Open circuit potential and leaching rate of pyrite in cupric chloride solution

As a refractory gold mineral, pyrite needs to be oxidised prior to gold leaching. In this study, the effect of [Cl^?] concentration (40.6–149.8?g/L), [Cu²⁺] concentration (0.8–31.6?g/L), pH (1.5–2.5) and temperature (25–90 –C) on the pyrite leaching rate was investigated. In addition, the open circuit potential (OCP) values of pyrite in cupric chloride solution were investigated. A linear regression model was constructed to predict pyrite dissolution rate i.e. corrosion current density. It was shown that the temperature had a significant positive effect on pyrite dissolution, while increased cupric ion concentration was also shown to provide some dissolution enhancement. According to the regression analysis, pH had no effect on the corrosion current density at OCP. Dissolution rates of pyrite varied between 0.05 and 2.9?µm/h. The activation energy values varied from 20 to 90?kJ/mol, indicated that the pyrite dissolution reaction rate was controlled by the chemical reaction or mixed mechanism rather than diffusion alone. The simultaneous increase in corrosion potential and corrosion current density indicated that the anodic pyrite dissolution reaction was rate determining at OCP.     

Leaching Kinetics of Bauxite in Hydrochloric Acid

The Bayer process is currently used to produce cell-grade alumina from bauxite. However, if the reactive silica content in the bauxite exceeds 7%, losses of caustic and aluminium as sodalite (2 Na₂0.2 Al₂O₃·3 SiO₂·2 H₂O) become economically unacceptable. As Australia possesses large tonnages of bauxite containing more than 7% reactive silica, a process for the production of cell-grade alumina horn these bauxites using hydrochloric acid leaching was developed by the authors.

The process consists of the following steps: calcination of bauxite, leaching of calcined bauxite in hydrochloric acid, filtration of residue, crystallization of A1C1₃.6 H₂O, decomposition of AlCl₃.6 H₂O crystals to produce A1₂O₃, regeneration of hydrochloric acid for recycle into the leaching step.

The kinetic studies of the dissolution showed that the extraction of aluminium is independent of the solid to leachant ratios studied. The acid concentration has a marked effect on leaching kinetics of bauxite and the leaching time decreases substantially as the concentration of acid increases. The order of the reaction is greater than one. The rate equation can be described by the following least squares line of best fit.

r = 4.1 × 10^?5 HCl^0.5 + 1.27 × 10^?5 HCl² where

r = the initial reaction rate, g Al extracted/sec.

Aluminium extraction greater than 90% can be achieved from bauxite in 4 h using 25% HCl while 30% acid is considered the maximum acid strength due to the crystallization of A1C1₃.6 H₂O from leach solutions.

The rate of leaching increases as the temperature of leaching is increased. An Arrhenius plot of the initial reaction rate (g Al extracted/sec) against the inverse absolute leaching temperature produced a straight line. The activation energy for the reaction was determined to be 83.3 kJ/mole with a standard deviation of 2.9 kJ/mole. The correlation between the initial reaction rate and the inverse absolute temperature can be described by the following regression equation:

r = 4.71×l0⁸e^83820/RT

The large activation energy value suggests that the reaction is chemically controlled rather than diffusion controlled. The additives such as NaCl and FeCI₃ do not effect the leaching rate.     

Dissolution of MoS2 concentrate using NaClO from 283 to 373 K

This study analyses the leaching process of molybdenite (MoS₂) concentrate using sodium hypochlorite (NaClO) at temperatures ranging from 283 to 373?K. The following variables were studied: leaching time, stirring velocity, temperature, NaClO concentration, NaOH concentration, particle size and liquid:solid ratio. The optimum parameters for molybdenite dissolution were: Time?=?0.81?h, stirring speed?=?800?rev?min^??1, temperature?=?303?K, NaClO concentration?=?1.49M, NaOH concentration?=?8.01M, particle size?=?45?μm and liquid:solid ratio?=?300?:?1.?Under these conditions, molybdenite dissolution reached around 99%, while Cu and Fe recovery were insignificant. Analysis using XRD, QUESCAM and SEM showed release indices and subsequent reaction of the MoS₂ and the constituent parts of Cu and Fe.     

The kinetics of dissolution of cubanite in aqueous acidic ferric sulfate solutions

When sintered disks of synthetic cubanite (CuFe₂S₃) were leached in acidified ferric sulfate solutions, the following reaction stoichiometry was observed: CuFe₂S₃+3Fe₂(SO₄)₃?CuSO₄+8FeSO₄+3S Over the temperature range 45° to 90°C, the reaction displayed linear kinetics that were interpreted as indicating rate control by some reaction occurring at the surface of the cubanite. The apparent activation energy for the dissolution process is 11.6±0.7 kcal per mole. The dissolution rate increases steadily with increasing ferric concentration, but decreases with increasing strengths of either H₂SO₄ or FeSO₄. The addition of either NaCl or HCl to the leaching solutions substantially catalyzes the rate of cubanite dissolution. Natural cubanite appears to dissolve like the synthetic material.     

Cupric chloride leaching of a complex copper/zinc/lead ore

A complex Cu/Zn/Pb ore from Cayeli, Turkey, was reacted with cupric chloride solutions under different conditions. Energies of activation were calculated for dissolution of copper (37 kJ mol^?1), iron (33 kJ mol^?1, zinc (26 kJ mol^?1) and lead (7.5 kJ mol^?1, values which indicate diffusion control of the reaction, probably through the sulphur layer formed round each particle. Particle size/leaching relationships corroborated microscopic assessments and indicated that chalcopyrite dissolved at a very low rate. Calculation of Fe:Cu ratios of metal leached showed considerable dissolution of pyrite from finely-ground ($\text{d}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 3?5 μ}\text{m}$) ore. Examination of residues using SEM X-ray fluorescence line scan techniques revealed little attack of large pyrite crystals, suggesting that fine pyrite particles in complex relationship with the sphalerite and chalcopyrite were dissolving.     

Leaching behaviour of low-grade copper ore in the presence of organic acid

This study investigated the kinetics of copper leaching from malachite ore using formic acid as an organic leaching reagent. The analysis was carried out to determine the effect of the following parameters: particle size, acid concentration, leaching time, formic acid/malachite ratio, reaction temperature and stirring speed. Based on the experimental results, the optimum leaching parameters are as follows: particle size, +75?30?µm; formic acid concentration, 0.4?mol?L^?1; leaching time, 90?min; formic acid (volume)/malachite (weight) ratio, 10?mL?g^?1; temperature, 25°C; and stirring speed, 200?rev min^?1. Copper extraction was 70.25% under optimum experimental conditions. Furthermore, a kinetic model was used to indicate the effects of these parameters on the leaching of copper from malachite ore in formic acid solution. It was observed that the leaching process was controlled by film diffusion through a product layer. In conclusion, formic acid can be utilised as an organic leaching reagent in the leaching process of other ores that are similar in structure and composition to malachite.     

Kinetics of Ammonia Leaching of Multimetal Sulphides

This paper briefly describes the studies carried out on oxidative ammonia leaching of Cu-Zn-Pb multimetal sulphides. Kinetics of zinc and copper dissolution were studied with ? 200 + 300 mesh BSS fraction and 1% solids in the slurry. It is observed that the dissolution of sphalerite proceeds by a phase boundary reaction model and that of copper via diffusion through product layer in the temperature range of 70-100°C. The rate equations for zinc and copper dissolution are given by:

1 ? (1 ? α)^1/3 = k _Zn[NH₃][pO₂]^1/2

1 ? 2/3α ? (1 2/3α )^2/3 = k^Cu[NH₃]²[pO₂]^1/2

where the symbols have the usual meanings.

Activation energies for zinc and copper dissolution reactions are estimated to be 66.5 and 55.4 kJ/mole, respectively. Activation energy values thus obtained are also comparable to those obtained using a differential approach.

The leaching results obtained with 10% solids using a wide range of particle size (? 140 + 500 mesh) indicate that copper dissolution is chemically controlled in ammonia as well as ammonia-ammonium sulphate medium in the temperature range of 115-135°C. However, at lower temperature (?55°C). the leaching reaction follows a diffusion model. Zinc dissolution data show deviations from the shrinking core model due to high extractions in the initial stages.     

The Dissolution of Sphalerite in Ferric Chloride Solutions

The kinetics of dissolution of both sintered sphalerite disks and untreated sphalerite particles in ferric chloride-hydrochloric acid solutions have been investigated. Over the temperature interval 25 to 100°C, the dissolution occurred according to a linear rate law and with an associated apparent activation energy of about 10 kcal/mole. Most of the oxidized sulfide ion reported as elemental sulfur in the leach residues. The leaching rate was independent of the disk rotation speed and this fact, together with various hydrodynamic calculations, indicated that the reaction was chemically controlled. The dissolution rate increased as the 0.36 power of the ferric chloride concentration and it also increased substantially in the presence of dissolved CuCl₂. The accumulation of the ferrous chloride reaction product severely retarded the leaching reaction, but the presence of dissolved zinc chloride only slightly impeded it. The leaching rate was relatively insensitive to low levels of HC1 (>1 M), but increased dramatically at higher acid concentrations because of direct acid attack of the ZnS.     

Cyanidation kinetics of silver telluride (Ag2Te)

The extraction of precious metals from tellurides by cyanidation is more difficult than when they are in their native form, nevertheless the reason for their refractory nature has not been adequately supported. In this study, the mechanism of the cyanidation kinetics of silver telluride (Ag₂Te) was investigated. For this purpose, cyanidation experiments were carried out to: (1) study the difference between the cyanidation kinetics of elemental silver and silver telluride; (2) study the effect of temperature (i.e. 20, 25, 27, 30, 35 and 40°C) on silver telluride dissolution; and (3) elucidate the kinetic mechanism of the silver telluride cyanidation. The results obtained showed that: (1) while 83.5% of elemental silver was dissolved in 8?h, only 13.2% of silver from silver telluride was dissolved in the same time; (2) temperature has an important effect on silver extraction from silver telluride, but a minor effect on tellurium dissolution; and (3) at temperatures between 20 and 27°C, the process was controlled by the chemical reaction with an apparent activation energy of 191.9?kJ?mol^?1, whereas at temperatures between 30 and 40°C, the process was controlled by diffusion through a Ag₅Te₃ layer of products with an apparent activation energy of 25.2?kJ?mol^?1.     

Leaching of silica from vanadium-bearing steel slag in sodium hydroxide solution

The work aims to selectively extract silica from vanadium-bearing steel slag by a leaching process. The effects of the particle size, the ratio of solid to liquid, the concentration of sodium hydroxide solution and the leaching temperature on the leaching behavior of silica from vanadium-bearing steel slag were investigated. The leaching kinetics of silica from vanadium-bearing steel slag in 30-50% w/w NaOH solutions was studied at 240 °C and the shrinking-core model was established to express the leaching kinetics of silica. The data showed that the leaching rate was controlled by the chemical reaction on the system interface and the activation energy for the process was found to be 36.4 kJ mol^− 1. By the leaching process, the majority of silica could be removed effectively from the vanadium-bearing steel slag and a residue with a low SiO₂ content of 4.28% and a high V₂O₅ content of 11.15% was obtained. Under these conditions there was partial dissolution of Al and slight dissolution of Cr, Mn and Ti.     

Decomposition of pyrite in acids by pressure leaching and anodization: the case for an electrochemical mechanism

Abstract

The oxygen pressure leaching of pyrite has been studied with regard to reaction mechanism by oxygen-18 tracer tests, electrochemical simulation and actual leaching experiments over a range of variables; temperature, 85 to 130°C; pressure, 0 to 976 psi O₂ (66.4 atm); and acid concentration, 0.01 to 3M H₂SO₄. The dissolution mechanism has been found to be electrochemical and is a potentiostatically controlled steady state between sulphate-forming and elemental sulphur-forming anodic reactions:

1. FeS₂+8H₂O→Fe⁺²+2SO₄⁼+16H⁺+14e^?

2. FeS₂+Fe⁺²+2SO^o+2e^?

Ferric ions are produced primarily by a slow homogeneous reaction:

4Fe⁺²+O₂+4H⁺→4Fe⁺³+2H₂O

The cathodic reaction initially involves the reduction of oxygen:

1/2O₂+2H⁺+2e^?→H₂O

This reaction is supplemented by cathodic reduction of ferric ions after these have built up to a significant concentration

The electrochemical model has been cited to explain the effect of oxygen pressure on the system and acid production or consumption by pyrite during leaching.

Résumé

On a étudié la lixiviation de la pyrite en autoclave par lapos;oxygène, s'arrêtant en particulier sur le mécanisme de réaction, la simulation électrochimique, la pression et les concentrations dans les limites suivantes: °C 85–130, O₂ O – 66.4 atm., concentration d'acide sulfurique 0.01 à3M. Le mécanisme

de la réaction est électrochimique. Elle est contrôlée par deux réactions anodiques en competition, l'une formant l'anion sulfate, l'autre donnant du soufre:

1. FeS₂ + 8H₂O → Fe⁺² + 2SO₄⁼ + 16H⁺ + 14e^?

2. FeS₂ → Fe⁺² + 2S^o + 2e^?

Les ions ferriques sont produits surtout par une lente réaction homogàne:

4Fe⁺² →O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O

La réaction cathodique initiale comprend la réduction de l'oxygène:

1/2O₂ + 2H ⁺ + 2e^?→ H₂O

A cette réaction s'ajoute la réduction cathodique des ions ferriques et celle-ci prend plus au moins d'importance selon la concentration des ions ferriques.

Le modèle électrochimique permet d'expliquer l'effet de la pression d'oxygene sur la reaction globale ainsi que, la production et la consommation de l'acide par la pyrite pendant la lixiviation.     

Kinetics and Mechanism of Chlorination of Alumina Grains with He–CO–Cl2 Gas Mixtures—II. Intrinsic Rates and Effectiveness Factors

Abstract

The experimental results reported in Part I[1] for the chlorination of porous samples composed of alumina grains are correlated here using a coupled pore-diffusion/surface-reaction model. The reactant gases CO and Cl₂ diffuse inwards through the interstices between the grains and simultaneously react with grain-surfaces producing AlCl₃ and CO₂, Values for the intrinsic rate constant, k_w, were deduced from the measured rates by correcting for the mass-transfer effects.

ln k _w = ?2.1535(±0.3729) ? 9615(±426)T^?1

where k_w is mole Al₂O₃ reacted per g of solid-grain-sample per atm² per s. The effectiveness factors for the chlorination reaction were small, ranging between 0.136 and 0.374, indicating that the pore-diffusion of reactant species is relatively slow as compared to the second-order surface reaction. The activation energy for chlorination was found to be 79.94 (± 3.54) kJ.mol^?1 in the temperature range 800–950°C.     

Kinetics of dissolution of synthetic millerite (β-NiS) in acidic potassium dichromate solutions

The dissolution rate of millerite (β-NiS) in sulphuric acid solutions containing potassium dichromate has been determined by measurement of the rate of nickel ion formation. The effects of temperature, dichromate ion concentration, pH of leaching solution and stirring speed have been investigated. Two distinct regions for dissolution of millerite were observed, below and above 50°C. The activation energy of dissolution is 23.9 and 54.6 kJ mol^?1 in these two regions, respectively. At dichromate concentrations lower than 0.05 M a first order dependence of dissolution rate on Cr₂O₇^2? ion concentration was observed, whereas for higher concentrations a half order dependence was established. The concentration of hydrogen ion affects nickel extraction from β-NiS at pH 1.4, but at lower pH this factor is less important.     

Oxidative leaching of an offgrade/complex copper concentrate in chloride lixiviants

Laboratory studies have been conducted on chloride leaching as a possible route for the simultaneous recovery of copper, zinc, and lead from an off grade and complex chalcopyrite concentrate (from Sikkim, India) associated with appreciable amounts of sphalerite, galena, and pyrite. The effects of temperature, concentration, and quantity of ferric chloride, stirring speed, and leaching time on metal dissolution have been investigated. Leaching tests have also been conducted with in-dividual (HC1, NaCl, CuCl₂, FeCl₃) and mixed chlorides (two-, three-, and four-component mix-tures). Results show the possibility of recovering not only 99 pct Cu and 89 pct Zn but also 82 pct Pb and 58 pct elemental S by treatment of the concentrate with 4 M FeCl₃ at 383 K (110 °C) for 7.2 ks (2 hours) employing 25 pct excess FeCl₃ and a stirring speed of 700 rev min⁻¹. Though 64 pct iron of the concentrate is found to dissolve, the pyrite seems to remain unattacked. Kinetic studies indicate that the chalcopyrite, sphalerite, and galena of the concen-trate dissolve simultaneously in the FeCl₃ lixiviant as if each mineral is separately leached, and the Cu and Zn dissolution reactions are under chemical control (linear kinetics). The addition of NaCl to the chloride lixiviants is found to be beneficial only up to a common salt concen-tration of 100 g/l. Leaching of the copper concentrate with CuCl₂ or mixed FeCl₃-CuCl₂-NaCl has not been as effective as its direct leaching with 4 M FeCl₃. N.V. NGOC, formerly Visiting Scientist with the Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University.     

Kinetics of sulfation of chalcopyrite with steam and oxygen in the presence of ferric oxide

The kinetics of suifation of chalcopyrite with/without ferric oxide addition has been studied in the fixed bed for the temperature range 673 to 773 K in the absence of external mass transfer effects such as particle size of ore and flow rate of oxidizing gases such as steam and oxygen. The suifation reaction was observed to be topochemical. The activation energy value of 30.5 kJ/mol was found when no catalytic addition was made. The rate of suifation increases with the addition of ferric oxide. The rate constant values obtained without and with 10 pct Fe₂O₃ were 5.5 × 10³ min^?1 and 7.00 × 10³ min^?1, respectively. The activation energy value for the roasting in the presence of the catalyst was 29.2 kJ/mol under these conditions. Examination of the kinetic data indicates that the reaction occurred on the surface of the mineral particles and proceeded through the reactant and product phase boundary. The sulfated products were also characterized by metallography, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and X-ray diffractometry (XRD) studies.     

Leaching of metallic silver with aqueous ozone

The recycling of silver from metallic scraps can be performed through O₃ leaching at an ambient temperature and low (∼0.1 M) H₂SO₄ concentration. The main by-product is O₂, which can be recycled to the O₃ generation or used as leaching agent in a pretreatment step. The stoichiometry and the effects of the stirring speed, ozone and acid concentration and temperature on the leaching of silver were investigated. Silver dissolved as Ag²⁺_(aq) in the range 10⁻³–1 M H₂SO₄, but for pH ≥4, insoluble Ag₂O₂ was the main reaction product. Kinetics appeared to be controlled by mass transfer of O_3(aq) to the solid–liquid interface, showing first order dependency with respect to [O₃]_aq and P_O3. Specific rates were only slightly dependent on the temperature in the interval 10–50 °C, but decreased at 60 °C due to the fall in O₃ solubility. The mass transfer coefficients showed an average activation energy of 17 kJ/mol. No significant effect of [H₂SO₄] on mass transfer coefficients was observed for 10⁻²–1 M. Leaching rate gradually diminished for pH >2, as a consequence of the influence of the [H⁺] in the transport control.     

Kinetic and Mechanism Studies Using Shrinking Core Model for Copper Leaching from Chalcopyrite in Methanesulfonic Acid with Hydrogen Peroxide

ABSTRACT

An investigation on copper leaching from a chalcopyrite concentrate in methanesulfonic acid (MSA) and hydrogen peroxide at 75°C was carried out. Periodic additions of H₂O₂ were applied to enhance chalcopyrite dissolution and the reaction mechanism was analyzed using a shrinking core model. The results revealed that compared with the addition of H₂O₂ at the very beginning, periodic additions of H₂O₂ enhanced copper extraction and leaching kinetics, and the rate-determining step shifted from the diffusion of oxidant to the surface chemical reaction. The reaction orders with respect to MSA and H₂O₂ were found to be 0.19 and 1.26, respectively, suggesting the leaching process was highly dependent on H₂O₂ concentration. Calculated activation energy between the temperature range of 25–75°C was 79.8 kJ/mol. Detailed study also indicated the reaction mechanism is diffusion-controlled through a protective sulfur layer at lower temperature and surface chemical reaction-controlled at temperatures higher than 55°C. Overall, the MSA–H₂O₂ leaching system is a green method for chalcopyrite leaching and a possible flowsheet in the industrial application with the periodic additions of H₂O₂.     

A study of chalcopyrite dissolution in acidic ferric nitrate by potentiometric titration

A pure chalcopyrite sample was studied using a potentiometric titration technique, which measures the oxidant consumption in the dissolution of sulphide mineral slurries in acidic ferric ion solutions. Rate measurements were made over a period of days at constant solution pH and oxidation potential at 25 and 40°C. Long term leaching followed the parabolic kinetics and dissolution stoichiometry previously described in the literature, but there was an initial reaction in which more iron than copper dissolved from the lattice and a metal-deficient “chalcopyrite-like” surface phase was rapidly formed. The subsequent dissolution reaction (observed specific reaction rate, $\text{s}{}_0\text{= 1.4 × 10}{}^{\mathrm{?11}}\text{mol cm}{}^{\mathrm{?2}}\text{s}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ at 25°C; activation energy, E_a = 14 kcal mol^?1) was far more sensitive to temperature change and much slower than the rate of pore diffusion of oxidant ($\text{s}{}_0\text{≈ 10}{}^{\mathrm{?7}}\text{mol cm}{}^{\mathrm{?2}}\text{s}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$, E_a ≈ 2 kcal mol^?1) and was interpreted in terms of a solid state diffusion step within the crystal lattice.     

Oxidative acid leaching of mechanically activated sphalerite

The pressure leaching kinetics of mechanically activated sphalerite was investigated in this work. X-ray diffraction and scanning electron microscopy were used to characterise the influences of crystalline structure and morphology, respectively. A laser particle size analyser and specific surface area tester were used to determine the particle size and specific surface area, respectively. Compared to the non-activated sample, the activated samples demonstrated distinct physicochemical properties with higher reaction efficiencies and increased Zn recovery ratios. The activation energy of sphalerite decreased from 69.96 to 45.91, 45.11, and 44.44?kJ?mol^?1 as the activation time increased from 0 to 30, 60, and 120?min, respectively. The reaction orders for the H₂SO₄ solutions of the sphalerite samples activated for 0, 30, 60, and 120?min were 1.832, 1.247, 1.214, and 1.085, respectively, which indicated that the dependency of the sphalerite leaching process on H₂SO₄ could be reduced by means of mechanical activation.