Bioleaching of Copper from Low Grade Scrap TV Circuit Boards Using Mesophilic Bacteria

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1 Laval University From the SelectedWorks of Ahmet Deniz Bas 2013 Bioleaching of Copper from Low Grade Scrap TV Circuit Boards Using Mesophilic Bacteria Ahmet Deniz Bas, Laval University Haci Deveci, Karadeniz Technical University Ersin Y Yazici, Karadeniz Technical University Available at:

2 Hydrometallurgy 138 (2013) Contents lists available at SciVerse ScienceDirect Hydrometallurgy journal homepage: Bioleaching of copper from low grade scrap TV circuit boards using mesophilic bacteria A.D. Bas, H. Deveci, E.Y. Yazici Division of Mineral and Coal Processing, Department of Mining Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey article info abstract Article history: Received 14 February 2013 Received in revised form 20 June 2013 Accepted 29 June 2013 Available online 4 July 2013 Keywords: WEEE Bioleaching Acidophilic bacteria Recycling Copper In this study, the extraction of copper from manufacturing scrap TV circuit boards (STVB) by bacteria-assisted leaching was studied using a mixed culture of mesophilic bacteria. Initial availability of soluble iron appeared to be of prime importance for the extraction of copper from STVB, which contains 43 g/ton Fe. In this respect, the addition of 1 8 g/l Fe(II) significantly improved (by up to 54%) the extraction of copper from STVB at the expense of the increased acid consumption. A new approach for the bioleaching process was adopted, which is based on the addition of pyrite (5 50 g/l pyrite concentrate (PyC)) as a source of iron and sulphur. Copper extraction was observed to enhance from 24% to 84% in the presence of 50 g/l PyC with a significant decrease (62%) in the consumption of acid. An increase in the size of inoculum from 10% to 50% v/v was also observed to improve the bioleaching rate of copper. The findings in the current study highlight the practical importance of the availability of iron (and hence, iron content of wastes) for the successful development of a bioleaching process for e-waste and the potential for utilisation of pyrite as a suitable source of iron and sulphur in bioleaching of e-waste Elsevier B.V. All rights reserved. 1. Introduction In recent years, the rapid development of technology has led to a drastic increase in diversity and consumption of electrical and electronic equipments with their reduced life span and concomitantly, generation of ever increasing amounts of end-of-life equipments, known as waste of electrical and electronic equipments (WEEE) or e-waste (EPA, 2008; Yazici et al., 2011a). WEEE is the fastest growing waste stream in the world. In the EU, about million tonnes of WEEE (in 2005) were estimated to be generated and this figure are forecast to reach 12.3 million tonnes by 2020 with an annual growth rate of % (Huisman et al., 2007). It is also estimated that approximately million tonnes of WEEE are globally generated (Huisman et al., 2007; UNEP, 2009). WEEE contains a variety of hazardous organic (chlorinated/brominated flame retardants) and inorganic (Hg, Pb, etc.) substances, which may cause some environmental problems when it is not properly managed (Robinson, 2009; Widmer et al., 2005; Yazici et al., 2010). On the other hand, WEEE can contain base (Cu, in particular) and precious metals (Au, Ag and Pd) in quantities even higher than the ores, which renders them as potentially an important secondary resource for these metals (Hagelüken, 2006; Huisman et al., 2007; Yazici et al., 2011a). Therefore, recycling of WEEE is of prime importance from both environmental and economic points of view as regulations have already been issued in many countries (EU, 2012; Terazono et al., 2006). Corresponding author. Tel.: ; fax: address: hdeveci@ktu.edu.tr (H. Deveci). Traditional physical separation, hydrometallurgical and pyrometallurgical processes can be exploited for the recovery of contained metal values from WEEE (Cui and Forssberg, 2003; Cui and Zhang, 2008; Tuncuk et al., 2012; Yazici and Deveci, 2009). Pyrometallurgical processes are extensively used for the treatment of WEEE; albeit, they suffer from the requirement for high precious metals grade (Au, in particular) in feed, environmental concerns associated with off-gas/ dust emissions and high cost of off-gas/dust treatment. Hydrometallurgical processes with their potential for the treatment of low grade WEEE and small scale applications have received particular interest in recent years (Tuncuk et al., 2012; UNEP, 2009). However, an innovative, cost-effective, environmentally friendly and effective process as alternative to pyrometallurgical processes has yet to be developed. Biohydrometallurgical processes are often promoted as low cost and eco-friendly processes for the treatment of low-grade ores and wastes. They are essentially hydrometallurgical processes using microorganisms (i.e. bacteria, archae archaea and fungi) to enhance the dissolution of metals from ores, concentrates and wastes. In these processes, the exploitation of microorganisms is based on their inherent characteristics to oxidise/utilise inorganic and organic substrates so as to generate lixiviant for dissolution of metals (Deveci and Ball, 2010; Jain and Sharma, 2006). A variety of bacterial and fungal cultures have been used for bioleaching of metals from WEEE (Bas et al., 2012; Brandl et al., 2001; Chi et al., 2011; Choi et al., 2004; Ilyas et al., 2007). Notwithstanding this, iron oxidising acidophiles such as mesophilic At. ferrooxidans and L. ferrooxidans, and thermophilic S. thermosulphidooxidans have received the most interest (Lee and Pandey, 2012). These bacteria oxidise Fe(II) X/$ see front matter 2013 Elsevier B.V. All rights reserved.

3 66 A.D. Bas et al. / Hydrometallurgy 138 (2013) (Eq. (1)) under suitable conditions leading to the generation of Fe(III), which is a powerful oxidant with a reduction potential of 0.77 V for most sulphide minerals and metals (Nemati et al., 1998). Since metals are present in native form and/or as alloys in e-waste e.g. printed circuit boards (PCBs), oxidative leaching conditions should be maintained for efficient extraction of base and precious metals. Ferric iron, which can be generated in-situ from the oxidation of ferrous iron by bacteria (Eq. (1)), is a suitable oxidising agent to leach copper from e-waste (Eq. (2)). The rate of bioleaching of copper from e-waste is apparently controlled intimately by the availability (i.e. initial concentration) and the rate of biooxidation of ferrous iron (i.e. the rate of generation of ferric iron) in the bioleaching environment. Many studies have already demonstrated the beneficial effect of external addition of Fe(II)/Fe(III) on the rate and extent of copper extraction (Choi et al., 2004; Lewis et al., 2011; Xiang et al., 2010; Yang et al., 2009; Zhu et al., 2011). Others have reported remarkably slow rate of metal extraction apparently due to the limited availability of soluble iron at the onset of bioleaching and/or low iron content of the e-waste sample used (Brandl et al., 2001; Ilyas et al., 2007, 2010; Willscher et al., 2007). Biooxidation of Fe(II) to Fe(III) is an acid consuming reaction (Eq. (1)) and the addition of acid is required to control ph (i.e. at b ph 1.8) and hence, the undesired precipitation of iron (Deveci et al., 2003). It can be inferred from these earlier studies that the external supply of iron and acid is required for efficient bioleaching of e-waste with low iron content, in particular. Recently, Ilyas et al. (2013) reported the use of FeS 2 and S 0 (1% w/v) as additional energy source during bioleaching of electronic scrap using moderately thermophilic bacteria. They attributed the improved bioleaching efficiency to the partial compensation of acid consumption by S 0 oxidation providing more suitable conditions for bacteria. 2Fe 2þ þ 1 = 2 O 2 þ H þ bacteria 2Fe 3þ þ H 2 O ð1þ Fig. 1. An illustration of preparation of STVBs for bioleaching tests, the feed and products at each stage of size reduction. separation of metals from the crushed product ( 8 mm) was not effective with the unacceptably low metal recovery (Yazici et al., 2011b). The STVB sample contains 11.2% Cu with only 43 g/ton Fe, which is of practical importance for the development of a bioleaching process. In view of the low iron content of STVBs (Table 1), the supplementary addition of pyrite (up to 50 g/l) as a source of iron and sulphur into the bioleaching media was tested. A pyrite concentrate (PyC) assaying 42.2% Fe, 0.6% Cu and 44.6%S (Table 1), which was obtained from Kure Copper Mine, Turkey was used. Consistent with its chemical composition, pyrite was identified to be the predominant phase in the concentrate. Cu 0 þ 2Fe 3þ Cu 2þ þ 2Fe 2þ ð2þ 2.2. Bacteria and growth media To date, studies have focused mainly on bioleaching of high grade e-waste such as personal computers and mobile phones. In this study, bioleaching of copper from low grade scrap TV circuit boards (STVB) with low iron content was investigated using a mixed mesophilic culture of acidophilic bacteria. Effects of initial concentration of iron (up to 8 g/l Fe(II)) and size of inoculum (10 50% v/v) on the rate and extent of copper extraction were studied. Furthermore, a novel approach was adopted for the bioleaching of STVBs in that the addition of pyrite to provide iron source and to reduce acid consumption in the bioleaching process was tested. 2. Experimental 2.1. E-waste and pyrite samples An 80-kg sample of manufacturing scrap TV circuit boards (STVB) was obtained from an electronics company (Vestel Electronics in Manisa/Turkey). STVB was received after having been subjected to heat treatment for the removal of solder and recovery of many components for re-use at the manufacturing plant site. The sample as received was initially crushed to 3.35 mm in a rotary cutting shredder and then, ground to 250 μm in a two-stage grinding process using an Ultra Centrifugal Mill (Retsch ZM 200) (Fig. 1). The ground sample was riffled and packed into 250-g portions prior to use in the bioleaching experiments (Bas, 2012). Chemical composition of STVB sample was determined by wet chemical analysis as shown in Table 1. Low content of precious metals of STVB renders it not a particularly suitable feedstock for pyrometallurgical processes. Earlier studies also demonstrated that physical A mixed culture of mesophilic bacteria, MES1 (At. ferrooxidans, L. ferrooxidans, At. thiooxidans), were used in this study (Bas, 2012; Deveci and Ball, 2010). The culture is routinely grown and maintained on pyritic materials (mill tailings or concentrate) (1 2% w/vol) using the growth media with a composition of (NH 4 ) 2 SO g/l, MgSO 4.7H 2 O 0.5 g/l, KH 2 PO g/l, KCl 0.1 g/l. Prior to the bioleaching tests, the culture was transferred onto ferrous iron (200 mm Fe(II)) and successively subcultured (at ph 1.7, 35 C and 170 rpm) to obtain an adapted and active bacterial population with a high capacity for iron oxidation. Bacteria grown on 200 mm Fe(II) were used in the bioleaching tests. The mixed culture was not further characterised prior to use in the bioleaching tests. Table 1 Chemical composition of STVB and pyrite concentrate used in this study. Metal STVB Pyrite concentrate (PyC) Cu (%) Fe (%) Zn (%) Al (%) Ca (%) Co (%) Ni (%) S (%) ND 44.6 As (g/ton) Cd (g/ton) Cr (g/ton) Au (g/ton) 0.14 ND Ag (g/ton) 48 6 ND: not determined.

4 A.D. Bas et al. / Hydrometallurgy 138 (2013) Bioleaching experiments Bioleaching tests were carried out in 250-mL Erlenmeyer shake flasks. The growth media (90 ml) and the STVB sample (1 g per 100 ml of bioleach solution i.e. 1% w/v) were added prior to the inoculation of the flasks. A stock solution of Fe(II) (1 M, ph 1.7) was used to prepare the growth media at the different initial concentrations (1 8 g/l) of Fe(II) tested. Freshly grown and active bacterial culture (~10 8 cells/ml) following the oxidation of 80 90% of 200 mm Fe(II) was used as inoculum (10 ml i.e. 10% v/v). Having been prepared in duplicate, the flasks were placed on an orbital shaker operated at 35 C and 170 rpm. Over the experimental period, the flasks were sampled at the predetermined intervals and these samples were used for the analysis of metals (Cu and Fe). ph and redox potential (E, mv vs Ag/AgCl) were also monitored over the bioleaching period and ph was controlled at ph 1.7 by the dropwise addition of concentrated H 2 SO 4, if deviated towards neutrality. On the termination of bioleaching tests, the residues were collected by filtration, dried for six hours at 105 C and then analysed for metals after having been digested in hot aqua-regia. The extraction of metals was calculated based on the residue analysis. Analysis of metals (Cu and Fe) from the solutions was performed using an atomic absorption spectrophotometer (Perkin Elmer AAnalyst 400). Concentration of ferrous iron was determined using KMnO 4 titration. 3. Results and discussion 3.1. Effect of initial concentration of Fe(II) Preliminary tests had shown that the bioleaching of copper from STVB proceeded at a discernibly slow rate. This was consistent with the low iron content (only 43 g/ton Fe) of STVB resulting concomitantly in low availability of soluble iron at the onset to support the bacterial growth and, hence, to efficiently drive the bioleaching process. Therefore, the effect of supplementary addition of Fe(II) (1 8 g/l) on the bioleaching of copper from STVB was studied. Fig. 2 demonstrates the substantial improvement in the extraction rate and extent of copper in the presence of 1 8 g/l Fe(II). Final extraction of copper was only ~18% in the control test where the dissolution process is driven by the chemical oxidation of copper with oxygen (Eq. (3)).Inthe presence of bacteria, the rate and extent of dissolution of copper from STVB were improved in the tests where no Fe(II) was externally added. This improvement can be attributed to the oxidation of Fe(II) available in the medium by bacteria (Eq. (1)) i.e. generation of Fe(III), which reacts with copper present in STVB sample (Eq. (2)). Cu 0 þ 1 = 2 O 2 þ 2H þ Cu 2þ þ H 2 O ð3þ It can be presumed that, in the bioleaching process, Fe(II) is inherently the only energy-yielding substrate for bacteria to support the growth. In this regard, the external addition and/or increasing the concentration of Fe(II) was observed to markedly enhance the dissolution rate and extent of copper (Fig. 2). To illustrate, the extraction of copper improved from 35% to 89% with increasing the concentration of Fe(II) from none to 8 g/l. It is pertinent to note that further increase in the concentration of Fe(II) to 12 g/l did not further enhance the bioleaching of copper (data not shown), which was similar to that at 8 g/l (i.e. bioleaching of copper was no longer limited by the initial availability of Fe(II) at N8 g/l). At excessively high levels of Fe(II), the bioleaching of copper could be limited by the strength of inoculum (i.e. size of active population of bacteria) relative to Fe(II) available, the availability/transfer of O 2 and CO 2, and other factors (Deveci and Phillips, 2010; Deveci et al., 2003). The increase in the availability of soluble iron appears to enhance the growth of bacteria with increased capacity for the generation of ferric iron in the bioleaching environment (Xiang et al., 2010; Zhu et al., 2011). It can be deduced from these findings that bioleaching rate and extent of copper from e-waste such as STVB is essentially controlled by the availability of soluble iron, which should be provided in sufficient quantity when iron content of the waste is low. Oxidation of Fe(II) (Eq. (1)) and Cu 0 (Eq. (3)) are acid consuming reactions. Accordingly, ph tended to increase during the bioleaching and control tests in which ph was controlled by the drop-wise addition of 18 M H 2 SO 4 at the pre-set level of ph 1.7. In the control and bioleaching tests, acid consumption (g H 2 SO 4 per g STVB) was determined as shown in Fig. 3. Increasing the availability (i.e. initial concentration) of Fe(II) was observed to result in an increase in the consumption of acid, which is consistent with the inherent peculiarity of the bioleaching process i.e. the dependence of dissolution of copper on the generation of Fe(III) by bacteria. To illustrate, the consumption of acid increased by 50% by increasing the external addition of Fe(II) from none to 8 g/l Effect of pyrite addition Considering the inherent characteristics of the bioleaching process (i.e. acid consuming and need for iron supplement), pyrite was used as a source of iron and acid since biooxidation of pyrite is an acid generating reaction with concurrent release of iron (Eqs. (4) and (5)). Fig. 4 illustrates the extraction of copper in the presence of up to 50 g/l pyrite concentrate (PyC). In the absence of PyC, only ~25% of copper was extracted from STVB over the leaching period (115 h). Addition of PyC was found to enhance the rate and extent of copper extraction, which increased further with increasing the concentration of pyrite (Fig. 4). At a maximum concentration of PyC tested (50 g/l), Fig. 2. Effect of external addition of Fe(II) on the extraction of copper from STVB sample (1% w/vol, ph 1.7, 35 C, duplicate experiments). Fig. 3. Effect of initial addition of Fe(II) on the consumption of acid (g concentrated sulphuric acid per g of STVB) in the bioleaching tests (1% w/v STVB, 10% v/v inoculum, ph 1.7, 35 C, duplicate experiments).

5 68 A.D. Bas et al. / Hydrometallurgy 138 (2013) Fig. 4. Effect of addition of pyrite (up to 50 g/l as concentrate, PyC) on the extraction of copper from STVB (1% w/v STVB, 10% v/v inoculum, ph 1.7, 35 C, duplicate experiments). around 57% of extraction of copper was already achieved over a bioleaching period of only 37 h. Thereafter, the extraction of copper slowed down with a final copper extraction of 83% over 115 h. 4FeS 2 þ 15O 2 þ 2H 2 O bacteria 4Fe 3þ þ 8SO 2 4 þ 4H þ FeS 2 þ 14Fe 3þ þ 8H 2 O 15Fe 2þ þ 2SO 4 2 þ 16H þ In the bioleaching of STVBs, the dissolution of copper is presumed to occur via oxidation by Fe(III) (Eq. (2)), which is (re)generated in-situ by biooxidation of Fe(II) (Eq. (1)). Fe(III) also reacts with pyrite (Eq. (5)) leading to the release of iron and acid, which appeared to be essential for effective dissolution of copper in the bioleaching process. In effect, copper extraction profiles were consistent with soluble iron profiles obtained at different concentrations of PyC (Fig. 5). The availability of iron increased with increasing the amount of PyC added, leading to higher extractions of copper (Fig. 4). ph and redox potential (E, mv vs Ag/AgCl) profiles are also shown in Fig. 6. Furthermore, the addition of pyrite was also noted to reduce the acid consumption e.g. by 62% reduction in the presence of 50 g/l PyC (Fig. 7). Recently, Ilyas et al. (2013) reported the use of FeS 2 and S 0 (1% w/v) as additional energy source during bioleaching of electronic scrap using moderately thermophilic bacteria. They noted comparatively higher bioleaching efficiency for metals (Cu, Zn, Ni, Al) in the presence of S 0 and attributed the beneficial effect of S 0 addition to compensation of acid consumption leading to the stabilisation of ph in favour of bacterial growth. ð4þ ð5þ Fig. 6. Redox potential and ph profiles during the bioleaching tests in the absence and presence of up to 50 g/l pyrite concentrate, PyC (1% w/v STVB, 10% v/v inoculum, ph 1.7, 35 C, duplicate experiments). It is pertinent to note that a set of chemical control tests in the presence/absence of pyrite (10 g/l PyC) were also performed to evaluate the contribution of bacteria and pyrite to the dissolution process (Fig. 8). Addition of PyC was also observed to promote the chemical dissolution of copper. This could be attributed to the chemical oxidation of Fe(II) released from PyC (i.e. generation of Fe(III)), which is catalysed by Cu 2+ (Eq. (6)) (Bas, 2012). In the absence of pyrite, chemical leaching of copper (Eq. (3)) would be limited by the oxidative action of oxygen. Fig. 8 also illustrates the contribution of bioleaching to the extraction of copper and the significance of initial concentration of iron for the extraction kinetics, consistent with the earlier findings (Figs. 2 and 4). 2Fe 2þ þ 1 = 2 O 2 þ 2H þ Cu2þ 3.3. Effect of Inoculum Size 2Fe 3þ þ H 2 O In bioleaching systems, inoculum size is an important parameter, which directly affects the rate and extent of bioleaching (Ilyas et al., 2010; Xu et al., 2010) since active number of bacteria increases with an increase in the concentration of inoculum. Fig. 9 illustrates the substantial increase particularly in the initial rate of bioleaching of copper with increasing the concentration of inoculum from 10 to 50% v/v. A 67% extraction of copper was achieved only in 37 h at an inoculum size of 50% v/v compared with only 15% extraction at 10% ð6þ Fig. 5. Concentration of iron in solution during the bioleaching tests in the absence and presence of up to 50 g/l pyrite concentrate, PyC (1% w/v STVB, 10% v/v inoculum, ph 1.7, 35 C, duplicate experiments). Fig. 7. Effect of addition of pyrite (as concentrate, PyC) on the consumption of acid (g concentrated sulphuric acid per g of STVB) in the bioleaching tests (1% w/v STVB, 10% v/v inoculum, ph 1.7, 35 C, duplicate experiments).

6 A.D. Bas et al. / Hydrometallurgy 138 (2013) Extraction of Cu (%) v/v inoculum. At the end of bioleaching period (115 h), copper extraction was almost complete at 50% v/v inoculum. This enhancement in the dissolution rate can be attributed to the rapid conversion of Fe(II) into Fe(III) by the increased size of bacterial population. The increased availability of Fe(II), which was carried over by inoculum, could have also contributed to the improvement in bioleaching rate. It should be noted that the rate of generation of Fe(III), which tends to increase with increasing inoculum size would be ultimately controlled by the transfer of O 2 and CO 2 into the bioleaching medium. 4. Conclusions Control, No PyC Control, 10 g/l PyC 10 g/l PyC 10 g/l PyC+200 mm Fe(II) Time in hours Fig. 8. Extraction of copper from STVB (1% w/v) in chemical control and bioleaching in the absence/presence of PyC (10 g/l) and 200 mm Fe(II) (10% v/v inoculum, ph 1.7, 35 C). In this study, bioleaching of copper from a low grade manufacturing scrap TV circuit boards (STVB) using a mixed culture of mesophilic bacteria was performed. The results have shown that the bioleaching process is based esentially on the ability of bacteria to oxidise ferrous iron i.e. to generate ferric iron within the leaching environment. It is therefore an acid consuming process. In this regard, the rate and extent of extraction of copper from STVB are controlled intimately by the initial availability of soluble iron. Due to the low iron content of STVB, the external addition of ferrous iron was required for efficient extraction of copper in the bioleaching process e.g. 35% Cu without external addition of Fe 2+ c.f. 89% Cu at 8 g/l Fe 2+. An increase in the size of inoculum from 10% to 50% v/v was also observed to enhance the bioleaching rate of copper. A new approach with the supplementary addition of pyrite as a source of iron and sulphur was demonstrated. Addition of pyrite (as concentrate (PyC)) up to 50 g/l was shown to substantially improve the extraction of copper from 25% (without PyC) to 83% with a concurrent reduction (by 62%) in the acid consumption. These Fig. 9. Effect of inoculum size (10 50% v/v) on the bioleaching of copper from STVB (1% w/v, 10 g/l PyC, ph 1.7, 35 C, duplicate experiments). findings suggested that the initial concentration of iron and size of inoculum are of practical importance for the development of an efficient bioleaching process for the treatment of e-waste such as STVB. Furthermore, pyrite can be utilised as a cheap source of sulphur (i.e. acid) and iron for bioleaching of non-sulphide wastes with low iron content. Acknowledgement The authors would like to acknowledge the Research Foundation of Karadeniz Technical University (Project Nos: 889 and 8647) and TUBITAK (Project No:109M111) for the financial support, Vestel Electronics (Turkey) and ETI Bakır for kindly providing scrap TV boards and pyrite concentrate, respectively. References Bas, A.D., Recovery of copper from waste circuit boards by bioleaching and chemical leaching methods. Karadeniz Technical University, Trabzon, Turkey (MSc. Thesis 113 pp. (in Turkish)). Bas, A.D., Yazici, E.Y., Deveci, H., Bacteria-assisted leaching of waste computer printed circuit boards. In: Özdağ, H., Bozkurt, V., İpek, H., Bilir, K. (Eds.), Proc. of XIII. Int. Mineral Processing Symposium (IMPS). Dept. of Mining Eng., Eskişehir Osmangazi Univ., Eskişehir, pp Brandl, H., Bosshard, R., Wegmann, M., Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi. 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