G.H. Kelsall

Acid leaching and dissolution of major sulphide ore minerals: processes and galvanic effects in complex systems

Abstract The kinetics and mechanisms of dissolution of the major base metal sulphide minerals, pyrite, chalcopyrite, galena and sphalerite in acidic (chloride) media have been investigated. Minerals were ground in air, then dissolved in air-equilibrated solutions at pH 2.5, while monitoring the redox potential. Solution samples were analysed by ICP-AES and HPLC, and surfaces of residual sulphides analysed using XPS. Dissolution of aerial oxidation products on pyrite particles in the first 15 min apparently led to a sulphur-rich surface, and was followed by slower dissolution of pyrite itself, driven by oxygen reduction, and resulting in net production of protons. Chalcopyrite dissolution resulted in a Cu, S-rich (near) surface layer, accompanied by net consumption of protons. Apparently incongruent dissolution of galena and sphalerite may reflect the formation of elemental S at the surface. The rates of dissolution of chalcopyrite, galena and sphalerite in the presence of pyrite were determined, respectively, as 18, 31 and 1.5 times more rapid than in single-mineral experiments. These data were consistent with galvanically-promoted mineral oxidation of the other sulphides in the presence of pyrite. In the case of galena, the experimental data suggested extensive release of Pb ions and development of a sulphur-rich surface during galvanically-promoted dissolution.

Electrochemical oxidation of pyrite (FeS2) in aqueous electrolytes

Abstract Electrochemical oxidation of pyrite (FeS2) in aqueous electrolytes has been investigated using electrochemical techniques, in situ Fourier transform infrared spectroscopy (FTIR), ex situ X-ray photoelectron spectroscopy (XPS) and ion chromatography (IC). The results show that when pyrite is polarised in the potential region of −0.35 to 0.25 V versus SCE in 1 M HCl at sweep rates ≥30 mV s−1, the redox process is a reversible electrochemical adsorption/desorption, whereas at sweep rates ≤10 mV s−1, substantial oxidation of the pyrite surface occurs and becomes partially irreversible. The very limited oxidation rate of n-type pyrite over a wide range of potentials may be attributed to its semiconducting properties, or the main part of the applied potential falling across the space charge region rather than the Helmholtz layer. The electrochemical oxidation of pyrite involves a complex series and parallel reaction steps, ultimately producing Fe2+/Fe3+ and S/S2O32−/HSO4−, depending on pH and potential. A mechanism for this process is proposed and discussed using energy band and molecular orbital theories.

Electrowinning coupled to gold leaching by electrogenerated chlorine: I. Au(III)Au(I) / Au kinetics in aqueous Cl2/Cl− electrolytes

The effects of Au(I): Au(III) molar ratio, pH and Cl− concentration on the voltammetric behaviour of a Pt electrode were investigated to enable the kinetics and mechanism of gold dissolution and deposition to be determined. At + 0.8 V vs. SCE gold dissolved exclusively as AuCl2− ions, the proportion of AuCl4− ions increasing at higher potentials. A steady state kinetic model of the coupled electrochemical and chemical reactions resulting in gold dissolution as AuCln− ions, is shown to be capable of predicting the experimentally determined reaction order (≈ 1.7) with respect to chloride ion concentration and the Tafel slope (≈ 80 mV decade−1 at pH 0). Gold electrodeposition and chlorine reduction were studied using a rotating disc electrode. Gold leaching by Cl2/HClO was found to occur at about a quarter of the mass transport controlled rate at pH 0; dissolution rates decreased with increasing pH. However, higher concentrations of HClO (7.5 < pH < 3.3) than Cl2 (pH < 3.3) are attainable, enabling compensation for the slower intrinsic gold dissolution kinetics at higher pHs and avoiding loss of gaseous chlorine in an open system, since the equilibrium pressure of Cl2O(g) above such solutions is much lower than that of Cl2(g).

Ozone generation via the electrolysis of fluoboric acid using glassy carbon anodes and air depolarized cathodes

An electrochemical ozone generation process was studied wherein glassy carbon anodes and air depolarized cathodes were used to produce ozone at concentrations much higher than those obtainable by conventional oxygen-fed corona discharge generators. A mathematical model of the build up of ozone concentration with time is presented and compared to experimental data. Products based on this technology show promise of decreased initial costs compared with corona discharge ozone generation; however, energy consumption per kg ozone is greater. Recent developments in the literature are reviewed.

Thermodynamics of Ti-H2O-F(-Fe) systems at 298 K

Abstract The thermodynamics of Ti-F-H 2 O systems are summarized in the form of activity-pH and potential-pH diagrams, calculated from recently published critically assessed standard Gibbs energies of formation of the various species involved; however, there is considerable uncertainty in those data. The main differences compared with previously published potential-pH diagrams for such systems are the exclusion of Ti 2+ ions, since there appears to be no evidence for their existence in aqueous solutions, and the inclusion of an area of stability for TiH 2 , which excludes that for elemental titanium. In electrolytes with fluoride activities > 0.01, TiF 2− 6 ions are predicted to predominate in moderately acidic solutions and are responsible for the corrosion of titanium metal in such electrolytes which destabilise otherwise passivating oxide films such as TiO 2 (c, hydrated). Diagrams are also presented including the non-stoichiometric oxides Ti 3 O 5 and Ti 4 O 7 , which have large areas of stability at intermediate potentials, though they are not stable if rutile is also considered. Potential-pH diagrams for Fe-Ti-H 2 O systems were also calculated, as ilmenite (FeTiO 3 ) is the primary mineral source of titanium, the iron being acid leached to produce TiO 2 .

The Influence of Electrochemical Potentials on the Friction and Wear of Iron and Iron Oxides in Aqueous Systems

This paper examines the influence of electrochemical potentials on the friction and wear of iron/iron and iron oxide/alumina oxide rubbing contacts, lubricated with aqueous fluids. The chemical nature of surface films on metals under electrode potential control has been investigated using simultaneous voltammetry, impedance spectroscopy and Fourier transform infrared (FTIR) microspectroscopy. This approach has enabled the friction and wear behavior of rubbing contacts with well-defined and controlled surface compositions to be investigated as a function of electrode potential. It has been shown that electrochemical potentials influence friction coefficient in two separate ways, i.e., by modifying the effective inter-surface normal force via electrochemical double layer effects and by controlling the surface chemistry and then the shear strengths of the films present on the rubbing surfaces. Presented at the 48th Annual Meeting in Calgary, Alberta, Canada May 17-20, 1993

Thermodynamics of ClH2O, BrH2O, IH2O, AuClH2O, AuBrH2O and AuIH2O systems at 298 K

The thermodynamics of halide-water and gold-halide-water systems are summarized in the form of potential-pH and activity-pH diagrams, calculated from recently published standard Gibbs energies of formation of the various species and phases considered. The stability of the gold-halide complexes increases in the order Cl < Br < I, which is reflected in the pH above which Au(OH)3 (c) is predicted to form, though even the chloride complexes are stable at pH < 7. These diagrams enable predictions to be made of the behaviour of gold in halide electrolytes; chloride is used in industrial gold electrorefining, and interest is increasing in coupled leach-electrowin processes involving electrogenerated chlorine, bromine and iodine.

Photoelectrophoresis of colloidal semiconductors. Part 1.—The technique and its applications

A Malvern Zetasizer IIc laser Doppler electrophoresis apparatus has been modified to enable the effect of illumination on electrophoretic mobilities of aqueous dispersions of semiconductor particles to be studied. The electrophoretic mobilities of TiO2 particles were determined in the dark and whilst exposed to monochromatic light, the wavelength of which was varied between experiments. The results showed that a positive charge accumulated on the particles in the absence of easily reducible or oxidizable species in solution. This effect was observed only when the particles were illuminated with photons of ultra-band gap energy, demonstrating unambiguously that it was related to the generation of electron—hole pairs and their subsequent reactions with Ti—O surface groups. Similar shifts were observed for suspensions of SnO2, Ti4O7 and Fe(CN)4–6 photosensitized TiO2 particles. No such shift was observed for CdS particles in the absence of any excess lattice ions, probably owing to decomposition of the particles to generate a surface layer of photo-inactive sulphur.

Chemical and electrochemical equilibria and kinetics in aqueous Cr(III)/Cr(II) Chloride solutions

Abstract The effects of Cr(III) speciation and electrolyte composition on the voltammetric behaviour of the Cr(III)/Cr(II) couple, were studied by voltammetry at a hanging mercury drop electrode in chloride and sulphate electrolytes, which were analysed by spectrophotometry. Previous authors on Cr(III)-Cl − electrochemistry appear not to have characterised their electrolytes adequately, ignoring the well known slow chemical kinetics of the system. The thermodynamics of the Cr/H 2 O-Cl system were summarised in the form of potential ( E h )-pH and spedation diagrams calculated from published Gibbs energy of formation data. These were used to provide predictions of equilibrium compositions in kinetic experiments, and, together with spectrophotometric measurements, for characterising electrolytes immediately prior to electrochemical experiments. The aquation and chloride complexation kinetics of Cr(III) species, studied by visible spectrophotometry, were found to be dependent on the H + and Cl − ion concentrations and occurred slowly even at 333 K. In strongly addic chloride solutions at 293 K, equilibrium solution compositions were not achieved even after several months. From results of cyclic voltammetry at a hanging mercury drop electrode, the heterogeneous rate constants were calculated for the CrCl + 2 /Cr 2+ CrCl 2+ /Cr 2+ and Cr 3+ /Cr 2+ couples, the kinetics of which were found to be irreversible or quasi-reversible, dependent on the initial Cr(III) species and the electrolyte composition. The reduction of CrCl + 2 ions occurred via a CrCl 2+ intermediate and successive elimination of Cl − ligands, the presence of which greatly enhanced the electrode kinetics.

Thick Antiwear Films in Elastohydrodynamic Contacts. Part I: Film Growth in Rolling/Sliding EHD Contacts

It has been previously reported in the literature that a particular, unnamed, phosphonate antiwear additive can enhance elastohydrodynamic film thicknesses on rolling metal surfaces at elevated temperatures. In this paper, several different pure phosphonate additives have been prepared and their performance in rolling contacts has been studied. All the phosphonates tested formed thick films from base oil solution in rolling EHD contacts. The rate of film formation varied with the alkyl or aryl groups present and with the temperature but, in some cases, films started to form at room temperature. These films grew steadily with running over several hours, to reach thicknesses of more than one-half micron, augmenting the normal elastohydrodynamic films by this amount. Further studies have demonstrated that these thick antiwear films can withstand up to 10 percent sliding and are formed in the presence of other additives. The implications of the phenomenon of thick antiwear films as a means of increasing speci...