Inhibition of lead solubilization during the leaching of gold and silver in ammoniacal thiosulfate solutions (effect of phosphate addition)

The chemistry of precious metal ammoniacal thiosulfate leaching is very complex, especially with additives, such as EDTA, which are used to stabilize the major reactants in this system. Lead, which is co-extracted in these solutions, can be a large consumer of both EDTA and thiosulfate, decreasing the leaching capacity for gold and silver. In this research, thermodynamic analyses for different compositions of the leaching solution are performed to determine their ability to extract lead, silver and gold. These are used to explain the leaching behavior observed for a sulfide concentrate and for a mineral, both with high silver and lead contents and the latter, with an elevated concentration of gold.     

A strategy to determine the potential interval for selective silver electrodeposition from ammoniacal thiosulfate solutions

A strategy based upon a thermodynamic analysis, followed by a microelectrolysis study, was used to determine the potential interval for the selective electrodeposition of silver from a typical thiosulfate leaching solution (0.001 M Ag(I), 0.05 M Cu(II)/Cu(I), 0.2 M S₂O₃²⁻, 0.025 M EDTA, 0.6 M NH₃). In the thermodynamic analysis presented in this work, the system behavior was shown to be highly sensitive to changes in the solution pH, potential and concentration of some of its components. This fact, in addition to the similarity between the silver and copper species, obligates a strict control of each of the system variables in order to achieve efficient separations only under these conditions can the direct electrodeposition of silver on an industrial level be a viable alternative. In the first stage of the microelectrolysis study, it was determined that stainless steel 304 (SAE 304) and aluminum (Al) promote the electroactivity of the ammoniacal thiosulfate solution and therefore, are not recommended as substrate for the silver reduction process. In contrast, the vitreous carbon (VC) and titanium (Ti) do not, and for that reason these two substrates were used in the study of the electrodeposition of silver and copper. In the second stage, it was shown that the potential window for selective silver electrodeposition is 50 mV larger on titanium than on the vitreous carbon electrode, and this window is smaller and shifted to more negative potentials, as the thiosulfate concentrations increases, just as the thermodynamics analysis predicted.     

氰化法、硫代硫酸盐法、硫脲法浸出某难浸银精矿比较研究

对某浮选银精矿采用氰化法、硫代硫酸盐法、硫脲法进行银浸出实验,以评估3种不同浸出方法对该银精矿浸出效果．研究结果表明：在最优的工艺参数下[氰化法：碱预处理后加入铵盐,氰化钠浓度0.15 mol/L,pH值为11,液固比（浸出液体积与试样质量之比,单位是mL/g,下同）2∶1,搅拌强度500 r/min,浸出时间为48 h;硫代硫酸盐法：氧化焙烧预处理,氨水浓度1.0 mol/L,硫酸铜浓度0.01 mol/L,硫代硫酸钠浓度0.2 mol/L,pH值为10,液固比4∶1,搅拌强度500 r/min,浸出时间12 h;硫脲法：氧化焙烧和稀硫酸预浸处理,Fe3+浓度7.5×10-3 mol/L,硫脲浓度0.18 mol/L,pH=1.0,液固比为3∶1,搅拌速度250 r/min,浸出时间24 h],硫代硫酸盐方法获得的Ag浸出率为78.58%,硫脲相应获得的Ag浸出率为62.43%,比氰化法获得的浸出率低约3.33%．     

Selective separation of nickel from cobalt in ammoniacal solutions by emulsion type liquid membranes using 8-hydroxyquinoline (8-HQ) as mobile carrier

The selective separation and concentration of nickel from ammoniacal solutions containing nickel and cobalt by an emulsion liquid membrane (ELM) technique using 8-hydroxyquinoline (8-HQ) as carrier has been examined. The emulsion liquid membrane consists of a diluent (kerosene), a surfactant (ECA 4360J), a carrier (8-HQ), and a stripping solution (0.025 M EDTA solution, buffered at pH 4.0). Cobalt (II) in the 6 M ammonia feed solution was oxidised to cobalt (III) by adding H₂O₂ and the pH adjusted to 10 with hydrochloric acid (HCl). The important variables were found to be membrane composition, ammonia concentration, diluent type, surfactant concentration, extractant concentration, EDTA concentration in the stripping solution, pH of the feed and the stripping solutions, phase ratio, and treatment ratio. It was possible to selectively extract 96.5 to 99.0% of nickel from a mixture of nickel and cobalt.     

The reduction of copper(II) and the oxidation of thiosulfate and oxysulfur anions in gold leaching solutions

Due to the increasing environmental and public concerns associated with cyanide leaching of gold, much of the current research in gold hydrometallurgy is focussed on alternative systems, such as thiosulfate. One of the major problems with the thiosulfate leaching system is the undesirable reaction between thiosulfate and copper(II). The current paper presents a study on the effect of various anions on the rate of this reaction. It will be shown that the presence of anions such as sulfate, chloride and phosphate significantly reduce the rate at which copper(II) is reduced by thiosulfate. The presence of other oxysulfur species such as sulfite, tetrathionate and trithionate were also studied. These species were found to be readily oxidised by copper(II) and hence greatly increased the rate of copper(II) reduction. The rate of copper(II) reduction by tetrathionate is relatively unaffected by the addition of anions; however, anion addition was found to significantly influence the rate of copper(II) reduction by sulfite and trithionate. Based on these results, inner sphere mechanisms were proposed for the reaction of copper(II) with thiosulfate, sulfite and trithionate. The kinetics of copper(II) reduction is thus very complex and can be represented by a number of parallel-series reaction sequences.     

Kinetics of leaching of copper metal in copper(II)-ammonium sulphate solutions as determined by the rotating disc method

The influence of the concentration of copper(II)-ammine-sulphate complexes, temperature and the concentration of ammonia on the velocity of the leaching of copper metal in sulphate solutions was investigated by application of the rotating disc method.The results show that the following factors determine the velocity of the process: transport of copper(II) complexes to the reaction surface and the rate of the surface reaction (mixed control).An attempt was made to derive a semi-empirical equation describing the kinetics of the process. The value of the diffusion coefficient, calculated on the basis of the derived formula and the experimental data, is equal to (1.2 ± 0.1) × 10^?9 m² s^?1 (for solutions containing from 0.0012 to about 1 mol/l copper, 6.5 mol/l ammonia and 1.2 mol/l sulphates; temperature 25°C).The diffusion activation energy, determined in the range of temperatures from 15 to 50°C equals 30.1 ± 1.7 kJ/mole. This is surprisingly high especially in comparison with the activation energy of viscous flow determined at the same conditions, which is 15.9 ± 0.4 kJ/mole.Discussion of the results leads to the conclusion that free ammonia (the stoichiometric excess in relation to the amount necessary for a complete bonding of copper in a complex) does not participate in the electrode reaction. However, it diminishes the velocity of transport of copper complexes to the reaction surface.     

The coprecipitation behavior of Co(II) and Ni(II) with Fe(III), Cr(III) and Al(III) from aqueous ammoniacal solutions

The coprecipitation of cobalt(II) and nickel(II) with iron(III), chromium(III) and aluminum(III) from ammoniacal solutions has been investigated. The coprecipitation behavior was found to be very sensitive to the solution pH and total ammonia concentration. Co(II) and Ni(II) can be precipitated from low ammonia concentration solutions but are readily redissolved at higher ammonia concentrations. The coprecipitate of divalent and trivalent species was found to contain very large amounts of the divalent metals (up to a mole ratio M M(II)/M(III) of 2.5) when aluminum was the trivalent species, whereas with iron(III) or chromium(III), the ratio was only 0.5.     

Selective extraction and separation of metals especially silver (and copper) using poly(2-S-vinyl-1,3,4-thiadiazole-5-thiol)

There have been many attempts to prepare polymeric reagents which are capable of selectively extracting metal ions such as those of silver. The title polymer (PVTT) was used to extract ions of Ag(I) and Cu from separate and mixed solutions. The polymer has a capacity of 1.86 mol kg^?1 of Ag(I) with respect to the dry polymer from silver nitrate solution and 0.88 mol kg^?1 of Cu from copper(II) chloride solution. In a mixed Ag(I) and Cu(II) nitrate solution only the Ag(I) is extracted. The polymer does have low capacities for many other ions including Fe(II), Fe(III), Co(II), Ni(II) and Zn(II). The time ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for the silver uptake on half the sites of the polymer is well under five minutes at pH 5. The polymer is cheap and stable and, for example, is not subject to hydrolysis under the conditions used for metal extraction and can be made from readily available chemicals. The silver can be eluted from the polymer-silver complex using KCN (0.1 M). The time ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for half of the silver to be eluted is under five minutes. The regenerated polymer can be used for repeated separations without loss of capacity. When copper ions are complexed by the polymer there is evidence that some, at least, of the ions are reduced to Cu(I). The time ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for copper extraction is approximately five minutes and the copper is probably eluted as a Cu(I) species.In the copper complex of 2-S-methyl-1,3,4-thiadiazole-5-thiol (MTT) the copper is present entirely as Cu(I). The bonding in this and the corresponding Ag complex is thought to involve the 2-S-methyl and 5-thiol sulphur atoms which gives rise to polymeric species with linear SMS stereochemistry. The bonding in the polymer-metal compounds is thought to be similar. The selectivity of the polymer for Ag(I) ions arises from the ability of the thiol group to bond to this ion which causes it to separate out of solution. The polymer is able to satisfy the stereochemical requirement of the silver ion. The thiadiazole ring is resistant to oxidation or reduction under the conditions required for metal extraction purposes. The presence of NH groups on the ring increases the hydrophilicity of the polymer and enables rapid diffusion of the aquated ions to the site of co-ordination. The regenerated polymer can be used for extracting Ag(I) ions from very dilute (5 mg/l solutions.     

Awaruite (Ni3Fe) as a nickel resource—leaching with ammoniacal–ammonium solution containing citrate and thiosulfate

A method was developed for extraction of nickel from ultramafic rocks containing awaruite, an alloy of nickel and iron (nominally Ni₃Fe). The leaching solution contained ammonia, ammonium, citrate and thiosulfate at moderately alkaline pH. Leaching studies were carried out on synthetic Ni₃Fe, josephinite (a naturally occurring material consisting mainly of awaruite from Oregon, USA) and serpentinite containing low levels of awaruite from British Columbia, Canada. In addition to citrate, several other carboxylic acid ligands were also found to be suitable for this process.     

Selective adsorption of chromium(VI) from zinc(II) and other metal ions using persimmon waste gel

An adsorption gel was prepared from persimmon waste, a cellulosic material, rich in polyphenolic compounds, which exhibits a high affinity for chromium(VI). It was prepared by cross-linking persimmon waste with concentrated sulphuric acid. Adsorption tests for different metal ions such as Cr(VI), Cr(III), Fe(III), Zn(II), Cd(II) and Pb(II) from aqueous solution at pH values ranging from (pH 1 to 5) found Cr(VI) to be selectively adsorbed on the cross-linked gel over the other metal ions studied. The adsorption isotherm of Cr(VI) followed the Langmuir type of adsorption and exhibited a maximum loading capacity of 7.18 mol kg^− 1 at pH 1. Selective removal of Cr(VI) from Zn(II) was successfully demonstrated by using a column packed with the persimmon waste gel.     

The solvent extraction of lead from chloride solutions using di(2-ethylhexyl)phosphoric acid

Lead has been extracted from chloride solutions into kerosene containing excess di(2-ethylhexyl)phosphoric acid, using various concentrations of metal and extractant. Extraction proceeded readily, but the results indicated that 1.6 hydrogen ions were exchanged per atom of lead extracted, rather than 2.0 as would be expected for a divalent metal ion. Extraction was unaffected by metal concentration over the range studied (0.1–1.0 g l^?1), and the results were highly consistent. The unique log D vs. pH relation allowed simple collection of data for contact stage evaluation by rearrangement of the familiar solvent extraction equations and use of empirically derived constants. An S-shaped extraction isotherm resulted, which limits the ease with which aqueous lead concentrations may be reduced by extraction with di(2-ethylhexyl)phosphoric acid.     

Acoustic startle electromyographic (EMG) activity indexed from an electroculographic (EOG) electrode placement: a methodological note

Acoustic startle EMG responses were indexed from a traditional EOG electrode placement in 11 young adults while they viewed slides which varied in affective valence. We found, replicating earlier work that subjects' startle responses became more augmented as the content of the slides became more negative. The advantages of using an EOG electrode placement for measuring acoustic startle responses are discussed.     

4-(3-吡啶基)-2-巯基咪唑修饰碳糊电极阳极溶出伏安法测定痕量银

报道了采用化合物4-(3-吡啶基)-2-巯基咪唑(PMI)修饰碳糊电极测定痕量银的阳极溶出伏安法。在0.1 mol/L的HNO3中,Ag+可以富集于PMI修饰电极表面,将介质交换至含I-的0.02 mol/L硫酸溶液中,-0.20 V还原30 s后再进行阳极溶出伏安测定,可以获得灵敏的银阳极溶出峰。一次导数峰电流与Ag+在8.0×10-10~4.0×10-6mol/L浓度范围内呈线性关系,检出限可达5.0×10-10mol/L(S/N=3)。采用本法不经过预分离,对成分复杂的定影液废液和锌合金等样品进行了直接测定,测定的回收率为92%~105%。     

Extraction of Fe(III) from hydrochloric acid solutions using Amberlite XAD-7 resin impregnated with trioctylphosphine oxide (Cyanex 921)

Ferric ions were efficiently removed from HCl solutions using Amberlite XAD-7 resin impregnated with trioctylphosphine oxide (Cyanex 921). Iron was removed under the form HFeCl₄ through direct binding on the resin or by extraction with Cyanex 921 involving a solvation mechanism. High concentrations of HCl and intermediary extractant loadings were required for maximum sorption efficiency and rationale use of the extractant. At intermediary extractant loading (in the range 300–450 mg Cyanex 921 g^− 1) the maximum sorption capacity increased with extractant loading. Maximum sorption capacity slightly increased with temperature, the reaction is endothermic and the enthalpy change was found close to − 30.8 kJ mol^− 1. Sorption isotherms were fitted with the Langmuir equation and maximum sorption capacity reached values as high as 20–22 mg Fe g^− 1 in 3 M HCl solutions. Despite the good fit of experimental data with the pseudo second-order rate equation, sorption kinetics was controlled by the resistance to intraparticle diffusion. The intraparticle diffusion coefficient (D_e) varying in the range 1.2 × 10^− 11–4.7 × 10^− 10 m² min^− 1 was found to increase with metal concentration and with temperature, while varying the extractant loading it reached a maximum at a loading close to 453 mg Cyanex 921 g^− 1. The desorption of Fe(III) can be achieved using 0.1 M solutions of nitric acid, sulfuric acid, sodium sulfate and even water, maintaining high efficiencies for sorption and desorption for at least 5 cycles.     

Selective removal of arsenic(V) from a molybdate plant liquor by precipitation of magnesium arsenate

Precipitation of Mg₃(AsO₄)₂ for the removal of arsenic (As) from a molybdenum oxide processing plant liquor containing 70.9 g/L Mo(VI) and 469 mg/L As(V) was performed. The Stabcal software was used to model the speciation and solubility equilibria, and to identify the pH conditions at which optimum precipitation can be carried out. The optimum pH range for As(V) removal is between pH 7.5 and 10.2. This avoids the need to adjust the liquor pH to affect the precipitation. By adding magnesium chloride or sulphate at a Mg:As molar ratio of at least 2:1, As(V) could be removed to less than 5 mg/L at pH 10.2 resulting in a pure Mo(VI) liquor from which a high purity product of 99.9% MoO₃ could be produced by acidification.     

Selective structure of the gene pool. III. Technology of determination from F(ST)-statistics using numerical resampling

The selection pressure on the ith gene was determined on the basis of the heterogeneity of interpopulation diversity FST(i), assumpting that the significant deviation of FST(i) (observed differentiation of the ith gene) from Fe (selectively neutral differentiation) indicates selection. A selectively neutral variation Fe was estimated from the mean FST, averaged over the total set of genes. Since the validity of the equation FST approximately Fe = (4NeMe + 1)-1 depends on the fulfillment of conditions of the theoretical model as well as on organization of sampling of genes and subpopulations rather than on statistical parameters of the gene sample, an additional correction by boot-strapping is suggested. Simulation of sampling of the gene pool using numerical resampling (10(6) samples from each of the 50 markers) allowed us to characterize the selective structure of the gene pool. This structure consists of the following: one class of selectively neutral genes, NEUTRAL (FST(i) approximately Fe); and two classes of selective genes, LOWER DIFF (FST(i) < Fe) and SUPER DIFF (FST(i) > Fe). Interclass regions N-LOWER and N-SUPER between the classes of neutral and selective genes were defined with the use of bootstrapping. The hypothesis on the presence or absence of selection cannot be formulated with regard to the genes included in the interclass regions. A general trend for all gene pools of the world was revealed: the ratio between the boundary values of selective classes and Fe is constant. On the basis of this, a rapid and simple technique for estimating selective values of genes was developed. The positions of more than 80 immunobiochemical gene markers in the selective structure of six gene pools of the world (Europe, Asia, Africa, Australia, America, and northeastern Eurasia) are presented. The selective structure of five subregions of northeastern Eurasia (Caucasus, Europe, Ural, Siberia, and Central Asia) are characterized as well.     

The selective recovery of cadmium(II) from sulfate solutions by a counter-current extraction–stripping process using a mixture of diisopropylsalicylic acid and Cyanex® 471X

The liquid–liquid extraction of cadmium(II) from solutions, obtained by leaching of deposited cadmium carbonate or cadmium cementation sponge and heavily contaminated with other components, especially zinc, has been studied. Initially the solutions were purified; iron and other contaminants were removed by oxyhydrolysis. An equimolar mixture of diisopropylsalicylic acid and Cyanex® 471X (0.5 mol/L/0.5 mol/L), diluted with Solvesso 150, was used as the solvent. Experiments were performed both, as a laboratory simulation of a counter-current system, and in continuous counter-current trials using mixer-settlers. Good separation of cadmium(II) and zinc(II) has been obtained.     

Palladium(II) complexes adsorption from the chloride solutions with macrocomponent addition using strongly basic anion exchange resins, type 1

The use of the strongly basic anion exchange resins, type 1 such as Lewatit MP-500 and Lewatit MP-500A for palladium(II) complexes adsorption has been investigated. The adsorption process was carried out from the chloride solutions with macrocomponent (sodium chloride) addition (x M HCl–1.0 M NaCl; x M HCl–2.0 M NaCl) where the concentration of hydrochloric acid was constant and equal to x = 0; 0.1; 0.5; 1.0; and 2.0 M, respectively. The breakthrough curves of Pd(II) were determined and the sorption parameters (weight and bed distribution coefficients, working anion exchange capacity) were calculated. The pseudo-second kinetic order was applied in kinetic studies as well as to calculate the kinetic parameters. The values of the working anion exchange capacities (0.029 g/cm³; 0.028 g/cm³) for Lewatit MP-500 and Lewatit MP-500A (0.028 g/cm³; 0.027 g/cm³) in the 1.0 M NaCl and 0.1 M HCl–1.0 M NaCl solutions, respectively are really close and in other solutions under discussion Lewatit MP-500 possess slightly higher values of capacities, and therefore is insignificantly more efficient in the adsorption process of palladium(II) ions than Lewatit MP-500A. The equilibrium adsorption capacities changed in the range 8.84–9.99 and 8.40–9.38 mg/g for Lewatit MP-500 as well as 8.12–9.57 and 7.26–8.85 mg/g for Lewatit MP-500A in the chloride x M HCl-1.0 M NaCl and x M HCl-2.0 M NaCl solutions, respectively. The adsorption process proceeds according to the pseudo-second kinetic order.     

Solvent extraction and separation of Y(III) from sulfate,nitrate and chloride solutions using PC88A diluted in kerosene

Solvent extraction of yttrium (Y) from different aqueous solutions using the organic extractant 2- ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC88A) diluted in kerosene was investigated. Solution chemistry considerations were proposed to identify the predominant species in different aqueous media (H₂SO₄, HCl and HNO₃). In general, nitric and hydrochloric acids were found to be more efficient than sulfuric acid for yttrium extraction. Separation studies showed that nitric acid provide higher separation of Y(III) from light REES while sulfuric acid works better when separation of Y(III) from heavy REEs is desired.