Mineralogical changes occurring during the ferric lon leaching of bornite

The reaction products formed during the leaching of bornite in either ferric chloride or ferric sulfate media depend on the leaching conditions as well as the particle size of the bornite. The extent of dissolution is always more vigorous in the ferric chloride system and increases with increasing temperature in either system. The reaction initially involves the rapid outward diffusion of copper to form slightly nonstoichiometric bornite (Cu_5-xFeS₄), chalcopyrite, and covellite. The non-stoichiometric bornite is progressively converted to a Cu₃FeS₄ phase, which varies considerably in its composition, and to covellite. Although the reaction at low temperature terminates at the Cu₃FeS₄ phase, leaching at higher temperatures results in further dissolution to elemental sulfur and soluble Cu²⁺ and Fe²⁺. The leaching ofmassive bornite illustrates the complexities of the leaching reaction more clearly than is observed for the finelypaniculate bornite. In leached massive bornite, a distinct covellite zone appears in the Cu₃FeS₄ phase; as well, chalcopyrite exsolution lamellae rimmed by a copper sulfide (possibly digenite) appear in the covellite zone, in the Cu₃FeS₄ phase, and in the nonstoichiometric bornite. The experimental leaching results, especially those involving massive bornite, are generally consistent with the mineralogical trends produced by supergene alteration of bornite ores, but a significant difference is that the Cu₃FeS₄ phase does not correspond closely to the mineral idaite.     

Thermal treatment of complex sulfide ores in N2 and H2 Atmospheres: A new approach for the extraction of their valuable elements

The thermal treatment of natural sulfidic minerals such as sphalerite, galena, chalcopyrite, and pyrite in an N₂ or H₂ atmosphere was studied to examine the nature of reactions taking place. Such treatments have the potential of avoiding sulfur dioxide production which is associated with the roasting of complex sulfide ores (CSOs). The thermal treatment of CSO concentrates at temperatures less than 1000 °C in a nitrogen atmosphere leads to the decomposition of the pyritic matrix to pyrrhotite and the volatilization of sulfur, galena, and some of the CSOs’ trace elements. Treating the CSO in a reducing atmosphere converted sphalerite to zinc and produced a solid containing Cu^o, Fe^o, and silicoaluminates. Selective dissolution of copper may be achieved by a hydrometallurgical process. Hydrogen sulfide could be reacted with pyrrhotite to form pyrite and hydrogen. A flow sheet is proposed. M.-CH. Meyer-Joly formerly Researcher with Mineral Processing and Environmental Engineering, Institut National Polytechnique de Lorraine K. MALAU formerly Researcher with Mineral Processing and Environmental Engineering, Institut National Polytechnique de Lorraine     

CHARACTERIZATION OF PRODUCTS AND ANALYSIS OF SULFUR VAPORIZATION FROM AN IRON SULFIDE ORE

The total extraction of sulfur by vaporization from iron sulfide ores leads to a residue that is constituted in great part of iron. The aim of this article is to follow up the secondary metals composition in the residue during the sulfur extraction. When the pyrrhotite is treated alone, copper is completely evaporated after Cu₂S decomposition. While in the presence of carbon, iron is present as the core form in the iron sulfide. In this case, the kinetic of vaporization is fast and no secondary metals trace is detected in the iron core.     

Sulfidation of chalcopyrite with elemental sulfur

The sulfidation of chalcopyrite concentrate with elemental sulfur was studied in the temperature range of 325 °C to 500 °C. The effects of temperature, time, and composition of the reactants on the sulfidation were determined. The X-ray diffraction (XRD) and light microscopic analyses showed that the sulfidation of chalcopyrite forms CuS and FeS₂ at temperatures below 400 °C. However, at temperatures above 400 °C, Cu₅FeS₆ and FeS₂ were formed. The sulfidation of chalcopyrite proceeds mainly through the gaseous phase, and temperature has a significant influence on the sulfidation rate in the range of 325 °C to 400 °C. The extraction of copper from the reacted material was determined by leaching in an H₂SO₄-NaCl-O₂ system. Over 90 pct of copper could be extracted by leaching at 100 °C for 60 minutes in the mentioned system.     

Oxidation of mixed copper-iron sulfide

. A rectangular plate of mixed copper-iron sulfide composed of bornite (Cu₅FeS₄) and troilite (FeS) was oxidized in an O₂-Ar mixed gas stream at 1023 to 1123 K. At the start of the oxidation, iron was preferentially oxidized with the rapid formation of a dense Fe₃O₄ layer of about 10 μm thickness on the sample surface, without the evolution of SO₂ gas. Following this reaction, layers of both Fe₃O₄ and Fe₂O₃ grew on the sulfide surface in accordance with the parabolic rate law. The diffusion of iron through the oxide layers was assumed to control the oxidation rate during this stage. The effect of oxygen partial pressure on the parabolic rate constants was minor and an apparent activation energy of 126 kJ/mol was obtained. During the later stages of the reaction, when the sulfur activity in the inner sulfide core increased, the oxidation proceeded irregularly to the interior of the remaining sulfide with the formation of a porous oxide and the evolution of gaseous SO2. The remaining sulfide core was found to be a mixture of bornite (Cu₅FeS₄) and djurleite (Cu_1.96S). H. TSUKADA, former Graduate Student at Kyoto University     

Oxidative roasting of covellite with minimal retardation from the CuO⋅CuSO4 film

Copper (II) sulfide can be efficiently converted to the oxide at lower temperatures than normally required in aerobic roasting by a new method involving programed environment roasting (PER). When heating was conducted in absence of oxygen up to about 650°C, and then nitrogen was replaced by air, the sulfide was easily converted to oxide without need for further increase in temperature. Traditional oxidative roasting of chalcocite required a temperature range of 800° to 850°C for conversion to tenorite. Unlike the situation with conventional roasting, CuSO₄ was not detected in the X-ray diffractogram of the product obtained with the PER method above 625°C. Also, the amount of CuO ⋅ CuSO₄ significantly decreased as the halt temperature in the PER process increased from 600° to 700°C. Apparently the shell of copper oxysulfate is impervious to oxygen and/or sulfur dioxide and delays the formation of tenorite until the sulfate and oxysulfate are decomposed. If the oxysulfate stage were bypassed with an inert atmosphere, then, even if small amounts of this salt were formed upon introducing the oxidant, it would decompose at an appreciable rate and the impedance of its thin film to gaseous transport would be considerably diminished. By contrast, the accelerating effect of externally added iron on the oxidative roasting of covellite was confined to the low temperature reactions and hence iron promoted the extent of sulfate formation. Iron did not, however, lower the thermal requirement for complete oxidation of CuS or Cu₂S because it had virtually no effect on the thermal decomposition of CuO ⋅CuSO₄.     

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.     

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.     

含金银高硫微细粒铜锌矿石浮选工艺试验研究

某含金银高硫微细粒铜锌矿石中有用矿物粒度微细,黄铜矿与闪锌矿、方铅矿、毒砂关系密切,且硫高达21.44%。针对该矿石性质特点,试验探索了铜锌优先浮选、铜锌等可浮浮选、铜锌硫等可浮浮选、铜锌混浮—铜锌精矿再磨—铜锌分离、铜锌硫混浮—精矿再磨—铜锌硫分离等5种选别流程。试验结果表明:铜锌硫混浮—精矿再磨—铜锌硫分离流程适宜处理该矿石,其技术指标较好;同时,硫精矿(金银粗精矿)采用湿法工艺进行处理,也取得了良好的技术指标。     

Fundamental studies of the contribution of galvanic interaction to acid-bacterial leaching of mixed metal sulfides

A systematic study has been made towards understanding the role of galvanic interactions in the leaching of sulfide minerals both in the absence and presence of bacteria. When two sulfide minerals are in contact with each other in an acid-aqueous solution, the mineral lower in the electromotive series dissolves rapidly while the one higher in the series is galvanically protected. To ascertain the magnitude of galvanic interaction when chalcopyrite (CuFeS₂) and pyrite (FeS₂) are in contact, potentiodynamic polarization measurements were carried out in 1 M H₂SO₄ and in Bryner and Anderson medium. The individual rest potentials of CuFeS₂ and FeS₂ were found to be 0.52 V and 0.63 V [vs. Standard Hydrogen Electrode (SHE)] respectively. The mixed potential of the CuFeS₂: FeS₂ couple was found to be 0.56 V and 0.28 V (vs. SHE) in 1 M H₂SO₄ and in Bryner and Anderson medium respectively. The corrosion current (5 μA/cm²) was calculated from the polarization curves. These results were found to be in agreement with the actual leaching rates of copper from contacting large mineral specimens composing the CuFeS₂/FeS₂ system.Both powdered samples as well as large mineral specimens of CuFeS₂, FeS₂ and ZnS were used to observe the effect of galvanic interactions on the process of leaching. Chalcopyrite and pyrite powders, as pulp mixtures, were leached under a variety of experimental conditions to optimize their ratio (total pulp density) and size fraction for efficient metal extraction by way of maximum contact with each other. A CuFeS₂:FeS₂ ratio of 5:5 (g/g) and size fraction of ?200 mesh was found to be the most preferable when leaching was carried out in the presence of T. ferrooxidans. The effect of temperature and thermophilic bacteria on the rate of leaching was also studied. The presence of pyrite in the optimized quantities considerably enhanced the rate of copper dissolution which was increased, by a factor of 2, to 15. Under these experimental conditions, E_h increased from 338 mV to 580 mV while pH dropped from 2.30 to 1.56.In order to have a definite control over surface area of the ores and a constant contact between two sulfide ores, large crystalline-mineral samples of CuFeS₂, FeS₂ and ZnS were leached singly as well as in intimate contact (coupled galvanically). This also made it possible to observe the surfaces of the specimens as the leaching proceeded in the presence of galvanic interaction. Coupled CuFeS₂/FeS₂, ZnS/FeS₂ and CuFeS₂/FeS₂/ZnS systems showed improved metal dissolution as compared to leaching of single large specimens.Scanning electron microscope observations showed that there were no major changes on pyrite surfaces when in galvanic contact with chalcopyrite and/or sphalerite. These observations have been supported by chemical data and by energy-dispersive X-ray analysis.     

Synthesis,evaluation of kinetic characteristics and investigation of apoptosis of Cu2+-modified ceria nano discs

Ceria nano discs were synthesized by the stepwise thermal decomposition strategy of the oxalate precursor. A series of Ce_1–xCu_xO₂ (x = 0, 0.02, 0.1, 0.2 and 0.3) nano sized oxide systems were prepared through thermal decomposition route. Kinetic characterization of formation of solid solution was made by isoconversional strategy under non-isothermal condition. Introduction of various reactant molar ratios of Cu²⁺:Ce⁴⁺ has a pivotal role in the creation of new oxygen vacancies, decomposition strategy, particle size and shape. Cu²⁺ doping (x = 0.02 and 0.1) damages the disc shaped morphology of ceria. Homogeneous distribution of Cu²⁺ on the oxalate precursor has a significant role in the catalyzing activity for the destruction of oxalate bond to oxide. 2 mol% doped Cu²⁺ promotes breaking of oxalate bonds in nitrogen atmosphere. In vitro cell viability assay illustrates enhanced toxicity to cancer cells with 10 mol% Cu²⁺ doped ceria.     

Influence of Cu Layer on Structural and Optical Properties of Copper Oxides Prepared as Stacks

Copper-layered copper oxide (CO) stacks were prepared. The X-ray diffraction (XRD) results showed polycrystalline nature and combination of mixed Cu₄O₃, CuO, Cu₂O, and Cu₈O phases with different orientations and also exhibited the growth of Cu₄O₃ phases when Cu layer coated over and sandwiched between copper oxides. The structural parameters pointed the influence of Cu layer on crystallite size, residual stress, microstrain, and dislocation density of the copper oxides with various stack configurations. In Cu₄O₃ phase, the crystallite size was high for Cu as base layer and low for Cu as interface between copper oxides. In addition, for the Cu₄O₃ phase, CO stacks showed low tensile stress than Cu layered CO stacks, and it was low for CuO phases in Cu layered CO stacks. Optical bandgaps were between 1.53 and 2.29 eV. CO/Cu stack prepared at room temperature and Cu/CO/Cu stack prepared at 573 K (300 °C) showed low and high bandgap values, respectively. Overall, the Cu layer influenced the structural parameters as well as the optical properties of CO stacks.     

Hydrothermal purification and enrichment of Chilean copper concentrates. Part 2: The behavior of the bulk concentrates

The hydrothermal treatment of Chilean Codelco-type copper concentrates with copper sulfate solutions was investigated as a mean of removal of impurities and subsequent increase of the copper assay. The behavior of the mineral phases (digenite, chalcopyrite, covellite, bornite, pyrite and sphalerite) was similar to those obtained in previous works from pure mineral samples. An almost complete transformation of bornite, chalcopirite, covellite and sphalerite into Cu_2 ? xS phases was obtained at 225 °C–240 °C. The highest degree of elimination (around 80%) of impurities was in Zn, Cd, Tl and Bi. An intermediate elimination (40–70%) was achieved for Pb and Te, with only moderate elimination (20–40%) of Mo, Hg, Sb and As. Temperature was the variable having the greatest influence on the elimination of the impurities. A concentrate containing 33% Cu, 33% S, 22% Fe and 2% Zn was converted to a highly enriched concentrate containing 70% Cu, 19% S and 3% Fe. The advantages of a concentrate of this type would include: (1) raising by more than twice the smelting capacity due to the high copper content, (2) generation of a minimum amount of slag, (3) reduction by almost 50% in sulfur emissions, (4) substantial reduction of wastes containing hazardous metals and, finally (5), retention of the option to hydrometallurgical copper recovery since the neo-formed Cu_2 ? xS phases are more reactive than chalcopyrite to the chemical or biochemical leaching.     

Interaction of copper sulfides with copper oxides in the molten state

Reactions of Cu₂S with Cu₂O, CuS with Cu₂O and CuS with CuO in the molten state were examined in the presence of one atmosphere of argon at 1200°C. A rate law of the form,r _SO2 =kN_SN_O was applicable for each reaction system studied. Comparison of the rate constants for the systems, under conditions of similar initial mole fraction of sulfur to mole fraction of oxygen ratios, showed that Cu₂O was much more reactive than CuO in its reaction with copper sulfides. These results are incorporated in a mechanism in which Cu₂O reacts with the sulfide in the rate determining step. Experiments carried out in the presence of oxygen indicated the importance of a CuO-Cu₂O equilibrium in the overall reaction mechanism.     

Direct observations of selective attachment of bacteria on low-grade sulfide ores and other mineral surfaces

The selective attachment of a thermophilic, pleomorphic autotroph which is similar to Sulfolobus acidocaldarius and tentatively designated Caldariella to the pyritic (FeS₂) or chalcopyritic (CuFeS₂) phases of a low grade copper orc was observed using a scanning electron microscope. The selective attachment was further confirmed by the identification of ore mineral phases by elemental X-ray mapping in conjunction with an energy-dispersive analyzer. These observations provide strong support for the direct contact mechanism of bacterial oxidation of minerals, particularly sulfide minerals during leaching. Preferential attachment of microorganisms to specific areas on turquoise was also observed. The bacteria were observed to attach adsorptively to the phase surface and the attachment seems to be quite tenacious. Some possible indication of the production of biomatter surrounding the attachment zone of microorganisms on turquoise was also observed. The thermophilic bacteria in some instances were observed to form a continuous layer of the microbes on pyrite phases.     

Dynamic Simulation of the Thermal Decomposition of Pyrite Under Vacuum

The ultrasoft pseudopotential plane wave method is applied to dynamic simulation of the thermal decomposition mechanism of FeS₂ under vacuum. The FeS₂ (100), (111), and (210) surface relaxation and the geometric and electronic structure of the reactants and products are calculated. The results indicate that FeS₂ (100) is the most preferred surface to dissociate and also the most common cleavage surface. The thermal decomposition mechanism of FeS₂ is explained by dynamic simulation on a micro stratum: in general, the S-Fe bond gradually elongated until it fractured, the S-S bond strengthened gradually, the S-Fe bond was cleaved to form S, the force is relatively weaker between different layers, and thermal decomposition occurred easily between the layers. Simultaneously, the intermediate products, such as Fe _x S _y , were formed. Evidence of Fe pyrolysis into Fe metal was not found, and the intermediate products decomposed further. The contributions of the p and d orbitals of Fe increased, whereas that of the s orbital decreased. The contributions of the s and p orbitals of S decreased. The results obtained from FeS₂ thermal decomposition experiments under vacuum and differential thermal analysis—thermogravimetry are consistent with the results of calculation and simulation.     

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.     

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.     

Mineralogical characterization of silver flotation concentrates made from zinc neutral leach residues

Silver flotation concentrates prepared from high-silver (1480 ppm Ag) and low-silver (300 ppm Ag) neutral leach residues have been examined mineralogically to determine the phases present and to elucidate the behavior of silver during zinc processing. The flotation concentrates consist principally of sphalerite although lesser amounts of zinc ferrite and PbSO₄, as well as traces of other phases, also are present. In the high-silver flotation concentrate, silver occurs mostly as Ag₂S or (Ag, Cu)₂S rims on sphalerite although (Ag, Cu)₂S inclusions within sphalerite also are present. Trace amounts of a Cu-Ag-S-Cl phase are present on rare copper oxide grains, and this silver-bearing phase may be a fine mixture of Ag₂S, AgCl, and Cu₂S. In the low-silver flotation concentrate, silver occurs mostly as Ag₂S although traces of silver-bearing CuS and Cu₂S also are present. The Ag₂S occurs as <1 μm particles disseminated in elemental sulfur-silica gel patches, as discontinuous rims or isolated patches on sphalerite grains, and as tiny free particles. Silver chloride was not detected. These studies suggest that silver dissolves during neutral leaching and subsequently reacts with sphalerite or other sulfides to form silver sulfide.     

Mineralogical characterization of silver flotation concentrates made from zinc neutral leach residues

Silver flotation concentrates prepared from high-silver (1480 ppm Ag) and low-silver (300 ppm Ag) neutral leach residues have been examined mineralogically to determine the phases present and to elucidate the behavior of silver during zinc processing. The flotation concentrates consist principally of sphalerite although lesser amounts of zinc ferrite and PbSO₄, as well as traces of other phases, also are present. In the high-silver flotation concentrate, silver occurs mostly as Ag₂S or (Ag, Cu)₂S rims on sphalerite although (Ag, Cu)₂S inclusions within sphalerite also are present. Trace amounts of a Cu-Ag-S-Cl phase are present on rare copper oxide grains, and this silver-bearing phase may be a fine mixture of Ag₂S, AgCl, and Cu₂S. In the low-silver flotation concentrate, silver occurs mostly as Ag₂S although traces of silver-bearing CuS and Cu₂S also are present. The Ag₂S occurs as <1 μm particles disseminated in elemental sulfur-silica gel patches, as discontinuous rims or isolated patches on sphalerite grains, and as tiny free particles. Silver chloride was not detected. These studies suggest that silver dissolves during neutral leaching and subsequently reacts with sphalerite or other sulfides to form silver sulfide.