Highlands Ranch, CO, United States
Highlands Ranch, CO, United States

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Li J.,University of Utah | Safarzadeh M.S.,University of Utah | Moats M.S.,University of Utah | Miller J.D.,University of Utah | And 3 more authors.
Hydrometallurgy | Year: 2012

The gold-thiocyanate system is of interest as it could offer a number of advantages over the gold-cyanide system such as acidic leaching and less toxicity. In the gold-thiocyanate system, the rate of leaching can be accelerated by employing ferric ion as an oxidant. Ultimately, this would lead to a contaminated gold-bearing leach solution that would need to be treated. In this regard, process alternatives for solution concentration and purification are considered including solvent extraction, cementation, and carbon adsorption. The effect of ferric ion on the extraction and stripping of gold was studied. It was demonstrated that ferric ion has an adverse effect on the rate of gold stripping with thiourea. This situation was overcome by controlling the system chemistry-relatively low SCN concentration (0.02 M) in aqueous phase and low amine concentration (0.01-0.02 M) in the organic phase. Strategies other than solvent extraction including cementation with iron and zinc as well as activated carbon adsorption were considered and results reported. Finally, based on this study a conceptual flowsheet is given for the recovery of gold through thiocyanate hydrometallurgy. © 2011 Elsevier B.V. All rights reserved.


Li J.,University of Utah | Safarzadeh M.S.,University of Utah | Moats M.S.,University of Utah | Miller J.D.,University of Utah | And 3 more authors.
Hydrometallurgy | Year: 2012

The effects of metal ions, minerals, temperature, thiocyanate concentration, activated carbon, and pH on the rate of thiocyanate oxidation were determined. The rate of ferrous ion generation from the redox reaction between thiocyanate and ferric ion was found to be significant at 50 °C. The reaction constant (k) at 25 °C was found to be 1.43 × 10 - 5 L 0.4 mol - 0.4 min - 1. Ferric oxidation of thiocyanate was sensitive to temperature with an activation energy of 76.4 kJ/mol, typical of homogenous chemical reactions. Based on the kinetic data, the empirical rate equation for thiocyanate consumption and/or ferrous ion generation was found to have the following form:d[Fe2 +]dt=- 8d[SC N-]dt=k[ SCN-]1.36[ Fe3+]0[ H+]0=k[ SCN-]1.36 Oxide minerals did not have a profound effect on the oxidation of thiocyanate by ferric ion. Sulfide minerals, especially pyrite and galena catalyzed the redox reaction. The addition of cupric ion resulted in the oxidation of thiocyanate and formation of an insoluble cuprous thiocyanate compound. © 2011 Elsevier B.V. All rights reserved.


Li J.,University of Utah | Safarzadeh M.S.,University of Utah | Moats M.S.,University of Utah | Miller J.D.,University of Utah | And 3 more authors.
Hydrometallurgy | Year: 2012

Thiocyanate has been identified and studied as a promising alternative lixiviant for gold in acidic solutions. E h-pH and ion species distribution diagrams for SCN-H 2O, Au-SCN-H 2O, Ag-SCN-H 2O, Cu-SCN-H 2O, and Fe-SCN-H 2O systems were constructed to predict the behavior of each metal ion in the thiocyanate system and also to explain the experimental results. Thermodynamic analyses suggest that gold can be leached by thiocyanate under appropriate leaching potentials, forming aurous or auric complexes with thiocyanate, depending on the thiocyanate concentration and leaching potential. According to species distribution diagrams, silver (I) and copper (I) form insoluble salts at moderate thiocyanate concentrations and are soluble at low and high thiocyanate concentrations. Ferric ion forms a series of complexes with thiocyanate. The study of the ferric ion effect indicates that gold can be leached in acid thiocyanate solution with ferric sulfate as the oxidant. Also the presence of excess ferric ion reduces the apparent thiocyanate activity for copper (I) and silver (I) dissolution. The findings of this thermodynamic assessment are useful in the analysis of some of the phenomena encountered in the leaching and recovery of gold from thiocyanate solutions as discussed in subsequent papers. © 2011 Elsevier B.V. All rights reserved.


Li J.,University of Utah | Safarzadeh M.S.,University of Utah | Moats M.S.,University of Utah | Miller J.D.,University of Utah | And 3 more authors.
Hydrometallurgy | Year: 2012

Solvent extraction with amines has been evaluated as a possible method to purify and concentrate gold from thiocyanate solutions. No significant difference was observed in gold extraction with commercially available tertiary amines (Alamine 336, Alamine 308, and Alamine 304-1). The effects of Alamine 336 and thiocyanate concentrations were examined for gold extraction. Gold extraction isotherms were established at different thiocyanate concentrations. The extraction and stripping of gold showed satisfactory results at an organic/aqueous (O/A) ratio of 1:5 for extraction (~ 100%) and 5:1 for stripping (~ 92%), in the presence of decanol. Possible solvent extraction reactions and stripping reactions with thiourea are discussed in order to better understand the system chemistry. Sodium hydroxide/thiocyanate, ammonium hydroxide, and acidic thiourea were examined for the stripping of gold from the loaded organic phase. Acidic thiourea gave the best results (with complete stripping of gold) under the conditions studied. © 2011 Elsevier B.V. All rights reserved.


Li J.,University of Utah | Safarzadeh M.S.,University of Utah | Moats M.S.,University of Utah | Miller J.D.,University of Utah | And 3 more authors.
Hydrometallurgy | Year: 2012

Acid thiocyanate leaching of gold was investigated in the presence of ferric sulfate as an oxidant. According to leaching kinetic studies the initial rate of gold leaching is slow, and not significantly dependent on thiocyanate (0.05-0.2 M) and ferric (0.1-1.0 g/L) concentrations. Ferrous and cupric ions had no effect on leaching kinetics under the conditions studied. In contrast, silver (I) and copper (I) ions significantly impeded the rate of gold leaching. The electrochemical experiments (linear sweep voltammetry and chronoamperometry) indicated that the anodic reaction for gold leaching in acid thiocyanate solutions is the limiting step for the leaching process. Gold dissolution and thiocyanate oxidation participate simultaneously in the anodic process. The addition of thiourea noticeably enhanced the rate of gold leaching. Fourier transform infrared spectroscopy (FTIR) studies demonstrated that thiocyanate and its complexes with the metal ions involved in the leaching systems (Fe (III), Cu (II), Cu (I) and Ag (I)) had very weak adsorption properties at the gold surface. © 2011 Elsevier B.V. All rights reserved.

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