Study of the mechanism of anodic dissolution of Cu2S

The anodic dissolution of Cu₂S in sulfuric acid solutions was studied under galvanostatic and potentiostatic conditions. The anodic products were studied by mineralogical and X-ray diffraction methods. In every case, the formation of a digenite Cu_1-8S layer is observed at the surface of Cu₂S according to 5Cu₂S → 5Cu_1.8S + Cu⁺⁺ + 2e A copper concentration gradient appears through the digenite layer whose thickness remains constant as soon as a Cu_1.1S layer appears at its own surface according to 3Cu_1.8S → 4Cu_1.1S + Cu++ + 2e If the electrolysis conditions are such that the anodic potential remains low, the next reaction to occur is 10Cu_1.1S → HCu⁺⁺ + 10S + 22e But if under galvanostatic conditions, the current density is high enough at a given temperature to reach the sharp rise in anodic potential, or if under potentiostatic conditions the potential is kept high, two other reactions are possible: 10Cu_1.1S → 10CuS + Cu⁺⁺ + 2e followed by CuS → Cu⁺⁺ + S + 2e Moreover, at high anodic potential, the following reaction occurs also to some extent CuS + 4H₂O ? Cu⁺⁺ + SO₄ ⁼ + 8H⁺ +8e resulting in a decrease in anodic current efficiency for the copper dissolution. From a more practical point of view, it was shown that it is possible to deplete virtually completely the copper content of the anode (residue at less than 0.5 pct Cu)keepingthe electrode potential at a low value (less than +650 mV/ENH). Providing the temperature is high enough (75°C at least), the mean current density remains near to 2 A/dm², a suitable value to obtain good cathodic deposits.     

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.     

Electrochemistry of Chalcocite/Heazlewoodite/Sulfhydril Collector Systems

Abstract

Rest potential and cyclic voltammetry studies were conducted on chalcocite and heazlewoodite as a function of pH in the presence and absence of the sulfllydryl collectors, sodium ethyl xanthate (EX) and the sodium salt ofmercaptobenzothiazole (MBT). The minerals showed a significant difference in their response both in the presence and absence of collectors. The voltammograms suggested that EX^? and MBT^? interact with the minerals in a similar manner. Most reactions of chalcocite and heazlewoodite in the presence and absence of EX were identified from the literature. For the case of MBT and chalcocite the following new reaction is proposed at E _e = ?0.060 V:

Cu₂S+2MBT^? → 2CuMBT+S⁰+2e^?.

Résumé

Des études du potentiel à repos et la voltammetrie cyclique ont été entreprisés sur le chalcocite et le heazlewoodite en fonction de pH en présence et en absence des collecteurs sulfhydryl sodium ethyl xanthate (EX) et le sel sodium de mercaptobenzothiazole (MBT). Les minéraux différents manifestent des réponses différentes, en présence et en absence des collecteurs. Les voltammogrammes indiquent que l'EX et I'MBT réagent avec les minéraux en façons semblables. La plupart des réactions du chalcocite et du heazlewoodite en présence et en absence de l'EX existent dans la littérature. Pour le cas de MBT/chalcocite à E _e = ?0.060 V, une nouvelle réaction est proposée:

Cu₂S+2MBT^? → 2CuMBT+S⁰+2e^?.     

Interaction of copper sulfides with copper oxides in the molten state

Reactions of Cu₂S with Cu₂O, CuS with Cu₂O and CuS with CuO in the molten state were examined in the presence of one atmosphere of argon at 1200°C. A rate law of the form,r _SO2 =kN_SN_O was applicable for each reaction system studied. Comparison of the rate constants for the systems, under conditions of similar initial mole fraction of sulfur to mole fraction of oxygen ratios, showed that Cu₂O was much more reactive than CuO in its reaction with copper sulfides. These results are incorporated in a mechanism in which Cu₂O reacts with the sulfide in the rate determining step. Experiments carried out in the presence of oxygen indicated the importance of a CuO-Cu₂O equilibrium in the overall reaction mechanism.     

The oxidation of thiosulfates with copper sulfate. Application to an industrial fixing bath

This paper presents the transformation of thiosulfate using Cu(II) salts, such as copper sulfate, at pH between 4 and 5. The nature and kinetics of this process were determined. In the experimental conditions employed, the reaction between thiosulfates and Cu(II) ions produces a precipitate of CuS and the remaining sulfur is oxidized to sulfate, according to the following stoichiometry: 1 mol thiosulfate reacts with 1 mol Cu²⁺ and 3 mol H₂O, generating 1 mol copper(II), 1 mol sulfate and 2 mol H₃O⁺. In the kinetic study, the apparent reaction order was ≈ 0 with respect to H₃O⁺ concentration, in the interval 1.0 · 10^? 4–1.0 · 10^? 5M H₃O⁺; of order 0.4 with respect to Cu²⁺ in the interval 0.21–0.85 g L^? 1 Cu²⁺; and of order 0 with respect to S₂O₃^2? in the interval 0.88–2 g L^?1 S₂O₃^2?. The apparent activation energy was 98 kJ mol^? 1 in the interval 15–40 °C. On the basis of this behavior an empirical mathematical model was established, that fits well with the experimental results. The thiosulfate transformation process using copper(II) sulfate was applied to an industrial fixing bath that proceeded from the photographic industry; after this, the resulting effluent contained less than 10 mg L^? 1 of thiosulfates.     

Kinetics of chlorination and oxychlorination of chromium (III) oxide

Thermogravimetric analysis (TGA) is used to study the kinetics of chlorination of Cr₂O₃ with Cl₂+N₂ and Cl₂+O₂ gas mixtures in the temperature range of 550 °C to 1000 °C. The reactivity of Cr₂O₃ toward the chlorine-oxygen gas mixture is higher than that toward the chlorine-nitrogen one. Chlorination of Cr₂O₃ proceeds with an apparent activation energy of about 86 kJ/mol between 550 °C and 1000 °C. The apparent reaction order with respect to chlorine is about 1.23 at 800 °C. At temperatures lower than 650 °C, the shrinking sphere model is the most appropriate for describing the reaction kinetics. Oxychlorination of Cr₂O₃ is characterized by an apparent activation energy of about 87 and 46 kJ/mol for temperatures lower than 650 °C and higher than 700 °C, respectively. At 800 °C and using a Cl₂+O₂ gas mixture, the maximum reaction rate is obtained when the Cl₂/O₂ molar ratio is equal to 4, confirming the formation of chromium oxychloride. At this temperature, the reaction orders with respect to chlorine, oxygen, and Cl₂+O₂ are about 1.08, 0.23, and 1.29, respectively. Mathematical fitting of the experimental data is discussed.     

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.     

Dissolution of Cu2S in aqueous edta solutions containing oxygen

The mechanism of the reactions taking place in the heterogeneous system: synthetic polydispersive Cu₂S-ethylediaminetetraacetic acid (EDTA)—O₂—H₂O has been investigated. The partial pressure of oxygen and pH of the solution were found to exert a significant effect on the process kinetics. The dissolution rate does not depend, in practice, on the agitation rate and the EDTA concentration exerts an influence only at higher partial oxygen pressures.Dissolution of Cu₂S in aqueous EDTA solutions proceeds in two steps with the formation of CuS as an intermediate. In acid and neutral solutions the final products of dissolution are elementary sulphur and Cu(EDTA)^2- complex ion. The activation energy ΔE = 10.4kJ/mol (2.4 kcal/mol) suggests a diffusion controlled process. In alkaline solutions sulphur is oxidized to the sulphate ion and the dissolution process is kinetically controlled, ΔE = 41.4 kJ/mol (9.9 kcal/mol).     

The reaction of Cu(II) with LIX 65N in homogeneous solution

The nature of the complexes formed between Cu(II) and LIX 65N (H₂L) in ethanol has been studied by UV-visible spectrophotometry and the kinetics of the reaction between Cu(II) and H₂L by stopped-flow spectrophotometry at 25°C. The major complex, Cu(HL)₂, has a formation constant β₂ = [Cu(HL)₂]/[Cu][HL^?]² of log β₂ = 26.5 l² mol^?2 in 0.04 M NaClO₄. An additional complex, probably Cu(HL)⁺, can be detected in the absence of added NaClO₄. A third complex, identified as Cu₂(HL)₂²⁺, was observed at higher copper concentrations with formation constant K_d = [Cu₂(HL)₂]/[Cu(HL)₂][Cu] 1.7 × 10⁴ l mol^?1 in 0.04 M NaCIO₄. The rate of reaction of Cu(II) with H₂L to form Cu(HL)₂ obeys the rate equation, d[Cu(HL)₂]/d t = K_rate[Cu]¹[H₂L]¹[H⁺]⁰ where K_rate has the value of 2.9 × 10⁴ l mol^?1 s^-1 in the absence of added electrolytes.     

Kinetic study of the chlorination of gallium oxide

The kinetics of the chlorination of gallium oxide in chlorine atmosphere was studied between 650 °C and 800 °C. The calculations of the Gibbs standard free energy variation with temperature for the reaction Ga₂O_3(S)+3Cl_{2 (g)}→2GaCl_3(g)+1.5O_{2 (g)} show that direct chlorination is favorable above 850 °C. Thermogravimetric experiments were performed under isothermal and nonisothermal conditions. The effect of temperature, gas flow rate, and Cl₂ partial pressure were studied. The solids were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The nonisothermal results showed that chlorination of Ga₂O₃ starts at approximately 650 °C, with a mass loss of 50 pct at 850 °C. The isothermal results between 650 °C and 800 °C indicated that the reaction rate increased with temperature. The correlation of the experimental data with different solid-gas reaction models showed that the results are adequately represented by the model proposed by Shieh and Lee: X=1−{1−b ₂₂[b ₂₁ t+e ^−b ₂₁ ^t−1]}^1/(1−γ). From this model, it was found that the rate of reaction for the chlorination of Ga₂O₃ is of the order 0.68 with respect to Cl₂ and the activation energy is 113.23 kJ/mol. On the other hand, the order of the activation rate of the interface surface is 0.111 with respect to Cl₂ and its activation energy is 23.81 kJ/mol.     

Dissolution of zinc ferrite samples in acids

The dissolution of rotating discs of synthetic zinc ferrite — the principal constituent of the ‘Moore Cake’ residue in zinc extraction plants — was studied in mineral acids, particularly in 1–5 N H₂SO₄ at 70–99°C. This dissolution was found to be directly proportional to the surface area, and the order of the zinc ferrite-sulphuric acid reaction with respect to proton activity, [H⁺], to be 0.6. The apparent energy of activation was established as 15 kcal/mole, and the chemical reaction on the solid surface as the rate-controlling step.What appeared to be ‘non-stoichiometric’ or preferential dissolution of zinc (over iron) from zinc ferrite was observed during the initial stages of reaction. This was attributed to the existence of trace amounts (undetectable by X-ray methods) of unreacted zinc oxide grains in the zinc ferrite matrix. This is, to our knowledge, the first time that electron microprobe analysis has been used to identify and analyse these grains. Prolonged sintering at 1200°C for 48 hours eliminated the ZnO phase.Dissolution of zinc ferrite in acid is stoichiometric. A typical dissolution rate is ～ 10^?8 mol cm^?2 sec^?1, which corresponds to almost complete extraction of zinc from ‘Moore Cake’ particles in 2–5 N H₂SO₄ solution at 95°C in 1–2 hours.     

Cuprous hydrometallurgy Part IV. Rates and equilibria in the reaction of copper sulphides with copper(II) sulphate in aqueous acetonitrile

Copper(II) sulphate in solutions of aqueous acetonitrile leaches copper from copper sulphides to form stable copper(I) sulphate solutions. Covellite and chalcopyrite are oxidised and leached more rapidly in the early stages of leaching with acidic CuSO₄/CH₃CN/H₂O than with acidic iron(III) sulphate in water. A redox equilibrium between copper(I) sulphate, copper(II) sulphate and partially leached solid copper sulphide, Cu_xS, is established. The equilibrium concentration of Cu₂SO₄ and the value of x in Cu_xS, in solutions saturated with CuSO₄, are interdependent if the acetonitrile concentration is constant. This behaviour is considered in terms of the structural and electrochemical changes which occur, in the solids Cu₂S and CuS, as leaching proceeds. According to the activities of acetonitrile, of copper(I) sulphate and of copper(II) sulphate, i.e. according to the redox potential of the solution, CuS either can be oxidised by copper(II) sulphate to a less copper-rich copper sulphide and even to sulphur, or reduced by copper(I) sulphate to a more copper-rich sulphide, up to Cu₂S, in acidic solutions containing CuSO₄, Cu₂SO₄, CH₃CN and water. This observation leads to an easy method of generating Cu₂S from CuS or from sulphur.     

Kinetics of the chlorination of Cu2S with Cl2 gas

Abstract

When Cu₂S is chlorinated with Cl₂ gas, CuS, CuCl and CuCl₂ are observed at various stages of the reaction. It is proposed that CuCl₂ and CuS are the first products, which then react to produee CuCl and elemental sulphur. The elemental sulphur is then chlorinated to S₂Cl₂, which acts as a liquid bridge for the transport of dissolved sulphur away from the reaction interface. CuCl₂ forms only when all of the sulphide has been eliminated, and the partial pressure of chlorine in the CuCl₂ layer can exceed that for the stability of CuCl. The activation energy proposed for the diffusion of sulphuralong the gradient in a pore filled with S₂Cl₂ is 8.6 ± 0.5 kcal.

Résumé

Quand Cu₂S est chloruré par Cl₂ gaz, on observe CuS, CuCl et CuCl₂ à divers stades de la réaction. L’hypothèse proposée est que le CuCl₂ et le CuS sont les premiers produits formés qui réagiront ensuite pour produire CuCl et S. Le soufre élémentaire est ensuite chloruré en S₂Cl₂, et agit alors comme un pont liquide pour éloigner le S dissout de l’interface de réaction. CuCl₂ se forme quand tout le sulfure a été éliminé, et la pression partielle de Cl₂ dans la couche de CuCl₂ peut excéder celle correspondant àCuCl stable. L’énergie d’activation proposée par la diffusion du soufre dans S₂Cl₂est 8.6 ± 0.5 kcal.     

Aqueous Oxidation of Iron Monosulfide (FeS) by Molecular Oxygen

Aqueous oxidation of iron monosulfide (FeS) by oxygen at initial pH between 2.5 and 5 was investigated in a closed system at different temperatures (25, 35, and 40°C). It was found that the rate of aqueous oxidation of FeS increases when initial [H⁺] and temperature increase. The reaction order with respect to [H⁺] was 0.16 ± 0.02 at 25°C. The activation energy was found to be 23 ± 5 kJ mol^–1 at initial pH 2.5. This value suggests that aqueous oxidation of FeS by oxygen is controlled by a mixed regime of diffusion and surface reaction control. FTIR analysis of the initial and reacted FeS samples has shown that during the aqueous oxidation of FeS by O₂ a sulfur-rich layer is formed on the mineral surface. The experimental results indicate that the protons adsorb on the mineral surface and catalyze Fe²⁺ release into solution (by Couloumbic repulsion) and S(-II) oxidation to higher oxidation states.     

Substitution Recovery of Gold from Copper-Ethylenediamine-Thiosulfate Leaching Solution with Copper Power

The substitution method of recovering gold from thiosulfate-ethylenediamine (en)-Cu²⁺ leaching solution using copper powder was studied. The effects of reaction time, stirring speed, pH, thiosulfate concentration, en/Cu²⁺ molar ratio, Cu/Au⁺ mass ratio, and temperature on gold recovery were systematically examined. The experimental results showed that reaction time, stirring speed, thiosulfate concentration, en/Cu²⁺ molar ratio, Cu/Au⁺ mass ratio, and temperature have a significant influence on the recovery rate of gold, whereas the pH has little effect. A high gold recovery rate of 95.38% was achieved in 0.2 mol/L thiosulfate at 40°C after 40 min with a stirring speed of 400 rpm, pH of 11, en/Cu²⁺ molar ratio of 6, and Cu/Au⁺ mass ratio of 150. A kinetic study revealed that the reduction of gold-thiosulfate complex ions (Au(S₂O₃) ₂ ^3- ) on the surface of copper powder follows a first-order kinetics model with an apparent activation energy of 39.82 kJ/mol.     

Active oxygen species and oxidation mechanism over Ce-doped LaMn_(0.8)Ni_(0.2)O_3/hierarchical ZSM-5 in pentanal oxidation

Hierarchical ZSM-5(HZ) molecular sieves based on fly ash were synthesized using a method combining water heat treatment with step-by-step calcination.The coupling catalysts between La_(1-x)Ce_xMn_(0.8)-Ni_(0.2)O_3(x ≤ 0.5) perovskites and HZ were prepared through the impregnation method,which were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),high-resolution transmission electron microscopy(HRTEM),N_2 adsorption,X-ray photoelectron spectroscopy(XPS),NH_3-temperature programmed desoprtion(NH_3-TPD),H_2-temperature programmed reduction(H_2-TPR) and O_2-TPD techniques and investigated regarding pentanal oxidation at 120-390℃ to explore the effects of Ce doping on the catalytic activity and the active oxygen species of the coupling catalysts,meanwhile,the reaction mechanism and pathway of pentanal oxidation were also studied.The results reveal that Ce substitution at La sites can change the electronic interactions between all the elements and promote the electronic transfer among La,Ce,Ni,Mn and HZ,influencing directly the physicochemical characteristics of the catalysts.Moreover,the amount and transfer ability of surface adsorbed oxygen(O_2~-and O~-)regarded as the reactive oxygen species and the low temperature reducibility are the main influence factors in pentanal oxidation.Additionally,La_(0.8)Ce_(0.2)Mn_(0.8)Ni_(0.2)O_3/HZ exhibits the best catalytic activity and deep oxidation capacity as well as a better water resistance due to its larger amount of surface adsorbed oxygen species and higher low temperature reducibility.What's more,appropriate Ce substitution can significantly enhance the amount of O_2~-ions,which can distinctly enhance the catalytic activity of the catalyst,and moderate acid strength and appropriate acid amount can also facilitate the improvement of the pentanal oxidation activity.It is found that there is a synergic catalytic effect between surface acidity and redox ability of the catalyst.According to the in situ DRIFTS and GC/MS analyses,pentanal can be oxidized gradually to CO_2 and H_2 O by the surface oxygen species with the form of adsorption in air following the Langmuir-Hinshelwood(L-H) reaction mechanism.Two reaction pathways for the pentanal oxidation process are proposed,and the conversion of the formates to carbonates may be one of the main rate-determining steps.     

Kinetics of the sulfidation of chalcopyrite with gaseous sulfur

The sulfidation of chalcopyrite with gaseous sulfur in the temperature range of 325 °C to 400 °C occurs with the formation of covellite and pyrite as the final products. The rate of sulfidation depends strongly on the temperature, with nearly complete conversion in less than 30 minutes at 400 °C. Microscopic analysis of partially and completely reacted particles showed that the sulfidation proceeded topochemically, with a shrinking core of unreacted chalcopyrite surrounded by successive layers of FeS₂ and CuS. The experimental data exhibited an induction period at the beginning of the reaction. An electrochemical mechanism is proposed for the sulfidation reaction, which involves simultaneous diffusion of Cu⁻ and electrons through the product layers. The rate data showed that the fraction reacted is well represented by a shrinking-core model controlled by the reaction occurring at the chalcopyrite-pyrite interface, resulting in the conversion-vs-time relationship 1−(1−X)^1/3=k(t−t _ind). An activation energy of 98.4 kJ/mol was determined for the temperature range of 325 °C to 400 °C.     

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.     

Kinetics of carbochlorination of chromium (III) oxide

Kinetics of the carbochlorination of Cr₂O₃ has been studied with Cl₂+CO gas mixtures between 500 °C to 900 °C using thermogravimetric analysis. The apparent activation energy is about 100 kJ/mol. Mathematical fitting of the experimental data suggests that the shrinking sphere model is the most adequate to describe the carbochlorination mechanism of chromium oxide and that is controlled by the chemical reaction. In the temperature range of 550 °C to 800 °C, the reaction order is about 1.34 and is independent of temperature. Changing the Cl₂+CO content from 15 to 100 pct increases the reaction rate and does not affect the reaction mechanism. Similarly, changing the ratio of Cl₂/(Cl₂+CO) from 0.125 to 0.857 does not modify the carbochlorination mechanism of Cr₂O₃. In these conditions, the reaction rate passes through a maximum when using a chlorinating gas mixture having a Cl₂/(Cl₂+CO) ratio of about 0.5.     

Vacuum Carbothermic Production of Aluminum and Al-Si Alloys From Kaolin Clay: A Thermodynamic Study

Examination of the thermodynamic constraints for the carbothermic reduction of iron-free kaolinite, Al₂Si₂O₅(OH)₄, or of its calcination product mullite, Al₆Si₂O₁₃, either at atmospheric pressure or under vacuum of 10^?3 to 10^?5 bar, indicates the conditions required at equilibrium to produce either elementary Al or Al-Si alloys. At atmospheric pressure, a very high temperature of 3200 K would be required to obtain from Al₂Si₂O₅(OH)₄ + 9C an Al-Si alloy with 39 wt.% Si. At 10^?4 bar and 1800 K, the predicted Al-Si alloy would contain 2.4 wt.% Si. From mullite, the reaction of Al₆Si₂O₁₃ + 13C at 10^?4 bar and either 1800 K or 2200 K should produce an Al-Si alloy with 0.65 or 24 wt.% Si. The CO produced by the carbothermic reactions may be by water-gas shift converted to syngas, and further either to methanol or by a Fischer–Tropsch reaction to liquid fuels or chemical intermediates. Concentrated solar energy may be used to supply the required process heat of these high-temperature reactions.