Leaching of crude titanium powder produced by metallothermic reduction : effects of leaching conditions on final powder quality

Abstract

A low-cost titanium production process, the CSIR-Ti powder process, which aims to produce titanium powder directly by metallothermic reduction of titanium tetrachloride with lithium, has been under development at the Council for Industrial and Scientific Research (CSIR). Crude titanium powder produced using the CSIR-Ti process is inevitably contaminated with by-products such as lithium chloride, lithium and titanium dichloride. These by-products tend to become sources of impurities in titanium powder, specifically oxygen and chloride impurities. The presence of oxygen and chloride impurities has marked effects on the mechanical properties of titanium finished products. Consequently, for the crude titanium powder to be rendered useful downstream, it must be purified and the by-products reduced to concentrations specified in the commercial standards. The present study was undertaken to examine whether acid leaching could be used to selectively dissolve and prevent hydrolysis of the by-products—specifically excess lithium and unreacted titanium dichloride in the crude titanium powder produced by the CSIR-Ti process. A further objective was to determine whether a purified product that meets both oxygen and total residual chloride content as specified by the standards can be achieved. The effects of key leaching variables and their interaction were also investigated to gain fundamental understanding of these effects on the by-products leaching behaviour. A literature study to select a suitable lixiviant and to establish the aqueous chemistry of the byproducts and their effect on the leaching conditions was undertaken. It showed that of the various acids suggested in the literature, hydrochloric acid was the cheapest and that it was more suited for the CSIR-Ti leaching process than nitric acid, due to the common ion chloride. This simplifies the leachate purification process downstream. The literature study established that Ti(II) has no aqueous chemistry but instead is oxidised to Ti(III) in solution. It was found that Ti(III) is easily oxidised to TiO2+ by dissolved oxygen and water. However, the oxidation rate was slow in hydrochloric acid solutions with the advantage that hydrolysis of the ions could be minimised and the precipitation of the oxides or oxychlorides prevented. It was further revealed that the lithium neutralisation reaction is highly exothermic, with the possibility of raising the leachate temperature to 60°C, resulting in the contamination of the titanium powder particles by the oxide layer and precipitated hydrolysis products. Batch leaching tests were carried out using factorial design of experiments to investigate the effect of initial hydrochloric acid concentration, which was estimated by varying the concentration between 0.032 M and 1 M; particle size, which was varied between −10 mm and +10 mm; and the initial temperature, varied between 14°C and 30°C. The resulting data were modelled and analysed using the analysis of variance statistical method. The solid residues were analysed for oxygen and total residual chloride content. The solid residue was also characterised by scanning electron microscopy (SEM) to examine the morphology of the leached particles. Leaching kinetics model fitting was also conducted. The statistical analysis showed that of the three factors investigated, temperature was the factor with the most statistical significance on both the oxygen and chloride concentration in the purified product, followed by particle size. The effect of acid concentration proved to be minimal, a phenomenon attributed to low concentrations of acid-consuming impurities, specifically excess lithium in the crude product. Thus, the two concentrations of hydrochloric acid investigated were found to be efficient to prevent hydrolysis product formation. Scanning electron micrographs revealed that crushing the crude product with a jaw crusher occluded crude titanium pores, thus locking in some by-products in addition to the pores locked by sintering during the metallothermic reduction. The observation showed that residual chloride impurities in the purified product are not just a consequence of hydrolysis products but also byproducts locked deeper in the pores of the product. Based on the parameter ranges evaluated in the study, a product that satisfied both oxygen and chloride standard specifications was achieved when the crude product was leached in both 1 M and 0.032 M initial HCl concentrations, temperature of 30°C and particle size of +10 mm. The combination of (−10 mm and 14°C) at all concentrations also yielded acceptable oxygen and chloride content levels. Overall, it was concluded from the present work that purification of crude CSIR-Ti product by leaching in dilute HCl is technically feasible.