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.     

Sulfuric acid oxygen-pressure leaching of Ni3S2 prepared by a wet process

Ni₃S₂ prepared by a wet process was easily leached as nickel sulfate at 383 K, p_o2 1 MPa, and sulfuric acid concentration of 0.1–0.15 mol L⁻¹. The leaching reaction proceeds through the intermediate formation of NiS prior to complete dissolution. A constant leaching rate was observed for most of the duration of the reaction, and this has been attributed to an increase in the specific surface area of the sulfide particles. A thin sulfur layer was formed on the sulfide; the diffusion of oxygen through the sulfur layer was found to be rate-determining.     

Autoclave leaching of molybdenite concentrates with catalytic additives of nitric acid

The scientific theoretical prerequisites for the use of small additives of nitric acid for the intensification of the autoclave oxidative leaching (AOL) of sulfide concentrates are checked in order to increase the degree of extraction of the base metal with a simultaneous decrease in the cost of the process. The technological parameters of the AOL conditions applied to molybdenite are also refined, and recommendations for a practical application of the apparatus of the process and for a decrease in its cost are made.     

The leaching of galena in cupric chloride media

The kinetics of dissolution of galena (PbS) in CuCl₂-HCl-NaCl media have been investigated using the rotating disk technique. Rapid nonlinear kinetics are observed over the temperature range 45 °C to 90 °C and for NaCl concentrations from 0 to 4.0 M. The leaching rate is in-dependent of CuCl₂ concentrations >0.1 M but increases with increasing concentrations of either HC1 or NaCl. The leaching rate decreases with the accumulation of either the PbCl₂ or CuCl reaction product in the leaching medium but is insensitive to the disk rotation speed. The apparent activation energy for the rate controlling process is 33 kJ/mol, and this value falls to about 15 kJ/mol when interpreted as a dissolution rate for PbCl₂, whose solubility increases with temperature. The above observations are shown to be consistent with rate control by the outward diffusion of the PbCl₂ and CuCl reaction products through the solution trapped in the pores of the constantly thickening elemental sulfur layer which forms on the surface of the galena. CANMET, Energy, Mines, and Resources Canada, 555 Booth Street, Ottawa, ON, K1A 0G1, Canada.     

The leaching of chalcopyrite with cupric chloride

A comparative study of electrochemical leaching and chemical leaching of chalcopyrite was performed mainly at 343 K to elucidate the leaching mechanism of chalcopyrite with CuCl₂. Also, the morphology of the leached chalcopyrite surface was studied by using a single chalcopyrite crystal. The leaching with CuCl₂ produced a porous elemental sulfur layer on the chalcopyrite surface, showing a similar morphology to that produced during leaching with FeCl₃. The leaching kinetics were found to be linear over an extended period, followed by an acceleration stage, as a result of an increase in the reaction surface area. The leaching rate of chalcopyrite was proportional to C(CuCl₂)^0.5, whereas it was inversely proportional to C(CuCl)^0.5. The mixed potential of chalcopyrite exhibited a 66 mV decade⁻¹ dependency upon C(CuCl₂), and—69 mV decade⁻¹ upon C(CuCl). Based on these observations together with other findings, an electrochemical mechanism involving the oxidation of chalcopyrite and CuCl ₋ ² and the reduction of CuCl⁺ was proposed. The Tafel plot between the mixed potential and the current density obtained by converting the rate of chemical leaching gave a straight line whose slope was in good agreement with that of the electrochemical leaching. These findings strongly support the electrochemical mechanism of chalcopyrite leaching with cupric chloride.     

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.     

The oxidation of liquid Ni3S2

Using a levitation technique, molten nickel sulfide droplets were oxidized at temperatures above 1500°C under oxygen potentials varying from 5 to 40 pct in He gas. To analyze the results, the oxidation process was divided into two stages. The first stage corresponded to the desulfurization of Ni₃S₂ by oxidation of the dissolved sulfur. In the second stage, a small amount of desulfurization, oxidation of nickel vapor and absorption of oxygen gas into the droplet occurred. Both stages were found to be controlled by mass transfer of oxygen within the gas boundary layer. Under conditions of high oxygen potential, a halo appeared around the levitated droplet during the initial period. This halo disappeared during desulfurization, but reappeared towards the end of the oxidation period. Formerly Post-Doctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto.     

The vaporization kinetics of Ni3S2

The vaporization of Ni₃S₂ was investigated at high temperatures by a levitation technique. Vaporization was controlled by a mass transfer process within a gas boundary layer around the Ni₃S₂ droplet. The vaporized S₂(g) and Ni(g) recombine within the gas boundary layer, and this recombination along with the condensation of Ni(g) was found to enhance the vaporization rate. A model has been developed to account for the experimentally-obtained mass transfer coefficients. Y. FUKUNAKA, formerly Research Associate, Department of Metallurgy and Materials Science, University of Toronto     

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.     

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.     

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.     

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.     

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 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.     

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.     

Kinetics of leaching of copper concentrates by nitric acid

Based on the study of the kinetics of leaching of copper concentrates by nitric acid, the limiting stages, reaction rate constants, activation energies, and general and partial orders of reactions of the components involved have been determined. An effect of films of reaction products on the limiting stage of the leaching of sulfides by nitric acid has been revealed. It has been shown that in the case of high oxidation potentials no films of elementary sulfur and iron oxide are formed, and the rate of the process is controlled by external diffusion.     

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     

Ni3S2-FeS-Cu2S冰镍体系的表面张力

运用几何模型 (Kohler及Toop模型 )和通用溶液模型 (Chou模型 ) ,由Ni3S2 FeS、FeS Cu2 S和Cu2 S Ni3S2 三个二元边界体系的表面张力计算了Ni3S2 FeS Cu2 S三元系表面张力。计算结果与实验值 (实验误差为± 2 5 % )十分吻合 ,1 4 73K下Ni3S2 FeS Cu2 S冰镍体系的等表面张力曲线有较高的可信度 ,对分析研究镍的闪速熔炼和转炉吹炼过程有重要意义。