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.     

The kinetics of dissolution of bornite in acidified ferric sulfate solutions

Sintered disks of synthetic bornite were dissolved in acidified ferric sulfate solutions at temperatures ranging from 5δ to 94δC. The dissolution occurs in two stages; in the first a nonstoichiometric bornite with up to 25 pct deficiency of copper is formed. In the second, the nonstoichiometric bornite is converted to chalcopyrite and elemental sulfur, which accumulates on the disk surface. At temperatures below 35°C, the reaction follows parabolic kinetics and stops at the nonstoichiometric bornite stage. At higher temperatures it continues through to chalcopyrite and follows linear kinetics. Both the parabolic and linear processes have activation energies of 5 to 6 kcal per mole. At higher temperatures the sensitivity of the reaction rate to changes in stirring velocity indicates control by diffusion through the liquid boundary layer. Natural and synthetic bornite dissolve by the same process and at essentially the same rate.     

A partial equilibrium model to characterize the precipitation of ferric ion during the leaching of chalcopyrite with ferric sulfate

A partial equilibrium model has been developed and used to characterize the conditions under which precipitation of ferric ion occurs during the dump leaching of chalcopyrite ores. The precipitates which have been considered include amorphous Fe(OH)₃, α-FeOOH (goethite), and Na⁺, K⁺, Ag⁺, Pb²⁺, and H₃O⁺ jarosites. Solution of the model equations makes possible the determination of the concentrations of the solution species during leaching of the mineral. The concentration product for Fe(OH)₃ (am) and α-FeOOH was calculated for changing solution concentrations and compared with the solubility product constants to determine when precipitation would be expected thermodynamically. The K⁺, Na⁺, Ag⁺, and Pb²⁺ concentrations that would be necessary to satisfy the solubility product constants for the corresponding jarosites were calculated for various initial concentrations and varying amounts of O₂ consumption. Formerly Graduate Assistant, Ames Laboratory USDOE and Department of Chemical Engineering, Iowa State University, Ames, IA 50011     

Model for the ferric chloride leaching of galena

A shrinking core model for the FeCl₃ leaching of galena (PbS), which accounts for the microstructure previously observed during this process, is presented. The microstructure is characterized by two distinct product layers—PbCl₂ in direct contact with the PbS core and S^o above the PbCl₂ layer. Rate equations for the evolution of the three solid phases and the PbS leaching conversion are derived for the case of flat plate geometry appropriate for the reaction of massive galena fragments. In the case of rate control by diffusion of the lead chloride reaction products through the S^o layer, the system is shown to exhibit parabolic behavior as long as the S^o layer is built up primarily at the PbCl₂-S^o interface. For the mechanism considered in this study, negative deviation from parabolic behavior is observed to increase as the amount of S^o forming at the external S^o-solution interface rises. When the system is controlled by surface reaction kinetics, the model predicts linear rates under all circumstances.     

The leaching of chalcopyrite with ferric chloride

A comparative study of electrochemical leaching and chemical leaching of chalcopyrite was done to elucidate the leaching mechanism of chalcopyrite with FeCl₃. The leaching rate of chalcopyrite exhibits a half order dependency on the FeCl₃ concentration, whereas it is independent of the FeCl₂ concentration. The mixed potential of chalcopyrite exhibits a 72 mV · decade⁻¹ dependency upon FeCl₃ concentration; no influence on the mixed potential was observed by the addition of FeCl₂. In FeCl₃ solutions acidified with HC1, the predominant chemical species of Fe(III) was found to be FeCl ₂ ^u+ from equilibrium calculations. The concentration of this species is approximately proportional to the amount of FeCl₃ added to the solutions. Based on these observations, an electrochemical mechanism is proposed which involves the oxidation of chalcopyrite and the reduction of FeCl ₂ ⁺ , the predominant species of Fe(III). By converting the leaching rate to electric current density,i, 140 mV · decade⁻¹ dependency of mixed potential,E, against logi is obtained. This dependency of the chemical leaching of chalcopyrite with FeCl₃ as well as its activation energy agree with those for electrochemical leaching. These findings strongly support the electrochemical mechanism of FeCl₃ leaching of chalcopyrite. Formerly Graduate Student, Kyoto University     

The leaching of chalcopyrite with ferric sulfate

The leaching kinetics of natural chalcopyrite crystals with ferric sulfate was studied. The morphology of the leached chalcopyrite and the electrochemical properties of chalcopyrite electrodes also were investigated. The leaching of chalcopyrite showed parabolic-like kinetics initially and then showed linear kinetics. In the initial stage, a dense sulfur layer formed on the chalcopyrite surface. The growth of the layer caused it to peel from the surface, leaving a rough surface. In the linear stage, no thick sulfur layer was observed. In this investigation, chalcopyrite leaching in the linear stage was principally studied. The apparent activation energy for chalcopyrite leaching was found to range from 76.8 to 87.7 kJ mol⁻¹, and this suggests that the leaching of chalcopyrite is chemically controlled. The leaching rate of chalcopyrite increases with an increase in Fe(SO₄)_1.5 concentration up to 0.1 mol dm⁻³, but a further increase of the Fe(SO₄)_1.5 concentration has little effect on the leaching rate. The dependency of the mixed potential upon Fe(SO₄)_1.5 concentration was found to be 79 mV decade⁻¹ from 0.01 mol dm⁻³ to 1 mol dm⁻³ Fe(SO₄)_1.5. Both the leaching rate and the mixed potential decreased with an increased FeSO₄ concentration. The anodic current of Fe(II) oxidation on the chalcopyrite surface in a sulfate medium was larger than that in a chloride medium.     

Reaction mechanism for the ferric chloride leaching of sphalerite

Reaction mechanisms for the ferric chloride leaching of sphalerite are proposed based on data obtained in leaching and dual cell experiments presented in this work and in a previous study. The results from the leaching experiments show that at low concentrations the rate is proportional to [Fe³⁺]_T ^0.5 and [Cl^-]_T ^0.43 but at higher concentrations the reaction order with respect to both [Fe³⁺]_T and [Cl^-]_T decreases. Using dual cell experiments which allow the half cell reactions to be separated, increased rates are observed when NaCl is added to the anolyte and to the catholyte. The increase in rate is attributed to a direct, anodic electrochemical reaction of Cl^- with the mineral. When NaCl is added only to the catholyte, a decrease in the rate is observed due to a decrease in theE ⁰ of the cathode which is attributed to the formation of ferric-chloro complexes. Several possible electrochemical mechanisms and mathematical models based on the Butler-Volmer relation are delineated, and of these, one model is selected which accounts for the experimentally observed changes in reaction order for both Fe³⁺ and Cl^-. This analysis incorporates a charge transfer process for each ion and an adsorption step for ferric and chloride ions. The inhibiting effect of Fe²⁺ noted by previous investigators is also accounted for through a similar model which includes back reaction kinetics for Fe²⁺. The proposed models successfully provide a theoretical basis for describing the role of Cl^-, Fe³⁺, and Fe²⁺ as well as their interrelationship in zinc sulfide leaching reactions. Possible applications of these results to chloride leaching systems involving other sulfides or complex sulfides are considered.     

Microstructural changes occurring during the gaseous reduction of magnetite

The reduction of dense magnetite samples in H₂/H₂O and H₂/N₂ gas mixtures between temperatures of 723 and 1373 K has been investigated. A detailed study of partially reduced samples using conventional metallographic and scanning electron microscopy (SEM) techniques has enabled the conditions for the formation of a number of different product microstructures to be clearly defined. The reaction mechanisms involved in various continuous and discontinuous growth processes are discussed. Formerly with the University of Queensland.     

The leaching of galena in ferric sulfate media

The leaching of galena (PbS) in ferric sulfate media was investigated over the temperature range 55 °C to 95 °C and for various Fe(SO₄)_1.5, H₂SO₄, FeSO₄, and MgSO₄ concentrations. Relatively slow kinetics were consistently observed; in most instances, the 1-2/3α-(1−α)^2/3 vs time relationship, indicative of a diffusion-controlled reaction, was closely obeyed. The diffusion-controlled kinetics were attributed to the formation of a tenacious layer of PbSO₄ and S⁰ on the surface of the galena. The generation and morphology of the reaction products were systematically determined by scanning electron microscopy, and complex growth mechanisms were illustrated. The leaching rate increased rapidly with increasing temperature, and the apparent activation energy is 61.2 kJ/mol. The rate increases as the 0.5 power of the ferric ion concentration but is nearly independent of the concentration of the FeSO₄ reaction product. The rate is insensitive to H₂SO₄ concentrations <0.1 M but increases at higher acid levels. The presence of neutral sulfates, such as MgSO₄, decreases the leaching rate to a modest extent.     

The leaching of nickeliferous laterite with ferric chloride

Several experiments were conducted to investigate the extraction of nickel from nickeliferous laterite by ferric chloride solutions as a function of pulp density, solution composition, and temperature. Solubility relationships for goethite and nickel laterite in aqueous solution were reviewed in terms of leaching rates and reaction mechanisms. Generally, the amount of nickel extracted increased with temperature, the amount of “free acid,” and ferric chloride concentration; however, the amount was inhibited by ferrous chloride. In this investigation, as much as 96 pct of the available nickel was extracted by ferric chloride solution. Nickel extraction was found to be more dependent on ferric chloride concentration than on the concentration of hydrochloric acid. Mechanistically, nickel extraction occurred by the formation of an intermediate ferric chloride complex, which was then hydrolyzed to hematite.     

The kinetics of leaching galena with ferric nitrate

The kinetics of leaching galena with ferric nitrate as oxidant has been studied. Experimental results indicate that the rate of galena dissolution is controlled by surface chemical reaction. Rate is proportional to the square root of the concentration of ferric ion. The addition of more than one mole/liter sodium nitrate decreases reaction rate. With nitrate additions below this concentration, rate either remains constant or is slightly enhanced. An activation energy of 47 kJ/mol was measured, and rate is proportional to the inverse of the initial size of galena particles. These results are explained in terms of mixed electrochemical control. The anodic reaction involves the oxidation of galena to lead ion and elemental sulfur, and the cathodic reaction involves the reduction of ferric ion to ferrous ion.     

Reaction mechanism for the acid ferric sulfate leaching of chalcopyrite

The acid ferric sulfate leaching of chalcopyrite, CuFeS₂ + 4Fe⁺³ = Cu⁺² + 5Fe⁺² + 2S⁰ was studied using monosize particles in a well stirred reactor at ambient pressure and dilute solid phase concentration in order to obtain fundamental details of the reaction kinetics. The principal rate limiting step for this electrochemical reaction appears to be a transport process through the elemental sulfur reaction product. This conclusion has been reached in other investigations and is supported by data from this investigation in which the reaction rate was found to have an inverse second order dependence on the initial particle diameter. Furthermore, the reaction kinetics were found to be independent of Fe⁺³, Fe⁺², Cu⁺² and H₂SO₄ in the range of additions studied. The unique aspect of this particular research effort is that data analysis, using the Wagner theory of oxidation, suggests that the rate limiting process may be the transport of electrons through the elemental sulfur layer. Predicted reaction rates calculated from first principles using the physicochemical properties of the system (conductivity of elemental sulfur and the free energy change for the reaction) agree satisfactorily with experimentally determined rates. Further evidence which supports this analysis includes an experimental activation energy of 20 kcal/mol (83.7 kJ/mol) which is approximately the same as the apparent activation energy for the transfer of electrons through elemental sulfur, 23 kcal/ mol (96.3 kJ/mol) calculated from both conductivity and electron mobility measurements reported in the literature. formerly Metallurgy Graduate Student, University of Utah.     

Morphology of chalcopyrite leaching in acidic ferric sulfate media

Two-dimensional computer simulations based on percolation theory were used to explain the morphology associated with atmospheric chalcopyrite leaching in acidic ferric sulfate solution. The aim of this study was to understand the differences in observed morphology between chalcopyrite residues leached with and without pyrite in the leach environment. The study of chalcopyrite morphology is of interest because there are no records of similar investigations available. Simulations showed high copper extractions from chalcopyrite when surface atoms were mobile leading to agglomeration of like atoms and the formation of highly porous mineral structures. High degrees of surface mobility are associated with active anodic behavior. The simulated morphology was consistent with previously observed residue morphology from chalcopyrite leach experiments in the presence of pyrite. Thus it was found that the enhanced recoveries and peculiar morphology observed during pyrite catalyzed leaching are attributable to active anodic behavior. Conversely, the simulations also showed that the recovery of copper was low when surface atoms were effectively locked in place resulting in mineral passivation. The simulation morphology obtained in this case was also consistent with experimental results of chalcopyrite leached without the presence of pyrite which have shown non-porous film like product layers.     

The leaching of sintered CuS disks with ferric chlorides

A comparative study of chemical and electrochemical leaching of a sintered disk of CuS was carried out at 343 to elucidate the leaching mechanism of CuS with FeCl₃. The leaching,rate of CuS exhibited a half order dependence on the FeCl₃ concentration, but the addition of FeCl₂ slightly reduced the leaching rate. The mixed potential of CuS exhibited a 53 mV decade^-1 dependence upon the FeCl₃ concentration; no significant effect on the mixed potential was observed by the addition of FeCl₂. Based on these observations, an electrochemical mechanism is proposed which involves the oxidation of CuS and the reduction of FeCl₂ ⁺ . The activation energy in chemical leaching of CuS was 46.7 kJ mol^-1, while in electrochemical leaching, it was 50.4 kJ mol^-1. By converting the leaching rate to electric current density,i, a 110 mV decade^-1 dependence of mixed potential, E, against log i is obtained. This dependence of the mixed potential during chemical leaching of CuS on FeCl₃ concentration agrees with the value theoretically expected from the electrochemical model of the leaching process. These findings strongly support an electrochemical mechanism for the FeCl₃ leaching of CuS. HIROSHI HIAI, formerly Graduate Student with Kyoto University     

Structural changes occurring during atresia in sheep ovarian follicles

The structural changes that characterize primary, secondary and tertiary atresia in sheep Graafian follicles have been studied by means of histological, histochemical and ultrastructural techniques. In primary atresia vacuoles representing swollen endoplasmic reticulum are prominent along the antral border together with disorganized granulosa cells containing pyknotic nuclei. Phagocytic cells, which increase in number as atresia progresses, were seen within the membrana granulosa and are considered to be transformed granulosa cells. Even in follicles classified as nonatretic, a few antral vacuoles and occasional pyknotic nuclei are present. During secondary atresia there is a large increase in the number of cells with pyknotic nuclei; many of these nuclei had been extruded and had fused to form the characteristic Feulgen-positive atretic bodies found along the edge of the antral cavity. These bodies usually have a diameter of up to 15 mum but occasionally reached as much as 400 mum. A second area of degeneration is frequently present in the membrana granulosa, two or three cell layers from the basal lamina, and it is at this level that exfoliation of granulosa cells occurs in tertiary atresia. In contrast to the membrana granulosa, there are during secondary atresia, only slight indications of degeneration in the cumulus. In tertiary atresia the membrana granulosa is highly disorganized; the atretic bodies are often fewer in number than at earlier stages. The basal lamina remains essentially intact. It is at this stage that the first clear signs of degeneration occur in the theca interna. Despite some disintegration of the cumulus, the integrity of the oocyte is maintained and its nucleus remains vesicular. Changes in the thecal microcirculation may plan a key role in atresia: adjacent to the basal lamina of non-atretic follicles, there is a well-developed capillary network which is significantly reduced as atresia progresses.     

The solubility of silver chloride in ferric chloride leaching media

The solubility of silver chloride in various FeCl₃-FeCl₂-HCl solutions has been measured over the temperature range 20–100°C; the densities of the associated saturated solutions have also been determined. The solubility increases systematically with either rising temperature or increasing concentrations of the constituent chlorides. The solubility is higher in a ferrous chloride medium than in an equivalent ferric chloride solution. The presence of CuCl₂ systematically raises the AgCl solubility, but increasing ZnCl₂ concentrations cause the solubility to decrease slightly to about 1.5 M ZnCl₂ and subsequently to increase gradually. The addition of NaCl to the iron chloride media substantially increases the AgCl solubility under all conditions. Although the solubility of AgCI is only a few mg/L in cold water, this increases to over 1 g/L in hot, moderately concentrated chloride media. Silver chloride solubilities at elevated temperatures are sufficiently high that silver solubility limitations should not be a problem in most commercial ferric chloride leaching processes.     

Reaction kinetics of the ferric chloride leaching of sphalerite—an experimental study

Chloride leaching processes have significant potential for treating complex sulfides. One advantage of chloride leaching is fast dissolution rates for most sulfide minerals. This experimental study is concerned with ferric chloride leaching of sphalerite, a common component of many complex concentrates. The effects of stirring, temperature, ferric ion concentration, and particle size have been examined. In addition, reaction residues at various levels of zinc extraction were examined by SEM, and the products of reaction were identified by energy dispersive X-ray analysis and X-ray diffraction. These observations indicated that the dissolution reaction is topochemical. Moreover, the leaching results fit a surface reaction control model. The activation energy was calculated to be 58.4 kJ/mole which is reasonable for a rate limiting surface reaction. The order of the reaction was 0.5 with respect to Fe³⁺ at low concentrations and zero at high concentrations. The change in reaction order occurred at similar Fe³⁺ concentrations for various particle sizes. This is believed to be indicative of an electrochemical reaction mechanism at low Fe³⁺ and an adsorption mechanism at higher Fe³⁺. A kinetic model for the ferric chloride leaching of sphalerite was also obtained for the lower Fe³⁺ concentrations and is given by: (ie5-01) This model is in excellent agreement with the experimental results for fractions of zinc extracted up to 0.95.     

The effect of chloride ion on the ferric chloride leaching of galena concentrate

Previous investigations of the ferric chloride brine leaching of galena concentrate have shown that additions of chloride ion result in accelerated dissolution rates. The current study has provided the necessary information to extend and modify these previous results by incorporating the important effect of chloride ion on the dissolution kinetics. As part of this study the solubility of lead chloride in ferric chloride-brine solutions has been determined and results indicate that additions of either FeCl₃ or NaCl increase the PbCl₂ solubility. This is attributed to the effect of complexing on the level of free chloride ion. In addition, the dissolution kinetics of elemental lead and lead chloride were also determined and compared with the kinetics of PbS dissolution. It is significant that the rate of dissolution of PbCl₂ decreases as the concentration of Cl⁻ is decreased and as the concentration of dissolved lead increases. These results along with SEM examination of partially reacted Pb shot show that solid PbCl₂ forms on the surface long before the bulk solution is saturated with lead. The PbCl₂ is proposed to form by a direct electrochemical reaction between Cl⁻ and PbS prior to the formation of dissolved lead. The reaction was determined to be first order with respect to Cl⁻ and closely obeys the following kinetic model based on a rate limiting charge transfer reaction at the surface: The model is in excellent agreement with experimental results up to about 95 pct reaction as long as the solubility of PbCl₂ is greater than about 0.051 M. Where these conditions are not met, deviation from the surface reaction model occurs due to the extremely slow dissolution rate of PbCl₂. Therefore the effect of Cl⁻ on the brine leaching of PbS is attributed to two factors, the direct reaction of Cl⁻ with the pbS surface and the effect of Cl⁻ on the dissolution rate of PbCl₂. The overall dissolution process is viewed as occurring in three stages; in the first stage the reaction is controlled by the surface reaction and described by the model above, then as solid PbCl₂ is produced the diffusion of Cl⁻ would be equal in importance with the surface reaction,i.e, the second stage. As the reaction proceeds further, a shift in the rate-limiting step from surface reaction to product layer or pore diffusion occurs, the third stage. Thus the rate-determining step would no longer be just the surface reaction as observed experimentally at longer reaction times. The practical implications of these results for the processing of a complex sulfide concentrate using sequential, selective, or total leach approaches are also discussed.