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Leaching behavior of nickel from waste multi-layer ceramic capacitors
Hydrometallurgy 86 (2007) 89 – 95 www.elsevier.com/locate/hydromet
Leaching behavior of nickel from waste multi-layer ceramic capacitors Eun-young Kim a,b , Jae-chun Lee b,⁎, Byung-Su Kim b , Min-Seuk Kim b , Jinki Jeong b a
b
Department of Resources Recycling, University of Science and Technology, Daejeon, Korea Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Gajeong-dong, Daejeon, 305-350, Korea Received 2 November 2005; received in revised form 9 June 2006; accepted 7 November 2006 Available online 11 January 2007
Abstract The leaching behavior of nickel present in waste multi-layer ceramic capacitors (MLCC) was studied using different acidic leaching reagents in a stirred batch reactor. The waste MLCC consisted of multi-layered ceramic BaTiO3 and inner nickel electrodes arranged like a sandwich. The effects of acid leaching reagent, its concentration, agitation speed, particle size, and leaching temperature on the leaching of nickel from waste MLCC were examined. HNO3 was found to be most effective leaching reagent in comparison to HCl and H2SO4. The fraction of Ni leached was found to be 97% for a pulp density of 5 g/L and temperature 90 °C in an agitation time of 90 min. The leaching rate of nickel was limited by the diffusion of HNO3 solution throughout thin channels that were formed between BaTiO3 layers during the leaching of nickel. The Jander equation was also found to fit well with the leaching rate data. The leaching of nickel follows a first order rate reaction with HNO3 concentration and has an activation energy of 37.6 kJ/mol (9.0 kcal/mol). © 2006 Elsevier B.V. All rights reserved. Keywords: Recycling; Multi-layer ceramic capacitor; Leaching of waste capacitor; Nickel leaching
1. Introduction Multi-layer ceramic capacitors (MLCC) are frequently used as an important part in various electric and electronic units. In general, it is estimated that the number of pieces of MLCC used are around 150 in a mobile phone, 200 in a PDA (Personal Digital Assistant), and 300 in a digital TV set (KETI, 2002). It is thus expected that the amount of waste MLCC generated will continue to increase with the increase in ⁎ Corresponding author. Tel.: +82 42 868 3613. E-mail address: [email protected] (J-C. Lee). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.11.007
consumption of electric and electronic units such as mobile phones, PDAs, and digital TVs (KETI, 2002). Recently, a large amount of waste MLCC is generated every year in Korea. During the manufacture of capacitors or repair of electric and electronic equipment, all defective MLCC are discarded as wastes and disposed in landfills. However, the toxicity of such waste MLCC must be taken into account due to the presence of heavy metals such as nickel, copper, and zinc. The disposal of waste MLCC will be dangerous for the environment, if improperly managed. A MLCC consists of many layers of ceramic BaTiO3 and inner electrode like a sandwich. BaTiO3 layers act
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Table 1 Chemical composition of the waste MLCC used in the study (wt.%)
Table 2 Nickel content according to the particle size of waste MLCC
Ba
Ti
Ni
Ca
Al
Mn
Zn
Mg
Cu
Particle size (μm)
Ni (wt.%)
37.79
10.49
14.92
0.72
0.20
0.08
0.07
0.003
0.0015
− 500/+300 − 300/+180 − 180/+90 − 90
15.23 12.86 10.45 10.16
as a dielectric substance of MLCC (Shin and Kim, 2001; KETI, 2002). The inner electrode was usually manufactured with palladium metal. But, as the price of palladium tremendously increased in recent years, it is replaced by nickel as inner electrode material. Therefore, waste MLCC contains considerable amounts of nickel present in metallic state. Recent trend shows more than 60% of MLCC produced in the world contained nickel as inner electrode material (Shin and Kim, 2001). As each MLCC contains about 15% metallic nickel, it is highly desirable to develop an effective process for the recovery of nickel from waste MLCC. The recovery of nickel from waste MLCC is not only important in view of environmental perspectives but also for the recycling of metallic value for economical gain. In view of the above, the leaching studies with nitric acid were carried out to examine the leaching behavior of nickel from waste MLCC generated in an electric and electronic company in Korea. The effects of leaching reagent, its concentration, agitation speed, particle size, and leaching temperature on the nickel leaching were investigated. The kinetics data reported in this paper were determined by minimizing the effects of boundary layer diffusion and particle size. The study is a part of a major research project aimed at the development of an effective process for the recovery of nickel from waste MLCC.
2.2. Procedures Leaching experiments were carried out in a 1 L three necked thermostatic Pyrex reactor with heating mantle. The reactor was fitted with a temperature controller, stirrer, and reflux condenser. The reflux condenser was inserted in one port to avoid vapor loss at high temperatures. Fig. 1 shows the schematic diagram of leaching apparatus used in this study. In a typical run, a 500 mL acid solution, i.e., 1 M HNO3, 1 M HCl or 0.5 M H2SO4 was poured into the reactor and allowed to reach thermal equilibrium. Then 2.5 g of the waste MLCC powder ranging from 180 to 300 μm was added into the reactor, and the stirrer was set to 400 rpm. Then 4 mL of leach liquor samples was taken periodically at a desired time (between 5 min and 90 min) interval with a syringe. The sample was filtered and, then 3 mL of the filtrate was diluted with 5% HCl solution. The samples were analyzed by using an atomic absorption spectrometer (Varian SpectrAA-400) and an inductively coupled plasma spectrometer (JY-38 plus). The waste MLCC sample and leach residue were also analyzed by using a scanning electron microscope (SEM, JEOL JSM-6400) equipped with an energy dispersive X-ray spectrometer and X-ray diffractometer (Rigaku D-max2500PC).
2. Experimental 2.1. Materials Waste MLCC samples supplied by an electric and electronic company of Korea were used for the experiments. The waste MLCC was crushed to a particle size less than 500 μm by using a hammer mill and screened into different size fractions with Tyler sieves. The chemical composition of the waste MLCC used is shown in Table 1. The waste MLCC contained 37.79% Ba, 17.49% Ti, and 14.92% Ni as major elements and oxygen and hydrocarbons as balance. The analysis of nickel content present in different particle size and mass fraction of the waste MLCC presented in Table 2, indicates the decrease in percentage nickel content with the decrease in particle size.
Fig. 1. Schematic diagram of the leaching apparatus. 1. Reactor, 2. Heating mantle, 3. Temperature controller, 4. Reflux condenser, 5. Stirrer, 6. Thermometer, 7. Mercury seal.
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3. Results and discussion 3.1. Effect of leaching reagents The effect of leaching reagents on leaching conversion was investigated at a constant temperature of 90°C. Fig. 2 presents the results of leaching experiments obtained with 1 M HCl, 0.5 M H2SO4, and 1 M HNO3 solutions. As shown in Fig. 2, the fraction of Ni leached with HNO3 solution is substantially higher than that leached with H2SO4 and HCl solutions. From these results, the nickel contained in waste MLCC was observed effectively to react with the HNO3 solution according to the following reaction suggested by Mecucci and Scott (2002): NiðsÞ þ 4HNO3ð1Þ ¼ NiðNO3 Þ2ðaqÞ þ 2H2 Oð1Þ þ 2NO2ðgÞ ð1Þ The fraction of nickel leached was 97% with 1M HNO3 at a temperature of 90°C in 90 min, while in similar experimental condition the fraction of nickel leached obtained was less than 40% with 1 M HCl and 0.5 M H2SO4. Results indicated that nitric acid is an effective reagent for Ni leaching from waste MLCC. So, leaching studies were carried out using nitric acid.
Fig. 3. Effect of agitation speed on the nickel leaching (HNO3 concentration: 1 M, leaching temperature: 50 °C, particle size: 300– 180 μm, pulp density: 5 g/L).
To investigate the influence of film boundary diffusion on the leaching rate of nickel, different agitation speeds between 100 and 600 rpm were tested at 50 °C and 1 M HNO3 concentration. Fig. 3 shows the effect of agitation speed on the fraction of Ni leached. The results
indicate that the leaching rate is independent in respect to agitation speed higher than 200 rpm. This finding thus suggested that 200 rpm is sufficient to provide adequate particle suspension. Therefore, in all subsequent experiments, a working agitation speed of 400 rpm was selected to ensure effective particle suspension. The effect of particle size on the fraction of Ni leached was examined by varying the particle size between 300–500 μm and − 90 μm, while all other variables such as the HNO3 concentration, agitation speed, and leaching temperature were kept constant. The results are given in Fig. 4. The results indicate that the leaching rate is almost independent of the particle size below 180–300 μm. Thus, for further experiments the
Fig. 2. Effect of leaching reagent on the nickel leaching (leaching temperature: 90 °C, agitation speed: 400 rpm, particle size: 300– 180 μm, pulp density: 5 g/L).
Fig. 4. Effect of particle size on the nickel leaching (HNO3 concentration: 1 M, leaching temperature: 50 °C, agitation speed: 400 rpm, pulp density: 5 g/L).
3.2. Effects of agitation speed and particle size
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Fig. 5. Effect of HNO3 concentration on the nickel leaching (leaching temperature: 50 °C, agitation speed: 400 rpm, particle size: 300– 180 μm, pulp density: 5 g/L).
Fig. 6. Effect of leaching temperature on the nickel leaching. (HNO3 concentration: 1 M, agitation speed: 400 rpm, particle size: 300– 180 μm, pulp density: 5 g/L).
particle size in the range of 180 to 300 μm was selected to minimize the particle size effect on the leaching experiment.
Microscopy) and X-ray diffraction. The X-ray diffraction peaks presented in Fig. 7 show the presence of BaTiO3 and Ni in waste MLCC particles before leaching. The X-ray peaks of the leach residue showed only the presence of BaTiO3. On the other hand, for a leaching period of 60 min with 2 M HNO3 at 50°C as shown in Fig 5, the fraction of Ni leached was found 96%, while the fractions of Ba and Ti leached were found 3%. Thus, it was considered that most of the nickel was leached by the HNO3, while most of BaTiO3 remained in the original particles without reacting with the HNO3 solution. The phenomenon was clearly verified by SEM analysis. Fig. 8 shows the polished sections of waste MLCC particles before and after the
3.3. Effect of HNO3 concentration The HNO3 concentration was varied between 0.1 and 2.0 M, while the leaching temperature was kept constant at 50 °C. The Ni leached vs. time curves for this temperature are given in Fig. 5. These curves show that the fraction of Ni leached increases with increase in the concentration of HNO3. As shown in the figure, the nickel leaching was found typically fast in the initial period up to 30 min, but slowed down to a lower rate with further increase of leaching time. The fraction of nickel leached was found to be more than 94% with HNO3 concentrations higher than 0.5 M and a temperature of 50 °C at agitation time of 90 min. 3.4. Effect of leaching temperature The effect of temperature on the nickel leaching was investigated by varying the temperature between 30 and 90°C, while all other parameters were kept constant. Fig. 6 shows the fraction of Ni leached from waste MLCC at 1 M HNO3. The result presented in Fig. 6 shows that the 97% fraction of Ni could be leached with 1 M HNO3 at a temperature of 90 °C in 30 min. 3.5. Interpretation of the results The waste MLCC particles before and after HNO3 leaching were analyzed by SEM (Scanning Electron
Fig. 7. X-ray diffraction patterns of the waste MLCC particle before and after leaching.
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Fig. 8. SEM images of the waste MLCC particle before and after leaching.
leaching of nickel. This figure shows that after the leaching, nickel layers in the particles disappeared, while the overall particle morphology and size remain unchanged, forming many thin channels. It was also observed that the width of channel is approximately 1 μm as shown in Fig. 8. Thus, it was considered that the diffusion of HNO3 solution throughout the thin channels formed between BaTiO3 layers should be a little difficult, and in addition, this structure might give molecular sieve ability as it has a very thin width (Hernández et al., 1986). Based on this observation, the interpretation of the rate data was carried out using a number of different rate expressions such as shrinking unreacted core expression, from which the Jander rate equation (Levenspiel, 1962; Hernández et al., 1986; Habashi, 1980a,b; Paspaliaris and Tsolakis, 1987) was proved to yield the best results. The rate equation was derived from the reactions controlled by a fluid diffusion mechanism. The applicability of this rate expression can be expected from the fact that the volume change of waste MLCC particles before and after the leaching is negligible, which can be seen in Fig. 8. In this rate expression for a disc-shaped particle, the fraction of Ni leached is related to leaching time by the expression: ½1−ð1−XNi Þ1=2 2 ¼ kapp t
(min − 1 ·mol −n ), [HNO3] is the concentration of HNO3, f designates the HNO3 concentration dependence of the rate, and n is the reaction order for the HNO3 concentration. It is apparent from Eq. (2) that a plot of [1 − (1 − X) 1/2 ] 2 vs. t should be linear with kapp as the slope. The validity of the Jander rate expression for extracting nickel from waste MLCC using HNO3 solution was verified by plotting the Ni leached–time curves of Figs. 5 and 6, according to Eq. (2), as shown in Figs. 9 and 10 with R 2 values of 0.94–0.99. This indicates the leaching rate is limited by diffusion through the channels of BaTiO3 layers that are not leached by HNO3, as explained previously. In Fig. 11, the values of kapp determined from the slopes of Fig. 9 were plotted against the HNO 3 concentration (R 2 = 0.97). A straight line is obtained, indicating that the nickel leaching is of first order with respect to the
ð2Þ
where, XNi is the fraction of Ni leached, t is the leaching time (min), and kapp is the apparent rate constant (min− 1) which is given by kapp ¼ bkf ð½HNO3 Þ ¼ bk½HNO3 n ðmin−1 Þ
ð3Þ
Here, b is the stoichiometry factor (b = 1/4 in this system, according to the formulation of Sohn, 1978) for the reaction (1), k is the intrinsic rate constant
Fig. 9. Plot of the results in Fig. 5 according to Eq. (2).
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Fig. 10. Plot of the results in Fig. 6 according to Eq. (2).
Fig. 12. Arrhenius plot of the rate constants.
entire range of HNO3 concentrations considered in this study. Thus, Eq. (3) can be rewritten as kapp ¼ ð1=4Þk½HNO3 ðmin−1 Þ
ð4Þ
Also, the values of k at different temperatures were calculated using Eq. (4) from the slopes of each straight line in Fig. 11. Fig. 12 is an Arrhenius plot of the rate constants (R2 = 0.94). The slope of the straight line placed through the experimental points yields an activation energy of 37.6 kJ/mol. The line through these data can be expressed by the following equation: k ¼ 3:03104 exp½−4520=T ðmin−1 mol−1 Þ
ð5Þ
for the leaching reaction of nickel contained in waste MLCC with HNO3 solution.
Here, the activation energy obtained is slightly high with compared to that reported in many cases of diffusion-controlled leaching. In this research work, the reason might be expected due to some forms of surface diffusion in thin channels formed between BaTiO3 layers during the nickel leaching, which has high activation energy. On the other hand, a number of researchers reported high activation energy similar to our research for the diffusion-controlled leaching. Hernández et al. (1986) reported that the magnesium leaching from sepiolite with sulfuric acid has an activation energy of 63.5 kJ/mol, which is controlled by diffusion of the acid solution. The activation energy for the diffusioncontrolled leaching of zinc from sphalerite concentrate in aqueous slurry by chlorine gas was also found to be 49.6 kJ/mol (Jena et al., 1999). In addition, it was reported that the activation energies for the diffusioncontrolled leaching of iron oxides with hydrochloric acid (Paspaliaris and Tsolakis, 1987) and of pyrite with chlorine gas in aqueous (Vasilakos and Gage, 1986) are 62 to 79 kJ/mol and 26.7 kJ/mol, respectively. Using Eqs. (2), (4), and (5), the leaching rate is represented by the following equation: ½1−ð1−XNi Þ1=2 2 ¼ kapp t
ð6Þ
with kapp = 7.58 × 103·[HNO3]·exp [− 4520 / T ] (min− 1). 4. Conclusions
Fig. 11. Dependence of kapp on the HNO3 concentration from the results shown in Fig. 5.
The leaching behavior of nickel contained in waste MLCC generated in the electric and electronic company in Korea was studied using a stirred batch reactor at the
E. Kim et al. / Hydrometallurgy 86 (2007) 89–95
leaching temperatures between 30 and 90 °C. The leaching rate of nickel in HNO3 solution was found to be much higher than that obtained with HCl and H2SO4 solutions. Nickel extraction was found to be 97% with 1 M HNO3 at 90 °C, 90 min reaction time, and 5 g/L pulp density. The leaching reaction follows the Jander rate expression of a disc-shaped particle. The rate of nickel leaching is limited by the diffusion of HNO3 solution throughout thin channels that were formed between BaTiO3 layers during the nickel leaching. Nickel leaching was first order with respect to HNO3 with an activation energy of 37.6 kJ/mol. The rate of the leaching reaction of nickel from waste MLCC using HNO3 solution could be represented by the following equation: ½1−ð1−XNi Þ1=2 2 kapp t with kapp = 7.58 × 103·[HNO3]·exp [− 4520 / T] (min− 1) Acknowledgements This paper is based on work supported by Resources Recycling R&D Center (Korea) under the project entitled “Development of total recycling technology for WEEE”. Special thanks are extended to Dr. M.K. Jha for the critical review of the manuscript. One of the authors (E. Kim) is grateful to the KIGAM for the award of a fellowship.
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References Habashi, F., 1980a. Principles of Extractive Metallurgy, vol. I. Gordon and Breach, New York, USA, pp. 111–169. Habashi, F., 1980b. Principles of Extractive Metallurgy, vol. I. Gordon and Breach, New York, USA, pp. 223–252. Hernández, L.G., et al., 1986. Preparation of amorphous silica by acid dissolution of sepiolite—kinetic and textural study. J. Colloid Interface Sci. 109 (1), 150–160. Jena, P.K., Barbosa-Filho, O., Vasoncelos, I.C., 1999. Studies on the kinetics of slurry chlorination of a sphalerite concentrate by chlorine gas. Hydrometallurgy 52, 111–122. KETI, 2002. The Market and technical news of MLCC. Biweekly Electronics Information, p. 5 (in Korean). Levenspiel, O., 1962. Chemical Reaction Engineering. John Wiley & Sons Inc., New York, USA, pp. 338–352. Mecucci, A., Scott, K., 2002. Leaching and electrochemical recovery of copper, lead and tin from scrap printed circuit boards. J. Chem. Technol. Biotechnol. 77, 449–457. Paspaliaris, Y., Tsolakis, Y., 1987. Reaction kinetics for the leaching of iron oxides in diasporic bauxite from the Parnassus–Giona Zone (Greece) by hydrochloric acid. Hydrometallurgy 19, 259–266. Shin, H.J., Kim, J.H., 2001. The present status and an overview of MLCC. Ceramist 4 (4) (in Korean). Sohn, H.Y., 1978. Law of additive reaction times in fluid–solid reactions. Metall. Trans., B, Process Metall. 9B (1), 89–96. Vasilakos, N.P., Gage, C., 1986. Slurry-phase chlorination of iron pyrite in aqueous and organic media. Fuel 65, 593–594.
Leaching behavior of nickel from waste multi-layer ceramic capacitors Eun-young Kim a,b , Jae-chun Lee b,⁎, Byung-Su Kim b , Min-Seuk Kim b , Jinki Jeong b a
b
Department of Resources Recycling, University of Science and Technology, Daejeon, Korea Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Gajeong-dong, Daejeon, 305-350, Korea Received 2 November 2005; received in revised form 9 June 2006; accepted 7 November 2006 Available online 11 January 2007
Abstract The leaching behavior of nickel present in waste multi-layer ceramic capacitors (MLCC) was studied using different acidic leaching reagents in a stirred batch reactor. The waste MLCC consisted of multi-layered ceramic BaTiO3 and inner nickel electrodes arranged like a sandwich. The effects of acid leaching reagent, its concentration, agitation speed, particle size, and leaching temperature on the leaching of nickel from waste MLCC were examined. HNO3 was found to be most effective leaching reagent in comparison to HCl and H2SO4. The fraction of Ni leached was found to be 97% for a pulp density of 5 g/L and temperature 90 °C in an agitation time of 90 min. The leaching rate of nickel was limited by the diffusion of HNO3 solution throughout thin channels that were formed between BaTiO3 layers during the leaching of nickel. The Jander equation was also found to fit well with the leaching rate data. The leaching of nickel follows a first order rate reaction with HNO3 concentration and has an activation energy of 37.6 kJ/mol (9.0 kcal/mol). © 2006 Elsevier B.V. All rights reserved. Keywords: Recycling; Multi-layer ceramic capacitor; Leaching of waste capacitor; Nickel leaching
1. Introduction Multi-layer ceramic capacitors (MLCC) are frequently used as an important part in various electric and electronic units. In general, it is estimated that the number of pieces of MLCC used are around 150 in a mobile phone, 200 in a PDA (Personal Digital Assistant), and 300 in a digital TV set (KETI, 2002). It is thus expected that the amount of waste MLCC generated will continue to increase with the increase in ⁎ Corresponding author. Tel.: +82 42 868 3613. E-mail address: [email protected] (J-C. Lee). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.11.007
consumption of electric and electronic units such as mobile phones, PDAs, and digital TVs (KETI, 2002). Recently, a large amount of waste MLCC is generated every year in Korea. During the manufacture of capacitors or repair of electric and electronic equipment, all defective MLCC are discarded as wastes and disposed in landfills. However, the toxicity of such waste MLCC must be taken into account due to the presence of heavy metals such as nickel, copper, and zinc. The disposal of waste MLCC will be dangerous for the environment, if improperly managed. A MLCC consists of many layers of ceramic BaTiO3 and inner electrode like a sandwich. BaTiO3 layers act
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Table 1 Chemical composition of the waste MLCC used in the study (wt.%)
Table 2 Nickel content according to the particle size of waste MLCC
Ba
Ti
Ni
Ca
Al
Mn
Zn
Mg
Cu
Particle size (μm)
Ni (wt.%)
37.79
10.49
14.92
0.72
0.20
0.08
0.07
0.003
0.0015
− 500/+300 − 300/+180 − 180/+90 − 90
15.23 12.86 10.45 10.16
as a dielectric substance of MLCC (Shin and Kim, 2001; KETI, 2002). The inner electrode was usually manufactured with palladium metal. But, as the price of palladium tremendously increased in recent years, it is replaced by nickel as inner electrode material. Therefore, waste MLCC contains considerable amounts of nickel present in metallic state. Recent trend shows more than 60% of MLCC produced in the world contained nickel as inner electrode material (Shin and Kim, 2001). As each MLCC contains about 15% metallic nickel, it is highly desirable to develop an effective process for the recovery of nickel from waste MLCC. The recovery of nickel from waste MLCC is not only important in view of environmental perspectives but also for the recycling of metallic value for economical gain. In view of the above, the leaching studies with nitric acid were carried out to examine the leaching behavior of nickel from waste MLCC generated in an electric and electronic company in Korea. The effects of leaching reagent, its concentration, agitation speed, particle size, and leaching temperature on the nickel leaching were investigated. The kinetics data reported in this paper were determined by minimizing the effects of boundary layer diffusion and particle size. The study is a part of a major research project aimed at the development of an effective process for the recovery of nickel from waste MLCC.
2.2. Procedures Leaching experiments were carried out in a 1 L three necked thermostatic Pyrex reactor with heating mantle. The reactor was fitted with a temperature controller, stirrer, and reflux condenser. The reflux condenser was inserted in one port to avoid vapor loss at high temperatures. Fig. 1 shows the schematic diagram of leaching apparatus used in this study. In a typical run, a 500 mL acid solution, i.e., 1 M HNO3, 1 M HCl or 0.5 M H2SO4 was poured into the reactor and allowed to reach thermal equilibrium. Then 2.5 g of the waste MLCC powder ranging from 180 to 300 μm was added into the reactor, and the stirrer was set to 400 rpm. Then 4 mL of leach liquor samples was taken periodically at a desired time (between 5 min and 90 min) interval with a syringe. The sample was filtered and, then 3 mL of the filtrate was diluted with 5% HCl solution. The samples were analyzed by using an atomic absorption spectrometer (Varian SpectrAA-400) and an inductively coupled plasma spectrometer (JY-38 plus). The waste MLCC sample and leach residue were also analyzed by using a scanning electron microscope (SEM, JEOL JSM-6400) equipped with an energy dispersive X-ray spectrometer and X-ray diffractometer (Rigaku D-max2500PC).
2. Experimental 2.1. Materials Waste MLCC samples supplied by an electric and electronic company of Korea were used for the experiments. The waste MLCC was crushed to a particle size less than 500 μm by using a hammer mill and screened into different size fractions with Tyler sieves. The chemical composition of the waste MLCC used is shown in Table 1. The waste MLCC contained 37.79% Ba, 17.49% Ti, and 14.92% Ni as major elements and oxygen and hydrocarbons as balance. The analysis of nickel content present in different particle size and mass fraction of the waste MLCC presented in Table 2, indicates the decrease in percentage nickel content with the decrease in particle size.
Fig. 1. Schematic diagram of the leaching apparatus. 1. Reactor, 2. Heating mantle, 3. Temperature controller, 4. Reflux condenser, 5. Stirrer, 6. Thermometer, 7. Mercury seal.
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3. Results and discussion 3.1. Effect of leaching reagents The effect of leaching reagents on leaching conversion was investigated at a constant temperature of 90°C. Fig. 2 presents the results of leaching experiments obtained with 1 M HCl, 0.5 M H2SO4, and 1 M HNO3 solutions. As shown in Fig. 2, the fraction of Ni leached with HNO3 solution is substantially higher than that leached with H2SO4 and HCl solutions. From these results, the nickel contained in waste MLCC was observed effectively to react with the HNO3 solution according to the following reaction suggested by Mecucci and Scott (2002): NiðsÞ þ 4HNO3ð1Þ ¼ NiðNO3 Þ2ðaqÞ þ 2H2 Oð1Þ þ 2NO2ðgÞ ð1Þ The fraction of nickel leached was 97% with 1M HNO3 at a temperature of 90°C in 90 min, while in similar experimental condition the fraction of nickel leached obtained was less than 40% with 1 M HCl and 0.5 M H2SO4. Results indicated that nitric acid is an effective reagent for Ni leaching from waste MLCC. So, leaching studies were carried out using nitric acid.
Fig. 3. Effect of agitation speed on the nickel leaching (HNO3 concentration: 1 M, leaching temperature: 50 °C, particle size: 300– 180 μm, pulp density: 5 g/L).
To investigate the influence of film boundary diffusion on the leaching rate of nickel, different agitation speeds between 100 and 600 rpm were tested at 50 °C and 1 M HNO3 concentration. Fig. 3 shows the effect of agitation speed on the fraction of Ni leached. The results
indicate that the leaching rate is independent in respect to agitation speed higher than 200 rpm. This finding thus suggested that 200 rpm is sufficient to provide adequate particle suspension. Therefore, in all subsequent experiments, a working agitation speed of 400 rpm was selected to ensure effective particle suspension. The effect of particle size on the fraction of Ni leached was examined by varying the particle size between 300–500 μm and − 90 μm, while all other variables such as the HNO3 concentration, agitation speed, and leaching temperature were kept constant. The results are given in Fig. 4. The results indicate that the leaching rate is almost independent of the particle size below 180–300 μm. Thus, for further experiments the
Fig. 2. Effect of leaching reagent on the nickel leaching (leaching temperature: 90 °C, agitation speed: 400 rpm, particle size: 300– 180 μm, pulp density: 5 g/L).
Fig. 4. Effect of particle size on the nickel leaching (HNO3 concentration: 1 M, leaching temperature: 50 °C, agitation speed: 400 rpm, pulp density: 5 g/L).
3.2. Effects of agitation speed and particle size
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Fig. 5. Effect of HNO3 concentration on the nickel leaching (leaching temperature: 50 °C, agitation speed: 400 rpm, particle size: 300– 180 μm, pulp density: 5 g/L).
Fig. 6. Effect of leaching temperature on the nickel leaching. (HNO3 concentration: 1 M, agitation speed: 400 rpm, particle size: 300– 180 μm, pulp density: 5 g/L).
particle size in the range of 180 to 300 μm was selected to minimize the particle size effect on the leaching experiment.
Microscopy) and X-ray diffraction. The X-ray diffraction peaks presented in Fig. 7 show the presence of BaTiO3 and Ni in waste MLCC particles before leaching. The X-ray peaks of the leach residue showed only the presence of BaTiO3. On the other hand, for a leaching period of 60 min with 2 M HNO3 at 50°C as shown in Fig 5, the fraction of Ni leached was found 96%, while the fractions of Ba and Ti leached were found 3%. Thus, it was considered that most of the nickel was leached by the HNO3, while most of BaTiO3 remained in the original particles without reacting with the HNO3 solution. The phenomenon was clearly verified by SEM analysis. Fig. 8 shows the polished sections of waste MLCC particles before and after the
3.3. Effect of HNO3 concentration The HNO3 concentration was varied between 0.1 and 2.0 M, while the leaching temperature was kept constant at 50 °C. The Ni leached vs. time curves for this temperature are given in Fig. 5. These curves show that the fraction of Ni leached increases with increase in the concentration of HNO3. As shown in the figure, the nickel leaching was found typically fast in the initial period up to 30 min, but slowed down to a lower rate with further increase of leaching time. The fraction of nickel leached was found to be more than 94% with HNO3 concentrations higher than 0.5 M and a temperature of 50 °C at agitation time of 90 min. 3.4. Effect of leaching temperature The effect of temperature on the nickel leaching was investigated by varying the temperature between 30 and 90°C, while all other parameters were kept constant. Fig. 6 shows the fraction of Ni leached from waste MLCC at 1 M HNO3. The result presented in Fig. 6 shows that the 97% fraction of Ni could be leached with 1 M HNO3 at a temperature of 90 °C in 30 min. 3.5. Interpretation of the results The waste MLCC particles before and after HNO3 leaching were analyzed by SEM (Scanning Electron
Fig. 7. X-ray diffraction patterns of the waste MLCC particle before and after leaching.
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Fig. 8. SEM images of the waste MLCC particle before and after leaching.
leaching of nickel. This figure shows that after the leaching, nickel layers in the particles disappeared, while the overall particle morphology and size remain unchanged, forming many thin channels. It was also observed that the width of channel is approximately 1 μm as shown in Fig. 8. Thus, it was considered that the diffusion of HNO3 solution throughout the thin channels formed between BaTiO3 layers should be a little difficult, and in addition, this structure might give molecular sieve ability as it has a very thin width (Hernández et al., 1986). Based on this observation, the interpretation of the rate data was carried out using a number of different rate expressions such as shrinking unreacted core expression, from which the Jander rate equation (Levenspiel, 1962; Hernández et al., 1986; Habashi, 1980a,b; Paspaliaris and Tsolakis, 1987) was proved to yield the best results. The rate equation was derived from the reactions controlled by a fluid diffusion mechanism. The applicability of this rate expression can be expected from the fact that the volume change of waste MLCC particles before and after the leaching is negligible, which can be seen in Fig. 8. In this rate expression for a disc-shaped particle, the fraction of Ni leached is related to leaching time by the expression: ½1−ð1−XNi Þ1=2 2 ¼ kapp t
(min − 1 ·mol −n ), [HNO3] is the concentration of HNO3, f designates the HNO3 concentration dependence of the rate, and n is the reaction order for the HNO3 concentration. It is apparent from Eq. (2) that a plot of [1 − (1 − X) 1/2 ] 2 vs. t should be linear with kapp as the slope. The validity of the Jander rate expression for extracting nickel from waste MLCC using HNO3 solution was verified by plotting the Ni leached–time curves of Figs. 5 and 6, according to Eq. (2), as shown in Figs. 9 and 10 with R 2 values of 0.94–0.99. This indicates the leaching rate is limited by diffusion through the channels of BaTiO3 layers that are not leached by HNO3, as explained previously. In Fig. 11, the values of kapp determined from the slopes of Fig. 9 were plotted against the HNO 3 concentration (R 2 = 0.97). A straight line is obtained, indicating that the nickel leaching is of first order with respect to the
ð2Þ
where, XNi is the fraction of Ni leached, t is the leaching time (min), and kapp is the apparent rate constant (min− 1) which is given by kapp ¼ bkf ð½HNO3 Þ ¼ bk½HNO3 n ðmin−1 Þ
ð3Þ
Here, b is the stoichiometry factor (b = 1/4 in this system, according to the formulation of Sohn, 1978) for the reaction (1), k is the intrinsic rate constant
Fig. 9. Plot of the results in Fig. 5 according to Eq. (2).
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Fig. 10. Plot of the results in Fig. 6 according to Eq. (2).
Fig. 12. Arrhenius plot of the rate constants.
entire range of HNO3 concentrations considered in this study. Thus, Eq. (3) can be rewritten as kapp ¼ ð1=4Þk½HNO3 ðmin−1 Þ
ð4Þ
Also, the values of k at different temperatures were calculated using Eq. (4) from the slopes of each straight line in Fig. 11. Fig. 12 is an Arrhenius plot of the rate constants (R2 = 0.94). The slope of the straight line placed through the experimental points yields an activation energy of 37.6 kJ/mol. The line through these data can be expressed by the following equation: k ¼ 3:03104 exp½−4520=T ðmin−1 mol−1 Þ
ð5Þ
for the leaching reaction of nickel contained in waste MLCC with HNO3 solution.
Here, the activation energy obtained is slightly high with compared to that reported in many cases of diffusion-controlled leaching. In this research work, the reason might be expected due to some forms of surface diffusion in thin channels formed between BaTiO3 layers during the nickel leaching, which has high activation energy. On the other hand, a number of researchers reported high activation energy similar to our research for the diffusion-controlled leaching. Hernández et al. (1986) reported that the magnesium leaching from sepiolite with sulfuric acid has an activation energy of 63.5 kJ/mol, which is controlled by diffusion of the acid solution. The activation energy for the diffusioncontrolled leaching of zinc from sphalerite concentrate in aqueous slurry by chlorine gas was also found to be 49.6 kJ/mol (Jena et al., 1999). In addition, it was reported that the activation energies for the diffusioncontrolled leaching of iron oxides with hydrochloric acid (Paspaliaris and Tsolakis, 1987) and of pyrite with chlorine gas in aqueous (Vasilakos and Gage, 1986) are 62 to 79 kJ/mol and 26.7 kJ/mol, respectively. Using Eqs. (2), (4), and (5), the leaching rate is represented by the following equation: ½1−ð1−XNi Þ1=2 2 ¼ kapp t
ð6Þ
with kapp = 7.58 × 103·[HNO3]·exp [− 4520 / T ] (min− 1). 4. Conclusions
Fig. 11. Dependence of kapp on the HNO3 concentration from the results shown in Fig. 5.
The leaching behavior of nickel contained in waste MLCC generated in the electric and electronic company in Korea was studied using a stirred batch reactor at the
E. Kim et al. / Hydrometallurgy 86 (2007) 89–95
leaching temperatures between 30 and 90 °C. The leaching rate of nickel in HNO3 solution was found to be much higher than that obtained with HCl and H2SO4 solutions. Nickel extraction was found to be 97% with 1 M HNO3 at 90 °C, 90 min reaction time, and 5 g/L pulp density. The leaching reaction follows the Jander rate expression of a disc-shaped particle. The rate of nickel leaching is limited by the diffusion of HNO3 solution throughout thin channels that were formed between BaTiO3 layers during the nickel leaching. Nickel leaching was first order with respect to HNO3 with an activation energy of 37.6 kJ/mol. The rate of the leaching reaction of nickel from waste MLCC using HNO3 solution could be represented by the following equation: ½1−ð1−XNi Þ1=2 2 kapp t with kapp = 7.58 × 103·[HNO3]·exp [− 4520 / T] (min− 1) Acknowledgements This paper is based on work supported by Resources Recycling R&D Center (Korea) under the project entitled “Development of total recycling technology for WEEE”. Special thanks are extended to Dr. M.K. Jha for the critical review of the manuscript. One of the authors (E. Kim) is grateful to the KIGAM for the award of a fellowship.
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