Oxidation Of High-Sulfur And High-Arsenic Refractory Gold Concentrates By Recycling Oxidant

The "easily cyanidable" gold ores with exploitable grades are becoming rapidly depleted around the world and more and more ore bodies are proved to be refractory to some extent. Since refractory gold ores are not amenable to direct cyanidation, particular pretreatment processes are required prior to conventional cyanidation in order to improve the recovery of gold. Numerous pretreatment processes have been developed to overcome the refractory nature of some gold ores and to increase the extraction of gold and silver. These techniques include pre-aeration、chlorinaton、roasting、pressure oxidation、bacterial oxidation、ultra-fine grinding、chemical oxidation、electrochemical oxidation and high-pressure cyanidation. However, these pretreatments often imply environmental problems, excessive capital requirements, excessive operation time, high operating and maintenance costs or highly trained operating personnel. These disadvantages, coupled with the depletion of easily leachable ores, are forcing the mineral industry to explore new options to treat the refractory gold and silver ores. More and more researchers and experts are exploring new projects to tackle this problem in order to obtain good economic benefits.Considering the above problems, the new technology which has independent intellectual property has been developed in this thesis. Prepared recycling oxidant is referred to as oxygen carrier in which oxygen has high solubility under atmosphere pressure. Oxidized residues are utilized to extract precious metals and prepare silica white; Oxidation solutions are prepared to produce high purity ferric oxide after purification; At the same time, recycling oxidant can be produced in the regeneration device. Therefore, the whole technology makes all the resources utilized and realizes cleaner production.In this thesis, the investigated refractory gold concentrates contain high content of sulfur and arsenic and thus require breakdown of sulfide matrix (arsenopyrite and pyrite) to liberate the gold for further extraction by cyanidation or other processes because the gold frequently is very fine and even "invisible" and encapsulated in these sulfide minerals. Chemical pre-oxidation method is currently one of the major pretreatment processes for refractory high-sulfur and high-arsenic gold concentrates (HGC). In this investigation, ozone and ferric sulfate were used as oxidants to pre-oxidize HGC in order to increase gold recovery. The degradation of gold inclusions such as pyrite and arsenopyrite led to improved gold recovery in the subsequent cyanidation process. Pre-oxidation of HGC by ozone and ferric sulfate in acidic solutions is a novel and efficient process for pre-treat refractory gold ores before gold extraction. First, gold concentrates were characterized by chemical analysis、XRD analysis、MLA analysis and SEM-EDS analysis. Combined with the cyanidation experiment, the conclusion can be made that invisible gold grains are encapsulated in mineral matrix:pyrite and arsenopyrite.Secondly, the orthogonal experiments with three different oxidants (ferric sulfate, the oxygen and ferric sulfate, ozone and ferric sulfate) were carried out to seek the optimum pre-oxidation conditions. The contrasted experimental conclusions suggested that pretreatment process with ozone and ferric ions for HGC resulted in best pre-oxidation effect. Afterwards, ferric sulfate and ozone were used as oxidants to investigate the effect of main factors on the oxidation of iron which represented the pre-oxidation effect of refractory gold concentrates. The results showed that the oxidation of iron was affected by temperature、pulp concentration、reaction time、stirring speed and ozone concentration. The oxidation of iron increased with the increase of temperature、reaction time、stirring speed and ozone concentration. The oxidation of iron increased with the decrease of pulp concentration. From factorial experiments, it was found that optimum conditions were:temperature of 100℃, time of 8 h, ratio of liquid to solid of 20, stirring speed of 869 rpm, ozone concentration of 54ppm and concentration of ferric sulfate of 0.7mol/L. Under these conditions, the proportions of pyrite oxidation and arsenopyrite oxidation were 75% and 72%, respectively. The gold extraction was 96.5%, about 77% greater than the gold extraction without pre-oxidation.Thirdly, Taguchi method of experimental is applied to optimize the HGC pre-oxidation efficiency using ferric ions and ozone. The effects of various factors influencing the performance characteristics were analyzed. Analysis of the signal to noise ratio has been successfully carried out to find the relative contribution and the optimum factor level combination for maximum ferric extraction. According to the ANOVA results, it can be inferred that temperature is the most predominant factor. The maximum ferric extraction was obtained by setting temperature 100℃, time 16h, liquid to solid ratio 8 and initial ferric sulfate concentration 1.05 mol/L. Cyanide leaching tests for the oxidized residues under the optimum conditions suggested that gold extraction increased by 77% resulting in an overall extraction of 96.5%. Since optimum conditions determined by Taguchi’s a method in a laboratory environment are reproducible in real production environments as well, the findings of the present laboratory scale study may be very useful for the pretreatment of HGC with ozone and ferric ions on an industrial-scale.Fourthly, this thesis applied three types of artificial neural networks (ANNs), namely FFBP (Feed Forward Back Propagation), RBNN (Radial Basis Function Based Neural Network) and GRNN (Generalized Regression Neural Networks), to establish the pre-oxidation model. Six operating parameters (concentration of ozone and ferric sulfate, liquid to solid ratio, oxygen amount, time, temperature) were employed as input to the network; the output of the network was ferric extraction. The results show that the FFBP fits into better estimations than the RBNN and is significantly superior to the GRNN. Our study has demonstrated that the ANN model was capable of predicting the nonlinear pre-oxidation process of refractory gold concentrate. In addition, the neural net weight matrix can be used to assess the relative importance of the various input variables on the output variables. Weight calculation of FFBP model suggested that operating temperature and ozone concentration with a relative importance of 23.9% and 21.6% appeared to be the most influential parameters in the pre-oxidation process of refractory gold ore. The comparison between measured and predicted values of ferric extraction by ANN and multivariate regression analysis (MVRA) indicates that:prediction of ferric extraction by neural network is very accurate and closer to measured values; the high coefficient of determination values shown by ANN as compared to MVRA indicates better prediction capability of ANN over MVRA.Fifthly, the dissolution kinetics of RGC in ozone and acidic ferric sulfate solution was also studied in this thesis. It was found that the reaction rate increases with increase in ozone concentration, reaction temperature and with decrease in solid to liquid ratio and particle size. Analysis of the kinetic data by different kinetic models indicates that the pre-oxidation process follows the shrinking core model with the surface chemical reaction as the rate controlling step. The activation energy was calculated to be 40.35kJ/mol, which is consistent with a chemically controlled reaction. Dissolution rate can be expressed by Equation:1-(1-α)(?)3=22303.97e-40.35 RT t.