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Effect of super gravity on successive precipitation and separation behaviors of rare earths in multi-components rare-earth system
Separation and Purification Technology 228 (2019) 115752
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Effect of super gravity on successive precipitation and separation behaviors of rare earths in multi-components rare-earth system
T
⁎
Xi Lan, Jintao Gao , Yu Du, Zhancheng Guo State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Super gravity Successive precipitation Successive separation REEs Multi-components Re-system
Re-concentrate is a complex multi-components Re-system consisting of Ce, La, Pr and Nd, which brings great difficulties for investigating the transformation and separation behaviors of REEs. The effect of super-gravity field on the mass transfer and phase separation behaviors of REEs was investigated in this paper, and the phenomenon of successive precipitation and separation of REEs was discovered. The Re3+ was firstly precipitated into Re-oxyfluoride phase with OF3-, which overcame the interface tension to be evidently separated into the bottom layer under the action of super gravity. Subsequently, the Re3+ were successively precipitated with FeO33-, SiO44- and PO43- into Re-ferrate phase and britholite phase, which were separated further into the middle and top layers along the super gravity direction. Moreover, various REEs were efficiently recovered into the different Re-phases, respectively. The theoretical and experimental results verified the significant enhancement of super gravity on the mass transfer and phase separation of REEs, and its efficient application in REEs recovery from multi-component Re-system.
1. Introduction Mass transfer and phase separation are usually proven to be the main restrictive links in the physical chemical processes, especially in the case of high-temperature [1]. Phase separation is a key step in many technologies, such as the mineral processing application [2], impurity removal and purification [3], resource recovery [4] and so on. Chen reported that it was difficult to achieve the interphase dissociation of fine mineral particles in the pulp resulting from the insufficient driving force compared to the greater interfacial resistance. [2] Lee found that the Al2O3 inclusions were inefficient to be removed from the metal due to its fine size and dispersed distribution in the molten metal. [3] Zhang proposed that the valuable elements were impossible to be recovered effectively from the slags and sludges at high temperatures because of the high viscosity of the slag melt. [4] Therefore, several intensifying methods such as microwave [5], electromagnetic [6] and gas agitation [7] have been proposed to be applied for enhancing the phase separation in the physical chemical processes. Compared to above strengthening methods, super gravity method is a more effective intensification technology, which has the characteristics of greatly enhancing the relative motion between various phases with different density [8]. In the early days, super gravity was mainly applied in the phase separation process of chemical engineering and
⁎
pharmacy at low temperatures [9,10]. Recently, the super gravity has been proposed to be used in some fields of high-temperature, such as preparing materials with special functions [11], improving cleanliness of the steel [12], and recovering valuable elements from slags [13,14], which reflects its broad adaptability in industry. Rare earth elements (REEs) are critical materials for wide-ranging applications in space and green technologies [15,16], renewable energy [17], metallurgy [18], agriculture [19], and so forth. The largest rareearth (Re) deposit worldwide is located in Bayan Obo, Mongolia, China [20]. Over the years, the Bayan Obo ore has been mainly exploited as iron ore, and the Re-resources have not been effectively utilized, most of that were converted into the Re-concentrates by means of mineral processing [21]. However, it is difficult to efficiently separate the massive REEs from the complex multi-component Re-system. Moreover, the mass transfer and phase separation behaviors of REEs in this multicomponent Re-system were difficult to be investigated because of the serious lack of thermodynamic data for REEs. This paper introduces the super-gravity field into the Re-concentrate, to investigate the effect of super gravity on the precipitation and separation behaviors of REEs in this complex multi-component Resystem. Moreover, the enhancement of super gravity on mass transfer and phase separation of REEs, and its efficient application in REEs recovery from the multi-component Re-system was verified through
Corresponding author. E-mail address: [email protected] (J. Gao).
https://doi.org/10.1016/j.seppur.2019.115752 Received 15 April 2019; Received in revised form 15 June 2019; Accepted 28 June 2019 Available online 29 June 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 228 (2019) 115752
X. Lan, et al.
Fig. 1. Sketch of the experimental installation: (1) heating furnace, (2) resistance wire, (3) thermocouple, (4) magnesia crucible, (5) conductive slip ring, (6) centrifugal axis, (7) counterweight, (8) temperature controller, (9) rotation rate controller.
Fig. 1, and the specific experimental procedures are as follows. Firstly, each 20 g of the Re-concentrate was filled into a magnesia crucible, which was melted completely at 1773 K for 30 mins in the constant temperature zone of the heating furnace of centrifugal device under an argon atmosphere. Subsequently, the centrifugal device was started at a gravity coefficient of G = 800 corresponding to the angular velocity of 1691 r/min, and the crucible was continuously cooled from 1773 K to 1373 K where the Re-concentrate fully solidified, with various cooling rates of 2 K/min, 5 K/min or 10 K/min, respectively. After the temperature decreasing to 1373 K, the rotation function of the centrifuge was turned off, and the water-quenching was performed immediately when the crucibles were taken out. Simultaneously, the parallel experiment was carried out under the condition of normal gravity through continuous cooling from 1773 K to 1373 K with 2 K/min. All of the samples with various cooling rates were divided into two halves on average along the longitudinal center line and observed using a Leica optical microscope (DM4M) to characterize the multilayer structure in the entire sample. Subsequently, the multilayer samples were crossly divided into three parts along the two interfaces and analyzed further by SEM-EDS, XRD and Raman spectrometer, to investigate the micro structures, mineral structures and mineral compositions of the various layers, respectively. Furthermore, the distribution of REEs in different layers was characterized by the electron probe microanalyzer (EPMA). The mass fractions of Ce2O3, La2O3, Pr6O11 and Nd2O3 in the different layers were detected with the XRF and ICP methods, and the recovery percentages of REEs in the different layers were acquired according to Eq. (1).
theoretical and experimental results of XRD, SEM-EDS, XRF, EPMA, and Raman spectroscopy, respectively. 2. Experimental 2.1. Instrumentation The experiments were carried out in a centrifugal device incorporating a heating function, that is illustrated in Fig. 1, for investigating the effect of super-gravity field on successive precipitation and separation behaviors of various REEs. The centrifugal device is mainly composed of a heating furnace and a counterweight, which rotates symmetrically from the vertical to the horizontal when the centrifugal device starts to operate. The experimental temperature is controlled using a type B thermocouple closed to the sample. A conductive slip ring provides feedback information from the thermocouple, real-time controlling the temperature of the sample rotating with high speed, within the observed accuracy range of ± 3 K. Different rotation rates are controlled by a variable speed motor, and the various angular velocities correspond to different gravity coefficients according to the Eq. (1). 2.2. Materials The Re-concentrate employed in this research was obtained through multi-stage beneficiation of the Bayan Obo ore in Inner Mongolia, China [22]. Its chemical compositions that detected by the XRF combined with ICP-AES methods are shown in Table 1. It was found that the Re-concentrate is a multi-component Re-system which consists of more than 13 species elements including four REEs of Ce, La, Pr and Nd, and the total mass fraction of REEs is up to 44.09 wt%. Moreover, there are more than 20 kinds of minerals with various elements composition, which are tightly integrated with each other and dispersed in the Reconcentrate, as the mineral phase dissociation analyzer (MLA) images shown in Fig. 2.
RRe =
mT × ωRe − T mT × ωRe − T + mM × ωRe − M + mB × ωRe − B
(1)
where, RRe is the recovery percentage of Ce, La, Pr, Nd in different layers (%); mT, mM, and mB are the mass of the top, middle, and bottom layers (kg); ωRe-T, ωRe-M, and ωRe-B are the mass fractions of Ce2O3, La2O3, Pr6O11, Nd2O3 in the top, middle, and bottom layers, respectively (wt%).
2.3. Experimental procedure The experiments were carried out in the centrifugal device shown in Table 1 Chemical compositions (wt%) of Bayan Obo Re-concentrate. Composition
CaO
P2O5
Fe2O3
SO3
F
SiO2
MgO
BaO
MnO
Ce2O3
La2O3
Pr6O11
Nd2O3
Content
14.34
9.63
8.09
4.93
8.41
1.09
3.46
1.59
0.37
26.42
11.84
2.99
2.84
2
Separation and Purification Technology 228 (2019) 115752
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Fig. 2. MLA images of Bayan Obo Re-concentrate.
3. Results and discussion
of REEs with decreasing temperature under the normal gravity, as shown in Fig. 3(a). By contrast, a multilayer sturcture consisting of three layers with two explicit interfaces appeared significantly in the samples that attained under super gravity, as presented in Fig. 3(b) to (d). Moreover, the three layers (bottom, middle and top) were presented as the obvious diverse macroscopic structures along the direction of super gravity.
3.1. Effects of super gravity on precipitation and separation behaviors of Rephases 3.1.1. Optical micrography analysis The Leica optical microscope was used to characterize accurately the multilayer structure in the samples attained under super gravity and that of the parallel sample. The overall optical micrographs montaged by over 20 images (magnification 5 × 10) of the samples attained at the various cooling rates of 10, 5 and 2 K/min are shown in Fig. 3. Apparently, various mineral phases were discretely distributed in the entire sample, and it was difficult to ascertain the precipitation behaviors
3.1.2. XRD analysis The mineral structures in different layers attained under super gravity and that of the parallel sample were characterized by XRD. As the XRD pattern of the sample with G = 1 shown in Fig. 4(a), the different Re-phases that precipitated from the molten Re-concentrate were
Fig. 3. Overall optical micrographs of the samples obtained under super gravity compared with normal gravity with various cooling rates: (a) G = 1, 2 K/min; (b), (c) and (d) G = 800, 2 K/min, 5 K/min and 10 K/min, respectively. 3
Separation and Purification Technology 228 (2019) 115752
X. Lan, et al.
Fig. 4. Comparation of XRD patterns for the samples obtained under super gravity and normal gravity with the cooling rate of 2 K/min: (a) G = 1, (b) G = 800.
equiaxed crystals, which were agglomerated into the dense structure under the action of super gravity. Subsequently, the Re3+ was precipitated further with FeO33- into the Re-ferrate with continuous decreasing of temperature, which were presented as the cube crystals and be separated to the middle layer under the action of super gravity. Finally, the Re3+ was precipitated into the britholite with SiO44- and PO43-, which showed a dendritic structure and stayed in the top layer. Comparing the sizes of Re-phases attained under super gravity and normal gravity, it was evident that the Re-phases were significantly grown into the larger crystals under the action of super gravity, as shown in Figs. 5 and 6. Moreover, compared with the crystal sizes of the different Re-phases attained under super gravity with various cooling rates as revealed in Fig. 6, the sizes of Re-oxyfluoride, Re-ferrate and britholite were increased from 2 μm, 100 μm and 200 μm to 5 μm, 400 μm and 1000 μm, respectively. It was indicated that the effect of super gravity on nucleation and growth of the three Re-crystals was improved significantly with the decreasing of cooling rates.
mixed together in the entire sample attained under normal gravity. On the contrary, each layer attained under super gravity was only composed of a single Re-phase, the top layer only consisted of britholite phase, the middle layer was the only Re-ferrate phase, while the bottom layer was the single Re-oxyfluoride phase, respectively, as presented in Fig. 4(b). 3.1.3. SEM-EDS analysis The microstructures and element composition of Re-phases in different layers attained under super gravity and that of normal gravity were determined further by SEM-EDS. From the SEM-EDS images of the sample with G = 1 as shown in Fig. 5, the various Re-phases were discretely distributed among the entire slag, it was indicated that the difference in density between them produced the insufficient driving force to drive any precipitated Re-phase moved and separated from each other in the molten Re-concentrate via free sedimentation. By contrast, the Re-phases were successively precipitated and separated into different layers under super gravity, as presented in Fig. 6. As accordingly verified, the Re3+ was firstly precipitated into the Reoxyfluoride with OF3- from the molten Re-concentrate. The driving force generated through super gravity was sufficient to overcome the solid-liquid interface tension and drive the Re-oxyfluoride evidently migrated and separated to bottom layer along the direction of supergravity. Moreover, the Re-oxyfluoride appeared as the obvious
3.1.4. Raman analysis The mineral compositions in the different layers obtained under super gravity were collected further using Raman spectrometer. Through comparing the exhibit several vibrational features in the different layers of the sample as shown in Fig. 7, the bands of britholite (440 cm−1, 856 cm−1, 959 cm−1), the Re-ferrate (674 cm−1, 1359 cm−1), and the Re-oxyfluoride (445 cm−1, 562 cm−1) were appeared on the upper, middle and bottom layers, respectively, which confirmed further the significant distinct in the mineral compositions of the separated Re-phases under the action of super gravity. 3.2. Theoretical verification and discussion The separation behavior of the Re-phases that precipitated from Reconcentrate melt under super gravity was theoretically verified further. Considering that the precipitated Re-particles in the melt are generally subjected to three lateral forces in the condition of super-gravity, including the centrifugal force far away from the centrifugal axis Fc (Eq. (2)), the frictional resistance Ff (Eq. (3)), and the buoyancy caused by centrifugation FB (Eq. (4)). Hence, the theoretical sedimentary time for Re-particles with different sizes in the melt under super gravity compared with that of normal gravity were conducted based on the theory of stratification and sedimentation [23].
Fc = m
Fig. 5. SEM-EDS images of the sample obtained under normal gravity with the cooling rate of 2 K/min: (a) SEM, (b), (c) and (d) EDS of EDS [Ce,La,Pr,Nd]FeO3 and Ca3[Ce,La,Nd,Pr]2[(Si,P)O4]3F, [Ce,La,Pr,Nd]3O4F3, respectively.
Ff = f 4
ν2 = mω2x = ρVω2x x
dx dt
(2) (3)
Separation and Purification Technology 228 (2019) 115752
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Fig. 6. SEM-EDS images of different layers in the samples obtained under super gravity with various cooling rates: Pt.1-Ca3[Ce,La,Nd,Pr]2[(Si,P)O4]3F, Pt.2[Ce,La,Pr,Nd]FeO3, Pt.3-[Ce,La,Pr,Nd]3O4F3.
sedimentation time τ /s
674
Intensity (a. u.)
856
440
959
Top layer
440
1359
Middle layer
6
10
5
10
4
10
3
10
2
10
1
10
0
562
Bottom layer
10
10 200
400
600
800
1000
1200
1400
d =10 ȝm d =50 ȝm d =100 ȝm d =200 ȝm d =500 ȝm
-1
1600
1
200
Raman shift (cm )
m ρ ω2x ρ 0
(4)
G=
When Fc = FB + Ff, the acceleration of the particle is zero, and the particle is shown to be an isokinetic motion, and the isokinetic velocity is as Eq. (5).
τ=
dx = dt
(
m 1− f
ρ0 ρ
)ω x
800
g 2 + (ω2x )2 g L dx dt
=
g2 + ( =
9ηL × 10−3 2g (ρ − ρ0 ) d 2G
g
N2π 2x 2 ) 900
(7)
(8)
where, m is the mass of Re-particles (kg), ν is the linear velocity (m/s), x is the distance from centrifugal axis to the center of sample (m), ω is the angular velocity (rad/s), ρ and ρ0 are density of the particles and melt (g/cm3), V is the volume of particles (m3), f is the friction coefficient, and dx is the sedimentation velocity (m/s), η is viscosity of the melt dt (Pa·s), L is the active length (m), G is gravity coefficient, g is the normal gravity acceleration (g = 9.8 m/s2), and d is equivalent spherical diameter of the particles (m).
2
(5)
According to the Stokes formula [24], the friction coefficient of the spheroidal particles f is as Eq. (6), combined with the gravity coefficient as calculated according to Eq. (7), the complete sedimentation time τ could be obtained as Eq. (8).
f = 6πηx
600
Fig. 8. Theoretical sedimentation time τ for Re-particles with different sizes in the Re-concentrate melt as a function of gravity coefficient.
Fig. 7. Raman spectra of different layers in the sample obtained under super gravity with G = 800 and 2 K/min.
FB =
400
Gravity coefficient G
-1
(6) 5
Separation and Purification Technology 228 (2019) 115752
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Fig. 9. Diagram of successive precipitation and separation process of different Re-phases in Re-concentrate melt under super gravity: (a) normal gravity, (b) to (d) super gravity.
Fig. 10. EPMA results of REEs contents in the various Re-phases attained under super gravity with G = 800 and 2 K/min.
Table 2 Mass fractions (wt%) and recovery percentages (%) of REEs in the various Re-phases attained under super gravity with G = 800 and 2 K/min. Layers
Top Middle Bottom
Mass fractions (wt%)
Recovery percentages (%)
∑ReO
Ce2O3
La2O3
Pr6O11
Nd2O3
RCe
RLa
RPr
RNd
2.92 52.41 80.14
1.07 3.18 71.87
0.66 39.76 1.05
0.50 4.80 3.87
0.69 4.67 3.34
0.95 2.83 96.21
0.16 94.68 3.75
8.10 48.38 36.52
4.39 60.31 45.30
The theoretical sedimentation time τ for Re-particles with different sizes in the Re-concentrate melt, using the experimental dimensional parameters and experimental measurement results (a density difference of 5 g/cm3, a sample active length of 3 cm, and a melt viscosity of 0.1 Pa·s), as a function of gravity coefficient G and particle diameters d are shown in Fig. 8. Compared with the theoretical sedimentation time for Re-particles with the minimum size of 10 μm is 276000 s in a normal gravity, which decreases significantly to 345 s when the gravity coefficient increased to G = 800. The theoretical results verified further the significant enhancement of super gravity on the motion and separation behavior of Re-phases in the melt. Accordingly, the successive precipitation and separation behaviors of various REEs in the condition of super gravity were described as Fig. 9. In a normal-gravity field, the various Re-phases that precipitated
from the melt were difficult to move and aggregate via the free sedimentation, which were dispersed among the whole slag melt, as presented in Fig. 9(a). Instead, the driving force Δρg was enhanced significantly in the case of a super-gravity field. As a result, the various Rephases that precipitated successively from the Re-concentrate melt with continuous decreasing temperature, were separated successively into the different layers under the action of super gravity, as shown in Fig. 9(b) to (d). In addition, the super gravity significantly enhanced the sizes and amounts of Re-crystals, that is because the interfacial tension between the Re-crystals and the slag melt is easily broken under the action of supergravity, so the agglomeration and growth is more likely to occur for the Re-crystals, and the directional migration of the Recrystals creates a better dynamic condition for the precipitation of the remaining REEs in the slag melt. 6
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3.3. Effects of super gravity on recovery behaviors of REEs
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The EPMA results (Ce, La, Pr and Nd) of the different Re-phases in the multilayer sample attained under super gravity are shown in Fig. 10. Considering the mass fractions and the recovery percentages of REEs in each Re-phase as listed in Table 2, the different REEs were recovered into the various Re- phases from the Re-concentrate as driven by super gravity, respectively. The cerium element was precipitated and recovered efficiently into the Re-oxyfluoride phase with a high recovery percentage of 96.21%, and the mass fraction of Ce2O3 in which was up to 71.87 wt%. The lanthanum element was recovered further into the Re-ferrate phase with a high recovery percentage of 94.68% and a mass fraction of 39.76 wt% for La2O3. In addition, some remaining REEs were recovered further as the britholite phase with a ∑ReO content of 2.92 wt%. 4. Conclusions In the present work, the effect of super gravity on the precipitation and separation behaviors of various REEs in the Re-concentrate was investigated. (1) The successive precipitation and separation phenomenon of REEs was discovered, and the multilayer sturcture with two explicit interfaces was obtained under super gravity. (2) The Re3+ were successively precipitated into Re-oxyfluoride phase, Re-ferrate phase, and britholite phase with OF3−, FeO33−, SiO44− and PO43−, which were successively separated to the bottom, middle, and top layers as driven by super gravity. (3) Various REEs were efficiently recovered into the different Rephases from the multi-component Re-system. Acknowledgements This study is supported by the National Natural Science Foundations of China (No. 51774037 and No. 51404025). References [1] H.X. Fu, S.T. Yang, Z.L. Xiu, Phase separation in a salting-out extraction system of ethanol–ammonium sulfate, Sep. Purif. Technol. 148 (2015) 32–37. [2] H.T. Chen, S.A. Ravishankar, R.S. Farinato, Rational polymer design for solid–liquid separations in mineral processing applications, Int. J. Miner. Process. 72 (2003) 75–86. [3] S.H. Lee, C. Tse, K.W. Yi, P. Misra, V. Chevrier, C. Orrling, S. Sridhar, A.W. Cram, Separation and dissolution of Al2O3 inclusions at slag/metal interfaces, J. NonCryst. Solids 282 (2001) 41–48.
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Effect of super gravity on successive precipitation and separation behaviors of rare earths in multi-components rare-earth system
T
⁎
Xi Lan, Jintao Gao , Yu Du, Zhancheng Guo State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Super gravity Successive precipitation Successive separation REEs Multi-components Re-system
Re-concentrate is a complex multi-components Re-system consisting of Ce, La, Pr and Nd, which brings great difficulties for investigating the transformation and separation behaviors of REEs. The effect of super-gravity field on the mass transfer and phase separation behaviors of REEs was investigated in this paper, and the phenomenon of successive precipitation and separation of REEs was discovered. The Re3+ was firstly precipitated into Re-oxyfluoride phase with OF3-, which overcame the interface tension to be evidently separated into the bottom layer under the action of super gravity. Subsequently, the Re3+ were successively precipitated with FeO33-, SiO44- and PO43- into Re-ferrate phase and britholite phase, which were separated further into the middle and top layers along the super gravity direction. Moreover, various REEs were efficiently recovered into the different Re-phases, respectively. The theoretical and experimental results verified the significant enhancement of super gravity on the mass transfer and phase separation of REEs, and its efficient application in REEs recovery from multi-component Re-system.
1. Introduction Mass transfer and phase separation are usually proven to be the main restrictive links in the physical chemical processes, especially in the case of high-temperature [1]. Phase separation is a key step in many technologies, such as the mineral processing application [2], impurity removal and purification [3], resource recovery [4] and so on. Chen reported that it was difficult to achieve the interphase dissociation of fine mineral particles in the pulp resulting from the insufficient driving force compared to the greater interfacial resistance. [2] Lee found that the Al2O3 inclusions were inefficient to be removed from the metal due to its fine size and dispersed distribution in the molten metal. [3] Zhang proposed that the valuable elements were impossible to be recovered effectively from the slags and sludges at high temperatures because of the high viscosity of the slag melt. [4] Therefore, several intensifying methods such as microwave [5], electromagnetic [6] and gas agitation [7] have been proposed to be applied for enhancing the phase separation in the physical chemical processes. Compared to above strengthening methods, super gravity method is a more effective intensification technology, which has the characteristics of greatly enhancing the relative motion between various phases with different density [8]. In the early days, super gravity was mainly applied in the phase separation process of chemical engineering and
⁎
pharmacy at low temperatures [9,10]. Recently, the super gravity has been proposed to be used in some fields of high-temperature, such as preparing materials with special functions [11], improving cleanliness of the steel [12], and recovering valuable elements from slags [13,14], which reflects its broad adaptability in industry. Rare earth elements (REEs) are critical materials for wide-ranging applications in space and green technologies [15,16], renewable energy [17], metallurgy [18], agriculture [19], and so forth. The largest rareearth (Re) deposit worldwide is located in Bayan Obo, Mongolia, China [20]. Over the years, the Bayan Obo ore has been mainly exploited as iron ore, and the Re-resources have not been effectively utilized, most of that were converted into the Re-concentrates by means of mineral processing [21]. However, it is difficult to efficiently separate the massive REEs from the complex multi-component Re-system. Moreover, the mass transfer and phase separation behaviors of REEs in this multicomponent Re-system were difficult to be investigated because of the serious lack of thermodynamic data for REEs. This paper introduces the super-gravity field into the Re-concentrate, to investigate the effect of super gravity on the precipitation and separation behaviors of REEs in this complex multi-component Resystem. Moreover, the enhancement of super gravity on mass transfer and phase separation of REEs, and its efficient application in REEs recovery from the multi-component Re-system was verified through
Corresponding author. E-mail address: [email protected] (J. Gao).
https://doi.org/10.1016/j.seppur.2019.115752 Received 15 April 2019; Received in revised form 15 June 2019; Accepted 28 June 2019 Available online 29 June 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Sketch of the experimental installation: (1) heating furnace, (2) resistance wire, (3) thermocouple, (4) magnesia crucible, (5) conductive slip ring, (6) centrifugal axis, (7) counterweight, (8) temperature controller, (9) rotation rate controller.
Fig. 1, and the specific experimental procedures are as follows. Firstly, each 20 g of the Re-concentrate was filled into a magnesia crucible, which was melted completely at 1773 K for 30 mins in the constant temperature zone of the heating furnace of centrifugal device under an argon atmosphere. Subsequently, the centrifugal device was started at a gravity coefficient of G = 800 corresponding to the angular velocity of 1691 r/min, and the crucible was continuously cooled from 1773 K to 1373 K where the Re-concentrate fully solidified, with various cooling rates of 2 K/min, 5 K/min or 10 K/min, respectively. After the temperature decreasing to 1373 K, the rotation function of the centrifuge was turned off, and the water-quenching was performed immediately when the crucibles were taken out. Simultaneously, the parallel experiment was carried out under the condition of normal gravity through continuous cooling from 1773 K to 1373 K with 2 K/min. All of the samples with various cooling rates were divided into two halves on average along the longitudinal center line and observed using a Leica optical microscope (DM4M) to characterize the multilayer structure in the entire sample. Subsequently, the multilayer samples were crossly divided into three parts along the two interfaces and analyzed further by SEM-EDS, XRD and Raman spectrometer, to investigate the micro structures, mineral structures and mineral compositions of the various layers, respectively. Furthermore, the distribution of REEs in different layers was characterized by the electron probe microanalyzer (EPMA). The mass fractions of Ce2O3, La2O3, Pr6O11 and Nd2O3 in the different layers were detected with the XRF and ICP methods, and the recovery percentages of REEs in the different layers were acquired according to Eq. (1).
theoretical and experimental results of XRD, SEM-EDS, XRF, EPMA, and Raman spectroscopy, respectively. 2. Experimental 2.1. Instrumentation The experiments were carried out in a centrifugal device incorporating a heating function, that is illustrated in Fig. 1, for investigating the effect of super-gravity field on successive precipitation and separation behaviors of various REEs. The centrifugal device is mainly composed of a heating furnace and a counterweight, which rotates symmetrically from the vertical to the horizontal when the centrifugal device starts to operate. The experimental temperature is controlled using a type B thermocouple closed to the sample. A conductive slip ring provides feedback information from the thermocouple, real-time controlling the temperature of the sample rotating with high speed, within the observed accuracy range of ± 3 K. Different rotation rates are controlled by a variable speed motor, and the various angular velocities correspond to different gravity coefficients according to the Eq. (1). 2.2. Materials The Re-concentrate employed in this research was obtained through multi-stage beneficiation of the Bayan Obo ore in Inner Mongolia, China [22]. Its chemical compositions that detected by the XRF combined with ICP-AES methods are shown in Table 1. It was found that the Re-concentrate is a multi-component Re-system which consists of more than 13 species elements including four REEs of Ce, La, Pr and Nd, and the total mass fraction of REEs is up to 44.09 wt%. Moreover, there are more than 20 kinds of minerals with various elements composition, which are tightly integrated with each other and dispersed in the Reconcentrate, as the mineral phase dissociation analyzer (MLA) images shown in Fig. 2.
RRe =
mT × ωRe − T mT × ωRe − T + mM × ωRe − M + mB × ωRe − B
(1)
where, RRe is the recovery percentage of Ce, La, Pr, Nd in different layers (%); mT, mM, and mB are the mass of the top, middle, and bottom layers (kg); ωRe-T, ωRe-M, and ωRe-B are the mass fractions of Ce2O3, La2O3, Pr6O11, Nd2O3 in the top, middle, and bottom layers, respectively (wt%).
2.3. Experimental procedure The experiments were carried out in the centrifugal device shown in Table 1 Chemical compositions (wt%) of Bayan Obo Re-concentrate. Composition
CaO
P2O5
Fe2O3
SO3
F
SiO2
MgO
BaO
MnO
Ce2O3
La2O3
Pr6O11
Nd2O3
Content
14.34
9.63
8.09
4.93
8.41
1.09
3.46
1.59
0.37
26.42
11.84
2.99
2.84
2
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Fig. 2. MLA images of Bayan Obo Re-concentrate.
3. Results and discussion
of REEs with decreasing temperature under the normal gravity, as shown in Fig. 3(a). By contrast, a multilayer sturcture consisting of three layers with two explicit interfaces appeared significantly in the samples that attained under super gravity, as presented in Fig. 3(b) to (d). Moreover, the three layers (bottom, middle and top) were presented as the obvious diverse macroscopic structures along the direction of super gravity.
3.1. Effects of super gravity on precipitation and separation behaviors of Rephases 3.1.1. Optical micrography analysis The Leica optical microscope was used to characterize accurately the multilayer structure in the samples attained under super gravity and that of the parallel sample. The overall optical micrographs montaged by over 20 images (magnification 5 × 10) of the samples attained at the various cooling rates of 10, 5 and 2 K/min are shown in Fig. 3. Apparently, various mineral phases were discretely distributed in the entire sample, and it was difficult to ascertain the precipitation behaviors
3.1.2. XRD analysis The mineral structures in different layers attained under super gravity and that of the parallel sample were characterized by XRD. As the XRD pattern of the sample with G = 1 shown in Fig. 4(a), the different Re-phases that precipitated from the molten Re-concentrate were
Fig. 3. Overall optical micrographs of the samples obtained under super gravity compared with normal gravity with various cooling rates: (a) G = 1, 2 K/min; (b), (c) and (d) G = 800, 2 K/min, 5 K/min and 10 K/min, respectively. 3
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Fig. 4. Comparation of XRD patterns for the samples obtained under super gravity and normal gravity with the cooling rate of 2 K/min: (a) G = 1, (b) G = 800.
equiaxed crystals, which were agglomerated into the dense structure under the action of super gravity. Subsequently, the Re3+ was precipitated further with FeO33- into the Re-ferrate with continuous decreasing of temperature, which were presented as the cube crystals and be separated to the middle layer under the action of super gravity. Finally, the Re3+ was precipitated into the britholite with SiO44- and PO43-, which showed a dendritic structure and stayed in the top layer. Comparing the sizes of Re-phases attained under super gravity and normal gravity, it was evident that the Re-phases were significantly grown into the larger crystals under the action of super gravity, as shown in Figs. 5 and 6. Moreover, compared with the crystal sizes of the different Re-phases attained under super gravity with various cooling rates as revealed in Fig. 6, the sizes of Re-oxyfluoride, Re-ferrate and britholite were increased from 2 μm, 100 μm and 200 μm to 5 μm, 400 μm and 1000 μm, respectively. It was indicated that the effect of super gravity on nucleation and growth of the three Re-crystals was improved significantly with the decreasing of cooling rates.
mixed together in the entire sample attained under normal gravity. On the contrary, each layer attained under super gravity was only composed of a single Re-phase, the top layer only consisted of britholite phase, the middle layer was the only Re-ferrate phase, while the bottom layer was the single Re-oxyfluoride phase, respectively, as presented in Fig. 4(b). 3.1.3. SEM-EDS analysis The microstructures and element composition of Re-phases in different layers attained under super gravity and that of normal gravity were determined further by SEM-EDS. From the SEM-EDS images of the sample with G = 1 as shown in Fig. 5, the various Re-phases were discretely distributed among the entire slag, it was indicated that the difference in density between them produced the insufficient driving force to drive any precipitated Re-phase moved and separated from each other in the molten Re-concentrate via free sedimentation. By contrast, the Re-phases were successively precipitated and separated into different layers under super gravity, as presented in Fig. 6. As accordingly verified, the Re3+ was firstly precipitated into the Reoxyfluoride with OF3- from the molten Re-concentrate. The driving force generated through super gravity was sufficient to overcome the solid-liquid interface tension and drive the Re-oxyfluoride evidently migrated and separated to bottom layer along the direction of supergravity. Moreover, the Re-oxyfluoride appeared as the obvious
3.1.4. Raman analysis The mineral compositions in the different layers obtained under super gravity were collected further using Raman spectrometer. Through comparing the exhibit several vibrational features in the different layers of the sample as shown in Fig. 7, the bands of britholite (440 cm−1, 856 cm−1, 959 cm−1), the Re-ferrate (674 cm−1, 1359 cm−1), and the Re-oxyfluoride (445 cm−1, 562 cm−1) were appeared on the upper, middle and bottom layers, respectively, which confirmed further the significant distinct in the mineral compositions of the separated Re-phases under the action of super gravity. 3.2. Theoretical verification and discussion The separation behavior of the Re-phases that precipitated from Reconcentrate melt under super gravity was theoretically verified further. Considering that the precipitated Re-particles in the melt are generally subjected to three lateral forces in the condition of super-gravity, including the centrifugal force far away from the centrifugal axis Fc (Eq. (2)), the frictional resistance Ff (Eq. (3)), and the buoyancy caused by centrifugation FB (Eq. (4)). Hence, the theoretical sedimentary time for Re-particles with different sizes in the melt under super gravity compared with that of normal gravity were conducted based on the theory of stratification and sedimentation [23].
Fc = m
Fig. 5. SEM-EDS images of the sample obtained under normal gravity with the cooling rate of 2 K/min: (a) SEM, (b), (c) and (d) EDS of EDS [Ce,La,Pr,Nd]FeO3 and Ca3[Ce,La,Nd,Pr]2[(Si,P)O4]3F, [Ce,La,Pr,Nd]3O4F3, respectively.
Ff = f 4
ν2 = mω2x = ρVω2x x
dx dt
(2) (3)
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Fig. 6. SEM-EDS images of different layers in the samples obtained under super gravity with various cooling rates: Pt.1-Ca3[Ce,La,Nd,Pr]2[(Si,P)O4]3F, Pt.2[Ce,La,Pr,Nd]FeO3, Pt.3-[Ce,La,Pr,Nd]3O4F3.
sedimentation time τ /s
674
Intensity (a. u.)
856
440
959
Top layer
440
1359
Middle layer
6
10
5
10
4
10
3
10
2
10
1
10
0
562
Bottom layer
10
10 200
400
600
800
1000
1200
1400
d =10 ȝm d =50 ȝm d =100 ȝm d =200 ȝm d =500 ȝm
-1
1600
1
200
Raman shift (cm )
m ρ ω2x ρ 0
(4)
G=
When Fc = FB + Ff, the acceleration of the particle is zero, and the particle is shown to be an isokinetic motion, and the isokinetic velocity is as Eq. (5).
τ=
dx = dt
(
m 1− f
ρ0 ρ
)ω x
800
g 2 + (ω2x )2 g L dx dt
=
g2 + ( =
9ηL × 10−3 2g (ρ − ρ0 ) d 2G
g
N2π 2x 2 ) 900
(7)
(8)
where, m is the mass of Re-particles (kg), ν is the linear velocity (m/s), x is the distance from centrifugal axis to the center of sample (m), ω is the angular velocity (rad/s), ρ and ρ0 are density of the particles and melt (g/cm3), V is the volume of particles (m3), f is the friction coefficient, and dx is the sedimentation velocity (m/s), η is viscosity of the melt dt (Pa·s), L is the active length (m), G is gravity coefficient, g is the normal gravity acceleration (g = 9.8 m/s2), and d is equivalent spherical diameter of the particles (m).
2
(5)
According to the Stokes formula [24], the friction coefficient of the spheroidal particles f is as Eq. (6), combined with the gravity coefficient as calculated according to Eq. (7), the complete sedimentation time τ could be obtained as Eq. (8).
f = 6πηx
600
Fig. 8. Theoretical sedimentation time τ for Re-particles with different sizes in the Re-concentrate melt as a function of gravity coefficient.
Fig. 7. Raman spectra of different layers in the sample obtained under super gravity with G = 800 and 2 K/min.
FB =
400
Gravity coefficient G
-1
(6) 5
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Fig. 9. Diagram of successive precipitation and separation process of different Re-phases in Re-concentrate melt under super gravity: (a) normal gravity, (b) to (d) super gravity.
Fig. 10. EPMA results of REEs contents in the various Re-phases attained under super gravity with G = 800 and 2 K/min.
Table 2 Mass fractions (wt%) and recovery percentages (%) of REEs in the various Re-phases attained under super gravity with G = 800 and 2 K/min. Layers
Top Middle Bottom
Mass fractions (wt%)
Recovery percentages (%)
∑ReO
Ce2O3
La2O3
Pr6O11
Nd2O3
RCe
RLa
RPr
RNd
2.92 52.41 80.14
1.07 3.18 71.87
0.66 39.76 1.05
0.50 4.80 3.87
0.69 4.67 3.34
0.95 2.83 96.21
0.16 94.68 3.75
8.10 48.38 36.52
4.39 60.31 45.30
The theoretical sedimentation time τ for Re-particles with different sizes in the Re-concentrate melt, using the experimental dimensional parameters and experimental measurement results (a density difference of 5 g/cm3, a sample active length of 3 cm, and a melt viscosity of 0.1 Pa·s), as a function of gravity coefficient G and particle diameters d are shown in Fig. 8. Compared with the theoretical sedimentation time for Re-particles with the minimum size of 10 μm is 276000 s in a normal gravity, which decreases significantly to 345 s when the gravity coefficient increased to G = 800. The theoretical results verified further the significant enhancement of super gravity on the motion and separation behavior of Re-phases in the melt. Accordingly, the successive precipitation and separation behaviors of various REEs in the condition of super gravity were described as Fig. 9. In a normal-gravity field, the various Re-phases that precipitated
from the melt were difficult to move and aggregate via the free sedimentation, which were dispersed among the whole slag melt, as presented in Fig. 9(a). Instead, the driving force Δρg was enhanced significantly in the case of a super-gravity field. As a result, the various Rephases that precipitated successively from the Re-concentrate melt with continuous decreasing temperature, were separated successively into the different layers under the action of super gravity, as shown in Fig. 9(b) to (d). In addition, the super gravity significantly enhanced the sizes and amounts of Re-crystals, that is because the interfacial tension between the Re-crystals and the slag melt is easily broken under the action of supergravity, so the agglomeration and growth is more likely to occur for the Re-crystals, and the directional migration of the Recrystals creates a better dynamic condition for the precipitation of the remaining REEs in the slag melt. 6
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3.3. Effects of super gravity on recovery behaviors of REEs
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The EPMA results (Ce, La, Pr and Nd) of the different Re-phases in the multilayer sample attained under super gravity are shown in Fig. 10. Considering the mass fractions and the recovery percentages of REEs in each Re-phase as listed in Table 2, the different REEs were recovered into the various Re- phases from the Re-concentrate as driven by super gravity, respectively. The cerium element was precipitated and recovered efficiently into the Re-oxyfluoride phase with a high recovery percentage of 96.21%, and the mass fraction of Ce2O3 in which was up to 71.87 wt%. The lanthanum element was recovered further into the Re-ferrate phase with a high recovery percentage of 94.68% and a mass fraction of 39.76 wt% for La2O3. In addition, some remaining REEs were recovered further as the britholite phase with a ∑ReO content of 2.92 wt%. 4. Conclusions In the present work, the effect of super gravity on the precipitation and separation behaviors of various REEs in the Re-concentrate was investigated. (1) The successive precipitation and separation phenomenon of REEs was discovered, and the multilayer sturcture with two explicit interfaces was obtained under super gravity. (2) The Re3+ were successively precipitated into Re-oxyfluoride phase, Re-ferrate phase, and britholite phase with OF3−, FeO33−, SiO44− and PO43−, which were successively separated to the bottom, middle, and top layers as driven by super gravity. (3) Various REEs were efficiently recovered into the different Rephases from the multi-component Re-system. Acknowledgements This study is supported by the National Natural Science Foundations of China (No. 51774037 and No. 51404025). References [1] H.X. Fu, S.T. Yang, Z.L. Xiu, Phase separation in a salting-out extraction system of ethanol–ammonium sulfate, Sep. Purif. Technol. 148 (2015) 32–37. [2] H.T. Chen, S.A. Ravishankar, R.S. Farinato, Rational polymer design for solid–liquid separations in mineral processing applications, Int. J. Miner. Process. 72 (2003) 75–86. [3] S.H. Lee, C. Tse, K.W. Yi, P. Misra, V. Chevrier, C. Orrling, S. Sridhar, A.W. Cram, Separation and dissolution of Al2O3 inclusions at slag/metal interfaces, J. NonCryst. Solids 282 (2001) 41–48.
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