G. Senanayake

A comparative study of leaching kinetics of limonitic laterite and synthetic iron oxides in sulfuric acid containing sulfur dioxide

Abstract Limonitic laterite ore of particle size 90–125 μm containing goethite, magnetite and hematite was leached for 6 h at a pulp density of 10% (wt/vol) in sulfuric acid in the absence or presence of sulfur dioxide at atmospheric pressure and 90 °C in a glass reactor vessel. The sulfur dioxide flow rate was kept at 0.6 L min−1 L−1 of slurry to maintain a constant SO2 concentration of ≈0.3 mol L−1 in solution, and the sulfuric acid concentration was varied between 0 and 0.72 mol L−1. The relative percentage extractions of Fe, Ni, Co and Mn indicate that the Fe and Ni extractions are inter-related at a ratio of Ni/Fe=0.7–0.9 and suggest the possibility of catalysis of manganese dissolution by solubilized iron(II). This leads to a Mn extraction of over 90% in less than 30 min compared with 20–40% Fe extraction in the same period, depending on the acid concentration. The initial rate of leaching of iron shows first-order dependence with respect to H+. Whilst the synthetic iron oxides leach according to the shrinking particle/sphere kinetic model, the results obtained in the first 4 h of laterite leaching can be described by a shrinking particle model with an insoluble product layer that retards the diffusion of H+ to the reaction sites at the interface. The heterogeneous rate constants for both models increase with the increase in H+ concentration. The effective diffusion coefficient of H+ (DH+) through the product layer (0.5×10−9 to 4×10−9 cm2 s−1), determined in the present study, is in the magnitude range of the reported data for DH+ in polycrystalline Fe3O4 and MnO2, but lower than DH+ in aqueous media, 9×10−5 cm2 s−1.

The role of ligands and oxidants in thiosulfate leaching of gold

The widely used cyanidation technology is not applicable to some gold ores due to economic or technical reasons when the cyanidation gives very low gold extractions. Despite the fact that the gold mining industry accounts for only 13% of total cyanide consumption there is growing public concern over the use of toxic cyanide for gold extraction. Many researchers have identified that thiosulfate leaching is the most likely alternative to cyanidation due to nontoxicity, low cost and other benefits. This article describes the current information on stoichiometric relationships, equilibria, reaction rates and mechanisms related to anodic or chemical oxidation of gold in thiosulfate, ammoniacal thiosulfate and ammoniacal copper(II) thiosulfate solutions in the presence or absence of oxygen.

The effects of dissolved oxygen and cyanide dosage on gold extraction from a pyrrhotite-rich ore

The results of cyanidation of a pre-oxidized pyrrhotite-rich gold ore in laboratory leaching experiments and in plant trials are presented to highlight the effects of particle size, oxygen and cyanide concentrations during pre-oxidation and cyanidation on the overall gold extraction. The Multi Mix Systems (MMS) unit for oxygen injection significantly enhances the mass transfer of reagents and allows the achievement of high gold extraction. Pre-oxidation of ore of P80 = 63 μm for 12 h followed by cyanidation at a molar ratio of [CN-]/[O2] ≈ 12 provided the best gold extraction in the first cyanidation tank. The %Au extracted in the first cyanidation tank reached a maximum of 83% when the product [CN-] × [O2] reached ≈ 5.6 mmol 2 L-2.

Speciation and reduction potentials of metal ions in concentrated chloride and sulfate solutions relevant to processing base metal sulfides

The speciation, Eh-pH and Eh-log aCl- dependence of Fe(III), Fe(II), Cu(II), Cu(I), Ag(I), Pb(II), Zn(II), Ni(II), As(III), Sb(III), and Bi(III) ions in practical (high ionic strength) sulfate and chloride solutions are discussed. The emphasis is placed on those ions which form strong sulfato-, chloro-, and hydroxo-complex species. Measured potentials are compared with potentials calculated from reported association and stability constants to test the applicability of these constants in nonideal solutions and to characterize predominant species.

Studies on the liquid junction potential corrections of electrolytes at aqueous + mixed solvent boundaries

Liquid junction potentials (EJ) generated at the boundaries of various salt bridge/mixed aqueous + organic solutions, have been determined by comparing the measured emf of silver cells with the calculated emf using literature values of transfer activity coefficients γt (Ag+) and γt (Cl−) based on the tetraphenylarsonium-tetraphenylborate (TATB) assumption. The EJ values have been analyzed in terms of the ion-solvent interaction, determined by ionic transfer Gibbs energies, and the so-called solvent-solvent interaction. The effect of varying the cation, anion, and concentration of bridge electrolyte, is assessed in aqueous acetonitrile (AN), ethanol (EtOH), and dimethylsulfoxide (DMSO). It is concluded that the previously reported solvent-solvent contribution to EJ is merely a correction factor to the ion-solvent interaction contribution due to solvent mixing at the boundary. Four salt bridges (sat. K2SO4 3.5 M KCl, 3.5 M NaClO4, and 0.1 M Et4N-picrate) are compared on the basis of EJ. The recommended salt bridge for use with mixed aqueous + organic solutions is 3.5 M NaClO4 in water. Appropriate EJ correction factors are presented for emf measurements in mixtures of AN + H2O, EtOH + H2O, and DMSO + H2O.

A Review of Ammoniacal Thiosulfate Leaching of Gold: An Update Useful for Further Research in Non-cyanide Gold Lixiviants

ABSTRACT This literature review provides an update of ammoniacal thiosulfate as a potential lixiviant of gold, the history of thiosulfate leaching and recent progress on thiosulfate leaching of gold, as well as the production, oxidation, and stabilization of thiosulfate. The leaching results of different types of gold ores including oxide, sulfide, carbonaceous, and refractory ores/concentrates and the fundamentals relevant to the oxidation of pure gold are summarized. The factors affecting gold leaching such as pH, temperature, and oxidants in leaching solutions, reagent concentrations, and foreign ions are presented to update the current understanding on gold dissolution in a Cu(II)-NH3-S2O32-system.

Molar volumes and heat capacities of electrolytes and ions in nonaqueous solvents: 1. Formamide

Apparent molar volumes and heat capacities of 27 electrolytes have been measured as a function of concentration in formamide at 25°C using a series-connected flow densimeter and Picker calorimeter system. These data were extrapolated to infinite dilution using the appropriate Debye–Hückel limiting slopes to give the corresponding standard partial molar quantities. Ionic volumes and heat capacities at infinite dilution were obtained by an appropriate assumption based on the reference electrolyte Ph4PBPh4 (TPTB). The ionic volumes, but not the heat capacities, agree reasonably well with previously published statistically based predictions. The values obtained are discussed in terms of simple models of electrolyte solution behavior and a number of interesting features are noted, including, possible dependencies of ionic volumes on solvent isothermal compressibility and of ionic heat capacities on solvent electron acceptor abilities.

Refining copper by the acetonitrile process

The Eh-pH diagrams, solubility and redox potentials of Cu, Ag, As, Sb, Bi, Pb and Fe are compared in concentrated aqueous chloride and sulfate solutions and in sulfate solutions containing acetonitrile. It is shown that the significant differences in the chemistry of these elements account for the differences in the purity of copper produced from chloride and sulfate electrolytes. A sample of crude copper crystals produced by the CLEAR Process was refined by dissolution and recovery in sulfate media using the Parker acetonitrile process. The Parker process offers an alternative method of copper recovery to electrorefining, and allows a range of Eh and pH conditions to be used to provide further discrimination of impurities. High-purity copper powder was obtained even in the presence of added impurities when Eh and pH were maintained near 300 mV and 1, respectively, during the refining step.

Ultrasonic velocity, conductivity, viscosity and calorimetric studies of copper(I) and sodium perchlorates in cyanobenzene, pyridine and cyanomethane

Ultrasonic velocities (u), densities (ρ), viscosities (η), molar conductivities (Λ) and enthalpies of solution (ΔsolH) of CuClO4 and NaClO4 have been measured in cyanobenzene, pyridine and cyanomethane at 298, 308 and 318 K. These data have been used to calculate isentropic compressibilities (κs) of the salts and limiting ionic conductivities (λoi), solvated ionic radii (ri) and solvation numbers (n) for Cu+, Na+ and ClO4–. Cu+ is highly solvated in all three solvents with solvation decreasing in the order pyridine > cyanomethane > cyanobenzene. Na+ in all solvents is relatively less solvated and the solvation decreases in the order cyanomethane cyanobenzene pyridine. The ClO4– ion is poorly solvated in all three solvents. κs and ΔsolHo data indicate that NaClO4, in spite of its poor solvation in pyridine, shows strong structural effects which are attributed to strong pyridine–pyridine interactions.

Competitive solvation and complexation of Cu(I), Cu(II), Pb(II), Zn(II), and Ag(I) in aqueous ethanol, acetonitrile, and dimethylsulfoxide solutions containing chloride ion with applications to hydrometallurgy

The changes in reduction potential and single ion activity of Cu(I), Cu(II), Pb(II), Zn(II), and Ag(I) have been measured in a range of aqueous ethanol (EtOH), acetonitrile (AN), and dimethylsulfoxide (DMSO) compositions containing excess chloride ion. The results are compared with changes in such solutions in the absence of chloride ion and with the changes in strong brines and rationalized in terms of the various competitive ion-solvent and ion-chloride interactions. Organic solvents are shown to generally enhance chloride ion activity and promote complex ion formation. But AN is a stronger ligand for Cu(I) and DMSO is a stronger ligand for Cu(II) and Zn(II) than is Cl− or the other solvents. The decrease in metal ion activity in mixed aqueous solvents containing Cl− is greater than that in concentrated aqueous chloride salt solutions, according to the strength of the chloro- or solvo-complex. These fundamental changes lead to applications in the extraction of metal ion complexes and promote the dissolution of AgCl, PbCl2, and CuCl in aqueous DMSO containing Cl−.