Bacteria-Assisted Leaching of Waste Computer Printed Circuit Boards

Transcription

1 Laval University From the SelectedWorks of Ahmet Deniz Bas 2012 Bacteria-Assisted Leaching of Waste Computer Printed Circuit Boards Ahmet Deniz Bas, Laval University Ersin y Yazici, Karadeniz Technical University Haci Deveci, Karadeniz Technical University Available at:

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3 Bacteria-Assisted Leaching of Waste Computer Printed Circuit Boards A. D. Baş, E. Y. Yazıcı, H. Deveci Mineral&Coal Processing Div., Dept. of Mining Eng.,Karadeniz Technical University, Trabzon, Turkey. ABSTRACT: In this study, bioleaching of copper from printed circuit boards (PCBs) of end-of-life computers was investigated. The iron and sulphur oxidising strains of acidophilic bacteria (Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans and Acidithiobacillus thiooxidans) were used in consortium at ph 1.70 and 35 C. The extraction of copper in the absence of bacteria was determined to be only ~20% apparently due to the limited availability of soluble iron in the media. Futher studies were, therefore, carried out to examine the effect of external addition of Fe 2+ (1-8 g/l) on the extraction of copper from PCB sample. A significant improvement in the rate and extent of copper extraction (i.e. from ~60% to ~95% in 90 hours) was noted to occur when the initial concentration of Fe 2+ was maintained at >1 g/l. The extraction of copper was also shown to significantly enhance by increasing the size of inoculum from 10% to 50% (vol/vol%). INTRODUCTION In recent years, a dramatic increase in the production and consumption of electrical and electronic equipments (EEEs) with a sharp decrease in their lifespan has led to the generation of ever increasing amounts of end-of-life products, known as waste of electrical and electronic equipments (WEEE) or e- waste. WEEE is the fastest growing waste stream in the world with a 3-5% increasing rate per year then municipal wastes (UNEP, 2009). Due to organic and inorganic hazardous materials present in e-waste, a proper management method is required. Since e-waste contain appreciable amounts of precious (Au, Ag, Pd etc.) and base (Cu, Pb, etc.) metals, it is potentially an important secondary sources of these metals (Hagelüken, 2006; Yazici et al., 2011). Pyrometallurgical processes are traditionally used to treat WEEE although they suffer from high treatment costs and environmental problems associated with the release of toxic gasses and dusts. Especially for the assessment of low grade e- wastes, hydrometallurgical methods are promoted as they are regarded to be cost effective, environmentally friendly and suitable for small scale applications (Yazıcı and Deveci, 2009). In recent years there has been a growing interest for the recovery of metals from e-wastes by biohydrometallurgical methods. They are essentially hydrometallurgical methods, which deal with extraction of metals from ores, concentrates and wastes using microorganisms (Rossi, 199). Bioleaching is a leaching process based on the utilization of acidophilic strains of mesophilic and moderately thermophilic bacteria, and extremely thermophilic archea to enhance the dissolution of metals (Deveci et al., 2003). The most important peculiarity of these bacteria/archea is their ability to oxidise Fe 2+ (Eq.1) leading to the generation of Fe 3+ under suitable conditions (Jensen and Webb, 1994; Nemati et al., 1998; Deveci and Phillips, 2010; Deveci, 2011). Ferric iron with a reduction potential of 0.77 V is a powerful oxidant for most sulphide minerals and metals. ( ) (1) Since metals are present in metallic forms or as alloys in e-waste, leaching of metals such as copper should be carried out under oxidising conditions. In this regard, ferric iron, which is in-situ generated by the bacteria (Eq.1), can be suitably utilised as the oxidising agent to leach copper from e-waste (Eq. 2). Ferrous iron that forms as a reaction product can be further oxidised to ferric iron by iron oxidising bacteria such as Acidithiobacillus ferrooxidans (At. ferrooxidans) and Leptosprillum ferrooxidans (L. ferrooxidans) (Eq.1) whereby high oxidising conditions i.e. high ratio of Fe 3+ /Fe 2+ ) is maintained. 435

4 Accordingly, the rate of leaching 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. Due to the low iron content (2.1% Fe) of many e-waste samples such as PCBs (Yazici et al., 2011), the external addition of ferrous iron may be necessary for the process to improve the growth and, hence, kinetics of bioleaching of copper. In this study, bioleaching of copper from waste PCBs of of end-of-life computers was investigated using a mixed mesophilic culture of acidophilic bacteria. The effect of initial addition of Fe 2+ and the size of inoculum on the extraction of copper were studied. MATERIAL AND METHOD PCB sample The waste printed circuit boards (PCBs) were collected from the end-of-life PC at Karadeniz Technical University. PCBs were first crushed to mm in a rotary cutting shredder and, then, ground to -250 µm in a two-stage grinding process using a Ultra Centrifugal Mill (Retsch ZM 200). Afterwards, the ground material was riffled and packed as 250-g portions prior to use in bioleaching experiments (Baş, 2012). Chemical composition of PCB sample is shown in Table 1. The sample contains high levels of copper (21% Cu), gold (86 g/t Au) and silver (694 g/t Ag). Bacteria and Growth Media A mixed culture of mesophilic bacteria, MES1 (At. ferrooxians, L. ferrooxidans, At. thiooxidans) were used in this study (Deveci, 2001). The culture, which had been routinely grown and maintained on ferrous iron was used in the bioleaching tests. The growth media used was composed 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. Bioleaching Experiments Ferrous sulphate (1 M Fe 2+ at ph 1.7) as a stock solution were prepared prior to use in the experiments. Bioleaching tests were carried out in a total volume of 100 ml leach solution in 250-ml shake flasks. Bacteria grown on 200 mm Fe 2+ were used as inoculum (10% vol/vol) in the bioleaching experiments, which were carried out at a pulp density of 1% w/vol. The growth media (90 ml) and the PCB sample (1 g) were added prior to the inoculation of the flasks. An orbital shaker maintained at 35 C was used to provide mixing for the flasks at a speed of 170 rpm. Over the experimental period, the top of the flasks were kept covered with foam bungs. Samples (1 ml) were removed from the flasks at the sampling intervals and used for the analysis of metals (Cu and Fe), ph and redox potential (E, mv vs Ag/AgCl) (2) Table 1. Chemical composition of the PCB sample used in this study (Deveci et al., 2010) Cu (%) 21 Fe (%) 2.1 Al (%) 1.3 Zn (%) 0.7 Pb (%) 2.7 Ni (%) 0.43 Sn (%) 4.9 Au (g /t) 86 Ag (g/ t)

5 were also monitored over the bioleaching period and ph was adjusted to the present level of ph 1.7 by the dropwise addition of concentrated H 2 SO 4 if deviated towards neutrality. On the termination of bioleaching tests, the flask contents were filtered to collect the residue, which was dried for six hours at 105 C. The residue was then digested in hot aqua-regia for the analysis of the metals remained. The metal extraction was determined based on the residue analysis. Analysis of metals (Cu, Fe and Ag) from the solutions was performed using an atomic absorption spectrophotometer (Perkin Elmer AAnalyst 400). Concentration of ferrous iron was determined using KMnO 4 titration. RESULTS Bacterial Oxidation of Ferrous Iron Before being used in the experiments, the mixed culture (MES1) was subcultured/adapted on 200 mm Fe 2+ to obtain an adapted bacterial population with a high capacity for iron oxidation. The biooxidation of ferrous iron in four successive subcultures is illustrated in Fig.1. In these tests, the culture was subcultured when the ~75-80% of ferrous iron was oxidized. During the oxidation of ferrous iron, ph of the solution was adjusted to 1.7 with H 2 SO 4 to prevent the formation of iron precipitates. The oxidation performance of the bacteria due to the adaptation and/or selection of more efficient bacterial strains for ferrous iron (Fig. 1). 200 Fe(II) mm Time in hours Figure 1. Oxidation of Fe 2+ (200 mm) in subsequent subculture (ph 1.7, 35 C) Ferrous iron was oxidised in 130 h in the first subculture compared with ~20 h in the fourth subculture (Fig. 1). This improvement in the oxidation performance of the culture can be attributed to the adaptation and/or selection of more efficient bacterial strains within the population for ferrous iron oxidation. This adapted culture was used in the bioleaching experiments. Bioleaching of Copper: Effect of Initial Concentration of Fe Subculture 2. Subculture 3. Subculture 4. Subculture Preliminary tests had demonstrated that the bioleaching of copper from the PCB sample was too slow presumably due to the low availability of soluble iron at the onset of the bioleaching process. Iron content of the PCB sample (2.1% Fe) appeared to be relatively low to provide soluble iron in sufficient quantity. Therefore, the external addition of ferrous iron (1-8 g/l Fe 2+ ) was tested in an attempt to improve the 437

6 kinetics of bioleaching of copper from the sample. Fig.2 demonstrates the effect of addition of ferrous iron on the extraction of copper from the PCB sample. Extraction of copper with only ~20% in 90 hours was slow in the chemical control test where the dissolution process is essentially controlled by the availability (i.e. transfer) of oxygen (Eq.3). This reaction is catalyzed by Cu 2+ (Dutrizac and MacDonald, 1974). In the presence of bacteria (i.e. no Fe 2+ addition), the rate and extent of dissolution of copper were significantly enhanced indicating the contribution of bacteria. This enhancement can be attributed to the ability of bacteria to continually oxidise Fe 2+ available in the medium into Fe 3+ (Eq.1), which acts as the oxidant for copper present in the PCB sample (Eq.2). It can be presumed that the dissolution copper is controlled by the availability of soluble iron in the bioleaching tests. In this regard, the external addition of Fe 2+ was observed to produce a positive effect on the dissolution rate of copper (Fig. 2). To illustrate, increasing the concentration of Fe 2+ from 1 g/l to 8 g/l improved the recovery of copper from 60% to 95%. This could be linked with the increase in soluble iron concentration which leads to enhance the generation of ferric iron (Xiang et al., 2010; Zhu et al., 2011). These findings have revealed that concentration of ferrous iron in bioleaching of e-waste plays an important role for the kinetics of metal dissolution. (3) Extraction of Cu (%) Control 0 g/l Fe(II) 1 g/l Fe(II) 4 g/l Fe(II) 8 g/l Fe(II) Time in hours Figure 2. Effect of initially addition of Fe 2+ on the extraction of copper from waste computer PCB sample (PCB: 1% w/vol, ph 1.7, 35 C) Oxidation of ferrous iron is an acid generation reaction (Eq.1) and ph tended to increase during the bioleaching tests. Therefore, concentrated H 2 SO 4 was added in a dropwise fashion to control ph at the pre-set level of 1.7. Figure 3. illustrates the effect of addition of Fe 2+ on the consumption of acid (g H 2 SO 4 per g PCB) in the bioleaching process Acid consumption was recorded to increase with an increase in the availability (i.e. initial concentration) of Fe 2+. To illustrate, the consumption of acid increased by 2,2-fold with increasing the external addition of Fe 2+ from none to 8 g/l. 438

7 Acid consumption (g H 2 SO 4 /g PCBs) g/l Fe(II) 1 g/l Fe(II) 4 g/l Fe(II) 8 g/l Fe(II) Figure 3. Effect of initial addition of Fe 2+ on acid consumption in the bioleaching process Bioleaching of Copper: Effect of Inoculum Size 0 Fig. 4 shows the effect of size of inoculum on the rate of copper extraction from the PCB sample. These tests were performed at an initial Fe 2+ concentration of 8 g/l. The initial bioleaching rate was noted to increase from 39 mg/l/h to 47 g/l/h with an increase in the size of inoculum from 10 to 50% vol/vol. Over the initial period of 15 h copper extraction was 45% and 65% at 10% vol/vol and 50% vol/vol inoculum, respectively. Notwithstanding this, following a bioleaching period of 50 h, the rate and extent of extraction of copper were similar (e.g % over 90 h) at both inoculum sizes tested (Fig.4). It is also pertinent to note that a considerable percent of copper was also leached in the control experiment despite the relatively slow release of copper in the absence of bacteria. The current findings are consistent with those of Xu et al. (2010). They also studied the effect of inoculum size (0-100%) on the copper extraction from printed circuit boards. Although 22% of copper was extracted in the control tests, 82% of copper was extracted at the inoculum size of 60%. At the maximum size of inoculum, 99% of copper was leached after 120 hours. The positive effect of increasing inoculum size on the copper extraction can be attributed to the increase in the number of active bacteria leading to the fast oxidation of ferrous iron within the system. Maximum dissolution rate of copper (mg/l/h) Control 10% Inoculum 50% Inoculum Figure 4. Effect of inoculum size on the dissolution rate of copper (PCB: 1% w/vol; Fe 2+ : 8 g/l; Control: no bacteria) 439

8 CONCLUSIONS In this study, bioleaching of copper from waste PCBs using a mixed culture of mesophilic bacteria was demonstrated. The bioleaching tests were essentially developed based on the ability of bacteria to oxidise ferrous iron i.e. to generate ferric iron within the leaching environment. Bioleaching tests have shown that the rate and extent of extraction of copper are limited by the initial availability of soluble iron. The external addition of ferrous iron appeared to substantially improve the extraction of copper in the bioleaching tests e.g. 65% Cu without external addition of Fe 2+ c.f. 95% Cu at 8 g/l Fe 2+. An increase in the size of inoculum was observed to enhance the rate of copper extraction, from 45% to 65% by the increase in the size of inoculum from 10% to 50% only in 15 hours. This study has shown that bioleaching can be effectively used for the extraction of copper from waste computer printed circuit boards. The current findings suggest that initial concentration of iron and the size of inoculum are of practical importance for the development of an efficient bioleaching process for the treatment of e-waste such as PCBs. ACKNOWLEDGEMENT The authors would like to acknowledge the financial support from the Research Foundation of Karadeniz Technical University (Project Nos: 889 and 8647) and Tubitak (Project No:109M111). REFERENCES Baş, A.D., Recovery of Copper from Electronic Wastes by Bioleaching and Chemical Leaching Methods, MSc. Thesis, Karadeniz Technical University, Trabzon (in Turkish). Deveci, H., Bacterial Leaching of Complex Zinc/Lead Sulphides Using Mesophilic and Thermophilic Bacteria, PhD. Thesis, University of Exeter. Deveci, H., Akcil, A. and Alp, I., Parameters for Control and Optimisation of Bioleaching of sulphide Minerals. Materials Science & Technology 2003 Symposium: Process Control and Optimization in Ferrous and Non Ferrous Industry, Kongoli F., Thomas B. (Eds.), Sawamiphakdi K., 9-12 November, Chicago, USA, pp Deveci, H. and Philips, C.V., Oxidation of Ferrous Iron by Mesophilic and Thermophilic Bacteria, Proceedings of XII th.international Mineral Processing Symposium IMPS 2010, Gülsoy, Ö., Ergün, L., Can, N.M., Çelik, İ.B. (Eds.), 6-8 October, Cappadocia, Nevşehir, Turkey, pp Deveci, H., Yazıcı, E.Y., Aydın, U., Yazıcı, R. and Akcil, A. U Extraction of Copper From Scrap TV Boards by Sulphuric Acid Leaching Under Oxidising Conditions. Proc. of Going Green-CARE INNOVATION 2010 Conference, 8-11 November, Vienna, Austria, paper no: 045. Deveci, H., Biooxidation of Refractory Gold Ores, New Frontiers in Gold & Silver Hydrometallurgy- Golden Day Conference 2011, Baş, A.D (Ed.)., May 4 th, Karadeniz Technical University, Trabzon. Dutrizac, J.E. and MacDonald, R.J.C., Ferric Ion As a Leaching Medium, Minerals Sci. Engng, 6 (2), April Hagelüken, C., Recycling of Electronic Scrap at Umicore Precious Metals Refining. Acta Metallurgica Slovaca, 12, Jensen, A.B. and Webb, C., Ferrous Sulphate Oxidation Using Thiobacillus Ferrooxidans: A Review, Process Biochemistry, 30 (3), pp Nemati, M., Harrison, S.T.L., Hansford, G.S. and Webb, C., Biological Oxidation of Ferrous Sulphate by Thiobacillus ferrooxidans: A Review on The Kinetic Aspects, Biochemical Engineering Journal, 1, pp Rossi, G., Biohydrometallurgy, McGraw-Hill Book Company GmbH, Hamburg-New York, ISBN

9 UNEP, Sustainable Innovation and Technology Transfer Industrial Sector Studies, Recycling from E-Waste to Resources, United Nations Environment Programme & United Nations University. Xiang, Y., Wu, P., Zhu, N., Zhang, T., L, W., W., J. and Li, P., Bioleaching of Copper From Waste Printed Circuit Boards by Bacterial Consortium Enriched From Acid Mine Drainage, Journal of Hazardous Materials, 184, pp Xu, Z., Yang, T., Yang, L. and Li, Y., Bioleaching Metals from Waste Printed Cicuit Boards and the Shapes of Microorganisms, Proceedings of XXV th. International Mineral Processing Congress (IMPC), 6-10 Sept., Brisbane, Australia, pp Yazıcı, E. Y., Baş, A. D. and Deveci, H.,2011. E-Mines, Mining Turkey, 19,66-70 (in Turkish) Yazıcı, E.Y., and Deveci, H., Recovery of Metals From E-wastes, Madencilik, 48, 3-18 (in Turkish). Zhu, N., Xiang, Y., Zhang, T., Wu, P., Dang, Z., Li, P. and Wu, J., Bioleaching of Metal Concentrates of Waste Printed Circuit Boards by Mixed Culture of Acidophilic Bacteria, Journal of Hazardous Materials, Vol. 192, pp