The cyanidation of silver metal: Review of kinetics and reaction mechanism

Published rate data are analysed for the chemical and electrochemical dissolution of silver metal from rotating discs in aerated/oxygenated cyanide solutions at ≈25 °C, pH 11 and different partial pressures of oxygen. The current status of the reaction mechanism is also reviewed. Speciation analysis of 0.01 mM silver(I) in 1–100 mM cyanide solutions shows that Ag(CN)₂⁻ is the predominant complex (50%) at cyanide concentrations < 20 mM. However, at higher cyanide concentrations, Ag(CN)₃²⁻ (up to 60%) and Ag(CN)₄³⁻ (up to 10%) can be formed. Thus, it is important to consider a silver(I) : cyanide ion ratio of 2 or 3 in the Levich equation to calculate the diffusion coefficient of cyanide ion. Likewise, it is important to consider a silver(I) : oxygen ratio of 1 : 0.5 to calculate the diffusion coefficient of oxygen. This indicates the reduction of oxygen to hydrogen peroxide in the surface reaction. Analysis of exchange current density data for silver oxidation as a function of cyanide concentration shows the involvement of between 1 and 2 cyanide ions in the surface reaction. The limiting rate of silver dissolution at high cyanide concentrations (2.5 × 10^− 5 mol m^− 2 s^− 1 at ≈21 kPa oxygen pressure) represents the maximum surface coverage by cyanide. This value is in close agreement with the rate constant of the surface reaction 4 × 10^− 5 mol m^− 2 s^− 1 based on the pure kinetic current of the mixed “charge transfer plus diffusion” model proposed by Li and Wadsworth [Li, J., Wadsworth, M.E., 1993. Electrochemical study of silver dissolution in cyanide solutions. J. Electrochem. Soc. 140, 1921–1927].     

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.     

Separation of aluminum from rare earth by solvent extraction with4-octyloxybenzoic acid

Leaching method is usually used to extract rare earth(RE) elements from ion adsorbed RE ores.In the leaching process,some impurities such as aluminum(Al) enter the leaching solution.The separation of Al from RE by carboxylic acid extractant 4-octyloxybenzoic acid(POOA) was studied in this article.By changing the pH value,temperature,solvent,saponification degree and other parameters,the extraction and separation performance of POOA in chloride system was systematically studied.Through specific e...     

Kinetics and deposit morphology of gold cemented on magnesium, aluminum, zinc, iron and copper from ammonium thiosulfate-ammonia solutions

The kinetics of gold cementation by magnesium aluminum, zinc, iron and copper at equal conditions as well as the morphology of cementation products and the dissolution of cementing agents were investigated. Both the effect of cementing agents on gold cementation rate and the dissolution of metals decrease in order Cu > Zn > Mg > Fe > Al. High dissolution of magnesium, zinc, iron and copper per mol deposited gold was found. Zinc is the most efficient cementing agent. Magnesium and iron can be used in gold cementation practice but the extremely slow cementation rate prevents the practical application of aluminum.Significant differences in the morphology of gold deposits on magnesium, aluminum, zinc, iron and copper were found: thick deposit layers with small or large cracks were observed on magnesium or zinc; porous layers of fern leaf-shaped dendrites on copper; compact smooth deposit of fine crystallites on iron and small very rare formations of fine dendrites on aluminum.     

Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior

Pressureless sintering and hot pressing experiments were conducted on elemental powder compacts of Fe-15.8 wt pct Al and Fe-32 wt pct Al, corresponding approximately to the compositions of stoichiometric Fe₃Al and FeAl, respectively. Upon heating near the melting point of aluminum, an exothermic reaction was initiated in the compacts, resulting in synthesis of the desired compounds with reaction times on the order of seconds. Thermal analysis and microstructural observations indicate the formation of a transient liquid phase during rapid exothermic compact heating. The mechanisms shown to be responsible for microstructural development include initial compound formation in the solid state, appearance of an aluminum-rich liquid at the aluminum particle sites, iron dissolution accompanied by outward spreading of the liquid, and subsequent precipitation of the iron-rich compounds. Apparent enthalpies of formation,ΔH _f ^°(298), estimated from reaction temperature measurements were −18 and −31.8 kJ/mol for Fe₃Al and FeAl, respectively. The influences of heating rate, green density, and aluminum particle size on sintered density were studied for pressureless reaction sintering in vacuum. The effects of processing variables on densification were explained as the net result of swelling during heating and subsequent shrinkage due to the transient liquid phase. Near full density Fe₃Al and FeAl compounds were obtained through the application of external pressures near 70 MPa during reaction in a hot press. These alloys were partially ordered, chemically homogeneous, and exhibited an equiaxed grain structure with an average grain size below 10μm. The Fe₃Al material exhibited significantly higher fracture strength and somewhat lower ductility than coarse-grained wrought material of the same composition.     

A review of effects of silver,lead, sulfide and carbonaceous matter on gold cyanidation and mechanistic interpretation

Despite the wealth of published data on the beneficial or detrimental effects of silver, lead, sulfide, and carbonaceous matter on the rate of gold cyanidation at an anode or by dissolved oxygen, the lack of comparative studies on relative effects has hampered rationalisation of the role of these activators or passivators of gold. In the present study, the published rate data per unit surface area of gold, silver, and gold–silver alloys based on electrochemical or chemical dissolution of rotating discs or foils of constant surface area in aerated cyanide solutions at ambient temperatures are analysed on the basis of the Levich equation. The current status of the reaction mechanism is also reviewed and updated on the basis of species distribution and potential–pH diagrams, stoichiometric factors, and interim chemical species of gold(I), silver(I), and lead(II). The anodic peak potentials of reported voltammograms closely follow the potential–pH lines of Au(I)/Au(0) and Pb(II)/Pb(0) couples. Despite the formation of stable complexes between lead(II), nitrate, and hydroxide ions, the total calculated soluble lead(II) in alkaline solutions of pH range 10–11 saturated with lead hydroxide is shown to be < 0.1 g/m³. A comparison of the reported diffusion coefficients of cyanide ions and dissolved oxygen with the values based on the Levich plots of reported rates reveals the rate-controlling stoichiometric M/CN or M/O₂ molar ratios. The difference between some of these ratios and the generally accepted ratios of M/CN = 1/2 and M/O₂ = 1/0.5 or 1/0.25 based on the formation of M(CN)₂⁻, H₂O₂ or OH⁻ in the overall cyanidation reaction is attributed to the oxidation of cyanide to cyanate and passivation due to the formation of gold hydroxides/oxides. The alloyed or dissolved silver and lead eliminate passivation due to the involvement of mixed hydroxo–cyano complexes of silver and lead ions in the surface reaction. Gold dissolution by oxygen in cyanide-rich solutions is limited by oxygen diffusion, but enhanced by the presence of a low concentration of sodium sulfide due to the involvement of hydrosulfide ion in the surface reaction. However, excess lead or sulfide retards gold cyanidation due to surface blockage by metallic lead, lead hydroxide, or due to passivation by Au₂S/S. Even low concentrations of hydrosulfide passivate gold–silver alloys due to the formation of Ag₂S. This can be eliminated by adding stoichiometric quantities of lead(II) to precipitate sulfide as PbS. Large stoichiometric ratios of O₂/M for the cyanidation of graphite coated gold appears to be a result of the enhanced oxidation of cyanide by oxygen or hydrogen peroxide, leading to a cyanide deficiency at the surface and passivation of gold by hydroxide/oxide. The presence of excess cyanide or lead(II) does not override this effect.     

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.     

Lithium recovery from highly concentrated solutions: Response surface methodology (RSM) process parameters optimization

Lithium recovery from aqueous solutions was studied by trapping lithium ions using a gel of aluminum hydroxide freshly prepared in situ under the reaction of a strong base. A 2^4 − 1 fractional factorial design and central composite design were employed for experimental design and analysis of the results. The combined effect of molar ratio Al/Li, pH and stirring time on lithium trapping at 25 °C was investigated and optimized using response surface methodology (RSM). The optimum values of these factors were found to be 4.7, 7.2 and 3 h respectively. In this case the lithium trapping percent is ∼ 95%. The obtained model is highly significant (F_obs > F_tabulate and low p-value) with a correlation coefficient of 96.64%. On the other hand, linear, quadratic and interaction terms in this model have the largest statistical effect on the response (confidence level = 99.9%).     

Diffusion of silicon in aluminum

Interdiffusion coefficients in Al-Si alloys were determined by Matano’s method in the tem-perature range from 753 to 893 K with the couple consisting of pure aluminum and an Al-Si alloy. Temperature dependence of the impurity diffusion coefficients of Si in Al, obtained by extrapolation of the concentration dependence of the interdiffusion coeffi-cient to zero mole fraction of Si, is given by the following equation: DSi/Al = (2.02^+0.97 _-0.66 × 10^-4 exp [-(136 ±3) kJ mol^-1/RT] m²/s. p ] The Kirkendall marker was found to move toward the Si-rich side, indicating that the Si atom diffuses faster than the Al atom in Al-Si alloys. From the interdiffusion coeffi-cient and the marker shift, the intrinsic diffusion coefficients were calculated. The difference in the activation energies (ΔQ) between the impurity diffusion of Si in Al and the self-diffusion of Al was estimated by means of the asymptotic oscillating po-tential and the Le Claire theory. The calculated value of ΔQ is in fair agreement with the experimental value. The vacancy-solute binding energy for Si in Al was also dis-cussed based on the diffusion data. formerly Undergraduate Student, Tohoku University     

Leaching kinetics of synthetic CaWO4 in HCl solutions containing H3PO4 as chelating agent

In this study, the dissolution kinetics of synthetically prepared CaWO₄ in HCl solutions containing H₃PO₄ was studied. The effects of process parameters such as stirring speed, temperature and acid concentrations on the dissolution rate of CaWO₄ were investigated. The reaction rate was found to be of 1 / 3 and 2 / 3 order with respect to HCl and H₃PO₄ concentrations, respectively, and the activation energy for the dissolution reaction to be 60 kJ mol^− 1. The rate equation for the dissolution reaction was derived using the Avrami equation and the rate determining step was the chemical reaction on the surface of solid particles.     

Molecular dynamics simulation of aluminum inhibited leaching during ion-adsorbed type rare earth ore leaching process

Molecular dynamics simulation was adopted to study the interaction between sulfosalicylic acid and aluminum,lanthanum and yttrium,and adsorption on kaolinite surfaces.A complexation reaction occurs between sulfosalicylic acid and aluminum,with an interaction energy of-10472.05 kJ/mol.O—Al covalent bonds are formed with a peak value of 7.93,while there is only weak adsorption between sulfosalicylic acid and rare earth ions.A hydrogen bonding reaction with 13605.82 kJ/mol energy occurs between sulfosalicylic acid and the surface of kaolinite(100).Thus,sulfosalicylic acid can form a complex with free aluminum ions,and can also be adsorbed on kaolinite by hydrogen bonding with aluminum in kaolinite(100) surfaces.Leaching of ion-adsorbed type rare earth ore was performed with aluminum inhibited,results show that when sulfosalicylic acid dosage increases from 0 to 0.15 wt%,aluminum ion concentration in the leaching solution decreases from 273.23 to 47.19 mg/L.And the effect of leaching pH value on the effect of sulfosalicylic acid on aluminum inhibition was studied,the result shows that,when the leaching pH value is 4.0—5.0,the rare earth leaching rate and the aluminum ion concentration basically remain unchanged.The molecular dynamics simulation results were verified by detection and analysis of XPS and SEM.     

On the solubility of aluminum in cryolitic melts

The solubility of aluminum in NaF-AlF₃-Al₂O₃ melts with various additives was found to increase with increasing NaF/AlF₃ molar ratio (CR) and increasing temperature and to decrease with additions of A1₂O₃, CaF₂, MgF₂, and LiF to the melts. With the use of literature data for the activities of NaF and A1F₃ in cryolitic melts, three dissolution reaction models were found to give a good fit to the experimental solubility data. According to the most probable of these models the total concentration of dissolved aluminum (aluminum and sodium species) is given by c_Al = c_Na(diss) + c_AlF2- + c_Al2F3- + c_Al3F4- + c_Al4F5- In NaF rich melts, aluminum will dominantly dissolve as sodium, while at cryolite ratios commonly used in aluminum electrowinning (CR = 2.25 to 2.7) the AlF _-2 ^- -ion is the predominant dissolved metal species. Other species (A1₂F₃ ^-, A1₃F₄-, A1₄F₅-) were found to be of some significance only in melts with high excess A1F₃ (CR < 2).     

Leaching and kinetic modelling of low-grade calcareous sphalerite in acidic ferric chloride solution

The leaching kinetics of a low grade-calcareous sphalerite concentrate containing 38% ankerite and assaying 32% Zn, 7% Pb and 2.2% Fe was studied in HCl–FeCl₃ solution. An L₁₆ (five factors in four levels) standard orthogonal array was employed to evaluate the effect of Fe(III) and HCl concentration, reaction temperature, solid-to-liquid ratio and particle size on the reaction rate of sphalerite. Statistical techniques were used to determine that pulp density and Fe(III) concentration were the most significant factors affecting the leaching kinetics and to determine the optimum conditions for dissolution. The kinetic data were analyzed with the shrinking particle and shrinking core models. A new variant of the shrinking core model (SCM) best fitted the kinetic data in which both the interfacial transfer and diffusion across the product layer affect the reaction rate. The orders of reaction with respect to (C_Fe³⁺), (C_HCl), and (S/L) were 0.86, 0.21 and − 1.54, respectively. The activation energy for the dissolution was found to be 49.2 kJ/mol and a semi-empirical rate equation was derived to describe the process. Similar kinetic behavior was observed during sphalerite dissolution in acidic ferric sulphate and ferric chloride solutions, but the reaction rate constants obtained by leaching in chloride solutions were about tenfold higher than those in sulphate solutions.     

Separation of zinc from spent zinc-carbon batteries by selective leaching with sodium hydroxide

Physical methods such as crushing and sieving followed by magnetic separation steps were applied to separate non-magnetic material (88.4 wt.%) from zinc-carbon spent batteries. The oversize material was processed by eddy current separation to recover zinc sheet, carbon rods, and plastics. The undersize fraction (− 2.36 mm) with a metal composition of 15.5% Zn, 17.5% Mn, and 1.4% Fe was used for the leaching experiments under different conditions such as concentration of sodium hydroxide, temperature, agitation speed, and pulp density. Selective leaching using 4 mol dm^− 3 NaOH at 100 g dm^− 3 solid/liquid ratio, 80 °C and 200 rpm gave a zinc extraction of 82% and a manganese extraction of less than 0.1%. An overall zinc recovery was about 88.5% by the processes described in this study.     

Kinetics of galena dissolution in nitric acid solutions with hydrogen peroxide

The extraction of lead from a galena concentrate in nitric acid solutions with additional hydrogen peroxide was studied taking stirring speed, temperature, hydrogen peroxide and nitric acid concentrations, and particle size as dissolution parameters. The dissolution curves followed the surface chemical reaction controlled shrinking core model over the whole range of parameters, except at high nitric acid concentrations where the reaction was diffusion-controlled. The activation energy of 42 kJ mol^− 1 and a linear relationship between rate and inverse particle size support the reaction controlled dissolution mechanism. Hydrogen peroxide addition accelerated the reaction compared with nitric acid alone. It was concluded that the dissolution process is favourable, since the acid consumed for oxidation of galena can easily be regenerated in the same reactor by means of hydrogen peroxide.     

On the solubility of aluminum in cryolitic melts

The solubility of aluminum in NaF-AlF₃-Al₂O₃ melts with various additives was found to increase with increasing NaF/AlF₃ molar ratio (CR) and increasing temperature and to decrease with additions of A1₂O₃, CaF₂, MgF₂, and LiF to the melts. With the use of literature data for the activities of NaF and A1F₃ in cryolitic melts, three dissolution reaction models were found to give a good fit to the experimental solubility data. According to the most probable of these models the total concentration of dissolved aluminum (aluminum and sodium species) is given by c_Al = c_Na(diss) + c_AlF2- + c_Al2F3- + c_Al3F4- + c_Al4F5- In NaF rich melts, aluminum will dominantly dissolve as sodium, while at cryolite ratios commonly used in aluminum electrowinning (CR = 2.25 to 2.7) the AlF _-2 ^- -ion is the predominant dissolved metal species. Other species (A1₂F₃ ^-, A1₃F₄-, A1₄F₅-) were found to be of some significance only in melts with high excess A1F₃ (CR < 2).     

Measurement of solid solubility of germanium in aluminum

The solid solubility of Ge in aluminum was determined by resistivity measurement at 77 K of Al-Ge alloys annealed at various temperatures for times sufficient to complete the precipitation of the equilibrium Ge phase. The solid solubility of Ge in aluminum was found to vary from 0.252 at. pct at 533 K to 1.82 at. pct at 693 K. From the temperature dependence of the solid solubilities, the excess entropy of mixing ΔS and heat of mixing ΔH associated with the formation of solid solution of Ge in aluminum were calculated to be 2.77 R and 38.5 kJ mol^-1, respectively. Using these values of ΔS and ΔH, the maximum solid solubility of Ge in aluminum was calculated to be 2.08 at. pct at the eutectic temperature 697 K. The magnitude Δρ _Ge ⁷⁷ /Δc due to 1 at. pct impurity was determined to be 8.60 nΩm/at. pct Ge. The resistivity of Al-1.32 at. pct Ge alloys aged at 573 and 623 K was measured at 77 K to determine the time variation of the average concentrationc of the solute in the matrix during coarsening of the Ge precipitates. The value ofc, calculated using the value of Δρ _Ge ⁷⁷ /Δc, decreased linearly in accordance with the t^-1/3 law. The concentration of the solute in the matrix, in equilibrium with the Ge particle of infinite size, was determined by extrapolating thec vs t^-1/3 plot to infinite time, in excellent agreement with the above solid solubility. Formerly Graduate Student, Department of Materials Science, Tohoku University     

Kinetic study of the dissolution of cuprite in oxyacid solutions

The rates of disproportionation dissolution of three artificial cuprite samples were measured in sulfuric acid and perchloric acid solutions, from which oxygen had been stripped, at different concentrations and temperatures. Samples were prepared by electrolysis, sintering of electro-lytically produced cuprite, and oxidation of hot-rolled copper sheets. The effects of sodium sulfate and sodium perchlorate on the dissolution of cuprite were also examined. Metal copper formed by the disproportionation reaction plays a role as a barrier for further dissolution of cuprite. Apparent activation energies were determined in the temperature range of 293 to 323 K for the initial stage of the disproportionation reaction of cuprites. The values ranged from 14.7 to 24.9 kJ mol^-1 in 0.003 mol dm^-3 sulfuric acid and perchloric acid and from 29.7 to 52.0 kJ mol^-1 in 0.1 mol dm^-3 sulfuric acid and perchloric acid. Judging from the effects of temper-ature, agitation speed, acid concentration, and common salt additions, it is concluded that the adsorption of H⁺ onto the surface site is important in determining the dissolution rate of cuprite in oxyacid solutions. The dissolution behaviors of different cuprite samples were also morpho-logically examined.     

Intrinsic kinetics of the reaction between zinc sulfide and water vapor

The reaction between zinc sulfide and water vapor is a component reaction in a new reaction scheme recently developed to transform zinc sulfide to zinc oxide through the use of lime and water vapor. The intrinsic kinetics of this reaction for ultrafinely ground (<1 μm) ZnS particles was determined by carrying out measurements in the absence of heat- and mass-transfer effects. The reaction products were identified to be ZnO and H₂S. The kinetics of the reaction can be represented by and 3.94<C _H2_O <9.84 mol/m³ withn=1.28 andk ₁=45.3 exp(−14600/T) m³·mol⁻¹·s⁻¹, whereX _B is the fractional conversion. DAESOO KIM, formerly Granduate Student at the University of Utah     

Creep of laminated aluminum composites

The creep behavior of a laminate system consisting of alternate layers of pure aluminum and SAP (sintered aluminum powder) sheet has been examined in the temperature range 323 to 473 K and in the stress range 35 to 68 MN m⁻². It was observed that secondary creep strain in the laminates was greater than in elemental SAP; the secondary creep strain rate in laminates was lower than that in pure aluminum and the creep rate decreased with increasing fracture of SAP. A stress exponent (n) value of ∼20 was observed for most of the laminates and was reasonably constant for 3, 5, 7, and 9 ply laminates and volume fractionsV _f ) in the range 0.3 <V _f < 0.65. For higher volume fractions of SAP the mechanical behavior of the laminates was similar to that of SAP. The experimental activation energy for creep of 30.5 ± 5 Kcal mol⁻¹ correlates well with that for self-diffusion in aluminum. Laminating induced appreciable ductility to the SAP. Formerly Postgraduate Research Student, Department of Metallurgy, University of Manchester/UMIST