Kinetics of dissolution of cobalt oxides in acidic media

The kinetics of dissolution of cobalt oxides Co₂O₃ and Co₃O₄ in aqueous solutions of acids (H₂SO₄, EDTA) is experimentally studied. Dissolution rate W increases with the temperature or the EDTA concentration. The reaction orders of dissolution for hydrogen ions in sulfuric acid and EDTA (dlogW/dpH = 0.5 ± 0.1) and for anions (dlogW/dlog[An ⁻] = 0.5 ± 0.1) are determined. A specific feature of the dissolution kinetics in EDTA is a maximum in the dissolution rate of the cobalt oxides at pH −1. The activation energy of the process E _a is 70 kJ/mol in H₂SO₄ and 60 kJ/mol in EDTA. The modeling of the process shows that the CoOH⁺ ion is a surface particle controlling the dissolution rate in mineral acids and the CoHY⁻ ion, in the complexone.     

Diffusion-limited sulfidation of wustite

The sulfidation of wustite in H₂S−H₂O−H₂−Ar atmospheres has been studied at temperatures of 700, 800, and 900°C with thermogravimetric techniques. Polycrystalline wustite wafers were equilibrated in a flowing H₂O−H₂−Ar gas stream and then sulfidizedin situ. During an initial transient stage a protective layer of FeS formed on the sample, and an intermediate layer of Fe₃O₄ formed between the FeO and FeS layers. Subsequently, the reaction followed a parabolic rate law. The parabolic rate constant varied from 0.22×10⁻² mg² cm⁻⁴ min⁻¹ at 700°C to 6.5×10⁻² mg² cm⁻⁴ min⁻¹ at 900°C. The reaction rate was limited by the diffusion of iron through the intermediate Fe₃O₄ layer which grew concurrently with the FeS layer and at the expense of the FeO core. After the FeO core was completely converted to Fe₃O₄, the process entered a passive stage during which no further mass changes could be detected. SCOTT McCORMICK, formerly Graduate Student, Purdue University is currently Assistant Professor, Department of Metallurgical and Materials Engineering, Illinois Institute of Technology, Chicago, Illinois 60616.     

The kinetics of the solvent extraction of cobalt (II) and lead (II) with versatic acid

In the solvent extraction of cobalt (II) and lead (II) from aqueous ammonium nitrate solutions with Versatic 10 in hexane, initial extraction rates were measured at 303 K using a Lewis-type stirred transfer cell. In the extraction of cobalt(II), the initial extraction rate expression was found to be described by N = κ₁C_AW/[H⁺] in the high concentration region of Versatic 10 and by N = κ_IIC_AWC_BO²/[H⁺]² in the low concentration region of Versatic 10. An interfacial reaction mechanism was proposed in order to give a reasonable interpretation of the observed rate expressions. In the extraction of lead(II), the initial extraction rate at low pH was found to be described also by the second rate expression shown above. At high pH, the rate is independent of hydrogen ion concentration. From this result, it was presumed that only the non-hydrated aquo-cation of lead(II) takes part in complex formation at the interface.     

Potentiostatic studies of the influence of temperature on lead-silver anodes during electrowinning and decay period

The electrochemical behaviour of the Pb-0.7% Ag anode has been investigated in zinc sulphuric acid electrolyte containing 5, 8, 12?g/L Mn²⁺ by electrochemical measurements at different temperatures of 35°C, 40°C, 45°C. It was observed by cyclic voltammetry before electrolysis that temperature increase from 35°C to 45°C resulted in a decrease in the oxidation peak I (Pb → PbSO₄) and an increase in the oxygen evolution peak II (2H₂O → O₂?+?4H⁺?+?4e). At 2.01?V/SHE, the current densities of Pb-0.7% Ag anode in sulphuric acid solution containing 8?g/L Mn²⁺ at 35°C, 40°C and 45°C increased from 26.5 at 35°C to 28.5 and 37.7?mA/cm² at 40 and 45°C, respectively. In the presence of ZnSO₄ addition to the sulphuric acid solution, the anodic current of oxygen evolution at 2.01?V/SHE decreased by ～ 18% at 40°C. After 2?h of potentiostatic polarisation in the presence of zinc sulphate addition at 2.01?V/SHE, electrochemical Noise Measurements showed that the admittance (corrosion rate) of the Pb-0.7% Ag anode in the zinc electrolyte increased with the increase in temperatures during decay. This tendency was also confirmed by linear polarisation and impedance measurements.     

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.     

Oxidation of dense iron sulfide

Oxidation of stoichiometric iron sulfide was investigated. Rectangular plates of dense FeS were oxidized in an Ar-O₂ gas mixture at 1023 to 1123 K. Oxygen partial pressure was varied between 1.01 × 10³ and 2.03 × 10⁴ Pa. During the initial five minutes of oxidation, a magnetite layer of about 10 μm in thickness was formed on the surface without the evolution of SO₂ gas. Diffusion of iron from the interior of the sulfide to the sulfide/magnetite interface controlled the oxidation rate. Mass transfer through the gaseous boundary layer at the sample surface also affects the oxidation rate at lower oxygen partial pressures. Following this rapid formation of magnetite, the magnetite layer continued to grow for several hours in accordance with the parabolic rate law. Diffusion of iron through the magnetite layer controlled the oxidation rate during this stage. A thin layer of hematite was also formed on the outer surface of magnetite. When the composition of the inner sulfide core reached Fe_0.9S, the oxidation proceeded irregularly into the interior of the remaining sulfide. Porous oxide was formed and SO₂ gas was evolved. Former Graduate Student     

Improving tantalum's oxidation resistance by Al+ ion implantation

Tantalum was implanted with 180 keV Al⁺ ions to fluences up to 3×10¹⁸ Al⁺/cm². Subsequent microchemical and microstructural observations showed that an amorphous layer covered the surface and extended to depths near 3000 Å for fluences above 2.4×10¹⁸ Al⁺/cm². The layer, comprised of ～70 at. pet Al and ～30 at. pet Ta, crystallized at temperatures above 500°C. Oxidation measurements, performed in one atmosphere of air and at temperatures below 600°C, showed that the layer stopped oxidation of the implanted tantalum, while unimplanted tantalum oxidized rapidly. The protection provided by the implantation deteriorated somewhat by temperatures near 735°C but still reduced the oxidation rate by a factor of 5. The deterioration is caused by localized rupturing of the implanted layer and the resulting oxidation of the underlying tantalum. At 910°C, the implanted tantalum oxidized almost as rapidly as unimplanted tantalum.     

Sequential Precipitation of Heavy Metals Using Sulfide-Laden Bioreactor Effluent in a pH Controlled System

Sulfide produced in an ethanol fed anaerobic baffled reactor, was utilized to precipitate metals separately in a pH controlled system. Sulfide produced in the reactor (780±27 mg/L S^2–) was transported with N₂ gas to the metal precipitation chamber at pH 2.5 to precipitate Cu²⁺ separately from Fe²⁺. Cu precipitation was completed at 98% efficiency within 5 min. The remaining Fe²⁺ was then precipitated at elevated pHs by mixing the reactor effluent. XRD patterns of the precipitates showed that copper was precipitated in the form of CuS with a particle size of 14–22 nm whereas iron was precipitated as FeS with 32-85 nm particle size.     

Extraction equilibrium conditions of beryllium and aluminium from a beryl ore for optimal industrial beryllium compound production

This study examines the extraction of beryllium and aluminium from a Nigerian beryl ore using Cyanex®272 in kerosene from an aqueous sulphate pregnant solution. Parameters such as extractant concentration and equilibrium pH that dictates the extraction yield were studied. Under the following conditions: temperature 27?±?2°C, phase ratio 1?1, about 45.50 and 46.76% of beryllium and aluminium were extracted by 0.15?mol?L^?1 Cyanex®272 concentration within 30?min. However, the extraction yield of beryllium and aluminium was increased to 91.68% and 97.89% at equilibrium pH of 3 and 4, respectively, for beryllium and aluminium at 27?±?2°C. A 0.05?mol?L^?1 H₂SO₄ solution was found to be adequate for the stripping of about 99.00% Be and 95.40% Al from the loaded organic phase. The pure solutions containing metal ions were accordingly beneficiated to obtain beryllium and aluminium compounds of industrial values.     

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     

Oxidation of low carbon steel in multicomponent gases: Part I. Reaction mechanisms during isothermal oxidation

The article describes rates of oxidation of low carbon steel in various nitrogen-based atmospheres of O₂, CO₂, and H₂O in the temperature range 800 °C to 1150 °C. In characterizing the oxidation process, the weight gains of the samples per unit surface area vs time data were analyzed. Reaction rates during oxidation in the binary atmospheres of CO₂-N₂ and H₂O-N₂ followed a linear rate law and were found to be proportional to the partial pressures of CO₂ or H₂O. These rates were controlled by rate of reactions at the oxide surface and were highly dependent on oxidation temperature. The activation energies of the phase boundary reactions obtained were approximately 274 and 264 kJ/mole, for oxidation in CO₂ and H₂O atmospheres, respectively. Oxidation in gases containing free oxygen showed that the main oxidizing agent was the free oxygen and that additions of CO₂ and H₂O had little effect on the magnitude of the initial oxidation rates. Experiments for oxidation in multicomponent gases showed that the overall oxidation rates were the additions of rates resulting from oxidation with the individual gaseous species O₂, CO₂, and H₂O. Oxidation in these atmospheres exhibited an initial linear rate law which gradually transformed into a parabolic. Examination of scale microstructure after 1 hour of oxidation showed that, for oxidation in carbon dioxide and water vapor atmospheres, only wustite was present, while in atmospheres containing free oxygen, all three iron oxides, wustite, magnetite, and hematite, were present.     

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).     

Kinetics of the water-gas shift reaction on an “FeO” surface

The rate of the water-gas shift reaction, CO₂ + H₂→ CO + H₂O, has been determined on “FeO” as a catalyst between 700 and 1000°C by conversion and isotope-exchange measurements. Results of the two methods are in fair agreement and are consistent with the rate equation, $$\dot n_{CO} /A = k'[a_O ](p_{CO_2 } - a_O p_{CO} ),$$ wherea _O is the oxygen activity on the “FeO” surface. The rate limiting step is the adsorption-dissociation of CO₂ on the surface. The dependence ofk’ on a_O may be empirically expressed ask’[a _O] =k _o a _O ^{- m}, wherem = 2/3 at 900°C. Possible causes for this behavior are discussed. Variation ofk _o with T yieldsk _o = 0.00717 exp(-124,700 ±6300 J/RT) mol/cm² s atm. The activation energy agrees well with those determined from thermogravimetric studies of the linear oxidation of α-Fe (120.9 kJ/mol) and the oxidation-reduction of “FeO” within its range of nonstoichiometry (117.6 kJ/mol). It differs significantly, however, from the activation energy of 226.8 kJ/mol previously obtained for surface controlled oxidation of γ-Fe.     

Effect of oxygen on the surface tension of liquid copper

The influence of oxygen on the surface tension of liquid copper has been determined by the sessile drop technique. The surface tension of pure liquid copper at 1108 °C is found to be equal to 1.320 ± 0.015 N/m. The effect of oxygen is investigated for partial pressures of oxygen ranging from 10?13 to 5 X 10?6 atm. The surface activity of oxygen is deduced to equal 3200 ± 600 N/m and the saturation adsorption to equal 5.72 X 10"6 mole/m², which corresponds to a saturation area of 29 ±5Ų per adsorbed oxygen atom. The adsorption of oxygen on liquid copper is consistent with the formation at the metal surface of a two-dimensional compound of stoichiometry Cu³O. It is also concluded that equivalent attractive forces operate between neighboring adsorbed atoms.     

New Aspects in the Theory of Mineral Flotation

Microcalorimetric measurements were carried out to explain reactions of H⁺/OH^? on the surface of three different groups of minerals: oxides, salt-type minerals, and kaolinite and feldspar. The results were compared to those of H⁺/OH^? adsorption density measurements and to the isoelectric point, obtained from electrophoretic mobility measurements at different pH.

In the system sodium dodecylsulfate/salt-type mineral we measured the lattice ion concentrations together with the surfactant consumption due to surfactant adsorption and surfactant precipitation with dissolved calcium ions at pH 10. The adsorption isotherms calculated from these data are compared with the surface properties of the minerals in a suspension.

At least the influence of counterions on the adsorption of cationic surfactants on quartz was determined both with and without constant ionic strength.     

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.     

Effect of active oxygen on the performance of Pt/CeO2 catalysts for CO oxidation

This study was focused on the influence of active oxygen on the performance of Pt/CeO_2 catalysts for CO oxidation. A series of CeO_2 supports with different contents of active oxygen were obtained by adding surfactant at different synthesis steps. 0.25 wt% Pt was loaded on these CeO_2 supports by incipientwetness impregnation methods. The catalysts were characterized by N2 adsorption, X-ray diffraction(XRD), high-resolution transmission electron microscopy(HRTEM), H_2 temperature-programmed reduction(H_2-TPR), dynamic oxygen storage capacity(DOSC) and in-situ DRIFTS technologies. For S-f supports, the surfactant was added into the solution before spray-drying in the synthesis process, which facilitates more active oxygen formation on the surface of CeO_2. After loading Pt, the more active oxygen on CeO_2 contributes to dispersing Pt species and enhancing the CO oxidation activity. As for the aged samples,Pt-R-h shows the highest activity above 190 ℃ because of the presence of more partly oxidized Pt~(δ+) species. Thus the activity is also influenced by the states of Pt and the Pt~(δ+) species may contribute to the high activity at elevated temperature.     

Selective Oxidation and Reactive Wetting during Galvanizing of a CMnAl TRIP-Assisted Steel

A transformation induced plasticity (TRIP)-assisted steel with 0.2 pct C, 1.5 pct Mn, and 1.5 pct Al was successfully galvanized using a thermal cycle previously shown to produce an excellent combination of strength and ductility. The steel surface chemistry and oxide morphology were determined as a function of process atmosphere oxygen partial pressure. For the 220 K (–53 °C) dew point (dp) + 20 pct H₂ atmosphere, the oxide morphology was a mixture of films and nodules. For the 243 K (–30 °C) dp + 5 pct H₂ atmosphere, nodules of MnO were found primarily at grain boundaries. For the 278 K (+5 °C) dp + 5 pct H₂ atmosphere, nodules of metallic Fe were found on the surface as a result of alloy element internal oxidation. The steel surface chemistry and oxide morphology were then related to the reactive wetting behavior during continuous hot dip galvanizing. Good wetting was obtained using the two lower oxygen partial pressure process atmospheres [220 K dp and 243 K dp (–53 °C dp and –30 °C dp)]. An increase in the number of bare spots was observed when using the higher oxygen partial pressure process atmosphere (+5 °C dp) due to the increased thickness of localized oxide films.     

Oxidation of ferrous sulfate in weakly acidic solution by gas bubbling

The oxidation of Fe²⁺ ion in aqueous solution in a pH region between 4.7 and 5.5 was studied. By supplying dilute NaOH solution from an automatic titrator, pH of the solution was maintained constant during the oxidation. The reaction is comprised of the sequential steps of the dissolution of gaseous oxygen and the oxidation of Fe²⁺ ion by dissolved oxygen. The latter reaction proceeds along two paths: homogeneous reaction in the solution (rate constant:k) and heterogeneous reaction on the surface of ferric hydroxide precipitate (rate constant:ks). The measured time variation of the concentrations of Fe²⁺ ion and dissolved oxygen was explained by simultaneous rate equations. Linear relationships were found between logk and pH and between logks and pH having slopes of 2 and unity, respectively. An activation energy of 103 kJ/mol was obtained fork. The overall rate of oxidation of Fe²⁺ ion was chemically controlled at pH lower than 5.0 and temperature lower than 298 K. On the other hand, it was controlled by both chemical reactions and the dissolution of oxygen at higher pH and temperature.     

Removal of SO2 with oxygen in the presence of Fe(III)

A laboratory study of the aqueous oxidation of SO₂ in the presence of Fe(III) and Fe(II) has been conducted. The SO₂ concentration was 3930 ppm (3.93 × 10⁻³ atm or 398 Pa) in a gas stream with nitrogen and oxygen. The oxygen pressure was varied from 0 to 0.203 atmosphere. The initial concentration of Fe(III) ranged from 10⁻³ to 5-10⁻³ molar while that of Fe(II) was 5 × 10⁻³ molar. The temperatures were 298, 309.2, and 317.5 K. The solution pH was 1.83. The oxidation of SO₂ is intensive and yields from 90 to 97 pct recovery of incoming SO₂ when 5 × 10⁻³ molar Fe(III) and an oxygen pressure above 0.057 atmosphere are applied at 298 K. The reaction mechanism has been explained by determining the rate constants of the oxidation reactions from a kinetic model. The rate constants show that SO₂ is mostly oxidized by oxygen through formation of ferric-sulfite complex and that regeneration of ferric ion is possible under a normal oxygen pressure. The activation energy of the oxidation has been determined and has been found to be 13.5 Kcal/mole.