Kinetics and mechanism of the conversion of covellite (CuS) to digenite (Cu1.8S)

Rates of conversion of covellite (CuS) to digenite (Cu_1.*S) in N₂ and various partial pressures of O₂ have been studied in the temperature range 260° to 400°C. The reaction is first order with respect to O₂ concentration in N₂?O₂ mixtures at 1 atm. The experimental activation energy for the conversion of CuS to Cu_1.8S is 23±3 kcal. In a nitrogen atmosphere, the conversion proceeds at a slower rate but with the same activation energy and leads to the formation of elemental sulfur instead of SO₂. An externally added small amount of iron powder (0.6 pct) increased the rate of conversion, but had no effect on the apparent activation energy. The rate-controlling reaction in the conversion mechanism is postulated to be Cu_1.8S→1.8Cu⁺+1.8e ^?+S at the Cu_1.8S/gas interface. Reduction of CuS by copper(I) ions and electrons occurs by a rapid reaction, 0.8 Cu⁺+0.8e ^?+CuS→Cu_1.8S at the CuS/Cu_1.8S interface, which steadily contracts with time.     

Investigating the surface of particles of ultradispersed copper powders obtained by gas-phase condensation

Ultradispersed powders of metal copper obtained by evaporation-condensation technology are investigated. It is shown that, depending on the evaporation rate, various dispersities and degrees of agglomeration of copper powders are attained with the conservation of the weight fraction of copper in them no lower than 99.0%. It is established using the XPES method that a 5- to 6-nm-thick CuO layer (on the surface of which there is a Cu₂O layer up to 1 nm thick) is always present on the surface of Cu particles of all powders under consideration. It is assumed that, because of a low residual air pressure in industrial installations, the surface layer of copper oxidizes to the lower oxide Cu₂O during the evaporation of metal, while oxide CuO is formed as a result of the decomposition of Cu₂O during the condensation of copper particles. The smaller the particle size of the powder is, the higher the content of oxides is.     

Pressure leaching of copper minerals in perchloric acid solutions

The leaching of covellite (CuS), chalcocite (Cu₂S), bornite (Cu₅FeS₄), and chalcopyrite (CuFeS₂) was carried out in a small, shaking autoclave in perchloric acid solutions using moderate pressures of oxygen. The temperature range of investigation was 105° to 140°C. It was found that covellite, chalcocite, and bornite leach at approximately similar rates, with chalcopyrite being an order of magnitude slower. It was found that chalcocite leaching can be divided into two stages; first, the rapid transformation to covellite with an activation energy of 1.8 kcal/mole, followed by a slower oxidation stage identified as covelite dissolution with an activation energy of 11.4 kcal/mole. These two stages of leaching were also observed in bornite with chalcocite (or digenite) and covellite appearing as an intermediate step. No such transformations were observed in covellite or chalcopyrite. Two separate reactions were recognized as occurring simultaneously for all four minerals during the oxidation process; an electrochemical reaction yielding elemental sulfur and probably accounting for pits produced on the mineral surface, and a chemical reaction producing sulfate. The first reaction dominates in strongly acidic conditions, being responsible for about 85 pct of the sulfur released from the mineral, but the ratio of sulfate to elemental sulfur formed increases with decreasing acidity. Above 120°C the general oxidation process appears to be inhibited by molten sulfur coating the mineral particles; the sulfate producing reaction, however, is not noticeably affected above this temperature. For chalcopyrite, activation energies were determined separately for the oxygen consumption reaction and for the production of sulfate, with values of 11.3 and 16.0 kcal/mole respectively. This paper is based upon a thesis submitted by F. LOEWEN in partial fulfillment of the requirements of the degree of M.A. Sc. in Metallurgical Engineering at The University of British Columbia.     

Phase diagram of Cu2O-CuO-Y2O3 system in air

The isobaric phase diagram of the Cu₂O-CuO-Y₂O₃ ternary system has been investigated under 0.21 atm oxygen partial pressure mainly by thermogravimetric and differential thermal analyses. The present system was confirmed to consist of four solid phases: Y₂O₃, Y₂Cu₂O₅, CuO, and Cu₂O. The Y₂Cu₂O₅ phase is formed by the peritectic reaction with Y₂O₃ at 1493 K. An univariant reaction, L ⇆5 Cu₂O + Y₂Cu₂O₅, appears at 1368 K. Cu₂O is stable at high temperatures and transforms to CuO at 1299 K.     

Leaching kinetics of copper from natural chalcocite in alkaline Na4EDTA solutions

The leaching behavior of copper from natural chalcocite (Cu₂S) particles in alkaline Na₄EDTA solutions containing oxygen was examined at atmospheric pressure. The EDTA leaching process took place with consecutive reactions, where the solid product of the first reaction, covellite (CuS), became the reactant for the second. The copper leached into the alkaline solutions was immediately consumed by the chelation of copper (II) with EDTA, and the mineral sulfur was completely oxidized to sulfate ion. The experimental data for the leaching rate of copper were analyzed with a familiar shrinking-particle model for reaction control. The conversion rate of chalcocite to covellite was found to be about 10 times as high as the dissolution rate of covellite. The time required for complete dissolution of covellite was directly proportional to the initial particle size and was inversely proportional to the square root of the product of the hydroxide ion concentration and the oxygen partial pressure, but it was independent of the Na4EDTA concentration in the presence of excess Na₄EDTA. The observed effects of the relevant operating variables on the dissolution rate were consistent with a kinetic model for electrochemical reaction control. The kinetic model was developed by applying the Butler-Volmer equation to the electrochemical process, in which the anodic reaction involves the oxidation of covellite to copper (II) ion and sulfate ion and the cathodic reaction involves the reduction of oxygen in alkaline solution. The rate equation allowed us to predict the time required for the complete leaching of copper from chalcocite in the alkaline Na4EDTA solutions.     

An extended Derjaguin-Landau-Verwey-Overbeek theory approach to determining the surface energy of copper oxide thin films prepared by reactive magnetron sputtering

Copper oxide films were sputter deposited on glass substrates by reactive rf magnetron sputtering, using a solid copper target and an argon-oxygen gas atmosphere. The films were characterized by scanning electron microscopy, atomic force microscopy, and spectrophotometry. The effect of input power and oxygen flow rate on the dispersive, polar, and acid-base (AB) components of the surface energy of the films was evaluated. The extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to analyze the factors controlling the interaction of copper oxide films with their environment. The components of the surface energy were determined by the Owens-Wendt and the Van Oss-Chaudhury-Good approaches, using water, ethylene glycol, and diiodomethane as the probe liquids. The Lifshitz-Van der Waals (LW) dispersive interaction force was found to be the major contributor to the surface energy of the films. In addition, we observed that other phases of copper oxide such Cu₂O and mixed Cu₂O/CuO are also hydrophobic. Black optically absorbing CuO has been previously identified as a hydrophobic phase. Transparent Cu₂O films hold substantial promise as antistiction coatings in micromirror arrays for micro-electromechanical systems applications. Copper oxide films also have potential aerospace applications as low friction coatings.     

Oxygen pressure dependence of Cu2O-CuO-Gd2O3 phase diagram

The phase diagram of the Gd-Cu-O system has been investigated under various oxygen partial pressures by thermal analysis. In this system, only one ternary compound Gd₂CuO₄ exists stably, depending on the oxygen partial pressure. This compound decomposes to Gd₂O3 and Cu₂O under low oxygen partial pressure at high temperature. The incongruent melting and decomposition temperatures of Gd₂CuO₄ and the temperature of eutectic reaction have been investigated as a function of oxygen partial pressure. The present ternary system includes two invariant reactions:L + Gd₂O₃ ⇆ Gd₂CuO₄ + Cu₂O at 1376 K under 0.07 atm O₂ (7.09 kPa) andL⇆ Gd₂CuO₄ + CuO + Cu₂O at 1321 K under 0.42 atm O₂ (42.56 kPa). The thermodynamic properties of the system have also been considered.     

A kinetic study of the leaching of chalcopyrite at elevated temperatures

A study of the rate of dissolution of chalcopyrite (CuFeS₂) in acidic solutions under oxygen overpressures was carried out by measuring the rate of formation of cupric ions in solution. Effects of temperature, oxygen partial pressure, surface area, and concentration of sulfuric acid were evaluated. A sized batch of chalcopyrite was leached in the temperature range of 125 to 175° and in the pressure range of 75 to 400 psi of oxygen. In 0.5N H₂SO₄ all products of reaction went into solution except for trace amounts of elemental sulfur. The dissolution of chalcopyrite followed linear kinetics and was essentially independent of hydrogen ion concentration for H₂SO₄ concentrations between 0.2 and 0.5 JV. The oxygen dependence indicated adsorption approaching limiting values with increasing oxygen pressure. The linear mechanism was explained in terms of steady-state adsorption of oxygen at the chalcopyrite surface followed by a surface reaction. The enthalpy of activation for adsorption of oxygen was found to be approximately 33 kcal per mole. An activation enthalpy of approximately 9 kcal per mole was observed for the surface reaction. Charge transfer reaction are not rate controlling in the process.     

Cuprous hydrometallurgy Part IV. Rates and equilibria in the reaction of copper sulphides with copper(II) sulphate in aqueous acetonitrile

Copper(II) sulphate in solutions of aqueous acetonitrile leaches copper from copper sulphides to form stable copper(I) sulphate solutions. Covellite and chalcopyrite are oxidised and leached more rapidly in the early stages of leaching with acidic CuSO₄/CH₃CN/H₂O than with acidic iron(III) sulphate in water. A redox equilibrium between copper(I) sulphate, copper(II) sulphate and partially leached solid copper sulphide, Cu_xS, is established. The equilibrium concentration of Cu₂SO₄ and the value of x in Cu_xS, in solutions saturated with CuSO₄, are interdependent if the acetonitrile concentration is constant. This behaviour is considered in terms of the structural and electrochemical changes which occur, in the solids Cu₂S and CuS, as leaching proceeds. According to the activities of acetonitrile, of copper(I) sulphate and of copper(II) sulphate, i.e. according to the redox potential of the solution, CuS either can be oxidised by copper(II) sulphate to a less copper-rich copper sulphide and even to sulphur, or reduced by copper(I) sulphate to a more copper-rich sulphide, up to Cu₂S, in acidic solutions containing CuSO₄, Cu₂SO₄, CH₃CN and water. This observation leads to an easy method of generating Cu₂S from CuS or from sulphur.     

Oxidation of molten copper matte

An amount of 80 mg of molten copper matte of a pseudo-ternary Cu₂S-FeS-Fe system contained in a slender alumina sample tube was oxidized at 1503 and 1533 K in a mixed O₂-Ar gas stream and the oxidation path was followed, comparing the overall rate of oxidation with the gaseous diffusion in the sample tube. The following successive reactions were found to be controlled by gas diffusion. Initially, Fe was oxidized to form FeO. After the melt composition reached a pseudo-ternary Cu₂S-FeS-FeO system, FeS was oxidized to form FeO. As the amount of FeO increased, Fe₃O₄ was also formed and subsequently Cu was produced by the oxidation of Cu₂S. In the latter stage, the Cu was oxidized, and the final product under the condition of gas diffusion control was composed of Cu₂O, Fe₃O₄, and CuFeO₂. On the other hand, the rate of formation of Fe₂O₃, CuO, and CuFe₂O₄ was much slower and they were not formed during the oxidation duration where the overall rate of oxidation was controlled by gas diffusion.     

Experimental investigation and three-dimensional computational fluid-dynamics modeling of the flash-converting furnace shaft: Part II. Formulation of three-dimensional computational fluid-dynamics model incorporating the particle-cloud description

A fluid-dynamics computer model of the flash-converting furnace shaft, which is based on basic principles, is presented. The model is fully three-dimensional and incorporates the transport of momentum, heat, and mass and the reaction kinetics between the gas and particles in a particle-laden turbulent gas jet. The k-ɛ model was used to describe gas-phase turbulence in an Eulerian framework. The particle-cloud model was used to track the particle phase in a Lagrangian framework. The coupling of gas and particle equations was performed through the source terms in the Eulerian gas-phase governing equations. Copper matte particles were represented as Cu₂S · yFeS _x . Based on experimental observation, the oxidation products were assumed to be Cu₂O, CuO, Fe₃O₄, and SO₂. A reaction mechanism involving the external mass transfer of oxygen from the gas to the particle surface and diffusion of the oxygen through the successive layers of Cu₂O-Fe₃O₄ and CuO-Fe₃O₄ was proposed. The predictions of the computer model were compared with the experimental data collected in a large laboratory furnace. Reasonable agreement between the model predictions and the measurements was obtained in terms of the fractional completion of the oxidation reactions and the sulfur remaining in the reacted particles. The relevance of the computational model for further analysis and optimization of an industrial flash-converting operation is discussed.     

Oxidation of copper ultrafine powders and nanopowders produced by vapor phase condensation

The oxidation of copper nanopowders fabricated by metal evaporation and condensation is studied. It is found that, in contrast to the oxidation of bulk copper samples, the oxidation of nanopowders can provide the formation of a CuO surface layer on copper particles in a reactor, bypassing the stage of Cu₂O formation. The Cu₂O oxide that forms on the surface of unoxidized copper powder particles spontaneously transforms almost completely into a more thermodynamically stable phase (CuO oxide) during storage in air under natural conditions.     

Dissolution of Cu2S in aqueous edta solutions containing oxygen

The mechanism of the reactions taking place in the heterogeneous system: synthetic polydispersive Cu₂S-ethylediaminetetraacetic acid (EDTA)—O₂—H₂O has been investigated. The partial pressure of oxygen and pH of the solution were found to exert a significant effect on the process kinetics. The dissolution rate does not depend, in practice, on the agitation rate and the EDTA concentration exerts an influence only at higher partial oxygen pressures.Dissolution of Cu₂S in aqueous EDTA solutions proceeds in two steps with the formation of CuS as an intermediate. In acid and neutral solutions the final products of dissolution are elementary sulphur and Cu(EDTA)^2- complex ion. The activation energy ΔE = 10.4kJ/mol (2.4 kcal/mol) suggests a diffusion controlled process. In alkaline solutions sulphur is oxidized to the sulphate ion and the dissolution process is kinetically controlled, ΔE = 41.4 kJ/mol (9.9 kcal/mol).     

Promotional effect of CO pretreatment on CuO/CeO2 catalyst for catalytic reduction of NO by CO

The CuO/CeO2 catalysts were investigated by means of X-ray diffraction(XRD),laser Raman spectroscopy(LRS),X-ray photoelectronic spectroscopy(XPS),temperature-programmed reduction(TPR),in situ Fourier transform infrared spectroscopy(FTIR) and NO+CO reaction.The results revealed that the low temperature(150 °C) catalytic performances were enhanced for CO pretreated samples.During CO pretreatment,the surface Cu+/Cu0 and oxygen vacancies on ceria surface were present.The low valence copper species activated the adsorbed CO and surface oxygen vacancies facilitated the NO dissociation.These effects in turn led to higher activities of CuO/CeO2 for NO reduction.The current study provided helpful understandings of active sites and reaction mechanism in NO+CO reaction.     

Kinetics and Mechanism of High-Temperature Oxidation in Air of Au - Cu Alloy

We have used thermogravimetry to study the kinetics of high-temperature (up to 800 °C) oxidation of the alloy 58.3 mass% Au - 41.7 mass% Cu with isothermal heating of the specimens. Using petrographic analysis of the oxide layers, we determined the reaction products. We have shown that up to 200 °C, the indicated alloy is not oxidized at all. More rapid oxidation of the alloy is observed at temperatures above 400 °C. Up to 500 °C, an inner layer consisting of Cu₂O predominates in the two-layer scale on the alloy, while the outer CuO layer has a significantly smaller thickness. At 600 °C, the upper layer of scale contains Cu₂O while the lower layer contains Cu₂O and gold. At higher temperatures, all the way up to 800 °C, the scale is two-layer as before but its upper layer contains CuO while its lower layer contains Cu₂O and small gold rods distributed in that oxide. Thus we have established three oxidation regions characterized by different scale phase compositions and different mechanisms for the process, mainly due to transition from an ordered state of the alloy (intermetallic AuCu₃) to a completely disordered solid solution of gold in copper. We used the Arrhenius equation to calculate the apparent activation energy for oxidation: E₁ = 20.4 kJ/mole for the temperature range 400–500 °C and E₂ = 9.5 kJ/mole for 600–800 °C. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 85–91, July–August, 2005.     

Kinetics of the sulfidation of chalcopyrite with gaseous sulfur

The sulfidation of chalcopyrite with gaseous sulfur in the temperature range of 325 °C to 400 °C occurs with the formation of covellite and pyrite as the final products. The rate of sulfidation depends strongly on the temperature, with nearly complete conversion in less than 30 minutes at 400 °C. Microscopic analysis of partially and completely reacted particles showed that the sulfidation proceeded topochemically, with a shrinking core of unreacted chalcopyrite surrounded by successive layers of FeS₂ and CuS. The experimental data exhibited an induction period at the beginning of the reaction. An electrochemical mechanism is proposed for the sulfidation reaction, which involves simultaneous diffusion of Cu⁻ and electrons through the product layers. The rate data showed that the fraction reacted is well represented by a shrinking-core model controlled by the reaction occurring at the chalcopyrite-pyrite interface, resulting in the conversion-vs-time relationship 1−(1−X)^1/3=k(t−t _ind). An activation energy of 98.4 kJ/mol was determined for the temperature range of 325 °C to 400 °C.     

Natural convective mass transfer rates between solid magnetite and molten mattes

A twin balance apparatus capable of continuously weighing both the solid and liquid phase was constructed to measure the magnetite-matte reaction. The rate of magnetite consumption during the reaction with iron sulfide increased from 2.5 to 10.8 g Fe₃O₄/min with increasing temperature from 1473 to 1623 K. In the magnetite-(Fe-S-O) melt reactions, the dissolution rate decreased from 5.4 to 1.5 g Fe₃O₄/min with increasing oxygen content in the bulk liquid phase. The rate approached zero when the bulk phase contained approximately 32 at. pct O which was close to the magnetite saturation limit. The rate of reaction between magnetite and liquids of the FeS-Cu₂S binary system decreased from 5.4 to 0.2 g Fe₃O₄/min with increasing Cu₂S content from 0 to 60 mol pct Cu₂S. Examination of the reaction rate data of the Fe-S-O system in conjunction with a magnetite pellet profile study and oxygen analyses in the matte showed that the dissolution mechanism was one of diffusion enhanced by natural convection. The results of reacting magnetite with FeS-Cu₂S melts suggested that the same mechanism operated. The industrial significance of this investigation is discussed briefly in relation, to the problem of the presence of solid magnetite in copper smelting processes. Formerly with the Department of Metallurgy and Materials Science, University of Toronto     

Diffusivity and solubility of oxygen in solid copper using potentiostatic and potentiometric techniques

The diffusivity and solubility of oxygen in solid copper have been determined in the temperature range 700 to 1030 °C using potentiostatic and potentiometric techniques. The results are summarized by the following equations: _Do ^Cu = _1.16-0.31 ^+0.42 × 10⁻² exp(−67300 ± 3000/RT) cm² per second; _No ^s = 154 exp(−149600/RT)atom fraction of oxygen where R is in joules/degree/mole. The experimentally determined value of the pre-exponential factor in the diffusivity equation is found to be consistent with Zener’s model for an interstitial diffusion mechanism. on leave of absence from the Banaras Hindu University, India     

Application of chronopotentiometric analysis to the anodic treatment of copper sulphides

The analytical techniques of chronopotentiometry have been applied to previously published data on the electroleaching of the copper sulphides chalcocite, Cu₂S, and digenite, Cu₉S₅. The results can be explained by a model that combines solid state diffusion with chemical interaction between the solid and the solution. Quantitative evaluation of the reactions indicates that the first stage of reaction is preferential dissolution of copper, resulting in a compound of approximately CuS composition, but the Cu:S ratio of which apparently decreases with increasing temperature of treatment. The second state of reaction is the decomposition of this material to cupric ion and sulfur.     

Equilibrium relations in the system nickel oxide-copper oxide

Phase relations in the system Cu-Ni-0 were studied atPo ₂ = 0.21 atm using DTA and quenching techniques. Results were expressed in the form of a NiO-CuO “binary” in which a eutectic occurs at 1090°C and 6 mol pct NiO. The activities of CuO in (Ni,Cu)O solid solutions were determined at 1024°C by establishing the oxygen partial pressures at which Cu₂O precipitates from members of this solution series.