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

106

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

107

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

108

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

109

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

110

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

111

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

112

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

113

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

114

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.

115

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

116

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

117

[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].

118

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

119

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

120

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