Purification of titanium sponge produced by lithiothermic reduction of titanium tetrachloride: Effect of leaching conditions

The CSIR-Ti process employs lithiothermic reduction of titanium tetrachloride feedstock to produce titanium sponge. The product is therefore contaminated by a range of lithium and chloride species. In this study we examine the effects of particle size, temperature, and HCl concentration as input leaching variables on the removal of chlorides from the crude titanium sponge. A review of the aqueous chloride chemistry of Li and Ti provided initial conditions for leaching of impurity species from the sponge. Experimental results confirm that the effectiveness of leaching and removal of dissolved impurities from the sponge are dependent on leaching kinetics, which are influenced by temperature, particle size, and morphology. Of the variables tested, reaction temperature had the strongest influence on the oxygen content of the leached product. The HCl lixiviant concentration had a negligible effect under the conditions tested. Leaching of crude titanium sponge (−10 mm size fraction after crushing) at 14°C in either 1 M or 0.032 M HCl yielded a titanium sponge product that met the ASTM standard specification for commercially pure Grade 1 titanium, i.e., oxygen content < 0.18 mass% and chloride content < 0.15 mass%.The CSIR and South African Department of Science and Innovation.https://journals.co.za/journal/saimmam2023Materials Science and Metallurgical Engineerin

Anisotropic optical response of the diamond (111)-2x1 surface

The optical properties of the 2$\times$1 reconstruction of the diamond (111) surface are investigated. The electronic structure and optical properties of the surface are studied using a microscopic tight-binding approach. We calculate the dielectric response describing the surface region and investigate the origin of the electronic transitions involving surface and bulk states. A large anisotropy in the surface dielectric response appears as a consequence of the asymmetric reconstruction on the surface plane, which gives rise to the zigzag Pandey chains. The results are presented in terms of the reflectance anisotropy and electron energy loss spectra. While our results are in good agreement with available experimental data, additional experiments are proposed in order to unambiguously determine the surface electronic structure of this interesting surface.Comment: REVTEX manuscript with 6 postscript figures, all included in uu file. Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.html Submitted to Phys. Rev.

Accurate measurement of polarization potentials during electrodeposition of nickel metal from sulphate electrolytes

Extraction of nickel from either laterite or sulphide ores involves various complex hydrometallurgical processes, the final step of which is often electrowinning to produce pure nickel metal. Methods to investigate, monitor, and control the electrowinning process are therefore of importance for producing nickel metal of desirable quality and purity. The significance of potential measurements during the electrowinning process is well established. Many potential measurement methods, however, are not sufficiently accurate or repeatable, and it is therefore often difficult to monitor and control the electrodeposition process accurately. The relationship between nucleation (deposition of the first nickel clusters to the cathode surface) and growth (as more and more nickel clusters deposit) during electrodeposition is paramount for controlling the formation and quality of the nickel metal electrodeposit. A need therefore still exists for accurate measurement of both nucleation and plating overpotentials during the early stages of electrodeposition. In this investigation, a polarization measurement technique was developed for nickel electrodeposition from sulphate electrolytes under typical commercial electrowinning conditions. This method is repeatable and accurate within approximately 10 mV, and enables the quick and concise monitoring of both nucleation and plating during nickel electrodeposition.This paper was first presented at the 3rd Young Professionals Conference, 9–10 March 2017, Innovation Hub, Pretoria, South Africa.http://www.saimm.co.za/journal-papersam2018Materials Science and Metallurgical Engineerin

Decomposition of hydrogen peroxide in alkaline cyanide solutions

Although oxygen is widely employed as the oxidant of choice in gold leaching by cyanide, its low aqueous solubility presents some drawbacks in practical application; hydrogen peroxide has therefore been considered as a possible alternative. The aim of this investigation was to study the catalytic decomposition of hydrogen peroxide, which generates an oxidizing intermediate species, and to understand its effect on cyanide destruction. Operating conditions that facilitated the effective decomposition of hydrogen peroxide were established by varying the pH and catalyst type and concentration. The oxidizing intermediate, detected using an indirect technique, was found to be the hydroxyl radical (OH˙). OH˙ is commonly generated in acidic solutions, but this work demonstrated that it is also produced at the alkaline pH values necessary for cyanide gold leaching. The effects of free and complexed iron and copper catalysts on the oxidation and consumption of hydrogen peroxide and cyanide were also investigated. It was shown that the cyano complexes of Fe(II) and Cu(I) are also effective as decomposition catalysts. Hydrogen peroxide concentrations above 0.01 M decreased the free cyanide concentration, which was attributed to the probable formation of the cyanate anion (CNO−). Although cyanide consumption increased due to its oxidation in the presence of OH˙, excessive cyanide consumption in the presence of copper was attributed primarily to its complexation by the unstable copper(I) cyanide species. Rate constants for the decompositions of H2O2 and cyanide by ferrocyanide and copper cyanide were calculated; the latter was identified as being a better catalyst.The South African Minerals to Metals Research Institutehttp://www.saimm.co.za/journal-papersam2019Materials Science and Metallurgical Engineerin

Effect of aluminium on polarisation parameters and deposit characteristics in typical nickel sulfate electrolyte for electrowinning applications

Impurities present in the sulfate electrolyte during nickel electrodeposition influence the electrowinning process and morphology of deposited nickel metal in various ways. The presence of aluminium can be either detrimental or beneficial, depending on its concentration. Effects of the presence of aluminium in the electrolyte during nickel electrowinning were investigated by evaluation of the relationship between nucleation and plating overpotentials, nickel morphology, deposit contamination, internal stress development, and buffer characteristics of the electrolyte. Results showed that aluminium present at concentrations of lower than 1250 mg/L was unfavourable for the electrowinning process and highly strained nickel deposits of poor morphology were produced; at higher concentrations, the presence of aluminium was advantageous for the morphology and internal stress, and correlated with increased buffer characteristics of the electrolyte.The Industrial Metals and Minerals Research Institute (IMMRI), based at the University of Pretoria.http://www.elsevier.com/locate/hydromet2021-06-01hj2020Materials Science and Metallurgical Engineerin

An empirical rate equation for the partial removal of manganese from solution using a gas mixture of sulfur dioxide and oxygen

The oxidation of soluble manganese by SO2-O2 gas mixtures is known to occur via complicated reaction pathways. An empirical equation was derived for the rate of manganese precipitation from solution as a function of the gas composition, the concentrations of dissolved sulfur and O2, and the solution temperature and pH. The relationships between the rate of manganese precipitation and each of these parameters were obtained from batch tests. The influence of SO2 was described by both the SO2 content of the gas mixture and the concentration of the dissolved sulfur arising from the dissolution of the SO2 gas. The rate of manganese precipitation was found to be first order with respect to the dissolved O2 concentration and inversely proportional to the pH of the solution. An Arrhenius relationship was used to relate the solution temperature to the rate of precipitation, from which an activation energy of 23.5 kJ/mol was calculated. The manganese(II) concentration had little influence on the rate at concentrations above 500 mg/L. The rate equations for each of the operating parameters were combined into a single empirical rate equation for manganese precipitation, which described all of the batch data and was also applicable to manganese precipitation in continuous mode