Kinetics of alkaline decomposition and cyanidation of argentian ammonium jarosite in lime medium

The alkaline decomposition of argentian ammonium jarosite in lime medium is characterized by an induction period and a conversion period in which the sulfate and ammonium ions pass to the solution whereas calcium is incorporated in the residue jointly with iron; this residue is amorphous in nature. The process is chemically controlled and the order of reaction with respect to the hydroxide concentration is 0.4; the activation energy is 70 kJ mol⁻¹. Cyanidation of argentian ammonium jarosite in lime medium presents the same reaction rate in the range of 0–10.2 mol m⁻³ CN⁻; in this range of concentration, the cyanide process can be described, as in other jarosites, in a two-step process: a step of alkaline decomposition that controls the overall process followed by a fast step of silver complexation. For higher cyanide concentration, the order of reaction with respect to cyanide is 0.65, and kinetic models of control by chemical reaction and diffusion control through the products layer both fit well; the activation energy obtained is 29 kJ mol⁻¹; this is indicative of a mixed control of the cyanidation process in the experimental conditions employed. The process is faster than was observed in ammonium jarosite generated in zinc hydrometallurgy (Industrial Minera México, San Luís Potosí, México); it seems that the reaction rate decreases when the substitution level in the jarosite lattice increases; this behavior is similar to that observed for synthetic potassium jarosite and arsenical potassium jarosite from gossan ores (Rio Tinto, Spain) presented in a previous paper.     

Characterization and alkaline decomposition–cyanidation kinetics of industrial ammonium jarosite in NaOH media

A complete characterization was carried out on a jarositic residue from the zinc industry. This residue consists of ammonium jarosite, with some contents of H₃O⁺, Ag⁺, Pb²⁺, Na⁺ and K⁺ in the alkaline “sites” and, Cu²⁺ and Zn²⁺ as a partial substitution of iron. The formula is: [Ag_0.001Na_0.07K_0.02Pb_0.007(NH₄)_0.59(H₃O)_0.31]Fe₃(SO₄)₂(OH)₆. Some contents of franklinite (ZnO^·Fe₂O₃), gunninguite (ZnSO₄·H₂O) and quartz were also detected. The jarosite is interconnected rhombohedral crystals of 1–2 μm, with a size distribution of particles of 2–100 μm, which could be described by the Rosin–Rammler model.The alkaline decomposition curves exhibit an induction period followed by a progressive conversion period; the experimental data are consistent with the spherical particle with shrinking core model for chemical control. The alkaline decomposition of the ammonium jarosite can be shown by the following stoichiometric formula:NH₄Fe₃(SO₄)₂(OH)_6(s)+3OH_(aq)⁻→(NH)_4(aq)⁺+3Fe(OH)_3(s)+2SO_4(aq)²⁻.The decomposition (NaOH) presents an order of reaction of 1.1 with respect to the [OH⁻] and an activation energy of 77 kJ mol⁻¹. In NaOH/CN⁻ media, the process is of 0.8 order with respect to the OH⁻ and 0.15 with respect to the CN⁻. The activation energy was 46 kJ mol⁻¹. Products obtained are amorphous. Franklinite was not affected during the decomposition process. The presence of this phase is indicative that the franklinite acted like a nucleus during the ammonium jarosite precipitation.     

Decomposition kinetics of industrial jarosite in alkaline media for the recovery of precious metals by cyanidation

In the present study, the aqueous-slurry decomposition kinetics of industrial jarosite in alkaline media for the recovery of silver by cyanidation was investigated. For this purpose, aqueous-slurry decomposition experiments, using both NaOH and Ca(OH)₂ as alkalinising agents, were carried out in order to (1) study the effect of pH (i.e. 8, 9, 10 and11), contact time and temperature (i.e. 30, 40, 60 and 70°C) on jarosite decomposition; (2) elucidate the rate-determining step of the process kinetics when using NaOH or Ca(OH)₂, by applying the shrinking core model and Arrhenius equation and (3) study the effect of the aqueous-slurry decomposition on the recovery of silver by cyanidation. Results showed that when NaOH was used, the decomposition process was controlled by the chemical reaction with an activation energy of 40.42?kJ?mol^?1, whereas when Ca(OH)₂ was used, the decomposition was controlled by diffusion through a porous layer of CaCO₃ with an activation energy of 21.72?kJ?mol^?1. The alkaline decomposition emerges as a necessary step in order to recover up to 74% of the silver contained in the jarosite by cyanidation.     

Kinetics of Alkaline Decomposition and Cyaniding of Argentian Rubidium Jarosite in NaOH Medium

The alkaline decomposition of Argentian rubidium jarosite in NaOH media is characterized by an induction period and a progressive conversion period in which the sulfate and rubidium ions pass to the solution, leaving an amorphous iron hydroxide residue. The process is chemically controlled and the order of reaction with respect to hydroxide concentration in the range of 1.75 and 20.4?mol OH^? m^?3 is 0.94, while activation energy in the range of temperatures of 298?K to 328?K (25?°C to 55?°C) is 91.3?kJ mol^?1. Cyaniding of Argentian rubidium jarosite in NaOH media presents a reaction order of 0 with respect to NaCN concentration (in the range of 5 to 41?mol m^?3) and an order of reaction of 0.62 with respect to hydroxide concentration, in the range of 1.1 and 30?mol [OH^?] m^?3. In this case, the cyaniding process can be described, as in other jarosites, as the following two-step process: (1) a step (slow) of alkaline decomposition that controls the overall process followed by (2) a fast step of silver complexation. The activation energy during cyaniding in the range of temperatures of 298?K to 333?K (25?°C to 60?°C) is 43.5?kJ mol^?1, which is characteristic of a process controlled by chemical reaction. These results are quite similar to that observed for several synthetic jarosites and that precipitated in a zinc hydrometallurgical plant (Industrial Minera México, San Luis Potosi).     

Cyanidation kinetics of silver telluride (Ag2Te)

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

Leaching of gold and palladium with aqueous ozone in dilute chloride media

The recycling of gold and palladium from metallic scraps can be carried out by ozone-leaching at ambient temperature and low (∼0.1 M) H⁺ and Cl⁻ concentrations. Rh and Pt remain un-reacted, whereas metals such as Cu, Ni, Ag, can be previously eliminated through O₂/H⁺ and O₂/O₃/H⁺ leaching pretreatments. Gold and palladium are dissolved in O₃/Cl⁻/H⁺ with formation of AuCl₄⁻ and PdCl₄²⁻. Leaching studies showed a passive region, basically located at < 0.01 and < 0.05 M Cl⁻ for Au and Pd, respectively. In the non-passive region, rates were only slightly dependent on either H⁺ and Cl⁻. Secondary formation of chlorine or hypochlorous acid was negligible at ≤ 0.1 M Cl⁻. Kinetics appeared to be controlled by mass transfer of O_3(aq) to the solid–liquid interface, showing first order dependency with respect to [O₃]_aq. Rates increased with temperature up to about 40 °C, but decreased at higher temperatures due to the fall in the O₃ solubility. The ozone mass transfer coefficients showed an activation energy < 20 kJ/mol. Gold leaching rate gradually diminished for pH > 2, as consequence of the influence of the [H⁺] on transfer control. The electric power consumption associated with O₃ generation was in the range 4–8 kWh/kg metal leached.     

Synthesis, Thermodynamic, and Kinetics of Rubidium Jarosite Decomposition in Calcium Hydroxide Solutions

Rubidium jarosite was synthesized as a single phase by precipitation from aqueous solution. X-ray diffraction and scanning electron microscopy energy-dispersive spectrometry analysis showed that the synthetic product is a solid rubidium jarosite phase formed in spherical particles with an average particle size of about 35???m. The chemical analysis showed an approximate formula of Rb_0.9432Fe₃(SO₄)_2.1245(OH)₆. The decomposition of jarosite in terms of solution pH was thermodynamically modeled using FACTSage by constructing the potential pH diagram at 298?K (25?°C). The E-pH diagram showed that the decomposition of jarosite leads to a goethite compound (FeO·OH) together with Rb⁺ and $ {\text{SO}}_{4}^{2 - } $ ions. The experimental Rb-jarosite decomposition was carried out in alkaline solutions with five different Ca(OH)₂ concentrations. The decomposition process showed a so-called ??induction period?? followed by a progressive conversion period where Rb⁺ and $ {\text{SO}}_{4}^{2 - } $ ions formed in the aqueous solutions, whereas calcium was incorporated in the solid residue and iron gave way to goethite. The kinetic analysis showed that this process can be represented by the shrinking core chemically controlled model with a reaction order with respect to Ca(OH)₂ equals 0.4342 and the calculated activation energy is 98.70?kJ mol^?C1.     

Kinetics of aluminum can dissolution in sec-butyl alcohol for aluminum sec-butoxide

A kinetic study of dissolution reaction of Al can was conducted for the synthesis of aluminum sec-butoxide (ASB). With the Al can scraps and sec-butyl alcohol (SBA) as reactants, the reaction was examined at the condition of 3 mol SBA/mol Al of stoichiometric ratio, adding 10^− 3 mol HgI₂/mol Al for catalyst and no agitation at the reaction temperature ranging from 80 to 100 °C. After the dissolution of 24 h at 100 ^oC, the reaction gave a 75% yield. A two-stage dissolution mechanism was proposed in which the dissolution rate is determined first by a chemical reaction and then by ash layer diffusion as the previous dissolution kinetics for the synthesis of AIP (Aluminum iso-propoxide) (Yoo, S.-J.,Yoon,H.-S., Jang, H.D., Lee,M.-J., Lee, S.-I.,Hong, S.-T., Park,H.S., 2007a. Dissolution kinetics of aluminum can in isopropyl alcohol for aluminum isopropoxide. Chem. Eng. J. 133, 79–84.). On the basis of the shrinking core model with the shape of flat plate, the first dissolution rate of Al can was controlled by chemical reaction. The concentration of SBA was largely changed during the dissolution reaction because it was added the stoichiometric ratio to the reactor. Therefore it was included as an integral term of the reaction time. By using the Arrhenius expression, the apparent activation energy of the first chemical reaction step was determined to be 200.5 kJ mol^− 1. In the second stage, the dissolution rate is controlled by diffusion control through the ash layer. The apparent activation energy of the second step was determined to be 101.8 kJ mol^− 1.     

The interaction of hydrogen with the interface of AI2O3 particles in iron

A mathematical model to calculate the trap binding energy and trap density is suggested considering the theories of hydrogen trapping and hydrogen retrapping. When iron containing 2.0 wt pct Al₂O₃ is heated with a uniform heating rate of 3 K-min^-1, a hydrogen peak is observed at 853 K in the evolution ratevs temperature plot. This is due to hydrogen evolution from the Al₂O₃/lattice interface. The trap activation energy and trap binding energy of hydrogen at the Al₂O₃/lattice interface are estimated as 79 kJ ⋅ mol^-1 and 71.4 kJ ⋅ mol^-1, respectively, fitting experimental data to the model. This indicates that the Al₂O₃/lattice interface acts as an irreversible trapping site for hydrogen. By combining the trap binding energy and trap activation energy, the energy level of hydrogen around the Al₂O₃/lattice interface is suggested. The saddle point energy of hydrogen at Al₂O₃/lattice interface, 7.56 kJ ⋅ mol^-1 is nearly equivalent to the activation energy for hydrogen diffusion through a normal lattice, 6.9 kJ ⋅ mol^-1. Formerly Graduate Student, Korea Advanced Institute of Science and Technology.     

Characterization and alkaline decomposition/cyanidation of beudantite–jarosite materials from Rio Tinto gossan ores

Jarosite-type minerals are the major silver carriers in the gossan ores from Rio Tinto (Spain). Two types of minerals were found: one corresponding to beudantite variable enriched in sulfate; the other is potassium jarosite containing various amounts of arsenate and lead. They are isostructural with cell parameters intermediate between those reported for end members. Silver is present in both jarosites as dilute solid solution (230 ppm Ag in average). The cyanidation of potassium jarosite in saturated Ca(OH)₂ at 70–100°C consists of two step in series: a slow step of alkaline decomposition followed by a fast step of Ag complexation from the decomposition solids. The alkaline decomposition is characterized by the simultaneous removal of sulfate and K ions and the formation of an amorphous hydroxy-arsenate of Fe, Pb and Ca. The kinetics are chemically controlled, with an activation energy of 86.5 kJ mol⁻¹. The nature of the alkaline decomposition of beudantite was similar but extremely slow at ≤100°C.     

Kinetics of leaching selenium from Ni-Mo ore smelter dust using sodium chlorate in a mixture of hydrochloric and sulfuric acids

The kinetics of leaching selenium from Ni-Mo ore smelter dust in H₂SO₄-HCl-H₂O system was investigated. The effects including leaching temperature and time, particle size of the smelting dust, stirring speed, acid concentration and the coefficient β (the molar ratio of sodium chlorate to selenium in the smelter dust) on leaching of selenium were studied. The results indicated that the leaching of selenium increased sharply with the increase of temperature. The leaching of selenium reached 98% at 95 °C and stirring speed of 350 rpm for 150 min with the particle size of − 0.15 mm, initial [H⁺] concentration of 8 mol/L, the solid/liquid ratio of 1:5 g/mL and the coefficient β of 3.33. The leaching process was controlled by the surface chemical reaction and the kinetics of leaching selenium from Ni-Mo ore smelter dust followed the model of “shrinking core”. The apparent activation energy of leaching selenium was determined to be 44.4 kJ/mol, which was consistent with the values of activation energy reported for the surface chemical reaction control. The kinetics equation of leaching selenium from Ni-Mo ore smelter dust was expressed as , which coincided with the experimental results.     

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

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

Enthalpies of formation of liquid (copper + manganese) alloys

The enthalpies of formation of liquid (Cu + Mn) alloys were measured in the isoperibolic heat-flux calorimeter at 1573 K in the entire range of compositions. The integral molar enthalpy of mixing was found to be negative in the range of molar fractions 0 < x _Mn < 0.31, with ΔH(min) = −0.69 ± 0.27 kJ mol⁻¹ at x _Mn = 0.12, and positive in the range 0.31 < x _Mn < 1, with ΔH(max) = 3.67 ± 0.36 kJ mol⁻¹ at x _Mn = 0.75. Limiting partial molar enthalpies of manganese and copper were calculated as = −18.0 ± 6.6 kJ mol⁻¹ and = 29.1 ± 4.9 kJ mol⁻¹, respectively. The results are discussed in comparison with the thermodynamic data available in the literature and the equilibrium phase diagram.     

Jarosite pseudomorph formation from arsenopyrite oxidation using Acidithiobacillus ferrooxidans

In this work, the oxidizing action of a native strain type A. ferrooxidans on a sulphide containing a predominance of arsenopyrite and pyrite has been evaluated. Incubation of the A. ferrooxidans strain in flasks containing 200 mL of T&K medium with the ore (particle size of 106 μm) at pulp density 8% (w/v) at 35 °C on a rotary shaker at 200 rpm resulted in preferential oxidation of the arsenopyrite and the mobilization of 88% of the arsenic in 25 days. Mineralogical characterization of the residue after biooxidation was carried out with FTIR, XRD and SEM/XEDS techniques. An in situ oxidation of the arsenopyrite is suggested on the basis of the frequent appearance of jarosite pseudomorph replacing arsenopyrite, in which the transformations Fe²⁺ → Fe³⁺, S^− 2 → S^+ 6 and As^− 1 → As^+ 3 → As^+ 5 occur for the most part without formation of soluble intermediates, resulting in a type of jarosite that typically contains high concentrations of arsenic (type A-jarosite). However, during pyrite oxidation, dissolution of the constituent Fe and S predominates, which is evidenced by corrosion of pyrite particles with formation of pits, generating a type of jarosite with high quantities of K (type B-jarosite). Lastly, a third type of jarosite (type C-jarosite) also precipitated forming a thin film that covered the grains of pyrite principally.     

The leaching kinetics of chalcopyrite (CuFeS2) in ammonium lodide solutions with iodine

The leaching kinetics of chalcopyrite (CuFeS₂) in ammonium iodide solutions with iodine has been studied using the rotating disc method. The variables studied include the concentrations of lixiviants, rotation speed, pH of the solution, reaction temperature, and reaction product layer. The leaching rate was found to be independent of the disc rotating speed. The apparent activation energy was measured to be about 50 kJ/mole from 16 °C to 35 °C, and 30.3 kJ/mole from 35 °C to 60 °C. The experimental findings were described by an electrochemical reaction-controlled kinetic model: rate =k [NH₃]^0.69[OH⁻]^0.42[I ₃ ⁻ ]^0.5.     

Oxidation of Fe (II) in sulfuric acid solutions with dissolved molecular oxygen

The oxidation of Fe(II) with dissolved molecular oxygen was studied in sulfuric acid solutions containing 0.2 mol · dm⁻³ FeSO₄ at temperatures ranging from 343 to 363 K. In solutions of sulfuric acid above 0.4 mol · dm⁻³, the oxidation of Fe (II) was found to proceed through two parallel paths. In one path the reaction rate was proportional to both [Fe⁻²⁺]² andp _{o 2} exhibiting an activation energy of 51.6 · kJ mol⁻¹. In another path the reaction rate was proportional to [Fe²⁺]², [SO ₄ ^{2 −}], andp _{o 2} with an activation energy of 144.6 kJ · mol⁻¹. A reaction mechanism in which the SO ₄ ^{2 −} ions play an important role was proposed for the oxidation of Fe(II). In dilute solutions of sulfuric acid below 0.4 mol · dm⁻³, the rate of the oxidation reaction was found to be proportional to both [Fe(II)]² andp _{o 2}, and was also affected by [H⁺] and [SO ₄ ^{2 −}]. The decrease in [H⁺] resulted in the increase of reaction rate. The discussion was further extended to the effect of Fe (III) on the oxidation reaction of Fe (II).     

Interdiffusion in Co-Mn alloys

Interdiffusion coefficient in cobalt-manganese alloys has been determined by Matano's method in the temperature range between 1133 and 1423 K on (pure Co)-(Co-30.28 at. pct Mn alloy) and (pure Co)-(Co-51.76 at. pct Mn alloy) couples. This, ∼D, has been found to increase with the increase of manganese content. However, the activation energy (∼Q) and frequency factor ( ₀) show a maximum at about 10 at. pct Mn. The concentration dependence of and has been discussed taking into account the thermodynamic properties of the alloy. The difference in between the ferro- and paramagnetic phases in Co-5 at. pct Mn alloy has been found to be 24 kJ/mol, which is larger, than that for the diffusion of Mn⁵⁴ in this alloy. Further it has been found that the Kirkendall marker moves toward manganese-rich side, showing that manganese atoms diffuse faster than cobalt atoms. From the marker shift, the intrinsic diffusion coefficients,D _Co andD _Mn, at 33 at. pct Mn have been determined as follows:D _Co=0.22×10⁻⁴ exp(−263 kJ mol⁻¹/RT) m²/s, andD _Mn=0.98×10⁻⁴ exp(−229 kJ mol⁻¹/RT) m²/s.     

An electrochemical investigation of the thermodynamic properties of the NaCl-AlCl3 system at subliquidus temperatures

The phase relations in the NaCl-AlCl₃ system ( ) have been determined in the temperature range from 373 to 623 K by isothermal equilibration, electrical conductivity, and electromotive force measurements. Only one ternary compound, NaAlCl₄, was found to be stable, with a melting point of 426 K. The standard Gibbs energy of formation of NaCl and NaAlCl₄ has been measured in the temperature range from 423 to 623 K by a novel galvanic cell technique involving in-situ electrogenerated chlorine electrode in the Na/β″-alumina/NaCl, NaAlCl₄/Cl₂,C and Al/NaCl, NaAlCl₄/Cl₂,C cells along with the Na/β″-alumina/NaCl,NaAlCl₄/Al cell. The Δ _f G _NaCl(s ^)/o and values have been calculated as −412.4+0.095 T (±1) kJ mol⁻¹ and −1117.5+0.2460 T (±2) kJ mol⁻¹, respectively. The standard entropy of NaAlCl₄ (s) at 298 K, computed from the results of the study and the auxiliary information from the literature (184 J K⁻¹ mol⁻¹), show good agreement with the estimated JANAF value (188.28 J K⁻¹ mol⁻¹). The enthalpy of formation of NaAlCl₄ (l) from NaCl (s) and AlCl₃ (s) at 550 K obtained in the present study (−1850 J mol⁻¹) is in agreement with that computed from the heat-capacity measurements (−1910 J mol⁻¹). The present measurements are unique, as a new electrochemical technique is employed in a cell with low-melting sodium chloroaluminate electrolyte to obtain the thermodynamic properties of NaCl and NaAlCl₄ at significantly low temperatures. The Gibbs energy of formation of NaCl (s) is, thus, measured at temperatures as low as 423 K by an electrochemical technique for the first time, in this work.     

Hematite formation from jarosite type compounds by hydrothermal conversion

Abstract

The hydrothermal conversion of K jarosite, Pb jarosite, Na jarosite, Na–Ag jarosite, AsO₄ containing Na jarosite and in situ formed K jarosite and Na jarosite to hematite was investigated. Potassium jarosite is the most stable jarosite species. Its conversion to hematite in the absence of Fe₂O₃ seed occurred only partially after 5 h reaction at >240°C. In the presence of Fe₂O₃ seed, the conversion to hematite was nearly complete within 2 h at 225°C and was complete at 240°C. The rate of K jarosite precipitation, in situ at 225°C in the presence of 50 g L^?1 Fe₂O₃ seed, is faster than its rate of hydrothermal conversion to hematite. In contrast, complete conversion of either Pb jarosite or Na–Pb jarosite to hematite and insoluble PbSO₄ occurs within 0·75 h at 225°C in the presence of 20 g L^?1 Fe₂O₃ seed. Dissolved Fe(SO₄)_1·5 either inhibits the conversion of Pb jarosite or forms Pb jarosite from any PbSO₄ generated. The hydrothermal conversion of Na–Ag jarosite to hematite was complete within 0·75 h at 225°C in the presence of 20 g L^?1 Fe₂O₃ seed. The Ag dissolved during hydrothermal conversion and reported to the final solution. However, the presence of sulphur or sulphide minerals caused the reprecipitation of the dissolved Ag. The conversion of AsO₄ containing Na jarosite at 225°C in the presence of 20 g L^?1 Fe₂O₃ seed was complete within 2 h, for H₂SO₄ concentrations <0·4M. Increasing AsO₄ contents in the Na jarosite resulted in a linear increase in the AsO₄ content of the hematite, and ～95% of the AsO₄ remained in the conversion product. Increasing temperatures and Fe₂O₃ seed additions significantly promote the hydrothermal conversion of in situ formed Na jarosite at 200–240°C. However, the conversion of previously synthesised Na jarosite seems to proceed to a greater degree than that of in situ formed Na jarosite.

On a examiné la conversion hydrothermale en hématite de la jarosite de K, de la jarosite de Pb, de la jarosite de Na, de la jarosite de Na-Ag, de la jarosite de Na contenant de l’AsO₄, et de la jarosite de K et de la jarosite de Na qui sont formées in situ. La jarosite de potassium est la plus stable des espèces de jarosite. Sa conversion en hématite ne se produisait que partiellement après 5 h de réaction à >240°C en l’absence d’amorce de Fe₂O₃. En présence d’amorce de Fe₂O₃, la conversion en hématite était presque complète à moins de 2 h à 225°C et était complète à 240°C. La vitesse de précipitation de la jarosite de K, in situ à 225°C en présence de 50 g L^?1 d’amorce de Fe₂O₃, est plus rapide que sa vitesse de conversion hydrothermale en hématite. Par contraste, la conversion complète soit de la jarosite de Pb ou de la jarosite de Na-Pb en hématite et en PbSO₄ insoluble se produit à moins de 0·75 h à 225°C en présence de 20 g L^?1 d’amorce de Fe₂O₃. Le Fe(SO₄)_1·5 dissous soit inhibe la conversion de la jarosite de Pb ou forme de la jarosite de Pb à partir de tout PbSO₄ produit. La conversion hydrothermale de la jarosite de Na-Ag en hématite était complète à moins de 0·75 h à 225°C en présence de 20 g L^?1 d’amorce de Fe₂O₃. L’Ag se dissolvait lors de la conversion hydrothermale et se rapportait dans la solution finale. Cependant, la présence de soufre ou de minéraux sulfurés avait pour résultat la re-précipitation de l’Ag dissous. La conversion de la jarosite de Na contenant de l’AsO₄ à 225°C en présence de 20 g L^?1 d’amorce de Fe₂O₃ était complète à moins de 2 h, avec des concentrations d’H₂SO₄ <0·4 M. L’augmentation de la teneur en AsO₄ de la jarosite de Na avait pour résultat une augmentation linéaire de la teneur en AsO₄ de l’hématite et ～95% de l’AsO₄ demeurait dans le produit de conversion. L’augmentation de la température et d’additions d’amorce de Fe₂O₃ favorisait significativement la conversion hydrothermale de la jarosite de Na qui est formée in situ à 220–240°C. Cependant, la conversion de la jarosite de Na synthétisée antérieurement semblait se produire à un plus grand degré que celle de la jarosite de Na qui est formée in situ.     

Creep characteristics of single crystalline Ni3Al(Ta,B)

The creep characteristics, including the nature of the creep transient after a stress reduction and activation energy for creep of single crystalline Ni₃Al(Ta,B) in the temperature range 1083 to 1388 K, were investigated. An inverse type of creep transient is exhibited during stress reduction tests in the creep regime where the stress exponent is equal to 3.2. The activation energy for creep in this regime is equal to 340 kJ mol⁻¹. A normal type of creep transient is observed during stress reduction tests in the regime where the stress exponent is equal to 4.3. The activation energy for creep in this regime is equal to 530 kJ mol⁻¹. The different transient creep behavior and activation energies for creep observed in this investigation are consistent with the previous suggestion that then = 4.3 regime is associated with creep controlled by dislocation climb, whereas then = 3.2 regime is associated with a viscous dislocation glide process for Ni₃Al at high temperatures.