산화환경에서 비가시성 gold의 거동과 회수에 대한 연구

Abstract

Part 1 A study on geochemical behavior and recovery of gold in streams from gold mines

Acid mine drainage(AMD) occurs from mine tailings around abandoned gold-mines, which generate Fe-hydroxides containing heavy metals as well as gold. This acid mine drainage and Fe-hydroxides create serious contamination in mine areas and drainage systems. Accordingly, this study is to identify the gold distribution and gold forms in acid mine drainage and the Fe-hydroxides present in the drainage system and, specifically, to confirm the possibility of gold recovery from the stream sediment by lead-fire assay. Acid mine drainage samples and stream sediment samples were collected along the acidic drainage system. The content of cations and anions from the acid mine drainage were analysed with AAS and IC. The content of ferrous and ferric ions in the acid mine drainage were determined by the phenanthroline method. The saturation index was obtained from the physical and chemical factors in the acid mine drainage. To identify the mineralogical and chemical characteristics of stream sediment, aqua regia digestion, the sequential extraction method, AAS, XRD and IC were applied. The pH range was from 3.00 to 3.19 and the ORP range was from 396 to 450 mV and the content of Fe2+ ions decreased with the distance downstream of the mine site, Fe3+ ions increased with the distance downstream in the acid mine drainage. The dissolved Au content in acid mine drainage ranged from 0.66 mg/L to 0.97 mg/L by AAS analysis. The Au content decreased or was not detected from the upstream to downstream area in the drainage system, which could be influenced by factors such as pH, ORP, precipitation and adsorption. In XRD analysis, quartz and goethite were identified in the stream sediment, and these peaks of intensity and crystalline shape were developed and enhanced in accordance with the upstream and downstream due to oxidation and hydrolysis. The Au in the stream sediment was presented in the form of various complex types, and Au species in associated complexes may be precipitated with an oxidation-reduction reaction. The precipitated Au complex species interact with the Fe-hydroxide with a difference in chemical bonds. It is suggest that the Au species should be dominated by AuCl4- species complexes due to the strong acid environment. The Au content of stream sediment ranged from 13.76 g/t to 22.85 g/t by AAS analysis, and this gold content decreased with the distance downstream. It is suggested that the Au could be precipitated by the Fe-hydroxide from acidic mine drainage as a complex form, and the negatively charged Au could interact with the positively charged goethite containing Fe-hydroxide due to the interaction of static electricity. In order to confirm the bonding conditions of Au in the stream sediment, the sequential extraction method used a five-step experiment. The results of the sequential extraction experiment showed that the Au appeared dominantly bonded to iron-manganese hydroxide, organic matter-sulfur and residue bonding. The gold content in stream sediment ranged from 0.05 g/t to 0.47 g/t, and the average Au content obtained was 0.174 g/t by lead-fire assay. It is confirmed that more gold recovery can be obtained from mine waste, if mineralogical and geological characteristics are considered for abandoned gold mine waste in future applications of the recoverable techniques. Part 2 Characteristics of the recovery and loss of gold in lead-fire assays

The lead-fire assay has been applied from long in the past to the present day due to advantages in separation and preconcentration for gold ore and concentrate samples. However a large amount of gold content is lost in the glassy slag during the lead-fire assay for the reasons that sample particle size, flux mixture ratio, and fusion temperature were not at optimum conditions. Accordingly, to determine the factors for maximum gold recovery and to identify the reasons for gold loss in the glass slag, the lead-fire assay has been applied in gold ore mineral samples and concentrate samples, respectively. Roasted samples and salt-roasted samples were prepared according to particle size, roasting temperature and salt-addition, then these samples were analysed using reflected-polarized microscopy, SEM/EDS, XRD, aqua regia digestion and lead-fire assay. The gold mineral samples from gold-bearing quartz vein in the Golden-sun mine(Moisan, -82 and -110 level) consist of pyrite, galena, chalcopyrite and quartz, which contain invisible gold. The gold mineral sample(-110 level) for grade averages 184.97 g/t. The results of the concentrate sample in the lead-fire assay showed that the best gold recovery parameters were when the particle size was at -325 mesh(220.25 g/t), roasting temperature was at 750℃(215.20 g/t) and salt addition was 20%(224.65 g/t). The reasons for the maximum gold recovery were due to the particle size decrease allowing increased surface area, the gold-bearing pyrite was effectively roasted at 750℃, and the gold-bearing sulfide mineral simultaneously occurred with the reactions of oxidation, sulfurization, chlorinization and evaporation with the addition of salt. The minimum gold loss parameters through the slag were when the particle size was at 200×325 mesh(3.93 g/t), the roasting temperature was at 750℃(6.69 g/t) and salt addition was 20%(3.55 g/t). The decrease in the gold loss content is confirmed with the increase of gold recovery in the slag. In XRD analysis, galena and lead were identified in the slag. The gold loss reason is probably the presence of galena and lead in the slag. The reason for the presence of galena in the slag is that galena may be formed in the fusion process as the sulfur source from the gold-bearing sulfide mineral and the Pb source from the litharge as a collector for gold. According to theory in the lead-fire assay method, the addition of litharge(oxide lead, PbO) should collect all of the gold particles and the collecting the gold the lead must to sink down to the lead button during the fusion process, and thus the galena and lead must not be present in the slag. It is confirmed that the gold loss is much greater in the slag sample when there is the presence of galena and lead than in the slag sample with an amorphous phase by XRD analysis. When the raw ore sample(-82 level) and concentrate sample(from -82 level) were applied in the lead-fire assay, the rate of gold loss in the slag was 19.86% in the raw ore sample and 12.56% in the concentrate sample, respectively. The gold loss was much greater in the raw ore sample than the concentrate sample. This was probably due to the composition of the raw ore sample formed of gangue mineral such as silicates and then the silicate mineral was not effectively decomposed in the fusion process. In the case of excess borax addition, it is suggested that the gold loss would be decreased in the concentrate sample because borax effectively decomposes pyrite. Accordingly, it is expected that a more economic effect can be accomplished in gold mines, if this technique is applied in the field as an optimization of the lead-fire assay in future gold analysis-recovery processes.