Extraction study of rare earth elements with N,N′-dibutyl –N,N′-di(1-methylheptyl)-diglycolamide from hydrochloric acid

NUCLEAR CHEMISTRY, RADIOCHEMISTRY, RADIOPHARMACEUTICALS AND NUCLEAR MEDICINE

Extraction study of rare earth elements with N,N′-dibutyl –N,N′-di(1-methylheptyl)-diglycolamide from hydrochloric acid

Guo-Jing Sun，

Jin-Hong Yang，

Hong-Xiao Yang，

Guo-Xin Sun，

Yu Cui

Nuclear Science and Techniques

Vol.27, No.3

Article number 75

Published in print 20 Jun 2016

Available online 16 May 2016

DOI：10.1007/s41365-016-0055-0

48300

A novel, unsymmetrical diglycolamide, N,N′-dibutyl –N,N’-di(1-methylheptyl)-diglycolamide (DBD1MHDGA), was synthesized. The extraction of rare earth elements (REEs) from a hydrochloric acid medium with DBD1MHDGA was investigated. The results of the extraction experiments indicated that the distribution ratios of RE (III) ions increase with an increase in HCl concentration, atomic number, and extractant concentration. The calculated thermodynamic data shows that the extraction process is an exothermic reaction. The organic phase loaded rare earth ions were characterized by infrared spectroscopy. The composition of the extracted complex was determined.

extractionrare earthhydrochloric acidDBD1MHDGA.

1 Introduction

Rare earth elements (REEs) have an irreplaceable position in industrial application due to their special and unique chemical properties^[1-3], and are widely used in many sectors, including metallurgy, nuclear technology, catalysts^[4,5], and military hardware^[6]. As a consequence, much attention has been paid to researching the separation of REEs. Solvent extraction has been considered to be the main technology in the separation of rare earth elements because of the advantages of the separation effects, continuous operation, and large production capacity^[7]. Studies on solvent extraction of REEs have been conducted with various extractants. Organophosphorus extrantants are the industrial extraction agents used nowadays in the extraction and separation of REES, such as TBP, P204, and Cyanex272^[8-11]. However, their separation factors for the neighboring REEs is not good enough, resulting in nearly a thousand extraction processing stages to obtain a single rare earth element with a high purity.

Recently, many new structural amide extractants have been synthesized and studied. They show excellent extractability for lanthanides and actinides from nitric acid media and have good application prospects in nuclear fuel reprocessing^[12,13]. Among the amide extractants, diglycolamides are regarded as promising extractants due to their three coordination groups, which can effectively capture metal ions^[14-19]. Vivek Chavan et al. explored the assembled diglycolamide for f-element ions at a high acidity and the result of the variation of D_Am as a function of the HNO₃ concentration shows that an acid medium plays an important role in the affinity of different diglycolamides towards f-element ions^[20]. The extraction behavior of lanthanides from a nitric acid medium was investigated using ten structurally related diglycolamides, and the distribution ratio of trivalent lanthanide ions studied as a function of HNO₃ concentration, diglycolamide concentration, and temperature increase with an increase in HNO₃ and diglycolamide concentrations and decrease with increasing temperature. Furthermore, the extraction of the studied Ln(III) was found to decrease slightly with an increase in alkyl chain length^[21].

Compared to the HNO₃ medium, the number extraction studies of rare earth elements with an HCl medium is not enough yet. To explore the extraction and separation performance of this kind of extractant in the hydrometallurgy of rare earth elements in a hydrochloric acid medium is of practical significance^[22-26]. S. Dutta et al. researched the separation of ⁹⁰Y from ⁹⁰Sr by solvent extraction using N,N,N’,N’-tetraoctyl diglycolamide (TODGA) as the extractant from both HCl and HNO₃ acid mediums. The extraction efficiency in the HNO₃ medium increased with acid concentration, up to 3M, and decreased thereafter. However, the extraction profiles in the HCl medium for both Y(III) and Sr(II) showed a continuous increase with increasing HCl concentration, and the separation factor values were also found to be much higher in the HCl medium compared to those obtained from an HNO₃ medium^[27]. The extraction of rare earth elements with N,N,N′, N′-tetrabutyl-diglycolamide (TBDGA) and N,N′-dimethyl- N,N′-diphenyl- diglycolamide (DMDPhDGA) from the hydrochloric acid medium was studied in our lab, and the results indicated that the rare earth elements can be effectively extracted with diglycolamide, but benzene rings as substituent groups were unfavorable in the extraction of rare earth elements^[28,29]. In the recent years, it has been proven that extraction behavior was greatly influenced by the nature of the alkyl groups of amides^[30]. The main aim of our project was to investigate new structural extractants and to clarify the relationship between the extracting agent structure and the extractability of rare earth elements.

In the present paper, the extraction of rare earth elements with N,N′-dibutyl–N,N′-di(1-methylheptyl)-diglycolamide (DBD1MHDGA) was investigated in a hydrochloric acid medium. The extraction mechanism for rare earth elements was discussed. The thermodynamic parameters and activity coefficients in the extracting process were also presented.

2 Experimental

2.1 Synthesis and Characterization of DBD1MHDGA

N,N′-dibutyl–N,N′-di(1-methylheptyl)-diglycolamide (DBD1MHDGA) was synthesized by a three-step process below (Fig. 1):

Fig. 1

The synthesis scheme of DBD1MHDGA

Firstly, PBr₃ (1 equiv) was added drop-wise into 2-octanol (1 equiv) at 0 °C. Then, the mixture was heated up to 50 °C and stirred for 2 hours. 2-bromo octane was purified by distillation (124~127 °C/0.09 MPa), and the yield was 55%. ¹HNMR (400MHz, CDCl₃): δ 4.109~4.159(m, 1H, -CH-Br), 1.697~1.868(m, 5H, -CH₂-CH-, -CH-CH₃), 1.349~1.511(m, 8H, -CH₂-), 0.854~0.945(m, 3H, -CH₃).

Secondly, 2-bromo octane (1 equiv), n-butylamine (10 equiv) was stirred at 110°C for 48 hours in 200 ml of autoclave. The unreacted n-butylamine was removed by distillation. The residue was washed with 0.05M NaOH solution, extracted with ethyl acetate. The organic phase was washed with 0.1M HCl solution, water, and brine and then dried over sodium sulfate. After removing the solvent, the product was obtained as light yellow oil. The yield was 80%. ¹HNMR (400MHz, CDCl₃): δ 2.525~2.639(m, 3H, -NH-CH₂-, -NH-CH-), 1.278~1.482(m, 14H, -CH₂-CH₂-N), 1.035~1.020(d, 3H, -CH-CH₃), 0.866~0.936(m, 6H, -CH₃).

The final step deals with the reaction of diglycolyl chloride (1equiv) with dialkyl amine (2.5equiv) in the presence of triethylamine (2.5equiv) dissolved in methylene chloride. The reaction was stirred at room temperature for 6 hours. Then the mixture was washed sequentially with 3 mol·dm^-3 HCl solution, 1 mol·dm^-3 NaOH solution, water, and brine, then dried over sodium sulfate. The crude product was achieved by removing the solvent. Purification of the crude product was carried out through column chromatography. The yield was 90%. ¹HNMR (400 MHz, CDCl₃), δ 4.148~4.410(s, 4H, -C-CH₂-O-), 2.997~3.712(m, 6H, -N-CH-), 1.255~2.046(m, 28H, -CH₂-CH₂-), 1.128~1.172(t, 6H, -CH-CH₃), 0.794~0.974(m, 12H, -CH₃). Infrared Spectrum (IR) (cm^-1/film): $ν_{- C H_{3}}$ , 2958; $ν_{- C H_{2} -}$ , 2927; $ν_{- C O -}$ , 1651 and $ν_{- C - O - C -}$ , 1117.

2.2 Extraction procedures

The extraction procedures were carried out as follows: equal volumes of rare earth elements solution (weighing the theoretical calculation of rare earth oxides, dissolved in hydrochloric acid, then evaporated to dryness, the residue was dissolved in ultrapure water, transferring this solution to volumetric flask) and DBD1MHDGA in the diluents (v_kerosene:v_n-octanol=7:3) were shaken in a constant temperature bath oscillator for 30 min, which was determined to be sufficient to obtain equilibrium. All the experiments were conducted at 25±0.2 °C, except when temperature effects were investigated. The two phases were then separated by centrifugation. The concentrations of REEs in the aqueous phase were determined by the Arsenazo-III (0.2% in water) visible spectrophotometric method (722s visible-infrared spectrophotometer, $λ$ =655nm)^[31]. The concentrations in the organic phase were calculated by subtracting the aqueous concentrations from the total initial aqueous concentration of rare earth. Distribution ratios (D) were calculated as the ratios of the concentrations of REEs in the organic phase to those in the aqueous phase.

3 Results and discussion

3.1 Effect of HCl concentration

The extraction of Nd(III), Sm(III), Gd(III), Dy(III), Er(III), and Yb(III) with different HCl concentrations in kerosene/n-octanol (v_kerosene:v_n-octanol=7:3) is shown in Fig. 2. The tendency shows that the distribution ratios of rare earth ions increase with an increase in hydrochloric acid concentration from 1 mol·dm^-3 to 5 mol·dm^-3, which reveals that DBD1MHDGA has good affinity for the rare earth elements studied. In addition, there is a steep rise in the distribution ratio over 3 mol·dm^-3 HCl for all the studied rare earth elements, except Nd(III). The results are in agreement with our previous findings ^[28]_.

Fig. 2

The influence of HCl concentration on the extraction of different metals

3.2 Effect of the concentration of DBD1MHDGA

The influence of extractant concentration on the extraction capability of different metals in kerosene/n-octanol (v_kerosene:v_n-octanol=7:3) was conducted. The concentrations of HCl and rare earth elements remained unchanged. The logarithm distribution ratio was plotted as a function of the logarithm concentration of DBD1MHDGA in Fig. 3. From Fig. 3, the extraction distribution ratio apparently rises with an increase in atomic number, which established that this extractant has better affinity for heavier lanthanides.

Fig. 3

The influence of extractant concentration on the extraction of different metals

Fig. 3 also shows the linear relationship of REE extraction with DBD1MHDGA concentration and the values of the slope for Nd(III), Sm(III), Gd(III), Dy(III), Er(III), and Yb(III), which are 2.10, 2.70, 2.93, 3.00, 2.88 and 2.86, respectively. It revealed that Nd³⁺ can combine with two molecules of DBD1MHDGA, and the others with three^[32]. Hence, the composition of the extracted complex of RE(III) can be expressed as NdCl₃·2DBD1MHDGA,MCl₃·3DBD1MHDGA in kerosene/n-octanol (v_kerosene: v_n-octanol=7:3). Therefore, the chemical reactions involved during the extraction of metal ions from a hydrochloric acid medium by DBD1MHDGA in different diluents can be described as:

Nd ​^{3 +} + {3Cl}^{-} + {2DBD1MHDGA}_{(O)} = {NdCl}_{3} \cdot {2DBD1MHDGA}_{(o)}

(1)

M ​^{3 +} + {3Cl}^{-} + {3DBD1MHDGA}_{(O)} = {MCl}_{3} \cdot {3DBD1MHDGA}_{(o)}

(2)

M=Sm, Gd, Er, Dy, Yb.

According to the Pitzer equation^[33,34]:

ln γ_{\pm} = | Z_{M} Z_{X} | f + m \frac{2 ν_{M} ν_{X}}{ν} B + \frac{3}{2} m^{2} [\frac{2 {(ν_{M} ν_{X})}^{\frac{3}{2}}}{ν}] C^{Θ},

(3)

ν = ν_{M} + ν_{X} .

(4)

From the Eq. (3) and (4), we can obtain the mean activity coefficient:

\ln γ_{\pm} = | Z_{H^{+}} Z_{C l^{-}} | f + m \frac{2 ν_{H^{+}} ν_{C l^{-}}}{ν} B + \frac{3}{2} m^{2} [\frac{2 {(ν_{H^{+}} ν_{C l^{-}})}^{\frac{3}{2}}}{ν}] C^{Θ},

(5)

\ln γ_{\pm} = f + m B + \frac{3}{2} m^{2} c^{Θ},

(6)

f = - A [\frac{I^{\frac{1}{2}}}{1 + b I^{\frac{1}{2}}} + \frac{2}{b} \ln (1 + b I^{\frac{1}{2}})],

(7)

B = 2 β^{(0)} + \frac{2 β^{(1)}}{α^{2} I} [1 - (1 + α I^{\frac{1}{2}} - \frac{1}{2} α I^{2}) \exp (- α I^{\frac{1}{2}})],

(8)

I = \frac{1}{2} \sum m_{i} z_{i} .

(9)

When C_HCl=1.0 mol·kg^-1, the value of I is 1.0 mol·kg^-1. (Here, A, B are the Debye- Hücke coefficients, m is the molar concentration, v is the valence state, I is the ionic strength, and Z is equal to the charge of the ion).

Then Eq. (7) can be represented as follows:

f = - A [\frac{1}{1 + b} + \frac{2}{b} \ln (1 + b)] .

(10)

The equation below can be obtained with the same method:

B = 2 β^{(0)} + \frac{2 β^{(1)}}{α^{2}} [1 - (1 + α - \frac{1}{2} α) \exp (- α)] .

(11)

When C_HCl<6 mol·kg^-1, b is known as 1.2, and A＝0.391, α＝2.0, β⁽⁰⁾(HCl)＝0.1775, β⁽¹⁾(HCl)＝0.2945, and C^Θ=0.00080.

Taking these constants into formulas (10), (11), and (6), the following values are obtained: B=0.4820，f＝-0.6915，and $γ_{\pm}^{}$ ＝0.812. Then, we deduced the related activity coefficients as $γ_{C l^{-}} = 0.9001$ and $γ_{M^{3 +}} = 0.9659$ . For chemical reactions (1) and (2): the conditional constants of extraction equilibrium can be described as:

K = \frac{{[{NdCl}_{3} \cdot 2DBD1MHDGA ​]}_{(o)}}{[{Nd}^{3 +}] {[{Cl}^{-}]}^{3} {[DBD1MHDGA]}_{(o)}^{2}} = \frac{D}{{[{Cl}^{-}]}^{3} {[DBD1MHDGA]}_{(o)}^{2}}

(12)

K = \frac{{[{MCl}_{3} \cdot 3DBD1MHDGA]}_{(o)}}{[M^{3 +}] {[{Cl}^{-}]}^{3} {[DBD1MHDGA]}_{(o)}^{3}} = \frac{D}{{[{Cl}^{-}]}^{3} {[DBD1MHDGA]}_{(o)}^{3}}

(13)

M=Sm, Gd, Er, Dy, Yb.

The equilibrium constants are:

K_{e x} = \frac{D}{{[DBD1MHDGA]}_{(o)}^{2} {[{Cl}^{-}]}^{3} γ_{{Nd}^{3 +}} γ_{{Cl}^{-}}^{3}},

(14)

K_{e x} = \frac{D}{{[DBD1MHDGA]}_{(o)}^{3} {[{Cl}^{-}]}^{3} γ_{M^{3 +}} γ_{{Cl}^{-}}^{3}} .

(15)

Let

Y_{M} = \frac{D}{{[{Cl}^{-}]}^{3} γ_{M^{3 +}} γ_{{Cl}^{-} .}^{3}}

(16)

Taking Eq.(15) into account for Eq.(13) and (14), the following equations are obtained:

\lg D = \lg K ex + 2 \lg {[DBD1MHDGA]}_{(o)} - \lg Y_{Nd}

(17)

\lg D = \lg K ex + 3 \lg {[DBD1MHDGA]}_{(o)} - \lg Y_{M} .

(18)

The values of logK and logK_ex can be calculated and shown in Table 1.

Values of logK and logK_ex of the extraction of rare earth elements (III)

M³⁺	logK	logK_ex
Nd³⁺	1.71±0.05	1.86±0.05
Sm³⁺	2.64±0.04	2.79±0.04
Gd³⁺	3.09±0.08	3.24±0.08
Dy³⁺	3.40±0.09	3.55±0.09
Er³⁺	3.34±0.05	3.49±0.05
Yb³⁺	3.52±0.13	3.67±0.13

3.3 Effect of temperature

The effect of temperature on extraction was determined by varying the temperature from 288K to 338K. Fig. 4 clarifies the influence of temperature on the values of D in kerosene/n-octanol (v_kerosene: v_n-octanol=7:3).

Fig. 4

Effect of temperature on the extraction of different metals

It is evident that the extraction of RE(III) decreased with an increase in temperature. The negative values of the enthalpy change for the extraction calculated from the slopes of the lines indicates the exothermic nature of the extraction process. The thermodynamic parameters for the reaction can be calculated using these equations below:

{[\frac{\partial \lg D}{\partial (\frac{1}{T})}]}_{P} = \frac{- Δ_{r} H_{m}^{θ}}{2.303 R},

(19)

Δ_{r} G_{m}^{θ} = - R T \ln K^{θ},

(20)

Δ r S_{m}^{θ} = \frac{(Δ_{r} H_{m}^{θ} - Δ_{r} G_{m}^{θ})}{T} .

(21)

The thermodynamic constants are shown in Table 2.

The thermodynamic data of the extraction of different RE(III) with DBD1MHDGA(298K)

M³⁺	$Δ_{r} H_{m}^{θ}$ (kJ·mol^-1)	$Δ_{r} G_{m}^{}^{θ}$ (kJ·mol^-1)	$Δ_{r} S_{m}^{θ}$ (J·mol^-1·K^-1)
Nd³⁺	-39.08	-7.93	-104.47
Sm³⁺	-57.18	-11.25	-154.08
Gd³⁺	-68.25	-14.84	-179.12
Dy³⁺	-75.87	-17.81	-194.74
Er³⁺	-67.20	-16.73	-169.28
Yb³⁺	-75.17	-17.70	-192.76

3.4 Spectroscopic studies of the extracted species

An analysis of the IR spectra of the loaded organic phase was also performed (Fig. 5). The spectra of DBD1MHDGA was studied before and after rare earth elements extraction from HCl. Compared to the free extractant, the carbonyl group (CO) stretching vibration of the extracted species was shifted from about 1651 cm⁻¹ to about 1605 cm⁻¹ . This data indicates that metal ions are mainly coordinated to the oxygen of the carbonyl group of DBD1MHDGA.

Fig. 5

The infrared spectra of organic phase loaded rare earth ions

3.5 Effect of diluents

The diluents play an important role in the solvent extraction process, which should have low volatility, low surface tension, and should be stable^[35]. In this context, by adjusting the volume ratio of kerosene/n-octanol, the extraction of Dy(III) with DBD1MHDGA was investigated.

3.5.1 Influence of acid concentration on the extraction of Dy(III) from the different ratios of kerosene/n-octanol.

The extraction efficiency for Dy(III) by DBD1MHDGA from different ratios of kerosene/n-octanol is shown in Fig. 6. The extraction capacity of DBD1MHDGA in different ratios of kerosene/n-octanol for Dy(III) increases in the order: kerosene/n-octanol (V_kerosene:V_n-octanol=7:3)>kerosene/n-octanol (V_kerosene:V_n-octanol=8:2)>kerosene/n-octanol (V_kerosene:V_n-octanol=9:1). The distribution ratio increases with an increase in the n-octanol volume fraction. Since the solvation ability of n-octanol to the ionic compounds is better than that of kerosene, increasing the volume fraction of n-octanol is favored in the extraction process.

Fig. 6

Influence of HCl concentration on the extraction capability of Dy(III) from different ratios of kerosene/n-octanol

3.5.2 Influence of extractant concentration on the extraction capability of Dy(III) from different ratios of kerosene/n-octanol

The extraction effect of Dy(III) with DBD1MHDGA from different ratios of kerosene/n-octanol was investigated. Fig. 7 shows that the distribution ratio of rare earth elements increased as the DBD1MHDGA concentration increased. The line slopes are near 3, which shows that the extraction complex is DyCl₃·3DBD1MHDGA.

Fig. 7

Influence of extractant concentration on the extraction capability of Dy(III) from different ratios of kerosene/n-octanol

The extraction constant of this equilibrium, K, and the constant of conditional equilibrium, K_ex, are obtained with the same computing method. The values are shown in Table 3. The addition, n-octanol promotes the extraction of REEs from the HCl media.

Data of logK and logK_ex of the extraction of Dy(III) with DBD1MHDGA

Diluents Kerosene/n-octanol	V:V=7:3	V:V=8:2	V:V=9:1
logK	3.40±0.09	3.11±0.10	2.66±0.06
logK_ex	3.55±0.09	3.26±0.10	2.81±0.06

3.5.3 Influence of temperature on the extraction capability of Dy(III) from different ratios of kerosene/n-octanol.

The influence of temperature on the extraction of Dy(III) from different ratios of kerosene/n-octanol has been studied.

Fig. 8 shows the plots of lgD versus 1000/T. The change of enthalpy (ΔH), the change of Gibbs free energy (ΔG), and the change of entropy (ΔS) can thus be obtained with equations (20) and (21). The values are given in Table 4.

Thermodynamic data of the extraction of

D y (I I I)

with DBD1MHDGA in different ratios of kerosene/n-octanol (298K)

Diluents Kerosene/n-octanol	$Δ_{r} H_{m}^{θ}$ (kJ·mol^-1)	$Δ_{r} G_{m}^{θ}$ (kJ·mol^-1)	$Δ_{r} S_{m}^{θ}$ (J·mol^-1·K^-1)
V:V=7:3	-75.87	-17.80	-194.73
V:V=8:2	-76.31	-18.61	-193.52
V:V=9:1	-73.43	-16.04	-192.48

Fig. 8

Influence of temperature on the extraction capability of Dy(III) from different ratios of kerosene/n-octanol

It can be seen that ΔH is smaller than zero, indicating that the extraction of Dy(III) in the kerosene/n-octanol system is an exothermic reaction.

The back extraction of rare earth elements was carried out with water. The back extraction efficiency from the organic phase was up to 98%, which shows that the stripping of rare earth ions studied is easy.

4 Conclusion

A novel diglycolamide, DBDIMHDGA, was synthesized. In the range of hydrochloric acid concentration studied, the distribution ratio of the extraction of rare earth elements with DBD1MHDGA increased with hydrochloric acid concentration. When the concentration of HCl is over 3 mol·dm^-3, the distribution ratio increased sharply. The sequence of the extraction ability of rare earth ions with DBDIMHDGA was in the order: Nd(III)< Gd(III), Er(III)< Dy(III) < Sm(III) < Yb(III), which shows that this extractant has a better affinity for heavier lanthanides.

The distribution ratio increases with the extractant concentration. The slope analysis indicated that the extraction chemical reactions proceeded as follows:

{Nd}^{3 +} + {3Cl}^{-} + {2DBD1MHDGA}_{(o)} = {NdCl}_{3} \cdot {2DBD1MHDGA}_{(o)}

M^{3 +} + {3Cl}^{-} + {3DBD1MHDGA}_{(o)} = {MCl}_{3} \cdot {3DBD1MHDGA}_{(o)}

M=Sm, Gd, Er, Dy, Yb.

The experiments involving the temperature effect indicated that the extraction reaction is an exothermic process. Increasing temperature had a negative effect on the extraction efficiency. IR spectra suggested that the extractant, DBD1MHDGA, coordinated with rare earth ions through the oxygen of the carbonyl group.

References

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