Effects of ore mineralogy on the microbial leaching of low grade complex sulphide ores

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1 Hydrometallurgy 86 (2007) Effects of ore mineralogy on the microbial leaching of low grade complex sulphide ores P.A. Olubambi a,b,, S. Ndlovu a, J.H. Potgieter a, J.O. Borode b a School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private bag 3, WITS 2050, Johannesburg, South Africa b Department of Metallurgical and Materials Engineering, Federal University of Technology, PMB 704, Akure, Nigeria Received 5 July 2006; received in revised form 5 September 2006; accepted 29 October 2006 Available online 10 January 2007 Abstract The effect of ore mineralogy on the microbial leaching of low grade complex sulphide ores was investigated by utilizing mineralogical data on the variations in mineral and phase distribution within particle sizes of 53, 53, 75 and 106 μm of a Nigerian ore consisting of siderite, sphalerite, galena and quartz, with traces of pyrite and chalcopyrite. Bioleaching was conducted using mixed cultures of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans in mechanically stirred glass reactors. The highest bioleaching recoveries were obtained at a particle size of 75 μm, while particle sizes of 106 μm gave the least recoveries. Mineral phases and morphologies of the bulk ore and the leached residues respectfully examined with XRD and SEM analysis showed differences both in phase and morphological changes, with the 75 μm having the highest transformation and attack. Higher silica contents which reduced acidity, iron mobility and oxidation led to lower recoveries at particle sizes of 53 and 53 μm Elsevier B.V. All rights reserved. Keywords: Sulphide ore; Ore mineralogy; Bioleaching; Particle size 1. Introduction Corresponding author. Tel.: ; fax: addresses: polubambi@prme.wits.ac.za, polubambi@yahoo.com (P.A. Olubambi). Sulphide ores exhibit close similarities and complexities in their mineralogical associations and properties, which often pose challenges during their hydrometallurgical processing. As a result of these challenges, increased efforts are recently focused on the application of biohydrometallurgical processing routes for recovering their constituent metals. Nevertheless, the complexities of these sulphide ores and their general low solubility also place a limitation during biohydrometallurgical processing. Amongst others, an approach to increase their solubility is to ensure that metal atoms within the ore come in contact with the organisms. This can be achieved through grinding of the ore fine enough to liberate the constituent mineral phases prior to microbial attack. However, crushing and grinding of ore is a significant capital and operational cost, and energy-inefficient process (Benzer, 2005; Bilgili and Scarlett, 2005). Hence, it is imperative to fully determine comminution parameters relevant to the crushing and milling of an ore to enable complete understanding of mineral liberation, breakage characteristics, and for efficiency determination (Olubambi et al., 2006). This can be effectively achieved through adequate particle size analysis as it is very useful in X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.hydromet

2 P.A. Olubambi et al. / Hydrometallurgy 86 (2007) the understanding of recovery processes. According to Nemati et al. (2000), particle size distribution of the mineral is an important parameter, which may influence both the activity and the bioleaching capacity of microbes, by affecting physical attrition, availability of mineral for leaching and mass transfer. The relevance of particle size on the microbial dissolution process has therefore drawn the attention of many researchers and several research reports have been made (Torma et al., 1972; Blancarte-Zurita et al., 1986; Gonzalez et al., 1999; Nemati et al., 2000; Deveci, 2002, 2004; Hossain et al., 2004). Despite the extent of these studies, it is observed that they were all centered on the physical influence of particle sizes on cell viability, cell growth, and bacterial mineral attachment/ detachment phenomena. They also, do not provide useful information that could aid full understanding of the underlying mineralogical basis for microbial attack. Barrett (2003) observed that particle size alone may not reveal enough information about a process to allow process optimization. According to Olubambi et al. (2006), particle size does not provide detailed mineralogical information on the distribution of the constituent minerals and elements within the various size ranges. While minerals response to breakage is influenced by their mineralogical properties, comminution in turn affects mineral and elemental distribution within particle size ranges. Mineralogical differences within varying particle sizes therefore affect their responses and behaviour in different conventional and bioleaching media. Owing to the differences in the mineralogical compositions at different particle sizes, there exist some variations in microbial mineral interaction. This might result from the differences in the electrochemical galvanic interactions, as galvanic interactions depend on the mineralogical association between the phases present (Cruz et al., 2005). Although studies on galvanic interactions in bioleaching systems have been reported (Mehta and Murr, 1982; Nowak et al., 1984; Paramguru and Nayak, 1996; Madhuchhanda et al., 2000; Valix et al., 2001; da Silva et al., 2003; Abraitis et al., 2003), it must be noted that these studies have not taken into account galvanic interactions at different particle sizes that might result from the difference in the mineralogical compositions. This study therefore aims to understand the influence of particle sizes on microbial leaching by investigating the effects of ore mineralogy on the bioleaching of low grade complex sulphide ores from the view point of mineralogical variations of the ores and phase distribution within the different particle size ranges. 2. Materials and methods 2.1. Materials and micro-organisms The complex sulphide ore used for this study was obtained from Ishiagu, in the South Eastern region of Nigeria. X-ray diffraction (XRD) analysis of the bulk ore revealed 42% siderite, 35% sphalerite, 11% galena and 8% quartz. Ore characterization and sample preparation methods for the bioleaching experiments have previously been reported in Olubambi et al. (2006). The micro-organisms used in the microbial leaching experiments were mixed cultures of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans. The strains were obtained from the Council for Minerals Technology (MINTEK), South Africa. The cultures were routinely sub-cultured in 9 K medium (Nestor et al., 2001) using ferrous sulphate as an energy source in an incubator shaker at 32 C and a ph of 2.0. The culture was filtered through a Whatman filter No1 to remove the precipitate and centrifuged at 6000 rpm for 30 min using a Sorval Centrifuge, Model RC 5C PLUS. The centrifuged cells were washed in sulphuric acid at ph 2.0. Washing and centrifugation were repeated three times before the cells were free from precipitates Microbial leaching experiments Bioleaching experiments were carried out on particle sizes of 53, 53, 75 and 106 μm in 1000 ml glass reactors using 6% vol/vol clean cell suspension of an initial population of cells/ml. The glass reactor had four holes for agitation, ph/temperature measurement, aeration and sampling. The reactor was placed in a water-bath and agitated at 150 rpm from above using mechanical stirrers with the solution temperature kept at C. Pulp density was kept at 10% wt/vol, while the initial ph was 2.0, but monitored and adjusted every 2 days. The morphologies of the particles were observed by SEM analysis before and after microbial attack, while the mineral phases of the bioleached residues were also identified by XRD. 3. Results and discussion 3.1. Ore mineralogy Major minerals identified by XRD analysis of the particle sizes include sphalerite, galena, siderite and quartz, with traces of chalcopyrite and pyrite. The sphalerite mineral in the ore occurs as ferrous sphalerite.

3 98 P.A. Olubambi et al. / Hydrometallurgy 86 (2007) Table 1 Variations in Zn and Fe in sphalerite within particle sizes Particle size (μm) Chemical composition of sphalerite 106 Zn Fe S 75 Zn Fe S 53 Zn Fe S 53 Zn Fe S There is a variation in ratios of zinc to iron at different particle sizes within the sphalerite phase; finer particles having lower percentages of iron, while coarser sphalerite grains contain more solid solution iron (Table 1). The results of the elemental composition of the ore with respect to the total elements present within the ore and their distributions within the particle sizes of 53,+53,+75,+106 μm and the bulk ore total, determined by XRF and ICP-OES are shown in Table 2 and have been discussed in detail previously in Olubambi et al. (2006) Bioleaching results Since there were variations in the mineralogical composition within the particle sizes, microbial leaching studies were aimed at investigating dissolution trends among the varied particle sizes with reference to zinc, copper and iron dissolution, as well as ph changes. Dissolution results are presented and discussed below Effects of ore mineralogy on zinc and copper dissolution The amounts of zinc and copper bioleached at the varied particle sizes are shown in Figs. 1 and 2. The results revealed that zinc dissolution was higher than copper dissolution, with respective optimal recoveries of 62.9 and 12.5% on the 21st day of bioleaching obtained at particle size of 75 μm. These results show some similarities and deviations from reported studies. Gonzalez et al. (1999) investigated the influence of particle size, ferric ion and suspended cells concentration on the attachment of Thiobacillus ferrooxidans to a high pyrite and enargite concentrate and reported a marked effect of particle size on the equilibrium concentrations. It was observed that smaller particles brought about higher amounts of attached cells and this was reported to be probably due to the increased surface area of the finer particles that gave an increase in the number of active attachment sites. Nemati et al. (2000) reported that although, decreasing the particle size enhanced the bioleaching rate of pyrite, there was however a threshold particle size value below which a further decrease in the size led to a decrease in the dissolution rate. According to Nemati et al. (2000), decreasing the particle size of pyrite below this threshold value did not improve the rate of bioleaching due to the fact that fine particles adversely influenced the activity of the cells by apparently damaging the structure of the cells, resulting in their inability to oxidize pyrite. This was also confirmed by Deveci (2004) who showed that the rate and extent of loss in cell viability appeared to increase with decreasing particle size. Since both higher and lower particle sizes have been reported to have inhibitory effects on both bacterial activity and ferrous oxidation, it is therefore believed that the dissolution process and trend will be best understood from a mineralogical perspective as dissolution of minerals depend mostly on their chemical and mineralogical compositions. From the elemental distribution in Table 2, larger particle sizes contained higher amounts of zinc and copper than at lower particle sizes, signifying higher amounts of sphalerite and chalcopyrite contents at larger particle sizes than at lower sizes. Thus, there would be higher amounts of both zinc and copper available for dissolution at higher particle sizes. Smaller particle sizes contained higher amounts of acid consuming siderite and galena which increases ph and favours precipitation, thereby lowering the tendencies for zinc and copper to dissolve. One might presume and conclude from the above that, highest dissolution occurs at larger particle sizes of 106 μm. However, it should be understood that, the physical influence of larger particle sizes combined with Table 2 % Composition and distribution of the elements (Olubambi et al., 2006) Sample X-ray Fluorescence Spectroscopy (XRF) Optical Emission Spectrometry (ICP-OES) O Zn Fe S Pb Si Mn Cu Mg Ca Al Cd Bulk ore total Size 106 μm Size 75 μm Size 53 μm Size 53 μm

4 P.A. Olubambi et al. / Hydrometallurgy 86 (2007) Fig. 1. Influence of particle size on zinc bioleaching. agitation could result in cell damage, deactivation, disintegration, inhibition of the attachment of bacteria to the minerals and the detachment of the cells from the mineral surface (d'hugues et al., 1997; Rossi, 2001; Deveci, 2002), and consequently leading to lower dissolutions. Highest dissolutions at particle sizes of 75 μm could be reasoned from the mineralogical compositions and distributions of the lower acid consuming siderite and galena, coupled with higher amount of Fe in the sphalerite matrix within the particle sizes of 75 μm. The high amount of Fe within the sphalerite therefore promoted the highest zinc and copper dissolutions. This could be due to the availability of ferrous ions for ferric ion oxidation, which would increase the oxidation of zinc and copper at the particle sizes of 75 μm having higher amounts of Fe. This acceleration of the dissolution arising from the presence of Fe within the sphalerite could also be explained from the semiconduction and electrochemistry view point. According to Crundwell (1988, 1987), the iron content within sphalerite has the effect of narrowing the band gap energy, and consequently, the rate of dissolution of sphalerite is therefore directly proportional to the concentration of constitutional iron impurity in the solid. The iron content hence, formed a narrow impurity band within the forbidden band gap of the sphalerite, which energetically favoured the transfer of electron between the d-orbital band and the oxidant than the transfer of electrons between the valence band and the oxidant. Lower dissolution of copper in comparison with zinc could be attributed to a galvanic reaction between chalcopyrite and sphalerite. With reference to standard hydrogen electrode (SHE), sphalerite and chalcopyrite depicts respective rest potential of 0.24 V and 0.52 V (Mehta and Murr, 1982). Owing to the difference in their equilibrium potentials, a galvanic couple and interaction occur at the interface between the grain boundaries of sphalerite and chalcopyrite within the bioleaching media. Sphalerite being the less noble mineral acts as the anode in the galvanic couple, while chalcopyrite being the nobler mineral becomes the cathode (Holmes and Crundwell, 1995). As a result of this galvanic reaction between chalcopyrite and sphalerite (Eqs. (1) and (2)), zinc tends to preferentially dissolve faster than copper (Berry et al., 1978), and this could therefore account for the presence of higher soluble zinc and lower copper (Tipre and Dave, 2004). ZnS Zn 2þ þ S 0 þ 2e Cu 2þ þ 2e Cu ð1þ ð2þ Effects of ore mineralogy on ph Changes in ph with time in the bioleaching experiments presented in Fig. 3 revealed a general Fig. 2. Influence of particle size on copper bioleaching. Fig. 3. Effect of particle size on ph.

5 100 P.A. Olubambi et al. / Hydrometallurgy 86 (2007) increase in ph at the initial stages of bioleaching, after which the ph decreased slowly. The initial increase in ph within first 6th and 8th days could be attributed to solubilization of galena and siderite (Eqs. (3) and (4)). It should be understood that the major mineral in the ore is siderite. Under acidic conditions of bioleaching, where carbonic acid is stable, siderite consumes much acid (Walder and Schuster, 1998). Therefore, the initial rise in ph could be assumed to be attributed to the dissolution of the soluble and high acid consuming siderite and galena, while the progressive decrease in ph could be as a result of iron hydrolysis, the consequent precipitation of ferric compounds, and the oxidation of sulphur. PbS þ 2H þ Pb 2þ H 2 S FeCO 3 þ H þ Fe 2þ þ HCO 3 : ð3þ ð4þ The variations in the mineralogical composition of the particle sizes (Table 2) also show that siderite content increased with reducing particle size. Particle sizes of 106 μm have the least siderite content while 53 μm has the highest. This indicates that the acid consumption effect of siderite had more effect on the ph at particle sizes 53 μm and 53 μm. It was observed that after 8 days of bioleaching, the ph of size 106 μm was higher than that of particle size 75 μm. This could be explained from the inhibiting effect that higher particle size have on bacterial attachment to mineral which could have reduced the amount of acid generated by the organisms. The lower surface area of larger particle sizes, for instance particle size fraction of 106 μm would result in a decrease in the number of active microbial attachment sites. Hence, it is believed that larger particles of 106 μm led to a decrease in acid production through bacterial minerals interaction, which therefore results in higher ph. The mineralogy of the ore also shows higher amount of acid consuming galena (Eq. (3)) and siderite (Eq. (4)), with their amounts increasing as particle size decreased (Table 2). This indicates that the neutralization process of galena increases as particle size is reduced, leading to higher ph at particle sizes of 53 and 53 μm. Since sphalerite occurs as ferrous sphalerite, Walder and Schuster (1998) observed that, when iron substitutes for zinc, sphalerite dissolution generates acid instead of being acid consuming, due to hydrolysis of the ferric phases (Eq. (5)). It could therefore be understood from Eq. (5) that, the higher amount of Fe within the sphalerite matrix, (i.e. at particle sizes of 75 and 106 μm), also contributed to the increased acidity. Zn ð1 xþ Fe ð1 yþ S þð2 y=2þo 2 þ yh 2 O ð1 xþzn 2þ þð1 yþfe 2þ þ SO 2 4 þ 2xH þ Effects of ore mineralogy on iron mobility The amount of iron dissolved and presented in Fig. 4 revealed an initial increase up until the 9th day of bioleaching after which there was a general decrease. The general dissolution trend shows differences compared with the research studies reported by Rodriguez et al. (2003) where there was an increase in iron dissolution throughout the bioleaching experiment for 40 days. It should be understood that, in this present study, iron dissolution was generally high, which could be reasoned from the solubility of the acid consuming siderite (Eq. (4)). The high content of soluble iron might not depend on the initial growth of the bacteria but presumably on the high solubility of siderite in acid medium. The discrepancies in the decrease in the Fe total after the 9th day observed in this study, compared with the reports by Third et al. (2000) and Rodriguez et al. (2003), could be reasoned from the view point of iron precipitating to form jarosite and other iron precipitates. Similar decrease in the concentration of iron was reported by Deveci et al. (2004), and was attributed to the precipitation of ferric iron and its probable accumulation in the system. It should have been expected that the decrease in the ph after the 6th and the 8th day would have resulted in an increase in the concentrations of iron in solution, but the lower iron dissolution observed could be due to the impervious and Fig. 4. Effect of particle size on Fe dissolution (note: iron precipitation was not accounted for).

6 P.A. Olubambi et al. / Hydrometallurgy 86 (2007) Table 3 Phases identified by XRD analysis of bioleached residues S/ N Solid residue (μm) Major phases (order of abundance) 1 53 KFe 3 (SO 4 ) 2 (OH) 6, (K,H 3 O)Fe 3 (SO 4 ) 2 (OH) 6, PbSO 4, PbS, ZnS, CuS, FeS 2, S,SiO KFe 3 (SO 4 ) 2 (OH) 6, PbSO 4, CuS, ZnS, PbS, SiO 2, K(AlSi 3 O 8 ) 3 75 KFe 3 (SO 4 ) 2 (OH) 6, Pb(SO 4 ), S, ZnS, (K,H 3 O)Fe 3 (SO 4 ) 2 (OH) 6, PbS, SiO 2. CuS KFe 3 (SO 4 ) 2 (OH) 6, Pb(SO 4 ), S, ZnS, PbS, SiO 2, FeCO 3, KAl 3 Si 3 O 10 (OH) 2, CuS insoluble types of the precipitate formed. This might lead to limiting the transfer of oxygen needed for both dissolution and oxidation, resulting in a decrease in further oxidation of iron and subsequently in the amounts and zinc and copper dissolved. Beyond the 9th day of bioleaching, the highest amount of iron dissolved was obtained at the end of the leaching period for the particle size of 75 μm while the lowest was recorded at 53 μm. Lower dissolutions of Fe at particle sizes of 53 and 53 μm could be attributed to higher precipitation of iron in the form of jarosite which reduced the presence of iron in the aqueous solution. It was observed from Table 2 that smaller particle sizes of the ore contained higher amounts of iron and silicon than larger particle sizes, signifying higher amounts of siderite and silica contents. It should therefore be assumed that, there would be higher amounts of iron available for dissolution at smaller particle sizes of 53 μm. However, the higher amounts of silica present at lower particle sizes of 53 and 53 μm than of 75 and 106 μm might lower the amounts of iron within the aqueous solution. According to Davis et al. (2001), silica causes a negative shift in iron mobilization, and hinders ferrous oxidation to Fe 3+ (Rushing, 2002). Hamade et al. (2000) also reported that silica reacts with iron to form amorphous iron-silicate gels in a microbiological photochemical mediated system, thus reducing ferrous oxidation. Hence, instead of Fe going into solution and oxidizing to ferric ion, it could possibly be precipitated in the presence of silica. Under acidic medium, Santelli et al. (2001) noted that iron oxidizing bacteria species of T. ferrooxidans could also suppress iron dissolution in Fe-silicate minerals. The highest Fe dissolution obtained at the particle sizes of 106 μm within the first 9 days could therefore be due to lower amounts of silica. The decrease after the 9th day in comparison with particle sizes of 75 μm could Fig. 5. SEM micrographs of different particle sizes before microbial attack (Magn. 3000).

7 102 P.A. Olubambi et al. / Hydrometallurgy 86 (2007) have resulted from the overall negative effects of larger particles on cells needed for dissolution. Besides the direct influence of silica on iron dissolution, silica could presumably have a negative effect on bacteria growth needed for ferrous oxidation. According to Derksen et al. (2000), silica decreases the volumetric oxygen transfer rate, because of its blocking effect at the gas liquid interface during agitation. Since bioleaching is aerobic, any interference with oxygen, invariably affects bacterial growth. This negative effect of silica on oxygen could inhibit the oxidation of Fe 2+ to Fe 3+ needed as an electron donor for microbial activity. It has also been shown that freely suspended biomass also undergoes rapid attrition in the presence of silica (Nemati et al., 2000; Scholtz et al., 1997). The overall effect of silica is the resultant decrease in soluble iron at particle size of 53 and 53 μm and the consequent lower zinc and copper dissolutions than at particle size of 75 μm Influence of mineralogy on precipitate formation XRD analysis of the residues (Table 3) show in the order of abundance, the presence of jarosite, anglesite, elemental sulphur, hydroxides, and little secondary sulphides of pyrite and covellite, with highest amounts of covellite in particle size of 53 and 53 μm. Since the interaction between microbes and minerals within an aqueous solvent involves a redox process, the oxidation product goes into aqueous solution while the reduced products at the cathode form an insoluble precipitate. These insoluble products therefore, build up a nonconducting product layer within the system. Higher precipitates (especially jarosite, anglesite and sulphur) at lower particle sizes resulting from their higher amounts of siderite and galena contents, therefore, hindered bacterial growth as a result of lack of ferrous ions in solution. Morphological features of the samples before and after bioleaching shown in Figs. 5 and 6 revealed that the morphologies of the ore were differently changed at the different particle sizes. All the surfaces of the grains were covered and coated with precipitated products. The precipitates cover on mineral surface shown in Fig. 6 revealed that dissolution might be hindered through the less-porous product layer over the mineral sulphides and the building up of the non-conducting product layers within the system. According to Mehta and Murr (1983), the nonconducting product layer reduces the contact between the minerals, thereby culminating in a considerable decrease in the galvanic effect that could promote zinc and copper dissolution, while Keeling et al. (2005) noted that the precipitate may coat the mineral surface and inhibit access of ferric ions to fresh sulphide Fig. 6. SEM micrographs of solid residues of bioleached particle sizes showing the coverage on precipitates on mineral surfaces (Magn. 3000).

8 P.A. Olubambi et al. / Hydrometallurgy 86 (2007) mineral. Kodali et al. (2004) also observed that jarosite caused an obstruction to mineral microbe contact by forming a mass transfer barrier to nutrients, oxygen, and carbon dioxide. The report by Gomez et al. (1999) indicates that jarosite formed in the presence of bacteria suppresses not only copper extraction from chalcopyrite, but also zinc extraction from sphalerite coexisting with chalcopyrite. Nevertheless, it could be concluded from this study that the higher zinc extraction by bacteria could be an indication that the coverage of mineral particles with jarosite has little influence on zinc leaching. The higher suppression of copper is therefore believed to be due to its special affinity for jarosite, which therefore leads to interference in copper immobilization. Brierley and Brierley (2001) and Acevedo (2000) also noted a decrease and fluctuations in soluble copper which was attributed to the locking of some of the extracted copper with the jarosite formed. 4. Conclusion This study has revealed the influence of particle size and ore mineralogy on microbial leaching of low grade complex sulphide ores. It shows that particle size distribution alone does not provide relevant mineralogical information for understanding bioleaching behaviour. Attention should not be solely given to particle sizes, but more to the mineralogical and elemental distribution within the sizes and the interaction of the minerals/phases within the ore. The mineralogical and elemental distribution within the various particle sizes not only affects the mineral microbe interaction and the galvanic interaction, but also the precipitate formation on the surfaces which in turn plays a role in the metal dissolution process. 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