Solvent Extraction of Lanthanum Ion from Chloride Medium by Di-(2-ethylhexyl) Phosphoric Acid with a Complexing Method

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International Journal of Nonferrous Metallurgy, 2013, 2, 75-79
http://dx.doi.org/10.4236/ijnm.2013.22010 Published Online April 2013 (http://www.scirp.org/journal/ijnm)
Solvent Extraction of Lanthanum Ion from Chloride
Medium by Di-(2-ethylhexyl) Phosphoric Acid with a
Complexing Method
Shaohua Yin, Wenyuan Wu*, Xue Bian, Yao Luo, Fengyun Zhang
School of Materials and Metallurgy, Northeastern University, Shenyang, China
Email: *wuwy@smm.neu.edu.cn
Received December 26, 2012; revised January 30, 2013; accepted February 10, 2013
Copyright © 2013 Shaohua Yin et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Solvent extraction experiments of La(III) with di-(2-ethylhexyl)phosphoric acid (P204) from chloride solution in the
presence of a complexing agent (lactic acid) have been performed. The effective separation factors can be achieved
when the complexing agent is added to the aqueous phase of the extraction system. The complexing agent lactic acid
can be effectively recycled using tributyl phosphate (TBP) as extractant, by the use of a countercurrent extraction proc-
ess, and the chemical oxygen demand (COD) value in the raffinate is 57.7 mg/L, which meets the emission standards of
pollutants from rare earths industry. Thus, the simple and environment-friendly complexing method has been proved to
be an effective strategy for separating light rare earths, and provides a positive influence on the purification of La(III).
Keywords: La(III); Extraction and Separation; P204 Extractant; Lactic Acid; TBP
1. Introduction
Rare earths are important elements, from an industrial
point of view. They are extensively used in the metal-
lurgy, lasers, magnets, and batteries. Lanthanum (La), for
example, one of the most abundant rare earths, is of cur-
rent commercial interest as it is used in the hydrogen
storage materials, various alloy materials, etc. With the
increasing demand for it, the separation and purification
of La(III) have gained considerable importance.
Actually, solvent extraction is a classical chemical
analytical method, playing an important role as a separa-
tion technique [1]. Saponified acidic extractant such as
di-2-ethylhexyl phosphoric acid (P204) has been used to
separate La(III) and rare earths under some conditions,
which can enhance the extraction capacity of acidic ex-
tractants, but the resulting loss of ammonium ion to
aqueous phase causes serious pollution [2,3], which be-
comes an important issue in the current rare-earth indus-
try. Therefore, it is essential to find a new path for sepa-
rating La(III) from the adjacent REs.
Up to date, much effort has been devoted to exploring
some new extraction systems or extractants superior to
the existing saponified extraction system, such as syner-
gistic extraction systems [4-6], ionic liquids (ILs) [7-9]
containing functional groups used in the rare-earth ions
extraction and separation. One of the most effective me-
thods for improving the separation is to add a water-solu-
ble complexing agent into the aqueous phase [10]. Su-
jatha studied the separation of Ce(III) and Nd(III) using
the glycine as complexing agent, and found the average
separation factor between these lanthanides was im-
proved from 3.2 to 3.8 [11]. Similar trends have been
found in the separation of the lanthanides using other
complexing agents such as EDTA, DTPA and HEDTA
[12-16]. However, the complexing agents have high cost
and are difficult to be recycled.
We investigated the extraction of La(III) using a com-
plexing agent citric acid with unsaponified P204, and
found the extraction effect was as good as the saponified
system [17]. However, some disadvantages still exist in
these processes, such as very high extraction acidity and
a difficult stripping. Hence, there is a growing interest in
the development of new systems using a complexing
agent hydroxy carboxylic acid for effective separation of
REs. As a kind of hydroxy carboxylic acid, lactic acid
(abbreviated as HLac) is similar to the citric acid, and is
currently a promising option for the hydrometallurgical
separation of the trivalent lanthanides. The separation of
Pr(III)/Ce(III) using the HLac has been investigated, and
the extraction performance is better than that without the
complexing agent [18].
*Corresponding author.
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S. H. YIN ET AL.
76
In this paper, the extraction of La(III) using the un-
saponified P204 in the presence of the HLac in the labo-
ratory was investigated. The various extraction effects on
different La(III) were reported and considered for the
separation of La(III) from the adjacent rare earths.
2. Materials and Methods
2.1. Materials
P204 and TBP supplied by Tianjin Kermel Chemical
Reagent Co. Ltd. were used without purification and di-
luted in sulphonating kerosene. A LaCl3 solution with a
concentration of 0.2 mol/L was prepared by dissolving
lanthanum carbonate with a certain proportion of HLac
and a small amount of HCl. Lactic acid was supplied by
Sinopharm Chemical Reagent Co. Ltd. All other chemi-
cals used were of analytical reagent grade.
Digital pH meter (pHs-3C, Shanghai Rex Instruments
Factory), calibrated daily with 4.01 and 6.86 standard
buffer solutions, was employed to measure pH values of
the aqueous phase. An Agilent 1100 model HPLC was
employed to measure the concentration of lactic acid in
the recycling experiments.
2.2. Solvent Extraction Procedure
For the equilibrium experiments, equal volumes (20 mL)
of the aqueous and P204 extractant were mixed and
shaken for 30 min at 298 ± 1 K using a mechanical shak-
er. After phase separation, the concentration of La3+ left
in the aqueous phase was analyzed by titration with a
standard solution of EDTA at pH 5.5 using xylenol or-
ange as an indicator, and that in the organic phase was
obtained by mass balance. Distribution ratio was ob-
tained by D = [La3+]o/[La3+]a, where “a” and “o” denote
aqueous phase and organic phase. pH value was deter-
mined after extraction and phase separation.
For the recycling experiments, equal volumes (10 mL)
of the raffinate after the above extraction and TBP ex-
tractant were mixed and shaken for 30 min at 298 ± 1 K
using a mechanical shaker. The HLac concentration was
analyzed by a HPLC method after entire stripping, and
the extraction efficiency E is defined as follows:





HLac HLac
%HLac
t
t
E
a
(1)
where [HLac]t and [HLac]a represent initial and strip
liquor concentrations of HLac in aqueous phase, respect-
ively.
3. Results and Discussion
3.1. The Effect of Acidity on Extraction of
La(III)
It should be noted that the pH value in the aqueous solu-
tion plays an important role in the extraction system with
acidic extractants. The plots of distribution ratio (D)
versus aqueous pH are shown in Figure 1. It can be seen
that the D shows an increasing trend with pH increasing
at a constant concentration of lactic acid.
3.2. The Effect of HLac Concentration on the
Extraction
The effect of HLac concentration on the extraction of
La3+ with P204 is studied and the results are shown in
Figure 2. From Figure 2, it can be found that the distri-
bution ratios of La(III) increase with the increasing of
lactic acid concentration, and D reaches largest at the 0.6
mol/L HLac concentration and initial aqueous pH 3.5.
This may be explained in terms of the buffering action of
HLac, which slows down the effect of higher acidity in
the raffinate on the distribution ratios.
In practical application, the effect of initial aqueous
pH is considered to be an important parameter, primarily
because the acidity control of the aqueous phase is one of
Figure 1. The effect of aqueous pH on the extraction of
La(III).
Figure 2. The effect of lactic acid concentration on the ex-
traction of La(III).
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S. H. YIN ET AL. 77
the key links in industrial processes. To our knowledge,
extraction abilities of acidic organophosphorous extrac-
tants are commonly effected by the hydrogen ion from
the hydroxyl group in P204 [19]. As a rule, the extraction
of the trivalent rare-earth ions with P204 is an ion-ex-
change mechanism (Equation (2)). The rare earths ex-
traction efficiency can be decreased due to the increase
of the acidity in the raffinate, which is caused by the hy-
drogen ions exchanged with rare earth ions. To overcome
the disadvantages, the acidic extractants saponified with
aqueous ammonia have been widely applied for the ex-
traction (Equation (3)), but saponification will result in
serious pollution, emulsification and the third phase for-
mation during extraction. Therefore, the careful control
of the acidity in the raffinate is a critical component to
the successful operation of the extraction process. The
application of lactic acid is an effective strategy for eli-
minating this phenomenon, as shown in Equation (4),
where “x” is the number of the ligand, and equals to 1, 2
or 3, and Lac is the lactate ion. The rare-earth ions in the
aqueous phase can form a variety of complexes in the
presence of lactic acid. However, a particular complex is
prominent in the feed solution under the experimental
conditions. After extraction, some hydrogen ions in the
raffinate can combine with the lactate, which reduces the
acidity in the aqueous phase and slows down the effect of
higher acidity in the raffinate on the distribution ratios.
On the other hand, no ammonium ion is released in the
extraction process, indicating that it is a better environ-
ment-friendly process than the saponification extraction
method.


3+ +
22 2
3
RE +3H A RE HA +3H (2)
+
22 3 2 4 2
HA 2NH HO 2NHA 2HO

  (3)






 
3-x +
22
x
+
3
RE Lac +3H A
=REA 3HA + 3 x H xHLac
o
a
oa

a
(4)
3.3. Separation Performance
It is of considerable interest to quantitatively compare the
separation ability. Under equilibrium conditions, the se-
paration factors of the adjacent rare earth elements, β,
can be defined as:
2
1
D
D

 (5)
where the D1 and D2 refer to the distribution ratios of
metal ion 1 and metal ion 2, respectively. Generally, the
value of D2 is larger than that of D1.
As can be seen from Table 1, the separation factors for
Ce/La, Pr/La and Nd/La increase with the aqueous pH
value increasing at HLac concentration of 0.6 mol/L. For
example, at pH 3.5 and HLac concentration of 0.6 mol/L
in the range studied, the separation factors for Ce/La,
Pr/La and Nd/La become values of 3.42, 6.98 and 11.16.
From the separation factors, we can come to the conclu-
sion that this system should be a more effective method
to separate La3+ from the other rare earths.
3.4. Recycling Experiments of the Complexing
Agent HLac
As for the above experimental results, this system could
be considered an efficient potential method for separating
REs. However, there is a standard about the waste-water
containing high organic loadings in the rare earths indus-
try, otherwise, it will affect the chemical oxygen demand
(COD). So the treatment of HLac from the extraction
system becomes an important issue. In this study, we
determine the optimum conditions by orthogonal test
firstly when the lactic acid concentration is 0.6 mol/L.
Using a countercurrent extraction process at a phase ratio
Vo:Vw = 1:1, t = 20˚C, pH = 0.6, and 75 vol% TBP in
kerosene, the HLac recovery reaches more than 99% for
10 extraction stages as shown in Table 2.
In order to evaluate whether the lactic acid concentra-
tion achieves the emission standards of pollutants from
rare earths industry or not, we calculate the COD in the
raffinate after the 11 extraction stages. As can be seen
from the Table 3, the COD value after the 11 extraction
stages can be up to 57.7 mg/L, which meets the emission
standards of pollutants from rare earths industry (COD ≤
80 mg/L), where theory of chemical oxygen demand of
HLac is 1.07 g/g [20]. The results indicate that the HLac
can be effectively recycled.
Table 1. Separation factors (Di/DLa) under the experimental
conditions.
pH 2 2.5 3 3.5
βCe/La 2.42 2.8 3 3.42
βPr/La 3.92 4.82 5.58 6.98
βNd/La 5.1 6.5 8.04 11.16
Table 2. Extraction efficiencies (E%) of each row.
Parameter Stages 1 2 3 4 5
Row 1 (E%) HLac 0 0.06 0.21 1.01 1.91
Row 2 (E%) HLac 0.05 0.13 0.65 1.57 3.8
Row 3 (E%) HLac 0.67 1.46 1.98 7.98 9.85
Parameter Stages 6 7 8 9 10
Row 1 (E%) HLac 22.22 59.65 84.81 97.31 99.19
Row 2 (E%) HLac 24.2 49.09 77.85 91.93 99.18
Row 3 (E%) HLac 15.23 41.78 67.53 88.53 99.19
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S. H. YIN ET AL.
78
Table 3. The experimental results of multistage counter-
current extraction.
Parameter 10 stages countercurrent extraction
Mass concentration Feed (g/L) E% Raffinate COD (mg/L)
HLac 54.048 99.2 0.4 428
Parameter 11 stages countercurrent extraction
E% Raffinate COD (mg/L)
99.9 0.054 57.7
4. Conclusions
The following conclusions are drawn:
1) The distribution ratios of the extraction of La(III) by
P204 increase with the increase of the pH value in the
feed solution and lactic acid concentration. The maxi-
mum separation factors of Ce/La, Pr/La and Nd/La be-
come values of 3.42, 6.98 and 11.16 at pH 3.5 and HLac
concentration of 0.6 mol/L.
2) The recycling experiments show that the complex-
ing agent lactic acid could be efficiently recycled, and the
COD in the raffinate meets the emission standards of
pollutants from rare earths industry.
5. Acknowledgements
Financial aid from the following programs is gratefully
acknowledged: the National Natural Science Foundation
of China (50974042, 51104040 and 51274060), the Na-
tional Program on Key Basic Research Project of China
(973 Program) (2012CBA01205), the National Key Tech-
nology Research and Development Program of the Minis-
try of Science and Technology of China (2012BAE01B00)
and the Scientific Research special Foundation of Doctor
subject of Chinese Universities (20100042110008).
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