methods and the design of novel processes and materials involving rare earth elments is the limited thermodynamic data available. In the present work, we apply a suite of methods to estimate the enthalpy of formation of several rare earth compounds, including a new method based on a linear relationship, established by the authors. Experimental values of the enthalpy of formation of LnCl3, LnOCl, LnPO4, Ln2O2S, Ln2O2CO3 and NaLnO2 were collated and used to assess the accuracy of the different methods, which were then used to predict values for compounds for which no data exists. It is shown that Mostafa´s et al1 group contribution method and the linear relationship proposed by the authors give the lowest mean absolute error (<9%). The volume based thermodynamics (VBT)2,3 method yields estimates with an absolute mean errors below 16.0% for LnPO4 and Ln2O2S, but above 26.0% for other compounds. Correction of the VBT method using an improved estimate of the Madelung energy for the calculation of the lattice enthalpy decreases the absolute mean error below 12.0% for all compounds except LnPO4. These complementary methods provide options for calculating the enthalpy of formation of rare earth compounds, depending on the experimental data available and desired accuracy

Nanoparticle corona artefacts derived from specimen preparation of particle suspensions

Progress in the implementation of nanoparticles for therapeutic applications will accelerate with an improved understanding of the interface between nanoparticle surfaces and the media they are dispersed in. We examine this interface by analytical scanning transmission electron microscopy and show that incorrect specimen preparation or analysis can induce an artefactual, nanoscale, calcium phosphate-rich, amorphous coating on nanoparticles dispersed in cell culture media. We report that this ionic coating can be induced on five different types of nanoparticles (Au, BaTiO3, ZnO, TiO2 and Fe2O3) when specimen preparation causes a significant rise in pH above physiological levels. Such a pH change reduces ionic solubility in the suspending media to permit precipitation of calcium phosphate. Finally, we demonstrate that there is no indication of a calcium-phosphorus-rich coating on BaTiO3 nanoparticles suspended in culture media when prepared without alteration of the pH of the suspending media and imaged by cryo-STEM. Therefore we recommend that future reports utilising nanoparticles dispersed in cell culture media monitor and report the pH of suspensions during sample preparation

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

Influence of the Alkali-promoted phase transformation in monazite for selective recovery of rare-oxides using deep eutectic solvents

This is the final version. Available on open access from Elsevier via the DOI in this recordThe physico-chemical changes occurring during the high-temperature phase transformation of monazite in the presence of Na2CO3 at 1000 °C for 2 h duration at monazite: Na2CO3 ratios between 1.0 and 5.0, were investigated. The formation of sodium lanthanide phosphates was prevalent above a monazite:alkali ratio of 2, however, below this ratio, the dephosphorization of monazite as Na3PO4 and solid solutions occur offering unique selectivity for rare-earth oxide separation from the mineral matrix. Cyclic voltammetry of pure CeO2, La2O3, Nd2O3, and PrO2/Pr2O3 was carried out in the deep eutectic solvent Ethaline (1:2 mixture of choline chloride and ethylene glycol) proving the electrochemical activity of these oxides. Electrodissolution of pure oxides and water-leached monazite after high-temperature reaction with a ratio of 1:1 was carried out in a 0.1 mol/L glucose solution in Ethaline showing a preferential solubility of 23.85% for pure Nd2O3. In contrast, pure oxides of CeO2, La2O3 and PrO2/Pr2O3 were found to be insoluble. We also observed that electrodissolution of the water leached monazite was not possible because of the inert behaviour of solid solutions. Avoiding cerium oxidation during the high-temperature process will lead to a method for further selectivity for rare-earth oxide processing using staged electro-chemical winning of oxides.Natural Environment Research Council (NERC)Ministry of Science, Innovation and University of Spai

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

An investigation on hydrofluoric (HF) acid-free extraction for niobium oxide (Nb2O5) and tantalum oxide (Ta2O5) from columbite/tantalite concentrates using alkali reductive roasting

Tantalum, niobium, and their oxides are important precursor materials, essential for high-temperature alloys and electronic devices. The primary hydrometallurgical extraction technique to extract tantalum and niobium from minerals involves hydrofluoric acid (HF) digestion of the concentrates, followed by solvent extraction as an oxide separation and purification step. Solvent extraction, on the other hand, releases organic solvents which are lost irreversibly via natural evaporation during the process. This research demonstrates a novel chemical process for the extraction and refining of columbite and tantalite concentrates (29% Ta2O5 and 16% Nb2O5). In this process, the concentrates are reduced using carbon and alkali in the temperature range of 800–950 °C, which helps in reducing and magnetically separating the iron oxides present in the concentrates as metallic iron. The remaining residue is rich in alkali complex (e.g., sodium tantalates and niobates) formed during the roasting process which was reclaimed as a purified mixture of oxides of Nb2O5 and Ta2O5, by using oxalic acid leaching, followed by sodium bisulphate roasting

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