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Biooxidation of arsenopyrite to improve gold cyanidation: study of some parameters and comparison with grinding

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Biooxidation of arsenopyrite to improve gold cyanidation: study of some parameters and comparison with grinding
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  InlEnlmllmRJwnI lol zyxwvutsrqp mmmw PRommE Int. .I. Miner. Process. 52 (1997) 65-80 Biooxidation of arsenopyrite to improve gold cyanidation: study of some parameters and comparison with grinding S. Ubaldini a, F. Vegli6 b, L. Toro b, C. Abbruzzese a, zyxwvutsrqponml a CNR, Institute of Mineral Processing, Via Bolognola 7, 00138 Rome, Italy b Dep. Chemical, Chemical Engineering and Materials, University of L’Aquila, 67100 Monteluco di Roio L’Aquila, Italy Received 19 May 1994; accepted 5 May 1997 zyxwvutsrqponmlkjihgfedcbaZYXWV  bstract Cyanide leaching is the traditional process for the extraction of gold from primary raw materials, When the gold host rock contains high concentrations of sulphides, an oxidative thermal or a chemical pretreatment is often needed to access precious metal particles. Biooxidative leaching may be an interesting alternative, if compared with the environmental effects and costs of the convlentional pretreatment processes, to liberate gold from the sulphide matrix and then to make it iamenable to cyanidation. In this work, bioleaching with Thiobacillus ferrooxidans and ThiobaciYus thiooxiduns has been investigated at a bench scale on a refractory gold-bearing arsenopyrite (2 g/t Au) ore coming from the Golciick mine (Turkey). The factors influencing the biooxidative pretreatment, in order to enhance the gold recovery in a conventional cyanidation process, were tested using a factorial plan of experiments. Direct cyanide leaching of the arsenopyrite ground to - 74 pm showed no gold dissolution at all, but with fine grinding to - 30 pm, gold recovery reached about 55.3 after 48 h of cyanidation. On the contrary, cyanidation accomplished on a 72 h bioleached arsenopyrite has allowed 84.3 of gold to be solubilized in 2 h, using the following bioleaching conditions: 20 pulp density, pH 2, stirring conditions 200 rpm, temperature 3O”C, time 3 days. Under these conditions, the preliminary tests in a semi-con- tinuous lab-scale microfermentor have been performed to evaluate the scale-up of the biooxidative process. It was possible to solubilise 95.2 Au after 48 h cyanidation for the samples bioleached during 3 days, and 96.8 Au when the time of bioleaching was 7 days. 0 1997 Elsevier Science B.V. Keywords: biooxidation; arsenopyrite; cyanidation; gold; grinding * Corresponding author. 0301-7516/97/ 17.00 0 1997 Elsevier Science B.V. All rights reserved. PIZ SO301-7516(97)00041-O  66 S. Ubaldini et al. /ht. J. Miner. Process. 52 1997) 65-80 zyxwvutsrqponmlkjihgfedcbaZY 1 Introduction Cyanidation remains still the main technology for the processing of gold-bearing ores, accounting for over 85 of gold currently extracted in the world (Lodeishchikov et al., 1976; Lesoille et al., 1977; Espiell et al., 1986). However, the efficiency of this process, in the case of refractory gold ores, is low. In fact, if the gold particles are included in sulphide minerals (pyrrhotite, pyrite, arsenopy- rite), the gold leaching agent cannot reach them: so they remain unattacked (Brown et al., 1983; Livesey-Goldblatt et al., 1983; Gasparrini, 1983; Veglio et al., 1993a). Therefore, an oxidative pretreatment is necessary to decrease the refractory properties of the ore due to the encapsulation of precious metal inside the sulphide matrix (Ab- bruzzese et al., 1994; Ubaldini et al., 1995). Roasting is sometimes used, but is highly energy-consuming and involves a costly off-gas neutralization system to prevent atmo- spheric pollution by the evolved arsenious and sulphur oxide gases (Arriagada and Osseo-Assare, 1982, 1984). Chemical oxidation to recover gold from arsenopyrite by means of pressure oxidation and oxidation with nitric acid, is also expensive (Ubaldini et al., 1994) requiring high temperatures, high pressures and corrosion-resistant materials (Iglesias and Carranza, 1994). The biooxidative pretreatment may be an interesting alternative route to the conven- tional oxidative process such as roasting, nitric acid oxidation and pressure leaching due to the environmental protection and the low costs (Napgal et al., 1993). Bioleaching is suitable for refractory minerals using Thiobacillus ferrooxidans and Thiobacillus thioox- iduns (Lizama and Suzuki, 1989). Bacteria oxidation liberates the entrapped gold particles, thereby rendering it amenable to the cyanidation process (Classen et al., 1993). Bacteria utilize the energy obtained from the oxidation of ferrous ions to drive its growth metabolism and to assimilate carbon dioxide as cellular matter (Napgal et al., 1993). It was demonstrated that, under typical bioreactor conditions, abiotic sulphide oxida- tion was largely limited to the oxidation of sulphitic sulphur to sulphate, suggesting that subsequent oxidation of the elementary sulphur to sulphate was microbial. It was also demonstrated that attached bacteria contributed substantially to the oxidation of ferrous ions in solution (Free et al., 1993). Since important reserves of arsenical gold resources occur in northwestern Canada, USA (Alaska, Nevada), South Africa, Zimbabwe, Turkey and Spain (Brown et al., 1982; Forschaug, 1983; Ubaldini et al., 19941, the efforts to develop new treatment technology (Attia et al., 1985; Rossi, 1988; Paponetti et al., 1990; Ubaldini and Abbruzzese, 1991) are justified. The biological treatment is usually carried out after a comminution operation and prior to cyanidation (Attia et al., 1985). In this paper biooxidation with Thiobacillus ferrooxidans and Thiobacillus thiooxi- duns is proposed as a preliminary treatment of a gold-bearing arsenopyrite (Lizama and Suzuki, 1989; Barrett et al., 1991; Iglesias et al., 1992; Miller and Hansford, 1992; Iglesias and Carranza, 1994).  S. Ubaldini et al. / Int. J. Miner. Process. 52 1997) 65-80 67 In the biooxidation of arsenopyrite the reaction mechanism is the following (Ubaldini et al., 1994): 2FeAsS + 6.50, + 3H,O + 2H,AsO, + 2FeS0, 2FeS0, + H,SO, + 0.50, -+ Fe,(SO,), + H,O The net reaction of the direct mechanism can be represented as: 2FeAsS + 70, + H,SO, + 2H,O + Fe,(SO,), + 2H,AsO, Arsenopyrite may also reacts with ferric ion, generating ferrous sulphate and arsenic acid (Iglesias and Carranza, 1994): 2FeAsS + Fe,(SO,), + 4H,O + 60, + 4FeS0, + H,SO, + 2H,AsO, The ferrous iron is then oxidized by the bacteria to ferric iron ready for further leaching of the sulphide minerals (Barrett et al., 1991; Ubaldini et al., 1992). This research represents a study of technical feasibility of the biooxidative pretreat- ment to gold recovery of the auriferous arsenopyrite from the Gijlci_ick mine. The experimental work was devoted to the determination of the main factors affecting bioleaching by means of a factorial experiment (Davies, 1979; Veglio et al., 1993b). 2. Experimental design 2.1. Materials and methods A sample of arsenopyrite from the G&tick mine in western Turkey was used in this study. It is a typical refractory gold ore, the particles of the precious metal being encapsulated in a quartzitic mass. The samples for the experimental tests were prepared by grinding and sieving to - 74 I-Lm. Table 1 Chemical analysis of the gold-bearing arsenopyrite from the Gal&k mine in Turkey Compounds % SiO Al,& Fe& CaO TiO, p*o, K,O CO, AS S Sb Ba Au (g/t) Ag (g/t) 88.55 3.70 2.13 1.02 0.23 0.03 0.86 0.10 1.47 0.63 0.055 0.013 2.0 1.1  68 S. Ubaldini et al. /ht. .I. Miner. Process. 52 1997) 65-80 The chemical analysis of the ore is reported in Table 1. The main component is represented by silica (88.55 SiO,) and gold assayed at 2 g/t: AAS and ICP-AES were used for the quantitative determinations. The minerals identified by X-ray diffrac- tometry were pyrite, arsenopyrite and quartz. Neither gold nor silver were detected by diffractometry. 2.2. Bioleaching experiments in shaken flasks The microorganisms used in this study were strains of Thiobacillus ferrooxidans and Thiobacillus thiooxidans, isolated from a sulphur spring near L’Aquila (Italy) and prepared according to Paponetti et al. (1991). The selection was evaluated by the solubilization capacity of the bacterial cultures versus time. The biomass content of the pulp was determined by the Thoma technique. The samples for cell count were prepared centrifuging 1 ml of bacteria culture for 5 min, to count only bacterial cells in solution (Pasquinelli, 1977). The bacterial activity was measured through the oxygen consumption by the Warburg method. The cultures were grown in a 9 K medium without ferrous sulphate as indicated by Toro et al. (1989). The batch tests were carried out in 300-ml glass flasks continuously shaken on a rotary incubator. In general, lo-20 g of mineral were placed into the shaken flask with 100 ml of cultural media. The suspensions were mixed in a shaker for 2-3 h for conditioning, and then were inoculated with bacteria. The pH was adjusted with H,SO,. The ages of the inoculum utilised were 3 and 7 days. The amount of the inoculum was 10 ml of the bacteria culture. In our experiments, the free cell concentrations were determined microscopically (from 200 to 400 enlargements) by direct cell count, using Thoma’s counting chamber haemacytometer, after dilution and colouration of the samples (Konishi et al., 1994). The initial cell concentration in the medium was about 1.39 . 10’ cells/ml suspension. Solution samples for chemical analysis were taken at various time intervals. Fe was analyzed by AAS and As by ICP-AES. To evaluate the percentage of total oxidation of the mineral, As and Fe precipitated during bioleaching tests were determined. Samples of 1 g of solid residue were treated Table 2 First factorial experiment: 5 factors and 2 levels investigated (25) Factors Levels A pulp content (%,) 10 20 B PH 2.0 2.5 C stirring condition (rpm) 200 250 D temperature PC) 30 35 E time of treatment (days) 3 7  S. Ubaldini et al. /ht. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON  Miner. Process. 52 1997) 65-80 69 Table 3 Second factorial experiment: 2 factors and 3 levels investigated (3’) Factors A B Levels 1 2 3 pulp content (%) 10 15 20 PH 2.0 2.25 2.5 for 30 min at room temperature, by 10 ml of HCl 5 M (Attia and El-Zeky, 1989). Then, the analysis of Fe and As was performed by AAS and by ICP-AES. At the end of the bioleaching experiments, the solid residues were filtered, dried, weighed and analyzed for gold before being submitted to cyanide leaching. On the basis of the previous experimental runs the factors and their values for the experiments were determined (Abbruzzese et al., 1994). Then, a study was conducted on the basis of a two-level factorial design. The factors (pulp content, pH, stirring condition, temperature and time of treatment) and the levels considered in the biological tests are shown in Table 2. These factors were considered important for the biooxidative process taking into consideration the results reported in the literature (Attia and El-Zeky, 1989; Abbruzzese et al., 1994; Niemela et al., 1994). The levels were selected in range of values considered suitable for the biooxidation. Considering the results obtained in the first experimental tests, further study was planned on the basis of a three-level factorial design, in order to evaluate the influence of pH (Lizama and Suzuki, 1988) and pulp content (Attia and El-Zeky, 1989; Ab- bruzzese et al., 1994) on the behaviour of the surface for a wider range of experimental conditions. The factors and the levels considered in the biological tests are shown in Table 3. 2.3. Bioleaching experiments in a lab-scale microfermentor On the basis of the results achieved with shaken flasks, experimental tests in semi-continuous lab-scale reactor (LKB 1601 microfermentor) were subsequently planned in order to study, preliminarily, the scale-up problem of the biooxidative process: mainly the effects of the aeration rate and of the treatment time on gold extraction (Liu et al., 1993). In fact, it is possible to observe that the oxygen mass transfer in the aerated reactor is lo-30 times higher than in the shaken flask (Veglio et al., 1995). A volume of 10 1 of reactive system for arsenopyrite bioleaching, was inoculated by 1 1 of microbial suspension in active growth in a 9 K medium without ferrous sulphate (see Section 2.2). The incubation was performed at 20 pulp density, pH 2, stirring conditions 200 rpm, temperature 3O”C, time 3 days and 7 days. Forced aeration was applied; the air flow was increased from 0.50 vvm to 0.91 vvm (vvm = rn3/min for m3). The effluents from the reactor were daily collected for chemical analysis. In the first experiment, the reactor was run for 7 days without inoculation (blank), while in all subsequent experiments the inoculum was introduced at the beginning.
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