| Although cyanidation has always been the dominated process of gold extraction for over100years, many countries and regions are now to avoid using cyanide as leaching agent for its harm to environment and ecosystem. In order to maintain the ecological balance of gold producing area, the development of new pollutantgold leaching process has received more and more attention. Among all the studied leaching substitute of cyanide, thiosulfate has been widely used for the following key advantages:1.Thiosulfate is low toxicity and friendly to the environment;2. Thiosulfate leaching is less sensitive to base metals, and yields better results in dealing with carbohydrate-or arsenic-containing gold ores than traditional cyanidation.3. The extraction time of thiosulfate leaching is shorter, and the leaching rate is higher than those of traditional cyanidation.4. Thiosulfate leaching, which has proved highly effective in alkaline environments, is less erosive to equipment. However, there is still a long way to go before the large-scale industrialization of thiosulfate leaching. The mechanism of solution chemistry during thiosulfate leaching has been known as acomplex process, which includes not only the complexing reaction between the thiosulfate ion and gold, but also a series of redox reactions conducted by S2O32-itself, so the reagent consumption is very large and the subsequent adsorption rate of gold is low. To solve these,both in China and abroad, the research in thiosulfate leaching is continuing, and has made certain achievements.This paper argues anexperimental study of thiosulfate leaching on a gold ore obtained in Yunnan province. Based on the theoretical analysis and experimental studies, a best process condition was determined with a high gold leaching rate of94.3%. To the subsequent gold adsorption process, three schemes have been proposed as follows:adsorption by coarse pored resin, adsorption by activated carbon and substitution by zinc powder.During the experiments, all the concentrations of gold are determined by atomic absorption spectrophotometry (AAS), and after extensive analysis the optimum conditions for analyzing have been established as follows:the wavelength of245.8nm, the acetylene flow of2.35L/min, the air flow of10.56L/min, crevice of2.8/1.34and the flame height of nearly9cm.By great deal of experimental study, the optimum conditions of agitated leaching resulting in a gold leaching rate of94.3%are below:the Cu2+concentration of0.005mol/L, the ethylenediamine concentration of0.06mol/L, the sodium thiosulfate concentration of0.5mol/L, the SAA dosage of15000g/t, and the stirring rate of150r/d.For the subsequent adsorption of gold in leaching solution, a synthetic resin, which was combined by activated carbon and the product of cross-linking reaction between beta-cyclodextrin and epoxy chloropropane, was first used as adsorbent, and the adsorption rate was only10.5%; Then D301coarse pored resin was used as adsorbent with a poor adsorption rate of8.21%; after that the activated carbon and its modified ones were both used as adsorbents, and the adsorption rate (28%) by modifiedactivated carbon was nearly20%more than that by activated carbon (9.5%), which both still did not meet the requirement of industrialized application. Finally, a substitutional reaction tests by zinc powder was conducted to recover the gold in leaching solution, and the replacement rate was only48%.Through all the leaching and adsorbing tests, this paper has brought a much deeper understanding of the mechanisms of thiosulfate leaching, and could be a reference in the future research. |
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