In this work, mesophilic and thermophilic cultures have been used in the presence

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1 Bioleaching of chalcopyrite in the presence of silver: solids characterization A. Alvarez, M.L. Blázquez, A. Ballester and F. González Departamento de Ciencia de Materiales. Facultad de Ciencias Químicas Universidad Complutense, Ciudad Universitaria Madrid, Spain M. Cruells and A. Roca Departament d' Enginyeria Quimica i MetaLlurgia, Facultat de Química Universitat de Barcelona Marti i Franquis Barcelona. Spain In this work, mesophilic and thermophilic cultures have been used in the presence of silver as a catalyst. Chalcopyrite and byproducts of bioleaching were characterized in order to: a) obtain infomtion about the kinetics control of the process. and b) explain the different behaviour observed during tlqe mineral attack With thermophilic microorganisms (68 C and Noms medium as nutrient) the reaction stopped at approximately 65% copper extraction. with and without Ag ion addition as a catalyst. The remaining chalcopyrite appears surrounded by hydronium jarosite (with ammonium ion in substitution in the "aikaline site"), goethite and elemental sulphur. With mesophilic cultures (35 C and 9K medium as nutrient) the reactionstopped at approximately 25% copper extraction (without Ag) and approxirnately. 85% extraction (with Ag). In both cases, the remaining chalcopyrite was surrounded by agglomerates of ammonium jarosite (with hydronium ion substitution in the "alkaline site") and some iron oxides and/or hydroxides. E. Salinas Instituto de Ciencias de la Tierra Universidad Autbnoma de Hidalgo Pachuca,,Hidalgo, México - I'nwccdii~gx ol- Coppr WCohrc 9Y r)iiiicmalinn~l Cuiii'crcncc Volunr IV-Hydix~iiriallu~ ol'coppr Ediicd hy S.K. Youii$. D.B. Drcisii~gcr. 1.1'. Hxkl and D.G. Dixtui 'Ihc hliiicr~ls. Mciiils 6 Mnizrialh Swiciy. ll)<r,

2 INTRODUCTION The concentration of complex sulphides by differential flotation gives nse to low recoveries due to the intergrowth of the ore mineralogical phases (1-3). The liberation sizes are normaily so fine that the reagent consumption is high and the metal recoveries are oílen low during flotation. Bulk flotation of complex sulphides makes it possible to obtain a bulk concentrate suitable for hydrometallurgical treatment (4-6). At this moment, the technical viability of this process is not satisfactory, due to the operational complexity to recover Zn, Cu, Pb, Ag and to remove elements such as As, Sb, and Hg. Additionally, control of iron in soiution is essential. On the other han4 the leaching of the concentrate with femc sulfate or femc chlonde leads to the formation of elemental sulphur or copper-nch polysulphide layers (7) which produces slow reaction rates and uicomplete copper extraction. In this regard bioleaching is an altemative process, because the oxidation of the elemental sulphur by microorganisms could increase the process eíñciency. However, until now, this process has been applied only to copper and uranium low-grade ores and also to rehtory gold ores (8-1 1). It has been proven that the addition of silver ions accelerate the chemical(12) and microbiological leaching of chalcopyrite (9). The catalytic effect of the silver has been attributed to the formation of a conducting Ag,S film on the chalcopyrite surface. The mechanism proposed establishes the following sequence of reactions: The role of bacteria in silver-catalysed chalcopyrite leaching has been related to the regeneration of the Fe (m) ion or the oxidation of the elemental sulphur. However, for reasons not yet completely understood, the copper extraction in biolpching systems with silver ions is higher with mesophilic than thermophilic rnicrorganisms (9,13). Messina (South Africa) were also used. Table 1 gives the chemical composition of both chalcopyrite samples, obtained by emission spectrometry using inductively coupled plasma (ICP) and electron probe microanalysis (EPMA). Table 1 - Quantitative Chemical Analysis of the Chalcopyrite Samples \ Sample Element Fe Cu S Zn Co R..' Messina EPMA %(wt) ICP %(wt) Rio Tinto EPMA %(wt) ICP %(wt) 'Residual insolubles in aqua regia According to the results obtajned by X-ray diffraction, both samples used in this investigation contain pnmarily chalcopyrite, with small amounts of quartz. Figure 1 shows a micrograph of massive chalcopyrite with some fissures and porosity. In the chalcopyrite matnx, two type of inclusions were found (Figure 2) : a) pm crystals of carrollite (Fe: 1-45 %; Cu: %, Co: %, S: % and Zn: %, by EPMA) and b) crystals of similar size of sphalente (Zn: %; S: 33 %, Fe: %; Co: %, by EPMA). The purpose of this work is to characterize chalcopyrite and byproducts kom bioleaching, using mesophilic and thermophilic cultures in the presence and absence of silver as a catalyst in order to obtain information about the kinetics control of the process, and to explain the different behaviour observed during the mineral attack. MATERIALS 1. the present work a chalcopyrite concentrate from Rio Tinto Mines (Huelva, Spain) was used, with a particle size of less than 100 pm. Massive samples of chalcopynte from Figure 1 - Micrograph of chalcopyrite Figure 2 - Micrograph of inclusions (1-carrollite; 2- sphalente)

3 Time (dayo) VOLUME IV HYDROMETALLURGY OF COPPER EXPENMENTAL PROCEDURES Two different bactenal cultures were used in the bioleaching tests: a) a mixed culture of mesophilic bacteria grown on 95 ml of 9K nutrient medium and 5 g of chalcopyrite concentrate as the energy source, in an orbital shaker at 35OC; b) a thermophilic culture of Sul/olobus BC grown on 95 ml of nutrient medium (0.4 g.l-' (NH,),SO,; 0.5 g.l.-l MgSO,.iHzO and 0.2 g.l" K,HPO,) and 5 g of chalcopyrite concentrate, in an orbital shaker at 68OC. Due to the toxicity of silver to acidophilic microorganisms, particularly thermophiles, both cultures were adapted to this cation, through subsequent cycles where the silver concentration was increased. Bioleaching experiments were carried out with the chalcopyrite concentrate in an orbital shaker rotating at 150 min", at 5% (w/v) pulp density and at 35 and 68 C using mesophilic and thermophilic cultures, respectively. 0.3 and 0.5 g Ag* / kg concentrate (1.4x104M and 2.3x104 M Ag*) as catalyst were used. Control tests were performed with ore and nutrient medium in stenle conditions and periodically adding 2 ml of 2% thymol in ethanol as a bactericide. The ph was adjusted to 2.0 and 1.S for mesophilic and thermophilic bacteria, respectively. Analysis of copper in solution was canied out by atomic absorption spectrometry (AAS). The solids characterization was performed by X-ray diffraction (XRD). optical microscopy. electron microscopy (SEM), X-ray dispersive energy (EDS), inductively coupled plasma (ICP), elemental analysis (EA) and electron probe microanalysis (EPMA). Similar tests were canied out on massive chalcopyrite and the characterization of the solids was determined by the sarne techniques as those used on the chalcopyrite concentrate. Figure 3 - Iniluence of Ag concentration on CuFeS, bioleaching with mesophilic bacteria [lnoculated : o with 0.5 g Ag'; A with 0.3 g Ag*; * without Ag'; and uninoculated tests: m with 0.3 g Ag*; without Ag*] Figure 4 shows a SEM-SE image of the reaction residue (with bactena and silver); chalcopycite particles (5-50 pm) are embedded in agglomerates of jarosite ( pm). Figure 5 shows a particle of chalcopyrite sumunded by a jarosite layer. RESULTS AND DISCUSSION Leaching of Chalcopyrite Concentrate with Mesophilic Cultures Figure 3 shows the effect of silver on chalcopyrite dissolution at 35OC. Cu extraction was 25% with bacteria and increased to 85% in the presence of Ag' and bacteria. The leach residues were charactenzed by XRD. For the residue from bioleaching in the presence of silver ions and bacteria, jarosite was detected as the major phase while the reflections of chalcopyrite had not totally disappeared. Some quartz and sulphur were also detected. Chalcopyite was detected as the major phase by XRD of the residue leached with bacteria and without silver. Some of the reflections of a jarosite phase were also present. Small arnounts of sulphur and quartz were also detected. Figure 4 - SEM image of CuFeS? leached Figure 5 - Particle of CuFeS2 sumunded with mesophilic cultures and Ag+ by a jarosite layer (SEM-SE)

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