M.J. Nicol

A determination of the products of reaction betweer various sulfide minerals and aqueous xanthate solution, and a correlation of the products with electrode rest potentials

The surface reaction products formed when sulfide minerals react with xanthates were analyzed by spectrophotometric methods. These products were found to be predominantly either metal xanthate or dixanthogen, depending on the particular sulfide mineral. Dixanthogen is formed on those minerals that, in a solution of xanthate, assume a rest potential greater than the equilibrium potential for the reduction of dixanthogen or xanthate.

The electrochemical behaviour of galena (lead sulphide) — I. Anodic dissolution

Steady state potentiostatic experiments on a galena rotating-disc electrode in perchlorate solutions of varying pH values indicated that the primary reaction governing the anodic dissolution is PbS → Pb2+ + S + 2e. A mechanism involving a sulphur intermediate is proposed for this process. Cyclic-voltammetric studies with the use of rotating ring-disc electrodes in perchlorate and sulphate solutions revealed that the pH value at the electrode surface has a marked effect on the anodic dissolution. The morphology of the anodically produced sulphur has a bearing on the active-passive behaviour of galena. The species responsible for the passivation of the galena surface in sulphate and perchlorate solutions are shown to be the basic lead sulphates.

Underpotential deposition and its relation to the anomalous deposition of metals in alloys

It has been long known that the codeposition of metals as alloys can often lead to reduction of the less noble coponent ion at potentials more positive than can be predicted from thermodynamic considerations only...

An electrochemical study of the dissolution of gold in thiosulfate solutions. Part I: Alkaline solutions

The anodic dissolution of gold in alkaline thiosulfate solutions has been studied by using a rotating gold disc. Experimental results have shown that the gold dissolves at measurable rates in thiosulfate solutions at potentials above 0.2 V. It has been confirmed that dissolution occurs in parallel with oxidation of thiosulfate with a dissolution current efficiency that varies with time and with the experimental conditions and which is generally in the range of 0.3 to 0.6. Although oxygen could be used as an oxidant for gold in the thiosulfate system, the rate of the cathodic reduction of oxygen in the relevant potential region is too slow for practical purposes. It was found that in the potential region investigated, thiosulfate ions undergo oxidative decomposition leaving a sulfur-like film on the surface of gold, which inhibits the rate of dissolution of gold and results in a low anodic current efficiency for the dissolution of gold. The factors (temperature, pH, thiosulfate concentration and ammonia) have been found to have positive effects on the kinetics of gold dissolution. The rates of gold dissolution in oxygenated alkaline thiosulfate solutions have been estimated to be much lower than in the copper–ammonia–thiosulfate and cyanide systems.

The anodic behaviour of gold

The dissolution of gold in alkaline solutions containing cyanide ions is of great importance and its application in the MacArthur-Forrest process towards the end of the last century revolutionized the extraction of gold from its ores. Because of this, and its use in many industrial processes involving gold, it has been widely studied. What is known — and still unknown — about this reaction constitutes the main theme of this second instalment of a review of the anodic behaviour of gold.

The role of EH measurements in the interpretation of the kinetics and mechanisms of the oxidation and leaching of sulphide minerals

Measurements have been made of the potentials of pyrite, arsenopyrite and chalcopyrite rotating disk electrodes in acid sulfate solutions containing iron(III) and iron(II). These potentials have been compared with those of a stationary platinum wire and glassy carbon (pyrite) and platinum (arsenopyrite and chalcopyrite) ring electrodes. It has been demonstrated that the potential measured in the bulk of the solution can only be approximated to that at the surface of the minerals under conditions of relatively high concentrations of iron(II). A theoretical treatment of the mixed potential model has enabled the difference between the bulk solution potential and the mineral potentials to be calculated from the electrochemical characteristics of the reactions involved. These calculated differences have been compared with the experimental values.

The anodic behaviour of gold: Part I — Oxidation in acidic solutions

The anodic behaviour of gold is of fundamental importance not only in processes involving the metal, such as its extraction, refining, electroplating, electroetching and electropolishing, but also in the many uses of gold in which its resistance to corrosion is critical. The first part of a review of this subject is presented here. Part II, dealing with reactions in alkaline media, will appear in the next issue of Gold Bulletin.

Recovering gold from thiosulfate leach pulps via ion exchange

Increasing environmental and occupational safety concerns about the use of cyanide in gold processing has increased interest in more acceptable alternative lixiviants, the most promising of which is thiosulfate. However, the thiosulfate process lacks a proven inpulp method of recovering the dissolved gold because activated carbon is not effective for the absorption of the gold-thiosulfate complex. This paper describes work aimed at evaluating the effectiveness of commercially available anion exchange resins for the recovery of gold from thiosulfate leach liquors and pulps.

The anodic dissolution of uranium dioxide—I in perchlorate solutions

The anodic dissolution of uranium dioxide in perchlorate solution has been studied. Steady-state and potentiostatic and cyclic voltammetric measurements have been shown to be consistent with a mechanism that involves successive electron-transfer reactions. The presence of insulating uranium (VI) films on the surface at high anodic potentials has been confirmed by measurements with ring-disc electrodes. A comparative study of the behaviour of several samples of differing stoichiometry, crystallinity and porosity has revealed the existence of considerable solid-state effects, the origin of which is not clear.

Some aspects of the electrochemistry of the flotation of pyrrhotite

The iron sulfide mineral, pyrrhotite (Fe(1−x)S), has long been known to be more difficult to recover by flotation from alkaline slurries than many other base metal sulfide minerals. This paper summarizes the results of an electrochemical study of the surface reactions that occur during the flotation of nickeliferous pyrrhotite in the recovery of nickel and the platinum group metals. Mixed potential measurements conducted with natural pyrrhotite electrodes in various stages of an operating flotation plant showed that the mineral potential is positive to the equilibrium potential of the xanthate/dixanthogen couple. Similar results were obtained during batch flotation experiments and in synthetic solutions in the laboratory. Cyclic voltammetric and potentiostatic current/time transient experiments were used to investigate the oxidation of pyrrhotite under various conditions. In addition, the reduction of oxygen, the reaction of copper ions and the oxidation of xanthate ions at the mineral surface were investigated. The formation of dixanthogen on pyrrhotite surfaces is thermodynamically favourable in plant flotation slurries. However the interaction with xanthate at pH values above 7 is inhibited by a surface species formed during the conditioning prior to xanthate addition. In acidic solutions copper ions react readily with pyrrhotite to form a species, possibly CuS that can be oxidized at potentials above 0.4 V. At pH 9 this species does not form and there is no electrochemical reaction between pyrrhotite and copper ions. The beneficial effects of copper ions to flotation performance appear to be related to an enhancement of the oxidation of xanthate.