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].     

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.     

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

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

Processing copper–vanadium precipitate formed from crude TiCl4 in titania and titanium sponge production

Copper wire is used to remove vanadium from crude TiCl₄ in titania and titanium sponge production which produces a copper–vanadium precipitate. The recovery of copper and vanadium from this precipitate was studied. Experiments found that the precipitate can be naturally oxidized by stacking for one month in air, converting > 90% metallic copper contained in the original precipitate into CuCl₂·2H₂O, Cu₂Cl(OH)₃ and Cu₂(OH)₃Cl. The copper oxy-chlorides were easily converted to Cu(OH)₂ by stirring in dilute NaOH at pH 11 and 80 °C under a liquid-to-solid ratio of 4:1. When the pH was lowered to about pH 2.5 by sulfuric acid, iron, titanium and vanadium oxides remained in the first acid leach residue and copper was selectively leached into solution. By evaporating and cooling the leach solution, a product of CuSO₄·5H₂O with 99.7% purity was obtained.To recover the vanadium, the filter cake was roasted with Na₂CO₃ at 700 °C for 3 h under the stoichiometric proportion of 2.5 for V. The calcine was then leached with water at 70 °C and NH₄VO₃ was precipitated by the addition of NH₄Cl. Calcination of NH₄VO₃ at 550 °C for 2 h produced V₂O₅ with a purity of 98.6%. After vanadium recovery, the residue was leached once again with sulfuric acid and the total recoveries of copper and vanadium were 98.6% and 95.7% respectively.     

Screening of bacterial cells for biosorption of oxyanions: Application of micro-PIXE for measurement of biosorption

An attempt was made to isolate bacterial strains capable of biologically removing tungstate (WO₄²⁻) and perrhenate (ReO₄⁻). Thirty-eight water samples were collected from various areas of Anzali lagoon, Iran. Initial screening of a total of 100 bacterial isolates, resulted in the selection of one isolate with maximum biosorption capacity of WO₄²⁻ and ReO₄⁻. It was tentatively identified as Bacillus sp. according to morphological and biochemical properties and named strain GT-83. WO₄²⁻ and ReO₄⁻ uptake by Bacillus sp. GT-83 involved both inactive and active phenomena. The amount of WO₄²⁻ (initial concentration 184 mg/l) removed from aqueous solution after 16 h by inactive and active phenomena was 26 and 194.5 μg/mg protein, respectively. The strain also removed ReO₄⁻ inactively (23 μg/mg protein). Bacillus sp. GT-83 tolerated high MIC of the oxyanions. The order of toxicity of the oxyanions to the bacterium was WO₄²⁻ > ReO₄⁻ in solid media. The effects of increasing metal concentrations on the growth rate were determined in order to obtain precise patterns of resistance in liquid cultures. From the results of the oxyanions toxicity, inhibitory concentrations in solid media were higher than those in liquid media. Oxyanions biosorption was determined during the course of growth. Bacillus sp. GT-83 was capable of removing WO₄²⁻ and ReO₄⁻ during the active growth cycle with a sorption capacity of 194.5 μg WO₄²⁻/mg protein and 137.1 μg ReO₄⁻/mg protein. In view of the results of oxyanions accumulation experiments, it was concluded that Bacillus sp. GT-83 was not only tolerant to oxyanions, but it also bound considerable amounts of WO₄²⁻ and ReO₄⁻ from the growth medium. The binding of tungsten and rhenium on the cell wall of Bacillus sp. GT-83 was confirmed with micro-PIXE.     

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

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

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%).     

Jarosite pseudomorph formation from arsenopyrite oxidation using Acidithiobacillus ferrooxidans

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

Electronic spectra of oxofluoride complexes of vanadium(V, IV, III) in halide melts

The electronic spectra of vanadium(V, IV, III) oxide solutions in chloride-fluoride melts have been measured. From the spectral data, the complex groups of vanadium ions in these melts are estimated. The solubility limits of VO₂ and V₂O₃ oxides in chloride-fluoride melts are found.     

Vaporization Studies from Slag Surfaces Using a Thin Film Technique

The investigations of vanadium vaporization from CaO-SiO₂-FeO-V₂O₅ thin film slags were conducted using the single hot thermocouple technique (SHTT) with air as the oxidizing atmosphere. The slag samples were analyzed after the experiments by SEM/EDX. The vanadium content was found to decrease as a function of time. The loss of vanadium from the slag film after 30 minutes of oxidation was approximately 18 pct and after 50 minutes, it was nearly 56 pct. The possible mechanism of vanadium loss would be the surface oxidation of vanadium oxide in the slag, VO _x to V⁵⁺, followed by surface evaporation of V₂O₅, which has a high vapor pressure at the experimental temperature.     

Intracellular recovery of gold by microbial reduction of AuCl4 ions using the anaerobic bacterium Shewanella algae

Microbial reduction and intracellular precipitation of gold was achieved at 25 °C and pH 7 by using the mesophilic anaerobic bacterium Shewanella algae with H₂ as the electron donor. The reductive precipitation of gold by S. algae was a fast process: 0.1–1 mol/m³ AuCl₄⁻ ions were completely reduced to insoluble gold within 30 min. The biogenic precipitates were crystalline gold nanoparticles of 10–20 nm present in the periplasmic space. The reducing power of S. algae at 3.2 × 10¹⁵ cells/m³ and 25 °C was comparable to that of aqueous citric acid solution (chemical reductant) at 20 mol/m³ and 50 °C. The intracellular recovery of gold is potentially attractive as an environmentally friendly alternative to conventional methods.     

Distribution of vanadium between SiO2 rich slags and carbon saturated liquid iron

In order to determine optimal slag compositions for the extraction of vanadium from hot metal to SiO₂ rich slags, vanadium distributions were determined between slags of the following systems: FeO*–SiO₂ [(%V) ã 1.7] FeO*–SiO₂(sat)-CaO,Na₂O,MgO,Al₂O₃,TiO₂ [(%V) ã 1.7] FeO*–SiO₂–VO_n [(%FeO*)/(%SiO₂) = 1.5–1.7] and carbon saturated liquid iron at 1 300°C using the solid iron foil technique. The distribution increased with FeO* content in the binary FeO*–SiO₂ system, while additions of CaO, Na₂O, MgO and Al₂O₃ to FeO*–SiO₂ based slags under SiO₂ saturation caused the distribution to decrease. A slight decrease in distribution was also observed with increasing vanadium oxide content in FeO*–SiO₂ based slags having a constant (%FeO*)/(%SiO₂) ratio. The highest vanadium distributions were found in SiO₂ saturated slags with high TiO₂ contents. Vanadium valencies in the slags were determined by a wet analytic titration technique and the results showed that V^III+ is predominant. It was suggested that the predominating ionic species of vanadium in SiO₂ saturated slags are V³⁺ and VO⁺ while a change towards VO^?₂ may occur for FeO rich slags.     

Biosorption of lanthanum and cerium from aqueous solutions by Platanus orientalis leaf powder

The biosorption of lanthanum and cerium by leaves powder of Platanus orientalis was separately determined at varying experimental conditions using a batch technique. The effects of initial pH, contact time, initial metal ion concentration and temperature were investigated. The Langmuir and Freundlich isotherms were applied to represent the adsorption process. Langmuir isotherm fits the experimental data quite well. The Langmuir monolayer capacity of the sorbent is 28.65mg g^− 1 and 32.05mg g^− 1 for La and Ce(III), respectively. Thermodynamic parameters such as standard enthalpy (ΔH°), entropy (ΔS°) and free energy (ΔG°) were calculated from the slope and intercept of the plots of lnK_d versus 1/T. The results indicated that sorption was endothermic and spontaneous in nature. The work reveals that powdered leaf of Platanus orientalis is a good choice as a biosorbent for the recovery of lanthanum and cerium from aqueous solution.     

Leaching of vanadium from carbonaceous shale

The leaching of vanadium from carbonaceous shale using dilute H₂SO₄ was investigated, and the mechanism of leaching determined. The results showed that higher leaching efficiency of vanadium was obtained by increasing initial concentration of H₂SO₄, raising leaching temperature and prolonging leaching time. Addition of ammonium fluoride also enhanced recovery. A recovery of 92% was obtained using a liquid to solid ratio 4:1, initial H₂SO₄ concentration 18%, NH₄F addition 4.8 wt.% of carbonaceous shale, leaching temperature 95 °C and contact time 8 h; the recovery was only 56% without NH₄F. The presence of NH₄F enhanced the leaching of vanadiferous mica.     

On the Formation of Vanadium Ferrites in CaO–SiO2–FeO–V2O5 Slags

Selective extraction of valuable elements (such as V, Cr, Mn, etc.) and their compounds from metallurgical slag is in a focus of many researchers. Although vanadium may be present in slag as oxides and/or complex spinels with Fe, Mn, etc. during an alloyed steel production, the majority of vanadium in metallurgical slags typically exists as V₂O₅, which comprises up to 3–5 wt% of the slag in some cases. Due to the vanadium toxicity, these slags are forbidden in many civil engineering applications. As a result, hundreds of thousand tonnes of V₂O₅‐bearing slags are landfilled every year. In the present work, the formation of vanadium ferrites (FeV₂O₄ and Fe₂VO₄) in synthesized CaO–SiO₂–FeO–V₂O₅ slags containing 5 wt% V₂O₅ was examined under different partial pressures of oxygen. For the current slag chemistry range, an XRD analysis confirmed the presence of vanadium ferrite in slag samples treated at 1773 K in an argon atmosphere (PO₂ = 10^?1–10^?2 Pa) while no solid was noted in samples treated in air. Results were discussed based on thermodynamic consistency.     

Effect of manganese and/or ceria loading on V_2O_5-MoO_3/TiO_2 NH_3 selective catalytic reduction catalyst

The effect of manganese and/or ceria loading of V_2 O_5-Mo_O_3/TiO_2 catalysts was investigated for selective catalytic reduction(SCR) of NO_x by NH_3.The manganese and/or ceria loaded V_2 O_5-MoO_3/TiO_2 catalysts we re prepared by the wetness impregnation method.The physicochemical characteristics of the catalysts were thoroughly characterized.The catalytic performance of 1.5 wt% V_2 O_5-3 wt% MoO_3/TiO_2(V1.5 Mo3/Ti) is greatly enhanced by addition of 2.5 wt% MnO_x and 3.0 wt% CeO_2(V1.5 Mo3 Mn2.5 Ce3/Ti) below450℃.Compared with the V1.5 Mo3/Ti catalyst with NO_x conversion of 75% at 275 ℃,V1.5 Mo3 Mn2.5 Ce3/Ti exhibits higher NO_x conversion of 84% with good resistance to SO_2 and H_2 O at a gas hourly space velocity value of 150000 h~(-1).The active manganese,cerium,molybdenum,and vanadium oxide species are highly dispersed on the catalyst surface and some synergistic effects exist among these species.Addition of MnO_x significantly enhances the redox ability of the cerium,vanadium,and molybdenum species.Addition of Ce increases the acidity of the catalyst.More active oxygen species,including surface chemisorbed oxygen,form with addition of Mn and/or Ce.Because of the synergistic effects,appropriate proportions of manganese in different valence states exist in the catalysts.In summary,the good redox ability and the strong acidity contribute to the high NH3-SCR activity and N2 selectivity of the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst in a wide temperature range.And the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst shows good resistance to H_2 O and SO2 in long-time catalytic testing,which can be ascribed to the highly sulfated species adsorbed on the catalyst.     

Sorption of rhenium and vanadium from mineralized solutions by fibrous ionites

Sorption of rhenium(VII) and vanadium(V) from mineralized sulfate-chloride solutions by fibrous ionites of the FIBAN series is investigated. Equilibrium, kinetic, and dynamic characteristics of sorption of Re and V by the FIBAN ionite of the AK-22 brand, which contains functional groups =NH₂, -NH₂, -COOH, and ≡N, are found. It is established that the maximal capacity of this reagent in regards to vanadium(V) is observed at pH = 4. Sorption isotherms of Re and V are linear, being described by the Henry equation with constants K _H = 1.36 ± 0.30 dm³/g (R ² = 0.995) and 674 ± 21 cm³/g (R ² = 0.999), respectively. Integral sorption kinetic curves are found under conditions of a limited solution volume, and effective diffusion coefficients of Re and V, which constituted 9.0 × 10^?13 and 7.5 × 10^?15 m²/s, respectively, are calculated allowing for the half-transformation time. The possibility of separating these metals in dynamic conditions is shown.     

Equilibrium slag losses in ferrovanadium production

Ferrovanadium is sometimes produced from V₂O₃ in electric arc furnaces, using aluminum as a reductant. The CaO fluxes the alumina, which forms during reduction of the vanadium oxide. Incomplete reduction of vanadium oxide from the slag is a significant cause of vanadium losses. To quantify factors that can affect the equilibrium vanadium loss, the vanadium oxide activity coefficient was measured experimentally for different slag compositions. Hydrogen-water mixtures were used to control the partial oxygen pressure (approximately 10⁻¹³ atm) ove CaO-Al₂O₃ slags contained in vanadium crucibles at 1700 °C; gas-phase mass transfer was controlled by jetting the gas mixture onto the slag surface. Manipulation of the redox conditions at a single slag composition and temperature showed that, as expected, the vanadium is present in the trivalent state in the slag. The slag basicity (CaO-Al₂O₃ ratio) was found to have a very strong effect on the activity coefficient of VO_1.5, with clear implications for the effect of plant practice on vanadium loss. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Recovery of vanadium during ammonium molybdate production using ion exchange

The recovery of vanadium in ammonium molybdate production with raw sodium molybdate solution was studied. Experimental results showed that the vanadium and some of impurities P, As and Si can be adsorbed along with the molybdenum from the feed liquid by the weak base resin D314 in the pH range of 2.5-3.5 and then they can be eluted from the loaded resin with ammonia liquor. The vanadium can partially natural-precipitate from the eluted solution. The lower the pH value is and, the longer the standing time is, the less the vanadium remained in the solution will be. Standing for 24 h in pH value 6.9, the vanadium in the solution was reduced to 0.51 g/L V₂O₅. It was found that the precipitate is ammonium isopoly-vanadate and it is impure. By washing the precipitate with hydrochloric acid and ammonia solution sequentially and then roasting at 500 °C for 2 h, the product of V₂O₅ with the purity 99.12% was obtained. The impurities P, As and Si in the stood solution were removed by purifying with MgCl₂ under the pH value range of 8.0-9.0 at 60-80 °C for about 2 h, while the removal of the vanadium in the solution was performed by adsorbing with the strong base resin D296 in pH value about 7.0. The devanadiumized solution can be used to produce high-quality ammonium molybdate. The loaded vanadium resin was eluted with HCl 6 mol/L and, the eluted solution was returned to adjust the sodium molybdate solution pH value. The vanadium can be effectively separated and recovered in the process.     

Hydrometallurgical extraction of zinc from Jordanian electric arc furnace dust

Zinc was extracted in a Jordanian Steel Plant using an electric arc furnace dust. Sulfuric, nitric and hydrochloric acids were used at different concentrations to recover zinc from dust particles. The highest zinc extraction was obtained at low acid concentration of less than 1 mol/L in the order of H₂SO₄ followed by HNO₃ and then HCl. The kinetics of zinc extraction using H₂SO₄ showed a maximum zinc recovery of 72% obtained by using 0.1 mol/L acid concentration, 900 rpm agitation speed and 50 °C.