Extraction of Neodymium (III) from acidic nitrate medium...
Transcript of Extraction of Neodymium (III) from acidic nitrate medium...
105
Extraction of Neodymium (III) from acidic nitrate medium
using Cyanex 921/Cyanex 923 in kerosene
5.1. Introduction
The organophosphorous extractants are a good choice for their solvation properties as
well as their chemical stability and low aqueous solubility. These commercial extractants have
been widely used for the extraction and separation of rare earths [182,183]. The extraction of
some rare earths from HNO3 solution using bifunctional neutral organophosphorous compounds
has been studied by Turanov et al [102]. The extraction of the trivalent middle rare earths from
chloride media by kerosene solutions of 2-ethylhexylphosphonic acid mono-2-
ethylhexyl ester as an extractant was studied. The order of extraction ability of middle RE by 1.0
mol/L HEHEPA in kerosene is Tb>Gd>Eu>Sm by Fontana et al [184]. The extraction of rare
earths using 1-hexyl-4-ethyloctyl isopropyl phosphonic acid (HEIPPA) in heptane was studied
by Lu et al [185]. The substitution of phenyl group by p-tolyl group resulted in enhancement of
extraction. Thermodynamic model was given by Mokili et al [94] for the extraction of Nd and Pr
nitrates from nitric acid with tri-n-butyl phosphate. The model was tested over a wide range of
lanthanide concentration. The extraction behavior of some trivalent lanthanides including Nd
was reported by Gupta et al [101] using Cyanex 923 as extractant in toluene from various
aqueous acid media and effective separation was achieved between lighter and heavier
lanthanides. In thiocyanate and nitrate media, the distribution ratios of trivalent lanthanides are
found to increase with decreasing ionic radii as reported by Reddy et al [98]. The extraction of
rare earths from chloride media was studied by Wang et al [186] and extractability was explained
on the basis of the extractant structure and it was found that oxygen donors have more affinity
towards the rare earths. The extraction and stripping of trivalent neodymium (III) by the neutral
extractant Cyanex 921 in kerosene from thiocyanate medium was studied by El-Hefny [187].
The effect of various diluents on the extraction of Pr (III) and Sm (III) by Cyanex 923 was
studied by El-Nadi [113]. He reported that extraction was found to increase with decrease in the
polarity of the diluents. Extraction of Yb (III) from sulphuric acid medium using Cyanex 923 in
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heptane at equilibrium pH 3.0 was reported by Wang et al [188]. The extracted species appears
to be YbSO4 (HSO4).2Cyanex 923 following salvation mechanism.
As there are few reports on the extraction of neodymium (III) by neutral
organophosphorous extractants, an attempt has been made to know the extractability of Cyanex
921 and Cyanex 923 for Nd (III) and to establish the optimum conditions for quantitative
extraction. The extraction of neodymium (III) from acidic nitrate medium using Cyanex
921/Cyanex 923 in kerosene was investigated. The effect of shaking time, nitric acid
concentration, nitrate ion concentration and extractant concentration, temperature, metal ion
concentration on the extraction of neodymium (III) were studied. Stripping experiments were
carried out to know the best stripping agent.
5.2. Results and Discussion
5.2.1. Effect of equilibration time
To know the effective shaking time for the extraction of 0.001 M Nd (III) from 0.001 M
HNO3 and 0.1 M KNO3 with 0.1 M Cyanex 921 and 0.1 M Cyanex 923, the equilibration time
was varied from 1 minute to 60 minutes.
Table 5.1. Effect of equilibration time on the extraction of 0.001 M Nd (III) from 0.001 M
HNO3 and 0.1 M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
Time in minutes % Extraction
Cyanex 921 Cyanex 923
1 25.0 19.5
2 30.0 25.9
5 40.0 36.7
10 46.0 38.8
15 55.1 45.0
20 55.4 44.9
25 55.1 44.9
30 55.0 37.5
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60 55.3 37.3
Extraction of metal increased from 25.0% to 55.2% with 0.1 M Cyanex 921 and from 19.5% to
45% with 0.1 M Cyanex 923, with increase in time from 1 minute to 15 minutes, and then
decreased with further increase in time (Table 5.1) (Figure 5.1.). Therefore, 15 minutes shaking
time was maintained as the equilibration time.
Figure 5.1. Plot of % extraction versus equilibration time for the extraction of 0.001 M Nd
(III) from 0.001 M HNO3 and 0.1 M KNO3 using 0.1 M Cyanex 921/ 0.1 M Cyanex 923
in kerosene.
5.2.2. Effect of nitric acid concentration
Effect of nitric acid concentration on the extraction of 0.001 M Nd (III) from 0.1 M
KNO3 was studied with 0.1 M Cyanex 921/Cyanex 923 in the concentration range of 0.001 M to
0
20
40
60
0 15 30 45 60
% E
xtr
act
ion
Time in minutes
■ Cyanex 921
♦ Cyanex 923
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0.04 M. It was found that the extraction of 0.001 M Nd (III) increased from 55.1% to 63.3% with
increase in nitric acid concentration from 0.001 M to 0.008 M with 0.1 M Cyanex 921, whereas
with 0.1 M Cyanex 923, the extraction increased from 45.0% to 46.8% up to nitric acid
concentration 0.004 M, then decreased with further increase in concentration upto 0.04 M (Table
5.2) (Figure 5.2). The decrease in extraction of metal is because of the extraction of acid by the
neutral extractants. The oxygenated solvents like organophosphorus esters solvate the proton and
form a neutral inorganic species resulting in the extraction of the acids [162].
Table 5.2. Effect of nitric acid concentration on the extraction of 0.001 M Nd (III) from 0.1
M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
[HNO3],M
Cyanex 921 Cyanex 923
D % Extraction D % Extraction
0.001 1.23 55.2 0.81 45.0
0.002 1.43 58.8 0.87 46.5
0.004 1.44 59.0 0.88 46.8
0.008 1.73 63.3 0.73 42.2
0.01 0.94 48.4 0.69 40.8
0.02 0.84 45.6 0.57 36.3
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0.04 0.60 37.5 0.51 33.7
Figure 5.2. Plot of % extraction versus [HNO3], M for the extraction of 0.001 M Nd (III)
from 0.1 M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
5.2.3. Effect of nitrate ion concentration
To know the involvement of nitrate ion in the extracted complex the concentration of
potassium nitrate (KNO3) was varied in the range of 0.01 M to 3.0 M for the extraction of 0.001
M Nd (III) from 0.001 M HNO3 using 0.1 M Cyanex 921/Cyanex 923 in kerosene. The results
showed that the extraction of 0.001 M Nd (III) increased from 17.3% to 90.9% with increase of
nitrate ion concentration from 0.01 M to 0.45 M with 0.1 M Cyanex 921, whereas from 2.0% to
85.6% with nitrate ion concentration in the range 0.01-1.0 M with 0.1 M Cyanex 923 and then
decreased with further increase in concentration with both the extractants (Table 3.3). The plot of
log D versus log [NO3-] yields a slope of 2.6 with Cyanex 921 and 2.8 with Cyanex 923, which
indicate the incorporation of three molecules of nitrate ions in the extracted complex (Figure
5.3). From table 5.3, it is observed that the addition of nitrate salt to the aqueous phase greatly
0
20
40
60
80
0 0.01 0.02 0.03 0.04
% E
xtr
act
ion
[HNO3],M
■ Cyanex 921
♦ Cyanex 923
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enhances the extraction. The salting-out effect is supposed to be due to increase in the
hydrodynamic activity of neodymium in the aqueous phase with increase in nitrate ion
concentration.
Table 5.3. Effect of nitrate ion concentration on the extraction of 0.001 M Nd (III) from
0.001 M HNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
[KNO3],M
Cyanex 921 Cyanex 923
D % Extraction D % Extraction
0.01 0.21 17.3 0.02 2.0
0.02 0.51 33.6 0.24 19.3
0.05 0.98 49.5 0.33 24.9
0.1 1.23 55.2 0.81 45.0
0.2 2.53 71.7 1.56 60.9
0.3 3.46 77.6 1.58 61.2
0.35 3.78 79.1 2.20 68.7
0.4 5.97 85.6 3.12 75.7
0.45 10.03 90.9 4.75 82.6
0.8 0.63 38.6 5.88 85.5
1.0 0.23 18.7 6.12 85.9
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3.0 0.11 9.9 1.10 52.4
With further increase in nitrate ion concentration, the activity of water molecule was decreased
resulting in decrease in metal extraction [166].
Figure 5.3. Plot of log D versus log [NO3-] for the extraction of 0.001 M Nd (III) from 0.001
M HNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
5.2.4. Effect of extractant concentration
The extractability of Cyanex 921 and Cyanex 923 were studied by varying Cyanex 921
concentration from 0.02 M to 0.5 M and Cyanex 923 concentration from 0.01 M to 1.0 M for the
extraction of 0.001 M Nd (III) from 0.001 M HNO3 and 0.1 M KNO3. The results obtained
y = 2.6x + 1.8
R² = 0.8
y = 2.8x + 1.6
R² = 0.9
0
0.6
1.2
1.8
2.4
-0.6 -0.3 0
log
D
log [NO3-]
Cyanex 921
Cyanex 923
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showed that the extraction of Nd (III) increased from 9.1% to 97.9% with increase in
concentration of Cyanex 921 up to 0.5 M, whereas with increase of Cyanex 923 concentration
from 0.01 M to 1.0 M, the extraction increased from 1.9% to 94.0% (Table 5.4). The plot of log
D versus log [extractant] yields a slope of 1.6 with Cyanex 921 and 2.4 with Cyanex 923, which
indicates the presence of two molecules of Cyanex 921 or Cyanex 923 in the extracted complex
(Figure 5.4).
Table 5.4. Effect of extractant concentration on the extraction of 0.001 M Nd (III) from
0.001 M HNO3 and 0.1 M KNO3.
[Cyanex 921/Cyanex 923], M
Cyanex 921 Cyanex 923
D % Extraction D % Extraction
0.02 0.10 9.1 0.02 1.9
0.04 0.15 13.0 0.04 3.8
0.05 0.23 18.7 0.05 4.7
0.08 0.47 31.9 0.07 6.5
0.1 1.23 55.2 0.81 45.0
0.15 1.30 56.5 0.91 47.6
0.2 2.05 67.2 1.10 52.4
0.25 2.57 71.9 2.25 69.2
0.3 3.97 79.9 3.12 75.7
0.4 5.88 85.5 6.20 86.1
0.45 6.25 86.2 6.31 86.3
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0.5 47.74 97.9 10.36 91.2
0.6 - - 13.22 93.0
0.8 - - 14.10 93.4
1.0 - - 15.81 94.0
Figure 5.4. Plot of log D versus log [Extractant] for the extraction of 0.001 M Nd (III) from
0.001 M HNO3 and 0.1 M KNO3 using Cyanex 921/Cyanex 923 in kerosene.
y = 1.6x + 1.4
R² = 0.8
y = 2.4x + 1.4
R² = 0.9
-3
-2
-1
0
1
2
-3 -2 -1 0 1
log D
log [Extractant]
Cyanex 921
Cyanex 923
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5.2.5. Extraction Equilibrium
On the basis of above slope analysis results the extraction equilibrium for the extraction
of Nd (III) from acidic nitrate medium may be represented as:
Nd(aq )3+ + 3NO3(aq )
− + 2L(org ) Nd(NO3)3. 2L(org ) (5.2.5.1)
where L is Cyanex 921 or Cyanex 923
The distribution ratio is given as
D = [Nd(NO3)3. 2L](org )/[Nd3+](aq ) (5.2.5.2)
The equilibrium constant Kex can be expressed as
Kex = [Nd(NO3)3. 2L](org )/[Nd3+](aq )[NO3−](aq )
3 [L](org )2 (5.2.5.3)
5.2.6. Effect of temperature
The effect of temperature on the extraction of 0.001 M Nd (III) from 0.001 M nitric acid
and 0.1 M KNO3 using 0.1 M Cyanex 921/Cyanex 923 in kerosene was studied in the
temperature range of 25˚C(298K)–65˚C(338K). It was found that percentage of the extraction
decreased from 55.2% to 8.2% with 0.1 M Cyanex 921, whereas with 0.1 M Cyanex 923,
extraction decreased from 45.0% to 17.3% with increase in temperature from 298K to 338K
(Table 5.5). The standard enthalpy change (∆H°) and entropy change (∆S°) were calculated by
plotting log Keq vs. 1000/T as shown in Figure 5.5 and by using the Van‟t Hoff equation (3.2.6.1)
as stated in chapter 3.
From the plot, ∆H° was found to be - 49.8 kJmol-1
and ∆S° was found to be -
73.3 J K-1
mol-1
for Cyanex 921, whereas for Cyanex 923, ∆H° and ∆S° were obtained as -
30.6 kJ mol-1
and -11.5 J K-1
mol-1
. The negative value of standard enthalpy change in both the
cases indicates that the reaction is exothermic. The negative value of standard entropy change
indicates the formation of a stable complex which makes the extraction system more ordered
resulting decrease in entropy value.
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Table 5.5. Effect of temperature on the extraction of 0.001 M Nd (III) from 0.001 M HNO3
and 0.1 M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
Temperature in Kelvin Cyanex 921 Cyanex 923
D %Extraction D % Extraction
298 1.23 55.2 0.81 45.0
308 0.43 30.1 0.34 25.4
318 0.37 27.0 0.29 22.5
328 0.19 15.9 0.24 19.3
338 0.09 8.2 0.21 17.3
Figure 5.5. Plot of log Keq versus 1000/T for the extraction of 0.001 M Nd (III) from 0.001
M HNO3 and 0.1 M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
5.2.7. Effect of diluents
The extraction of Nd (III) has been studied using different organic diluents, namely
toluene, benzene, xylene and kerosene, keeping other parameters fixed i.e. temperature, 0.001 M
y = 2.6x - 3.8
R² = 0.961
y = 1.6x - 0.6
R² = 0.92
0
1
2
3
4
5
6
2.9 3 3.1 3.2 3.3 3.4
log
Keq
1000/T
Cyanex 921
Cyanex 923
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Nd (III), 0.001 M HNO3, 0.1 M KNO3, 0.1 M Cyanex 921 in all diluents and contact time of 15
minutes.
Table 5.6. Effect of diluents on the extraction of 0.001 M Nd (III) from 0.001 M HNO3 and
0.1 M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923.
Name of diluents Cyanex 921 Cyanex 923
D % Extraction D % Extraction
Kerosene 1.23 55.2 0.81 45.0
Benzene 0.16 13.8 0.08 7.4
Toluene 0.04 3.8 Turbidity -
Xylene 0.30 23.1 Turbidity -
Chloroform 0.21 17.3 0.38 27.5
Results are presented in Table 3.6. It is observed that although benzene and toluene have
dielectric constant of the same order of magnitude as that of kerosene, both is less effective for
extraction of Nd (III). This may be due to the lower solubility of the extracted species in these
diluents as compared to that in kerosene and also because of the different intermolecular diluent,
diluent and solvent, diluent interactions that occur in aromatic and non- aromatic diluents [189].
It is clear from Table 5.6 that kerosene is the best diluent for the extraction system.
5.2.8. Effect of metal ion concentration
The extraction of Nd (III) from 0.001 M HNO3 and 0.1 M KNO3 with 0.1 M Cyanex
921/Cyanex 923 was studied by varying the concentration of metal ion from 0.001–0.1 M. After
the analysis it was found that the extraction increased from 55.2% to 73.7% using 0.1 M Cyanex
921 and from 45.0% to 80.5% using 0.1 M Cyanex 923 with increase in concentration of metal
ion from 0.001 M to 0.05 M, then decreased with further increase in metal ion concentration
(Figure 5.6).
Table 5.7. Effect of metal ion concentration on the extraction of Nd (III) from 0.001 M
HNO3 and 0.1 M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene
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[Nd(III)],M
Cyanex 921 Cyanex 923
D % Extraction D % Extraction
0.001 1.23 55.2 0.81 45.0
0.002 1.65 62.3 0.92 48.0
0.005 2.0 66.6 1.01 50.2
0.008 2.25 69.2 1.14 53.3
0.01 2.63 72.4 3.91 79.6
0.02 2.71 73.0 4.05 80.2
0.05 2.81 73.7 4.12 80.5
0.08 1.56 61.0 2.01 66.7
0.1 0.86 46.1 0.75 42.9
The data are given in table 5.7. The decrease in extraction of metal is may be due to the
unavailability of extractant to extract the metal of high concentration [167].
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Figure 5.6. Plot of % extraction versus [Nd(III)],M for the extraction of Nd (III) from 0.001
M HNO3 and 0.1 M KNO3 using 0.1 M Cyanex 921/0.1 M Cyanex 923 in kerosene.
5.2.9. Stripping
The loaded organic phase containing 0.141 gpl of Nd (III) in 0.5 M Cyanex 921 and
0.135 gpl in 1.0 M Cyanex 923 collected after extraction were stripped using various
concentrations of Na2CO3, NaOH, HCl and H2SO4 in equal phase ratio to find a suitable
stripping agent. The aqueous phase was analyzed for the metal concentration (Table 5.8). It was
found that the stripping was 100% with 0.01 M HCl and 0.002 M H2SO4 with 0.5M Cyanex 921
whereas 0.008 M HCl and 0.04 M H2SO4 were found to be effective stripping solutions for the
recovery of 100% metal from the loaded organic phase of 1.0 M Cyanex 923 (Figure 5.7).
0
20
40
60
80
100
0 0.02 0.04 0.06 0.08 0.1 0.12
% E
xtr
act
ion
[Nd(III)], M
■ Cyanex 921
♦ Cyanex 923
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Table 5.8. Stripping of Nd (III) from the loaded organic phase of 0.5 M Cyanex 921 and 1.0
M Cyanex 923 in kerosene.
Loaded organic phase 0.5 M Cyanex 921 1.0 M Cyanex 923
[Stripping solution],M HCl H2SO4 HCl H2SO4
% Stripping
0.002 70.0 100.0 47.5 43.6
0.005 89.5 85.0 68.0 59.0
0.008 92.0 89.8 100.0 73.6
0.01 100.0 72.0 87.5 85.4
0.02 97.4 65.0 79.7 98.0
0.04 96.5 64.0 74.2 100.0
0.05 95.0 62.0 66.2 86.9
0.1 94.0 59.0 59.7 68.4
Figure 5.7. Plot of % stripping versus [Acid], M for recovery of Nd (III) from the loaded
organic phase of 0.5 M Cyanex 921 and 1.0 M Cyanex 923 in kerosene.
0
20
40
60
80
100
120
0 0.02 0.04 0.06 0.08 0.1 0.12
% S
trip
pin
g
[Acid],M
■ Cyanex 921 + HCl ♦ Cyanex 921 + H2SO4
▲Cyanex 923 + HCl ● Cyanex 923 + H2SO4
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5.3. Conclusions
The distribution data for the extraction of 0.001 M Nd (III) from acidic nitrate medium
showed that extraction was quantitative (97.9%) with 0.5 M Cyanex 921 and of 94.0%
with 1.0 M Cyanex 923 in kerosene.
From slope analysis, the composition of the extracted species is proposed to be
Nd(NO3)3.2L, where L is Cyanex 921 or Cyanex 923.
The negative values of standard enthalpy change indicated extraction process was
exothermic. The negative value standard entropy change confirmed the formation of a
stable complex.
0.01 M HCl and 0.002 M H2SO4 can effectively strip neodymium from the loaded
organic phase of 0.5 M Cyanex 921 and whereas 0.008 M HCl and 0.04 M H2SO4 were
found to be effective stripping solutions for the recovery of 100% metal from the loaded
organic phase of 1.0 M Cyanex 923.
xxx