Dissolution kinetics of malachite in ammonia/ammonium carbonate leaching

The leaching of oxide copper ore containing malachite, which is the unique copper mineral in the ore, by aqueous ammonia solution has been studied. The effect of leaching time, ammonium hydroxide, and ammonium carbonate concentration, pH, [NH₃]/[NH₄⁺] ratio, stirring speed, solid/liquid ratio, particle size, and temperature were investigated. The main important parameters in ammonia leaching of malachite ore are determined as leaching time, ammonia/ammonium concentration ratio, pH, solid/liquid ratio, leaching temperature, and particle size. Optimum leaching conditions from malachite ore by ammonia/ammonium carbonate solution are found as ammonia/ammonium carbonate concentrations: 5 M NH₄OH+0.3 M (NH₄)₂CO₃; solid/liquid ratio: 1:10 g/mL; leaching times: 120 min; stirring speed: 300 rpm; leaching temperature: 25 °C; particle size finer than 450 μm. More than 98% of copper was effectively recovered. During the leaching, copper dissolves as in the form of Cu(NH₃)₄⁺² complex ion, whereas gangue minerals do not react with ammonia. It was determined that interface transfer and diffusion across the product layer control the leaching process. The activation energy for dissolution was found to be 15 kJ mol⁻¹.     

The oxidation of thiosulfates with copper sulfate. Application to an industrial fixing bath

This paper presents the transformation of thiosulfate using Cu(II) salts, such as copper sulfate, at pH between 4 and 5. The nature and kinetics of this process were determined. In the experimental conditions employed, the reaction between thiosulfates and Cu(II) ions produces a precipitate of CuS and the remaining sulfur is oxidized to sulfate, according to the following stoichiometry: 1 mol thiosulfate reacts with 1 mol Cu²⁺ and 3 mol H₂O, generating 1 mol copper(II), 1 mol sulfate and 2 mol H₃O⁺. In the kinetic study, the apparent reaction order was ≈ 0 with respect to H₃O⁺ concentration, in the interval 1.0 · 10^? 4–1.0 · 10^? 5M H₃O⁺; of order 0.4 with respect to Cu²⁺ in the interval 0.21–0.85 g L^? 1 Cu²⁺; and of order 0 with respect to S₂O₃^2? in the interval 0.88–2 g L^?1 S₂O₃^2?. The apparent activation energy was 98 kJ mol^? 1 in the interval 15–40 °C. On the basis of this behavior an empirical mathematical model was established, that fits well with the experimental results. The thiosulfate transformation process using copper(II) sulfate was applied to an industrial fixing bath that proceeded from the photographic industry; after this, the resulting effluent contained less than 10 mg L^? 1 of thiosulfates.     

197Au Mössbauer study of copper refinery anode slimes

Copper refinery anode slimes are abundantly produced during the electrolytic refining of copper. Although the slimes contain significant and economically recoverable amounts of gold and silver, the chemical state of the gold has not been fully identified. In the present work, the chemical form of gold in a copper anode, in a raw slime, and in slimes treated by different leaching procedures has been investigated by Mössbauer spectroscopy with the 77.3 keVγ-rays of¹⁹⁷Au. The Mössbauer spectrum of the anode is typical of a dilute Au:Cu alloy. The spectrum of the raw slime consists of two components, namely, a single, rather broad line with an isomer shift (IS) of about ?0.3 mm/s relative to a Pt metal source and a quadrupole doublet with an IS of + 1.2 mm/s and a quadrupole splitting of 5.0 mm/s. The single line component can be attributed to a gold-rich alloy, with an approximate composition of Au₆₀Ag{n40} or Au₈₀Cu₂₀ if it is a binary alloy, or to a ternary Au-Ag-Cu alloy of appropriate composition. The parameters of the quadrupole doublet match those of Ag₃AuSe₂ (fischesserite) or related Ag_2?xAu_xSe compounds. In these compounds, the gold atoms are coordinated by two selenium atoms in a linear arrangement, as is typical for Au(I). It was found that the ratio between the concentrations of the metallic phase and the selenide strongly depends on the leaching conditions. The measurement of the Lamb-Mössbauer factor of fischesserite is also reported.     

Thermochemistry of binary liquid gold alloys: The systems gold-copper and gold-silver at 1379 K

The enthalpies of mixing of the binary liquid alloys of gold with copper and silver have been measured by high temperature reaction calorimetry. The experimental results are described by the following analytical expressions which were derived by a least squares treatment of the data: Au-Cu: ΔH_mix = X_AuX_Cu(-28,821 - 2,468 X^Au + 9,541 X_Au ²) J mol^-1 Au-Ag: ΔH_mix = X_AuX_Ag(-15,820 - 529 X_Au @#@) J mol^-1 The results are compared with excess Gibbs energy data from the published literature to yield approximate excess entropies of mixing. For the gold-copper system the observed parabolic dependence of the enthalpy interaction parameter on composition is well accounted for by the quasi-chemical theory.     

A Review of Ammoniacal Thiosulfate Leaching of Gold: An Update Useful for Further Research in Non-cyanide Gold Lixiviants

This literature review provides an update of ammoniacal thiosulfate as a potential lixiviant of gold, the history of thiosulfate leaching and recent progress on thiosulfate leaching of gold, as well as the production, oxidation, and stabilization of thiosulfate. The leaching results of different types of gold ores including oxide, sulfide, carbonaceous, and refractory ores/concentrates and the fundamentals relevant to the oxidation of pure gold are summarized. The factors affecting gold leaching such as pH, temperature, and oxidants in leaching solutions, reagent concentrations, and foreign ions are presented to update the current understanding on gold dissolution in a Cu(II)-NH₃-S₂O₃^2-system.     

Study of the mechanism of anodic dissolution of Cu2S

The anodic dissolution of Cu₂S in sulfuric acid solutions was studied under galvanostatic and potentiostatic conditions. The anodic products were studied by mineralogical and X-ray diffraction methods. In every case, the formation of a digenite Cu_1-8S layer is observed at the surface of Cu₂S according to 5Cu₂S → 5Cu_1.8S + Cu⁺⁺ + 2e A copper concentration gradient appears through the digenite layer whose thickness remains constant as soon as a Cu_1.1S layer appears at its own surface according to 3Cu_1.8S → 4Cu_1.1S + Cu++ + 2e If the electrolysis conditions are such that the anodic potential remains low, the next reaction to occur is 10Cu_1.1S → HCu⁺⁺ + 10S + 22e But if under galvanostatic conditions, the current density is high enough at a given temperature to reach the sharp rise in anodic potential, or if under potentiostatic conditions the potential is kept high, two other reactions are possible: 10Cu_1.1S → 10CuS + Cu⁺⁺ + 2e followed by CuS → Cu⁺⁺ + S + 2e Moreover, at high anodic potential, the following reaction occurs also to some extent CuS + 4H₂O ? Cu⁺⁺ + SO₄ ⁼ + 8H⁺ +8e resulting in a decrease in anodic current efficiency for the copper dissolution. From a more practical point of view, it was shown that it is possible to deplete virtually completely the copper content of the anode (residue at less than 0.5 pct Cu)keepingthe electrode potential at a low value (less than +650 mV/ENH). Providing the temperature is high enough (75°C at least), the mean current density remains near to 2 A/dm², a suitable value to obtain good cathodic deposits.     

The redox equilibria of copper ions in the molten silicate fluxes as a measure of basicity

The redox equilibria of copper ions in the molten NaO_0.5-SiO₂, NaO_0.5-NaF-SiO₂, NaO_0.5-CaO-SiO₂, CaO-CaF₂-SiO₂, CaO-MgO-SiO₂, and CaO-SiO₂ systems have been measured in order to seek a new measure of flux basicity. The Cu²⁺/Cu⁺ ratio decreases with increasing the content of basic oxide. The correlations between the Cu²⁺/Cu⁺ ratio and other refining indexes, such as carbonate, sulfide, and phosphate capacities, CaO activity, and theoretical optical basicity are discussed.     

Leaching of bornite in acidified ferric chloride solutions

In an acidified ferric chloride solution, bornite leaches in two stages of reaction with the first being relatively much more rapid than the second; the first terminates at 28 pct copper dissolution. The first-stage dissolution reaction is electrochemical and is mixed kinetics-controlled; ferric-ion transfer through the solution boundary layer and reduction on the surface to release Cu²⁺ into solution are both important in controlling the rate. The concentration of labile Cu⁺ in the bornite lattice governs the potential of the surface reaction, and, once Cu⁺ is depleted from the original bornite, stage-I reaction ceases. The solid reaction intermediate formed is Cu₃FeS₄. Minute subcrystallites formed at the latter part of stage I leach topochemically in stage II. This reaction which commences at 28 pct Cu dissolution is characterized by a change in mechanism at about 40 pct copper dissolution, though the overall chemical equation for reaction is unchanged in stage II; cupric and ferrous ions and sulfur as a solid residue are products of reaction. The region 28 to about 40 pct Cu dissolution is designated as a transition period to stage-II reaction. Reaction rate in this period is interpreted as being controlled by reduction of Fe³⁺ on active product sulfur surface sites, and hence the reaction rate is controlled by the rate of nucleation and growth of sulfur on the Cu₃FeS₄ intermediate surfaces. Strain in the Cu₃FeS₄ crystal lattice is released during this period by diffusion from the lattice of Cu⁺ remaining from the labile copper initially present in the bornite. After about 40 pct Cu dissolution the rate of reaction is controlled by diffusion through the fully formed sulfur layer in an equiaxial geometrically controlled reaction.     

Ammonia,oxidation leaching of chalcopyrite —surface deposit effects

The reaction for the ammonia, oxidation leaching of chalcopyrite, CuFeS₂ + 4NH₃ + 17/4 O₂ + 2 OH_- = Cu(NH₃)₄ ⁺² + l/2Fe₂O₃ + 2 SO₄ + H₂O was studied using monosize particles in an intensely stirred reactor at moderate oxygen pressures. For dilute solids concentration, the rate is controlled by an electrochemical surface reaction. Under conditions of low stirring speeds and low oxygen pressure, the hematite reaction product passivates the surface and the reaction virtually stops. Even though stirring speed influences the rate of the electrochemical reaction, this effect is due to changes in the morphology of the hematite deposit which alters the surface reaction kinetics, rather than being indicative of mass transfer limitations. Formerly Metallurgy Graduate Student, University of Utah,     

硫氰酸铵-十四烷基三甲基溴化铵微晶吸附体系浮选分离铜

建立了硫氰酸铵-十四烷基三甲基溴化铵微晶吸附体系浮选分离铜的新方法。探讨了硫氰酸铵溶液用量、十四烷基三甲基溴化铵（TTMAB）溶液用量和酸度等因素对Cu²⁺浮选率的影响,讨论了Cu²⁺的浮选分离机理。结果表明,在最佳条件下,由Cu²⁺,SCN^-和十四烷基三甲基溴化铵阳离子(TTMAB⁺)形成的不溶于水的三元缔合物(TTMAB)₂ ［Cu(SCN)₄］被定量吸附在TTMAB⁺与SCN^-作用产生的微晶物质TTMAB⁺·SCN^-表面,且被浮选至水相上形成界面清晰的液-固两相,而Fe³⁺、Co²⁺、Ni²⁺、Cd²⁺、Mn²⁺、Al³⁺等离子仍然留在水相中,实现了Cu²⁺与这些离子的定量分离。据此建立了硫氰酸铵-十四烷基三甲基溴化铵微晶吸附体系浮选分离铜的新方法,进行了合成水样中Cu²⁺的定量浮选分离,浮选率为93.4%～103.6%。     

The reduction of sulfide minerals by hydrogen in the presence of lime

Calcium oxide favors greatly the reduction of metal sulfides by H₂ at 800°C. Copper sulfides and copper-iron sulfides are reduced rapidly to metals: Cu₂S+H₂+CaO→2Cu+CaS+H₂O CuS+H₂+CaO→Cu+CaS+H₂O Cu₅FeS₄+4H₂+4CaO→5Cu+ Fe+4CaS+4H₂O The reduction of chalcopyrite takes place according to any of the following reactions depending on the amount of CaO added: Cu₂S·Fe₂S₃+H₂+CaO→Cu₂S·2FeS+CaS+H₂O Cu₂S·Fe₂S₃+3H₂+3CaO→Cu₂S+2Fe+3CaS+3H₂O Cu₂S·Fe₂S₃+4H₂+4CaO→2Cu+2Fe+4CaS+4H₂O Gravity and magnetic methods were not successful in separating the reaction products. However, leaching by dilute HCl was successful in eliminating CaS and iron (as FeS as well as metal) leaving behind Cu₂S concentrate or metallic copper, respectively. Reduction of molybdenite is also influenced by the presence of CaO, but that of pentlandite is not. FATHI HABASHI and RAYMOND DUGADALE, formerly with the Research Division of the Anaconda Company, Tucson, Arizona     

Au/La-CeOx catalyst for CO oxidation:Effect of different atmospheres pretreatment on gold state—Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

So far,it is still a controversial issue which status of gold species is a better active site for catalyzing CO oxidation.Herein,the influence of the different atmospheres pretreatment(oxidative and reductive)on gold state of Au/La-CeOx(1 wt%gold loading)catalyst during CO oxidation was studied.The changes of Au species were monitored by combined in situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS)and X-ray photoelectron spectroscopy(XPS).For the sample pretreated with oxidative atmosphere,the data show that the initial Au³⁺is transformed to Au^δ+(0<δ<1)during CO oxidation,which is a key step to lead to higher reactivity.For the sample after reductive atmosphere pretreatment,Au^δ+is mixed with a small amount of Au⁰which can be converted to Au^δ+with the increase of temperature in reaction.Meanwhile,the sample always maintains high activity during the reaction.Therefore,the Au?+obtained by reductive pretreatment is more active than the Au³⁺obtained by oxidative treatment in catalyzing CO oxidation.     

197 Au Mössbauer study of the gold species adsorbed on carbon from cyanide solutions

The gold species present on activated carbon after adsorption from solutions of Au(CN)₂ ^? have been studied by¹⁹⁷Au Mössbauer spectroscopy as a function of the pH value of the solution, the loading of the carbon, the coadsorption of polyvalent cations, and the treatment of the samples after adsorption. The gold was found to be adsorbed mainly as Au(CN)₂ ^?. Coadsorbed polyvalent cations (Ca^²+, Gd^³+) have no influence on the Mössbauer parameters of the adsorbed gold complex. After adsorption from acidic solutions (pH ? 4), one finds a substantial amount of adsorbed gold with Mössbauer parameters similar to those of crystalline AuCN. Presumably, this gold is bound in Au_x(CN)_x+1 oligomers which form during drying. An additional product with Mössbauer parameters close to those of KAu(CN)₂Cl₂ was observed on dried samples after adsorption at pH 1. A minor gold species with an uncommonly small electric quadrupole splitting was found on wet carbons but disappeared on drying.     

A thermodynamic study of digenite solid solution (Cu2-δS) at 600 to 1000°C and a statistical thermodynamic critique on general nonstoichiometry

A very accurate experimental method to determine the composition of metal-saturated sulfides has been developed and applied to Cu_2-δS. The technique consists of using a high-sensitivity thermobalance which operates in a controlled atmosphere of H₂-H₂S gas. By this method it was shown that Cu-saturated Cu_xS is stoichiometric(x= 2.0000 ± 0.0002) between 700 and 1000°C. The free energy of formation for Cu₂S was found to be: 2Cu(s) + 1/2 S₂(g) = Cu_2.0000S(s), ΔG⁰ = -30,610 + 6.80T (cal/mole). The sulfur partial pressure was determined over nonstoichiometric Cu_2-δS for sulfur contents up to 21 pct. From the result, thermodynamic functions such as activity of copper, heats and entropies of solution were calculated as a function of nonstoichiometric composition. It was thermo-dynamically demonstrated that previous models dealing with nonstoichiometric oxides (or sulfides) based on the law of mass action,i.e., δ = const ie 67 01 (or ie 67 02), are inconsis-tent as they fail to satisfy the Gibbs-Duhem relation and also fail to account for the dis-sociation pressure over the stoichiometric composition. To resolve this dilemma, a sta-tistical thermodynamic method of constructing the grand partition function was introduced. Stoichiometry and nonstoichiometry of the Cu_2-δS were then explained by postulating an ionic crystal consisting of Cu⁺, Cu²⁺, Cu⁰ and neutral vacancies in the copper sublattice and S²⁻ and neutral vacancies in the sulfur sublattice. Formerly Postdoctorate Fellow, National Research Council of Canada, Ottawa, Ontario     

Adsorption of gold on activated carbon in bromide solutions

Adsorption of gold on activated carbon from gold bromide solutions was examined as a function of pH, particle size, amount of carbon, initial gold bromide concentration, bromine concentration, temperature, and various cations and anions. The pH did not have any effect on gold adsorption below pH = 6. Above pH = 6, pH had a dramatically negative effect. The adsorption occurred according to a first-order reaction-rate model. The reaction-rate constant was proportional to the inverse of carbon particle radius, the amount of carbon, and independent of the initial gold concentration. Temperature had very little effect on the rate of gold adsorption. Cations (Cu²⁺, Fe³⁺, Ca²⁺, Mg²⁺, and Na⁺) and anions (SO ₄ ² , Cl^-, Br^-) did not have any effect on gold adsorption. Free bromine had a very negative effect on gold uptake. Gold bromide is very stable and stays in this form on the carbon surface. Contrary to this, gold chloride is unstable, and elemental gold precipitates on the carbon surface. Loading capacity of carbon depended on the presence of free bromine. In the presence of free bromine, it was further dependent on temperature (increased with increasing temperature). AtT = 25 °C, gold loading was 27 to 64 kg Au/t carbon. In bromine-free solutions, gold loading was 174 to 315 kg Au/t carbon.     

The electrochemical oxidation of chalcopyrite in ammoniacal solutions

The anodic dissolution of chalcopyrite in ammoniacal solutions was investigated using electrochemical methods. At low overvoltages, the formation of a copper deficient sulfide layer, Cu_1-xFeS₂ through a charge transfer reaction is proposed based upon the dependence of the rest or open circuit potential on solution composition and the presence of a Tafel region of appropriate slope. In addition, a current peak that occurs at 10⁻⁴ A/cm² is a function of the square root of the voltage scanning speed and is explained in terms of a charge transfer reaction. At larger overvoltages, constant potential experiments and mass balances performed at various anodic potentials indicate that the dissolution is consistent with the overall reaction, CuFeS₂ + 4NH₃ + 9OH^- = Cu(NH₃) ₄ ⁺² + Fe(OH)₃ + S₂O ₃ ⁼ + 3H₂O +9e ^-, although some copper may be released to solution in the cuprous state and some ferrous iron has been identified in the product film. Currentvs time data taken during constant potential experiments were found to obey a linear rate relationship. This was interpreted in terms of the formation of a layer of constant thickness which is corroded at the outer interface at the same rate it is formed at the inner interface.. The model proposed is typical of the corrosion of some metals. An examination of the polarization curves shows the dissolution reaction to be first order with respect to [OH^-]. The lack of dependence on [Cu²⁺] indicates that the catalytic effect of cupric ion during oxygen pressure leaching is related only to the cathodic reduction of O₂ in agreement with the results of previous investigations.     

Sequential Precipitation of Heavy Metals Using Sulfide-Laden Bioreactor Effluent in a pH Controlled System

Sulfide produced in an ethanol fed anaerobic baffled reactor, was utilized to precipitate metals separately in a pH controlled system. Sulfide produced in the reactor (780±27 mg/L S^2–) was transported with N₂ gas to the metal precipitation chamber at pH 2.5 to precipitate Cu²⁺ separately from Fe²⁺. Cu precipitation was completed at 98% efficiency within 5 min. The remaining Fe²⁺ was then precipitated at elevated pHs by mixing the reactor effluent. XRD patterns of the precipitates showed that copper was precipitated in the form of CuS with a particle size of 14–22 nm whereas iron was precipitated as FeS with 32-85 nm particle size.     

Thermodynamics of titanium alloys: I. Development of a triple Knudsen cell and its use to study the activity of copper in the Ti−Cu system

A new experimental technique for the determination of thermodynamic activity of alloys has been developed utilizing a triple Knudsen cell with a pure enriched natural isotope as the standard reference state. The alloy for which activity is to be determined is placed in one of the two effusion chambers of the triple cell and the pure isotopic standard in the other. The molecular beams from each chamber effuse into a third upper chamber and through a collimating hole into the ion source of a Bendix time-of-flight mass spectrometer. Since the recorded intensities are proportional to the vapor pressures within the chambers, a simple calculation based upon their ratio gives a direct determination of the activity of the solute in the alloy. The accuracy of the triple cell technique for the experimental determination of activities was checked by the measurement of the copper activity in a Ni?Cu alloy containing 30.6 wt pct Cu. A value ofRT γ_Cu=1778±142 cal per g-atom at 1450°K was obtained, which is in excellent agreement with values in the literature. The activity of copper in the bcc β phase when then determined for four Ti?Cu alloys (X _Cu=0.033 to 0.085), using pure enriched Cu⁶⁵ as the standard reference state. The composition range investigated was limited to alloys belowX _Cu=0.10, the maximum solubility within the experimental temperature range between 1423° and 1573°K, the activity of copper in the bcc β phase can be expressed using pure liquid copper as the standard state by the following equation:RT Ina _Cu=RT InX _Cu+X _Ti ² (1.92±0.30) in kcal per g-atom. The activity of titanium is given byRT Ina _Ti=RT InX _Ti+X _Cu ² (1.92±0.30) in kcal per g-atom.     

Electrochemistry of Chalcocite/Heazlewoodite/Sulfhydril Collector Systems

Abstract

Rest potential and cyclic voltammetry studies were conducted on chalcocite and heazlewoodite as a function of pH in the presence and absence of the sulfllydryl collectors, sodium ethyl xanthate (EX) and the sodium salt ofmercaptobenzothiazole (MBT). The minerals showed a significant difference in their response both in the presence and absence of collectors. The voltammograms suggested that EX^? and MBT^? interact with the minerals in a similar manner. Most reactions of chalcocite and heazlewoodite in the presence and absence of EX were identified from the literature. For the case of MBT and chalcocite the following new reaction is proposed at E _e = ?0.060 V:

Cu₂S+2MBT^? → 2CuMBT+S⁰+2e^?.

Résumé

Des études du potentiel à repos et la voltammetrie cyclique ont été entreprisés sur le chalcocite et le heazlewoodite en fonction de pH en présence et en absence des collecteurs sulfhydryl sodium ethyl xanthate (EX) et le sel sodium de mercaptobenzothiazole (MBT). Les minéraux différents manifestent des réponses différentes, en présence et en absence des collecteurs. Les voltammogrammes indiquent que l'EX et I'MBT réagent avec les minéraux en façons semblables. La plupart des réactions du chalcocite et du heazlewoodite en présence et en absence de l'EX existent dans la littérature. Pour le cas de MBT/chalcocite à E _e = ?0.060 V, une nouvelle réaction est proposée:

Cu₂S+2MBT^? → 2CuMBT+S⁰+2e^?.     

Dissolution of bornite in sulfuric acid using oxygen as oxidant

Leaching of natural bornite in a sulfuric acid solution with oxygen as oxidant was investigated using the parameters: temperature, particle size, initial concentration of ferrous, ferric and cupric ions, and using microscopic, X-ray and electronprobe microanalysis to characterize the reaction products. Additionally, stirring rate, pH and P_O2 were varied. Dissolution curves for percent copper extracted as a function of time were sigmoidal in shape with three distinct periods of reaction: induction, autocatalytic and post-autocatalytic which levelled off at 28% dissolution of copper. The length of the induction period was not reproducible, causing the dissolution curves to be shifted with respect to time. The dissolution curves in the autocatalytic and post-autocatalytic regions were reproducible, and this property was utilized to treat much of the kinetic data. The iron dissolution curves had four dissolution regions. An initial small but rapid release of iron to solution preceded the three periods just given for copper dissolution. Aside from this initial iron release, the iron and copper dissolution curves were almost identical.Stirring rate had no effect on dissolution of copper above 400 min^?1 nor did oxygen flow rate in the range 20–40 cm³/min. Dissolution rate was slightly dependent on oxygen partial pressure for P_O2 < 0.67. Hydrogen ion concentration had no effect except that sufficient acid was required to prevent hydrolysis and precipitation of iron salts.The dissolution rate was directly dependent on the reciprocal of particle diameter indicating possible surface chemical reaction control, but the activation energy of 35.9 kJ/mol (8.58 kcal/mol) for the autocatalytic region of copper dissolution is slightly too small for that, though not unreasonable. Initial addition of Fe²⁺ had a rather complex effect and markedly enhanced dissolution of copper, as also did initial addition of Fe³⁺. Microscopic analysis showed nuclei of two new phases, covellite and Cu₃FeS₄, in the induction region. The new phases grow rapidly in the autocatalytic stage, which is controlled by nuclei formation and chemical reaction. The post-autocatalytic region is characterized by complete transformation of bornite into covellite on the particle surfaces and Cu₃FeS₄ as an internal product with an X-ray spectrum very similar to that of chalcopyrite. The post-autocatalytic region is controlled by autocatalytic growth of newly formed phases. Further reaction beyond the autocatalytic region (percent copper dissolution > 28%) occurs so slowly with oxygen as oxidant that it was not studied.The rate of copper dissolution appears to be controlled by the rate of iron dissolution. Using that and the other experimental evidence a mechanism for reaction is proposed in which iron-deficient bornite, Cu₅Fe_?S₄, is formed on the surface by initial preferential iron dissolution. Labile Cu⁺ diffuses into this from Cu₅Fe_?SO₄ and unreacted bornite to produce CuS on the surface. Depletion of labile Cu⁺ ions from Cu₅FeS₄ produces Cu₃FeS₄ in the interior of the mineral particles.