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 (0.05–0.2M) and ferric (0.1–1.0g/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.

Thiocyanate has been identified and studied as a promising alternative lixiviant for gold in acidic solutions. Eh–pH and ion species distribution diagrams for SCN–H2O, Au–SCN–H2O, Ag–SCN–H2O, Cu–SCN–H2O, and Fe–SCN–H2O 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.

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.

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 (0.02M) in aqueous phase and low amine concentration (0.01–0.02M) 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.

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 degrees C. The reaction constant (k) at 25 degrees C was found to be 1.43 x 10(-5) LOA mol(-0.4) m(-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 = -8 d[SCN-]/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. (C) 2011 Elsevier B.V All rights reserved.

The low potential hydrophobic state of pyrite in amyl xanthate (PAX) flotation with nitrogen is of particular interest with regard to the N2TEC flotation technology currently being used for the recovery of auriferous pyrite at Newmont's Lone Tree Plant in Nevada. Initially, the N2TEC system had been found to operate satisfactorily, but cyanide in the flotation mill water appeared to be responsible for a loss in pyrite recovery. This supposition was confirmed with laboratory experiments, and a program was initiated to study flotation chemistry variables by electrochemically controlled contact angle measurements. Experimental results show that activation of pyrite in such cyanide solutions can be achieved more effectively with lead than with copper. Subsequently, based on these fundamental studies, significant improvement at the Lone Tree Plant was achieved by lead activation, in which case the recovery increased to expected levels.The effect of activator is particularly significant not only with respect to pyrite depression by residual cyanide, but also with respect to collector (PAX) consumption and the initial state of the pyrite surface. Experimental results show the importance of the pyrite surface state and the rather interesting features of the activation process. (C) 2005 Elsevier Ltd. All rights reserved.

The influence of pyrite pre-oxidation in alkaline solutions on gold recovery by cyanidation from Twin Creek refractory gold ore in which pyrite was identified as the major sulfide mineral has been investigated with the aid of electrochemical measurements, leaching experiments, and direct analysis of reaction products for selected residues. It was found that gold recovery by cyanidation in bottle roll experiments mainly depended on the extent of pyrite pre-oxidation. The rate of pyrite oxidation in alkaline solutions measured by electrochemical measurements, including chronoamperometry and linear sweep voltammetry, increased with an increase in pH, potential, and temperature. All alkaline reagents used for the electrochemical measurements, NaOH, NH4OH, Na2CO3 and Ca(OH)(2), showed a similar effect on pyrite oxidation kinetics. However, the results of alkaline pre-oxidation for pyrite of the Twin Creek refractory gold ore suggested that NaOH and Na2CO3/Ca(OH)(2) were superior to Ca(OH)(2). Without pre-oxidation, cyanide leachable gold was found to be only 20% which could be increased to 70% under appropriate pre-oxidation conditions. At the same time, cyanide consumption decreased from 2.5 kg/t ore to 1.5 kg/t ore.Selected residues after pre-oxidation and cyanidation were examined by X-ray diffraction. Backscattered electron images of pyrite particles in these residues were taken. The reaction products at the surface of pyrite particles were found to be iron-, silicon-, and calcium-bearing compounds with variable amounts of sulfur as determined by X-ray energy dispersion analysis. Additionally, some mineral fines, such as aluminum and/or potassium-bearing minerals, were found to be present at the partially oxidized pyrite surface. (C) 2005 Elsevier Ltd. All rights reserved.

Thiocyanate was evaluated for gold recovery from low-grade refractory gold ores following biooxidation for an acidic heap leaching approach. One of the important features in using thiocyanate is that the leaching can be performed in acidic media, thus avoiding problems related to neutralization and material handling as leaching with alkaline cyanide. Also, the acidic leaching enables the use of ferric ion or bioleach solution as oxidants. In this study, Newmont-Nevada low-grade refractory sulfidic ore was biooxidized in columns at a particle size of 100% passing 102 mm. Thiocyanate leach on the biooxidized ore sample was performed on both ground samples in pulps and coarse material in columns using ferric ion as an oxidant. The variables of thiocyanate and ferric ion concentrations were compared for effect on leaching. Thiocyanate leach results are encouraging and indicate potential for development of a practical process. Gold extractions by thiocyanate are comparable to cyanide leach. Thiocyanate consumption was higher than cyanide, but was in an acceptable level.

Thiosulfate is an alternative lixiviant for the leaching of precious metals. Research and development of thiosulfate leach technology has been prompted by environmental concerns with the use of cyanide and problems associated with the processing of difficult-to-treat ores. The Newmont Mining, Nevada, operation has successfully developed a combined biooxidation–thiosulfate heap-leaching process for carbonaceous/high sulfide ores and a direct thiosulfate heap-leaching process for carbonaceous/low sulfide ores. The chemistry of copper catalytic thiosulfate leach is a complicated system due to the simultaneous presence of ammonia, thiosulfate and Cu(II)–Cu(I) redox couple, especially for a heap leach operation. In this paper, factors affecting both the leach reaction and the stability of the lixiviant are discussed relating to heap-leaching operation from a solution chemistry perspective.