Michael Nicol

The role of lead ions in the cyanidation of gold has been extensively studied in this investigation. In addition to a speciation analysis using accepted thermodynamic data, the equilibrium potential for the deposition of lead under cyanidation conditions has been estimated. This has been used in conjunction with other electrochemical measurements to confirm that bulk lead metal is not deposited on a gold surface during cyanidation. The underpotential deposition of lead has been confirmed and it has been shown that an incomplete layer of lead is responsible for the increased rates in the presence of lead. The detrimental effect of high lead concentrations appears to be the result of the formation of a complete lead layer on the gold surface.
A less extensive study of the effect of sulfide ions on the rate of cyanidation has confirmed that low (micro-molar) concentrations of sulfide ions enhance the dissolution of gold in cyanide solutions but inhibit the rate of dissolution at high concentrations. The detrimental effect is associated with the anodic oxidation of gold and the formation of a passivating layer of Au2S is possibly responsible. There does not appear to be a significant effect of sulfide on the kinetics of the cathodic reduction of oxygen. A gold alloy containing 10 % silver is less susceptible to the inhibitory effect of sulfide ions.

In a study (Part II) aimed at the simultaneous heap leaching of copper (predominantly chalcopyrite), gold and uranium from an ore, it was found that this could be achieved using chlorate as the oxidant in chloride solutions. As a result, a more detailed study has been made using electrochemical and batch leach tests on pure chalcopyrite electrodes under typical heap leach conditions.

The use of chlorate at moderately low concentrations has shown that enhanced rates of dissolution of chalcopyrite can be maintained over periods up to 4 days at ambient temperatures in acidic chloride solutions. The rate of dissolution is enhanced at higher acid concentrations. The rate in the presence of chlorate does not appear to depend on the iron(III) (and presumably the copper(II)) concentration.

The observed rates are significantly greater than those predicted for oxidative dissolution from the electrochemistry and the mixed potential model. An alternative non-oxidative mechanism has been revisited to account for this difference.

A preliminary measurement has been made of the kinetics of the oxidation of hydrogen sufide and iron(III) by low concentrations of chlorine produced by reaction of chlorate with chloride ions. These reactions are rapid, and theory shows that they would support the proposed non-oxidative mechanism.

This study of the simultaneous leaching of copper, gold and uranium from a flotation concentrate is aimed at establishing the conditions that could be applied in a possible heap leach process that would recover all three metals (plus silver, if present) in a single step. The results obtained in this study confirm the results obtained in Part I (Nicol et al., 2024) that enhanced rates for leaching of chalcopyrite from flotation concentrates from two different sources can be obtained in acidic chloride solutions in the presence of chlorate ions. The measurements of the solution potentials during leaching with chlorate are similar to those obtained previously and confirm that chlorine generated by the chlorate-chloride reaction is the active oxidant.
Uranium is dissolved even in the absence of chlorate by iron(III) as the oxidant. The latter is re-generated by reaction of iron(II) with chlorate. The dissolution of gold requires higher potentials, and it is only leached when the potential approaches 1.0 V in concentrated chloride solutions. At low chloride concentrations (1 M), gold is not dissolved because of apparent passivation.
An electrochemical study of the behaviour of gold in chloride solutions in the presence of sulfide ions has shown that the passivation is probably the result of the formation of a layer of Au2S on the gold surface at low potentials before the addition of chlorate to the leach solutions i.e., before the generation of sufficient chlorine concentrations. At potentials above about 0.95 V Au2S is unstable being oxidised to AuCl2− and elemental sulfur thereby eliminating passivation.
•Enhanced rates for leaching of chalcopyrite in acidic chloride solutions with chlorate.•Chlorine generated by the chlorate-chloride reaction is the active oxidant.•The dissolution of gold requires higher potentials of at least 1.0 V.•At low chloride concentrations (1 M), gold is not dissolved because of apparent passivation.•Passivation is probably the result of the formation of a layer of Au2S on the gold surface

Despite the fact that the cyanidation process for the recovery of gold was patented almost 140 years ago, understanding of the dissolution of gold has proved to be particularly elusive. While it is widely accepted to be a typical electrochemically-based corrosion process, details of the published kinetics of the anodic and cathodic reactions have been found to be very variable with little agreement between many studies, predominantly of the anodic reaction.

In this series of three papers, an attempt has been made to shed some light on the electrochemistry of gold in cyanide solutions. Unlike many previous studies, the use of gold‑silver alloys and an electrode fabricated using gold from an actual plant have been used in addition to pure gold. Solutions from two gold plants have also been compared to pure solutions.

The first of these papers summarizes and compares the literature on the anodic characteristics of gold dissolution in cyanide solutions. The use of the mixed-potential model to describe the kinetics of gold dissolution and the important measurement of mixed potentials under typical cyanidation conditions are described in detail with an emphasis on the possible passivation of gold and the effect of the metal and solution composition on the passivation process.

A number of small scale dissolution experiments using various gold materials and solutions has been undertaken in conjunction with mixed potential and anodic voltammetric measurements. This study has shown that electrochemical studies using pure gold electrodes in pure solutions cannot be used to predict the reactivity of gold under actual plant conditions.

An extensive study has been undertaken of the reduction of oxygen on various gold electrodes under conditions appropriate to the cyanide leaching of gold. Voltammetric studies with various gold electrodes and ring-disk electrode measurements with pure gold have demonstrated that the 2-step reduction involving peroxide as an intermediate is complex with variable extents of production of peroxide and reduction to water depending on the electrode material and solution purity. There is thus no simple stoichiometry for the reduction of oxygen although the use of a 2-electron reduction is probably the best approximation under plant conditions. This is contrary to the often accepted 4-electron process.

The nature of the gold surface is important with lower reactivity of gold‑silver alloy and plant electrodes relative to pure gold. The sensitivity of oxygen to the presence of underpotentially deposited lead has been demonstrated in solutions contaminated with lead ions. Lead ions have a significant catalytic effect on the kinetics of oxygen reduction that can be attributed to the known effect of underpotentially deposited lead on the reduction of oxygen on gold electrodes in acidic and highly alkaline solutions. Thus, both the anodic dissolution of gold and the cathodic reduction of oxygen are catalysed in the presence of low concentrations of lead ions.

In the intensive cyanidation of gravity gold concentrates, sodium m-nitrobenzene sulfonate (NBS) is often used to supplement dissolved oxygen as the oxidant in the process. A previous paper presented the results of a largely electrochemical study of the behaviour of NBS during cyanidation of gold. The results confirmed that NBS acts as an oxidant in the cyanidation of gold and that the mixed potential model can be applied to describe the mechanism of its action. This paper explores the corresponding oxidation of sulfide minerals, that inevitably are contained in gold concentrates, by either dissolved oxygen or NBS. Using electrochemical techniques it was found that dissolved oxygen is effective in the oxidation of several sulfide minerals at pH values between 9 and 11. The effect of cyanide on both the anodic and cathodic processes has been studied. NBS has been found to be ineffective as an oxidant for all minerals tested except galena, even in the presence of cyanide.

In the intensive cyanidation of gravity gold concentrates, sodium m-nitrobenzene sulfonate (NBS) is often used to supplement dissolved oxygen as the oxidant in the process. This paper presents the results of a largely electrochemical study of the behaviour of NBS during cyanidation. The results have confirmed that NBS acts as an oxidant in the cyanidation of gold and that the mixed potential model can be applied to describe the mechanism of its action.

The mixed potential is a good initial indicator of the rate of gold dissolution and, as expected, the anodic dissolution of pure gold in cyanide solutions is characterized by passivation at potentials above about −0.35 V.

The reduction of oxygen under the conditions of the present study occurs in two 2-electron steps with peroxide as an intermediate. Dissolution of gold occurs at potentials in the diffusion-controlled region for the first step. The cathodic reduction of NBS occurs in the same potential region as the reduction of oxygen. The reaction is first order in the concentration of NBS and is largely independent of the pH. The stoichiometry of the reaction involves six moles of gold per mole of NBS confirming that the amine is the final product of reduction of NBS.

Rates of gold dissolution in various solutions have been measured using a calibrated linear polarisation method. The rate increases approximately linearly with increasing NBS concentration and is independent of pH. The rate in 0.5 g/L NBS is approximately the same as in oxygenated solutions.

A relatively simple titration has been adapted for use in determining NBS concentrations.

This study has shown that the use of methods that measure the initial rates of dissolution of gold using iron(III) as the oxidant under ambient conditions can be deceptive in that, in the absence of efficient methods to re-oxidize iron(II) to iron(III), the rates rapidly decrease with time resulting in predictable poor recoveries. Nevertheless, the results obtained have confirmed that gold(I) is the main product of dissolution with iron(III) as the oxidant and that the initial rate increases parabolically with increasing chloride concentration, is proportional to the square root of the iron(III) concentration and is independent of the acidity. These observations are consistent with the effects of chloride and acid concentrations on the anodic dissolution of gold. The mixed potential model of oxidative dissolution was shown to be operative in this system. The rate of homogeneous re-oxidation of iron(II) to iron(III) by dissolved oxygen under ambient conditions is too slow to be of value in sustaining the high potentials required. On the other hand, the use of low concentrations of chlorate have been shown to be effective in sustaining the reaction as a result of the generation of small quantities of chlorine by oxidation of chloride.

A detailed study has been made of the effect of the addition of iodine/iodide on the electrochemistry of the dissolution of chalcopyrite in acidic sulfate media containing iron(III). Mixed potential measurements have revealed that the potential is higher with iodine as an oxidant compared to iron(III) although the solution potentials are similar. Potentiostatic measurements in the absence of the oxidants at the measured mixed potentials have shown that chalcopyrite should dissolve some five times faster with iodine as the oxidant. Linear sweep voltammetry has confirmed the much higher reversibility of the iodine/iodide couple on a chalcopyrite surface than that of the iron(III)/iron(II) couple. Limited dissolution tests have confirmed that chalcopyrite dissolves more rapidly with iodine as the oxidant at rates similar to those predicted from the electrochemical measurements.
A preliminary spectrophotometric study of the kinetics of the oxidation of iodide by iron(III) in sulfate media has shown that the rate is significantly slower than previously assumed and that the reaction occurs in two stages with tri-iodide as the initial product that is slowly oxidised to iodine. At the low concentrations necessary for economic application, iodide ions do not effect the rate of oxidation of iron(II) by dissolved oxygen in chloride solutions.

It is suggested that the use of iodide as a redox mediator during heap leaching of chalcopyrite ores by acidic ferric sulfate lixiviants is unlikely to change current problems with the slow rate of dissolution under ambient conditions.

The role of galvanic coupling of chalcopyrite and pyrite in the enhanced rate of dissolution of chalcopyrite in acidic sulfate and chloride solutions at 30 °C has been defined in a detailed electrochemical study. The results indicate that, under ambient conditions, the effect of galvanic contact with pyrite only marginally increases the rate of dissolution of chalcopyrite in both sulfate and chloride solutions containing iron(III) despite significant increases in the mixed potential of chalcopyrite. This has been shown to be due to the fact that the effect of potential on the rate of anodic oxidation of chalcopyrite is small in the relevant potential under these conditions. This is confirmed by the observation that the mixed potential and, therefore, the galvanic current is largely unaffected by a 4-fold decrease in pyrite surface area. The galvanic current transients compared well with potentiostatic current/time transients for chalcopyrite at the mixed potential observed during coupling. In the case of copper(II) as the oxidant in chloride solutions, the galvanic currents are small due largely to the fact that the increase in chalcopyrite potential and decrease in pyrite potential are small as a result of the lower reduction potential of the copper(II)/copper(I) couple than the iron(III)/iron(II) couple. The galvanic currents with dissolved oxygen as the oxidant are significantly lower than with iron(III) as the oxidant and confirm the well-known fact that oxygen is less effective than iron(III) as an oxidant for chalcopyrite even when galvanic coupled to pyrite. The electrochemical data obtained in this study indicate that the small galvanic currents observed do likely not account for the relatively large increases in rate previously observed in agitated leaching of chalcopyrite/pyrite slurries at elevated temperatures.