Solvent Extraction Research and Development, Japan, Vol. 14, 105 – 113 (2007)

Separation and Recovery Process for Rare Earth Metals from
Fluorescence Material Wastes Using Solvent Extraction
Takahide NAKAMURA, Syouhei NISHIHAMA,* and Kazuharu YOSHIZUKA
Department of Chemical Processes and Environments, The University of Kitakyushu,
Hibikino 1-1, Kitakyushu 808-0135, JapanThe separation and recovery of rare earth metals from fluorescence material waste
was investigated by using solvent extraction with 2-ethylhexyl phosphonic acidmono-2-ethylhexyl ester in kerosene. Extraction equilibrium expressions were
established for the trivalent metals (La, Ce, Eu, Tb, Y and Al) and divalent metals(Ca, Sr and Ba) for both single and multi components systems. The processsimulation for the selective recovery of Eu, Tb and Y revealed that effective
separation and recovery of Eu, Tb and Y can be achieved in two steps in acounter-current mixer-settler cascade from the alkaline earth metals (Ca, Sr and Ba)
and La and Ce.

1. Introduction
Rare metals are basic materials for the production of high technology goods and systems, andtheir demand at high purity has therefore been increasing in recent years. The rare earth metals are also
widely used as fluorescent materials for lamp, CRT and plasma displays, and for the permanent magnet
motors for hybrid cars [1]. In Japan, most rare earth metals are now imported from China, and the valueof these metals is rising with increasing demand [2]. As the development of effective recovery processes
for rare earth metals is a quite attractive issue from the view point of resources sustainability, therecovery of these metals from increasing amounts of fluorescent wastes is worthy of study.
Solvent extraction is one of the important separation and recovery methods for valuable metals,
such as vanadium [3], molybdenum [4], indium [5] and rare earth metals [6]. Although solvent
extraction has been recently avoided due to the usage of organic solvents, it is still attractive from theview points of simplicity, speediness and easiness of scale-up. Kubota et al. investigated the recovery ofrare earth metals from CRT waste from TVs by solvent extraction with calix[4]arene derivatives as
extractants. They demonstrated that Y and Eu can be separated even in the presence of excess amountsof Zn and Al [7]. Takahashi et al. also investigated the separation and recovery of rare earth metals fromfluorescence lamp waste. They demonstrated that recovery of rare earth metals of high purity, mainly Y,
Eu and La, can be achieved by using ion exchange and/or solvent extraction methods [8]. However, nopractical rare earth metal recycle process using solvent extraction has been commercialized due to the- 105 -

issue of cost performance. The optimum conditions for solvent extraction for the separation andrecovery of rare earth metals from these wastes should be therefore established.
Process simulation is well known to be effective for determining the appropriate conditions aswell as for predicting the operational conditions. Several studies have been made using computer
simulation of counter-current solvent extraction processes for metal separation. Tanaka et al. reported
on the role of the extraction equilibrium constant in counter-current multistage solvent
extraction-stripping processes [9]. Bagacki et al. has also investigated the effect of various phenomena
on metal extraction with chelate type reagents in both counter-current and cross-current extraction
systems [10]. Nishihama et al. have developed a process simulation method for a counter-current
mixer-settler cascade, based on extraction equilibrium and stoichiometric relationships [11]. An
effective separation and recovery process for rare earth metals could be developed by using processsimulation.
In this study, a recovery process for rare earth metals from spent fluorescent material by solvent
extraction was investigated, employing the common commercial extractant, PC-88A (2-ethylhexyl
phosphonic acid mono-2-ethylhexyl ester). Extraction equilibrium formulations for the metals,
contained in the phosphor sludge, were established. On the basis of the extraction equilibrium studies,
process simulations for the mutual separation of rare earth metals from an artificial leaching solution of
the phosphor sludge, listed in Table 1 [12], was carried out. The present work particularly focuses on theselective recovery of Eu, Tb and Y.
Table 1 Composition of fluorescence tube material wastes††

Elements

Y

La

P

Sr

Ca

Ba

Tb

Eu

Cl

Al

Mass %

31.9

11.8

7.4

6.4

5.5

3.4

2.7

2.2

1.2

0.8

Values are the analytical results from X-ray fluorescence measurements [12].
2. Experimental

Extraction tests were carried out by the conventional batchwise method. The commercial
extractant, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (marketed as PC-88A by Daihachi
Chemical Industry Co. Ltd.), was used as received. Organic solutions were prepared by dissolving
PC-88A in kerosene. Aqueous metal solutions were prepared by dissolving metal nitrate salts indeionized water. In single component systems, concentrations for divalent and trivalent metals were set
at 5.0 × 10-3 and 1.0 × 10-3 mol/L, respectively. In multi component systems, mixed solutions of La (8.5
× 10-3 mol/L), Ce (4.3 × 10-4 mol/L), Eu (1.5 × 10-3 mol/L), Tb (1.7 × 10-3 mol/L), Y (3.6 × 10-2 mol/L),
Al (3.0 × 10-3 mol/L), Ca (1.4 × 10-2 mol/L), Sr (7.3 × 10-3 mol/L) and Ba (7.3 × 10-3 mol/L) were used,based on the metals listed in Table 1. The concentration of PC-88A was set at 1.0 mol/L( [(RH) 2 ] = 0.5 mol/L ) for the divalent metals and 0.2 mol/L ( [(RH) 2 ] = 0.1 mol/L ) for the trivalent
metals for the single component systems, and was set at 0.5 mol/L ( [(RH) 2 ] = 0.25 mol/L ) for the multicomponent systems. The pH was adjusted by adding the appropriate amount of nitric acid or sodiumhydroxide to the aqueous metal solution. Equal volumes (10 mL) of the aqueous and organic solutions
were shaken vigorously at 298 K for more than 3 h to attain equilibrium. After phase separation, theequilibrium pH value was measured by a pH meter (HORIBA, F-23). The metal concentrations in the- 106 -

aqueous solutions were analyzed using an inductively coupled plasma atomic emission spectrometer(ICP-AES, SHIMADZU, ICPS-7500), and those in the organic solutions were calculated from thematerial balance.
3. Results and Discussion
3.1 Extraction Equilibrium Formulation
The extraction equilibrium expressions for PC-88A with trivalent metals (rare earth metals andAl) and divalent alkaline earth metals have been reported [13]. Fig. 1 shows the effect of theequilibrium pH on the distribution ratio (D) of the metals. Straight lines with slopes of 3 and 2 fortrivalent and divalent metals were obtained, respectively. The extraction equilibria of these metals aredefined in Eqs. (1) and (2), respectively [13].
M 3+ + 3(RH)2 ⇔ MR 3 (RH)3 + 3H + ; Kex,RE,Al(1)

AE 2+ + 3(RH)2 ⇔ AER 2 (RH)4 + 2H + ; Kex,AE(2)

where, RE, AE and (RH)2 represent rare earth,alkaline earth metals and the dimeric species of
PC-88A, respectively. The extraction equilibrium
constants are therefore defined as Eqs. (3) and (4).
[ MR 3 (RH) 3 ][H + ]3
K ex,RE,Al =[M 3+ ][(RH) 2 ]3slope=2

102

(3)

+ 2

[AER 2 (RH) 4 ][H ][M 2+ ][(RH) 2 ]3slope=3101

D(-)

K ex,AE =

103

(4)

La
Ce
Eu
Tb
Y
Al
Ca
Sr
Ba

100
10-1

The extraction equilibrium constants, determinedusing the above equations, are summarized in Table
2. The lines in Fig. 1 are the calculated values ofthe distribution ratios with the obtained extraction
equilibrium constants. The experimental data areseen to cluster on the lines drawn with thecalculated values.
Batchwise extractions from the artificial
leaching solution with PC-88A were also carriedout. Fig. 2 shows the effect of the pH value on the
distribution ratio of each metal in the multi

10-2
10-3
0

1

2

3

4

pHeq ( - )Figure 1 Effect of the equilibrium pH on the
distribution ratios in single metal systems;
comparison of observed data with prediction
shown by the lines

Table 2 Extraction equilibrium constants for PC-88A/kerosene
Metal

La

Kex (–)

3.29×10Metal

Ca

Kex (L/mol)

3.12×10Ce-3

2.23×10

Eu
-2

Sr
-4

2.81×10

4.02×10

-1

Ba
-6

5

2.08×10-6- 107 -

Tb

Y

Al

4.26

1.11×10

2.56×10-1

components system. Since Sr and Ba are hardly
extracted at all, the data for these two metals areomitted from Fig. 2. Since the distribution ratios
calculated by assuming no interaction among themetals are also shown by the lines, theexperimental data are seen to almost scatteraround the calculated lines. This indicates thatthe extraction equilibrium expressions obtainedin single metal systems can be applied to multicomponent system. In the multi component
system, extraction of the divalent metals is
suppressed compared with the extraction in the
single metal systems. Since this is due to thecompetition in the extraction of coexisting metals
in the multi component system, separation fromthe divalent alkaline metals is easily achieved.

103

Y Tb Eu Al Ce

D(-)

102

Ca

101

La

100
10-1
Keys as Fig. 1

10-2
0

1

2

3

4

5

pHeq ( - )Figure 2 Effect of equilibrium pH on the
distribution ratios in a multi metal system;comparison of observed data with prediction
shown by the lines

Table 3 Simulation of batchwise extraction and scrubbing†
SectionEx #1(pHeq=2)‡

Sc #1(pHeq=1.1)

§

Sc #2
(pHeq=1.4)

La

Ce
-3

Eu

4.28×10-4

1.45×10Tb-3

1.70×10Y-3

3.59×10Al-22.96×10-3[M]feed (mol/L)

8.50×10[M ] ( mol/L )Purity (%)

6.64×10-3

4.11×10-4

1.45×10-3

1.70×10-3

3.59×10-2

2.96×10-3

13.52

0.84

2.95

3.46

73.11

6.03

[M] (mol/L)

6.54×10-3

3.73×10-4

5.12×10-4

8.35×10-5

6.98×10-4

1.37×10-3[M ] ( mol/L )Purity (%)

9.77×10-5

3.78×10-5

9.35×10-4

1.62×10-3

3.52×10-2

1.59×10-3

0.25

0.10

2.37

4.09

89.16

4.03

[M] (mol/L)

8.71×10-5

2.07×10-5

5.87×10-5

1.02×10-5

8.52×10-5

1.52×10-4[M ] ( mol/L )Purity (%)

1.06×10-5
0.03

1.71×10-5

8.76×10-4

1.61×10-3

3.51×10-2

1.44×10-3

0.04

2.24

4.11

89.89

3.69

[(RH) 2 ] = 0.25 mol/L .
Organic feed of Sc #1 was organic phase of Ex #1.
§
Organic feed of Sc #2 was organic phase of Sc #1.
†‡

3.2 Simulation for Separation and Recovery of Rare Earth Metals
On the basis of these results, process simulation for separation and recovery of rare earth metals
from the fluorescence material wastes was carried out. From the experimental results in Fig. 2,batchwise extraction from the leaching solution is effective for separation from co-existing alkaline
earth metals.

- 108 -

Figure 3 shows the calculated results for the batchwise extraction of rare earth metals from theartificial leaching solution. This demonstrates that trivalent metals are easily extracted, while divalent
alkaline earth metals are poorly extracted, as expected from the experimental results. When theextraction is carried out at an equilibrium pH = 2, most of the trivalent metals are quantitatively
extracted into the organic phase, while most of the alkaline earth metals remain in the aqueous phase.
The effect of the equilibrium pH on the percentage scrubbing and purity in the organic phase isshown in Fig. 4. At pH = 1.1, most of La and Ce can be scrubbed off into the aqueous phase, while mostof the other metals remain in the organic phase. To improve the removal of La and Ce, additional
scrubbing may be carried out at pH = 1.4. Overall results of the simulation for batch extraction andscrubbing are also summarized in Table 3.
The process simulation for the separation of Eu/Tb/Y/Al was then investigated in acounter-current mixer-settler cascade. Figure 5 shows the schematic flowsheet of the cascade. Detailed
information on the calculation method for the counter-current mixer-settler cascade has been provided
previously [11].
100

Purity in Org. Phase ( % )

80

La
Ce
Eu
Tb
Y
Al
Ca
Sr
Ba

60

20

0

80
60

20

0

100

100

80

80

Scrubbing ( % )Extraction ( % )Purity in Org. Phase ( % )

100

60
40
20

La
Ce
Eu
Tb
Y
Al

60
40
20
0

0
0

1

2

3

0

4

1

2

3

4

pHeq ( - )pHeq ( - )Figure 3 Calculation results of the effect of pH

Figure 4 Calculation results of the effect of pH

on percentage extraction and purity of metals inon percentage scrubbing and purity of metals.

the organic phase. [(RH) 2 ] = 0.25mol/L[(RH) 2 ] = 0.25mol/L- 109 -

Org. output soln.

Feed soln.( F )[Mi]P[Mi]feed

0.04

Extr. Stage P
0.03[M] ( mol/L )[Mi]P[Mi]P-1[Mi]p+1[Mi]p

Extr. Stage p[Mi]p-1

[Mi]p

[Mi]1

[Mi]2

0.02

0.01

0.00

Extr. Stage 10Raffinate

10

15

Figure 6 Simulation of the separation of Y from a
Eu/Tb/Y/Al

Figure 5 Schematic flowsheet for thecounter-current mixer-settler cascade

5

Stage number p

[Mi]1

[Mi]0
Org. input soln.( E )

Eu
Tb
Y
Al

mixtureinacounter-current

mixer-settler cascade with 15 extraction stages at
F/E = 1. [(RH) 2 ] = 0.15mol/L and pHeq = 1.02

The distribution ratios for the metals (Eu, Tb, Y and Al) in stage p are expressed by Eq. (5) as
this can be calculated with the extraction equilibrium formulations.
[M i ] p = [M i ] p ⋅ Di, p( i = Eu, Tb, Y and Al)(5)

The mass balance of each metal in stage p is expressed by Eq. (6).
F ⋅ [M i ] p +1 = F ⋅ [M i ] p + E ⋅ [M i ] p − E ⋅ [M i ] p −1

(6)

The overall mass balance of the each metal is shown in Eq. (7).
F ⋅ [M i ]feed + E ⋅ [M i ]0 = F ⋅ [M i ]1 + E ⋅ [M i ] p(7)

In this simulation, the input values for the flow ratio (F/E = 1), [(RH) 2 ] feed (= 0.15 mol/L) , and a 15
stage cascade were applied. The equilibrium pH values of each stage are fixed to obtain effective
extraction.
The calculation results are showed in Fig. 6. Although co-extraction of Tb together with Y took
place in the early stages, the ion exchange reaction between extracted Tb and Y in the aqueous phaseprogresses from the middle of the cascade, to achieve the effective separation of Y without the need fora scrubbing section. The detailed results are summarized in Table 4. Although 36.3 % of the Tb, against
leaching solution, was contained in the organic phase, this was ignored to permit construction of asimple process in the present work. If the recovery of Tb in the organic solution is to be considered, theorganic phase should be scrubbed to remove co-extracted Tb.
The separation of Tb from the raffinate using a 10 stage counter-current mixer-settler cascade- 110 -

was then investigated. The input flow ratio

0.0012

values (F/E = 1) and [(RH ) 2 ]feed (=0.005

0.0010mol/L) were used. The calculation results
respectively.

Althoughthe[M] ( mol/L )are shown and listed in Fig. 7 and Table 4,
percentage

recovery of Tb was 58.1 % from theleaching solution, a purity of 85.7% Tb was

0.0008Eu
Tb
Al

0.00060.0004
obtained.
After this extraction step, the0.0002
mixture of Eu and Al still remained in the
0.0000raffinate. Since the separation factor of Eu
from Al is extremely small, precipitative
0
5
10
separation of Eu with oxalic acid isStage number pconsidered to be an effective separation
Figure 7 Simulation for the separation of Tb from amethod [14]. When over three equivalent
Eu/Tb/Al mixture in a counter-current mixer-settler
amounts of oxalic acid (6.8 × 10-3 mol/L)
cascade with 10 extraction stages at F/E = 1.
was added to an actual aqueous solution of[(RH) 2 ] = 0.005mol/L and pHeq =2.60
an Eu/Al mixture ([Eu] = 8.4 × 10-4 mol/Land [Al] = 1.1 × 10-3 mol/L), a white
precipitate was rapidly obtained. When the composition of the precipitate was analyzed after dissolution
with nitric acid, no Al was detected. The separation of Eu/Al is therefore easily achieved by this
precipitation method. The overall flow chart for the present proposed separation process is shown in Fig.
8.

Table 4 Simulation of a counter-current mixer-settler cascade for the separation of Eu/Tb/Y/Al
Section

MS #1

F/E

input

1

Stage

[(RH) 2 ]feed

number(mol/L)15

1.5×10-1

output

MS #2

input
output

pHeq

1.022

1

10

5.0×10-3
2.601

[Eu]

[Tb]

[Y]

[Al]

[Eu ]

[Tb ]

[Y]

[Al]

(mol/L)(mol/L)(mol/L)(mol/L)(mol/L)(mol/L)(mol/L)(mol/L)

8.76×10-4

1.61×10-3

3.51×10-2

1.44×10-3

0

0

0

0

-4

-4

-5

-3

3.17×10

6.17×10-2

3.51×10

3.32×10-5

9.89×10

8.44×10-4

9.89×10-40

1.41×10-3

0

0

0

0

-4

-6

0

1.32×10-3

7.98×10-5

9.87×10-4

0

8.46×10-5

1.67×10

1.41×10-4

8.44×107.65×10

1.54×10-5

4. Conclusion
The separation and recovery of rare earth metals from fluorescent material wastes by using solvent
extraction with PC-88A in kerosene was investigated as follows:(1) Extraction equilibrium expressions for the trivalent (La, Ce, Eu, Tb, Y and Al) and divalent metals(Ca, Sr and Ba) were established, and these equilibrium expressions can be applied to the multicomponent system.
(2) Using process simulation, the effective separation and recovery of Eu, Tb and Y can be achieved by- 111 -

two steps in a counter-current mixer-settler cascade, together with precipitation with oxalic acid,followed by batchwise extraction-scrubbing processes.
(3) The separation and recovery process developed in the present work is based on solvent extraction
and precipitation, which is feasible for the recovery of valuable resources at industrial scale.

FluorescencesludgeLeaching with HNO3
Leaching solution
NaOH
pH control
pH=2.0

Solvent Extraction (Ex #1)

Aqueous phase

La, Ce, Eu,
Tb, Y, Al

Organic phase

Ca, Sr, Ba
Scrubbing
pH=1.1 (Sc #1)
pH=1.4 (Sc #2)

Eu, Tb, Y, Al

Organic phase

Aqueous phase
La, Ce, Eu, Tb

Counter-current mixer
settler cascade (MS #1)
pH=1.02
15 Stages, E/F=1

Organic phase

Raffinate

Eu, Tb, Al

Recovery of Y

Counter-current mixer
settler cascade (MS #2)
pH=2.60
10 Stages, E/F=1

Recovery = 97.8 %Purity = 98.1 %Organic phase

Eu, AlRaffinateOxalic acid

Recovery of Tb

Precipitation

Recovery = 58.1 %Purity = 85.7 %Recovery of Eu
Recovery = 52.8 %Purity ≒ 100 %Figure 8 The overall flow chart of the present separation process. The
recovery is calculated against the leaching solution compositions- 112 -

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- 113 -