Kinetic model for the cyanidation of silver ammonium jarosite in NaOH medium

A synthesis of silver ammonium jarosite has been carried out obtaining a single-phase product with the formula: [(NH₄)_0.71(H₃O)_0.25Ag_0.040]Fe_2.85(SO₄)₂(OH)_5.50. The product consists on compact spherical aggregates of rhombohedral crystals. The nature and kinetics of alkaline decomposition and also of cyanidation have been determined. In both processes an induction period followed by a conversion period have been observed. During decomposition, the inverse of the induction period is proportional to [OH⁻]^0.75 and an apparent activation energy of 80 kJ mol^− 1 was obtained; during the conversion period, the process is of 0.6 order (OH⁻ concentration) and an activation energy of 60 kJ mol^− 1 was obtained. During cyanidation, the inverse of the induction period is proportional to [CN⁻]^0.5 and an apparent activation energy of 54 kJ mol^− 1 was obtained; during the conversion period the process is of 0 order (CN⁻ concentration) and an activation energy of 52 kJ mol^− 1 was obtained. Results obtained are consistent with the spherical particle model with decreasing core and chemical control, in the experimental conditions employed. For both processes and in the basis of the behaviour described, two mathematical models, including the induction and conversion periods, were established, that fits well with the experimental results obtained. Cyanidation rate of different jarosite materials in NaOH media have also been established: this reaction rate at 50 °C is very high for potassium jarosite, high and similar for argentojarosite and ammonium jarosite, lower for industrial ammonium jarosite and negligible for natural arsenical potassium jarosite and beudantite. These results confirm that the reaction rate of cyanidation decreases when the substitution level in the jarosite lattice increases.     

Rubidium jarosite and thallium jarosite — new synthetic jarosite-type compounds and their structures

Rubidium jarosite (RbFe₃(SO₄)₂(OH)₆) and thallium jarosite (TlFe₃(SO₄)₂(OH)₆) were synthesized as single phase products by precipitation from aqueous solution. Hydronium ion (H₃O⁺) substitutes for part of the “alkali” metal in these compounds. Both jarosites are hexagonal (R3m) and have similar unit cell dimensions. During heating rubidium jarosite undergoes two major decompositions; initially water is evolved and subsequently sulphur oxides are emitted. Thallium jarosite decomposes in three principal stages during programmed heating. The first two stages are similar to the decomposition of rubidium jarosite; the third decomposition involves the breakdown of thallium sulphate and the subsequent sublimation of thallous oxide.     

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.     

The co-precipitation of copper and zinc with lead jarosite

The formation of lead jarosite, Pb_0.5Fe₃(SO₄)₂(OH)₆, in the presence of dissolved copper and/or zinc results in a significant substitution of these metals in the jarosite phase; the co-precipitation is most pronounced in sulphate media but also occurs, to a lesser degree, in chloride solutions. The copper and/or zinc substitute for iron, and under extreme conditions the product approaches beaverite, Pb(Cu,Zn)Fe₂(SO₄)₂(OH)₆, in structure and composition. The extent of co-precipitation increases sharply with increasing concentrations of dissolved CuSO₄ or ZnSO₄ and slightly with either an increasing stoichiometric ratio of PbSO₄/Fe³⁺ or increasing ionic strength. The co-precipitation of copper or zinc is not significantly affected by acid concentration although the yield of product declines with increasing concentration of H₂SO₄. The extent of reaction is relatively insensitive to reaction temperatures in the range 130–180°C and to reaction times in excess of 2 h. Copper is strongly co-precipitated in preference to zinc from solutions containing both metals. Other divalent base metals such as Co, Ni and Mn are also co-precipitated with lead jarosite although not to the same degree as copper or zinc.     

Chemical conversions of osmium (VI) sulfite complexes in ammonia–sulfate solutions

It has been found that osmium (VI) sulfite complexes having composition [OsO₂(SO₃)₂(H₂O)₂]²⁻ in ammonia and sulfuric acid solutions enter into reaction of intrasphere substitution resulting in the formation of new complexes of osmium (VI). The rate of their formation depends on the concentration of constituents in the system and the temperature of solutions. Water-soluble ammonia–sulfite complexes of osmium (VI) (final form is [OsO₂(SO₃)₂(NH₃)₂]²⁻) are formed in ammonia-sulfate solutions. These complexes are converted into sulfate derivatives—water-soluble ([OsO₂(SO₃)(SO₄)(NH₃)₂]²⁻) and insoluble ([OsO₂(SO₄)₂(NH₃)₂]²⁻) in solutions containing (NH₄)₂SO₄ and H₂SO₄.     

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.     

Speciation of the Fe(II)–Fe(III)–H2SO4–H2O system at 25 and 50 °C

This work presents theoretical and experimental results on the speciation of the Fe(II)–Fe(III)–H₂SO₄–H₂O system in concentrated solutions (up to 2.2 m H₂SO₄ and 1.3 m Fe). The aim was to study the chemical equilibria of iron at 25 and 50 °C in synthetic aqueous sulphuric acid solutions that contain dissolved ferric and ferrous ion species. Raman spectroscopy, volumetric titration and conductivity measurements have been carried out in order to study the presence of specific ions and to characterize the ionic equilibrium. A thermochemical equilibrium model incorporating an extended Debye–Hückel relationship was used to calculate the activities of ionic species in solution. Model calculations were compared with experimental results. Model simulations indicate that anions, cations and neutral complexes coexisted in the studied system, where the dominant species were HSO₄⁻, H⁺, Fe²⁺ and FeH(SO₄)₂⁰. This indicated that these solutions showed a high buffer capacity due to the existence of bisulphate ions (HSO₄⁻), which presented the highest concentration. A decrease in the concentration of H⁺ and Fe³⁺ took place with increasing temperature due to the formation of complex species. Standard equilibrium constants for the formation of FeH(SO₄)₂⁰ were obtained in this work: log K_f⁰ = 8.1 ± 0.3 at 25 °C and 10.0 ± 0.3 at 50 °C.     

Crystal Chemistry of Iron in Non-Metamict Chevkinite-（Ce） ：Valence State and Site Occupation Proportions

Chevkiniteis one of the commonrare earth acces-sory minerals found in a wide occurrence of paragene-sis . About80analyses of samples in the chevkinitegroup have beenreportedto contain FeO(assuming∑Fe as FeO) rangingfrom7.11%to16·88%[1].How-ever ,in mine…     

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.     

The behavior of thallium during jarosite precipitation

Jarosite precipitation provides an effective means of eliminating thallium from zinc processing circuits, and a systematic study of the extent and mechanism of thallium removal during the precipitation of ammonium, sodium, and potassium jarosites was carried out. Thallium (as Tl⁺) substitutes for the “alkali” ion in the jarosite structure. Nearly ideal jarosite solid solutions are formed with potassium, but thallium is preferentially precipitated relative to either ammonium or sodium. Approximately 80 pct of the dissolved thallium precipitates during the formation of ammonium jarosite; the extent of thallium removal is virtually independent of thallium concentrations in the 0 to 3000 mg/L Tl range and of the presence of 75 g/L of dissolved Zn. Although the deportment of thallium is nearly independent of (NH₄)₂SO₄ or Na₂SO₄ concentrations >0.1 M, the precipitates made from more dilute media are relatively enriched in thallium. Likewise, the precipitates made from dilute ferric ion media are also Tl-rich. Low solution pH values or low temperatures both significantly reduce the amount of jarosite formed, but the precipitates made under these conditions are enriched in thallium. Furthermore, because thallium jarosite is more stable than the ammonium or sodium analogues, the initially formed precipitates are consistently Tl rich. The presence of jarosite seed accelerates the precipitation reaction, but dilutes the thallium content of the product. The results suggest that most of the thallium in a hydrometallurgical zinc circuit could be selectively precipitated in a small amount of jarosite, by carrying out the precipitation reaction for a short time in the absence of seed and from solutions having low alkali concentrations.     

Physico-chemical properties of copper electrolytes

A systematic study was undertaken to determine the diffusion coefficient(D) for Cu⁺² in the CuSO₄-H₂SO₄ system at different Cu and H₂SO₄ concentrations and temperatures. An empirical equation for predicting theD value was developed and checked for its validity. Conductivities, densities, and viscosities of copper electrolytes were measured in a wide range of Cu and H₂SO₄ concentrations and temperatures and the results are reported. Such information also was gen-erated for complex solutions containing the impurities Ni, Co, Fe⁺², Fe⁺³, and Mn. These prop-erties were calculated further using empirical equations and compared with the measured values.     

The action of sulfur trioxide on chalcopyrite

Chalcopyrite reacts readily with SO₃ at about 100°C to form water-soluble sulfates; the reaction is approximately: 3CuFeS₂+26SO₃→3CuSO₄+FeSO₄+Fe₂(SO₄)₃+25SO₂ The presence of about 4 pct O₂ in the gas phase greatly accelerates the reaction presumably due to the complete transformation of ferrous into ferric sulfate in an extremely porous form: 2CuFeS₂+17SO₃+1/2O₂→2CuSO₄+Fe₂(SO₄)₃+16SO₂ A stoichiometric mixture of SO₂+1/2O₂ behaves towards chalcopyrite in nearly the same way as SO₃ although only in the temperature range 350° to 700°C.     

Preparation and Electrorheological Property of Y4O（OH）9（NO3）-NH4NO3 Materials

The new electrorheological (ER) material, a particle material composed of Y4O(OH)9(NO3) and NH4NO3, was obtained.They display better ER performance.The shear stress of the suspension of Y4O(OH)9(NO3)(NH4NO3)2.8 material in dimethyl silicone oil reaches 1469 Pa at an electric field strength (E) of 4.2 kV·mm-1 and the shear rate (γ) of 150 s-1.The relative shear stress, τE/τ0 (τE and τ0 are the shear stresses at E=4.2 and 0 kV·mm-1, respectively), is up to 29, which is 19 times that of pure Y2O3 material.The dielectric and conductive property of the materials play important roles in the modification of the ER effect of the particle materials.The researches on these new ER materials are very useful for obtaining a better understanding on the mechanism of the ER effect and finding an ideal ER material.     

Cathodic deposition of copper in the presence of aqueous sulfurous acid,bivalent aquacobalt ion,or both using a stainless steel cathode and a graphite anode

A study on cathodic deposition of copper in acidic aqueous sulfate solution has been carried out using a stainless steel cathode and a graphite anode. The individual and the combined effects of added [H₂SO₃·aq] and [Co²⁺·aq] on cathode potential, current efficiency, crystal orientations, and deposit morphology have been investigated and are compared. The maximum decrease of ≿50 pct in cathode potential is more pronounced in the presence of ≿10.25 g/L of H₂SO₃ alone in the electrolyte than that (≿30 pct) in the presence of ≿100 ppm of added Co²⁺ (aq) alone; however, the presence of added Co²⁺ (aq) along with H₂SO₃ (aq) does not cause further decrease in cathode potential in comparison to that observed in the presence of only H₂SO₃ (aq) in the electrolyte. The current efficiency is found to decrease in the presence of [H₂SO₃·aq] in the range of 1.32 to 30.75 g/L or in the presence of added [Co²⁺·aq] in the range of ≿10 ppm to 600 ppm, while the decrease of about 4 pct in current efficiency is more pronounced in the presence of only H₂SO₃ (aq) in the electrolyte, it is about 2 pct in the presence of only added Co²⁺ (aq) in the same electrolyte. The addition of Co²⁺ (aq) to the electrolyte containing H₂SO₃ (aq) does not alter the current efficiency (94 pct) of copper at the cathode. The linear sweep voltammetry (LSV) method was used to study the effect of added [H₂SO₃·aq], [Co²⁺·aq], or both, on the copper deposition at the cathode. The presence of each of these two additives or both causes a depolarization effect; the extent of the depolarization depends on the concentration of H₂SO₃ (aq), Co²⁺ (aq), and the current density. X-ray diffraction (XRD) data suggest that there is a change in the order of the preferred crystal orientations (viz., from the (220) plane in the absence of added H₂SO₃ (aq) and Co²⁺ (aq) to the (111) plane in the presence of added H₂SO₃ (aq) and Co²⁺ (aq) in the electrolyte solution) due to a change in the preferred plane of relative crystal growth. Results of scanning electron microscopy (SEM) indicate that cathode deposits of better surface morphology due to small-sized crystallites are found in the presence of added H₂SO₃ (aq)+Co²⁺ (aq) in the electrolyte solution. 1000 ppm=1.0 g/L=1.0 × 10⁻³ kg dm⁻³     

Ammonia,oxidation leaching of chalcopyrite —reaction kinetics

The reaction for the ammonia, oxidation leaching of chalcopyrite, CuFeS₂ + 4NH₃ + 17/4 O₂ + 2 OH^- ⇌ Cu(NH₃)⁺² ₄2 + l/2Fe₂O₃ + 2 SO₄ + H₂O was studied using monosize particles in an intensely stirred reactor under moderate pressures to determine the important chemical factors which govern the kinetic response of the system. The reaction kinetics were studied at dilute solid phase concentration so that oxygen transport at the gas/liquid interface would not limit the rate. A catalytic electrochemical surface reaction was shown to control the reaction kinetics with the reaction rate determined by the following equation derived from electrochemical considerations: dα/dl=127 f/do (OH_- ^1/2 (k₁PO₁/1+k₂PO₁ (k₁+k₂(Cu⁺²)_o+k’₂α)(1-α)^2/3 (K P 1/2 Excellent agreement between theory and experiment was obtained both with regard to apparent reaction orders for oxygen, cupric, and hydroxyl, and with regard to geometric factors that influence the reaction rate. Further support for the reaction mechanism included an activation energy of approximately 10 kcal/mole obtained under a variety of experimental conditions and the fact that the initial reaction rate constant was several orders of magnitude less than predicted mass transfer coefficients. Formerly Metallurgy Graduate Student, University of Utah     

Corrosion behavior of austenitic alloy 690 under anodic and cathodic potentials

The corrosion behavior of austenitic alloy 690 in a solution-annealed condition has been evaluated with the application of anodic as well as cathodic potentials in an acidic chloride solution at room temperature (RT). In a 0.5M H₂SO₄ + 0.5M NaCl solution, the alloy displayed active-passive pitting behavior with the application of an anodic potential. Surface films, formed at the onset and later stage of the passive region, were characterized using X-ray photoelectron spectroscopy (XPS). The XPS revealed that the surface film formed at the onset of passivity (+ 100 mV SCE) consisted of Cr(OH)₃, without any Fe⁺³/Fe⁺². The presence of nickel in the film was found in a transition state of Ni⁺² and Ni⁰. The passive film formed at the higher anodic potential (+ 700 mV SCE) consisted of Cr₂O₃ without any Fe⁺³/Fe⁺² or even Ni⁺²/Ni⁰. Microscopic studies of alloy 690 after anodic polarization in an acidic chloride solution revealed pitting, which was found to be initiated at large, faceted TiN-type inclusions. The susceptibility of the alloy to hydrogen embrittlement has been investigated by conducting cathodic charging of the tensile samples in a 0.5M H₂SO₄ solution at RT and by subsequent tensile testing of the charged samples in air at a strain rate of 1.3 × 10⁻⁴ s⁻¹ up to fracture. An indication toward hydrogen-induced ductility loss was noticed for the samples of the alloy, which is believed to be attributable to a hydrogen-enhanced microvoid growth process. Since the microvoid growth process occurs at the last stage of fracture, the effect of hydrogen on the ductility of the alloy is little.     

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.     

Effects of iron and temperature on solubility of light rare earth sulfates in multicomponent system of Fe2(SO4)3-H3PO4-H2SO4 synthetic solution

Numerous light rare earth elements (LREE) minerals containing Fe and P were processed by sulfuric acid roasting method, and the leaching solution mainly comprises LREE sulfate, Fe₂(SO₄)₃, H₃PO₄, and H₂SO₄, however, the solubility data of LREE sulfates in this system is few. This work studies the solubility of LREE sulfates in independent LREE sulfate system RE₂(SO₄)₃-Fe₂(SO₄)₃-H₃PO₄-H₂SO₄ (RE = La, Ce, Pr or Nd) and mixed LREE sulfates system (La,Ce,Pr,Nd)₂(SO₄)₃-Fe₂(SO₄)₃-H₃PO₄-H₂SO₄ at different temperature (25–65 °C) and concentrations of Fe₂(SO₄)₃ (Fe₂O₃, 0–50.13 g/L), H₂SO₄ (0.5 mol/L), and H₃PO₄ (P₂O₅, 20.34 g/L) based on the industrial operating condition at low liquid and solid ratio 2:1. The solubility of each LREE sulfate in the independent system (La₂O₃, 12.25–20.88 g/L; CeO₂, 41.93–62.35 g/L; Pr₆O₁₁, 37.34–56.69 g/L; Nd₂O₃, 26.60–37.63 g/L) is much higher than that of the mixed system (La₂O₃, 6.95–11.03 g/L; CeO₂, 10.63–21.51 g/L; Pr₆O₁₁, 11.56–20.36 g/L; Nd₂O₃, 12.36–19.79 g/L) under the same other conditions. The results also indicate that, in the two systems, both Fe and the temperature have negative effects on the solubility of LREE sulfates. That may occur due to the complication reactions between the complexes of RESO₄⁺ and Fe(SO₄)₂^–. However, the influence degree of temperature and iron concentration on the LREE sulfates solubility varies in the two systems and among different LREE species. This research is of theoretical significance for optimizing the conditions of the sulfuric acid process for recovering the LREE from the mixed LREE bearing minerals as well as the single LREE containing secondary rare earth scraps.     

Leaching of metallic silver with aqueous ozone

The recycling of silver from metallic scraps can be performed through O₃ leaching at an ambient temperature and low (∼0.1 M) H₂SO₄ concentration. The main by-product is O₂, which can be recycled to the O₃ generation or used as leaching agent in a pretreatment step. The stoichiometry and the effects of the stirring speed, ozone and acid concentration and temperature on the leaching of silver were investigated. Silver dissolved as Ag²⁺_(aq) in the range 10⁻³–1 M H₂SO₄, but for pH ≥4, insoluble Ag₂O₂ was the main reaction product. 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 and P_O3. Specific rates were only slightly dependent on the temperature in the interval 10–50 °C, but decreased at 60 °C due to the fall in O₃ solubility. The mass transfer coefficients showed an average activation energy of 17 kJ/mol. No significant effect of [H₂SO₄] on mass transfer coefficients was observed for 10⁻²–1 M. Leaching rate gradually diminished for pH >2, as a consequence of the influence of the [H⁺] in the transport control.