Biolixiviacion 2 by micromecuis

Minerals Engineering 24 (2011) 1219–1222

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Technical Note

Bioleaching of gold and copper from waste mobile phone PCBs by using a cyanogenic bacterium q Tran D. Chi a,b, Jae-chun Lee b,⇑, B.D. Pandey b,c, Kyoungkeun Yoo d, Jinki Jeong b a

Resources Recycling, University of Science and Technology, Daejeon 305-350, Republic of Korea Mineral Resources Research Div., Korea Institute of Geoscience & Mineral Resources (KIGAM), Daejeon 305-350, Republic of Korea c National Metallurgical Laboratory, CSIR, Jamshedpur 831 007, India d Department of Energy & Resources Engineering, Korea Maritime University, Busan 606-791, Republic of Korea b

a r t i c l e

i n f o

Article history: Available online 8 June 2011 Keywords: Bioleaching Chromobacterium violaceum Waste PCBs Leaching of gold and copper

a b s t r a c t Chromobacterium violaceum (C. violaceum), a cyanide generating bacterium has been used to leach out gold and copper from the waste mobile phone printed circuit boards (PCBs) containing 34.5% Cu and 0.025% Au in YP (yeast extract and polypeptone with glycine) medium. The bioleaching was carried out in an incubator shaker (150 rpm) at 30 °C and 15 g/L pulp density in the pH range 8–11. Dissolution of gold and copper increased from 7.78% (0.225 ppm) to 10.8% (0.46 ppm) and 4.9% (419 ppm) to 11.4% (879 ppm) in 8 days with increase in pH from 8 to 11 and 8 to 10 respectively. Supplementing oxygen with 0.004% (v/v) H2O2 increased the copper leaching to 24.6% (1743 ppm) at pH 10 in 8 days whereas improvement in gold leaching was insigniﬁcant with the recovery of 11.31% Au at pH 11.0. The waste PCBs can thus be recycled in environmental friendly manner. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The waste mobile phone PCBs, the rich source of metals containing copper and precious metals such as gold and silver, may be processed to ensure resource recycling and reduce environmental degradation. The cyanidation process is mostly used in gold metallurgy (Marsden and House, 1960). With the success of biobeneﬁciation of refractory ores (Iglesias and Carranza, 1994) using Bacillus sp. followed by cyanidation, an alternate process may be explored using cyanogenic bacteria to extract gold, although the reactive mechanisms for the two processes are different. Amongst the bacteria (Tânia et al., 2004), Chromobacterium violaceum, a mesophilic, gram-negative and facultative anaerobe can leach gold with cyanide generated from glycine during the growth and early stationary phase of bacteria (Knowles and Bunch, 1986). Earlier, bioleaching of gold from powder (Kita et al., 2006) and ores (Lawson et al., 1999) was investigated. Mention may be made of bioprocessing of secondary resources (Brandl et al., 2008) including waste PCBs (Faramazi, 2004) and initial study from the authors laboratory (Chi et al., in press). The reaction of cyanide with gold and copper is thermodynamically (Iglesias and Carranza, 1994) favourable with oxygen as an oxidant. As metal leaching with a cyanogenic bacteria is inhibited (Kita et al., 2008) with q Paper presented at BioHydromet’10 held at Cape Town, SA during November 8–9, 2010. ⇑ Corresponding author. E-mail address: jclee@kigam.re.kr (J.-c. Lee).

the consumption of dissolved oxygen (DO) for bacterial respiration during the growth phase, this research is focused on the leaching of gold and copper from the waste mobile phone PCBs using C. violaceum in absence/presence of H2O2 to supplement the DO level.

2. Materials and methods C. violaceum used in this study was supplied by Korean Collection for Type Cultures (KCTC), KRIBB, Daejeon. It was revived in a medium comprising of 5 g/L polypeptone and 3 g/L beef extract in 100 mL solution at 6.8 pH and 30 °C in an incubator (150 rpm) for over 2 days and bacterial population was calculated as reported elsewhere (Chi et al., in press). Waste mobile phones of different models were supplied by the recycling companies in Korea. Mobile phones were manually dismantled to separate cover and PCBs and cut to 1 mm 1 mm size by scissor. The sample dissolved in aqua regia and analyzed by Atomic Absorption spectrometer (AAS PerkinElmer 400) had 58% total metal content with 34.52% Cu and 0.025% Au. Bioleaching was carried out in sterilized Erlenmeyer ﬂasks (250 mL) taking 200 mL of YP media (5 g/L yeast extract, 10 g/L polypeptone and 5 g/L glycine) and 1 g/L MgSO4 7H2O at 15 g/L pulp density while adding 1 mL C. violaceum culture under log phase. The ﬂasks were incubated at 30 °C and the desired pH in an orbital motion shaker (150 rpm). DO level was raised by adding 1 mL H2O2 concentration: 0.00–0.005% (v/v) into the medium after 24 h of leaching when DO level decreased to a minimal value. H2O2

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by bacteria without PCBs was determined (Clescert et al., 1998) by UV/VIS spectrometer (1601, Shimadzu) at 580.5 nm using pyridine and barbituric acid. DO and pH were determined by DO meter (HQ40d, Hach) and pH meter (Orion-720A), respectively. All data given in the text are based on the average of duplicate set of experiments.

3. Results and discussion pH = 7.4 pH = 8.0 pH = 8.5 pH = 9.0 pH = 10.0 pH = 11.0

3.1. Leaching by cyanogenic bacteria

Time (days) Fig. 1. Effect of pH on cyanide generation in YP medium at 30 °C.

was added at every 1 h interval for 8 h each day and cell population was measured from the supernatant drawn (1 mL) after 10 min of H2O2 addition. The leach liquor was analyzed for gold and copper and the material balance was computed by analyzing the residue dissolved in aqua regia. The concentration of cyanide generated

Gold leaching efficiency (%)

(a) 12 10

pH = 8 pH = 8.5 pH = 9 pH = 10 pH = 11

The cyanide generation by C. violaceum was estimated (Fig. 1) at different pH (7.4–11.0) in the YP media since gold leaching was effective (Marsden and House, 1960) at pH above 10.0. Cyanide level was high (68 ppm) at pH 9.0 in 5 days which dropped to 56 ppm in 7 days. At higher pH of 10.0 and 11.0 the cyanide generation was lower in 5 days but it remained almost steady thereafter till 7 days; cyanide level (54 ppm) at pH 10.0 in 7 days closely followed to that of pH 9.0 because of its stability for longer duration (Knowles and Bunch, 1986). Bacterial population (data not given) was found to be 4.18 109 cells/mL in 5 days time at pH 9.0 which dropped to 9.9 108 cells/mL and 8.7 108 cells/mL at pH 10.0 and 11.0, respectively. DO concentration changed with time as almost entire DO initially present (6.8 ppm) was consumed by bacteria in 24 h for its

Dissolved oxygen concentration (ppm)

Cyanide concentration (ppm)

0.00% 0.0025% 0.003% 0.004% 0.005%

Time (days) 12

4.5

Cell population (x109cells/mL)

Copper leaching efficiency (%)

(b)

Time (min)

pH = 8.0 pH = 8.5 pH = 9 pH = 10 pH = 11

10 8 6 4 2

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

Time (days) Fig. 2. Effect of pH on bioleaching efﬁciency of gold and copper in YP medium at 30 °C.

0.0025 0.003

0.004

0.01

0.1

0.3

H2O2 concentration (%v/v) Fig. 3. DO concentration and bacterial cell counts in YP medium with various H2O2 concentrations at pH 9.0 in presence of C. violaceum.

T.D. Chi et al. / Minerals Engineering 24 (2011) 1219–1222

respiration which remained at the low level (0.06 ppm). Gold leaching (Fig. 2) increased from 7.78% to 10.9% with increase in pH from 8.0 to 11.0 in 8 days (Fig. 2a), whereas for copper it was 4.9% to 11.4% with increase in pH from 8.0 to 10.0 (Fig. 2b). Leaching was optimum at pH 11.0 and 10.0 with 0.46 ppm Au and 879 ppm Cu, respectively. The Eh–pH diagramme (Marsden and House, 1960) show that CuðCNÞ 2 is more stable at pH less than 3 9.0 while CuðCNÞ 3 and CuðCNÞ4 will be formed in larger amounts at pH above 9.0 in presence of high cyanide; formation of higher cyanide complexes will be unlikely with low cyanide. Thus, high stability of metal cyanide complexes (Rees and van Deventer, 1999) and increased stability of HCN at pH > 10 improved the metal leaching at higher pH. As regards gold, AuðCNÞ 2 is formed at higher pH and higher DO is reported to favour gold dissolution (Kita et al., 2008). High amount of copper dissolved (879 ppm at pH 10.0) compared to the cyanide generation (54 ppm CN in 8 days) in the growth study (Fig. 1) may be the result of continuous reaction and simultaneous generation of cyanide matching the requirements of metal dissolved. This indicated that the total amount of cyanide generated during leaching was considerably higher than that estimated in the growth study.

Addition of H2O2 to increase DO during bioleaching may harm bacteria (Kita et al., 2008) at the higher doses. Fig. 3 shows that

Gold leaching efficiency (%)

(a)

pH 8.5 pH 9.0 pH 10.0 pH 11.0

Time (days)

Copper leaching efficiency (%)

(b)

ð1Þ

As the Eh at pH 11.0 was found to be <20 mV and <40 mV at pH 10.0 in 8 days under the conditions of Fig. 4 in spite of H2O2 addition, gold leaching may not be facilitated by CuðCNÞ 2 . As an alternative, gold present in PCBs as bonding wire and coated thin ﬁlm can be beneﬁciated (Lee and Jeong, 2010) and enriched (80–85%) in the non-metallic portion with a little metallics, and can be leached with C. violaceum to recycle the waste PCBs. 4. Conclusion

DO was maximum (22 ppm) when 0.005% (v/v) H2O2 was added as compared to 12 ppm DO with 0.004% H2O2, but bacterial population (Fig. 3) was seriously affected beyond 0.004% H2O2. Increasing H2O2 from 0.0025% to 0.004% marginally affected cell population from 4.1 109 to 3.8 109 cells/mL and therefore, addition of 0.004% H2O2 was considered optimum. Addition of 0.004% H2O2 increased copper leaching (Fig. 4) from 7.23% (463 ppm) to 24.6% (1734 ppm) with increase in pH from 8.5 to 10.0 in 8 days. Even at pH 9.0, 23.54 % Cu (1518 ppm) was leached in 8 days. Copper leaching decreased to a slightly lower value (22.4%) at higher pH (11.0). However, gold recovery was not that much improved as it increased only marginally from 8.1% to 11.32% with the rise in pH from 8.5 to 11.0; the leaching efﬁciency of gold (Fig. 4a) was nearly same (11.21%) at pH 10.0 in presence of H2O2. As reported by Kita et al. (2008), DO played a more important role than cyanide resulting in higher copper leaching. Preferential copper dissolution compared to gold with high DO could be attributed to the galvanic interaction; gold being nobler (E0Au3þ =Au : 1:52 V) than copper (E0Cu2þ =Cu : 0:34 V). For gold leaching with cyanide released from CuðCNÞ 2 by reaction (1), higher Eh (>250 mV) and pH (>10.5) need to be maintained (Rees and van Deventer, 1999).

2Au þ 2CuðCNÞ 2 þ O2 þ 2H2 O ! 2AuðCNÞ 2 þ 2CuðOHÞ2

3.2. Bioleaching in presence of hydrogen peroxide

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Copper and gold from the waste mobile phone PCBs can be recovered by the cyanide generated in the YP media by C. violaceum. The DO played a deﬁnite role during the leaching which decreased signiﬁcantly within 24 h because of respiration of bacteria. Adding 0.004% H2O2 increased DO without seriously affecting bacteria and improved copper leaching from 11.4% to 24.6% at pH 10.0 and gold recovery only marginally from 10.8% to 11.31% at pH 11.0. Preferred copper leaching than gold may be the result of high copper present in the sample consuming cyanide produced at higher DO level. To improve gold bioleaching copper content must be decreased by a suitable method prior to bacterial leaching.

Acknowledgements 20

This paper is based on work supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the project entitled ‘‘Development of New Technology for the Recycling of Rare Metals from Urbane Ore’’. One of the authors (Dr. B.D. Pandey) is thankful to the Korean Federation of Science and Technology Societies for awarding Brain Pool Scientist.

pH 8.5 pH 9.0 pH 10.0 pH 11.0

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Time (days) Fig. 4. Effect of pH in presence of 0.004% (v/v) H2O2 on bioleaching efﬁciency of gold and copper in YP medium at 30 °C.

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