Enrichment of trace rare earth elements from the leaching liquor of ion-absorption minerals using a solid complex centrifugal separation process

Article abstract of DOI:10.1039/c7gc03674d

A novel solid complex centrifugal separation (SCCS) process has been developed to enrich trace rare earth (RE) elements from the leaching liquor of ion-absorption RE minerals. When compared to liquid-liquid centrifugal extraction (LLCE), the proposed proc

Full text of DOI:10.1039/c7gc03674d

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Enrichment of trace rare earth elements from leaching liquor of
ion-absorption minerals by solid complex centrifugal separation
process
Received 00th January 20xx,
Accepted 00th January 20xx
Yanliang Wang,a,b Xiangguang Guo,a,b Yanfeng Bi,c Jia Su,a,b Weichang Kongd and Xiaoqi Suna,b*
DOI: 10.1039/x0xx00000x
A novel solid complex centrifugal separation (SCCS) process has been developed to enrich trace rare earth (RE) elements
from the leaching liquor of ion-absorption RE minerals. Compared to liquid-liquid centrifugal extraction (LLCE), the
proposed process employed 100 % extractant without volatile diluent for the RE enrichment, which led to a much shorter
equilibrium time of 5 min. Taking into account their abilities to form solid complexes with RE ions from aqueous phase,
some alkyl phenoxy carboxylic acid derivatives including p-tert-octylphenoxy acetic acid (POAA), iso-propanoic acid (POPA)
and iso-butyric acid (POBA) were synthesized and used as the solid extractants. The SCCS process included the following
steps. Firstly, the solid extractants with a saponification degree of 80 % were mixed with 0.5 g/L RE solution at a
liquid/solid phase ratio of 200/1 to obtain the solid RE complexes. Secondly, the solid RE complexes were separated with
aqueous phases using a liquid/solid centrifugal separator. Finally, high concentration of RE solution (>200 g/L) was
obtained by the stripping of solid RE complexes with concentrated HCl. In the SCCS process, precipitation rate of more
than 95.4 % and stripping rate of nearly 100 % for RE could be achieved. Water solubilities of the prepared solid
extractants in raffinate solution were tested to be lower than 32.8 ppm at 25 oC and mass losses were determined as 0.6 %
for each cycle. The obtained high concentration RE solution with the purity of 96.9 wt.% can be used directly as a feed
solution for the next individual RE element separation.
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minerals at low grade of 0.05-0.3 wt.%, it was difficult to
enrich and recover the RE elements through the conventional
physical beneficiation methods such as gravity separation,
1 Introduction
It is known that the RE elements (15 lanthanides plus yttrium
and scandium) have been applied in many high-tech products
relying on their unique electronic, magnetic, and optical
properties.1-3 Heavy RE elements were relatively scarce, such
as Y, Dy and Ho-Lu. However, the Dy element was
indispensable for enhancing magnetic coercivity in
conventional NdFeB permanent magnets.4, 5 As for Lu, the PET
detector technology relied on Lu oxyorthosilicate scintillation
crystals.6, 7 Ion-absorption RE minerals are the most significant
primary heavy RE resource, which was found mainly in South
China. Owing to existing as the ion-exchangeable phase in the
magnetic separation and flotation.8-11 Ammonium sulphate
solution was used to leach the ion-absorption type RE
minerals, however, the RE ions accompanied with non-RE
metallic species, i.e. Al3+, Fe3+ and Ca2+, in the leaching liquor
with low concentration of 0.1-5 g/L.12 With the continuous
exploitation of ion-absorption minerals, RE grade of the
mineral reduced obviously, resulting in the decrease of RE
concentration in the leaching liquor and increase of the non-RE
metallic species.13
RE feed solution for the extraction separation in
workshop should be at high concentration. Three major
enrichment and recover processes for the trace RE elements
from leaching liquor of ion-absorption minerals used in RE
industry are summarized as follows.
(1) Oxalic acid precipitation process. It was often used in
industrial operation due to the process simplicity and effective
recovery of RE elements. RE elements in the leaching liquor
could be precipitated using oxalic acid (H2C2O4) after in-situ
leaching of them from ion-absorption minerals by acidic
(NH4)2SO4. However, owing to its toxicity and water solubility
(14.3 g/100 g water at 30oC), the regeneration of H2C2O4 was
quite difficult which did not conform to the development trend
of green process.14-16 Recently, static magnetic field enhanced
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precipitation with oxalic acid was applied to the precipitation
To develop an efficient and sustainable enrichment process
of RE elements from leaching liquor of ion-absorption minerals. for the trace RE ions from leaching liquor of ion-absorption
Compared to the non-magnetization, the results of magnetic minerals, a solid complex centrifugal separation (SCCS) method has
processes appeared to be better and its optimal precipitation been developed in the present study. It worthwhile to mention
condition was obtained at the pH value of precipitation that there was a well-established chelating method to recover
solution at 2, RE2O3:H2C2O4 equalled to 2.5:1 and a magnetic trace metals in the field of analytical chemistry.28 Similarly, its
strength at 400 kA/m.17 The H2C2O4 precipitation reaction can principle is that metal salt compounds can be produced when
be written as the following equation:
mixing complexing agents containing coordination groups with
trace metal ions. The metal salts can be easily dissolved in
organic solvents but not dissolved in water. However, the
significant difference between SCCS method developed in the
present study and chelating method is that the former is a
solid-liquid separation without organic solvent, and the latter
is a liquid-liquid separation which depends on the diluent of
organic solvent.
2RE3+ +3H2C2O4 →RE2(CO3)3 ↓ +6H+
(1)
(2) NH4HCO3 precipitation process. Because NH4HCO3
was a cheap, non-toxic and easily available agricultural
chemical product, this method had better economic
performance. However, the precipitation of low concentration
RE elements with NH4HCO3 as a precipitant was difficult to
form crystalline RE carbonate, which made subsequent
19
The NH4HCO3
solid/liquid phase separation difficult.18,
2 Experimental
precipitation reaction can be written as follows:
2RE3+ +6NH4HCO3 →RE2(CO3)3 ↓+3CO2 ↑+3H O+6NH4+
2.1 Materials and reagents
(2)
2
Raw materials for the syntheses of solid extractants including
(3) Liquid-liquid extraction process. The traditional liquid-
liquid extraction (LLE) showed excellent results for the
enrichment and recovery of RE which adopted organic
phosphonic acids, alkylated carboxylic acids and phenoxy
p-
tert-octylphenol (m.p. 84oC), chloroacetic acid, 2-chloropropionic
acid and 2-bromo butyric acid were purchased from Chendu Xiya
Reagent Chemical Technology Co., Ltd, China. NaOH, aqueous
ammonia, ethanol and ammonium sulphate were purchased from
Sinopharm Chemical Reagent Co., Ltd. Naphthenic acid (NA) was
kindly provided by Shanghai Rare Earth Chemical Co. Ltd. of China.
Ion-absorption RE minerals were provided by Longyan Rare-
earth Development Co., Ltd., China. RE oxides (La2O3-Lu2O3, plus
Y2O3) were purchased from Fujian Changting Golden Dragon Rare
Earth Co., Ltd., China, among which La2O3-Dy2O3, Tm2O3-Lu2O3 and
Y2O3 were of high grade (99.99 %), and the purities of Ho2O3 and
Er2O3 were 99.9 %. All the other chemicals were used without
further purification.
carboxylic acids as extractants and HCl as
a stripping
reagent.20-23 Very recently, a novel extraction method for the
separation and enrichment of RE elements has been reported
that a three-stage HEH(EHP) extraction at VA/VO = 10/1 and
three-stage HDEHP stepwise extraction at VA/VO = 25/1 were
required to enrich heavy and light RE elements. The total
recovery for RE elements reached 99 % and the heavy and light
RE elements concentrations were up to 240 and 200 g/L after
stripping with 6 mol/L HCl, respectively.24 A new bubbling
organic liquid membrane extraction process was also reported
Inductively Coupled Plasma Optical Emission Spectroscopy
to extract and enrich RE element
s with extremely low
(ICP-OES) Horiba Ultima
2 was used to determine the
concentrations from the acidic sulphate leach solutions of ion-
absorption type RE minerals.25 Two processes by liquid-liquid
centrifugal extraction (LLCE) were previously reported that
recovering RE from wet-process phosphoric acid (WPA) using
concentrations of RE elements in aqueous phase. The 1H and 13C
NMR spectra were obtained with an AV III-500 Bruker
spectrometer. The Scanning Electron Microscope (SEM) was
collected on Hitachi SU1510, 30 KV. Particle sizes were measured by
Brookhaven NanoBrook Omni, USA. Fourier Transform Infra-Red
(FT-IR) spectra were obtained by using a Nicolet iS50 spectrometer
(Thermo Scientific) in the range 600-4000 cm-1. Ultraviolet-Visible
(UV-Vis) diffuse reflectance spectra were collected using Agilent
Cary 5000 UV-Vis Spectrophotometer. Carbon and hydrogen in the
organic samples were analysed by Elementar Vario micro cube,
Germany.
26
di-(2-ethylhexyl) phosphoric acid (D2EHPA) and enriching RE
from ion-adsorption minerals using naphthenic acid (NA) 27
,
respectively. Compared with mixer-settlers widely applied in
the RE hydrometallurgical industry, the centrifugal contactor
revealed advantages such as low holdup volume, excellent
phase separation, high mass transfer efficiency, etc. Neglecting
the dimerization effect of the acid extractants, the extraction
reactions can be written as follows:
RE3+ +3HL(org) →REL3(org) +3H+
2.2. Synthesis and characterization of extractants
(3)
Alkyl phenoxy carboxylic acids including
p-tert-octylphenoxy acetic
Although the LLE and LLCE have been proven to be reliable
and efficient techniques, unexpected emulsification may
appear under low acidity conditions and lead to the difficulties
in liquid-liquid phase separation. The proportion of the diluent
in the organic phase is greater than 60 %, resulting in fire risks
and environmental hazards of Volatile Organic Compounds
(VOCs).
acid (POAA), tert-octylphenoxy iso-propanoic acid (POPA) and p-
p
-
tert-octylphenoxy iso-butyric acid (POBA) were synthesized and
their molecular structures are shown in Fig. 1.
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temperature. The experiments were repeated at least twice. The
extraction rate (E) for LLE, precipitation rate (P) for SCCS and the
amount of RE precipitation per gram of solid extractant (Q) are
defined as follows.
Cini − Ceq
E% = P% =
×100
(4)
(5)
Cini
OR
ꢄꢅꢄꢆꢃꢇꢈꢉ∙ꢊꢋꢈ
ꢀ ꢁ ꢂꢃ
POAA : R= CH2COOH
POPA : R= CH(CH3)COOH
POBA : R= CH(C2H5)COOH
ꢌ
where Cini and Ceq are the initial and equilibrium concentration of RE
element in aqueous phase, respectively. Vaq and m represent the
aqueous phase volume and the mass of extractant, respectively.
Fig. 1 Molecular structures of POAA, POPA and POBA (Ellipsoid mirror
isomer at 30 %).
They were prepared in laboratory scale via Williamson reaction
and the following synthetic routes are relatively simple and green:
(1) 0.2 mol 4-tert-octylphenol, 0.21 mol chloroacetic / propionic /
butyric acid, 0.41 mol NaOH were mixed together in alcohol media.
The mixture was stirred at 110 oC for 6 hours. A certain amount of
NaOH was added to control the system pH at 10-11 throughout the
reaction. (2) The resultant solution was sequentially neutralized
with 6 mol/L HCl, extracted with diethyl ether, washed with
deionized water, and evaporated on a rotary evaporator until dry.
(3) The products were purified by recrystallization in n-hexane. As
for a large-scale preparation, the reaction time is enough for 1.5
hours.
3 Results and discussion
3.1 Comparison of LLE with SCCS
LLE is a common separation method and widely used for the
extraction and separation of RE elements.29, 30 Extractant for
LLE should be diluted by inert solvent such as sulfonated
kerosene to reduce the viscosity of extractant and increase the
solubility of extraction complex in the organic phase. However,
some problems gradually emerged during the industrial
practices, such as emulsification of organic phase leads to the
difficulties in phase separation. The solid extracting complex
during the LLE process need to be avoided. Otherwise, it will
continue to plug the pipe of the equipment and cause the loss
of the extractant. As for the structural optimum design of
extractant, there are two methods to ensure the extractant
keep liquid phase. Firstly, increasing the disorder degree of the
extractant structure. For example, choosing high degree of
branching groups as the carbon chains of extractants.
Secondly, mixing two, three or four extractants together to
reduce the melting point of the mixture based on the principle
of eutectic mixture principle. For example, Cyanex 923
comprises a mixture of four trialkyl phosphine oxides with the
general formula R3PO (14 %), R2R'PO (42 %), RR'2PO (31 %) and
R'3PO (8 %), in which R denotes n-octyl and R' stands for the n-
hexyl group. When the composition reduces from 100 % to 14
p-tert-octylphenoxy acetic acid (POAA): Final yield 83 %.
Purity >99 % (acid-base titration method). 1H NMR (500 MHz,
CDCl3) δ 0.68 (s, 9H), 1.30 (s, 6H), 1.69(s, 2H), 4.62 (s, 2H), 6.89
(d, J = 8.7 Hz, 2H), 7.27(d, J = 8.6 Hz, 2H), 12.95 (s, 1H); 13C
NMR (500 MHz, CDCl3) δ32.0，32.1, 32.5, 38.1, 56.8, 64.9,
o
114.0, 127.3, 142.4, 155.8, 170.8. Melting point 124 C. Water
o
solubility 33.3 ppm at 25 C (UV-Vis Spectrophotometry).
p-tert-octylphenoxy iso-propanoic acid (POPA): Final yield 80
%. Purity >98 %. 1H NMR (500 MHz, CDCl3) δ 1.34 (s, 15H), 1.65
(d, J = 6.9 Hz, 3H), 1.69, (s, 2H), 4.78 (q, J = 6.9 Hz, 1H), 6.82 (d,
J = 8.8 Hz, 2H), 7.28 (d, J = 8.9 Hz, 2H); 13C NMR (500 MHz,
CDCl3) δ13.7, 26.8, 27.6, 27.6, 33.3, 52.3, 67.6, 109.8, 122.6,
o
139.1, 149.9, 172.3. Melting point 109 C. Water solubility 31.5
ppm at 25 oC.
o
%, the melting point correspondingly reduced from 52 C to -5
oC.31 As a novel extraction and separation system, the
extractants were selected in a diametrically opposite way in this
paper. As shown in Fig.1, the selected alkyl phenoxy carboxylic
acids including POAA, POPA and POBA are solid extractants
with symmetrically carbon chains which may increase melting
p-tert-octylphenoxy iso-butyric acid (POBA): Final yield 68 %.
Purity >98 %. 1H NMR (500MHz, CDCl3) δ 0.74 (s, 9H), 1.13 (s, 3H),
1.37(s, 6H), 1.72(s, 2H), 2.08(m, 2H), 4.62(t, 1H), 6.86(m, 2H), 7.31
(m, 2H); 13C NMR (500MHz, CDCl3) δ 9.6, 26.1, 31.6, 32.4, 38.1, 57.0,
114.5, 127.3, 143.6, 155.2, 177.5. Melting point 79 oC. Water
o
points and improve crystallization performances.
The
solubility 25.2 ppm at 25 C.
extractants were 100 % used without adding dilute of LLE such
as kerosene. In addition, the formations of solid complexes
were utilized to achieve the rapid separation of solid/liquid
phase.
2.3. Extraction procedure and data treatment
SCCS experiments were carried out by mixing the saponified
extractants and RE solution together to form solid complex phase
at room temperature. The mixture was centrifuged, and the solid
phase was separated from the liquid phase. The main technical
parameters of liquid/solid centrifugal separator are given as follows:
effective volume of the wheel hub was 300 mL, maximum speed
was 6000 rpm, separation factor (centrifugal force / gravity) was
2014, material of filter cloth was polypropylene fiber, pore of filter
cloth was 300 mesh, processing capacity was 100 L/h.
Taking yttrium as an example, the extraction and
precipitation efficiency of RE ion by LLE (NA) and SCCS (POAA,
POPA, POBA) methods were compared at room temperature
and the results are shown in Fig.2. For traditional LLE system, 40
min was found to be enough for the extraction equilibriums of RE
element. As for the SCCS system, the equilibrium behaviour was
found to be quite different from that of traditional LLE and less
than 5 min was needed to attain the equilibrium. Without the
resistance effect of diluent, the shorter equilibrium time during
SCCS can be attributed to the quick complexation reaction of
saponified extractants and RE ion in the aqueous phase. The
To make a comparison, LLE experiments were performed by
contacting organic phase containing extractants and dilutes with RE
aqueous solution for 30 min in
a vibrating mixer at room
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amounts of RE precipitation per gram of solid extractants (Q) were 3.2 The formation mechanism of solid complexes
also calculated. Due to the smallest molecular weight of POAA
The extraction of trivalent RE elements with carboxylic acid or
among the SCCS, the unit mass of POAA revealed the highest RE
precipitation amount of 97.6 mg/g. Water contents of solid
complexes including POAA-RE, POPA-RE and POBA-RE
complexes were tested as 32.7 %, 77.4 % and 79.4 %,
respectively.
phosphonic acid was commonly known as the mechanism of cation
exchange reaction.32 As a weak acid, the acidic extractant itself
extracts only a very small amount of RE ions. To extract RE elements
quantitatively in aqueous phase, the saponification by acid-base
neutralization reactions are necessary to break the dimer of acidic
extractants. The effect of saponification degree in the precipitation
of RE elements by POAA, POPA and POBA were investigated and the
results are shown in Fig.4. The precipitation rates of RE elements
with POAA, POPA and POBA all quantitatively increase with the
increasing saponification degree over the range of 0-80 %. The
value of slope follows the sequence POAA > POPA > POBA, which
are coincided with the pKa sequences of POAA (pKa = 4.88), POPA
(pKa = 5.19) and POPA (pKa = 5.68). In addition, the formation of
100
80
60
40
20
0
100
80
60
40
20
0
d
a
b
c
Q at equilibrium, mg/g
97.6 91.5 88.1 80.7
Water contents of solid complex, %
(a) POAA
(b) POPA
(c) POBA
(d) NA
solid complex,
extractants,
H+, were calculated, and the results are also shown in
Table 1. The ratios of
H+/ RE3+ keep about 3 at different
Δ
RE3+, and the reduction of H+ in the solid
Δ
32.7 77.4
79.4
N/A
Δ
Δ
saponification degrees, which means that the precipitation rates of
RE elements are quantitative under the current saponification
condition. The water solubility of POAA, POPA and POBA were
tested as 33.3, 31.5 and 25.2 ppm at 25 oC, respectively. With the
increases of saponification degrees during the precipitation
processes, the water solubilities of POAA, POPA and POBA slightly
increased. The saponification and precipitation reaction can be
written as the following equations.
0
20
40
60
80
100
120
Reaction time, min.
Fig. 2 Extraction and precipitation efficiency of RE with (a) POAA, (b)
POPA, (c) POBA and (d) NA by LLE and SCCS methods. Aqueous phase: RE
concentration = 0.969 g/L. liquid/solid phase ratio = 200/1. CNA = 0.10
mol/L in 14.5 mL kerosene, saponification degree = 80 %.
To reveal the precipitation abilities and selectivities of SCCS
methods using POAA, POPA and POBA, their precipitation rates for
different RE ions were compared. As can be seen in Fig. 3, POAA
indicates better precipitation abilities for the RE ions before Y,
POPA and POBA show better precipitation abilities for the RE ions
after Y. Generally, the extractants reveal better precipitation
abilities for light RE ions than heavy RE ions. For example, the
extraction order of POAA follows the approximate reverse
sequence, i.e., Sm > Pr > Nd > Ce > Eu > La > Gd > Tb > Dy > Ho > Tm
> Er > Yb > Lu > Y.
OH− +HL →L− +H2O
(6)
500
100
400
80
300
200
100
0
60
40
20
0
80
70
60
50
POAA Slope=1.26 R2=0.998
POPASlope=1.23 R2=0.997
POBASlope=1.20 R2=0.995
0
20
40
60
80
40
Saponification degree, %
(a) POAA
(b) POPA
(c) POBA
Fig. 4 The effect of saponification degree on the precipitation of RE by POAA,
POPA and POBA. Aqueous phase: RE concentration = 1.11 g/L, liquid/solid
phase ratio = 200/1. saponification degree = 0-80 %.
30
20
La Ce Pr NdSm Eu Gd Tb Dy Ho Y Er Tm Yb Lu
RE elements
Table 1 The relationship between the formation of solid complex,
and the reduction of H+ in the solid extractants, H+.
Δ ,
RE3+
Δ
Fig. 3 Precipitation efficiency of mixed RE ions with (a) POAA, (b) POPA and
(c) POBA. Aqueous phase: ΣRE = 23.4 g/L (8.8×10-3 mol/L each RE ion).
Liquid/solid phase ratio = 9.5/1, saponification degree = 75 %.
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ΔRE3+ ×10-4 mol
ΔH+/ΔRE3+
ΔH+×10-4
(f) POBA-RE
(e) POPA-RE
(d) POAA-RE
Sap.
Rate, %
mole
POAA POPA POBA POAA POPA POBA
15
25
50
60
65
2.25
3.75
7.50
9.00
9.75
0.731 0.746 0.790 3.1
3.0
2.9
3.1
3.0
3.0
2.9
3.0
3.1
3.0
3.1
1.16
2.39
3.02
3.21
1.28
2.44
2.98
3.22
1.25
2.46
3.04
3.15
3.2
3.1
3.0
3.0
5.0
4.5
4.0
3.5
(c) POBA
(b) POPA
(a) POAA
70
80
10.5
12.0
3.54
3.90
3.57
3.81
3.49
3.71
3.0
3.1
2.9
3.1
3.0
3.2
3.3 Characterization of POAA-RE, POPA-RE and POBA-RE
complexes.
5.0
4.5
4.0
3.5
δ, ppm
FT-IR spectra of POAA, POPA and POBA before and after
precipitation reaction in the range of 750-3600 cm-1 are depicted in
Fig.5 (a-c), (d-f), respectively. The IR spectra obtained from POAA,
POPA and POBA clearly demonstrate that they are typical carboxylic
acids. The bands at 1743, 1720, and 1717 cm-1 are assigned to the
stretching vibrations of C=O of COOH in the molecules of POAA,
Fig. 6 1H NMR spectra for -CH2, two -CH groups in (a) POAA, (b) POPA, (c)
POBA and corresponding RE complexes (d) POAA-RE, (e) POPA-RE and (f)
POBA-RE.
The surface morphologies of POAA-RE, POPA-RE and POBA-RE
POPA and POBA, respectively, which are consistent with the complexes were probed using Scanning Electron Microscopy (SEM).
single crystal structures (Fig. S1). It can be explained that for As shown in Fig.7(a-c), the POAA-RE, POPA-RE and POBA-RE
stretching vibration, the stronger the hydrogen bond is, the complexes were presented as layered structure when adding
wider the band is and the greater the displacement is in the 1.5×10-3 mole saponified POAA/POPA/POBA into 8.85×10-3 mol/L
direction of low wavenumber due to the hydrogen bonding RECl3 solution. If the RE concentration changes even lower, as
effect between molecules.33 The C=O asymmetric stretching shown in Fig.7(d-f), the POAA-RE, POPA-RE and POBA-RE complexes
vibrations occur at 1610, 1612, and 1614 cm-1 indicating the appeared as rod-like structures.
presence of carboxylate groups in the structures of POAA-RE, POPA-
RE and POBA-RE, respectively.
(f) POBA-RE
1614
(e) POPA-RE
1612
(d) POAA-RE
1610
(c) POBA
1718
(b) POPA
1720
(a) POAA
1743
3500
3000
2500
2000
1500
1000
Wavenumber, cm-1
Fig. 5 IR spectra of (a) POAA, (b)POPA, (c) POBA, (d) POAA-RE, (e) POPA-RE
and (f) POBA-RE.
1H NMR spectra were also employed to characterize POAA,
POPA, POBA and corresponding complexes with RE ions before and
after precipitation reactions. As shown in Fig.6, the 1H NMR
characteristic peaks for one -CH2 group and two -CH groups in the
molecules of POAA, POPA and POBA are 4.62, 4.75/4.77 and 4.58,
whereas the peaks are shifted to 4.04, 4.46 and 4.39 after SCCS,
respectively. Such changes can be ascribed to the coordination
interactions between the acid molecules and RE ions.
Fig. 7 SEM photographs of POAA-RE, POPA-RE and POBA-RE complexes. (a-c)
Aqueous phase: RE concentration = 1.40 g/L, Vaq = 40 mL. Liquid/solid phase
ratio = 100/1. (d-f) Aqueous phase: RE concentration = 0.140 g/L, Vaq = 40 mL.
Liquid/solid phase ratio = 1000/1.
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3.4 Pilot test
7
Pilot tests were carried out to verification the current method for
the enrichment of trace RE elements in leaching liquor from ion-
absorption RE minerals and the schematic description of SCCS
procedure is shown in Fig.8. Firstly, the solid extractants were
prepared and saponified into anionic surfactants with alkali, such as
sodium hydroxide and ammonia, which could be fully break the
dimers of acidic extractants and contact with the aqueous phase.
Secondly, insoluble solid complexes were formed by adding the
saponified extractants into low concentration RE solution.
Therefore, RE elements were transferred from aqueous phase into
solid phase. Solid complexes could be separated from raffinate
solution containing non-RE impurities easily with liquid/solid
centrifugal separator. Finally, high concentration RE solution was
obtained by the stripping of solid complexes with concentrated HCl.
The extractant and (NH4)2SO4 in the raffinate solution were
designed to be recycled. Considering the excellent amount of RE
precipitation per gram(Q) and water contents of solid complexes,
POAA was selected as the extractants in the pilot test.
0.3
0.2
0.1
0.0
6
5
4
3
2
1
0
(c) Fresh
(d) Recycled
0
100
200
300
400
Leaching time, min
(a) Fresh
(b) Recycled
0
10
20
30
40
50
60
70
Retention volume,mL
Fig.9 Leaching test of ion-absorption RE minerals by (a) Fresh (NH4)2SO4,
20.0 g/L and (b) Recycled (NH4)2SO4, 20.2 g/L. The leaching kinetics tests
were carried out in a PMMA column with inner diameter of 3.2 cm at room
temperature, filling amount of ion-absorption RE ore is 100 g, filling height 9
cm. The linear relationship between 1-2/3α-(1-α) 2/3 and leaching time by (c)
Fresh (NH4)2SO4 with a slope of 9.76×10-4 and R2 of 0.987 and (d) Recycled
(NH4)2SO4 with a slope of 8.36×10-4 and R2 of 0.991 were also determined.
In the precipitation section at large liquid/solid phase ratio of
200/1, 2000mL RE leaching liquor were prepared as the feed
solution and the RE concentration was diluted as 0.473 g/L. Two
dodges of 10g POAA were selected and saponified with NaOH
(Process-I) and NH3·H2O (Process-II). Solid complexes were formed
by adding the saponified extractants into RE Feed solution. After
removing raffinate solution with liquid/solid centrifugal separator,
14.7 and 14.8 g solid RE complexes were obtained and the
precipitation rates were determined as 95.6 % and 95.5 % in
Process-I and II, respectively.
Fig.8 Schematic description of SCCS process for the enrichment of RE
elements from leaching liquor of ion-absorption minerals.
Table 2. Composition of the ion-absorption RE mineral and products.
RE products, %
Ingredients
RE mineral, %
Process-I
25.3
Process-II
23.4
The ion-absorption RE minerals were collected at Longyan of
Fujian Province, China, and the partitioning of ion-exchangeable
phase from the minerals has been determined as 0.114 %. The
composition of ion-absorption RE mineral are shown in Table 2.
The proportion of heavy RE elements containing Gd-Lu in the ion-
adsorption RE mineral accounts for 14.2 % which is much higher
than other types of RE minerals such as bastnaesite and monazite.
Leaching test was carried out to obtain RE leaching liquor of
ion-absorption minerals. Nowadays, RE concentrate is recovered by
in-situ leaching process by 1-4 wt.% (NH4)2SO4 solution in the
industry. The leaching process of minerals is a kind of ion-
exchangeable process between the positive ions in the solution and
the clay minerals. The linear relationship between 1-2/3α-(1-α)2/3
and leaching time, as shown in Fig. 9(a), suggests that the leaching
Y2O3/REO
La2O3/REO
CeO2/REO
Pr6O11/REO
Nd2O3/REO
Sm2O3/REO
Eu2O3/REO
Gd2O3/REO
Tb4O7/REO
Dy2O3/REO
Ho2O3/REO
Er2O3/REO
Tm2O3/REO
Yb2O3/REO
Lu2O3/REO
ΣREO
25.17
24.70
4.54
5.91
20.50
4.44
0.50
4.60
0.72
4.05
0.79
2.05
0.25
1.57
0.21
0.114
13.0
<0.20
11.7
23.3
23.9
3.77
4.16
5.87
6.38
20.6
21.6
4.83
5.05
0.576
4.71
0.587
4.62
0.790
4.51
0.763
4.26
0.854
2.28
0.795
2.09
0.300
1.96
0.258
1.76
0.319
213.0*
6.29*
0.028*
0.59*
0.279
201.7*
7.00*
0.026*
0.50*
kinetics abide by the inner-diffusion-controlled process of spherical
35
particles,34,
equation.
the kinetics equation is given by the following
Al2O3
Fe2O3
CaO
2/3
(8)
1-2
α
/3- (1-
α
)
= K ⋅t
* unit: g/L.
where α is the leachability at a given time and K is the rate
constant.
6 | J. Name., 2012, 00, 1-3
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Table 3 Material balance calculation of RE during the SCCS process.
m, g
/or V, mL
2000
2000
14.7
Material
Balance, %
(100)
4.42
(100)
Precipitation rate
/ Stripping rate, %
Process-I
Procedure
CRE, g/L
mRE, g
Feed
Raffinates
Solid complex
Stripping solution
Washing solution
0.473
0.0209
0.0615
213.0
2.16
0.946
0.0418
0.904
0.895
0.0216
Extraction Section
95.6
Stripping section
4.2
10
99.0
2.39
101.4
m, g
or V, mL
2000
2000
14.8
Material
Balance, %
(100)
4.50
(100)
Precipitation rate
/ Stripping rate, %
Process-II
Procedure
CRE, g/L
mRE, g
Feed
Raffinates
Solid complex
Stripping solution
Washing solution
0.473
0.0215
0.061
201.7
4.82
0.946
0.0430
0.903
0.847
0.0482
Extraction Section
95.5
Stripping section
4.2
10
93.8
5.34
99.1
Particle morphologies and sizes of the solid complexes were The stripping experiments were carried out in flasks at liquid/solid
measured, and the results were shown in Fig.10. As a comparation,
precipitate powders of RE2(C2O4)3 and RE2(CO3)3, using H2C2O4 and
NH4HCO3 as precipitants, were also prepared under the same
condition. The extractant POAA were saponified with NaOH and
NH3·H2O in Process-I and II, respectively. It revealed that surface
morphologies of POAA-RE with the particle sizes of 40-50 μm were
quite similar. As for RE2(C2O4)3 and RE2(CO3)3, their particle sizes
were tested as about 6 and 2 μm, respectively. The superior particle
size of POAA-RE made a significant contribution to the separation of
solid phase and liquid phase.
phase ratio of about 1/2.7. After heating the mixture of solid RE
complexes and 5.5 mL concentrate HCl up to 100 oC, high
concentration of 213.0 and 201.7 g/L RE solution were obtained.
Then the extractants were washed by 10mL deionized water to
remove the entrained RE ions. Total stripping rates were calculated
as 100.0 % and 99.2 % in Process-I and II, respectively. Water
solubilities of solid extractants in the raffinate solutions were tested
to be lower than 32.8 ppm at 25 oC, and their mass losses were
calculated as 0.6 % during the SCCS processes. Enrichment factors
of RE elements were calculated as 450 and 426 times through
process I and II, respectively. Detailed material balance calculations
of RE elements are shown in Table 3. The high concentration RE
solution with the purity of 96.9 wt.% can be directly used as feed
solution for the workshop of individual RE separation. To avoid the
diffusion of volatile hydrochloric acid into the atmosphere, the
absorption device is urgently needed in the stripping section when
the mixture is heated to 100 oC. Acid mist absorbers made of glass
fiber reinforced plastics(FRP) are recommended to ensure the
waste gas after purification meet the requirements of the Rare
Earth Industrial Pollutant Discharge Standard of China (GB26451-
2011, HCl<60 mg/m3).
In view of the convenience of separation, the equipment of
liquid/solid centrifugal separator was employed which contributed
to improving the production efficiency. A larger scale liquid/solid
centrifugal separator with wheel hub volume of 150
L is
recommended to be installed in the RE mining area, which has the
processing capacity of 1200 m3 leaching liquor of ion-absorption RE
minerals per day. For a lower occupied area of the equipment, it
may be installed on a removable mining truck to reduce the
construction cost of mining enterprise.
The extractant can be regenerated and the raffinate solution
containing 7.23 and 8.26 g/L (NH4)2SO4 can be recycling utilized by
adding (NH4)2SO4 to 20 g/L, the leaching test with recycled
(NH4)2SO4 was also shown in Fig.9 (b). The leaching of RE elements
from ion-absorption RE minerals using regenerated raffinate
solution containing (NH4)2SO4 also abide by the inner-diffusion-
controlled law and shown linear relationship between 1-2/3α-(1-α)
2/3 and leaching time. The regenerated POAA was characterized by
IR and elemental analysis. As shown in Fig.11, the characteristic
peak at 1743 cm-1 which assigned to the stretching vibration of C=O
of COOH are consistent with the molecular structure before and
after SCCS process. Elemental analysis was also conducted and the
results showed that there was no obvious change of the carbon and
hydrogen contents when the POAA was regenerated. Comparing
with common RE precipitation processes by H2C2O4 and NH4HCO3,
the reuse of extractant also revealed the potential to reduce
production costs.
d
c
b a
0.1
1
10
Diameter,
100
1000
ꢀm
Fig.10 Particle morphologies and sizes of the solid precipitates. (a) POAA-RE,
POAA was saponified with NaOH. (b) POAA-RE, POAA was saponified with
NH3 H2O. (c) RE2(C2O4)3, H2C2O4 was used as precipitant. (d) RE2(CO3)3,
NH4HCO3 was used as precipitant.
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Acknowledgements
(b) Regenerated
POAA
This work was supported by “Hundreds of Talents Program” and
Science and Technology Service Network Initiative from the Chinese
Academy of Sciences, Science and Technology Major Projects of
Fujian Province (2015HZ0001-3) and Natural Science Foundation of
Fujian Province (2016J0102). The authors would like to thank
Longyan Rare-earth Development Co., Ltd (China) for supplying ion-
absorption rare earth minerals.
1743
1743
(a) POAA
Elements in the compands,%
(a) C73.05 H9.25
(b) C73.08 H9.49
Calc. C72.69 H9.15
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This article presents an efficient and sustainable procedure for
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Graphical abstract