Kinetics of dissolution of synthetic heazlewoodite (Ni3S2) in nitric acid solutions

The dissolution rate of heazlewoodite in nitric acid solution has been determined. The effects of nitric acid concentration, temperature, particle size, stirring intensity and addition of Cu²⁺ ions have been investigated. Solid residues after leaching were examined by SEM, X-ray diffraction and chemical analysis. In the solutions containing less than 2.0 M HNO₃, dissolution was observed to be completely inhibited after 30 min leaching time, and the rate of hydrogen sulphide production was faster than its oxidation to S⁰ and HSO₄^?. In 3 M HNO₃, an abrupt increase in dissolution rate of Ni₃S₂ was found. Two different regions of the dissolution of heazlewoodite were observed below and above 50°C. At temperatures below 50°C, the dissolution rate was very slow, even in 3.0 M HNO₃ solution, and H₂S gas was evolved. Above 50°C, the dissolution rate rapidly increased. Over the temperature interval 60–90°C in 3.0 M HNO₃ dissolution followed a linear rate law, and the activation energy was found to be 42.1 kJ mol^?1. Most of the oxidized sulphide ion was found in the solution as sulphate. The leaching rate was independent of stirring speed. The rate-controlling step of the Ni₃S₂ dissolution is the oxidation of hydrogen sulphide to elemental sulphur or sulphate ions on the Ni₃S₂ surface. Addition of small amounts of Cu²⁺ ions to the nitric acid acted as catalyst for the dissolution of Ni₃S₂. Bubbling air through the leach suspension increased the dissolution rate of Ni₃S₂ in solutions containing less than 2.0 M HNO₃.     

Silver ion catalysis in nitric acid dissolution of Ni3S2

The dissolution of Ni₃S₂ in nitric acid solutions in the presence of silver ions was investigated. The effect of stirring, particle size, temperature and silver ion concentration were examined. In addition, the reaction residues at various levels of nickel extraction were examined by SEM, X-ray diffraction, electron microprobe and chemical analysis. These observations indicated that the dissolution reaction is topochemical and fits a surface reaction control model. The activation energy was calculated to be 52.6 ± 3.6 kJ/mol, which is reasonable for a rate-limiting surface reaction. The order of the reaction was 0.2 with respect to Ag⁺ concentration. An electrochemical reaction between nitric acid and the intermediate product Ag₂S occurring on the surface of Ni₃S₂ appears to be the rate-determining step.     

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.     

Kinetics of Ni<Subscript>3</Subscript>S<Subscript>2</Subscript> sulfide dissolution in solutions of sulfuric and hydrochloric acids

The kinetics of Ni₃S₂ sulfide (heazlewoodite) dissolution in solutions of hydrochloric and sulfuric acids is studied. The process under study in the temperature range of 30–90°C is found to occur in a kinetic regime and is controlled by the corresponding chemical reactions of the Ni₃S₂ decomposition by solutions of inorganic acids (E _a = 67–92 kJ/mol, or 16–22 kcal/mol). The only exception is the Ni₃S₂-HCl system at elevated temperatures (60–90°C). In this case, the apparent activation energy decreases sharply to 8.8 kJ/mol (2.1 kcal/mol), which is explained by the catalytic effect of gaseous chlorine formed under these conditions. The studies performed are related to the physicochemical substantiation of the hydrometallurgical processing of the copper-nickel converter mattes produced in the industrial cycle of the Norilsk Mining Company.     

Enhancement of chalcopyrite leaching by ferrous ions in acidic ferric sulfate solutions

The effects of ferrous ions on chalcopyrite oxidation with ferric ions in 0.1 mol dm⁻³ sulfuric acid solutions were investigated by leaching experiments at 303 K in nitrogen. With high cupric ion concentrations, the chalcopyrite oxidation was enhanced by high concentrations of ferrous ions and copper extraction was mainly controlled by the concentration ratio of ferrous to ferric ions or the redox potential of solutions. Ferrous ions, however, suppressed the chalcopyrite oxidation when cupric ion concentrations were low. A reaction model, which involves chalcopyrite reduction to intermediate Cu₂S by ferrous ions and oxidation of the Cu₂S by ferric ions, was proposed to interpret the results.     

A rotating disk study of silver dissolution with thiourea in the presence of ferric sulfate

The rotating disk technique was used to study silver dissolution with thiourea as a function of sulfuric acid, ferric sulfate, and thiourea concentrations. The effect of many foreign ions (Mn, Cu, Co, Ca, Na,etc.) and various additives was also examined. The dissolution of silver was zero order with sulfuric acid, first order with ferric sulfate, and second order with thiourea. Among the foreign ions, copper had a dramatically negative effect. The strong oxidants such as hydrogen peroxide and manganese dioxide were also detrimental for silver dissolution. According to the temperature effect studied (5 °C to 35 °C), the activation energy was 22.6 kJ/ mole. Silver does not dissolve with thiourea in the absence of ferric ions. Sulfuric acid does not participate in the dissolution reaction. The most important parameter for silver dissolution is the ferric sulfate/thiourea ratio. In excess ferric sulfate, a solid silver-thiourea complex is formed, which precludes transfer of silver into solution. In excess thiourea, the free thiourea reacts with formed solid silver-thiourea complex, and silver goes into the solution, predominantly as the dimers of AgTU⁺ ₃ complexes. The solid silver-thiourea complex in question was characterized by various spectroscopic, microscopic, and chemical analysis techniques. According to chemical composition, it corresponds to Ag₂SO₂·3TUH₂O compound. Formerly Graduate Student, University of Idaho     

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.     

Pyrite oxidation with ozone: stoichiometry and kinetics

One of the most frequent causes of refractoriness in precious metals leaching is their occlusion or fine dissemination into a pyritic matrix. This study experimentally explores the acid leaching of pyrite with ozone, suggests the stoichiometry of the reaction, estimates its activation energy and defines the effect of the main variables on the leaching kinetics. The results of stoichiometry tests allow establishing that one mole of pyrite requires 7.7 moles of ozone to produce one mole of ferric ion and 2 moles of HSO₄? ions. A decrease in the particle size, solution pH and solids’ concentration of the leaching system increases pyrite dissolution. The type of acid (nitric, sulphuric and hydrochloric) does not affect pyrite dissolution rate. Up to 60% of pyrite is dissolved when the optimal experimental conditions are employed (1?g pyrite (?25?µm), 800?mL of 0.18?M of H₂SO₄, 800?rev?min^?1, 1.2?L?min^?1 gas stream O₂/O₃ with 0.079?g O₃?L^?1 and 25°C). The apparent activation energy of the pyrite-ozone reaction is 14.92?kJ?mol^?1, and the absence of a passive layer on the pyrite surface and the linearity of the dissolution profiles suggest that the dissolution kinetics is controlled by the chemical reaction.     

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.     

Kinetics of Lime-Enhanced Hydrogen Reduction of Solid Nickel Sulphide

Investigations have been carried out on the hydrogen reduction of solid nickel sulphide (β-NI₃S₂) in the presence of lime. The effects of the charge composition, temperature (500-700°C). hydrogen flow rate, time of reduction and particle size of the sulphide have been studied. Lime was found to tremendously enhance the reduction process and drastically stifle H₂S emission into the off-gas. Temperature as well as hydrogen flow rate were found to affect the reduction process and best results were achieved (in static bed experiments) with 200% CaO addition at 630°C in 2 hr employing a hydrogen flow rate of 0.2 l/min. Thermodynamic considerations and several experimental findings indicate that the progress of the Ni₃S₂-CaO-H₂ reaction is governed by the intrinsic kinetics of the Ni₃S₂-H₂ reaction. Kinetic analysis reveals the observance of Jander's linear rate equation indicative of phase boundary control at the sulphide/gas interface. Scanning electron microscopic studies on the reduced nickel sulphide pellets show that like in solid state transformations, discontinuous precipitation (cellular morphology) is exhibited.     

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.     

Phase constitution of the Ni−Cr−S alloy system between 600° and 850°C

The diffusion couple method and electron probe microanalysis have been used to determine the ternary isotherms of Ni?Cr?S at 600°, 700°, 800°, and 850°C for sulfur compositions lying below 60 at. pct. It has been found that the solid and liquid phases Ni_3+x S₂ have a very shallow penetration into the diagram whereas the Ni_1?x S, Cr₃S₄, and Cr₂S₃ phases penetrate very deeply into the diagram. The phase compositions NiCr₂S₄ and NiCrS₃ have been detected near the limit of the latter two phase fields. The binary phases Cr₂S₃, Cr₃S₄, Cr₅S₆, Cr₇S₈, and CrS have all been identified and their existence ranges and free energies of formation have been established at 700°C. The phase diagram observed and proposed is consistent with the sequence of free energies of formation of nickel and chromium sulfides.     

An investigation of the thermodynamics and kinetics of the ferric chloride brine leaching of galena concentrate

The solution thermodynamics of acidified ferric chloride brine lixiviants and the dissolution kinetics of a galena concentrate in such solutions have been investigated. The distribution of the various metal chloro complexes calculated from available thermodynamic data shows that the distribution is shifted to the higher complexes, predominantly FeCl ₃ ^o , FeCl ₂ ^o , and PbCl ₄ ⁼ , as the total Cl^? concentration increases, and that the distribution is unaffected by the extent of reaction. The dissolution of PbS concentrate is presented as a competition between a nonoxidative reaction with H⁺ and the oxidative reaction with ferric ion. Acid dissolution of PbS predominates when the activity ratio of hydrogen ion to ferric ion is high. Under these conditions H₂S is produced. When the activity ratio of hydrogen ion to ferric ion is low, and especially when the concentration of Fe³⁺ is greater than 0.15 M, oxidative dissolution of PbS becomes the controlling reaction. The dissolution can be represented by a shrinking core model with a surface chemical reaction as the rate controlling step. This is supported by the activation energy of 72.1 kJ/mole and the dependence of the rate on the inverse of the particle radius. The following rate equation was found to be in excellent agreement with the experimentally observed leaching behavior for 0.15 to 0.6 M [Fe⁺³] _T up to approximately 90 to 95 pet extraction: $$1 - \left( {1 - \alpha } \right)^{1/3} = \left[ {\frac{{2.3 x 10^{12} }}{{r_0 }}\left[ {{\text{Fe}}^{{\text{ + 3}}} } \right]_T^{0.21} \exp \left( {\frac{{ - 72100}}{{{\text{R}}T}}} \right)} \right]t$$ The rate deviates from the 0.21 order for Fe⁺³ concentrations greater than 0.6 M. The deviation from the surface model at higher values of PbS conversion is due to the presence of solid PbCl₂ in the pores of the reacting particles.     

La Lixiviation de concentres de sulfures de nickel par le chlorure ferrique

The effectiveness of ferric chloride leaching of nickel sulphide concentrates and of a nickel sulphide matte has been demonstrated. Ferric chloride concentrations ranged from 1 M to 5.7 M for treatment of the concentrates and 0.6 M to 3.8 M for that of the matte; experiments were carried out at 80 ± 1°C.The rate of the reaction was markedly slower after the dissolution of ～ 70% of the Ni. Concentrations of ferric chloride have a significant effect upon the quantity of elemental sulphur obtained. During leaching, a part of the iron from the leachant precipitated out. β-NiS is formed during the treatment of a nickel sulphide matte with ferric chloride.The reaction was followed by continuous measurements of pH, conductivity, redox potential and spectrophotometric intermittent measurements concerning nickel concentrations.     

The precipitation of Ni3S2 from sulfate solutions

The formation conditions for the recovery of nickel from sulfate solutions as Ni3S2 have been investigated; this sulfide is more reactive than NiS in subsequent leaching operations. Hydrogen sulfide gas at atmospheric pressure is introduced into a NiSO4-Na2SO4-MgSO4-Al2(SO4)3 solution in the presence of reduced iron powder. Although the formation of Ni3S2 is compctitive with that of NiS, the nickel precipitation efficiency and the ratio of nickel as Ni3S2 to the total nickel precipitated reached 99.5 to 99.9 and 90 to 95 pct, respectively, under the following conditions: 363 K, Ph2s 31 kPa, Ni²⁺ 4.0 g · dm^-3, 3[Fe^o]/[Ni²⁺] 1.25 to 1.5, H2S flow rate 70 to 100 cm³ · min^-1, and 45 to 60 minutes retention time. Selective formation of Ni3S2 is achieved within 10 minutes, and a reaction on the surface of the iron is rate-determining during the early stages of precipitation. Since the iron is almost totally consumed after 1 to 2 hours of reaction, the precipitated Ni₃S₂ is gradually converted to NiS. Calculations considering the buffer action of sulfate ion and sulfate complex formation with polyvalent metal cations as well as with nickel ions confirmed that significant nickel precipitation as Ni₃S₂ should occur under the test conditions.     

Leaching and kinetic modelling of low-grade calcareous sphalerite in acidic ferric chloride solution

The leaching kinetics of a low grade-calcareous sphalerite concentrate containing 38% ankerite and assaying 32% Zn, 7% Pb and 2.2% Fe was studied in HCl–FeCl₃ solution. An L₁₆ (five factors in four levels) standard orthogonal array was employed to evaluate the effect of Fe(III) and HCl concentration, reaction temperature, solid-to-liquid ratio and particle size on the reaction rate of sphalerite. Statistical techniques were used to determine that pulp density and Fe(III) concentration were the most significant factors affecting the leaching kinetics and to determine the optimum conditions for dissolution. The kinetic data were analyzed with the shrinking particle and shrinking core models. A new variant of the shrinking core model (SCM) best fitted the kinetic data in which both the interfacial transfer and diffusion across the product layer affect the reaction rate. The orders of reaction with respect to (C_Fe³⁺), (C_HCl), and (S/L) were 0.86, 0.21 and − 1.54, respectively. The activation energy for the dissolution was found to be 49.2 kJ/mol and a semi-empirical rate equation was derived to describe the process. Similar kinetic behavior was observed during sphalerite dissolution in acidic ferric sulphate and ferric chloride solutions, but the reaction rate constants obtained by leaching in chloride solutions were about tenfold higher than those in sulphate solutions.     

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.     

Kinetics and mechanism of the oxidative dissolution of a zinc sulphide concentrate in ferric sulphate solutions

The kinetics of the oxidative dissolution of a zinc sulphide (sphalerite) concentrate was studied. It was observed that the dissolution of the concentrate continued beyond 90% conversion in two hours at 80°C. The kinetics of dissolution are successfully described by an electrochemical mechanism in which the charge transfer from the solid to the oxidant is rate-limiting. The rate of reaction is proportional to the sum of the concentrations of the Fe³⁺ (aq) and FeHSO₄²⁺ complexes with a reaction order of one-half. The addition of Fe (II) to the solution had an indirect effect on the reaction rate, by decreasing the concentrations of the electro-active ions. Addition of ZnSO₄ did not affect the reaction rate.     

Oxidative Leaching of Chalcopyrite by Cupric Ion in Chloride Media

In this study, chalcopyrite concentrate produced in Sarcheshmeh copper plant was subjected to oxidative leaching investigation by cupric ion to determine the effect of several parameters on the copper and iron dissolution, including temperature of leaching, time of holding and cupric concentration as oxidant agent in a range of 38–97 °C, 1.5–8.5 h and 0.2–0.8 M, respectively. The leaching media was chloride providing with 3 M HCl 37% and CuCl₂. The experiments were designed by central composite design method. The dissolution of copper and iron was examined. The maximum dissolution of copper 62.64%, was obtained at 85 °C, 7 h and oxidant concentration of 0.7 M. The kinetics model of chalcopyrite leaching and an optimized condition with maximum dissolution of copper and minimum dissolution of iron was obtained by Minitab^®18 software. Additionally, the thermodynamics of leaching was investigated by Pourbaix diagrams of copper and iron composition, using HSC Chemistry6 software. It was found that the oxidative leaching process is controlled by diffusion through passivation layer with an activation energy of 19.57 kJ/mol.

     

Stability of Ni3(Al,Ti) Gamma Prime Precipitates in a Nickel-Based Superalloy Inconel X-750 Under Heavy Ion Irradiation

Phase stability of Ni₃(Al, Ti) precipitates in Inconel X-750 under cascade damage was studied using heavy ion irradiation with transmission electron microscope (TEM) in situ observations. From 333 K to 673 K (60 °C to 400 °C), ordered Ni₃(Al, Ti) precipitates became completely disordered at low irradiation dose of 0.06 displacement per atom (dpa). At higher dose, a trend of precipitate dissolution occurring under disordered state was observed, which is due to the ballistic mixing effect by irradiation. However, at temperatures greater than 773 K (500 °C), the precipitates stayed ordered up to 5.4 dpa, supporting the view that irradiation-induced disordering/dissolution and thermal recovery reach a balance between 673 K and 773 K (400 °C and 500 °C). Effects of Ti/Al ratio and irradiation dose rate are also discussed.