Milton E. Wadsworth

Reaction mechanism for the acid ferric sulfate leaching of chalcopyrite

The acid ferric sulfate leaching of chalcopyrite, CuFeS2 + 4Fe+3 = Cu+2 + 5Fe+2 + 2S0 was studied using monosize particles in a well stirred reactor at ambient pressure and dilute solid phase concentration in order to obtain fundamental details of the reaction kinetics. The principal rate limiting step for this electrochemical reaction appears to be a transport process through the elemental sulfur reaction product. This conclusion has been reached in other investigations and is supported by data from this investigation in which the reaction rate was found to have an inverse second order dependence on the initial particle diameter. Furthermore, the reaction kinetics were found to be independent of Fe+3, Fe+2, Cu+2 and H2SO4 in the range of additions studied. The unique aspect of this particular research effort is that data analysis, using the Wagner theory of oxidation, suggests that the rate limiting process may be the transport of electrons through the elemental sulfur layer. Predicted reaction rates calculated from first principles using the physicochemical properties of the system (conductivity of elemental sulfur and the free energy change for the reaction) agree satisfactorily with experimentally determined rates. Further evidence which supports this analysis includes an experimental activation energy of 20 kcal/mol (83.7 kJ/mol) which is approximately the same as the apparent activation energy for the transfer of electrons through elemental sulfur, 23 kcal/ mol (96.3 kJ/mol) calculated from both conductivity and electron mobility measurements reported in the literature.

Passive and Transpassive Anodic Behavior of Chalcopyrite in Acid Solutions

The electrochemical oxidation of CuFeS2 in various acid solutions was studied using electrodes made from massive samples. The primary techniques employed were potentiodynamic polarization and constant potential experiments supplemented by capacitance measurements. It was the purpose of this study to investigate the behavior of: (1) several sources of CuFeS2 in H2SO4 electrolytes, and (2) a single source of CuFeS2 in various dilute acids. Electrochemical characterization of CuFeS2 from various locations was performed in 1 M H2SO4 which showed significant differences in their behavior. All samples exhibited passive-like response during anodic polarization. The current density in this passive region was reproducible and showed differences of up to two orders of magnitude between samples from different sources which has been attributed mainly to the presence of impurities in some of the samples. During anodic polarization CuFeS2 was found to be sensitive to pH at higher potential, but insensitive at low potential in sulfate solution. In addition, current decay measurements at constant potential in the low potential-passive region were found to follow the Sato-Cohen (logarithmic) model for solid film formation. Based on current and mass balance measurements, two intermediate sulfide phases appeared to form in the sequence CuFeS2→S1→S2. At higher potentials, in the transpassive region, the observed increase in current is compatible with the decomposition of water to form chemisorbed oxygen which releases copper and forms sulfate ions.

In-place leaching of primary sulfide ores: Laboratory leaching data and kinetics model

Experimental results obtained in laboratory leaching studies of primary copper sulfide ore in sulfuric acid systems pressurized with oxygen are interpreted by a computerized geometric model involving the movement of a reaction zone through the ore fragments. Physical properties of the ore, including size, shape, and mineral content, are considered. The leaching mechanism involves mixed kinetics and includes a surface reaction within a moving reaction zone plus pore diffusion of dissolved oxygen through the reacted portion of the ore fragment to the reaction zone. The results are applicable to conditions that would exist innuclear solution mining or similar processes in which the ore is converted into rubble and then inundated by a leach solution adequately supplied with oxidants. Experimental results at 90°C are correlated with the model, which includes temperature-dependent parameters.

Galvanic conversion of chalcopyrite

Galvanic interaction between particulate chalcopyrite (CuFeS2) and copper results in the rapid conversion of chalcopyrite to chalcocite. The effects of temperature, surface area, concentration of sulfuric acid and agitation were systematically evaluated. The kinetics were found to be controlled by a steady-state current flow controlled by the effective anodic and cathodic surface areas involved in the galvanic couple. The experimental activation energy was 11.5 and stoichiometric data and reaction products have been characterized. The overall kinetic system has been evaluated based upon an electrochemical model.

Gold dissolution and activation in cyanide solution: kinetics and mechanism

Abstract The rate of gold dissolution was measured in clear solution using a gold rotating disk electrode. The data, obtained at various cyanide concentrations, are in agreement with literature observations. The dissolution rates were independent of electrode rotation speed for air-saturated solutions and cyanide concentrations above 5 mol m−3 and were well below external mass transfer-limited rates for cyanide and oxygen. An activator molecule, NMI, was shown to increase the rate of dissolution and served to illustrate the importance of surface reactions in the dissolution mechanism. With increasing activator concentration, the dissolution rate first increased to a maximum value and then decreased. The optimum activator concentration at the maximum dissolution rate was found to be a function of the cyanide level. The results suggest competitive adsorption between the activator and cyanide species. A dissolution mechanism in which the active site contains two (or more) gold atoms is proposed. Dissolution kinetics are controlled by reactions on the crystalline gold surface in regions independent of potential and identified by measured potentials when both cyanide and oxygen are present. The data are explained by crystalline surface mass transfer away from the active surface site followed by charge transfer and dissolution. Model predictions are in reasonable agreement with observations. The proposed mechanism is based solely on kinetics measurements and calls attention to the need for further mechanistic research.

Application of mixed potential theory in hydrometallurgy

Abstract The concept of mixed potentials has been applied to explain important aspects of corrosion for many years. In many hydrometallurgical systems, reactions occur by means of electrochemical mechanisms. Application of mixed potential theory to explain the kinetics of oxidative and reductive leaching processes is a useful concept in explaining observed results qualitatively. It explains the often observed rapid shift from one kinetic regime to another and provides basic information regarding sequential phase changes. Quantitative application of mixed potential theory requires a knowledge of individual electrode kinetics. From the individual electrode kinetics, mixed potentials and overall reaction kinetics may be calculated. Examples are presented for contact reduction, cyanidation and sulfide leaching.

Characterization of surface layers formed during pyrite oxidation

Surface electrochemical reactions of pyrite have been studied using cyclic voltammetry, in situ laser Raman spectroscopy and potentiostatic measurements. A surface sulfur layer was identified on pyrite surfaces during transpassive oxidation. Sulfur layer formation at high anodic potentials was confirmed by subsequent surface reactions during the cathodic and return anodic cycle in cyclic voltammetry and by in situ Raman spectroscopy. Initially, polysulfides (FeSx of variable x) form as intermediates during anodic oxidation in both acidic and basic solutions. The nature of these intermediates depends upon pH and the rate of surface film growth. The end product of the film growth is an active form of sulfur, S0act, which is readily reducible. Raman spectra obtained during an extended period of time indicated the film was mainly S0act. The conditions such as applied potentials and pH that lead to the build-up of the surface oxidation layer have been determined. Film growth kinetics obey the paralinear rate law, indicating uniform film growth of the sulfur-rich layer to a steady state thickness. The thickness is controlled by the simultaneous rate of diffusion of cations through the polysulfide layer, with parabolic rate of constant kp, and the rate oxidation of the active-sulfur outer layer, with the linear rate constant k1, to soluble sulfur species in solution. The effects of temperature, applied potential and pH on kp and k1 were determined using the paralinear rate law.

The effect of electrolyte composition on the cathodic reduction of CuFeS2

Abstract The electrochemical reduction of CuFeS 2 mineral electrodes has been investigated by performing cathodic polarization curves, constant potential experiments, and cyclic polarization curves in various electrolytes. The effects of H 2 SO 4 , Fe 2+ and Cu 2+ concentrations have been examined as well as the effects of various dissolved gases, air, O 2 , H 2 S and N 2 . Decreasing H 2 SO 4 only shifts the curve to more negative potentials but initial concentrations of 0.36 M Fe 2+ cause up to a ten-fold enhancement in the observed current. The presence of oxygen or Cu 2+ also leads to an increase in the reduction current but in either case further increases in current are observed when Fe 2+ is added to the electrolyte. Chalcocite (Cu 2 S) or djurleite (Cu 1.96 S) have been identified as products of reaction, although it is possible that a thin layer of a different copper-iron sulfide may form as an intermediate based on the color changes which appear after cathodic excursions. In addition, scanning Auger microprobe analyses of these surfaces show an increasing Cu:Fe ratio as the time at potential is increased. The results also indicate that the solid product layer is porous. A mechanism which accounts for the observed current increase in the presence of Fe 2+ is proposed which involves a redox reaction between dissolved Fe 2+ and cupric ion in the copper sulfide product layer.

Transpassive Oxidation of Pyrite

The electrochemical behavior of mineral and coal pyrites in basic borate/sulfate solutions was investigated using cyclic voltammetry with both stationary and rotating disk electrode. Emphasis was centered on transpassive oxidation. In the transpassive region, 0.4 to 0.8 V (SCE), aggressive oxidation of pyrite occurred. The reaction products in this region are Fe(III) oxides, sulfate ion, and partially oxidized sulfur intermediates. The formation of sulfur and polysulfides was identified by in situ Raman spectroscopy. Exposure of pyrite to anodic potentials higher than the transpassive region resulted in rapid oxidation of sulfur intermediates to sulfate ion. The effect of electrode rotation speed, electrode precondition time, and upper potential of the scan in the transpassive region was observed to be critical to the formation of sulfur intermediates. Sulfur intermediates, formed in the transpassive region, dramatically affected subsequent oxidation reactions occurring in the lower potential region. The magnitude of two dominant oxidation peaks, a ferrous hydroxide peak and an iron sulfide peak, observed in this region correlated directly with the quantity of sulfur intermediates formed in the transpassive region. This effect was less pronounced for coal pyrites compared to mineral pyrite.

Application of Raman spectroscopy to metal-sulfide surface analysis

The surface products of electrochemically oxidized pyrite (FeS(2)) are investigated as a function of applied potential by using Raman spectroscopy. The parameters necessary for sulfur formation on the pyrite surface were determined. An optical multichannel apparatus, consisting of an argon laser, a triple spectrograph, and a charge-coupled-device detector, was utilized for the Raman measurements. The advantages of this system for surface characterization include high resolution and high sensitivity as well as the capability of identifying compounds and making in-situ measurements.