A rotating disk study of silver dissolution with thiourea in the presence of ferric sulfate

The rotating disk technique was used to study silver dissolution with thiourea as a function of sulfuric acid, ferric sulfate, and thiourea concentrations. The effect of many foreign ions (Mn, Cu, Co, Ca, Na,etc.) and various additives was also examined. The dissolution of silver was zero order with sulfuric acid, first order with ferric sulfate, and second order with thiourea. Among the foreign ions, copper had a dramatically negative effect. The strong oxidants such as hydrogen peroxide and manganese dioxide were also detrimental for silver dissolution. According to the temperature effect studied (5 °C to 35 °C), the activation energy was 22.6 kJ/ mole. Silver does not dissolve with thiourea in the absence of ferric ions. Sulfuric acid does not participate in the dissolution reaction. The most important parameter for silver dissolution is the ferric sulfate/thiourea ratio. In excess ferric sulfate, a solid silver-thiourea complex is formed, which precludes transfer of silver into solution. In excess thiourea, the free thiourea reacts with formed solid silver-thiourea complex, and silver goes into the solution, predominantly as the dimers of AgTU⁺ ₃ complexes. The solid silver-thiourea complex in question was characterized by various spectroscopic, microscopic, and chemical analysis techniques. According to chemical composition, it corresponds to Ag₂SO₂·3TUH₂O compound. Formerly Graduate Student, University of Idaho     

The dissolution of gold in acidic solutions of thiourea

In order to evaluate the potential of acidic thiourea as a reagent for leaching gold, a study was made of the dissolution of gold in acidic solutions of thiourea containing various oxidants. Experiments were conducted on rotating disks of pure gold and on ground gold ores. The chemical oxidants used included iron(III), hydrogen peroxide, oxygen and formamidine disulphide; the latter reagent was formed in situ by the action of both hydrogen peroxide and dissolved oxygen on thiourea. Gold was observed to dissolve in these solutions at rates which approached the limiting diffusion controlled rate. Iron(III) as the oxidant caused the most rapid initial rate of dissolution of gold, but this rate soon decreased because of the reaction between iron(III) and thiourea; this resulted in the consumption of an excessive amount of thiourea which made the use of iron(III) as the oxidant unattractive in any ore leaching system based upon the use of thiourea as leaching agent.The results observed in the rotating disk study were applied to the leaching of crushed ores. A large proportion of the oxidant necessary for the extraction of the gold was derived from the ore itself; the remainder of the oxidant required could be supplied as hydrogen peroxide during preparation of the leach liquor, and by agitation of the slurry by a flow of air. When solutions containing 1.2 M thiourea were used it was possible to extract the gold from an ore within one hour; under these conditions the consumption of thiourea was about 1.4 kg thiourea per ton of ore treated. This figure could be reduced to 0.4 kg thiourea/ton if 0.1 M thiourea was used; complete extraction of the gold then occurred within eight hours.Gold can be leached at a much greater rate by acidic solutions of thiourea than is possible by conventional cyanidation techniques. However on economic grounds the latter technique must be preferred unless a really rapid rate of dissolution of gold is required.     

硫代硫酸盐提金理论研究：金的阳极溶解

本文研究了金在硫代硫酸盐溶液中的阳极溶解行为,结果表明,在氨性硫代硫酸盐溶液中金的阳极溶解机理为NH_3优先扩散至金粒表面与金离子络合,生成的金氨络离子进入溶液后被S_2O_3~(2-)取代,形成更稳定的金硫代硫酸根络离子。试验研究及理论分析证实和支持了这一机理。     

Anodic dissolution of silver, copper, palladium and gold in dimethylsulfoxide-halide solutions

The anodic behaviour of silver, copper, palladium, gold, platinum and glassy carbon (GC) electrodes has been studied in concentrated halide solutions in dimethylsulfoxide (DMSO) using linear sweep voltammetry, coulometry, chronoamperometry and X-ray diffraction. Parallel studies in perchlorate and aqueous solutions were used to interpret the data. Only solution oxidation was observed at Pt and GC, but Ag, Cu, Pd and Au were found to oxidize more readily in DMSO than in aqueous solution. The relative merits of DMSO and aqueous halide solutions for the electrorefining of these metals are discussed briefly.     

A kinetic study on nonoxidative dissolution of sphalerite in aqueous hydrochloric acid solutions

The nonoxidative leaching of sphalerite in aqueous acidic solutions was studied from a kinetic point of view. Also the selective nonoxidation leaching in a hydrochloric acid solution containing a large amount of sodium chloride was examined for a Pb-Zn sulfide bulk concentrate. The dissolution rates of sphalerites from five different mines appeared to be controlled by a chemical reaction on the surface of sphalerite. The dissolution rate of sphalerite is of the first order with respect to the hydrogen ion activity of the solutions. It is also considerably affected by the iron content of the sphalerite sample; a linear relationship was observed between iron content of the sphalerite and its dissolution rate. The addition of sodium chloride to the hydrochloric acid solutions greatly enhanced dissolution rates. Compared to the dissolution rates of galena, which were reported in a previous paper, the dissolution rates of sphalerite were found to be far slower. The difference in the dissolution rates between these two minerals becomes greater with the addition of sodium chloride to the hydrchloric acid solutions. Based on these findings, the selective leaching of Pb-Zn bulk concentrate in a hydrochloric acid solution containing a large amount of sodium chloride was examined. The experimental results clearly showed that the galena was selectively leached, leaving a residue of sphalerite. NORIO MISAKI formerly Graduate student, Kyoto 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.     

An electrochemical study on the dissolution of gold and copper from gold/copper alloys

The dissolution behavior of gold and copper from their elemental states and from gold/copper alloys in cyanide solutions has been investigated using a rotating-disc electrode. The anodic and cathodic reactions were studied separately, and the resulting polarization curves were combined to examine the overall dissolution reactions. The dissolution rate of pure gold in the aerated cyanide solutions was inhibited by the anodic passivation on the gold surface, whereas the dissolution rate of pure copper was found to be mass-transfer controlled. On the other hand, the dissolution of gold and copper from the alloys was partially controlled by electrochemical reaction and largely by mass transfer.     

硫代硫酸盐提金理论研究—金溶解动力学

采用腐蚀电化学方法研究了金在硫代硫酸盐溶液中溶解的动力学。无铜、氨时,金的溶解活化能为27.99kJ/mol,Cu(NH_3)_4~(2+)为0.01mol/L,总氨浓度为0.5mol/L时活化能降为15.54kJ/mol;在0.001～0.1mol/L范围内,Cu(NH_3)_4~(2+)的反应级数为1。动力学研究的结果进一步揭示了铜、氨在浸金过程中的催化作用,并再次证实了作者提出的氨性硫代硫酸盐溶液浸金的电化学—催化机理。     

Reaction mechanism of gold dissolution with bromine

The dissolution of gold with elemental bromine was studied by using a rotating disc technique. The main parameters studied were bromine and bromide concentrations, stirring speed, pH, and temperature. The effect of various salts, manganese, and hydrogen peroxide was also examined. The dissolution kinetics of gold with Br₂ and NaBr mixture is complex. The reaction mechanism is a function of solution composition, which determines the kind of adsorbing species. For an excess concentration of bromide ions, the rate expression is Rate = (2k _cl7 k _al6)^1/2 K ₁₅ [Br ₃ ⁻ ] and for an excess concentration of bromine, the rate expression is Rate = (2k _c27 k _a29)^1/2 [Br⁻]^1/2 {K₂₅ [Br₂]³/(1 +K ₂₅ [Br₂]³)}^1/2 Gold in bromine solutions dissolves according to electrochemical/chemical (EC) mechanisms. The electrochemical component of the mechanism is responsible for the formation of AuBr₂. In the chemical component of the mechanism, this monovalent gold bromide disproportionates into gold and stable AuBr ₄ ⁻ , which reports into solution. With respect to pH, there are two characteristic dissolution regions. In the pH range of 1 to 7, gold dissolution rates were insensitive to pH. Above pH 7, gold dissolution rates decreased with increase of pH.     

金矿封闭溶样法的优化

封闭溶矿体系由HNO3-NH4HF2-KMnO4-NaCl调整为HCl-NaClO3-NH4HF2,消除了传统封闭溶样法去除碳(包括石墨)、硫、碳酸盐等过程中可能出现的样品迸溅外溢、溶矿困难而导致分析测定结果偏低的不利因素,除硫速度和体系的反应体积易于控制,新生态[Cl]的产生速度和浓度大大增加,增加了反应容器内的压强值,进一步提高了矿石溶解效率,降低了对环境的污染程度。     

从稀贵液中直接电沉积金的研究

用电极过程动力学对从氰化贵液中直接电沉积金过程进行分析,可得出提高Au（CN）2离子与阴极碰撞几率和减小析H2副反应是提高电沉积金效率的关键因素。采用多孔电极、析出H2小的电极材料和分两段电解措施,使从低浓度含金贵液中直接电沉积金的效率达到98％以上。     

Mechanism and kinetics of sulfite-thiosulfate dissolution of gold

Dissolution mechanisms of noble metals in aqueous solutions of sodium thiosulfate, sodium sulfite, and their mixture under standard conditions (without heating the medium) in alkali and acidic media are described. The conditions of intensifying the dissolution process are evaluated. The reaction rates are investigated. Based on the obtained data, it is concluded that it is rational to apply the sulfite-thiosulfate solvents of noble metals to recover them from concentrates under the usual conditions.     

Kinetics of galena dissolution in ferric chloride solutions

A leaching investigation of galena with ferric chloride has been carried out as a function of concentration of ferric chloride and sodium chloride, temperature, and particle size. Three size fractions were considered in this investigation, namely, 48 × 65, 35 × 48, and 28 × 35 mesh. The concentration ranges of ferric chloride and sodium chloride used in this investigation were 0 to 0.25 M and 0 to 3 M, respectively. The reaction rate mechanism has been discussed in terms of a shrinking core model developed for cubic systems. Mass transport of ferric chlorocomplex through the product sulfur layer appears to be responsible for establishing the overall leaching rate under most of the conditions used in this investigation. The apparent activation energy for the leaching of 28 × 35 mesh galena with 0.1 M FeCl₃, 1 M HC1, and 3.0 M NaCl was found to be about 8.05 kcal/mol (33.7 kJ/mol), which was partially contributed by diffusion and partially by the heat of reaction of the formation of ferric chlorocomplexes. Rate of dissolution at both 50° and 90 °C is greatly affected by ferric chloride concentration up to 0.2 M and is essentially constant with ferric chloride concentration above this value.     

Effect of sulfides on gold dissolution in ammoniacal thiosulfate medium

To understand how various sulfide minerals affect the dissolution behavior of gold in the ammoniacal thiosulfate leaching system, an extensive study has been carried out on gold leaching in the presence of sulfides using pure gold plates. Special emphasis has been placed on gold leaching in association with sulfide dissolution, thiosulfate decomposition, and dissolved oxygen depletion in leach solutions. The results demonstrated that the leaching behavior of gold depended strongly on the solubilities of the sulfides, the thiosulfate decomposition, and the oxygen concentration in slurries. Gold dissolution was enhanced or diminished, depending on the sulfide types and the sulfide concentrations in slurries. An increase in the stirring speed accelerated the gold dissolution rates due to the improved mass transfer occurring in the gold leaching process and the increased dissolved oxygen content in leach solutions. The addition of sulfate in the sulfide slurries increased the gold leaching rates because of the depression of the sulfide dissolution in the leaching systems. Topological studies by scanning electron microscopy (SEM) demonstrated that the existence of passivating layers at the leached gold surfaces could result in the retardation of gold dissolution in the presence of sulfide minerals. An erratum to this article is available at .     

A rotating disk of gas and dust around a young counterpart to beta Pictoris

beta Pictoris is the best known example of a main-sequence star encircled by a tenuous disk. Optical and infrared images of beta Pic suggest that the disk is composed of dust grains which have been interpreted as the debris generated by the disruption of the asteroid-sized remnants of planet-formation processes. The star itself is relatively old, with an age in excess of 100 Myr. Here we present high-resolution millimetre-wave images of continuum and molecular-line emission from dust and gas surrounding a much younger star, MWC480: the stellar properties of MWC480 are similar to those of beta Pic, but its age is just 6Myr. The morphology of the circumstellar material and a comparison with the predictions of kinematic modelling indicate the presence of a rotating disk, gravitationally bound to the star. Moreover, the mass of the disk is greater than the minimum required to form a planetary system like our own. We therefore suggest that the disk around the young star MWC480 could be a progenitor of debris disks of the type associated with older stars such as beta Pic, and so holds much promise for the study of both the origin of debris disks and the early stages of the formation of planetary systems.     

A fundamental study of raceway size in two dimensions

Blast furnace raceway zones are formed by the force of the air blast injected through the tuyeres evacuating a region of the packed bed directly in front of these tuyeres. Raceway depths in blast furnaces have been historically predicted through the use of empirical correlations based on measurements on cold and hot models. These correlations are not found to be universal in application, however, with many researchers finding only fair agreement between their experi-mental data and the correlations proposed by other researchers. We present here an alternative physical mechanism approach for raceway formation based on examination of the fundamental properties of the system. The study includes two-dimensional experiments where raceway depths and shapes are measured and an accompanying theoretical and numerical analysis of the under-lying mechanisms. Gas flow distributions around the raceway zone are also examined. It is found that the raceway size for given blast conditions and particle properties is such that the total gas drag on the solids vertically above the raceway balances the solid bed weight, with some allowance for solid holdup by walls. The formulation of this theory leads to the further conclusion that the total surface area of the raceway walls as a fraction of the horizontal cross-sectional area of the container or furnace is a unique function of three factors: (a) the gas flow rate as a fraction of the gas flow rate required to fluidize the bed, (b) a particle Reynolds number calculated at the particle's incipient fluidization velocity, and (c) the shape of the horizontal cross section of the bed.     

封闭溶样-原子吸收法测定钼精矿中的金

矿样经550℃焙烧1h后,用NH3·H2O溶解处理,过滤除去钼;残渣中的金经王水封闭溶样法分解后,利用活性炭吸附柱动态吸附,采用原子吸收法测定。对5个样品平行测定10次,平均值与火试金法测定结果相吻合,RSD为O．35％～1．47％,加入标准物质回收率为99．0％～102．0％。     

A study on the dissolution of ilmenite sand

Preliminary experiments on the dissolution of titanium and iron from ilmenite sand (< 130 μm particles) in various inoganic acids indicate that only hydrofluoric acid is effective. Leaching in hydrofluoric acid has been investigated as regards contact time, acid concentration, solid to liquid phase ratio, number of leaching stages and temperature. About 81% titanium is dissolved within 5 h by 6.4 M hydrofluoric acid at boiling point, while only 26 iron is dissolved. Multi-stage leaching is not so advantageous, but the extent of leaching increases greatly with temperature. The optimum solid to liquid phase ratio is found to be 1/20 kg/l.     

Rates of dissolution of rotating iron cylinders in liquid copper and cu-fe alloys

Iron cylinders with molybdenum capped ends are rotated at speeds of 260, 570, and 835 rpm in liquid copper and Cu-Fe alloys maintained at 1220°, 1300°C, and 1370°C under argon at 1 atm pressure. The dependence of the dissolution rate of the cylinders on the concentration of iron in the bulk liquid is observed. The solution-rate constants defined by an approximate form of the Berthoud equation vary from 7 × 10^-3 to 30 × 10^-3 cm.s^-1. There is a linear relation between the logarithm of the rate constant and the reciprocal of absolute temperature for each rotational speed. The rate constant is found to vary with the 0.85 to the 0.96 power of the Reynolds number in the range 6500 〈 Re 〈 22000. This suggests that the dissolution process is diffusion controlled. The dependence of the dissolution rate on the activity of iron in the bulk liquid is observed. Oxygen increases markedly the dissolution rate, whereas sulfur does not.