Solvent extraction and molecular modeling studies of Dy(III) from chloride solutions using acidic extractants

Journal Pre-proof Solvent extraction and molecular modeling studies of Dy(III) from chloride solutions using acidic extractants

V. Agarwal, M.S. Safarzadeh PII:

S0167-7322(19)35918-5

DOI:

https://doi.org/10.1016/j.molliq.2020.112452

Reference:

MOLLIQ 112452

To appear in:

Journal of Molecular Liquids

Received date:

25 October 2019

Revised date:

10 December 2019

Accepted date:

2 January 2020

Please cite this article as: V. Agarwal and M.S. Safarzadeh, Solvent extraction and molecular modeling studies of Dy(III) from chloride solutions using acidic extractants, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2020.112452

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© 2018 Published by Elsevier.

Journal Pre-proof

Solvent extraction and molecular modeling studies of Dy(III) from chloride solutions using acidic extractants V. Agarwal, M.S. Safarzadeh*

Department of Materials and Metallurgical Engineering, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, SD 57701-3995, USA

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*Corresponding author: [email protected]

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Tel.: +1 605 394 1284; fax: +1 605 394 3369

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Abstract

In the present work, solvent extraction of Dy(III) from chloride solutions using 2-

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ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) and Cyanex 572 (mixture of phosphonic and phosphinic acid extractants) diluted in kerosene has been reported. Pourbaix

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and speciation diagrams were plotted to understand the chemistry and formation of different species of Dy(III) in chloride solutions. Effect of different extraction parameters such as pH (1-

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5), extractant concentration (1-60 mM) and chloride concentration (0.05-1 M) were investigated. PC88A was found to extract Dy(III) ions better than Cyanex 572 under similar conditions.

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Experimental results showed that Dy(III) extraction reaction occurred via cation exchange mechanism, which was further supported by Fourier-transform infrared spectroscopy (FTIR) studies of organic solutions loaded with Dy(III). Separation studies of Dy(III) from a mixed solution of 16 rare earth elements (REEs) indicate that PC88A works better for light REEs/Dy separation while selectivity of middle and heavy REEs over Dy(III) are superior with Cyanex 572. Molecular modeling was employed to calculate the free energies of Dy ions in aqueous and organic phases (consisting of phosphonic acid in heptane), which supported the experimental results. Keywords: Solvent extraction; molecular modeling; FTIR; PC88A; Cyanex 572; dysprosium

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Journal Pre-proof 1. Introduction Dysprosium is an important rare earth metal and its global demand has significantly increased recently. Dysprosium is used in alloys for permanent NdFeB magnets and is regarded as a critical element by European Union and the United States due to the shortage in supply and high demand (Binnemans et al., 2013). Recycling of dysprosium has become an important concern in many countries and hydrometallurgical extraction and separation of dysprosium seems to be a prominent solution. Solvent extraction is the well-known and commonly used method in industry for metal recovery from complex solutions (Xie et al., 2014). Many

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researchers have investigated the solvent extraction of dysprosium using different organic

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extractants. The most common acidic organophosphorus extractants used are di-(2-ethylhexyl)phosphoric acid (DEHPA) (Mohammadi et al., 2015; Thakur, 2000; Yoon et al., 2016), 2-

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ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) (Huang and Tanaka, 2010; Mishra

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et al., 2000; Mohammadi et al., 2015; Singh et al., 2008; Thakur, 2000; Yoon et al., 2016) and bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) (Wang et al., 2006; Padhan and Sarangi,

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2019).

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Kuźnik (1981) studied the solvent extraction of dysprosium(III) and ytterbium(III) from l-(2pyridylazo)-2-naphthol (PAN) diluted in carbon tetrachloride. The extraction of ytterbium from

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dysprosium was found to be easier and the separation factor between ytterbium and dysprosium was found to be 3. Mishra et al. (2000) investigated the simultaneous purification of dysprosium

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and terbium using PC88A diluted in paraffinic kerosene, while Thakur (2000) studied the separation of dysprosium and yttrium using DEHPA and PC88A. These researchers (Mishra et al., 2000; Thakur, 2000) developed empirical mathematical models to predict the distribution behavior at different conditions and also developed computer programs with the help of the developed model to calculate the concentration of metal ion in aqueous and organic phases at various extraction stages. The authors reported that ~93% of Dy could be recovered as Dy of ~97% purity through the process. Additionally, Tb of ~83% purity and yttrium of ~93% purity were also obtained.

Solvent extraction processing to produce nuclear-grade dysprosium oxide (Dy2O3) using PC88A from a crude REE concentrate containing yttrium oxide (~67%) and dysprosium oxide 2

Journal Pre-proof (~22%) has been reported (Singh et al., 2008). Solvent extraction of Dy(III) from nitric acid solution using PC88A diluted in Shellsol D70 was investigated (Huang and Tanaka, 2010). Mohammadi et al. (2015) studied the separation of Nd(III), Dy(III), and Y(III) from hydrochloric acid solutions using DEHPA and PC88A and reported that better separation of Y(III) from Dy(III) was achieved from a mixture of DEHPA and PC88A at low extractant concentration (0.06 and 0.09 M), while PC88A worked better at higher extractant concentration (0.15 M). DEHPA was found to provide a better separation of Nd(III) from Y(III) and Dy(III). Yoon et al. (2016) studied the solvent extraction and separation of Dy(III) and Nd(III) using DEHPA and

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PC88A from permanent magnet scrap leach solution and concluded that both PC88A and

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DEHPA were feasible for REE extraction but PC88A was found to be better for separation

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purposes.

Aforementioned acidic extractants showed potential in separating REEs; however, separation

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of REEs is still considered a challenging task owning to their similar physical and chemical

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properties. Therefore, interest in the development of novel extractants towards the effective separation of REEs is still growing. Cytec has launched a novel phosphorous-based chelating

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extractant named Cyanex 572 to improve the separation efficiency amongst REEs compared to traditional phosphonic acid extractants. Cyanex 572 is a mixture of PC88A (25-75%), Cyanex 272 (25-75%), and diisobtylene (1-2%). It provides advantage in reducing process cost by

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minimizing acid requirement during stripping stage (McCallum et al., 2014).

The literature is sparse on the solvent extraction of Dy(III) using Cyanex 572. Comparative study of Cyanex 272 and Cyanex 572 was reported for the separation of Nd from a mixed solution of Nd, Tb and Dy by pertraction (Pavon et al., 2017). Another comparative study of Dy(III) and Y(III) from nitric and hydrochloric acid solutions using Cyanex 572 reported that saponification of Cyanex 572 does not favor the extraction of Y(III) and Dy(III) (Hefny et al., 2018). Raji et al. (2018) reported extraction of Dy ions in the presence of Nd ions from acidic chloride solutions through emulsion liquid membrane using Cyanex 572.

The present work is directed to investigate and compare the solvent extraction of Dy(III) from chloride solutions using PC88A and Cyanex 572. Computational modeling was performed 3

Journal Pre-proof to calculate free energies of Dy(III) in the aqueous phase and in the organic phase containing phosphonic acid. Infrared spectroscopy results of Dy with PC88A and Cyanex 572 were compared and possible reaction mechanism based on experimental and FTIR results were proposed. Separation studies of Dy(III) from a mixed solution containing 16 REEs in a chloride matrix using PC88A and Cyanex 572 were reported.

2. Experimental

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2.1. Materials and reagents

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The organic reagents: PC88A (≥ 95 wt%) and Cyanex 572 (≥ 95 wt%) were supplied by Daihachi Chemical Industry and Cytec® Industries, respectively. Kerosene for extraction

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experiments or hexane for FTIR measurements (both obtained from Sigma Aldrich, reagent

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grade) were employed as the diluents for organic reagents. Stock solutions of Dy(III) were prepared by diluting the required amount of standard solution of Dy(III) of 1000 mg/L (Inorganic

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Ventures) diluted in 10 vol% hydrochloric acid. For separation studies, a synthetic solution containing 16 REEs [La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III),

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Y(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III), and Sc(III)] of 1000 mg/L each diluted in 10 vol% hydrochloric acid was used and purchased from Inorganic Ventures. Hydrochloric acid

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(37.2%) or ammonium hydroxide (NH4OH) solutions obtained from Fisher Scientific were used 600R pH meter.

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to adjust the pH of aqueous solutions. The pH of aqueous solutions was measured using a PHB-

2.2. Extraction experiments

The extraction experiments were carried out by contacting equal volumes of organic and aqueous solutions (15 mL) in a 100 mL separatory funnel at unit phase ratio for 10 min (previous experiments showed that 5 min was adequate to attain the equilibrium) using a Lab Companion SI-300 benchtop incubated shaker. All the experiments were performed at room temperature (25 ± 2 °C). After phase separation, the aqueous phase was collected for equilibrium pH (using a PHB-600R pH meter) measurement. The collected aqueous phase was further diluted in hydrochloric acid solutions (2 vol%) and REEs(III) concentration was analyzed using Agilent 4

Journal Pre-proof 7900 ICP-MS inductively coupled plasma-mass spectrometer (ICP-MS). For FTIR measurements, 10 mM of both extractants (PC88A and Cyanex 572) were diluted in hexane and loaded with 3 mM Dy(III) using unit phase ratio and room temperature at initial pH of 5. Organic phase was loaded until saturation was achieved by repeating extraction experiment 2 times with fresh aqueous phase in each stage. FTIR measurements were performed using Shimadzu FTIR instrument in the range of 600-4000 cm-1.

The extraction percentage (E%) and distribution coefficient (D) of Dy(III) at equilibrium

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[Dy]i − [Dy]eq. ) × 100 [Dy]i

(1)

[Dy]i − [Dy]eq. Vaq )× [Dy]eq. Vorg

(2)

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D=(

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E% = (

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were calculated using the following expressions:

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where [Dy]i is the initial dysprosium concentration in the aqueous solution and [Dy] eq. is the equilibrium dysprosium concentration remaining in the raffinate. Vaq and Vorg (mL) represent the

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volume of aqueous phase and organic phase, respectively. Separation factor (β), which

expressed as: 𝛽M2 = M1

D2 D1

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determines selectivity of a metal ion over another metal ion of the extraction system, can be

(3)

where D2 and D1 are distribution coefficients of the metal ions: M2 and M1, respectively.

3. Solution chemistry of Dy(III) in chloride media Pourbaix diagrams of dysprosium in hydrochloric acid solution were constructed using STABCAL® software (Huang, 2012) as shown in Figure 1. According to Figure 1a, free species of trivalent dysprosium ion (Dy(III)) predominates in pH range of 1 to ~5, while dysprosium 5

Journal Pre-proof forms a cationic oxide species (DyO+) at pH values greater than 5 at low chloride concentrations (0.05 M). At higher chloride concentration (1 M), even at low pH values (≤5), a cationic chloride species of dysprosium (DyCl2+) predominates (Figure 1b) while at higher pH values (≥5), similar to Figure 1a, cationic species of dysprosium oxide (DyO+) predominates. Complexation of dysprosium at higher chloride concentration could be further explained by speciation diagram of dysprosium, which is shown in Figure 2 for the chloride concentration range of 0.001 M to 3 M. The formation of different species in the diagram was calculated based on the following

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equations: ∆𝐺 0 = −0.408 kcal/mol; Kf = 199

(4)

Dy 3+ + 2Cl− → DyCl+ 2

∆𝐺 0 = 0.044 kcal/mol; Kf = 0.93

(5)

Dy 3+ + 3Cl− → DyCl3

∆𝐺 0 = 0.567 kcal/mol; Kf = 0.38

(6)

Dy 3+ + 4Cl− → DyCl− 4

∆𝐺 0 = 1.149 kcal/mol; Kf = 0.14

(7)

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Dy 3+ + Cl− → DyCl2+

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Figure 2 also shows that at low chloride concentrations (0.001-0.5 M), free species of Dy(III) predominates, while by increasing chloride concentration up to 1 M, cationic species of

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dysprosium chloride DyCl2+ start to predominate (~44%) as was also shown by Pourbaix diagram

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dysprosium.

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(Figure 1). Further increasing the chloride concentration (1-3 M) results in more complexation of

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Journal Pre-proof Figure 1. Pourbaix diagrams for dysprosium in hydrochloric acid solution at constant chloride ion concentration of (A) 0.05 M and (B) 1 M (concentration of total dissolved dysprosium ions =

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0.25 mM). Activity coefficients were assumed to be equal to unity.

Figure 2. Speciation diagram for dysprosium at different chloride concentration (concentration

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of total dissolved dysprosium ions = 0.1 M, pH = 2).

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4. Results and discussion 4.1. Extraction isotherms

Different solvent extraction parameters such as pH, extractant concentration and ionic strength were varied to optimize the Dy(III) extraction from chloride media. The influence of initial pH in the range of 1 to 5 was investigated on Dy(III) extraction using PC88A and Cyanex 572 (Figure 3). Figure 3a demonstrates that Dy(III) extraction efficiency continuously increased as initial pH of aqueous phase increased and reached completion at initial pH value of 5 using both extractants. This behavior of Dy(III) was expected due to the cation exchange reaction

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Journal Pre-proof between the Dy(III) and acidic extractants PC88A and Cyanex 572, as shown in Equation 8 (Agarwal et al., 2017): M n+ + n(𝑅𝐻)𝑜𝑟𝑔 ⇄ (M𝑅𝑛 )𝑜𝑟𝑔 + nH +

(8)

where Mn+, RH, and MR n represent the metal ion, the acidic extractant before deprotonation and the metal complex formed as a result of extraction reaction, respectively. At higher acidic conditions, acidic extractants interacts with metal ions as a conjugated base (electron donor),

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therefore, extraction of metal ions in the organic phase was not favored at low pH values (Kislik,

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2002).

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The corresponding equilibrium pH values of initial pH 5 were 2.56 and 2.63 for PC88A and Cyanex 572, respectively. The equilibrium pH shift was higher for PC88A, suggesting better

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extraction of Dy(III) compared to Cyanex 572 under similar experimental conditions. However,

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for better comparison of extraction performance between PC88A and Cyanex 572, pH0.5 values (pH of 50% metal extraction) of respective extractants were obtained from Figure 3a, indicating that pH0.5 for Dy(III) extraction using PC88A (1.31) was lower than that for Cyanex 572 (1.44),

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which confirms PC88A is a better extractant for Dy(III) extraction compared to Cyanex 572. Using data in Figure 3a, a plot of equilibrium pH vs. logarithm of distribution ratio of Dy(III)

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using PC88A and Cyanex 572 from chloride solutions is shown in Figure 3b. Slope analysis

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results demonstrated the linear relationship between distribution ratio of Dy(III) with equilibrium pH for PC88A and Cyanex 572 with slopes of 2.3 and 2.7, respectively. Similar slopes are reported in the literature (Quinn et al., 2015).

The effect of extractant concentration (1-60 mM) on the extraction of Dy(III) is shown in Figure 3c. Extraction of Dy(III) increased at elevated extractant concentration due to increase in availability of extractant molecules per Dy(III). Plotting logarithm of distribution coefficient against logarithm of extractant concentration (Figure 3c) resulted in a linear relationship with slopes of 2.5 and 2.9 using PC88A and Cyanex 572, respectively. Again, the slope values were lower than the expected value of 3 and similar results were also reported by other researchers for REEs (Quinn et al., 2015, Hefny et al., 2018). The possible reason of this behavior could be 8

Journal Pre-proof either change in the speciation of extractant molecule as suggested by Quin et al (2015), where PC88A could exist as dimer or trimer depending on its concentration. Based on the dimerization constant (K2) value of PC88A in kerosene at room temperature (Log K2 = 4.09) in literature (Kolarik, 2010), it was calculated that most of the PC88A molecules exist in dimer form (~9097%) under the investigated conditions. However, the speciation of dysprosium ions cannot be ignored at the same time as shown in Pourbaix and speciation diagrams (Figures 1 and 2), where dysprosium could exist as different cationic species. These cationic species or chloride complexation will influence the extraction reaction and could result in the lower stoichiometry

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association of dysprosium with acidic extractants should the extraction reaction be influenced by

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reactions 4-7. Nevertheless, distribution coefficients for Dy extraction were higher with PC88A,

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again proving PC88A as a better extractant for dysprosium extraction than Cyanex 572.

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Journal Pre-proof Figure 3. (A) equilibrium pH against extraction of Dy(III), (B) equilibrium pH against logarithm of distribution coefficient of Dy(III), (C) extractant concentration vs. distribution coefficient of Dy(III), and (D) chloride ion concentration against Dy(III) extraction using PC88A and Cyanex 572 dissolved in kerosene (pH of 2, [Dy(III)] = 0.25 mM, [Extractant] = 30 mM, 10 min contact time, unit phase ratio, and room temperature).

Figure 3d shows the effect of chloride concentration (0.05 M to 1 M) in the aqueous phase on the extraction of Dy(III) at constant extractant concentration (30 mM)The chloride

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concentration was adjusted in the aqueous solution by adding required amount of sodium

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chloride. The results illustrate that increasing chloride concentration affected the extraction of dysprosium negatively using PC88A and Cyanex 572. Although this effect was minimal for

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PC88A extraction of Dy(III) (~99%-96%), the extraction decreased from ~96% to ~85% when

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Cyanex 572 was used. Davies equation was applied to ascertain ionic strength and activity coefficients of Dy(III) at initial pH value of 2 and at 0.26 mM of Dy concentration in chloride

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media (Equation 9) (Butler, 1968) to explain this behavior. The calculated values of ionic

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strength and activity coefficients are reported in Table 1.

√I logΥ = −0.5z 2 ( − 0.30I) 1 + √I

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(9)

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where Υ𝑖 , z, and I represent activity coefficient, ionic charge, and the ionic strength, respectively. According to calculated values in Table 1, the activity coefficient of Dy(III) decreased from 0.23 to 0.12 at elevated chloride concentration (1 M), suggesting decreased activity of Dy(III) at higher chloride concentration, which resulted in lower extraction of Dy(III). These results are also in accordance with the speciation diagram (Figure 2), where it was shown that at 1 M chloride concentration, complexation of dysprosium ions occurs and DyCl2+ species predominates. Similar extraction behavior was observed by Mishra et al. (2018) for Eu(III) with DEHPA. Table 1. Ionic strengths and activity coefficients for Dy(III) in aqueous solutions at different chloride concentrations (pH = 2, [Dy] = 0.26 mM).

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Anion (concentration) Ionic strength (I) Activity coefficient (Υ) Chloride (0.05 M)

0.03

0.23

Chloride (1.0 M)

1.0

0.12

4.2. FTIR studies The characterization of 10 mM PC88A and Cyanex 572 in hexane before and after

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loading of Dy(III) ions was performed using infrared spectra analysis as shown in Figure 4. Previous research (Agarwal et al., 2018) indicated that characteristic absorption bands of P=O

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stretch for PC88A and Cyanex 572 were attributed at 1197 cm-1 and 1172 cm-1, respectively before loading. Similarly, P-O-H vibration peaks for PC88A and Cyanex 572 were reported at

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1032 cm-1 and 1029 cm-1, respectively.

Figure 4 demonstrated that loading of Dy(III) ions resulted in shifting of P=O stretch

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from 1197 cm-1 to 1164 cm-1 for PC88A, while same shift from 1172 cm-1 to 1138 cm-1 was observed in Cyanex 572 case. Shifts in the P=O stretch suggest possible interaction or

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association of Dy(III) ions with oxygen atom in the organophosphorus extractants structure. Moreover, peaks attributed to P-O-H vibration disappeared with Dy(III) loading in both

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extractants, suggesting that the extraction of Dy(III) with PC88A and Cyanex 572 is governed by

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cation exchange mechanism indicated by Equation 8.

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Figure 4. The FTIR spectra of PC88A and Cyanex 572 in hexane before and after loading with

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Dy(III) (pH = 5, [Dy]i = 3 mM, and [Extractant] = 10 mM).

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4.3. Separation studies

Choice of extractant is highly dependent on selectivity of metal ion of interest with other

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impurities or metal ions present in the same solution. To investigate the selectivity of Dy(III) over other REEs, separation performances of PC88A and Cyanex 572 extractants were

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investigated and compared in a mixed chloride based solution of 16 REEs (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu and Sc). Figure 5 shows the logarithm of separation factors calculated for each REE with respect to Dy. The results show that PC88A can separate Dy(III) from light REEs (La, Ce, Pr, Nd) more effectively than Cyanex 572. However, Cyanex 572 showed better separation performance of Dy from the middle (Sm, Eu and Gd) and heavy REEs (Tb, Y, Ho, Er, Tm, Yb, Lu) and scandium. Therefore, Cyanex 572 could be a potential extractant for the separation of Dy(IIII) from other REEs because it is well known that separation of Dy(III) – a heavy REE, is difficult from other heavy REEs because of similar ionic radii.

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Figure 5. Separation factors of dysprosium from different REEs from chloride matrix using

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PC88A and Cyanex 572 in kerosene (pH = 2, [Extractant] = 60 mM, [Cl-] = 0.05 M, [REE] = 20

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mg/L, O/A = 1:1).

5. Computational Modeling

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In our previous publication (Agarwal et al., 2018), Ab Initio quantum computations utilizing

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density functional theory (DFT) were performed to understand the solvent extraction behavior of Eu(III) from water into an organic phase consisting of heptane and a simplified phosphonic acid structure of PC88A named dimethyl phosphonic acid (DMPA) using Gaussian 09 software. Similar free energy calculations in aqueous and organic phases were carried out for Dy(III) solvent extraction system using DMPA.

Figure 6 and Table 2 illustrate the results of optimized geometry of dysprosium (first hydration shell in water). Furthermore, hydration energies were also calculated based on optimized geometry. Figure 6 shows that Dy(III) interacts with eight water molecules in lowest energy configuration and the geometry at minimal energy was found to be square anti-prism. Former research showed that similar energy optimization of REEs having lager ionic radii than 13

Journal Pre-proof Dy such as Eu (Agarwal et al., 2018) and La (Agarwal et al., 2019) interact with nine water

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molecules.

Figure 6. The optimized square anti-prism geometry of the hydrated Dy(III) ion (Gaussian 09

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software with the energy functional uB3LYP and the large core quasi-relativistic ECP MWB52).

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Table 2. Free energies of hydration calculated for Dy(III) and Eu(III).

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3+ G(Mgas REE phase ) G(H2 O)8,9 cation (au) (au) -35.5 -611.3 Dy(III) Eu(III) -33.8 -687.7 (Agarwal et al., 2018)

calculated G(M 3+ (H2 O)8,9 ) ∆Ghydration (au) (kcal/mol) -611.3 -852

-687.7

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Similar energy calculations were performed for Dy(III) in organic phase using a simple model representing PC88A in the DFT calculations, named DMPA (dimethyl phosphonic acid) as reported earlier (Agarwal et al., 2018). Figure 7 shows the optimized geometry for Dy(III) ions with DMPA anions, indicating that one Dy ion interacts with three DMPA anions. This interaction results in release of three hydrogen ions, which is in accordance with the experimental and FTIR results. Table 3 summarizes the modeling results for the extraction model of Dy(III) with DMPA anions and a comparison of obtained results with Eu(III) results reported

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Journal Pre-proof earlier (Agarwal et al., 2018). Free energies calculated for Dy(III)-DMPA complex was found to be less than the Eu-DMPA complex which suggests stronger interaction of DMPA or PC88A molecules with Dy ion compared to Eu ions. Therefore, these results imply that under similar conditions, PC88A should extract Dy(III) ions more efficiently than Eu(III) ions, which has

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already been reported in the literature (Sato 1989).

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Figure 7. Optimized geometry of interaction of Dy(III) with three DMPA anions in heptane.

REE Cation Dy(III) (present work) Eu(III) (Agarwal et al., 2018)

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Table 3. Free energies of extraction with DMPA for Dy(III) and Eu(III). Hydrated REE cation free energy in water (au)

DMPA dimer free energy in heptane (au)

REE (DMPA)3 free energy in heptane (au)

Hydrated proton H+ (H7O3+) free energy in water (au)

∆G0 (kcal)

-724.6

-1294.9

-1977.9

-229.6

33

-722.8

-1294.9

-1976.1

-229.6

38

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6. Conclusions Based on the results of this research, the following points are demonstrated: 

Pourbaix and speciation diagrams of Dy(III) showed that at high chloride concentrations, complexation of Dy ions follows.



PC88A extracts Dy ions more effectively than Cyanex 572 extractant under similar conditions.



Slope analysis results revealed that the stoichiometry of Dy extraction reaction with

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investigated extractants was lower than the expected value of 3, possibly due to complexation of Dy with chloride ions.

FTIR results illustrated significant shifts in characteristic absorption bands of P=O stretch

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

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and weakening of P-OH vibration bands in both PC88A and Cyanex 572 with Dy(III) loading. This behavior supports the assumption of cation exchange mechanism of Dy 

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extraction reaction with PC88A and Cyanex 572.

PC88A outperformed Cyanex 572 in separating Dy ions from light REEs while Cyanex



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572 separation performance was better in separating Dy from middle and heavy REEs. Free energies of Dy were calculated using molecular modeling and results from

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molecular modeling supported experimental and FTIR results.

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7. Acknowledgements

The authors would like to thank Dr. John Bendler for performing the modeling work. Cytec Industries is acknowledged for supplying a commercial sample of Cyanex 572. 8. References Agarwal, V., Safarzadeh, M. S., & Galvin, J. (2017). An extension of Free's extraction isotherm for the solvent extraction of cations using acidic extractants. International Journal of Mineral Processing, 167, 86-94. Agarwal, V., Safarzadeh, M. S., & Bendler, J. T. (2018). Solvent extraction of Eu(III) from hydrochloric acid solutions using PC88A and Cyanex 572 in kerosene. Hydrometallurgy, 177, 152-160.

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Journal Pre-proof Agarwal, V., Safarzadeh, M. S., & Galvin, J. (2019). A comparative study of the solvent extraction of lanthanum(III) from different acid solutions. Mineral Processing and Extractive Metallurgy, 1-8. Binnemans, K., Jones, P. T., Blanpain, B., Van Gerven, T., Yang, Y., Walton, A., & Buchert, M. (2013). Recycling of rare earths: a critical review. Journal of cleaner production, 51, 1-22. Butler, J. N. (1968). The thermodynamic activity of calcium ion in sodium chloride-calcium chloride electrolytes. Biophysical Journal, 8(12), 1426-1433.

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El-Hefny, N. E., Gasser, M. S., Emam, S. S., Mahmoud, W. H., & Aly, H. F. (2018). Comparative studies on Y(III) and Dy(III) extraction from hydrochloric and nitric acids by Cyanex 572 as a novel extractant. Journal of Rare Earths, 36(12), 1342-1350.

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Huang, H. H. (2012). Stabcal brief instructions: Set up, run and quick learn. Montana Tech, The University of Montana, Butte, MT, USA.

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Huang, Y., & Tanaka, M. (2010). Solvent extraction equilibrium of dysprosium(III) from nitric acid solutions with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester. Transactions of Nonferrous Metals Society of China, 20(4), 707-711.

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Journal Pre-proof Authors statement Please find submitted herewith our manuscript, “Solvent extraction and molecular modeling studies of Dy(III) from chloride solutions using acidic extractants” for publication in Journal of

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Molecular Liquids.

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Journal Pre-proof Conflict of interest

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There are no conflicts of interest in this paper.

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Journal Pre-proof Highlights Solvent extraction of Dy(III) from chloride solutions was studied.

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PC88A extracts Dy ions more effectively than Cyanex 572.

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PC88A outperformed Cyanex 572 in separating Dy ions from light REEs.

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Standard Gibbs energies of Dy were calculated using molecular modeling.

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