Oxygen pressure leaching of white metal

Sulfuric acid–oxygen pressure leaching of white metal was investigated at laboratory scale as an alternative to the pyrometallurgical Peirce–Smith converting to produce metallic copper. The main variables studied were temperature, concentration of sulfuric acid, partial pressure of oxygen, and time of leaching. The results indicated that most of the sulfur in the white metal is oxidized to sulfate in the range 105 °C to 150 °C. The concentration of sulfuric acid over 0.05 M, and oxygen partial pressure over 608 kPa had little effect on the dissolution of copper. The temperature was the most significant variable; below 130 °C copper dissolution was incomplete after 5 h of leaching while at 150 °C the dissolution was complete in 90 min. The dissolution of white metal proceeded in two stages through the formation of CuS as an intermediate compound and the kinetics of copper dissolution was analyzed by considering two consecutive reactions controlled by chemical reaction. The activation energies for the first and second stages were determined as 55 and 88 kJ/mol, respectively.     

Intrinsic Kinetics of Chlorination of WO<Subscript>3</Subscript> Particles With Cl<Subscript>2</Subscript> Gas Between 973 K and 1223 K (700 °C and 950 °C)

Chlorination is one of the methods applied in extractive metallurgy for the treatment of minerals to obtain valuable metals, such as titanium and zirconium. The possibility of applying chlorination metallurgy to other metals such as tungsten was the major aim of this study. The kinetics of the chlorination of tungsten oxide (WO₃) particles has been investigated by thermogravimetry between 973 K and 1223 K (700 °C and 950 °C) and for partial pressures of chlorine ranging from 15 to 70 kPa. The starting temperature for the reaction of WO₃ with chlorine is determined to be about 920 K (647 °C). The influence of chlorine diffusion through the bulk gas phase and through the particle interstices in the overall rate was analyzed. In the absence of these two mass-transfer steps, a reaction order of 0.5 with respect to chlorine partial pressure, and an activation energy of 183 kJ/mol were determined. For tungsten oxide particles of less than 50-μm size, a complete rate expression has been obtained.     

The kinetics of molybdenum dioxide oxidation

Powdered molybdenum dioxide was oxidized to MoO₃ in the temperature range 390 to 465°C under oxygen partial pressures of 0.016 and 0.18 atmospheres^* and under 0.009 atmospheres in the temperature interval 465 to 529°C. The course of the reaction was followed by observing weight change with time. Parabolic kinetics were evident for oxidation below 460°C. Above 460°C linear kinetics were observed. The partial pressure dependence at 407°C and 455°C was found to be approximately \(P_{O_2 }^{1/5} \) . Oxidation tests were restricted to an oxygen partial pressure of 0.009 atmosphere in the temperature range between 460°C and 530°C since above this partial pressure excessive heating occurred. For the low temperature range the oxidation was explained in terms of the diffusion of mono-and divalent oxygen interstitials. Activation enthalpies of 159±8 and 105±8 kJ/mole were obtained respectively for parabolic and linear rate processes.     

High-Temperature Volatilization Mechanism of Stibnite in Nitrogen-Oxygen Atmospheres

The volatilization of stibnite (Sb₂S₃) in nitrogen and mixtures of nitrogen-oxygen was investigated in the temperature range 973 K to 1423 K (700 °C to 1150 °C). The overall volatilization reaction study was carried out using a thermogravimetric analysis technique under various gas flow rates. The results indicated that in an inert atmosphere, stibnite can be volatilized most efficiently as Sb₂S₃(g) with a linear rate up to about 1173 K (900 °C). At temperatures above 1223 K (950 °C), stibnite decomposes to antimony and sulfur gas, impairing the antimony volatilization. For linear behavior in nitrogen gas, kinetic constants were determined, and an activation energy of 134 kJ/mol was calculated for the volatilization reaction. However, in the presence of oxygen, antimony can be volatilized efficiently as valentinite (Sb₂O₃) at low oxygen concentrations (approximately 1 to 5 pct) at approximately 1173 K to 1223 K (900 °C to 950 °C); otherwise, at higher partial pressures of oxygen, the volatilization of antimony is limited by the formation of nonvolatile cervantite (SbO₂). In highly oxidizing atmospheres, a high vaporization of antimony could be achieved only at temperatures higher than 1423 K (1150 °C) where cervantite becomes unstable and decomposes into SbO(g) and 0.5O₂(g).     

Brief review of oxidation kinetics of copper at 350 °C to 1050 °C

Copper’s oxication mechanism and purity effects were elucidated by oxidizing 99.99 pct (4N), 99.9999 pct (6N), and floating zone refined (>99.9999 pct) specimens in 0.1 MPa oxygen at 350 °C to 1050 °C. Throughout the temperature range, the oxidation kinetics for all specimens obeys the parabolic oxidation rate law. The Cu₂O scale grows predominantly, and the rate-determining step is concluded to be outward diffusion of copper atoms in Cu₂O. The activation energy at high temperatures, where the lattice diffusion predominates, is 173 kJ/mol, but it becomes lower at intermediate temperatures and even lower at low temperatures because of the contribution of the grain boundary diffusion. At high temperatures, oxidation kinetics is almost uninfluenced by purity, but the lattice-diffusion temperature range is wider for higher-purity copper. At intermediate temperatures, copper oxidation is enhanced because trace impurities can impede growth of Cu₂O grains to facilitate grain boundary diffusion. At low temperatures, grain boundary diffusion is possibly hindered by impurities segregated at grain boundaries.     

Initial-stage Sintering Kinetics of Nanocrystalline Tungsten

Initial-stage sintering kinetics of nanocrystalline tungsten has been studied in the temperature range of 1273–1473 K (1000–1200 °C). Nanocrystalline tungsten sinters initially through a grain boundary diffusion mechanism. The calculated activation energy was 388 ± 11 kJ/mol at low temperatures (1273–1373 K (1000–1100 °C)) and 409 ± 7 kJ/mol at high temperatures (1373–1473 K (1100–1200 °C)), which are close to the experimentally measured activation energy for grain boundary diffusion (385 kJ/mol).     

Kinetics of pyrite oxidation in sodium hydroxide solutions

The kinetics of pyrite oxidation in sodium hydroxide solution were investigated in a stirred reactor, under temperatures ranging from 50 °C to 85 °C, oxygen partial pressures of up to 1 atm, particle size fractions from -150 + 106 to -38 + 10μm (-100 + 150 mesh to -400 mesh + 10 μ), and pH values of up to 12.5. The surface reaction is represented by the rate equation:-dN/dt = Sbk″pO^0.5 ₂[oH^{- 0.25}/(1 +k‴ pO₂ ^0.5) where N represents moles of pyrite, S is the surface area of the solid particles,k″ andk″ are constants,b is a stoichiometric factor, pO₂ is the oxygen partial pressure, and [OH^-] is the hydroxyl ion concentration. The corresponding fractional conversion (X) vs time behavior follows the shrinking particle model for chemical reaction control: 1 - (1 -X)^1/3 =k _ct The rate increases with the reciprocal of particle size and has an activation energy of 55.6 kJ/mol (13.6 kcal/mol). The relationship between reaction rate and oxygen partial pressure resembles a Langmuir-type equation and thus suggests that the reaction involves adsorption or desorption of oxygen at the interface. The square-root rate law may be due to the adsorption of a dissociated oxygen molecule. The observed apparent reaction order with respect to the hydroxyl ion concentration is a result of a complex combination of processes involving the oxidation and nydrolysis of iron, oxidation and hydrolysis of sulfur, and the oxygen reduction. Formerly Graduate Student, Department of Mineral Engineering, Pennsylvania State University     

Leaching kinetics and stoichiometry of pyrite oxidation from a pyrite–marcasite concentrate in acid ferric sulfate media

A pyrite concentrate with minor marcasite was oxidized in an acidic ferric sulfate medium at temperatures from 45 to 75 °C and at constant potentials corresponding to Fe(III) to Fe(II) ratios from 10 to 300. Potassium permanganate (KMnO₄) was found to be both a suitable oxidant for controlling the solution potential and a convenient and reliable indicator of leaching progress.The stoichiometry of pyrite oxidation was found to be essentially independent of temperature and only slightly dependent on solution potential over the range of conditions studied. Each unit of sulfide sulfur oxidized yielded 64 ± 2% sulfate, the rest elemental sulfur as discrete particles approximately 2 μm in diameter.The pyrite oxidation rate was very sensitive to the temperature, giving a large activation energy (83 kJ/mol), and was proportional to the Fe(III)/Fe(II) concentration ratio to the power of 0.57. The shrinking sphere model fitted very well the changing grain topology. A single mathematical expression combines the thermal, chemical, and topological functions to predict the pyrite conversion as a function of the known temperature, ferric concentration, ferrous concentration, particle size, and time. The model predictions are excellent over the range of conditions tested.     

Gas phase embrittlement of nickel by sulfur

Nickel and nickel-base alloys are extremely sensitive to embrittlement and enhanced crack propagation due to sulfur, which may contaminate during melting, processing, or service. This paper reports the results of a systematic study of the effect of sulfur partial pressure and exposure temperature on intergranular penetration and tensile embrittlement in Ni270. For exposures between 450 °C and 900 °C, a maximum embrittlement was observed at about 700 °C. Partial pressures of sulfur exceeding about 5 × 10^-8 atm were embrittling. Kinetic measurements, based on the depth of postexposure intergranular fracture, gave an activation energy of 74 kJ/mol in the temperature range 450 °C to 800 °C. This value is considerably less than the value reported for matrix sulfur diffusion and also less than that reported for intergranular oxygen diffusion. Auger spectroscopy confirmed high local concentrations of sulfur at grain boundaries. In only one case, under the most severe sulfur exposure, was indirect evidence of sulfide formation observed; clearly, the embrittlement was a consequence of intergranular diffusion and segregation of elemental sulfur. A simple composite model for the tensile strength and ductility of partially embrittled specimens, in terms of sulfur penetration distance, satisfactorily accounted for the experimental measurements. Formerly with the Materials Engineering Department, Rensselaer Polytechnic Institute.     

Hydrogen Reduction Kinetics of Magnetite Concentrate Particles Relevant to a Novel Flash Ironmaking Process

A novel ironmaking technology is under development at the University of Utah. The purpose of this research was to determine comprehensive kinetics of the flash reduction reaction of magnetite concentrate particles by hydrogen. Experiments were carried out in the temperature range of 1423 K to 1673 K (1150 °C to 1400 °C) with the other experimental variables being hydrogen partial pressure and particle size. The nucleation and growth kinetics expression was found to describe the reduction rate of fine concentrate particles and the reduction kinetics had a 1/2-order dependence on hydrogen partial pressure and an activation energy of 463 kJ/mol. Unexpectedly, large concentrate particles reacted faster at 1423 K and 1473 K (1150 °C and 1200 °C), but the effect of particle size was negligible when the reduction temperature was above 1573 K (1300 °C). A complete reaction rate expression incorporating all these factors was formulated.     

Oxidation Kinetics of a Pb-30 At. Pct In Alloy

The solid-state oxidation kinetics of a Pb-30 at. pct In alloy were studied from 22 °C to 175 °C using Auger electron spectroscopy (AES) combined with argon ion sputtering. At 22 °C, oxidation followed a direct logarithmic relationship. The data were interpreted in terms of a model presented previously. [2] The predicted value of the model parameter using the structural data of In₂O₃ is within a factor of 2 from the experimentally determined value. At temperatures higher than 100 °C, the oxidation kinetics changed to a different mechanism. Rapid oxidation occurred initially, followed by slower parabolic oxidation. The oxide formed was found to be In₂O₃ with possible existence of a metastable indium suboxide near the oxide/alloy interface. These data were described quantitatively by the model proposed by Smeltzeret al. [5] The model assumes the existence of short-circuit diffusion in addition to lattice diffusion. The degree of short-circuit diffusion decreases exponentially with time. The activation energy for the parabolic oxidation was found to be 30.8 ± 4 kJ/mol (0.32 eV). The rather low activation energy was rationalized by the fact that the diffusion of oxygen vacancies in In₂O₃ responsible for the parabolic oxidation occurred in the extrinsic region. This rationalization was made by analyzing the defect structure of In₂O₃ using oxygen diffusion data and the enthalpy of forming oxygen vacancies in In₂O₃ reported in the literature. [10,11,12]     

A Study on Alpha-Case Depth in Ti-6Al-2Sn-4Zr-2Mo

Isothermal oxidation experiments in air were performed on Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) with a bimodal microstructure in the temperature range 811 K to 922 K (538 °C to 649 °C) for up to 500 hours, and α-case depths were quantified using metallography. Alpha-case depth followed a parabolic variation with time. Alpha-case depths in excess of 10 μm formed above 811 K (538 °C) and 100-hour exposures. An activation energy of 244 kJ/mol was estimated for diffusion of oxygen in the α phase of Ti-6242.     

Kinetics and Reaction Mechanisms of High-Temperature Flash Oxidation of Molybdenite

The kinetics and reaction mechanism of the flash oxidation of +35/–53 μm molybdenite particles in air, as well as in 25, 50, and 100 pct oxygen higher than 800 K, has been investigated using a stagnant gas reactor and a laminar flow reactor coupled to a fast-response, two-wavelength pyrometer. The changes in the morphology and in the chemical composition of partially reacted particles were also investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and electron microprobe. High-speed photography was also used to characterize the particle combustion phenomena. The effects of oxygen concentration and gas temperature on ignition and peak combustion temperatures were studied. The experimental results indicate that MoS₂ goes through a process of ignition/combustion with the formation of gaseous MoO₃ and SO₂ with no evidence of formation of a molten phase, although the reacting molybdenite particles reach temperatures much higher than their melting temperature. This effect may be a result of the combustion of gaseous sulfur from partial decomposition of molybdenite to Mo₂S₃ under a high gas temperature and 100 pct oxygen. In some cases, the partial fragmentation and distortion of particles also takes place. The transformation can be approximated to the unreacted core model with chemical control and with activation energy of 104.0 ± 4 kJ/mol at the actual temperature of the reacting particles. The reaction was found to be first order with respect to the oxygen concentration. The rate constant calculated at the actual temperatures of the reacting particles shows a good agreement with kinetic data obtained at lower temperatures. The ignition temperature of molybdenite shows an inverse relationship with the gas temperature and oxygen content, with the lowest ignition temperature of 1120 K for 100 pct oxygen. Increasing the oxygen content from 21 to 100 pct increases the particle combustion temperature from 1600 K to more than 2600 K. A high oxygen content also resulted in a change of the reaction mechanism from relatively constant combustion temperatures in air to much faster transient combustion pulses in pure oxygen.     

Creep and oxygen diffusion in magnetite

The creep rate of polycrystalline Fe₃O₄ has been measured as a fonction of stress and oxygen partial pressure in the temperature range 480–1100°C. A regime of power law creep is found at high stress, with a stress exponent of ≈- 3.1 and an activation energy of 264 kJ/mol. A regime of diffusional flow is found at lower stresses and is interpreted as Nabarro-Herring creep. The data for the two regimes are combined to deduce an oxygen diffusion coefficient of ≈-10⁻⁵ exp(−264 kJ/mol/RT) m²s⁻¹, with oxygen vacancies suggested as the mobile species.     

Oxidation of Fe-V Melts Under CO<Subscript>2</Subscript>-O<Subscript>2</Subscript> Gas Mixtures

The oxidation mechanism of liquid Fe-V alloys with V content from 5 to 20 mass pct under different oxygen partial pressures using CO₂-O₂ mixtures with CO₂ varying from 80 pct to 100 pct was investigated by thermogravimetric analysis between 1823 K and 1923 K (1550 °C and 1650 °C). The products after oxidation were identified by scanning electron microscopy energy-dispersive spectrograph and X-ray diffraction. The results indicate that the oxidation process can be divided into the following steps: an apparent incubation period, followed by a chemical reaction step with a transition step before the reaction, and diffusion as the last stage. At the initial stage, a period of slow mass increase was observed that could be attributed to possible oxygen dissolution in the liquid iron-vanadium coupled with the vaporization of V₂O. The length of this period increased with increasing temperature as well as vanadium content in the melt and decreased with increasing oxygen partial pressure of the oxidant gas. This analysis was followed by a region of chemical oxidation. The oxidation rate increased with the increase of the O₂ ratio in the CO₂-O₂ gas mixtures. During the final stage, the oxidation seemed to proceed with the diffusion of oxygen through the product layer to the reaction front. The Arrhenius activation energies for chemical reaction and diffusion were calculated, and kinetic equations for various steps were setup to describe the experimental results. The transition from one reaction mechanism to the next was described mathematically as mixed-control equations. Thus, uniform kinetic equations have been setup that could simulate the experimental results with good precision.     

Pressure oxidation leaching of chalcopyrite: Part II: Comparison of medium temperature kinetics and products and effect of chloride ion

Pressure oxidation reaction kinetics and products are compared for the recovery of copper from a chalcopyrite concentrate under medium temperature conditions at 125–150 °C, as patented and promoted by various commercial operations, normally referred to as the Anglo/UBC, CESL and NSC (nitrogen species catalysed) processes. The main aim was to compare the effect of additives on the dispersion of molten sulfur and recovery of copper, and develop a better understanding of the role of chloride ion.Greater than 94% copper was extracted from the concentrate under all process conditions (except NSC) within 30 min. The extraction of the remaining copper in some instances took substantially longer due to poor dispersion of sulfur and agglomeration of the sulfide minerals. The partial oxidation of sulfide to elemental sulfur was about 70–80% at 150 °C and was greater in the presence of chloride ion and at higher initial acidity. Iron was leached and re-precipitated forming a number of different phases depending upon the acidity and salinity. These phases including hematite, jarosite and goethite were characterised using Quantitative X-ray Diffraction (QXRD) analysis. Hematite formation was favoured at higher temperatures (≥ 150 °C), low acidity and low to moderate salinity. Goethite formation was favoured at lower temperatures (< 150 °C) and by low acidity and low salinity. Jarosite was formed under conditions of moderate to high acidity and its formation was enhanced/stabilised in the presence of sodium ions. It was demonstrated that the basic copper salt, antlerite, could be produced under conditions of low acidity.Chloride ion addition and high acid concentrations enhanced copper extraction kinetics and recovery, and inhibited the oxidation of sulfur to sulfate. This suggests that chloride ion is adsorbed on the sulfur surface to restrict direct oxygen reaction. Chloride ion also enhances the anodic oxidation of mineral sulfides and the dispersion of molten sulfur.