Investigation On Electrochemical Kinetics And Application For Co-investigation Of Gold Leaching

Being the predominant approach applied for gold leaching, cyanidation has always been concerned for its low rate. Investigation on intensification of gold leaching is of great importance for increaseing leaching rate as well as for raising technical level of gold production. In this thesis paper, a technical idea named co-intensification was proposed, thereby simultaneously intensification of anodic and cathodic processes to promote gold leaching was investigated on the basis of hydrogen peroxide assisted leaching. By using steady-state linear potential-dynamic scanning, the effect of single electrode processes on gold anodic dissolution in the cyanidation system was revealed through systematically investigation of kinetics characteristics of electrode processes for gold cyanidation intensified by heavy metals and construction of mixed potential model, by which the electrochemical kinetics for co-intensification of gold cyanidation is established. On this basis, investigations on co-intensified leaching of three different types of gold ores were carried out, and remarkable co-intensification effects were obtained.Concerning conventional canide system, anodic kinetics of gold dissolution is significantly affected by either cyanide concentration or pH. Under low cyanide concentration (0.002M), there exists a limited current density for anodic dissolution of gold, the value of which approaches limited transferring current density. No noticeable passivation is observed and the process is controlled by transfer of CN^-. When cyanide concentration increases, gold anodic dissolution potential decreases, current density increases, and anodic peak is observed, wich suggests that the rate control style is transformed. However, peak potential and peak cureent density deacreas with the increase of cyanide concentration. Increase of pH causes increase in dissolution potential and decrease in active dissolution current and peak current, showing the information of upgraded passivation. Higher temperature may decrease the dissolution potential and increase dissolution rate with little chang for peak potential. The equilibrium activated energy W_α⁰ is calculated to be about 40kJ/mol with measurements made in solution of 0.02M cyanide. Therefore, chemical oxidation of gold in cyanide solution of higher concentration is controlled by surface reaction, and anodic dissolution in the active potential range controlled by electrical reaction. This is further conformed by the fact that electrode rotary causes no substantial effect on anodic current.Addition of small amount of heavy metal ions such as Pb²⁺,Bi³⁺,Tl⁺,Hg²⁺,Ag⁺ bring about great increase in anodic dissolution rate of gold. Bi³⁺,Tl⁺,Hg²⁺ also show effect on decrease of passivation to some extent. Ag⁺doesn't increase anodic cureent in potential range on left side of the peak, but causes the current to further increase with potential move positively after the peak and intensified gold anodic dissolution by avoiding passivation. Pb²⁺,Bi³⁺,Tl⁺,Hg²⁺ significantly reduce the equilibrium activated energy W_α⁰ , raise the rate ofelectrochemical reaction step to the extent that rate control style be transformed, so that the rate control style is transformed from electrochemical control to mixed control. In addition, these heavy metals also markedly change the effect pattern of cyanide concentration and reduce effect degree of pH. As cyanide concentration increases, peak current is significantly raised and peak potential keeps unmoved, which results in enlargement of active dissolution potential range.The intensifying effects of the heavy metals are strangely related to their concentration. Pb²⁺,Ag⁺,Bi³⁺ show best effects in the concentrations of 10^-6M,10^-5M,10^-5M respectively. Further increase of their contration may cause reduce to the raised peak cureents. Even though, Tl⁺ can keep the peakcurrent increasing with its concentration up to 10^-3M. As for Hg⁺, when its concentration further increase above 10^-5M, peak current shows little decrease, but positive shift of equilibrium potential and peak potential is resulted.Cathodic reduction of oxygen on gold electrode is a transfer controlled process. Increase of oxygen concentration, rotary speed, temperature, and decrease of pH may cause increase of cathodic current density. The cathodic process proceeds mainly by two-electon 2e pattern. The total transferred electron number increases with the negative shift of potential and decrease of pH. When pH reaches 13.0, The total transferred electron number keeps close to 2e in the hole tested potential range, as the linear polarization curve come up with an obvious platform.All the above mentioned heavy metal ions Pb²⁺,Bi³⁺,Tl⁺,Hg²⁺,Ag⁺ show positive effects on cathodic reduction of oxygen, and the effects rise with the increases their concentration. The mechanism of the promotion lies in the increase of total transferred electron number, which makes the linear polarization curves show platform that represent limited current densities. Besides, heavy metals also raise the rate of electrochemical step of the anodic reduction of oxygen. Addition of a certain amount of hydrogen peroxide is confirmed to be an effective way for intensification of cathodic process on gold electrode, which leads not only greatly higher cathodic current than that of pure oxygen reduction, but also positive shift of cathdic equilibrium potential.In order to illuminate the interaction of the individual electrodic reactions in electrochemical dissolution system, mixed potential model, which serves as the theoretical foundation for the technical ideo of co-intensification, is constructed by combining all the linear polarization curves for individual electrodic reactions and total curves for anodic reactions and cathodic reactions. As the model shows, in a electrochemical dissolution system, simultaneously occuring anodic reactions are of restrictive effect on the anodic dissolution reaction, while all the cathodic reactions may promote the main dissolution reaction. During intensification on electrochemical dissolution, negative shift of the mixed potential suggests that the raise degree of anodic process be greater than that of cathodic process, and the controlling extent of cathodic reactions on dissolution increases. On the contrary, positive shift of the mixed potential suggests that the raise degree of cathodic process be greater than that of anodic process, and the controlling extent of anodic reactions on dissolution increases.According to mixed potential model, gold dissolution in the abcent of intensifying reagents is controlled by cathodic process, being an oxygen transfer controlled reaction with an activated energy of 7.99kJ/mol. Being co-intensified by a heavy metal associated with hydrogen peroxide, gold dissolution rate is much more higher than those of singlehanded intensification either by heavy a metal on anodic process or hydrogen on cathodic process. Too high hydrogen peroxide concentration may cause passivation while co-intensified with Pb or Bi³⁺. However, owing to the enlargement effect of active potential range and current peak on anodic dissolution, passivation doesn't occur with increase of hydrogen peroxide concentration while co-intensified with Tl⁺ or Hg²⁺. As for the co-intensification with Ag⁺, gold dissolution rate keep inceasing with hydrogen peroxide concentration and no passivation occur as well.On cyanide leaching of gold from ores, simultaneously addition of one of the five heavy metals and hydrogen peroxide achieves co-intensifying effect, which turn up to be noticeable for the three tested ores, i.e. a refractory sulphide gold ore, an easily leachable sulphide gold ore, and a low grade oxide gold ore.For the refractory sulphide gold ore, singlehanded intensification with a heavy metal has no obvious effect, while co-intensifying effect may be obtained and gold leaching rate is cosiderablely increased by using Bi³⁺, Tl⁺, or Hg²⁺ in association with hydrogen peroxide. For easily leachable sulphide gold ore, gold leaching may be accelerated significantly by singlehanded intensification improve either with heavy a meatal or with hydrogen peroxide and, marked effects are also achieved for co-intensificacaiton in which gold leaching rates turn up to be much higher than those in singlehanded intensifications with heavy metals or hydrogen peroxide. For low grade oxide gold ore, Bi³⁺ and Tl⁺ have noticeable effect while used either for singlehanded intensification or in co-intensification.