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Kinetics on leaching rare earth from the weathered crust elution-deposited rare earth ores with ammonium sulfate solution
Hydrometallurgy 101 (2010) 166–170
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / h y d r o m e t
Kinetics on leaching rare earth from the weathered crust elution-deposited rare earth ores with ammonium sulfate solution Tian Jun a,b,⁎, Yin Jingqun b, Chi Ruan a,c, Rao Guohua b, Jiang Mintao b, Ouyang Kexian b a b c
School of Resources Processing and Bioengineering, Central South University, Changsha, 410083, China Institute of Applied Chemistry, Jiangxi Academy of Science, Nanchang, 330029, China Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology,Wuhan, 430073, China
a r t i c l e
i n f o
Article history: Received 3 August 2009 Received in revised form 28 December 2009 Accepted 5 January 2010 Available online 11 January 2010 Keywords: Rare earth Leaching Kinetics Clays minerals
a b s t r a c t In this paper, the kinetic of leaching rare earth from the weathered crust elution-deposited rare earth ores with ammonium sulfate solution was investigated. The effects of the main leaching parameters such as temperature and ore particle size on leaching process were examined and discussed in order to elucidate the kinetics and mechanism of leaching rare earth. It was found that the higher the leaching temperature or the smaller the ore particle size, the faster the leaching progress. The leaching mechanism was analyzed with different kinetics models. The results show that the leaching process can be described by the shrinking-core model, the leaching rate was controlled by the inner diffusion of reactants and leaching products through mineral porous layer, the leaching process follows the kinetic model 1 − 2/3α − (1 − α)2/3 = kt, the apparent activation energy was 9.24 kJ/mol obtained from calculating the experimental data and the order of the ore particle size found to be approximately 2. An empirical equation relating the rare earth leached rate constant 9240 with ore particle size and leaching temperature was established as 1–2 = 3α–ð1–αÞ2 = 3 = 1:50⋅r0−2 ⋅e RT ⋅t: © 2010 Elsevier B.V. All rights reserved.
1. Introduction The weathered crust elution-deposited rare earth ore is a kind of rich rare earth clay minerals. It only existed and was discovered in China, mainly located in Jiangxi, Fujian, Hunan, Guangdong, Yunnan and Guangxi provinces. This rare earth deposits grades are low. It contains rich medium and heavy rare earth elements and is the main resources of mid and heavy rare earth in the world (Chi and Tian, 2008). The metallogenetic mechanism of this kind ore could be due to granodiorite and volcanic rocks were weathered in humid and warm climate, being transformed into clay minerals such as kaolinite, halloysite and montmorillonite. During the same progressive geological weathering leaching process, the rare earth minerals (e.g., bastnasite) were also weathered in original rocks, and the releasing rare earth ions formed rare earth aqua and rare earth hydroxyl ions adsorbed on clay minerals with migrating flow natural water(Wan and Liu, 2005). So the rare earth in the weathered crust elutiondeposited rare earth ore mainly exists with the ion-exchangeable phase adsorbed on clay minerals (Chi et al., 2005). This kind of aqueous and hydroxyl aqueous adsorption module has stable chemical properties, don't hydrolyze and revolute in neutral water. Conventional physical processing methods, such as flotation, magnetic, gravity and electrostatic separation were found to be ineffective ⁎ Corresponding author. Institute of Applied Chemistry, Jiangxi Academy of Science, Nanchang, 330029, China. E-mail address: [email protected] (T. Jun). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.01.001
in extraction rare earth from such sources, but can be leached by ionexchange method with cation based on the rare earth ions adsorbed characteristics (He et al., 1983). Since the weathered crust elution-deposited rare earth ores in China have been discovered at 1969, the scientists and engineers of China have been engaged in research and development of a series of hydrometallurgy for the special weathered crust elution-deposited rare earth ores in China (Huang et al., 2005). Although the three generation leaching process such as the first generation leaching process in pool with NaCl solution, the second generation leaching process in pool with (NH4)2SO4 solution and the third generation insitu leaching process have been developed along the green chemical leaching approach according to the ion-exchange property (Tian et al., 2009). However, many years industrial practice have shown that the leaching effect not only be controlled by the properties of the rare earth ore, the leaching reagent and its concentration, but also be influenced by the hydrodynamics, the kinetics and the mass transfer of the leaching process (Tian et al., 2001). So far there is little knowledge on the leaching kinetics of the weathered crust elutiondeposited rare earth ore. In recent years, with the application and development of the green chemistry of extraction process, especially popularization of the insitu leaching process, the high selectivity leaching, high performance and low consumption extraction technology becomes more and more important and stringent in industrial design (Chi et al., 2006, 2002; Lasheen et al., 2009). In order to know the mechanism of the rare earth leaching and choose the more suitable technology of extraction
T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
167
rare earth from this ore, it is necessary to investigate the leaching kinetics. It would be useful to provide a scientific approach to and a theoretic basis for leaching rare earth from this ore with high performance and low consumption, and be applied to optimize the rare earth extraction conditions, to take mathematics simulation on the leaching process and to improve the rare earth received rate in extraction process.
As shown in Table 3, the partitioning of the LN rare earth ore contains mainly heavy rare earth, and the yttrium element account for 68.94% and hardly any light rare earth elements (Kul et al., 2008). The ore sample was sieved using standard sieves, giving different particle size fractions for the kinetics experiments.
2. Experimental
The leaching kinetics experiments were carried out in a 500mL trimouth glass flask partially immersed in a heated water bath and temperature was controlled within ±1 °C, stirring of this mixture was achieved by using stirrer impeller driven by a variable speed motor. The tri-mouth glass flask was fitted with a thermometer, shaft stirrer and a reflux condenser(Ashraf et al., 2005; Núñez et al., 1994a,b). The leaching tests were conducted in an aforementioned reactor at constant temperature fixed anywhere in the range of room temperature to 95 °C. A total of 250 ml of known concentration ammonium sulfate solution was taken in the reactor and preheated to desired leach temperature, then a known weight (50 g) and particle size range of sample portions of ore was placed into the reactor, and rapidly stirred for various leaching time. After completion of each given time experiment, from which the samples were collected by a measuring pipe, and rapidly transferred to a measuring flask to dilute 100 time with deionized distilled water and cool down to room temperature, then this slurry solution was filtered and the filtrate was analyzed the rare earth concentration by EDTA titration methods. From the analysis results the rare earth leached fraction (α) can calculated according to the following equation:
2.1. The characterization and composition of experimental rare earth ore The experimental weathered crust elution-deposited rare earth ores sample is original rare earth ores collected from Zudong Rare Earth Mine area located in the Longnan County (LN), Jiangxi province, China, which mainly contains heavy rare earth. The main chemical composition of ore sample is listed in Table 1. Table 1 shows that the LN rare earth ore contains only 0.102% rare earth. The LN rare earth deposits grades are low. It indicated that extraction rare earth from this ore is difficult. The existing state of rare earth in the LN rare earth ore was determined with the sequential fractionating extraction methods(Chi et al., 2005), It could be divided into the following four forms: (a) aqueous soluble phase rare earth, the aqueous soluble rare earth and rare earth hydroxyl aqueous ions which were free or individual form without being adsorbed by clay minerals during weathering and weathering-leaching processes; (b) ion-exchangeable phase rare earth, the rare earth exist as rare earth ions adsorbed by clay minerals with aqueous soluble rare earth and rare earth hydroxyl cation states; (c) colloid sediment phase rare earth, the rare earth exist as insoluble rare earth oxides or hydroxyl compounds bonding with, or depositing on oxide minerals; (d) mineral phase rare earth, which includes rare earth mineral crystals of ionic compounds as well as diffusion of rare earth ions to displace mineral crystals in the ores. The results are shown in Table 2. The rare earth exists mainly with ion-exchangeable phase, which account for 80.62% from Table 2, and less than 20% mineral phase and colloid sediment phase, and hardly any aqueous soluble phase. The partitioning of the ion-exchangeable phase from the LN rare earth ore has been determined by Jiangxi Analytical and Testing Center, Nanchang, China with ICP-MS (Inductively Coupled Plasma Mass Spectrometry, SCIEX ELAN 9000 type), the result shows analysis as Table 3. Table 1 The main chemical composition of LN rare earth ore sample (mass fraction %). Element
RE
Al
Fe
Mn
Ca
Mg
SiO2
Loss
Original rare earth ore
0.102
14.66
3.24
0.04
0.32
0.57
60.4
20.37
Table 2 Existing state of rare earth in the LN rare earth ore (mass fraction %). Phase state
Aqueous soluble Ion-exchangeable Colloidal sediment Mineral
Phase 6.15 × 10−3 distribution
80.62
5.30
13.36
Table 3 The partitioning of the LN rare earth ore (mass fraction %). Element
La2O3
CeO2
Pr6O11
Nd2O3
Sm2O3
Eu2O3
Gd2O3
Tb4O7
Content (%)
1.56
0.085
0.65
3.20
2.42
0.50
4.48
0.95
Element
Dy2O3
Ho2O3
Er2O3
Tm2O3
Yb2O3
Lu2O3
Y2O3
Content (%)
6.90
1.56
4.33
0.55
3.48
0.50
68.94
(continued on next page)
2.2. Apparatus and experimental procedure
α=
εt ε0
ð5Þ
where: ε1 is total amount of leaching out rare earth from reaction time t = 0 to reaction time t = t, ε0 is total amount of rare earth present in the original ore sample. 2.3. Analytic methods The rare earth partitioning has been determined by Jiangxi Analytical and Testing Center, Nanchang, China with ICP-MS (Inductively Coupled Plasma Mass Spectrometry, SCIEX ELAN 9000 type) by acidification of the ore samples after being fused with sodium peroxide and sodium hydroxide. The rare earth content of minerals phase and the total rare earth content of the ore has been determined by arsenazo (III) spectrophosphorimetry by acidifying and purifying the ore sample after being fused with sodium peroxide and sodium hydroxide (Vilímec and Jakubec, 1987). The RE3+ contents in leach liquor were determined by titration with EDTA using xylenol orange as indicator and hexamethylene tetramine as buffer. The rare earth content of ion-exchangeable phase analytical methods: 500 g ore sample was taken in a column of 50 mm in diameter and leached with 2% (NH4)2SO4 solution. The leach liquor was collected together and been determined with the same as the rare earth ion contents. The rare earth content of colloidal sediment phase analytical methods: 500 g ore sample was leached with 2.5 L 0.5 mol/L NH2OH. HCl + 2.0 mol/LHCl as extraction reagent for 1 h under stirring. Leaching product was filtered, cross-flowed 5 times at the same condition, The leach liquor was collected together and been determined with the same as the rare earth content of minerals phase (Chi et al., 2005). Deionized distilled water and reagent grade chemicals were used in all the experiments.
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T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
3. Results and discussion 3.1. Effect of leaching temperature on rare earth leaching The leaching reactions of the LN rare earth ore with ammonium sulfate are as follows (Chi and Tian, 2008): −
þ
−
fA14 ½Si4 O10 ðOHÞ8 gm ·nREðsÞ þ3nNH4ðaqÞ ⇌fA14 ½Si4 O10 ðOHÞ8 gm þ
3+
3+
×3nNH4ðsÞ þnREðaqÞ where: s and aq is solid phase and aqueous phase respectively. It shows that the leaching process is a typical noncatalytic heterogeneous reactions in solid–liquid systems (Levenspiel, 1999).To investigate the leaching process and mechanism of rare earth from LN rare earth ore with ammonium sulfate solution, the effect of temperature on the leaching rate was examined in the range of 25– 95 °C with 2.0% (W/V) concentration ammonium sulfate solution for different leaching times under the conditions of 2.0 mm average particle size and 1:5 solid: liquid ratio. The results about effect of leaching temperature on rare earth leaching are shown in Fig. 1. As seen in Fig. 1, the leaching fraction (α) of rare earth increases with increasing leaching temperature and leaching time prolonging in the initial stage. It indicates that rare earth leaching behaviour is controlled kinetics. After 8 min, the leaching rate increases slowly, suggesting that the leaching process approaches to an equilibrium. 3.2. Leaching kinetics equation If the rare earth ore particles were considered as spherical particles, the leaching process can be described with the shrinking core model and can be divided into four models (Chen et al., 1998): (1) Chemical reaction controls: 1−ð1−αÞ
1=3
= k1 t
(2) Outer diffusion controls: 1=3 = k2 t 1−ð1−αÞ
ð1Þ
where: k1, k2, k3 and k4 are chemical constants for different control step, respectively; α is the rare earth leached fraction; t is real leaching time (min.); C0 is the initial concentration of leaching reagent; r0 is the initial radius of the ore sample particle; p is molar density of the sample particle; M is the mass of the sample particle. The kinetics type can be determined by treating of the experimental data. For the kinetics type of outer diffusion or chemical reaction controlling, the plot of 1 − (1 − α)1/3 vs. time (t) is a straight line which passes 0 point, or the kinetics type of inner diffusion controlling, the plot of 1 − 2/3α − (1 − α)2/3 vs. time (t) is a straight line which passes 0 point, or the kinetics type of mixed controlling, the plot of 1 − (1 − α)1/3 vs. time (t) is not a straight line which passes 0 point, neither is the plot of 1 − (1 − α)1/3 vs. time (t) (Levenspiel, 1999). The kinetics equation is obtained by trying trial or error method from the experiment data in Fig. 1 (Habashi, 1980). Aforementioned Eqs. (1), (2) and (4) were examined but did not fit the data well and only the Eq. (3) were examined fit the data best. A series of straight linear relationship with a correlation coefficient R2 = 0.9997 can be obtained with plotting 1 − 2/3α − (1 − α)2/3 vs. t as shown in Fig. 2. Therefore the kinetics equation can be expressed as:
1−2 = 3α−ð1−αÞ
2=3
= k3 t
ð5Þ
where k3 is the linear rate constant (min−1). Its values are 0.01154(20 °C), 0.01484(45 °C), 0.01850(70 °C) and 0.02625(95 °C) respectively. It is observed from the plots that the experimental data fit well into the shrinking core model. This suggests that the leaching rate is controlled by inner diffusion of reactants and leaching products through mineral porous layer during the kinetics district (Zhou et al., 2005). Experimental data fit well into the shrinking core model approach in which a variable activation energy term was introduced. Kinetic analysis of the experimental results reveals that the associated energy and the rate-controlling mechanism are dependent upon both ore sample property as well as the leaching reaction temperature.
ð2Þ 3.3. Leaching apparent activation energy
(3) Inner diffusion controls: 2 2=3 = k3 t 1− α−ð1−αÞ 3
ð3Þ
(4) Mixed controls: 1−ð1−αÞ
1=3
= k4
C0 M t r0 p
Fig. 1. Rare earth leaching fraction (α) under different temperature.
ð4Þ
The K3 values of leaching process at different temperature were calculated from the slopes of the straight lines given in Fig. 2, the Arrhenius diagram (Plots of lnk3 vs 1/T) for the four temperatures for the leaching rare earth from LN rare earth ore with 2.0% (W/V) concentration (NH4)2SO4 solution is linear as shown in Fig. 3.
Fig. 2. Plots of 1 − 2/3α − (1 − α)2/3 vs. time for different temperatures.
T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
Fig. 3. Arrhenius plot of leaching rare earth from LN the rare earth ore.
Fig. 5. Plots of 1 − 2/3a − (1 − a)2/3 vs. time for different particle size.
According to Arrhenius equation, this is represented as: E −RT
k3 = Ae
169
ð6Þ
where k3 is the rate constant (min−1), A is the preexponential factor, R is the universal gas constant(J·mol−1·k−1), T is leaching reaction absolute temperature (Kelvin, K), and E is activation energy (kJ/mol). The apparent activation energy of the leaching process was determined from the slope of the straight line in Fig. 3 to be 9.24 kJ/mol. It also proves that leaching process is controlled by inner diffusion as its activation energy is between 4 kJ/mol to 12 kJ/mol (Aydogan et al., 2006). 3.4. Effect of particle size of LN rare earth ore on rare earth leaching In order to further confirm that the leaching process is inner diffusion controlled and establish mechanism of leaching rare earth, The LN rare earth ore sample was sieved into four different particle size fractions (1.8–2.1, 2.1–2.8, 2.8–4.0 and 4.0–5.2 mm). Then each size fraction was leached at 25 °C with 2.0% (W/V) concentration ammonium sulfate solution for different leaching times respectively. A number of curve about the effect of particle size of LN rare earth ore on rare earth leaching is shown in Fig. 4. The leaching rate increases with the decreasing of particle size of the LN rare earth ore as shown in Fig. 4. It further proves that the leaching process is controlled by inner diffusion. From the experimental data in Fig. 4, a set of straight line relationship can also be obtained by plotting 1 − 2/3α − (1 − α)2/3 vs. t as shown in Fig. 5. It is evident that for the entire size fraction
Fig. 4. Rare earth leaching fraction (α) under different particle size of ore sample.
examined the rare earth leaching process can be well described with inner diffusion control model. The Arrhenius equation can also be expanded and represented as follows:
E n −RT
ð7Þ
k3 = A′ r0 e
In this equation, r0 is the radius of ore particle, n is the order of the ore particle size. The K3 values of leaching process at different particle size of LN rare earth ore with 2.0% (W/V) concentration ammonium sulfate solution were calculated from these slopes of the straight lines give in Fig. 5. It is an interesting and intriguing result that a straight line can be obtained with plotting the lnK values vs. lnr0, as shown in Fig. 6. the linear relationship between the rate constant and the inverse particle size also support the fact that leaching rate is controlled by the inner diffusion. From the line relationship between lnK and lnr0 potted in Fig. 6, the n and A′ value can be calculated from the slope and intercept of the straight line in Fig. 6, which are −1.89 (n ≈ −2) and 1.50 respectively. Then the k3 values can be obtained as the following equation[19–22]:
−2
9240
k3 = 1:50⋅r0 ⋅e RT
Fig. 6. The relationship between lnK and lnr0 plot at 25 °C.
ð8Þ
170
T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
The empirical equation of the leaching kinetics is established as: 1–2 = 3α–ð1–αÞ
2=3
−2
9240
= 1:50⋅r0 ⋅e RT ⋅t:
ð9Þ
4. Conclusions The rare earth leaching rate increase with increasing leaching temperature and time, during the initial stages of leaching rate was controlled by kinetics. The kinetics tests about the factors of temperature and particle size of the rare earth ore on leaching ratio show that the leaching process followed the shrinking core model. The leaching kinetics is controlled by diffusion of porous solid layer. The apparent activation energy was 9.24 kJ/mol, and the order of the ore particle size found to be approximately 2. Then empirical equation of the leaching 9240 kinetics is established as: 1–2 = 3α–ð1–αÞ2 = 3 = 1:50⋅r0−2 ⋅e RT ⋅t Acknowledgments The authors gratefully acknowledge the financial support of the Jiangxi Province Nature Science Foundation (Project No.: Q959612, Q972026), the National Nature Science Foundation of China (Project No.: 50664004, 50574069) and the National Science Foundation for Distinguished Youth Scholar (59725408). The authors express their profound gratitude to the editor and reviewers of Hydrometallurgy for corrections and suggestions. References Ashraf, M., Zafar, Z.I., Ansari, T.M., 2005. Selective leaching kinetics and upgrading of low-grade calcareous phosphate rock in succinic acid. Hydrometallurgy 80 (4), 286–292. Aydogan, S., Ucar, G., Canbazoglu, M., 2006. Dissolution kinetics of chalcopyrite in acidic potassium dichromate solution. Hydrometallurgy 81 (1), 45–51.
Chen, J., Yang, S., Ke, J., 1998. The Studies and Progress of Metallurgy. China Metallurgy industry press, Beijing. Chi, R., Tian, J., 2008. Weathered Crust Elution-Deposited Rare Earth Ores. Nova Science Publishers, Inc., New York. Chi, R., et al., 2002. Kinetics of manganese reduction leaching from weathered rareearth mud with sodium sulfite. Metallurgical and Materials Transactions. B, Process Metallurgy and Materials Processing Science 33 (1), 41–46. Chi, R., et al., 2005. Existing State and Partitioning of Rare Earth on Weathered Ores. Journal of Rare Earths 23 (6), 756–759. Chi, R., et al., 2006. Kinetics of rare earth leaching from a mangnese-removed weathered rare-earth mud in hydrochloric acid solutions. Separation Science and Technology 41 (6), 1099–1113. Habashi, F., 1980. Principles of Extractive Metallurgy. : General Principles, vol. 1. Gordon and Breach, New York. He, L., Feng, T., Fu, S., 1983. Study on process of extraction of rare earths from the ionadsorption type rare earth ore ore by (NH4)2SO4 leaching. Chinese Rare Earths 4 (3), 1–5. Huang, X., et al., 2005. Development of rare earth hydrometallurgy technology in China. Journal of Rare Earths 23 (1), 1–5. Kul, M., Topkaya, Y., Karakaya, I., 2008. Rare earth double sulfates from preconcentrated bastnasite. Hydrometallurgy 93 (3–4), 129–135. Lasheen, T.A., Hazek, M.N.E., Helal, A.S., 2009. Kinetics of reductive leaching of manganese oxide ore with molasses in nitric acid solution. Hydrometallurgy 98 (314–317). Levenspiel, O., 1999. Chemical Reaction Engineering. John Wiley & Sons, Inc, New York. Núñez, C., Cruells, M., García-Soto, L., 1994a. A general shrinking-particle model for the chemical dissolution of all types of cylinders and discs. Hydrometallurgy 36 (3), 285–294. Núñez, C., Viñals, J., Roca, A., García-Soto, L., 1994b. A general shrinking-particle model for the chemical dissolution of crystalline forms. Hydrometallurgy 36 (1), 1–17. Tian, J., et al., 2001. Leaching hydrodynamics of weathered elution-deposited rare earth ore. Transactions of Nonferrous Metals Society of China 11 (3), 434–437. Tian, J., Chi, R., Yin, J., 2009. Process of Leaching Rare Earths from the Weatherd Crust Elution-deposited Rare Earth Ore. 2009's Fifth International Conference on Hydrometallurgy, Zhangjiajie, China, p. B-55. Vilímec, J., Jakubec, K., 1987. Direct spectrophotometric determination of rare earths in organic extracts with chlorophosphonazo III. Microchemical Journal 35 (3), 325–327. Wan, Y., Liu, C., 2005. Study on adsorption of rare earth elements by kaolinite. Journal of Rare Earths 23 (3), 377–381. Zhou, H., Zheng, S., Zhang, Y., Yi, D., 2005. A kinetic study of the leaching of a low-grade niobium–tantalum ore by concentrated KOH solution. Hydrometallurgy 80 (3), 170–178.
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / h y d r o m e t
Kinetics on leaching rare earth from the weathered crust elution-deposited rare earth ores with ammonium sulfate solution Tian Jun a,b,⁎, Yin Jingqun b, Chi Ruan a,c, Rao Guohua b, Jiang Mintao b, Ouyang Kexian b a b c
School of Resources Processing and Bioengineering, Central South University, Changsha, 410083, China Institute of Applied Chemistry, Jiangxi Academy of Science, Nanchang, 330029, China Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology,Wuhan, 430073, China
a r t i c l e
i n f o
Article history: Received 3 August 2009 Received in revised form 28 December 2009 Accepted 5 January 2010 Available online 11 January 2010 Keywords: Rare earth Leaching Kinetics Clays minerals
a b s t r a c t In this paper, the kinetic of leaching rare earth from the weathered crust elution-deposited rare earth ores with ammonium sulfate solution was investigated. The effects of the main leaching parameters such as temperature and ore particle size on leaching process were examined and discussed in order to elucidate the kinetics and mechanism of leaching rare earth. It was found that the higher the leaching temperature or the smaller the ore particle size, the faster the leaching progress. The leaching mechanism was analyzed with different kinetics models. The results show that the leaching process can be described by the shrinking-core model, the leaching rate was controlled by the inner diffusion of reactants and leaching products through mineral porous layer, the leaching process follows the kinetic model 1 − 2/3α − (1 − α)2/3 = kt, the apparent activation energy was 9.24 kJ/mol obtained from calculating the experimental data and the order of the ore particle size found to be approximately 2. An empirical equation relating the rare earth leached rate constant 9240 with ore particle size and leaching temperature was established as 1–2 = 3α–ð1–αÞ2 = 3 = 1:50⋅r0−2 ⋅e RT ⋅t: © 2010 Elsevier B.V. All rights reserved.
1. Introduction The weathered crust elution-deposited rare earth ore is a kind of rich rare earth clay minerals. It only existed and was discovered in China, mainly located in Jiangxi, Fujian, Hunan, Guangdong, Yunnan and Guangxi provinces. This rare earth deposits grades are low. It contains rich medium and heavy rare earth elements and is the main resources of mid and heavy rare earth in the world (Chi and Tian, 2008). The metallogenetic mechanism of this kind ore could be due to granodiorite and volcanic rocks were weathered in humid and warm climate, being transformed into clay minerals such as kaolinite, halloysite and montmorillonite. During the same progressive geological weathering leaching process, the rare earth minerals (e.g., bastnasite) were also weathered in original rocks, and the releasing rare earth ions formed rare earth aqua and rare earth hydroxyl ions adsorbed on clay minerals with migrating flow natural water(Wan and Liu, 2005). So the rare earth in the weathered crust elutiondeposited rare earth ore mainly exists with the ion-exchangeable phase adsorbed on clay minerals (Chi et al., 2005). This kind of aqueous and hydroxyl aqueous adsorption module has stable chemical properties, don't hydrolyze and revolute in neutral water. Conventional physical processing methods, such as flotation, magnetic, gravity and electrostatic separation were found to be ineffective ⁎ Corresponding author. Institute of Applied Chemistry, Jiangxi Academy of Science, Nanchang, 330029, China. E-mail address: [email protected] (T. Jun). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.01.001
in extraction rare earth from such sources, but can be leached by ionexchange method with cation based on the rare earth ions adsorbed characteristics (He et al., 1983). Since the weathered crust elution-deposited rare earth ores in China have been discovered at 1969, the scientists and engineers of China have been engaged in research and development of a series of hydrometallurgy for the special weathered crust elution-deposited rare earth ores in China (Huang et al., 2005). Although the three generation leaching process such as the first generation leaching process in pool with NaCl solution, the second generation leaching process in pool with (NH4)2SO4 solution and the third generation insitu leaching process have been developed along the green chemical leaching approach according to the ion-exchange property (Tian et al., 2009). However, many years industrial practice have shown that the leaching effect not only be controlled by the properties of the rare earth ore, the leaching reagent and its concentration, but also be influenced by the hydrodynamics, the kinetics and the mass transfer of the leaching process (Tian et al., 2001). So far there is little knowledge on the leaching kinetics of the weathered crust elutiondeposited rare earth ore. In recent years, with the application and development of the green chemistry of extraction process, especially popularization of the insitu leaching process, the high selectivity leaching, high performance and low consumption extraction technology becomes more and more important and stringent in industrial design (Chi et al., 2006, 2002; Lasheen et al., 2009). In order to know the mechanism of the rare earth leaching and choose the more suitable technology of extraction
T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
167
rare earth from this ore, it is necessary to investigate the leaching kinetics. It would be useful to provide a scientific approach to and a theoretic basis for leaching rare earth from this ore with high performance and low consumption, and be applied to optimize the rare earth extraction conditions, to take mathematics simulation on the leaching process and to improve the rare earth received rate in extraction process.
As shown in Table 3, the partitioning of the LN rare earth ore contains mainly heavy rare earth, and the yttrium element account for 68.94% and hardly any light rare earth elements (Kul et al., 2008). The ore sample was sieved using standard sieves, giving different particle size fractions for the kinetics experiments.
2. Experimental
The leaching kinetics experiments were carried out in a 500mL trimouth glass flask partially immersed in a heated water bath and temperature was controlled within ±1 °C, stirring of this mixture was achieved by using stirrer impeller driven by a variable speed motor. The tri-mouth glass flask was fitted with a thermometer, shaft stirrer and a reflux condenser(Ashraf et al., 2005; Núñez et al., 1994a,b). The leaching tests were conducted in an aforementioned reactor at constant temperature fixed anywhere in the range of room temperature to 95 °C. A total of 250 ml of known concentration ammonium sulfate solution was taken in the reactor and preheated to desired leach temperature, then a known weight (50 g) and particle size range of sample portions of ore was placed into the reactor, and rapidly stirred for various leaching time. After completion of each given time experiment, from which the samples were collected by a measuring pipe, and rapidly transferred to a measuring flask to dilute 100 time with deionized distilled water and cool down to room temperature, then this slurry solution was filtered and the filtrate was analyzed the rare earth concentration by EDTA titration methods. From the analysis results the rare earth leached fraction (α) can calculated according to the following equation:
2.1. The characterization and composition of experimental rare earth ore The experimental weathered crust elution-deposited rare earth ores sample is original rare earth ores collected from Zudong Rare Earth Mine area located in the Longnan County (LN), Jiangxi province, China, which mainly contains heavy rare earth. The main chemical composition of ore sample is listed in Table 1. Table 1 shows that the LN rare earth ore contains only 0.102% rare earth. The LN rare earth deposits grades are low. It indicated that extraction rare earth from this ore is difficult. The existing state of rare earth in the LN rare earth ore was determined with the sequential fractionating extraction methods(Chi et al., 2005), It could be divided into the following four forms: (a) aqueous soluble phase rare earth, the aqueous soluble rare earth and rare earth hydroxyl aqueous ions which were free or individual form without being adsorbed by clay minerals during weathering and weathering-leaching processes; (b) ion-exchangeable phase rare earth, the rare earth exist as rare earth ions adsorbed by clay minerals with aqueous soluble rare earth and rare earth hydroxyl cation states; (c) colloid sediment phase rare earth, the rare earth exist as insoluble rare earth oxides or hydroxyl compounds bonding with, or depositing on oxide minerals; (d) mineral phase rare earth, which includes rare earth mineral crystals of ionic compounds as well as diffusion of rare earth ions to displace mineral crystals in the ores. The results are shown in Table 2. The rare earth exists mainly with ion-exchangeable phase, which account for 80.62% from Table 2, and less than 20% mineral phase and colloid sediment phase, and hardly any aqueous soluble phase. The partitioning of the ion-exchangeable phase from the LN rare earth ore has been determined by Jiangxi Analytical and Testing Center, Nanchang, China with ICP-MS (Inductively Coupled Plasma Mass Spectrometry, SCIEX ELAN 9000 type), the result shows analysis as Table 3. Table 1 The main chemical composition of LN rare earth ore sample (mass fraction %). Element
RE
Al
Fe
Mn
Ca
Mg
SiO2
Loss
Original rare earth ore
0.102
14.66
3.24
0.04
0.32
0.57
60.4
20.37
Table 2 Existing state of rare earth in the LN rare earth ore (mass fraction %). Phase state
Aqueous soluble Ion-exchangeable Colloidal sediment Mineral
Phase 6.15 × 10−3 distribution
80.62
5.30
13.36
Table 3 The partitioning of the LN rare earth ore (mass fraction %). Element
La2O3
CeO2
Pr6O11
Nd2O3
Sm2O3
Eu2O3
Gd2O3
Tb4O7
Content (%)
1.56
0.085
0.65
3.20
2.42
0.50
4.48
0.95
Element
Dy2O3
Ho2O3
Er2O3
Tm2O3
Yb2O3
Lu2O3
Y2O3
Content (%)
6.90
1.56
4.33
0.55
3.48
0.50
68.94
(continued on next page)
2.2. Apparatus and experimental procedure
α=
εt ε0
ð5Þ
where: ε1 is total amount of leaching out rare earth from reaction time t = 0 to reaction time t = t, ε0 is total amount of rare earth present in the original ore sample. 2.3. Analytic methods The rare earth partitioning has been determined by Jiangxi Analytical and Testing Center, Nanchang, China with ICP-MS (Inductively Coupled Plasma Mass Spectrometry, SCIEX ELAN 9000 type) by acidification of the ore samples after being fused with sodium peroxide and sodium hydroxide. The rare earth content of minerals phase and the total rare earth content of the ore has been determined by arsenazo (III) spectrophosphorimetry by acidifying and purifying the ore sample after being fused with sodium peroxide and sodium hydroxide (Vilímec and Jakubec, 1987). The RE3+ contents in leach liquor were determined by titration with EDTA using xylenol orange as indicator and hexamethylene tetramine as buffer. The rare earth content of ion-exchangeable phase analytical methods: 500 g ore sample was taken in a column of 50 mm in diameter and leached with 2% (NH4)2SO4 solution. The leach liquor was collected together and been determined with the same as the rare earth ion contents. The rare earth content of colloidal sediment phase analytical methods: 500 g ore sample was leached with 2.5 L 0.5 mol/L NH2OH. HCl + 2.0 mol/LHCl as extraction reagent for 1 h under stirring. Leaching product was filtered, cross-flowed 5 times at the same condition, The leach liquor was collected together and been determined with the same as the rare earth content of minerals phase (Chi et al., 2005). Deionized distilled water and reagent grade chemicals were used in all the experiments.
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T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
3. Results and discussion 3.1. Effect of leaching temperature on rare earth leaching The leaching reactions of the LN rare earth ore with ammonium sulfate are as follows (Chi and Tian, 2008): −
þ
−
fA14 ½Si4 O10 ðOHÞ8 gm ·nREðsÞ þ3nNH4ðaqÞ ⇌fA14 ½Si4 O10 ðOHÞ8 gm þ
3+
3+
×3nNH4ðsÞ þnREðaqÞ where: s and aq is solid phase and aqueous phase respectively. It shows that the leaching process is a typical noncatalytic heterogeneous reactions in solid–liquid systems (Levenspiel, 1999).To investigate the leaching process and mechanism of rare earth from LN rare earth ore with ammonium sulfate solution, the effect of temperature on the leaching rate was examined in the range of 25– 95 °C with 2.0% (W/V) concentration ammonium sulfate solution for different leaching times under the conditions of 2.0 mm average particle size and 1:5 solid: liquid ratio. The results about effect of leaching temperature on rare earth leaching are shown in Fig. 1. As seen in Fig. 1, the leaching fraction (α) of rare earth increases with increasing leaching temperature and leaching time prolonging in the initial stage. It indicates that rare earth leaching behaviour is controlled kinetics. After 8 min, the leaching rate increases slowly, suggesting that the leaching process approaches to an equilibrium. 3.2. Leaching kinetics equation If the rare earth ore particles were considered as spherical particles, the leaching process can be described with the shrinking core model and can be divided into four models (Chen et al., 1998): (1) Chemical reaction controls: 1−ð1−αÞ
1=3
= k1 t
(2) Outer diffusion controls: 1=3 = k2 t 1−ð1−αÞ
ð1Þ
where: k1, k2, k3 and k4 are chemical constants for different control step, respectively; α is the rare earth leached fraction; t is real leaching time (min.); C0 is the initial concentration of leaching reagent; r0 is the initial radius of the ore sample particle; p is molar density of the sample particle; M is the mass of the sample particle. The kinetics type can be determined by treating of the experimental data. For the kinetics type of outer diffusion or chemical reaction controlling, the plot of 1 − (1 − α)1/3 vs. time (t) is a straight line which passes 0 point, or the kinetics type of inner diffusion controlling, the plot of 1 − 2/3α − (1 − α)2/3 vs. time (t) is a straight line which passes 0 point, or the kinetics type of mixed controlling, the plot of 1 − (1 − α)1/3 vs. time (t) is not a straight line which passes 0 point, neither is the plot of 1 − (1 − α)1/3 vs. time (t) (Levenspiel, 1999). The kinetics equation is obtained by trying trial or error method from the experiment data in Fig. 1 (Habashi, 1980). Aforementioned Eqs. (1), (2) and (4) were examined but did not fit the data well and only the Eq. (3) were examined fit the data best. A series of straight linear relationship with a correlation coefficient R2 = 0.9997 can be obtained with plotting 1 − 2/3α − (1 − α)2/3 vs. t as shown in Fig. 2. Therefore the kinetics equation can be expressed as:
1−2 = 3α−ð1−αÞ
2=3
= k3 t
ð5Þ
where k3 is the linear rate constant (min−1). Its values are 0.01154(20 °C), 0.01484(45 °C), 0.01850(70 °C) and 0.02625(95 °C) respectively. It is observed from the plots that the experimental data fit well into the shrinking core model. This suggests that the leaching rate is controlled by inner diffusion of reactants and leaching products through mineral porous layer during the kinetics district (Zhou et al., 2005). Experimental data fit well into the shrinking core model approach in which a variable activation energy term was introduced. Kinetic analysis of the experimental results reveals that the associated energy and the rate-controlling mechanism are dependent upon both ore sample property as well as the leaching reaction temperature.
ð2Þ 3.3. Leaching apparent activation energy
(3) Inner diffusion controls: 2 2=3 = k3 t 1− α−ð1−αÞ 3
ð3Þ
(4) Mixed controls: 1−ð1−αÞ
1=3
= k4
C0 M t r0 p
Fig. 1. Rare earth leaching fraction (α) under different temperature.
ð4Þ
The K3 values of leaching process at different temperature were calculated from the slopes of the straight lines given in Fig. 2, the Arrhenius diagram (Plots of lnk3 vs 1/T) for the four temperatures for the leaching rare earth from LN rare earth ore with 2.0% (W/V) concentration (NH4)2SO4 solution is linear as shown in Fig. 3.
Fig. 2. Plots of 1 − 2/3α − (1 − α)2/3 vs. time for different temperatures.
T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
Fig. 3. Arrhenius plot of leaching rare earth from LN the rare earth ore.
Fig. 5. Plots of 1 − 2/3a − (1 − a)2/3 vs. time for different particle size.
According to Arrhenius equation, this is represented as: E −RT
k3 = Ae
169
ð6Þ
where k3 is the rate constant (min−1), A is the preexponential factor, R is the universal gas constant(J·mol−1·k−1), T is leaching reaction absolute temperature (Kelvin, K), and E is activation energy (kJ/mol). The apparent activation energy of the leaching process was determined from the slope of the straight line in Fig. 3 to be 9.24 kJ/mol. It also proves that leaching process is controlled by inner diffusion as its activation energy is between 4 kJ/mol to 12 kJ/mol (Aydogan et al., 2006). 3.4. Effect of particle size of LN rare earth ore on rare earth leaching In order to further confirm that the leaching process is inner diffusion controlled and establish mechanism of leaching rare earth, The LN rare earth ore sample was sieved into four different particle size fractions (1.8–2.1, 2.1–2.8, 2.8–4.0 and 4.0–5.2 mm). Then each size fraction was leached at 25 °C with 2.0% (W/V) concentration ammonium sulfate solution for different leaching times respectively. A number of curve about the effect of particle size of LN rare earth ore on rare earth leaching is shown in Fig. 4. The leaching rate increases with the decreasing of particle size of the LN rare earth ore as shown in Fig. 4. It further proves that the leaching process is controlled by inner diffusion. From the experimental data in Fig. 4, a set of straight line relationship can also be obtained by plotting 1 − 2/3α − (1 − α)2/3 vs. t as shown in Fig. 5. It is evident that for the entire size fraction
Fig. 4. Rare earth leaching fraction (α) under different particle size of ore sample.
examined the rare earth leaching process can be well described with inner diffusion control model. The Arrhenius equation can also be expanded and represented as follows:
E n −RT
ð7Þ
k3 = A′ r0 e
In this equation, r0 is the radius of ore particle, n is the order of the ore particle size. The K3 values of leaching process at different particle size of LN rare earth ore with 2.0% (W/V) concentration ammonium sulfate solution were calculated from these slopes of the straight lines give in Fig. 5. It is an interesting and intriguing result that a straight line can be obtained with plotting the lnK values vs. lnr0, as shown in Fig. 6. the linear relationship between the rate constant and the inverse particle size also support the fact that leaching rate is controlled by the inner diffusion. From the line relationship between lnK and lnr0 potted in Fig. 6, the n and A′ value can be calculated from the slope and intercept of the straight line in Fig. 6, which are −1.89 (n ≈ −2) and 1.50 respectively. Then the k3 values can be obtained as the following equation[19–22]:
−2
9240
k3 = 1:50⋅r0 ⋅e RT
Fig. 6. The relationship between lnK and lnr0 plot at 25 °C.
ð8Þ
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T. Jun et al. / Hydrometallurgy 101 (2010) 166–170
The empirical equation of the leaching kinetics is established as: 1–2 = 3α–ð1–αÞ
2=3
−2
9240
= 1:50⋅r0 ⋅e RT ⋅t:
ð9Þ
4. Conclusions The rare earth leaching rate increase with increasing leaching temperature and time, during the initial stages of leaching rate was controlled by kinetics. The kinetics tests about the factors of temperature and particle size of the rare earth ore on leaching ratio show that the leaching process followed the shrinking core model. The leaching kinetics is controlled by diffusion of porous solid layer. The apparent activation energy was 9.24 kJ/mol, and the order of the ore particle size found to be approximately 2. Then empirical equation of the leaching 9240 kinetics is established as: 1–2 = 3α–ð1–αÞ2 = 3 = 1:50⋅r0−2 ⋅e RT ⋅t Acknowledgments The authors gratefully acknowledge the financial support of the Jiangxi Province Nature Science Foundation (Project No.: Q959612, Q972026), the National Nature Science Foundation of China (Project No.: 50664004, 50574069) and the National Science Foundation for Distinguished Youth Scholar (59725408). The authors express their profound gratitude to the editor and reviewers of Hydrometallurgy for corrections and suggestions. References Ashraf, M., Zafar, Z.I., Ansari, T.M., 2005. Selective leaching kinetics and upgrading of low-grade calcareous phosphate rock in succinic acid. Hydrometallurgy 80 (4), 286–292. Aydogan, S., Ucar, G., Canbazoglu, M., 2006. Dissolution kinetics of chalcopyrite in acidic potassium dichromate solution. Hydrometallurgy 81 (1), 45–51.
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