Mechanism of gold dissolving in alkaline thiourea solution

Reaction mechanism of gold dissolving in alkaline thiourea solution was studied by electrochemical methods, such as cyclic voltammetry, chronopotentiometry, AC impedance, linear sweep voltammetry. Apparent activation energy of anodic process of gold electrode dissolving in alkaline thiourea solution is 14.91 kJ/moh Rate determining step is the process of gold thiourea complex diffusing away from electrode surface to solution. The results of AC impedance and chronopotentiometry indicate that thiourea adsorbs on gold electrode surface before dissolving in solution. There does not exist proceeding chemical reactions. Formamidine disulfide, the decomposed product of thiourea, does not participate the process of gold dissolution and thiourea complex. Species with electro-activity produced in the process of electrode reaction adsorbs on the electrode surface. In alkaline thiourea solution, gold dissolving mechanism undergoes the following courses： adsorption of thiourea on electrode surface; charge transfer from gold atom to thiourea molecule; Au[SC（NH2）2]ads^＋ receiving a thiourea molecule and forming stable Au[SC（NH2）2]2^＋; and then Au[SC（NH2）2]2^＋ diffusing away from the electrode surface to solution, the last step is the rate-determining one.

Electrochemical kinetics of gold dissolving in alkaline thiourea solution

Kinetic parameters of the electrode reactions were measured by investigating steady-state current-potential behaviors. The results show that the apparent transfer coefficient of anodic process is 0.0582, diffusion coefficient of thiourea gold complex is 6.04×10~ -6 cm^2/s,anodic reaction order of thiourea is 2.0183, and anodic reaction order of OH^- is 0.0166. The theoretical kinetics equation of gold dissolving in alkaline thiourea solution is deduced, which indicates that anodic reaction order of thiourea is 2, and anodic reaction order of OH^- is 0. The theoretical values of the kinetic parameters are consistent with experimental values very well. The correctness of the mechanism is further demonstrated using apparent transfer coefficient according to the electrochemical dynamic equation of multi-electron reaction.

DISSOLUTION THEORY OF GOLD IN ALKALINE THIOUREA SOLUTION(Ⅰ) Anodic Behavior on Gold in Alkaline Thiourea Solution Containing Na_2SO_3

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Dissolution theory of gold in alkaline thiourea solution(Ⅲ) ——Thermodynamics on dissolution of gold in alkaline thiourea solution containing Na _2SO_3

The oxidants of gold were investigated in an alkaline thiourea solution containing Na 2CO 3, in which Na 2S 2O 8 is a proper oxidant for dissolving gold because of in such homogeneous sulfur system coexisting complex agent, oxidant and stabilizing agent. The thermodynamic analyses were conducted on the dissolving of gold in the alkaline thiourea solution containing Na 2SO 3 by the oxidants oxygen or Na 2S 2O 8. The results show that the possibility of gold dissolution reduces with increasing pH value, while oxygen acts as oxidant; and when Na 2S 2O 8 acts as the oxidant of gold in the alkaline thiourea solution of pH 12.5, decomposition potential of thiourea decreases from the standard value 0.42 V to -0.32 V, also the dissolution trend of gold enhances with shifting the mix potential of the solution to the positive direction.

Optimization of efficient stable reagent of alkaline thiourea solution for gold leaching

Na2SiO3 and Na2SeO3 were chosen as stable reagents of alkaline thiourea solution substituting Na2SO3, according to the structure-property relationship between the stability of alkaline thiourea and the structure of thiourea and sulfite ion, and the effect of the stable reagents on stability of alkaline thiourea was investigated. The results show that contrary to Na2SeO3, Na2SO3 and Na2SiO3 affect the stability of alkaline thiourea solution remarkably. The stable effect of Na2SiO3 on alkaline thiourea is obviously better than that of Na2SO3. The stable reagents Na2SO3 and Na2SiO3 decrease the decomposition rate of alkaline thiourea solution greatly, and the decomposition rate of alkaline thiourea reduces from （72.5%） to （33.8%） with addition of (0.3 mol·L-1) Na2SiO3. Dissolution currents of gold in the alkaline thiourea solution containing Na2SO3 and Na2SiO3 are （2.0） (mA·cm-2) and （3.5） (mA·cm-2) at the potential of 0.42 V, respectively, and Na2SO3 is consumed excessively due to the oxidation reaction of Na2SO3 occurring in the studied potential range. Na2SiO3 is an efficient stable reagent of alkaline thiourea solution, and gold dissolution is accelerated much more obviously by Na2SiO3 than by Na2SO3.

Study on Mechanism of Gold Leaching by Thiourea in the Presence of Sodium Sulphite

The linear potential sweep voltammetry and electrochemical methods were used to study the mechanism and kinetics of gold dissolution in thiourea solution in the absence and presence of sodium sulphite.The results show that in the absence of sodium sulfite the dissolution rate of gold in thiourea solution is relatively slow and it increases with the increase of the concentration of thiourea and decreases with pH. Thiourea is easily oxidized to formamidine disulphide, which reduces the leaching efficiency and increases reagent consumption. In the presence of sodium sulphite, the effective concentration of thiourea is increased and the dissolution rate of gold in thiourea solution is greatly increased. The activation energy required for gold dissolution in thiourea solution with sodium sulphite is 20.9 kJ·mol -1 which is much lower than 55.0 kJ·mol -1 in the absence of sodium sulphite. It is evident that sodium sulphite has a significant effect on the electrochemical reaction kinetics of gold in thiourea solution.

Recovery of gold from refractory gold ores: Effect of pyrite on the stability of the thiourea leaching system

The extraction of gold from refractory gold ores(RGOs)without side reactions is an extremely promising endeavor.However,most RGOs contain large amounts of sulfide,such as pyrite.Thus,investigation of the influence of sulfide on the gold leaching process is important to maximize the utilization of RGOs.In this work,the effects of pyrite on the stability of the thiourea system were systematically investigated under different conditions.Results showed that the decomposition rate of thiourea was accelerated sharply in the presence of pyrite.The effect of pyrite on gold recovery in thiourea leaching systems was then confirmed via a series of experiments.The decomposition efficiency of thiourea decreased by 40%and the recovery efficiency of gold increased by 56%after the removal of sulfide by roasting.Under optimal conditions,the efficiency of the gold recovery system increased to 83.69%and only 57.92%of thiourea decomposition was observed.The high consumption of thiourea by the leaching system may be attributed to not only adsorption by mineral particles but also catalytic decomposition by some impurities in the ores,such as pyrite and soluble ferric oxide.

Dissolution Behavior of Gold in Alkaline Media Using Thiourea

In this work the dissolutive behavior of gold in alkaline medium using thiourea (TU), under different variables, was studied in a theoretical and experimental way, in order to determine the conditions under which it is feasible to dissolve gold in thiourea-alkaline medium. A thermodynamic study was conducted by chemical speciation using the method of Ro-jas-Hernández, together with an electrochemical study where the electric potential was swept in the anodic direction. The main results of the thermodynamic study were that formamidine disulfide (FDS) and sulfinic compounds (S.C.) prevail at alkaline pH;by increasing the initial concen-tration of thiourea at alkaline pH, the presence of the gold complex is al-most zero for any initial concentration of thiourea. By including sodium sulfite in the gold-thiourea system, it was possible to obtain the Au(I)-TU complex at alkaline pH, with a presence of 95.13%. Electrochemical tests allowed verifying that in the absence of sodium sulfite the dissolution of gold in an alkaline medium is very slow but adding sodium sulfite im-provements become evident in the dissolution of the metal. Therefore, sodium sulfite catalyzes the gold dissolution process and stabilizes the thiourea. With this study it was possible to establish the feasibility of using thiourea in an alkaline medium for the dissolution of gold, and the conditions under which it is possible to dissolve the gold in that medium. With these fundamentals and conditions, it is now possible to move forward to test this system for minerals and/or concentrates containing gold.

STUDY ON SELECTIVE DISSOLUTION OF GOLD IN ALKALINE THIOUREA MEDIA

Gold dissolves easily in an alkaline thiourea solution with Na2SO3 and Na2S2O8, where the accompanied elements of Au, such as Ag, Cu, Ni and Fe can hardly dissolve. It was considered that electrochemical reduction and catalytic action of SO32- prevent thiourea from decomposing irreversibly and accelerates dissolution of gold.The S2O82- as an oxidant can control the potential so that formamidine disulfide may form efficiently. The dissolution of gold is mainly through oxidation of Au by formamidine disulfide and complexation of thiourea with An+,and not direct oxidation of S2O82-. Thus,sufficient formamidine disulfide is necessary to accelerate the dissolution of gold. The reason for Ag, Cu hardly being dissoluble is that the black layers of Ag2S and Cu1.92S form on the surface of Ag and Cu. The metal Ni and Fe do not dissolve in the alkaline media due to their sulfide films passivation samely.

Effects of sulfurous acid on anodic process of gold electrode in thiourea solution

The electrochemistry behaviors of gold electrode in thiourea solution were studied by using electrochemical techniques, such as potentiodynamic, voltammogram and current step. A catalytical electrochemical-reduction mechanism was proposed to identify the anodic oxidation of gold in the thiourea solution. The results indicate that the decomposition of thiourea occurs when the electrode potential is higher than 640 mV. The addition of sulfurous acid presen a very positive effect on the anodic process of gold electrode.The anodic oxidation rate of gold is 5 times faster than that without sulfurous acid.The passivity of gold electrode is attributed to the accumulation of elemental sulfur on the surface of gold.The sulfurous acid reacts with the oxidation product （（SC（NH）NH2）2） of thiourea,which can decrease the decomposition of thiourea and improves its stability.

Anodic dissolution of gold in alkaline solutions containing thiourea, thiosulfate and sulfiteions

Gold dissolves electrochemically in alkaline solutions containing ligands to form complex ions with gold ion. Therefore, selective leaching of noble metals is expected without dissolution of base metals such as steels, aluminum alloys in scrap treatment. Gold electrodes were investigated using linear sweep voltammetry, EQCM method and potentiostatic electrolysis in alkaline solutions containing thiourea, Na 2SO 3 and Na 2S 2O 3. The solution composition, electrode potential affect gold dissolution rate and current efficiency. The gold dissolved from anode electrode forms complex ions, suspension particles as compound precipitates and deposits on cathode electrode as a metal. Anodic efficiency for gold dissolution is between 10% and 22%. This is caused by the oxidation decomposition of sulfite ions and thiourea. The stability of the alkaline solution containing these elements was also estimated by capillary electrophoresis technique.

Dynamics of anodic process of gold electrode in thiourea solution in the presence of sulfur dioxide

The electrochemical kinetics equation of the oxidation of gold in thiourea solution in the presence of sulfur dioxide was given out. The kinetic parameters are determined as: the transmission coefficient β =0.477; when the reaction time is zero, the initial over potential of the oxidation of gold in acid thiourea solution is 0.267 V. The dynamics of anodic process of gold electrode was studied by using Tafel plots. In the solution containing 5×10 -6 mol/L sulfur dioxide the activated energy is 46.11 kJ/mol. When the electrode potential is higher than 270 mV, the gold is oxidized by thiourea. The addition of H 2SO 3 can obviously improve the corrosion speed of gold.

THIOUREA LEACHING OF GOLD FROM A CALCINE OF GOLD-BEARING ARSENICAL PYRITE CONCENTRATE

The thiourea leaching of gold from the calcine of gold-bearing arsenical pyrite concentrate of Kangjiawan mine was studied.The effects of the leaching time,the concentrations of thiourea,hydrochloric acid and initial ferric ion on the leaching recovery of gold were investigated,and a regressive model has been established,which showed that the concentrations of thiourea and hydrochloric acid were the most important factors,leaching time the second,and concentration of the initial ferric ion the least・Under the optimal conditions,that is,temperature 50℃,the ratio of liquid to solid 4:1,thiourea 12g/L,hydrochloric acid 1 mol/L,and initial ferric ion 1 g/L,the leaching recoveries of gold and silver were more than 81%and 73%,respectively.The increase of silver recovery and the reduction of thiourea consumption could be attained when sodium sulphite was added.

SELECTIVE DISSOLUTION OF GOLD IN AN ALKALINETHIOUREA SOLUTION BY ELECTROLYSIS

Anodic polarization behavior of gold, silver, copper, nickel and iron in potentiostatic condition has been examined in an alkaline aqueous thiourea solution, where gold is hardly dissoluble normally. The addition of Na2SO3 into the solution can accelerate anodic dissolution of gold. The factors affecting selective dissolution of gold in the alkaline thiourea solution by electrolysis have been studied, and the optimum condition was obtained. In 0.1 mol/L thiourea solution of pH 12. 5 containing 0. 5 mol/L Na2SO3 and 2. 5% acetone, at the potential of 0. 34 V vs NHE, at the temperature of 323 K, the dissolved mass of gold anode with the exposed area of 1. 0 cm2 reached more than 300 mg·dm-3 within 30 min, and other metals such as silver, copper, nickel and iron could hardly dissolve.

Electrochemical adsorption and passivation on gold surface in alkaline thiourea solutions

An electrochemical method was used to examine Au surface absorption and passivation under the most commonly employed pH conditions in an alkaline Au leaching system.The polarization at different pH values was obtained through the current step curve.At pH 10,no absorption layer was formed.When the pH was increased to 11,an absorption layer was formed through the Au electrode reaction.At pH 12,the entire system could not be stabilized,even after long durations because of the thiourea oxidation and decomposition.The samples were analyzed by scanning electron microscopy(SEM),energy-dispersive X-ray spectroscopy(EDS),and atomic force microscopy(AFM).AFM observations of the Au plating surface and SEM-EDS analyses for the Au-coated graphite surface indicated that a passivation membrane was formed on the Au surface after its use in the alkaline thiourea Au leaching system.Two-step leaching was used to confirm the conclusions drawn from the experimental results.The twostage experimental results further confirmed the existence of a passivation membrane.

LEACHING OF PRECIOUS METALS FROM COMPLEX SULPHIDE ORES——2.DIRECT LEACHING OF Cu SULFIDE CONCENTRATE BY ACIDIC THIOUREA SOLUTIONS

Recovery of gold and silver from a sulphide concentrate by thiourea leaching was investigat-ed.The effects of various parameters on the process,such as oxidants,the initial concentrationof thiourea and acidity in solutions as well as reaction temperature,were systematically exam-ined.From the results it was confirmed that(a)oxidants had no significant influence on the dis-solution of gold and silver at a fixed initial concentration of thiourea;(b)the initial concentra-tion of thiourea and acidity affected the dissolution rate only in the earliest period of leaching;(c)the rate of the process was controlled by reagent diffusion.The optimum operations wereworked out for the thiourea leaching process of the concentrate.

Application of diazo-thiourea and gold nano-particles in the design of a highly sensitive and selective DNA biosensor

An effective procedure for constructing a DNA biosensor is developed based on covalent immobilization of NH2 labeled,single strand DNA（NH2-ssDNA） onto a self-assembled diazo-thiourea and gold nanoparticles modified Au electrode（diazo-thiourea/GNM/Au）.Gold nano-particles expand the electrode surface area and increase the amount of immobilized thiourea and single stranded DNA（ssDNA） onto the electrode surface.Diazo-thiourea film provides a surface with high conductibility for electron transfer and a bed for the covalent coupling of NH2-ssDNA onto the electrode surface.The immobilization and hybridization of the probe DNA on the modified electrode is studied by differential pulse voltammetry（DPV） using methylene blue（MB） as a well-known electrochemical hybridization indicator.The linear range for the determination of complementary target ssDNA is from 9.5（±0.1） × 10^-13 mol/L to1.2（±0.2） x 10^-9 mol/L with a detection limit of 1.2（±0.1） 〉 10^-13 mol/L.

Enhanced Sensitivity and Selectivity of Chemosensor for Malonate by Anchoring on Gold Nanoparticles

A novel organic-inorganic nanohybrided receptor 4 functionalized with bis-thiourea arms and then assembled on gold nanoparticles was synthesized. The preorganized system possesses phenylthiourea units for the spectropho- tometric sensing of dicarboxylates, especially malonate, based on changes in the surface plasmon absorption of the gold nanoparticles （GNPs）. The intensity of the absorbance band increases gradually with the concentration of di- carboxylates increasing. But such an ion-selective change in the plasmon band was not observed in control tests carried out by mono-thiourea-modified GNPs, receptor 3 and the free receptor 2. As it is shown from the association constants, derived from quantitative titrations, receptor 4 can selectively recognize dicarboxylate anions of shorter carbon chain, and has the highest affinity to malonate. The interaction properties for anions of receptor 4 were evaluated by lH NMR and UV-vis spectroscopic methods.

黄钾铁矾对硫脲浸金的影响及草酸盐的作用机理

研究生物氧化产生的黄钾铁矾对酸性硫脲浸金的影响,并探讨草酸添加剂的作用机制。结果表明,黄钾铁矾会增加硫脲的消耗和阻碍金的溶解。黄钾铁矾能显著提高硫脲的分解,阻碍金的溶解。黄钾铁矾造成不利影响的主要原因可能是酸性溶液中黄钾铁矾释放的铁离子使溶液浆电位增加,从而加速硫脲的分解,并加剧硫脲分解产生的钝化物种对金表面的钝化作用。添加草酸盐可以有效削弱黄钾铁矾的不利影响,表现为明显降低硫脲的消耗和促进金的溶解。通过溶液电位、Zeta电位和XPS分析,揭示草酸的可能作用机理。草酸盐不仅可以通过与Fe^(3+)配位减弱Fe^(3+)与硫脲之间的相互作用,还可以通过无差别吸附和静电斥力阻止钝化产物在金表面包裹。

生物质吸附剂吸附回收硫脲废液中金的研究

提出一种以毛竹为原料制备生物质吸附剂、选择性地从硫脲废液中吸附回收金的方法。竹炭在H_(2)O-CO_(2)气氛中活化后表面的纳米孔洞比单独使用CO_(2)活化的活性炭孔洞数量更多且尺寸更小,大大增加了吸附材料的比表面积。探究了吸附材料种类、pH值、吸附剂用量、吸附温度、吸附时间对硫脲废液中金吸附脱除的影响,得到适宜的吸附条件为:pH=6、吸附剂用量8 g/L、吸附温度25℃、时间3.5 h,其中采用H_(2)O-CO_(2)协同活化的活性炭吸附效果较好,可吸附脱除硫脲废液中96%以上的金。采用吸附等温模型拟合了H_(2)O-CO_(2)协同活化的活性炭在硫脲废液中吸附金的过程,结果表明,协同活化的活性炭吸附硫脲废液中金的过程更符合Freundlich模型,为物理吸附过程。