Adsorption of gold on activated carbon in bromide solutions

Adsorption of gold on activated carbon from gold bromide solutions was examined as a function of pH, particle size, amount of carbon, initial gold bromide concentration, bromine concentration, temperature, and various cations and anions. The pH did not have any effect on gold adsorption below pH = 6. Above pH = 6, pH had a dramatically negative effect. The adsorption occurred according to a first-order reaction-rate model. The reaction-rate constant was proportional to the inverse of carbon particle radius, the amount of carbon, and independent of the initial gold concentration. Temperature had very little effect on the rate of gold adsorption. Cations (Cu²⁺, Fe³⁺, Ca²⁺, Mg²⁺, and Na⁺) and anions (SO ₄ ² , Cl^-, Br^-) did not have any effect on gold adsorption. Free bromine had a very negative effect on gold uptake. Gold bromide is very stable and stays in this form on the carbon surface. Contrary to this, gold chloride is unstable, and elemental gold precipitates on the carbon surface. Loading capacity of carbon depended on the presence of free bromine. In the presence of free bromine, it was further dependent on temperature (increased with increasing temperature). AtT = 25 °C, gold loading was 27 to 64 kg Au/t carbon. In bromine-free solutions, gold loading was 174 to 315 kg Au/t carbon.     

Kinetics of pyrite oxidation in sodium carbonate solutions

The kinetics of pyrite oxidation in sodium carbonate solutions were investigated in a stirred vessel, under temperatures ranging from 50 °C to 85 °C, oxygen partial pressures from 0 to 1 atm, particle size fractions from −150 + 106 to −38 + 10 μm (−100 + 150 Mesh to −400 Mesh + 10 μm) and pH values of up to 12.5. The rate of the oxidation reaction is described by the following expression:−dN/dt = SbkpO ₂ ^0.5 [OH₋]^0.1 whereN represents moles of pyrite,S is the surface area of the solid particles,b is a stoichiometric factor,k is an apparent rate constant, pO```2`` is the oxygen partial pressure, and [OH⁻] is the hydroxyl ion concentration. The experimental data were fitted by a stochastic model for chemically controlled reactions, represented by the following fractional conversion(X) vs time (t) equation: (1−X)^−2/3−1 =k _STt The assumption behind this model,i.e., surface heterogeneity leading to preferential dissolution, is supported by the micrographs of reacted pyrite particles, showing pits created by localized dissolution beneath an oxide layer. In addition to the surface texture, the magnitude of the activation energy (60.9 kJ/mol or 14.6 ± 2.7 kcal/mol), the independence of rate on the stirring speed, the inverse relationship between the rate constant and the initial particle diameter, and the fractional reaction orders are also in agreement with a mechanism controlled by chemical reaction.     

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.     

Investigation into relative stability of structural and electron characteristics of gold complexes [AuX2]? (X = SCN, CN, F, Cl, Br, I) and [Au(SC(NH2)2)2]+ by methods of quantum chemistry

Quantum chemical calculations of the electron structure of complex ions of gold Au⁺ are performed taking into account the solvent effects. The account of the solvent influence provided the quality agreement of the calculated series of deformation energies of the coordination bond (ΔE ₀) with the experimental series of stability constants of gold complexes (SCN ∼ Cl < Br < I ≪ CN). Populations of outer orbitals and the effective atomic charge of gold in the [Au(X)₂]⁻ complexes are calculated. The effective atomic charges of gold in the series under consideration vary from 0.096e for the fluoride complex to 0.396e for the thiocarbamide complex, with a formal oxidation level of gold of +1. The results of calculations make it possible to eliminate the apparent contradictions between the experimental data acquired by various methods and to interpret them in the framework of notions of gold sorption in the composition of the [Au(CN)₂]⁻ complex.     

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.     

Kinetics of reduction of gold(III) complexes using H2O2

In this article, the effect of different kinetic parameters, namely, temperature, pH, and reductant concentration, on the rate of Au(III) reduction from aqueous chloride solutions by H₂O₂ was investigated. The possible mechanism of complex [Au(OH)₄]⁻ ion reduction by hydrogen dioxide is also discussed and the model mechanism based on experimental data is postulated. On the basis of the suggested mechanism, the rate equation for Au precipitation is given in the form , in which respective rate constants k ₁, k ₃, and k ₅ were determined experimentally and are given in the text.     

Studies on kinetics of low-temperature chlorination of ZrO2 by gaseous carbon tetrachloride

Studies on the kinetics of chlorination of ZrO₂. powder by carbon tetrachloride vapor in mixture with nitrogen in a low-temperature range of 650 to 825 K at different periods and partial pressures of carbon tetrachloride were carried out. The chlorination results at 650 and 675 K seem to follow a diffusion-controlled reaction model of Jander’s type: [1−(1−R)^1/3]² =k ₁t whereR is the fraction of ZrO₂ chlorinated in timet andk ₁ is the rate constant. The approximate activation energy of the process was calculated fromk ₁ values at the previously mentioned two temperatures and found to be 278 kJ/mole. For the chlorination in the temperature range of 700 to 750 K, the topochemical reaction model 1−(1−R) ^1/3 =k ₂t was followed. The rate constant,k ₂, was found to be proportional to the partial pressure of carbon tetrachloride. The activation energy of this reaction was calculated to be 154 kJ/mole. In the temperature range of 775 to 825 K, the rate of chlorination was found to be directly proportional to the time of chlorination following Langmuir’s Adsorption Isotherm. Because of the very high rate of chlorination and thermodynamic possibility of decomposition of CCl₄ above 773 K, the rate-controlling step has been suggested to be the decomposition of the adsorbed complex formed by ZrO₂ with carbon and chlorine atoms, obtained from the decomposition of CCl₄ vapor. The activation energy of the process was 54 kJ/mole. In view of nearly complete chlorination of ZrO₂ by CCl₄ in a very short period of about 15 minutes, at a temperature around 800 K and lesser possibility of formation of toxic product gases, the process is recommended for commercial application.     

Diffusion of vanadium,chromium, and manganese in copper

The diffusion coefficients of vanadium, chromium and manaanese in copper have been determined by the residual activity method with radioactive tracers V⁴⁸, Cr⁵¹ and Mn⁵⁴ in the temperature ranges between 955 and 1342 K, between 999 and 1338 K and between 971 and 1253 K, respectively. The temperature dependence of the diffusion coefficients is expressed by the following Arrhenius equations along with the probable errors:D _V/Cu = (2.48 _-0.44 ^+0.53 ) x 10⁻⁴ exp [-(215 ± 2) kJ mol⁻¹/RT] m² per s,D _Cr/Cu = (0.337 _-0.090 ^+0.124 ) x 10⁻⁴ exp [-(195 ± 3) kJ mol⁻¹/RT] m² per s,D _Mn/Cu = (1.02 _-0.18 ^+0.22 ) x 10⁻⁴ exp [-(200 ± 2) kJ mol⁻¹/RT] m² per s. Anomalous penetration profiles for the diffusion of Cr⁵¹ and Mn⁵⁴ in the present results suggest that experimental results onD _Cr/Cu andD _Mn/Cu in the past have been influenced by oxidation and evaporation of the chemically active radiotracers during annealing for diffusion. formerly Graduate Student, Tohoku University     

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).     

Leaching kinetics of digenite concentrate in oxygenated chloride media at ambient pressure

The leaching of digenite concentrate in CuCl₂-HCl-NaCl oxygenated solutions is very rapid. From the effect of variables on the leaching rate and measurements of the concentrations of cuprous and cupric species in the solution as a function of time, it was concluded that the leaching in O₂ atmosphere proceeds by the attack of cupric ions on the copper sulfides to produce cuprous ions which are subsequently oxidized to cupric by the O₂ present in the system. The kinetic study showed that the leaching proceeds in two sequential stages. In the first stage, the digenite is transformed to covellite, and in the second stage, the covellite is dissolved to copper and elemental sulfur. In the first stage, the fraction of copper extracted varied linearly with time according to α=k _l t, whereas in the second stage, the dissolution of covellite was well represented by a shrinking core model controlled by diffusion through a porous product layer kinetic equation: 1−2/3α _cv−(1−α _cv)^2/3=k _cv t. The calculated activation energies were 15.8 and 80.0 kJ/mol for the first and second stages, respectively. These results were explained by an electrochemical mechanism of digenite dissolution.     

Electrochemical Phenomena in MnO2-Fes2 Leaching in Dilute HCl. Part 3: Manganese Dissolution from Indian Ocean Nodules

Abstract

The present paper deals with the kinetics and mechanism of MnO₂ dissolution from sea bed manganese nodules in the presence of pyrite. Manganese (iv) was found to dissolve mostly through reduction by ferrous ions generated by the oxidation of pyrite by ferric ions which was considered to take place through two reactions, one producing S° and the other [SO₄^2?]. These two reactions were found to take place at a mole proportion of about 20% and 80%, respectively, but in terms of electron involvement the participation of the former is insignificant. The dissolution rates obtained from leaching studies were analysed based on the kinetic expressions derived previously and were found to confirm the theoretical basis. The kinetics and mechanism vary depending on whether or not sufficient acid is present. In the presence of sufficient acid the expression {[Fe³⁺]/(Rate)² = K ₃[Fe²⁺] + K ₄} is valid and in depleted acid conditions {rate = K ₆[H⁺][Fe²⁺])^1/2} holds good.

Résumé

Le présent document traite de la cinétique et du mécanisme de dissolution, en présence de pyrite, du MnO₂ de nodules de manganèse du fond de la mer. On a trouve que le manganèse (iv) se dissolvait en grande partie par réduction par des ions ferreux générés par l'oxydation de la pyrite par des ions ferriques. On considère que ceci se produit par l'intermédiaire de deux réactions. l'une produisant S° et l'autre, [SO₄^2?]. Ces deux réctions se produisent à des proportions, en mol, d'environ 20% et 80%, respectivement, mais en ce qui a trait à l'impliçation d'électron, la participation de la première est insignifiante. On a analysé les taux de dissolution obtenus des études de lessivage en se basant sur les expresions cinétiques dérivées auparavant, confirmant la base théorique. La cine[Faac]tique et Ie mécanisme varient dépendant qu'il y a ou non suffisamment d'acide présent. En présence de suffisamment d'acide, l'expression {[Fe³⁺]/(taux)² = K ₃[Fe²⁺] + K₄} est valide et, en conditions appauvries en acide, {taux = K ₆[H⁺][Fe²⁺]^1/2} tient.     

Dissolution of columbite and tantalite in acidic fluoride media

The dissolution reactions of Nb and Ta from columbite and tantalite in the aqueous solutions of HF, HF-HC1, NH₄F-HCI, HF-H₂SO₄, and NH₄F-H₂SO₄ were kinetically studied in the temperature range of 333 to 353 K. The presence of both H⁺ and F^- in the leachant is necessary for the fast dissolution of columbite and tantalite. The increase in these ion concentrations and the elevation of temperature are effective in increasing the dissolution rate. The initial dissolution rate of Nb from columbite under the conditions employed in this work can be expressed as follows:R _i =k _o a(H⁺)^1.2 C(F^2212;)^1.1 exp(−E _a /RT) where an apparent activation energy,E _a , ranges from 53.9 kJ mol⁻¹ to 65.5 kJ mol⁻¹. Formerly Graduate Student, Department of Metallurgy, Kyoto University, Japan     

Dissolution of columbite and tantalite in acidic fluoride media

The dissolution reactions of Nb and Ta from columbite and tantalite in the aqueous solutions of HF, HF-HC1, NH₄F-HCI, HF-H₂SO₄, and NH₄F-H₂SO₄ were kinetically studied in the temperature range of 333 to 353 K. The presence of both H⁺ and F^- in the leachant is necessary for the fast dissolution of columbite and tantalite. The increase in these ion concentrations and the elevation of temperature are effective in increasing the dissolution rate. The initial dissolution rate of Nb from columbite under the conditions employed in this work can be expressed as follows:R _i =k _o a(H⁺)^1.2 C(F^2212;)^1.1 exp(−E _a /RT) where an apparent activation energy,E _a , ranges from 53.9 kJ mol⁻¹ to 65.5 kJ mol⁻¹. Formerly Graduate Student, Department of Metallurgy, Kyoto University, Japan     

Kinetics of reduction of hematite with hydrogen gas at modest temperatures

The kinetics of hydrogen reduction of thin, dense strips of hematite were investigated in the range 245 °C to 482 °C. Pure hydrogen gas at 1 atm was used as the reducing agent. Because of the relative thinness (only 136 /μm thick) of the specimens used, the pore-diffusion of gases offered no significant resistance to the reduction process. The interfacial-reaction-rate constantk _s ^* , which has been corrected for film-mass-transfer effects, is found to be given by logk _s ^* = −1.032 (±0.138) -[7860 (±200)]/2.303r where k _s ^* is in g · atom O · cm⁻² · s⁻¹ · atm⁻¹. The activation energy for the reduction process is found to be 65,325 (±1650) J · mol⁻¹; the rate-controlling step appears to be the Fe₃O₄ → Fe conversion step.     

Deformation twinning of martensite

Low temperature strength properties, deformation modes, and the transition from slip to twinning were studied in Fe-Ni massive-martensites as a function of martensite structural features. Specimens with block widthsd (mm) from coarse (d ^−1/2 = 1.0) to fine (d ^−1/2 = 6.5) were tensile tested at 4 and 77 K. Twinning was the dominant mode at 4 K at all block sizes. The twin-yield stress (σ _T) increased with decreasing block size:σ _T = σ_iT + k_Td^−1/2 At 77 K the mode changed from twinning to slip atd = 0.05. Slip was less sensitive tod than twinning, withk _T at 4 K twicek _Y at 77 K. The initial twin load-drop stress also increased withd ^−1/2 at 77 K. Twins were thin, banded, undeviated by the substructure, and restricted to specific blocks. The twinning plane was {21l}. Deformation is discussed in terms of the unique massive-martensite substructure as compared with that of other ferrous BCC alloys.     

Morphology and composition of the precipitate obtained from the [Ag(S2O3)2]3?-S2O 32? -solutions by cementation with magnesium scrap

The morphology and composition of the sediment obtained by cementation with magnesium scrap from solutions 0.0025–0.01M [Ag(S₂O₃)₂]³⁻ + 0.5M S₂O₃²⁻ at t = 20–40°C is studied. It is shown that a disperse sediment of silver and sulfur is formed in a wide range of concentrations. This sediment is easily detached from the magnesium surface. In this case, the content of sulfur in it decreases as the [Ag(S₂O₃)₂]³⁻ concentration and temperature are increased. It is established that the final product of cementation of the [Ag(S₂O₃)₂]³⁻-S₂O₃²⁻ solution by the composition corresponds to Ag₂S.     

Calculation of the acid-base equilibrium constants at the alumina/electrolyte interface from the ph dependence of the adsorption of singly charged ions (Na<Superscript>+</Superscript>, Cl<Superscript>−</Superscript>)

A procedure was proposed for the calculation of the acid-base equilibrium constants at an alumina/electrolyte interface from experimental data on the adsorption of singly charged ions (Na⁺, Cl⁻) at various pH values. The calculated constants (pK ₁⁰= 4.1, pK ₂⁰= 11.9, pK ₃⁰= 8.3, and pK ₄⁰= 7.7) are shown to agree with the values obtained from an experimental pH dependence of the electrokinetic potential and the results of potentiometric titration of Al₂O₃ suspensions.     

The oxidation-reduction kinetics of palladium powder

The cyclic oxidation-reduction of submicrometer sized palladium powder was investigated over the temperature range 848 to 923 K. The total oxygen uptake was found to decrease with increasing number of cycles as a consequence of sintering. Furthermore, sintering was found to be restricted to the reduction steps in these cycles. The relationships for the rate constants of the oxidation and reduction processes are, respectively, K_x, = (1.04)10⁶ exp[-(74.1)10³/RT],k _r = (7.63)10¹² exp[—(207.9)10³/RT]. The activation energies for the oxidation of palladium powder and the reduction of palladium oxide are 74.1 and 207.9 kJ mol⁻¹, respectively.     

Deuterium exchange studies of the interfacial rate of reaction of water vapor with silica-saturated iron silicate melts

Measurements have been made of the rate of dissociation of H₂O on silica-saturated iron silicate melts at 1300 °C and 1400 °C by the HDO-H₂ deuterium exchange technique. The general rate equations for the technique are developed, and the available information on the associated isotope equilibria is briefly reviewed. The rate is deduced to be first order with respect to the pressure of water vapor, and the apparent first-order rate constant is found to be essentially inversely proportional to the activity of oxygen in the melt over the range studied;i.e., pH ₂O/ pH₂ − 0.9 to 18 at 1400 °C. Comparison with the rates deduced by Sasaki and Belton from measurements of the steady-state oxygen activity of the melts in flowing H₂O-CO mixtures at 1250 °C leads to the conclusion that interfacial rates of oxidation (or reduction) of the melts in H₂O-H₂ atmospheres are given by the rate lawv = k₁(pH₂O(a′_o) −pH ₂) over the range of conditions covered by the two sets of experiments. The terma’o is the oxygen activity of the melt, defined as the equilibriumpH ₂O/pH₂ ratio, and k₁, in units of mol cm⁻²s⁻² atm⁻¹, is given by the expression: log k¹ = −6700/T − 0.08 to within a factor of about 2 over the temperature range of 1250 °C to 1400 °C. Formerly Postdoctoral Fellow, Department of Metallurgy, University of Newcastle     

Caustic Stress Corrosion Cracking of Mild Steel

The stress corrosion cracking (SCC) behavior of cold worked mild steel in hot, aqueous, 33 pct NaOH solutions was studied with prefatigue cracked double cantilever beam specimens. SCC kinetics were studied under freely corroding potentials (E _corr ≈ −1.00 V_SHE) and potentiostatic potentials of −0.76 V_SHE near the active-passive transition. The pH of the liquid within the crack was determined and fractography was studied by scanning electron microscopy. Cracking was transgranular atE _corr, intergranular at −0.76 V_SHE, and produced no detectable change in crack liquid pH from that of the bulk solution. Crack rates were dependent upon temperature, potential, and stress intensity (K ₁). The apparent activation energy in Region II, where crack growth rate was independent ofK, was ∼ 24kJ/mol for both cracking modes. This was considered to be due to mixed rate control involving activation polarization and mass transport processes. The mechanism of cracking was entirely consistent with metal dissolution at –0.76 V_SHE and may involve hydrogen embrittlement and/or dissolution effects atE _corr. DOUGLAS SINGBEIL, formerly Research Student, University of British Columbia, is Research Scientist, Pulp and Paper Research Institute of Canada, 570-Blvd. St. Jean, Pointe Claire, Quebec, Canada H9R 3J9.