Gretchen T. Lapidus

The leaching of silver sulfide with the thiosulfate–ammonia–cupric ion system

Abstract In recent years, there has been renewed interest in the use of thiosulfate as a substitute for cyanide in gold and silver leaching. Copper has been recognized to act as a catalyst in the process, and ammonia to stabilize the system. However, discrepancies exist in the literature concerning the oxidizing agent and, specifically in the case of silver sulfide (argentite), whether the leaching is oxidative or by substitution. The distinct mechanisms have implications concerning the feasibility of the process. The present study elucidates the chemistry of the leach through calculation of the species distribution and experiments on pure phase argentite, and also determines the role of the ammonia/thiosulfate concentration ratio. The experimental results on the pure phase show that the cupric ion reacts with thiosulfate to form the tetrathionate ion and a cuprous thiosulfate or ammonia complex, which liberates the silver by substitution in the solid phase. The results of leaching studies from a mineral concentrate indicate that, in addition to the cuprous ion concentration, the ammonia/thiosulfate ratio influences the silver extraction velocity: silver solubility and extraction are favored at low values of this quantity. A mathematical model for silver leaching in this system, which considers redox and complex equilibria, was developed. The experimental extractions were similar to those predicted by the model for low ammonia concentrations. Divergence from the simulated behavior was probably due to the slow reduction of the cupric ion at the high ammonia concentration (1 M).

Cyanidation kinetics of silver sulfide

Abstract In the present study, silver sulfide cyanide leaching was initially studied using synthetic Ag2S and oxygen. Qualitative and quantitative chemical analyses were employed to determine which species (S2−, S2O32−, SO32−, SO42−, H2O2, or SCN−) were present in solution. The results made it possible to establish the stoichiometric relationships of the reactions under consideration. It was found that thiosulfate ion is the dominant sulfur species. In the second part, the kinetics of silver extraction were determined for a mineral concentrate. The sodium cyanide and oxygen concentrations were varied to define the diffusional and reaction phenomena that occur during leaching at constant temperature and pressure. A mathematical model for silver leaching by cyanidation, which considers the redox reaction and complex equilibria in the solution phase, was developed. Silver sulfide leaching from mineral particles was found to be a kinetically controlled process, strongly influenced by solubility phenomena. The experimental results agreed with those predicted by the proposed model.

Modelling of nickel permeation through a supported liquid membrane

Abstract This paper presents a mathematical model for the permeation of nickel(II) ions from sulfate solutions through supported liquid membranes (SLM) which contain di(2-ethylhexyl) phosphoric acid (D2EHPA) as the carrier, dissolved in kerosene. This process of facilitated transport, based on membrane technology, is a variation on the conventional technique of solvent extraction and may be described mathematically using Ficks second law. The equations for permeation velocity are derived considering the diffusion of D2EHPA and its metallic complexes through the liquid membrane. In this work, the system is considered to be in a transient state, and chemical reaction between nickel and the carrier to take place only at the solvent–aqueous interfaces. Model concentration profiles are obtained for the nickel ions, from which extraction velocities are predicted. The experimental and simulated nickel extractions showed similar tendencies for a high nickel concentration, neutralized case. The model results indicate that high initial nickel concentrations and large membrane thicknesses both have detrimental effects on nickel extraction and stripping.

Model for heap leaching of gold ores by cyanidation

Abstract The objective of this study is to develop a model which describes the heap leaching process for gold extraction from non-refractory ores. The model considers the cyanidation reaction in a bed of uniformly sized spherical porous mineral particles, through which the leaching solution trickles. The leaching agents diffuse into the porous particles to react with the grains of metallic gold. The model considers the effects of transport mechanisms on the process and uses an electrochemical mechanism to describe gold cyanidation at the grain level. Mass transfer of the reactants and products in the heap are described using the concepts developed in research on trickle bed reactors. The effective wetting of the particles is estimated with correlations and employed to calculate an apparent effective diffusivity for the different species within the partially wetted particles. Numerical solution of the model equations allows the description of reactant and product concentration profiles within the particles and in the heap. From these profiles, mass balances are performed to show how the gold reacts and is transported until it finally leaves the heap. The effects of different parameters, such as mineral porosity, particle radius, leaching solution feed rate, heap height and concentration of reagents, on the gold extraction rate are determined. The results show that the rate-controlling step in this process, under normal operating conditions, is the diffusion of reactants within the porous particles. For this reason, particle radius, mineral porosity and solution flow rate greatly influence the overall leaching rate. One of the most important findings of this study is that the time required for the gold to react with cyanide and oxygen within the mineral is small compared to the time for the gold cyanide complex to diffuse out of the ore particle and leave the heap.

Mathematical modelling of metal leaching in nonporous minerals

Abstract A model based on the shrinking-core principle is presented for three cases of oxidative metal leaching from nonporous mineral particles. Several limitations inherent in the assumptions of the traditional shrinking-core model, such as those of oxidation agent in excess and first-order irreversible kinetics, are overcome. In each case, an unconventional behavior observed in experimental data is explained through numerical leaching simulations.

The effect of product solubility on the leaching kinetics of non-porous minerals

Abstract An extension of the shrinking core model which accounts for product mass transfer and diffusion control caused by product solubility limitations is proposed. Both the cases of a dilute and a concentrated slurry are considered. Experimental evidence of mixed kinetics involving both oxidizing agent and product diffusion control are presented for the ammoniacal leaching of zinc with cupric chloride. The activation energies calculated indicate the influence of solubility limitations.

Copper leaching from electronic waste for the improvement of gold recycling

Gold recovery from electronic waste material with high copper content was investigated at ambient conditions. A chemical preliminary treatment was found necessary to remove the large quantities of copper before the precious metal can be extracted. For this purpose inorganic acids (HCl, HNO3 and H2SO4) and two organic substances EDTA and citrate, were tested. The effect of auxiliary oxidants such as air, ozone and peroxide hydroxide was studied. In pretreatments with peroxide and HCl or citrate, copper extractions greater than 90% were achieved. In the second leaching stage for gold recovery, the solid residue of the copper extraction was contacted with thiourea solutions, resulting in greater than 90% gold removal after only one hour of reaction.

Unsteady-state model for gold cyanidation on a rotating disk

Abstract The purpose of this study is to model the unsteady-state gold leaching behavior on a rotating disk in cyanide and air solutions. Employing a modified version of the kinetic expression established by Wadsworth and the accepted mass transport equations, computer simulations at different solution conditions were performed and compared to results obtained experimentally by other authors for similar systems. The simulations reproduce the effects of high solution pH, sulfide, Pb 2+ and lime concentrations, as well as the change over from O 2 to CN − control with air pressure and cyanide concentration.

Effect of temperature on copper, iron and lead leaching from e-waste using citrate solutions

E-waste is a potential source of large quantities of metals. The ability of citrate solutions to recover base metals from these materials has been demonstrated. In the present study, the effect of the temperature on base metal leaching capacity by the citrate solutions is determined. The material employed consisted of a mechanically prepared, gravity concentrated e-waste, with a metallic content greater than 90%. The leaching conditions were selected based on previous research performed by the authors (0.5 M sodium citrate, pH 4.5 and 20 g per liter e-waste concentrate). Leaching tests were performed at temperatures between 0° and 70 °C. The initial leaching rates for the three metals increased with temperature. However, these tapered off with time for temperatures above 30 °C, which can be associated to citrate destruction.

Closed circuit recovery of copper, lead and iron from electronic waste with citrate solutions

An integral closed circuit hydrometallurgical process is presented for base metal recovery from electronic waste. The leaching medium consists of a sodium citrate solution, from which base metals are retrieved by direct electrowinning, and the barren solution is recycled back to the leaching stage. This leaching-electrowinning cycle was repeated four times. The redox properties of the fresh citrate solution, as well as the leach liquors, were characterized by cyclic voltammetry to determine adequate conditions for metal reduction, as well as to limit citrate degradation. The leaching efficiency of electronic waste, employing the same solution after four complete cycles was 71, 83 and 94% for copper, iron and lead, respectively, compared to the original leach with fresh citrate solution.