Journal of Hazardous Materials 279 (2014) 384–388
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Rare earth element recycling from waste nickel-metal hydride batteries Xiuli Yang a,b,∗ , Junwei Zhang a , Xihui Fang a a b
Jiangxi Key Laboratory of Mining Engineering, Jiangxi University of Science and Technology, Jiangxi 341000, China Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology, Jiangxi 341000, China
h i g h l i g h t s • • • • •
Leaching kinetics of REEs has rarely been reported. A new method, including hydrochloric acid leaching and oxalic acid precipitation, was proposed. REEs recovery rate of 95.16% and pure rare earth oxides of 99% were obtained. Leaching process was controlled by chemical reaction. The kinetic equation was determined.
a r t i c l e
i n f o
Article history: Received 5 May 2014 Received in revised form 17 June 2014 Accepted 15 July 2014 Available online 21 July 2014 Keywords: Rare earth element Hydrochloric acid Oxalic acid Waste nickel-metal hydride battery Recycling
a b s t r a c t With an increase in number of waste nickel-metal hydride batteries, and because of the importance of rare earth elements, the recycling of rare earth elements is becoming increasingly important. In this paper, we investigate the effects of temperature, hydrochloric acid concentration, and leaching time to optimize leaching conditions and determine leach kinetics. The results indicate that an increase in temperature, hydrochloric acid concentration, and leaching time enhance the leaching rate of rare earth elements. A maximum rare earth elements recovery of 95.16% was achieved at optimal leaching conditions of 70 ◦ C, solid/liquid ratio of 1:10, 20% hydrochloric acid concentration, −74 m particle size, and 100 min leaching time. The experimental data were best fitted by a chemical reaction-controlled model. The activation energy was 43.98 kJ/mol and the reaction order for hydrochloric acid concentration was 0.64. The kinetic 1/3 0.64 equation for the leaching process was found to be: 1 − (1 − x) = A/r0 [HCl] exp −439,800 t. After 8.314T leaching and filtration, by adding saturated oxalic solution to the filtrate, rare earth element oxalates were obtained. After removing impurities by adding ammonia, filtering, washing with dilute hydrochloric acid, and calcining at 810 ◦ C, a final product of 99% pure rare earth oxides was obtained. © 2014 Elsevier B.V. All rights reserved.
1. Introduction China is one of the largest global producers of batteries at almost one third of the global output. The annual output of small nickelmetal hydride (Ni-MH) batteries alone reaches 0.16 billion per year. Generally, Ni-MH batteries have a useful life of two years [1], resulting in the wastage of a large quantity of these batteries every year. Eight to ten percent rare earth elements (REEs) exist in the anode of battery in the form of hydroxide, oxide, and pure metal.
∗ Corresponding author at: Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology, Jiangxi 341000, China. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.jhazmat.2014.07.027 0304-3894/© 2014 Elsevier B.V. All rights reserved.
The REEs include mainly lanthanum, cerium, praseodymium, and neodymium. To date, the recycling of REEs from waste Ni-MH batteries and the leaching kinetics of REEs have rarely been reported on. Pyro- and hydrometallurgical processes have been considered for REE recycling [2–6]. However, REEs in ashes cannot be recovered in the pyrometallurgy process, and serious pollution results. Compared with pyrometallurgy, the hydrometallurgical approach has more advantages, such as lower operating temperatures, lower energy, and harmless gas emissions. In 2006, Gerasimova et al. proposed sulfuric acid leaching and Na2 CO3 /NaOH precipitation for REE recycling from waste Ni-MH batteries [7]. However, because of the similar solubilities of REE carbonates and carbonate impurities in water and common acids, they are difficult to separate and the final REE product purity is reduced. In 2008, Mei et al. proposed
X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388
385
Hydrochloric and oxalic acids (both analytical reagent grade) were supplied by Ganzhou Chemical Reagent Company, Jiangxi, China. Distilled water was used in the experiments. 2.2. Procedure
Fig. 1. Flow chart of REEs recycling from waste nickel-metal hydride batteries.
Table 1 Chemical composition of the anode material of waste nickel-metal hydride batteries (wt.%). Ni
Cu
Co
Mn
K
Al
Ce
La
Nd
Pr
35.02
7.08
6.96
3.75
1.06
0.98
11.8
7.56
3.94
1.04
dilute sulfuric acid leaching and anhydrous sodium sulfate precipitation for REE recycling from waste Ni-MH batteries [8] with a reaction equation as follows: RE2 (SO4 )3 + Na2 SO4 + xH2 O → RE2 (SO4 ) · Na2 SO4 · xH2 O
(1)
Eq. (1) shows that the introduction of sodium affects the recycled REE quality. Moreover, double salt formation between REE and nickel and/or alkali metals in the sulfuric acid leach decreases REE recovery. Therefore, a new method was proposed, which includes hydrochloric acid leaching and oxalic acid precipitation. In this method, hydrochloric acid and oxalic acid replace sulfuric acid and sodium carbonate and sodium hydroxide or anhydrous sodium sulfate, respectively. The reaction equations are as follows: RE + 6H+ → RE3+ + 3H2
(2)
RE2 O3 + 6H+ → 2RE3+ + 3H2 O
(3)
RE(OH)3 + 3H+ → RE3+ + 3H2 O
(4)
RE3+ + H2 C2 O4 + H2 O → RE2 (C2 O4 ) · nH2 O + H+
(5)
According to the above analysis, the processing flow to recover rare earths from waste nickel-metal hydride batteries is proposed, as shown in Fig. 1. Firstly, unwanted material in waste Ni-MH batteries is leached using hydrochloric acid. After filtration, oxalic acid is added to the filtrate, and rare earth oxalates, RE2 (C2 O4 )·nH2 O, result. After roasting, pure anhydrous rare earth oxalate is obtained. 2. Materials and methods 2.1. Materials We collected waste Ni-MH batteries from Jiangxi, China. The chemical composition of the unwanted material in the waste batteries as analyzed by inductively coupled plasma-atomic emission spectroscopy is shown in Table 1. Prior to analysis, samples were roasted and smelted with an appropriate amount of sodium carbonate and boric acid at 1000 ◦ C for 20 min, and then dissolved using hydrochloric acid solution in a 1:1 (m/v) ratio. Lastly, the solution was diluted to a suitable concentration for analysis.
The flow chart of the proposed method for recycling REEs from waste Ni-MH batteries is shown in Fig. 1. Firstly, the unwanted material, which had been separated manually from the waste NiMH batteries, was dried at 100 ◦ C for 1 h. The dried and cut material was ground to −74 m. The ground unwanted material (50 g) and hydrochloric acid (of different concentrations) was placed into a beaker at different solid/liquid (S/L) ratios, and then stirred constantly at 450 rpm. After reaction, the insoluble matter was filtered and oxalic acid was added to 100 ml of the filtrate. After oxalate precipitation and calcination, rare earth oxides were obtained. The leaching efficiency was calculated from: x% =
1−
m × xRE mo × xoRE
× 100
(6)
where x is the REE leaching efficiency, mr the mass of leach residue (g), xRE the fraction of REEs in the leaching residue, mo the mass of unwanted material in the Ni-MH batteries (g), and xoRE is the REE fraction in the unwanted material from the Ni-MH batteries. 2.3. Analysis To examine phase changes of unreacted and reacted unwanted material in waste Ni-MH batteries, samples were analyzed by Philips X-ray diffractometry with Cu K␣ radiation generated at 40 kV and 150 mA. X-ray diffraction spectra were obtained in the 5–90◦ range of 2 and the diffraction patterns obtained were compared with the Joint Committee on Powder Diffraction Standards archives in the PDXL software program. Chemical composition was analyzed by inductively coupled plasma-atomic emission spectroscopy (Perkin Elmer Co., Ltd., OPTIMA 7000DV). 3. Results and discussion 3.1. Hydrochloric acid leaching 3.1.1. Effect of temperature on leaching rate of rare earth elements The effect of temperature on REE leaching rate was tested for the −74 m particle size fraction using a 20% hydrochloric acid solution and 450 rpm stirring speed from 50 to 70 ◦ C. The results in Fig. 2 indicate that temperature has an appreciable effect on REE leaching. As temperature increases, the time required to extract maximum REEs decreases. This increase may be because of an increase in volatilization of hydrochloric acid with the increase in temperature. To obtain the kinetic equation and apparent activation energy, experimental data were correlated to various kinetics models for liquid/solid reactions. For a liquid/solid reaction system, the reaction rate is generally controlled by diffusion through the liquid film, diffusion through the product layer, chemical reaction at the solid particle surface or a mixture of diffusion and chemical reactions [9–11]. The shrinking core model considers that the leaching process is controlled by one of these steps. When either diffusion or the surface chemical reaction is the slowest step, the shrinking core model equations can be expressed as follows [10]: 1 − 3(1 − x)2/3 + 2(1 − x) = kd t
(7)
1 − (1 − x)1/3 = kr t
(8)
386
X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388 1.0 100
80
1-3(1-x) +2(1-x)
60
2/3
Leaching rate of rare elements (%)
(a) 0.8
40
70 °C 60 °C 55 °C 50 °C
20
70 °C 60 °C 55 °C 50 °C
0.6
0.4
2
R =0.956 2 R =0.979 2 R =0.988 2 R =0.978
0.2
0.0 0 0
10
20
30
40
50
60
70
80
90
100 110 120 130
0
20
40
60
Time (min)
80
100
80
100
120
Time (min)
Fig. 2. Effect of temperature on leaching rate of REEs (20% hydrochloric acid concentration, S/L: 1:10, particle size: −74 m, stirring speed: 450 rpm).
0.8
(b) 0.7
k = A × exp −
RT
1/3
0.5 0.4 0.3
0.1 0.0 0
(9)
where k is the rate constant for surface reaction (s−1 ), T the reaction temperature (K), R the universal gas constant, 8.314 J/(K mol) and Ea is the activation energy (J/mol). Fig. 4 presents the linear Arrhenius plot of ln k vs. 1/T with slope (−Ea /R). The apparent activation energy was calculated to be 43.98 kJ/mol. 3.1.2. Effect of hydrochloric acid concentration on leaching rate of rare earth elements Fig. 5 shows the effect of acid concentration on REE leaching rate as a function of leaching time for a particle size of −74 m, at 70 ◦ C, and a S/L ratio of 1:10 (g/ml). The leaching of REEs was strongly dependent on the increase in acid concentration. Insufficient acid existed at low acid concentration (5–15%), and only 45.67–87.12% of the REEs were extracted. The percentage leached improves to 98.89% when the acid concentration is increased to 20%. This is attributed to the increasing H+ concentration that results in further dissolution of the REE-containing material. Highest REE extractions were obtained for experiments carried out for 100 min. After this period, REE extraction values showed a linear trend with respect to reaction time. To determine the reaction order, the experimental data in Fig. 5 were analyzed based on Eq. (8), with results shown in Fig. 6. There is a good correlation between experimental data and Eq. (8), indicating that the leaching process was controlled by a surface chemical reaction. The reaction order can be obtained via the logarithmic relationship between the overall rate constant obtained from Fig. 6 and the hydrochloric acid concentration. Results are summarized in Fig. 7, from which the reaction order of the process was obtained as 0.64.
2
R =0.997 2 R =0.991 2 R =0.993 2 R =0.992
0.2
20
40
60
120
Time (min) Fig. 3. Plot of shrinking core model with time at various temperatures (a) diffusion model, (b) chemical reaction model (20% hydrochloric acid concentration, S/L: 1:10, particle size: −74 m, stirring speed: 450 rpm).
According to Eqs. (8) and (9), the kinetic equation can be expressed as follows: 1 − (1 − x)1/3 = kr t =
kC n t A n C exp = r0 r0
−E a
RT
t
(10)
-4.8 -4.9 -5.0 -5.1 -5.2
Ln(k)
E a
70 °C 60 °C 55 °C 50 °C
0.6
1-(1-x)
where x is the leaching rate of REEs (%), t the reaction time (min), and kd and kr are rate constants calculated from Eqs. (7) and (8), respectively. To determine the main controlling step, the experimental data in Fig. 2 were analyzed based on Eqs. (7) and (8), with results shown in Fig. 3(a) and (b), respectively. Eq. (7) does not fit the experimental data well (see Fig. 3(a)) but there is a good correlation between the experimental data and Eq. (8) (see Fig. 3(b)). The leaching kinetics can therefore be described by the shrinking core model with chemical reaction. The apparent activation energy was determined based on Eq. (9):
Slope=-5.29
-5.3 -5.4 -5.5 -5.6 -5.7 -5.8 -5.9 2.90
2.92
2.94
2.96
2.98
3.00
3.02
3.04
3.06
3.08
3.10
3.12
-1
1000/T (K ) Fig. 4. Arrhenius plot for rare elements leaching (20% hydrochloric acid concentration, S/L: 1:10, particle size: −74 m, stirring speed: 450 rpm).
X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388
1400
La2O3 Ce2O3
1200
80
Pr2O3 Nd2O3
1000 60
Intensity(A.U.)
Leaching rate of rare earth elements (%)
100
387
40
5% 10% 15% 20%
20
800 600 400 200
0 0
10
20
30
40
50
60
70
80
90
100 110 120 130
0
Time (min)
0
Fig. 5. Effect of acid concentration on leaching rate of rare earth elements (S/L: 1:10, temperature: 70 ◦ C, particle size: −74 m, stirring speed: 450 rpm)
10
20
30
40
50
60
70
80
90
2-theta Fig. 8. XRD pattern of the product.
where C is the hydrochloric acid concentration (%), the mineral particle density (g/cm3 ), and r0 is the initial solid particle radius (m). Based on the activation energy and reaction order obtained, the kinetic equation for the hydrochloric acid leaching of REEs can be expressed as follows:
0.75
0.60
5% 10% 15% 20%
1-(1-x)
1/3
0.45
2
R =0.995 2 R =0.991 2 R =0.994 2 R =0.996
1 − (1 − x)1/3 =
0.30
−439, 800 8.314T
t
(11)
3.2. Preparation of REE oxides
0.15
0.00
0
10
20
30
40
50
60
70
80
90
100
110
Time (min) Fig. 6. Plot of chemical reaction mode with time at various hydrochloric acid concentrations (S/L: 1:10, temperature: 70 ◦ C, particle size: −74 m, stirring speed: 450 rpm).
-2.2
-2.3
To obtain the REE oxides, after leaching and filtering, saturated oxalic acid was added gradually into the classified filtrate. NaOH solution (0.8%) was used to adjust the solution to a final pH of 0.4–0.6. NaF (1.0 mol/L) was used as indicator to determine the endpoint of complete precipitation. After complete precipitation and aging, REE oxalate, which included some nickel, copper, and cobalt ions, was obtained. After filtering, the REE oxalates were dissolved in distilled water (S/L = 1:5). According to Eqs. (12)–(15), after ammonia addition, stable [Cu(NH3 )4 ]2+ , [Ni(NH3 )6 ]2+ , and [Co(NH3 )6 ]3+ can be obtained, and these impurities can be removed by adding ammonia. After removing the impurities and filtering again, the REE oxalates were washed several times using dilute hydrochloric acid and pure REE oxalates were obtained. After calcination at 810 ◦ C, a final product of 99% pure rare earth oxides was obtained, as observed from the X-ray diffractogram of the product in Fig. 8. The total REE recovery was 95.16%. Cu2+ + 4NH3 → [Cu(NH3 )4 ]2+
Slope=0.64
Log(k)
A [HCl]0.64 exp r0
Ni
-2.4
2+
+ 6NH3 → [Ni(NH3 )6 ]
2+
Co2+ + 6NH3 → [Co(NH3 )6 ]2+ 2[Co(NH3 )6 ]2+ +
-2.5
1 O2 + H2 O → 2[Co(NH3 )6 ]3+ + 2OH− 2
(12) (13) (14) (15)
4. Conclusions
-2.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Log(C) Fig. 7. Plot of log–log plot of overall rate constant vs. hydrochloric acid concentration (S/L: 1:10, temperature: 70 ◦ C, particle size: −74 m, stirring speed: 450 rpm).
The following conclusions, can be deduced from previous results and discussions: (1) Temperature, hydrochloric acid concentration, and leaching time had a significant effect on REE recycling from waste Ni-MH batteries. A maximum REE recovery rate of 95.16% was obtained from the sample of particle size −74 m by leaching with 20% hydrochloric acid for 100 min at 70 ◦ C with a 1:10 S/L ratio and
388
X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388
agitation at 450 rpm. Under optimal conditions, 99% pure rare earth oxides were obtained. (2) According to the kinetic study, the leaching process was controlled by chemical reaction. Under experimental conditions, the activation energy and reaction order with hydrochloric acid concentration were 43.98 kJ/mol and 0.64, respectively. The kinetic equation was found to be: 1 − (1 − x)1/3 = −439,800 0.64 exp 8.314T t. A/r0 [HCl] Acknowledgements This work was financially supported by Doctor Initial Funding Project (No. 3401223165) and College Project (No. 3200826334) of Jiangxi University of Science and Technology of China. References [1] 2012–2017 report of market research and investment consultant of nickelmetal hydride batteries in China, 2012. [2] F. Shi, Rare Earth Metallurgy Technology, Metallurgy Industry Press, Beijing, 2009.
[3] H. Liu, S. Zhang, D. Pan, J. Tian, M. Yang, M. Wu, Rare earth elements: recycling from waste phosphor by dual hydrochloric acid dissolution, J. Hazard. Mater. 272 (2014) 96–101. [4] R. Otto, A. Wojtalewicz-Kasprzak, Method for recovery of rare earths from fluorescent lamps, US Patent 8628734 (2014). [5] D.G. Porob, A.M. Srivastava, P.K. Nammalwar, G.C. Ramachandran, H.A. Comanzo, Rare earth recovery from fluorescent material and associated method, US Patent 8137645 (2012). [6] F. Yang, F. Kubota, Y. Baba, N. Kamiya, M. Goto, Selective extraction and recovery of rare earth metals from phosphor powders in waste fluorescent lamps using an ionic liquid system, J. Hazard. Mater. 254–255 (2013) 79–88. [7] L.G. Gerasimova, A.I. Nikolaev, M.V. Maslova, R.F. Okhrimenko, Recycling permanent-magnet production wastes containing rare-earth elements, Steel Transl. 36 (2006) 71–73. [8] G.J. Mei, Y. Xia, W. Shi, B. Liu, Recovery of rare earth from spent MH-Ni battery negative electrode, Environ. Prot. Chem. Ind. 28 (2008) 70–73. [9] M. Gharabaghi, M. Irannajad, A.R. Azadmehr, Leaching kinetics of nickel extraction from hazardous waste by sulphuric acid and optimization dissolution conditions, Chem. Eng. Res. Des. 91 (2013) 325–331. [10] O. Levenspiel, Chemical Reaction Engineering, Wiley, New York, 1998. [11] Z.I. Zafar, Determination of semi empirical kinetic model for dissolution of bauxite ore with sulfuric acid: parametric cumulative effect on the Arrhenius parameters, Chem. Eng. J. 141 (2008) 233–241.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Rare earth element recycling from waste nickel-metal hydride batteries Xiuli Yang a,b,∗ , Junwei Zhang a , Xihui Fang a a b
Jiangxi Key Laboratory of Mining Engineering, Jiangxi University of Science and Technology, Jiangxi 341000, China Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology, Jiangxi 341000, China
h i g h l i g h t s • • • • •
Leaching kinetics of REEs has rarely been reported. A new method, including hydrochloric acid leaching and oxalic acid precipitation, was proposed. REEs recovery rate of 95.16% and pure rare earth oxides of 99% were obtained. Leaching process was controlled by chemical reaction. The kinetic equation was determined.
a r t i c l e
i n f o
Article history: Received 5 May 2014 Received in revised form 17 June 2014 Accepted 15 July 2014 Available online 21 July 2014 Keywords: Rare earth element Hydrochloric acid Oxalic acid Waste nickel-metal hydride battery Recycling
a b s t r a c t With an increase in number of waste nickel-metal hydride batteries, and because of the importance of rare earth elements, the recycling of rare earth elements is becoming increasingly important. In this paper, we investigate the effects of temperature, hydrochloric acid concentration, and leaching time to optimize leaching conditions and determine leach kinetics. The results indicate that an increase in temperature, hydrochloric acid concentration, and leaching time enhance the leaching rate of rare earth elements. A maximum rare earth elements recovery of 95.16% was achieved at optimal leaching conditions of 70 ◦ C, solid/liquid ratio of 1:10, 20% hydrochloric acid concentration, −74 m particle size, and 100 min leaching time. The experimental data were best fitted by a chemical reaction-controlled model. The activation energy was 43.98 kJ/mol and the reaction order for hydrochloric acid concentration was 0.64. The kinetic 1/3 0.64 equation for the leaching process was found to be: 1 − (1 − x) = A/r0 [HCl] exp −439,800 t. After 8.314T leaching and filtration, by adding saturated oxalic solution to the filtrate, rare earth element oxalates were obtained. After removing impurities by adding ammonia, filtering, washing with dilute hydrochloric acid, and calcining at 810 ◦ C, a final product of 99% pure rare earth oxides was obtained. © 2014 Elsevier B.V. All rights reserved.
1. Introduction China is one of the largest global producers of batteries at almost one third of the global output. The annual output of small nickelmetal hydride (Ni-MH) batteries alone reaches 0.16 billion per year. Generally, Ni-MH batteries have a useful life of two years [1], resulting in the wastage of a large quantity of these batteries every year. Eight to ten percent rare earth elements (REEs) exist in the anode of battery in the form of hydroxide, oxide, and pure metal.
∗ Corresponding author at: Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology, Jiangxi 341000, China. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.jhazmat.2014.07.027 0304-3894/© 2014 Elsevier B.V. All rights reserved.
The REEs include mainly lanthanum, cerium, praseodymium, and neodymium. To date, the recycling of REEs from waste Ni-MH batteries and the leaching kinetics of REEs have rarely been reported on. Pyro- and hydrometallurgical processes have been considered for REE recycling [2–6]. However, REEs in ashes cannot be recovered in the pyrometallurgy process, and serious pollution results. Compared with pyrometallurgy, the hydrometallurgical approach has more advantages, such as lower operating temperatures, lower energy, and harmless gas emissions. In 2006, Gerasimova et al. proposed sulfuric acid leaching and Na2 CO3 /NaOH precipitation for REE recycling from waste Ni-MH batteries [7]. However, because of the similar solubilities of REE carbonates and carbonate impurities in water and common acids, they are difficult to separate and the final REE product purity is reduced. In 2008, Mei et al. proposed
X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388
385
Hydrochloric and oxalic acids (both analytical reagent grade) were supplied by Ganzhou Chemical Reagent Company, Jiangxi, China. Distilled water was used in the experiments. 2.2. Procedure
Fig. 1. Flow chart of REEs recycling from waste nickel-metal hydride batteries.
Table 1 Chemical composition of the anode material of waste nickel-metal hydride batteries (wt.%). Ni
Cu
Co
Mn
K
Al
Ce
La
Nd
Pr
35.02
7.08
6.96
3.75
1.06
0.98
11.8
7.56
3.94
1.04
dilute sulfuric acid leaching and anhydrous sodium sulfate precipitation for REE recycling from waste Ni-MH batteries [8] with a reaction equation as follows: RE2 (SO4 )3 + Na2 SO4 + xH2 O → RE2 (SO4 ) · Na2 SO4 · xH2 O
(1)
Eq. (1) shows that the introduction of sodium affects the recycled REE quality. Moreover, double salt formation between REE and nickel and/or alkali metals in the sulfuric acid leach decreases REE recovery. Therefore, a new method was proposed, which includes hydrochloric acid leaching and oxalic acid precipitation. In this method, hydrochloric acid and oxalic acid replace sulfuric acid and sodium carbonate and sodium hydroxide or anhydrous sodium sulfate, respectively. The reaction equations are as follows: RE + 6H+ → RE3+ + 3H2
(2)
RE2 O3 + 6H+ → 2RE3+ + 3H2 O
(3)
RE(OH)3 + 3H+ → RE3+ + 3H2 O
(4)
RE3+ + H2 C2 O4 + H2 O → RE2 (C2 O4 ) · nH2 O + H+
(5)
According to the above analysis, the processing flow to recover rare earths from waste nickel-metal hydride batteries is proposed, as shown in Fig. 1. Firstly, unwanted material in waste Ni-MH batteries is leached using hydrochloric acid. After filtration, oxalic acid is added to the filtrate, and rare earth oxalates, RE2 (C2 O4 )·nH2 O, result. After roasting, pure anhydrous rare earth oxalate is obtained. 2. Materials and methods 2.1. Materials We collected waste Ni-MH batteries from Jiangxi, China. The chemical composition of the unwanted material in the waste batteries as analyzed by inductively coupled plasma-atomic emission spectroscopy is shown in Table 1. Prior to analysis, samples were roasted and smelted with an appropriate amount of sodium carbonate and boric acid at 1000 ◦ C for 20 min, and then dissolved using hydrochloric acid solution in a 1:1 (m/v) ratio. Lastly, the solution was diluted to a suitable concentration for analysis.
The flow chart of the proposed method for recycling REEs from waste Ni-MH batteries is shown in Fig. 1. Firstly, the unwanted material, which had been separated manually from the waste NiMH batteries, was dried at 100 ◦ C for 1 h. The dried and cut material was ground to −74 m. The ground unwanted material (50 g) and hydrochloric acid (of different concentrations) was placed into a beaker at different solid/liquid (S/L) ratios, and then stirred constantly at 450 rpm. After reaction, the insoluble matter was filtered and oxalic acid was added to 100 ml of the filtrate. After oxalate precipitation and calcination, rare earth oxides were obtained. The leaching efficiency was calculated from: x% =
1−
m × xRE mo × xoRE
× 100
(6)
where x is the REE leaching efficiency, mr the mass of leach residue (g), xRE the fraction of REEs in the leaching residue, mo the mass of unwanted material in the Ni-MH batteries (g), and xoRE is the REE fraction in the unwanted material from the Ni-MH batteries. 2.3. Analysis To examine phase changes of unreacted and reacted unwanted material in waste Ni-MH batteries, samples were analyzed by Philips X-ray diffractometry with Cu K␣ radiation generated at 40 kV and 150 mA. X-ray diffraction spectra were obtained in the 5–90◦ range of 2 and the diffraction patterns obtained were compared with the Joint Committee on Powder Diffraction Standards archives in the PDXL software program. Chemical composition was analyzed by inductively coupled plasma-atomic emission spectroscopy (Perkin Elmer Co., Ltd., OPTIMA 7000DV). 3. Results and discussion 3.1. Hydrochloric acid leaching 3.1.1. Effect of temperature on leaching rate of rare earth elements The effect of temperature on REE leaching rate was tested for the −74 m particle size fraction using a 20% hydrochloric acid solution and 450 rpm stirring speed from 50 to 70 ◦ C. The results in Fig. 2 indicate that temperature has an appreciable effect on REE leaching. As temperature increases, the time required to extract maximum REEs decreases. This increase may be because of an increase in volatilization of hydrochloric acid with the increase in temperature. To obtain the kinetic equation and apparent activation energy, experimental data were correlated to various kinetics models for liquid/solid reactions. For a liquid/solid reaction system, the reaction rate is generally controlled by diffusion through the liquid film, diffusion through the product layer, chemical reaction at the solid particle surface or a mixture of diffusion and chemical reactions [9–11]. The shrinking core model considers that the leaching process is controlled by one of these steps. When either diffusion or the surface chemical reaction is the slowest step, the shrinking core model equations can be expressed as follows [10]: 1 − 3(1 − x)2/3 + 2(1 − x) = kd t
(7)
1 − (1 − x)1/3 = kr t
(8)
386
X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388 1.0 100
80
1-3(1-x) +2(1-x)
60
2/3
Leaching rate of rare elements (%)
(a) 0.8
40
70 °C 60 °C 55 °C 50 °C
20
70 °C 60 °C 55 °C 50 °C
0.6
0.4
2
R =0.956 2 R =0.979 2 R =0.988 2 R =0.978
0.2
0.0 0 0
10
20
30
40
50
60
70
80
90
100 110 120 130
0
20
40
60
Time (min)
80
100
80
100
120
Time (min)
Fig. 2. Effect of temperature on leaching rate of REEs (20% hydrochloric acid concentration, S/L: 1:10, particle size: −74 m, stirring speed: 450 rpm).
0.8
(b) 0.7
k = A × exp −
RT
1/3
0.5 0.4 0.3
0.1 0.0 0
(9)
where k is the rate constant for surface reaction (s−1 ), T the reaction temperature (K), R the universal gas constant, 8.314 J/(K mol) and Ea is the activation energy (J/mol). Fig. 4 presents the linear Arrhenius plot of ln k vs. 1/T with slope (−Ea /R). The apparent activation energy was calculated to be 43.98 kJ/mol. 3.1.2. Effect of hydrochloric acid concentration on leaching rate of rare earth elements Fig. 5 shows the effect of acid concentration on REE leaching rate as a function of leaching time for a particle size of −74 m, at 70 ◦ C, and a S/L ratio of 1:10 (g/ml). The leaching of REEs was strongly dependent on the increase in acid concentration. Insufficient acid existed at low acid concentration (5–15%), and only 45.67–87.12% of the REEs were extracted. The percentage leached improves to 98.89% when the acid concentration is increased to 20%. This is attributed to the increasing H+ concentration that results in further dissolution of the REE-containing material. Highest REE extractions were obtained for experiments carried out for 100 min. After this period, REE extraction values showed a linear trend with respect to reaction time. To determine the reaction order, the experimental data in Fig. 5 were analyzed based on Eq. (8), with results shown in Fig. 6. There is a good correlation between experimental data and Eq. (8), indicating that the leaching process was controlled by a surface chemical reaction. The reaction order can be obtained via the logarithmic relationship between the overall rate constant obtained from Fig. 6 and the hydrochloric acid concentration. Results are summarized in Fig. 7, from which the reaction order of the process was obtained as 0.64.
2
R =0.997 2 R =0.991 2 R =0.993 2 R =0.992
0.2
20
40
60
120
Time (min) Fig. 3. Plot of shrinking core model with time at various temperatures (a) diffusion model, (b) chemical reaction model (20% hydrochloric acid concentration, S/L: 1:10, particle size: −74 m, stirring speed: 450 rpm).
According to Eqs. (8) and (9), the kinetic equation can be expressed as follows: 1 − (1 − x)1/3 = kr t =
kC n t A n C exp = r0 r0
−E a
RT
t
(10)
-4.8 -4.9 -5.0 -5.1 -5.2
Ln(k)
E a
70 °C 60 °C 55 °C 50 °C
0.6
1-(1-x)
where x is the leaching rate of REEs (%), t the reaction time (min), and kd and kr are rate constants calculated from Eqs. (7) and (8), respectively. To determine the main controlling step, the experimental data in Fig. 2 were analyzed based on Eqs. (7) and (8), with results shown in Fig. 3(a) and (b), respectively. Eq. (7) does not fit the experimental data well (see Fig. 3(a)) but there is a good correlation between the experimental data and Eq. (8) (see Fig. 3(b)). The leaching kinetics can therefore be described by the shrinking core model with chemical reaction. The apparent activation energy was determined based on Eq. (9):
Slope=-5.29
-5.3 -5.4 -5.5 -5.6 -5.7 -5.8 -5.9 2.90
2.92
2.94
2.96
2.98
3.00
3.02
3.04
3.06
3.08
3.10
3.12
-1
1000/T (K ) Fig. 4. Arrhenius plot for rare elements leaching (20% hydrochloric acid concentration, S/L: 1:10, particle size: −74 m, stirring speed: 450 rpm).
X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388
1400
La2O3 Ce2O3
1200
80
Pr2O3 Nd2O3
1000 60
Intensity(A.U.)
Leaching rate of rare earth elements (%)
100
387
40
5% 10% 15% 20%
20
800 600 400 200
0 0
10
20
30
40
50
60
70
80
90
100 110 120 130
0
Time (min)
0
Fig. 5. Effect of acid concentration on leaching rate of rare earth elements (S/L: 1:10, temperature: 70 ◦ C, particle size: −74 m, stirring speed: 450 rpm)
10
20
30
40
50
60
70
80
90
2-theta Fig. 8. XRD pattern of the product.
where C is the hydrochloric acid concentration (%), the mineral particle density (g/cm3 ), and r0 is the initial solid particle radius (m). Based on the activation energy and reaction order obtained, the kinetic equation for the hydrochloric acid leaching of REEs can be expressed as follows:
0.75
0.60
5% 10% 15% 20%
1-(1-x)
1/3
0.45
2
R =0.995 2 R =0.991 2 R =0.994 2 R =0.996
1 − (1 − x)1/3 =
0.30
−439, 800 8.314T
t
(11)
3.2. Preparation of REE oxides
0.15
0.00
0
10
20
30
40
50
60
70
80
90
100
110
Time (min) Fig. 6. Plot of chemical reaction mode with time at various hydrochloric acid concentrations (S/L: 1:10, temperature: 70 ◦ C, particle size: −74 m, stirring speed: 450 rpm).
-2.2
-2.3
To obtain the REE oxides, after leaching and filtering, saturated oxalic acid was added gradually into the classified filtrate. NaOH solution (0.8%) was used to adjust the solution to a final pH of 0.4–0.6. NaF (1.0 mol/L) was used as indicator to determine the endpoint of complete precipitation. After complete precipitation and aging, REE oxalate, which included some nickel, copper, and cobalt ions, was obtained. After filtering, the REE oxalates were dissolved in distilled water (S/L = 1:5). According to Eqs. (12)–(15), after ammonia addition, stable [Cu(NH3 )4 ]2+ , [Ni(NH3 )6 ]2+ , and [Co(NH3 )6 ]3+ can be obtained, and these impurities can be removed by adding ammonia. After removing the impurities and filtering again, the REE oxalates were washed several times using dilute hydrochloric acid and pure REE oxalates were obtained. After calcination at 810 ◦ C, a final product of 99% pure rare earth oxides was obtained, as observed from the X-ray diffractogram of the product in Fig. 8. The total REE recovery was 95.16%. Cu2+ + 4NH3 → [Cu(NH3 )4 ]2+
Slope=0.64
Log(k)
A [HCl]0.64 exp r0
Ni
-2.4
2+
+ 6NH3 → [Ni(NH3 )6 ]
2+
Co2+ + 6NH3 → [Co(NH3 )6 ]2+ 2[Co(NH3 )6 ]2+ +
-2.5
1 O2 + H2 O → 2[Co(NH3 )6 ]3+ + 2OH− 2
(12) (13) (14) (15)
4. Conclusions
-2.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Log(C) Fig. 7. Plot of log–log plot of overall rate constant vs. hydrochloric acid concentration (S/L: 1:10, temperature: 70 ◦ C, particle size: −74 m, stirring speed: 450 rpm).
The following conclusions, can be deduced from previous results and discussions: (1) Temperature, hydrochloric acid concentration, and leaching time had a significant effect on REE recycling from waste Ni-MH batteries. A maximum REE recovery rate of 95.16% was obtained from the sample of particle size −74 m by leaching with 20% hydrochloric acid for 100 min at 70 ◦ C with a 1:10 S/L ratio and
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X. Yang et al. / Journal of Hazardous Materials 279 (2014) 384–388
agitation at 450 rpm. Under optimal conditions, 99% pure rare earth oxides were obtained. (2) According to the kinetic study, the leaching process was controlled by chemical reaction. Under experimental conditions, the activation energy and reaction order with hydrochloric acid concentration were 43.98 kJ/mol and 0.64, respectively. The kinetic equation was found to be: 1 − (1 − x)1/3 = −439,800 0.64 exp 8.314T t. A/r0 [HCl] Acknowledgements This work was financially supported by Doctor Initial Funding Project (No. 3401223165) and College Project (No. 3200826334) of Jiangxi University of Science and Technology of China. References [1] 2012–2017 report of market research and investment consultant of nickelmetal hydride batteries in China, 2012. [2] F. Shi, Rare Earth Metallurgy Technology, Metallurgy Industry Press, Beijing, 2009.
[3] H. Liu, S. Zhang, D. Pan, J. Tian, M. Yang, M. Wu, Rare earth elements: recycling from waste phosphor by dual hydrochloric acid dissolution, J. Hazard. Mater. 272 (2014) 96–101. [4] R. Otto, A. Wojtalewicz-Kasprzak, Method for recovery of rare earths from fluorescent lamps, US Patent 8628734 (2014). [5] D.G. Porob, A.M. Srivastava, P.K. Nammalwar, G.C. Ramachandran, H.A. Comanzo, Rare earth recovery from fluorescent material and associated method, US Patent 8137645 (2012). [6] F. Yang, F. Kubota, Y. Baba, N. Kamiya, M. Goto, Selective extraction and recovery of rare earth metals from phosphor powders in waste fluorescent lamps using an ionic liquid system, J. Hazard. Mater. 254–255 (2013) 79–88. [7] L.G. Gerasimova, A.I. Nikolaev, M.V. Maslova, R.F. Okhrimenko, Recycling permanent-magnet production wastes containing rare-earth elements, Steel Transl. 36 (2006) 71–73. [8] G.J. Mei, Y. Xia, W. Shi, B. Liu, Recovery of rare earth from spent MH-Ni battery negative electrode, Environ. Prot. Chem. Ind. 28 (2008) 70–73. [9] M. Gharabaghi, M. Irannajad, A.R. Azadmehr, Leaching kinetics of nickel extraction from hazardous waste by sulphuric acid and optimization dissolution conditions, Chem. Eng. Res. Des. 91 (2013) 325–331. [10] O. Levenspiel, Chemical Reaction Engineering, Wiley, New York, 1998. [11] Z.I. Zafar, Determination of semi empirical kinetic model for dissolution of bauxite ore with sulfuric acid: parametric cumulative effect on the Arrhenius parameters, Chem. Eng. J. 141 (2008) 233–241.
