D.M. Muir

Thiosulfate leaching of gold—A review

Abstract The ammoniacal thiosulfate leaching process for gold and silver extraction has been reviewed in terms of leaching mechanism, thermodynamics, thiosulfate stability, and gold recovery options. The application to different ore types and process options have also been discussed. The thiosulfate leaching process it catalysed by copper and has several advantages over the conventional cyanidation process. Thiosulfate leaching can be considered a non-toxic process, the gold dissolution rates can be faster than conventional cyanidation and, due to the decreased interference of foreign cations, high gold recoveries can be obtained from the thiosulfate leaching of complex and carbonaceous-type ores. In addition, thiosulfate can be cheaper than cyanide. The chemistry of the ammonia-thiosulfate - copper system is complicated due to the simultaneous presence of complexing ligands such as ammonia and thiosulfate, the Cu(II)Cu(I) redox couple and the stability of thiosulfate in solution. However, by maintaining suitable concentrations of thiosulfate, ammonia, copper and oxygen in the leach solution, and consequently, suitable Eh and pH conditions, thiosulfate leaching can be made practical. Generally the thiosulfate leaching conditions reported in the literature are severe with high reagent consumption. Further investigations are required on leaching under low reagent concentrations over extended periods where reagent consumption is low. For high grade ores or refractory sulfide ores where some pretreatment processing is required to liberate gold, the in-situ generation of thiosulfate should be investigated in more detail. This may lessen thiosulfate consumption and liberate more gold through the oxidation of host sulfide minerals. Cementation (or metal displacement), resins and to a limited extent, activated carbon can be used to recovery gold from thiosulfate solutions. The use of resins to recover gold from solution appears to show some promise, however more work is required to develop suitable elution and recovery methods, and greater selectivity over copper. While difficulties remain to be overcome, thiosulfate leaching has considerable potential as an effective and less hazardous procedure for gold and silver extraction from auriferous ores.

Pressure Acid Leaching of Nickel Laterites: A Review

Abstract A review of the literature over the past 30 years on the processing of nickel laterites by high temperature acid leaching has been carried out to provide a better understanding of the mineralogy, leaching process chemistry and effect of operating conditions on nickel recovery, residue properties and scaling. Particular attention is paid to the leaching experience of the commercial Moa Bay plant and to the recently reported testwork and flowsheets associated with the three Western Australia lalerite plants that will be operating in 1999. It is shown that laterites can vary significantly in their mineralogy according to location, climate and depth, and that the main host minerals for nickel and cobalt can be either goethite (iron oxide) or nontronite (clay) or manganese oxides. The mechanism of leaching involves acid dissolution of the host mineral lattice followed by hydrolysis and precipitation (transformation) of a variety of insoluble oxides and sulphates of iron, aluminium and silica under the high temperature conditions. Optimum leaching conditions and final liquor composition varies according to the ore mineralogy. More fundamental studies have demonstrated that the rate of leaching and character of the residue is dependent upon the level of Mg, Mn and Cr in the ore, the Eh of the slurry and salinity of the process water. A number of studies are reviewed on the chemistry and precipitation of iron, aluminium, magnesium and silica to understand how the process conditions affect the solubility of the species and the nature of the scale which they form. Early work at Moa Bay indicates that the incorporation of chromium into alunite scale also affects the incorporation of silica and nickel and the settling of the residues. Various types of scale have been identified during different stages of leaching and possible means of minimising scale are discussed. The clay-rich nickel laterites in Western Australia differ from Moa Bay laterite in mineralogy and have comparatively high silica and low chromium content. Since no commercial plant has previously processed such ores or used saline process water, there is little published in this area. It is therefore recommended that further research be carried out on understanding the process chemistry and species equilibrium from various ore types under autoclave conditions.

Pressure acid leaching of arid-region nickel laterite ore: Part I: effect of water quality

The effect of water salinity on the reactions occurring during pressure acid leaching of an arid-region laterite ore, using hypersaline water, seawater, sub-potable water and tap water, is examined. Particular emphasis is placed on the mineralogy of the residue and its implications with regard to residue volume/mass, overall acid consumption and nickel extraction. Analysis of a pressure acid leach residue by electron microprobe indicates that the residual nickel is present in phases that contain silicon and varying concentrations of aluminium, but are deficient in sulphur. Incomplete extraction of nickel from the ore may not be attributed to any one mineral phase.

Oxidative precipitation of manganese with SO2/O2 and separation from cobalt and nickel

The oxidation of Mn(II) and precipitation as MnO2/Mn2O3occurs spontaneously using SO2/O2 gas mixtures in the pH range 1-6 and temperature range 25-80 °C. The rate of Mn(II) oxidation with SO2/O2 is first order with respect to SO2 partial pressure up to 5.7% SO2 at 80 °C and half order with respect to [H+]. The rate of oxidation of Mn(II) is slow at pH 4. Any Fe(II) is oxidised and precipitated before Mn(II). Low concentrations of manganese (0.01 M) can be selectively oxidised and precipitated from cobalt and nickel solutions at pH 3-4 with less than 1% co-precipitation of cobalt and nickel in the laboratory batch tests. The selectivity of manganese precipitation at pH 3-4 is consistent with thermodynamic data for the Mn-Ni/Co-H2O systems.

Dissolution of metal ferrites and iron oxides by HCl under oxidising and reducing conditions

Abstract A study has been carried out on the relative rates of dissolution of copper, zinc, and nickel ferrites in HCl solutions and on the effect of various oxidant and reductant species in solution. The results are compared with the rates of dissolution of hematite, magnetite, and nickel oxide, and correlated with their electrochemical behaviour. Cyclic voltammograms of the metal ferrites and oxides show oxidation or reduction peaks corresponding to changes in the oxidation state of the metal in the lattice. It is shown that the relative reactivity in 1 M HCl at 25° is in the order Fe3O4 ⪢ ZnFe2O4 > CuFe2O4 > Fe2O3 > NiFe2O4. The leaching of the metal ferrites and iron oxides is enhanced by the presence of 0.01 M Cu(I) or Sn(II) but the leaching of NiO is enhanced by oxidants such as 0.01 M Cu(II) in chloride media.

Pressure acid leaching of arid-region nickel laterite ore: Part II. Effect of ore type

A comparison of the leach chemistry and residue mineralogy has been carried out on the pressure acid leaching of nontronite, limonite and saprolite ores, using hypersaline water. Results are also compared with a typical arid-region laterite feed from Bulong, which consists of a blend of these ore types. Particular emphasis is placed on the influence of ore type on liquor analysis of iron, aluminium and magnesium, residue mineralogy and nickel extraction. Microprobe evidence is presented that incomplete nickel extraction results from the presence of unreacted minor phases present in the original ore, or from the presence of nickel in the amorphous silica, in apparent association with magnesium.

SO2/O2 as an oxidant in hydrometallurgy

Oxygen mixed with SO2 can function as a powerful oxidant in acid solution. This paper summarises basic studies on the mechanism of SO2/O2 oxidation, the kinetics of oxidation in acidic media, and potential applications in hydrometallurgy. The SO2/O2 system readily oxidises Fe(II) to Fe(III) and provides a simple method of regenerating Fe(III) for leaching minerals like Cu2S or U3O8. It also oxidises As(III), which in the presence of iron(III) enables a stable ferric arsenate compound to be precipitated. A method for the leaching and removal of arsenic from smelter fumes is proposed. Oxidation of Mn(II) to Mn(III)/Mn(IV) oxides by SO2/O2 allows manganese impurity to be removed from leach liquors and electrolytes.

Iron(II) oxidation by SO2/O2 in acidic media:: Part I. Kinetics and mechanism

The kinetics of iron(II) oxidation by SO2/O2 has been studied at 80°C as a means of generating Fe(III) and H2SO4 for subsequent leaching reactions. It is shown that both Fe(III) and S(IV) species are involved in the initial step of the oxidation. The rate of Fe(II) oxidation can be expressed by the following equation: r=kobs[Fe(III)][S(IV)]/fobs([H+]) for 0–0.02 M Fe(III) at the optimum SO2/O2 ratio, where fobs([H+]) is a function of pH. At 80°C, the optimum gas composition of SO2 is around 2% SO2 and both Fe(II) and SO2 were oxidised with the Fe(III)/H2SO4 ratio of about 2. At a SO2/O2 ratio above 9% SO2 and pH 1, dithionate was detected, and the proportion of dithionate to sulphate increased with higher SO2/O2 ratio. A radical chain reaction mechanism is proposed, involving a slow rate of formation of the ferric sulphite complex FeSO3+and decomposition to produce the sulphite radical SO3⋅−. This is followed by a fast reaction with O2 to form the peroxo-monosulphate radical SO5⋅−, which is responsible for the autoxidation of Fe(II). The form of the derived rate expression and the predicted ratio of Fe(III)/H2SO4 from the proposed mechanism essentially agree with the experimental results.

Studies on role of oxygen in the adsorption of Au(CN) 2 − and Ag(CN) 2 − onto activated carbon

Comparative studies of the adsorption of Au(CN)2−, Ag(CN)2−, and Hg(CN)2− onto activated carbon (Norit R2020) have suggested that oxygen and oxygen containing surface functional groups play a role in the adsorption process of Au(CN)2− and Ag(CN)2− but not in the adsorption of Hg(CN)2−. Adsorption of Au(CN)2− and Ag(CN)2− on carbon degassed at 950 °C under 10−5 torr (1.33 × 10−3 P) vacuum is decreased by 50 pct compared with the adsorption on normal activated carbon. However, in the presence of oxygen in solution, the degassed carbon adsorbs Au(CCN)2− to the same extent as normal carbon. The effect of organic solvents and the variation in the potential of the two types of carbon upon adsorption of Au(CN)2− were also investigated. These results indicate that activated carbon behaves like an ion-exchange resin but is capable of oxidizing cyanide and cyanide complexes by chemisorbed oxygen. A dual mechanism for the adsorption of Au(CN)2− and Ag(CN)2− onto activated carbon is therefore proposed, in which cyanide complexes adsorb on carbon by anion exchange with OH− followed by partial oxidative decomposition of Au(CN)2− or Ag(CN)2− to insoluble AuCN or AgCN.

Speciation and reduction potentials of metal ions in concentrated chloride and sulfate solutions relevant to processing base metal sulfides

The speciation, Eh-pH and Eh-log aCl- dependence of Fe(III), Fe(II), Cu(II), Cu(I), Ag(I), Pb(II), Zn(II), Ni(II), As(III), Sb(III), and Bi(III) ions in practical (high ionic strength) sulfate and chloride solutions are discussed. The emphasis is placed on those ions which form strong sulfato-, chloro-, and hydroxo-complex species. Measured potentials are compared with potentials calculated from reported association and stability constants to test the applicability of these constants in nonideal solutions and to characterize predominant species.