Supercritical Extraction of Lanthanide Tributyl Phosphate Complexes

Review pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2019, 58, 9199−9211

Supercritical Extraction of Lanthanide Tributyl Phosphate Complexes: Current Status and Future Directions L. K. Sinclair,*,† J. W. Tester,‡ J. F. H. Thompson,§ and R. V. Fox∥ †

CF Technologies, Inc., 1 Westinghouse Plaza, Building D, Hyde Park, Massachusetts 02136, United States Smith School of Chemical and Biomolecular Engineering, Cornell University, Snee Hall, Ithaca, New York 14853, United States § School of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, New York 14853, United States ∥ Idaho National Laboratory, 775 University Boulevard, Idaho Falls, Idaho 83415, United States Downloaded via GUILFORD COLG on August 1, 2019 at 06:17:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Many researchers have studied the extraction of lanthanides with tributyl phosphate in supercritical carbon dioxide. Potential applications include the enhanced extraction or separation of lanthanides from ores and recycled materials by making use of the unique solvation properties of supercritical CO2. In some cases, tributyl phosphate has been used to extract lanthanides from their solid nitrate salt form or from nitrate solutions. In other cases, tributyl phosphate/nitric acid adducts have been used to extract lanthanides from oxides, hydroxides, ores, phosphors, magnets, and waste batteries. Flow-through-type experiments have been useful for measuring extraction kinetics for various lanthanide-containing materials. Equilibrium -type experiments have helped to show the effect of different parameters on phase equilibria, often making use of spectroscopy to measure supercritical lanthanide concentrations in situ. Several studies have noted that extraction decreases when more water is added to the system; this is likely due to the condensation of aqueous droplets, which segregate lanthanides and thus inhibit extraction. It is proposed that varying degrees of water dissolution account for the inconsistent effect of pressure or temperature on extraction across various studies.

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nitrates.12−20 In 2009, a pilot plant demonstrated the supercritical extraction of uranium from incinerator fly ash.21 This early work in actinide extraction laid the foundation for lanthanide extraction with the TBP/supercritical CO2 system. These types of extraction systems are the focus of this Review. Whereas lanthanides are critical for modern technologies including catalysts, glass, phosphors, and magnets,22 conventional extraction and separation methods have enormous challenges. Conventional extraction generally involves ore concentration, followed by high-temperature digestion and leaching processes to generate an impure aqueous solution. Solvent extraction may then be used as a bulk purification process to produce a mixed lanthanide material, which can be further treated with ion exchange to produce high-purity (up to 99.99999%) individual metal products.23,24 Alternatively, solvent extraction can be used to generate relatively pure (up to 99.999%) individual metals by exploiting the slightly different binding affinities due to the decreasing ionic radius across the lanthanide series.23−25 Because these affinity differences are so minor, only slight separation can be achieved in each stage, necessitating tens to hundreds of stages to effectively separate adjacent lanthanides.25

INTRODUCTION

Background. Supercritical carbon dioxide is defined as CO2 above its critical temperature of 31 °C and its critical pressure of 7.39 MPa. Under those conditions, CO2 has a density and solvation power similar to those of many liquid solvents. Supercritical carbon dioxide has several desirable characteristics, including tunable density/solvation properties and gas-like diffusion rates, which facilitate penetration into solid matrices. Supercritical carbon dioxide has been used commercially for decades as a solvent in the food, chemical, and materials industries. For example, it has been adopted as a solvent to remove caffeine from coffee beans and tea.1 Starting in the early 1990s, several researchers began exploring the use of supercritical CO2 as an extraction medium for metals.2−5 To dissolve a metal species in supercritical CO2, the metal must be charge neutral and coordinatively satisfied. This can be achieved through bonding with negatively charged ligands or through bonding with a combination of anions and neutral ligands. The resulting metal−ligand complex must be nonpolar, so it can dissolve in the nonpolar CO2 phase. Early on, many researchers began to focus on actinide extraction with tributyl phosphate (TBP). This was motivated by the desire for a fission product separation process that does not generate contaminated organic wastes. Researchers demonstrated the extraction of uranium and other actinides from aqueous solutions6−11 and from solid oxides or © 2019 American Chemical Society

Received: Revised: Accepted: Published: 9199

March 2, 2019 May 4, 2019 May 8, 2019 May 22, 2019 DOI: 10.1021/acs.iecr.9b01193 Ind. Eng. Chem. Res. 2019, 58, 9199−9211

Review

Industrial & Engineering Chemistry Research

indicate that bidentate coordination of the nitrate groups dominates, especially in low-water environments.28,30,31 When these complexes are exposed to TBP, the polar phosphate group on the TBP molecule substitutes for a coordinated water molecule around the metal cation. As more TBP molecules surround the central ion, the complex is progressively dehydrated. Although there is some dispute on this topic, most studies suggest that lanthanides complexed with three or more TBP molecules have fully dehydrated inner coordination spheres, although some dynamic exchange of water in and out of the inner coordination sphere is possible.27,28,32−34 As more TBP molecules substitute for coordinated water molecules, the complex becomes better shielded with nonpolar butyl groups and eventually becomes soluble in the organic or supercritical phase. TBP binding affinity increases with atomic number due to the stronger binding forces associated with the smaller atomic radius of the lanthanide. (Depending on the acid concentration, binding affinity may begin decreasing again toward the end of the lanthanide series.)23,24,35 It is generally accepted that the 1:3 complex is extracted into the organic phase in conventional liquid/liquid systems.23,24,36 However, some studies have pointed to other stoichiometries in organic and supercritical systems. Some studies have suggested that lanthanides can be extracted as 1:4 complexes, especially at low water availability.28,37 Some supercritical studies have also provided evidence that heavier lanthanides such as Ho, Yb, and Lu can also be extracted as 1:2 complexes.38,39 This could be due to the aforementioned lower coordination numbers for heavier lanthanides. It is not clear whether the complex stoichiometry is dependent on the nature of the nonpolar phase (for example, supercritical CO2 versus organic solvent). TBP/H2O, TBP/HNO3, and TBP/H2O/HNO3 complexes of various stoichiometries can also be formed.40 Each of these complexes will partition in some way between the phases present in the system, which can include supercritical, solid, or aqueous phases. Mechanism of Extraction from Solid or Aqueous Nitrates. Researchers have used CO2 and TBP to extract lanthanides from aqueous nitrate solutions38,41,42 and from solid nitrate salts.37,39,43 TBP is a slightly stronger Lewis base than water and is able to displace the coordinated water molecules surrounding a metal nitrate.44 This organometallic complex has nonpolar butyl groups facing outward, thus rendering it soluble in the organic phase. The reaction can be represented as follows

If proven viable, supercritical extraction has the potential to replace the leaching or solvent extraction steps in conventional lanthanide ore processing. Ores would undergo conventional concentration and digestion steps to isolate and convert lanthanide minerals to an acid-soluble form. Supercritical extraction could be used to extract the lanthanides out of these solids directly, or the solids could be leached and the lanthanides could be extracted from the solution. After extraction, the lanthanides could be precipitated via stripping or pressure reduction (which lowers solute solubility in supercritical CO2), and the CO2 can be recycled back to the extraction step. Several different mechanisms could potentially provide for lanthanide separation. If the various metal species have differing kinetics or equilibrium distributions during extraction or stripping, then this could be exploited to achieve some degree of separation. Another potential separation mechanism involves the tunable solvation properties of supercritical CO2: If the various lanthanide complexes have different relationships between pressure and solubility, then sequential pressure increases or reductions could be used to separate the lanthanides from each other. If proven viable, a supercritical extraction and separation process could present several advantages over conventional leaching/solvent extraction technology: • Reduced residence time requirements for each extraction stage due to the high diffusivity of supercritical carbon dioxide • Reduction in acid consumption by precipitating lanthanides from the supercritical phase via pressure reduction rather than acid stripping • Elimination of flammability issues through the replacement of organic solvents such as kerosene with nonflammable CO2 Of course, the need for high-pressure equipment presents the greatest challenge for the commercial adoption of supercritical CO2 for lanthanide extraction. Purpose and Scope. The purpose of this Review is to summarize the body of work on the supercritical extraction of lanthanide tributyl phosphate complexes. First, fundamental mechanisms of extraction are discussed. Next, the results of past studies are tabulated. These studies are organized by type of experiment. Finally, the key results and conclusions of these studies are compared, with particular attention to the role of water, the separation of lanthanides, and the effect of temperature and pressure. Several studies have found contradictory effects of temperature and pressure on extraction, and an explanation is proposed to account for these inconsistent findings.

Ln 3 +(s/aq) + 3NO3−(s/aq) + y H2 O(s/aq) + z TBP(Sc)

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→ Ln(NO3)3 ·z TBP ·y H2 O(Sc)

MECHANISM Structures of Complexes. Lanthanide cations have been found to have coordination numbers of 8−10 in several experimental26,27 and computational28,29 studies. It is thought that lighter lanthanides (La to Eu) have a coordination number of 9 in a nitrate solution, whereas heavier lanthanides (Dy to Lu) have a coordination number of 8. (Coordination numbers of both 8 and 9 are thought to be possible for the middle lanthanides.)30 In nitrate solution, these inner coordination spheres are populated with water at low nitrate concentrations, with nitrate anions gradually occupying more of the inner coordination sphere as ionic strength increases.27 Most studies

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Mechanism of Extraction from Other Materials. In a slight variation of this process, an adduct can be made by agitating TBP with nitric acid in various volume ratios and isolating the organic phase if separation occurs. The adduct can be dissolved in supercritical CO2 and used to extract lanthanides from acid-soluble minerals such as oxides and hydroxides.44−50 In these systems, the extraction mechanism involves the reaction of the lanthanide minerals with nitric acid to form lanthanide nitrates, followed by the complexation of the nitrate salt with TBP. Equations 2 and 3 illustrate this mechanism for oxides and hydroxides, respectively. 9200

DOI: 10.1021/acs.iecr.9b01193 Ind. Eng. Chem. Res. 2019, 58, 9199−9211

9201

Nd2O3, ZrO2, MoO3, and RuO2

Phosphors (Y and Eu oxides, La and Ce phosphates, and Tb in unidentified form)

Shimizu 200552

10−3 mol each Gd2O3 and SrO or 10−3 mol each Gd2O3 and ZrO2

Tomioka 200056

Tomioka 200247

10−3 mol each of Nd2O3 and Gd2O3

Solution with 6 M HNO3, 3 M LiNO3, and 3 × 10−4 M each of La, Ce, Sm, Eu, Dy, Gd, Yb, and Lu

starting material

Tomioka 199848

Laintz 199438

source

Adducts prepared using TBP and 15.5 M HNO3. Adduct stoichiometry was TBP(HNO3)1.3(H2O)0.4

Adducts prepared using anhydrous TBP and 15.5 M HNO3. Adduct was 2.4 M HNO3

Adduct prepared using TBP and 3 M HNO3. Adduct was 1.3 M HNO3

Adduct prepared using TBP and 3 M HNO3. Adduct was 1.3 M HNO3

TBP

adduct/extractant

Table 1. Summary of Flow-Through Extraction Experiments

Starting material added to extraction vessel and pressurized with CO2. CO2 (1.0−1.5 mL/min) and adduct (0.97 g/min) pumped in and combined in mixing joint. Outlet collected to calculate recovery 0.01 or 0.001 mol of starting material was loaded into cell. CO2 (2.2−2.5 mL/min) and adduct (0.5 mL/min) pumped continuously through cell. Outlet captured to calculate recovery 20 mg of starting material and 2 mL of complex placed in reaction cell. Another cell containing 8 mL of TBP was upstream of the reaction cell. CO2 fed through system (2 mL/min)

40 mL of adduct added to equilibrium vessel and starting material added to extraction vessel. CO2 introduced at 1.6−2.0 mL/min. Outlet collected to calculate recovery

8 mL of solution was added to extraction vessel. CO2 containing TBP at 10, 20, or 30 vol % was added to cell and mixed in static mode, followed by a dynamic extraction with a flow rate of 2 mL/min. Depleted solution in extraction vessel analyzed to calculate recovery

system description

15 MPa, 60 °C

12 MPa, 40 °C

12 MPa, 40 °C

12 or 15 MPa, 40 ° C

35 MPa, 60 °C

pressure and temperature

30−120 min

30−300 min

15−90 min

10 min static extraction, then 60 min dynamic extraction

15 min static extraction, then 30 min dynamic extraction

contact time

Nd2O3 alone: 66% In the presence of ZrO2: 51% In the presence of MoO3: 19% In the presence of RuO2: 50% After 120 min: 99.7% for Y, 99.8% for Eu, 60 min: >96% for Nd

best conditions and corresponding recovery

observations Extraction decreased slightly with higher pressure (15−30 MPa) Extraction increased slightly with temperature (40−60 °C) First-order kinetic model proposed Decrease in static extraction recovery above about 4 M HNO3 adduct Flushing system with TBP after static extraction improved recovery, especially for the higher acidity adducts Static extraction recovery was independent of temperature (40−60 ° C) and pressure (15−30 MPa) Dynamic extraction decreased slightly with higher pressure (15−30 MPa) Dynamic extraction increased slightly with temperature (40−60 °C) Droplet condensation proposed Failed to extract CeO2

Industrial & Engineering Chemistry Research Review

9202


9203

Treated bastnäsite concentrate (La, Ce, Nd, Pr oxides or hydroxides, with Ca, Ba, and Sr as the main impurities)

Sinclair 201750

La2O3, Nd2O3, Eu2O3, and Y2O3 mixture and waste phosphors containing La, Eu, Gd, and Y

Mixture of lanthanide oxides (Y2O3, CeO2, Eu2O3, Tb2O3, and Dy2O3)

Baek 201644

Mastrolitti 201753

Neodymium oxide converted to nitrates

starting material

Zhu 201149

source

Table 1. continued

Adducts formed from TBP and 15 M HNO3. More TBP added to organic phase. Adduct was 2.4 wt % water

Adducts made with TBP and fuming nitric acid. Stoichiometries ranged from TBP(HNO3)0.63(H2O)0.2 to TBP(HNO3)2.44(H2O)0.86. Adduct acidities ranged from 2 to 6 M HNO3

Adducts made with dry TBP and concentrated nitric acid (TBP(HNO3)1.7(H2O)0.6) or wet TBP and fuming nitric acid (TBP(HNO3)5.2(H2O)1.7)

TBP

adduct/extractant

180−2000 mg of oxide mixture (or 250 mg of phosphors) and 10 mL of adduct added to cell. Cell was stirred for a certain static reaction time, and then cell contents were discharged into a trap solution. Trap solution assayed to determine recovery

Approximately 0.1 g starting material loaded into column packed with silanized glass wool. CO2 flow introduced at 3 mL/min, with adduct at varying flow rates. Adduct was 1−6 mol % in CO2. Outlet samples collected at various time points to obtain kinetic curve

1.29 × 10−4 mol of each starting material placed in extraction column and kept in place using silanized glass wool. CO2 introduced at 3 mL/min and adduct was pumped in at varying flow rates (4.9 or 8.7 mol % for TBP(HNO3)1.7(H2O)0.6, 1.9 or 5.0 mol % for TBP (HNO3)5.2(H2O)1.7). Outlet captured to calculate recovery

Same dynamic extraction system as previous study.46 3.0 mL/min CO2 flow rate. 0.4 mL/min TBP flow rate

system description

15 MPa, 50 °C

34 MPa, 65 °C

34.5 MPa, 65 °C

25 MPa, 50 °C


10−120 min

15−120 min

90 min (plus 45 min of flushing with CO2)

20−180 min

contact time

250 mg phosphors with 1 h contact time (recoveries are estimated from graph): 4%

160 mg of oxide mixture with >40 min contact time (recoveries are approximate): 45% for La, 90% for Nd, 100% for Eu, and 100% for Y

Roasted bastnäsite with TBP(HNO3)1.38(H2O)0.46 at 5.0 mol % after 120 min: 72% for La, 96% for Ce, 88% for Pr, and 90% for Nd NaOH digested bastnäsite with TBP(HNO3)1.38(H2O)0.46 at 5.1 mol % after 90 min: 93% for La, 100% for Ce, 99% for Pr, and 101% for Nd

TBP(HNO3)1.7(H2O)0.6 at 4.9 mol %: >99% for Y, 0.12% for Ce, >99% for Eu, 92.1% for Tb, and 98.5% for Dy

>60 min extraction: >95% for Nd


observations Both commercial and converted Nd2O3 samples were fully extracted within ∼1 h Extraction increased with atomic number Lower acidity adduct showed higher extraction Lower adduct concentration resulted in better extraction Failed to extract CeO2 Peak extraction of Ce, Nd, and Pr with 3 to 4 M HNO3 adduct Extraction increased with adduct concentration in the range of 1−6 mol % in CO2 Extraction of Ba and Sr was generally below detection limit. Extraction of Ca was about 50% Extraction increased with time in the range of 10− 40 min but remained constant with greater extraction time Extraction decreased with mass of starting material in



NiMH battery anode (dismantled and ground). The anode consists of a metal alloy containing La, Ce, Pr, and Nd

NdFeB batteries (demagnetized, coating removed, and ground). The magnet contained Fe, Nd, Pr, and minor Dy. Supplemental tests with mixture of Nd2O3, Fe2O3, and FeO

Zhang 201854

starting material

Yao 201851

source

Table 1. continued

Adducts formed with TBP and 10.4, 13.05, or 15.7 M HNO3. Stoichiometries were TBP(H2O)0.387(HNO3)1.224, TBP(H2O)0.405(HNO3)1.352, and TBP(H2O)0.560(HNO3)1.969. Methanol added in some tests

Adducts formed with TBP and 10.4 or 15.7 M HNO3. Stoichiometries were TBP(HNO3)1.745(H2O)0.52 and TBP (HNO3)1.171(H2O)0.384. Methanol added in some tests

adduct/extractant

Starting material and 5 mL of adduct placed in reaction cell. The cell was pressurized with CO2, agitated, and heated to the desired temperature. After a set time, the cell was depressurized and the outlet was collected

Starting material and adduct placed in the reaction cell. The cell was pressurized with CO2, agitated, and heated to the desired temperature. After a set time, the cell was then depressurized and the outlet was collected

system description

20.7−31 MPa, 35−55 ° C

20.7 or 31.0 MPa, 35−55 ° C


60, 90, or 120 min

60 or 120 min

contact time

TBP(H2O)0.560(HNO3)1.969 at 10 mL adduct/g starting material, 1500 rpm, 20.7 MPa, 55 °C, 2 wt % methanol added, 1 h: 94% for Nd, 91% for Pr, 98% for Dy, and 62% for Fe

for La, 46% for Gd, 50% for Eu, and 50% for Y TBP(HNO3)1.745(H2O)0.52 with 10 mL of adduct/g starting material, 2 mol % methanol added, 31 MPa, 2 h: 86% for La, 86% for Ce, 88% for Pr, and 90% for Nd


observations the range of 180−2000 mg More acidic adduct yielded better extraction Extraction increased with pressure (20.7 vs 31.0 MPa) Extraction was relatively independent of temperature (35 vs 55 °C) Extraction improved with 2 mol % methanol addition Increased adduct concentration (5−10 mL adduct/g starting material) yielded better lanthanide extraction More acidic adduct yielded better lanthanide extraction Fe extraction increased with residence time (1 to 2 h) and pressure (20.7−31 MPa) Fe extraction inhibited by methanol (0− 2 wt %) and by increased agitation rate (750−1500 rpm)


9204


Nd(NO3)3·6H2O, Pr(NO3)3·6H2O, Ho(NO3)3·5H2O Ce(NO3)3·6H2O, Ce(NH4)2(NO3)6·6H2O

Zhu 201643

Nd(NO3)3·6H2O and Ho(NO3)3·5H2O

Metal nitrates (0.00875 to >0.035 mmol) added to cell, along with TBP at 0.1−15 mmol/mmol of metal. The cell was pressurized with CO2 and stirred. Supercritical metal concentration measured with UV/vis spectroscopy Approximately 0.1 g of each lanthanide nitrate placed in cell. CO2 was pumped in and the cell was heated to the desired temperature. TBP was pumped in and the cell was stirred. Supercritical metal concentration measured with UV/vis spectroscopy

Nd(NO3)3·6H2O, Pr(NO3)3·6H2O, and Ho(NO3)3·5H2O

Fox 2004; Fox 200537,39

Sinclair 201857

20 mL of starting solution and TBP (3.3−6.2 mol %) loaded into cell, pressurized, and stirred. After equilibration, an aqueous sample was taken at constant pressure

One solution containing 0−5 M HNO3 and 0.06 M Nd nitrate and one solution containing 1 M HNO3 and 0.06 M of La, Ce, Nd, Sm, Eu, or Yb

Joung 200042

Approximately 8 mM Nd(NO3)3·6H2O, 8 mM Ho(NO3)3·5H2O, 8−80 mM TBP, and 0−200 mM water added to cell. The cell was pressurized with CO2 and stirred. Supercritical Nd, Ho, and water concentration measured with UV/vis spectroscopy

Cell was half-filled with starting solution. TBP and CO2 were mixed in the pump at 6−20 mol %, then fed into the cell, shaken, and equilibrated overnight. Aqueous phase sampled at constant pressure

Solution containing 0.05 M lanthanide nitrate (La, Ce, Pr, Nd, Gd, Ho, Er, Yb, and/or Y), 0.1 M HNO3, and 3 M NaNO3

Dehghani 199641

system description

starting material

source

Table 2. Summary of Equilibrium Extraction Experiments

17−28 MPa, 60 °C

14−19 MPa, 45−60 °C

27.5 and 31 MPa, 35 °C

50 mL

41.2 mL

3.5 mL

62 mL

0.75−23 MPa, 40 °C

10−25 MPa, 40−60 °C

vessel volume 75 or 150 mL

pressure and temperature observations

Equilibrium behavior of La with TBP was the same in CO2 and hexane Extraction was highest for Ho/Er Extraction of Gd increased with pressure (0.75−23 MPa), Yb decreased, and La was unchanged Extraction of Nd was independent of aqueous acidity (0−5 M HNO3) at 40, 50, or 60 °C Extraction of Nd with TBP-saturated CO2 of varying pressure (10−25 MPa) found that extraction decreased with pressure. Extraction was nearly constant across lanthanide series at 40 °C and 25 MPa Evidence of four TBP molecules for every Nd(NO3)3 or Pr(NO3)3. For HO(NO3)3, either two or four TBP molecules depending on amount of TBP available Solubility increased with pressure between 14 and 16 MPa, then leveled off between 16 and 19 MPa Extraction increased slightly with temperature (45−60 °C) Ho was extracted preferentially over Nd Nd and Pr did not separate Nd showed greater extraction when in the presence of Ce3+ and Ce4+ Ho was preferentially extracted over Nd Extraction was greatest when water concentration was minimized Separation was greatest when water concentration was minimized


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Review


Figure 1. Equilibrium extraction of Nd with TBP.

extraction of lanthanides with TBP. These studies are summarized in Table 1. In general, these experiments involve loading a lanthanide-containing starting material (solid or liquid) into a reaction chamber. If the starting material is a nitrate salt or nitrate solution, then CO2 and TBP are introduced to extract the metal nitrates (see eq 1). If the starting material is a lanthanide oxide or hydroxide, then CO2 and a TBP/HNO3 adduct are introduced to generate a nitrate and then extract with TBP (see eqs 2 and 3). Percent extraction can then be measured by directing the outlet fluid into a trap solution44−54,56 or by measuring the remaining lanthanides in the solid/liquid phase left in the cell.38,45,46,49 Many studies have distinguished between “dynamic extraction” and “static extraction”. In static extractions, CO2 and the TBP/adduct are introduced into the high-pressure vessel containing the starting material. The components are then allowed to stir for a set period of time; then, the outlet valve is opened and the supercritical phase is allowed to leave the vessel. The compartment may be flushed for a period of time with CO2 to remove all dissolved species in the supercritical phase. Static extractions are not ideal for determining reaction kinetics because these types of experiments inherently involve both a closed stirring stage and an open flushing stage. It is therefore difficult to determine how much of the extraction occurred during each stage. In dynamic extractions, a lanthanide-containing starting material (solid or liquid) is loaded into a high-pressure cell or column; then, supercritical CO2 containing dissolved TBP or adduct is pumped through the cell continuously. Ideally, a constant TBP or adduct concentration is maintained throughout the experiment. Some studies have included chambers upstream of the reaction cell where the supercritical phase can be mixed to ensure the full dissolution of any TBP or adduct, thus maintaining a constant concentration.44,50 Dynamic extractions can be used for determining the kinetics of extraction from various lanthanide-containing materials.46,49,50,53 First-order kinetic models have been fit to kinetic extraction data from dynamic systems,46,50 but further study is required to elucidate the mechanism and rate-limiting steps. Equilibrium Extraction Experiments. Several researchers have sought to establish the fundamental phase equilibrium behavior of the lanthanide/TBP/supercritical CO2 system. These studies are summarized in Table 2. These are modeled

0.5Ln2O3(s) + z TBP(HNO3)x (H 2O)y(Sc) → (TBP)z Ln(NO3)3(Sc) + (zx − 3)HNO3(aq) + (zy + 1.5)H 2O(aq)

(2)

Ln(OH)3(s) + z TBP(HNO3)x (H 2O)y(Sc) → (TBP)z Ln(NO3)3(Sc) + (zx − 3)HNO3(aq) + (zy + 3)H 2O(aq)

(3)

Different adduct compositions can be made by adjusting the volume ratio of nitric acid and TBP or adjusting the molarity of the nitric acid used. Many studies have looked at how extraction is affected by different adduct compositions.44,45,50,51 FTIR and mole ratio studies of these adducts have found that low-acidity adducts mainly consist of one TBP associated with one HNO3. At HNO3/TBP molar ratios greater than 1, complexes with two (or three) HNO3 molecules associated with a single TBP begin to form, possibly with water associated with the complex as well.44 The coexistence of TBP(HNO3), TBP(HNO3)2, TBP(HNO3)(H2O), and TBP(HNO3)(H2O)2 at higher acidities is corroborated by molecular dynamics simulations.40 These same complexes are also known to form in TBP solvent extraction systems.14,24 In addition to this work on pure oxides and hydroxides, a small number of studies have used TBP/nitric acid adducts in supercritical CO2 to extract lanthanides from real ores or recycled materials. These include extraction from pretreated bastnäsite,50 fluorescent lamp phosphors,52,53 nickel metal hydride batteries,51 and neodymium−iron−boron magnets.54 Supercritical lanthanide recovery from coal fly ash has also been proposed.55

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SUMMARY OF PAST STUDIES Past studies are listed in this section, organized by type of experimental setup (flow-through or equilibrium study). In the following sections, the results of these studies are discussed as they relate to the effect of water, the selectivity, and the effects of temperature and pressure. Flow-Through Extraction Experiments. Flow-through experiments are the most common way to test the supercritical 9206


Review


Figure 2. Equilibrium extraction of Ho with TBP.

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EFFECT OF WATER Extraction from aqueous solutions tends to have lower recovery than extraction from solids. (This can be seen in the low concentrations measured by Dehghani 1996 and Joung 2000 in Figures 1 and 2.) This is likely because as more water is added to the system, the lanthanide activity in the aqueous phase is reduced, resulting in less extraction. A secondary effect of water addition is the partial dissolution of water into the supercritical phase, both as free water and as a 1:1 complex with TBP.57 This reduces TBP availability to extract lanthanides and may also inhibit lanthanide extraction through the alteration of the supercritical phase polarity. The effect of water on the extent of extraction can be seen in flow-through studies as well. There have been several flowthrough studies in which TBP/HNO3/H2O adducts with a high water content were used to extract lanthanide oxides or hydroxides. All have found that extraction was limited, leading some to postulate the formation of water droplets.44,45,48,50,52,54,56 These authors propose that if lanthanide nitrates become dissolved in these water droplets, then this can limit extraction into the supercritical phase. One study provided further evidence of droplet formation by showing that further flushing with TBP after extraction recovered significant additional lanthanides, especially in cases where a water-rich adduct had been used.45 This evidence of droplet formation has led some researchers to ensure that water in adducts is kept at a minimum by drying TBP with molecular sieves or using fuming nitric acid.44,50

on liquid/liquid equilibrium studies, which have long been used to establish separation between adjacent lanthanides and establish the number of equilibrium stages required for effective separation.23−25 Previous supercritical equilibrium studies focused on the equilibria between supercritical CO2 and dilute solutions containing lanthanide nitrates in the presence of TBP.41,42 In those studies, the aqueous phase was sampled once the system reached equilibrium, allowing for the supercritical concentration to be calculated by mass balance. More recent studies have focused on the equilibrium between supercritical CO2 and solid lanthanide nitrates in the presence of TBP.37,39,43,57 Those studies have all used in situ UV/vis spectroscopy to measure supercritical metal concentrations without the need for sampling. Those studies focused on Pr, Nd, and Ho because those lanthanides have sharp UV/vis absorption peaks at 446, 584, and 451 nm, respectively. The absorption coefficient has been found to be largely independent of temperature and pressure, which simplifies concentration measurements.43 One study57 recently added to this technique by also measuring water concentrations in supercritical CO2 via the 2ν3 overtone band at 1387 nm. This allowed for simultaneous equilibrium measurements of lanthanides and water. The equilibrium extraction of Nd and Ho is graphed in Figures 1 and 2, respectively. Although a range of pressures from 15 to 30 MPa is included, equilibrium extraction changes little in this pressure range.42,43,57 Similarly, although a mix of temperatures in the range of 35 to 60 °C is included, extraction changes little over this temperature range.42,43 The data show, as expected, that aqueous solutions of Nd or Ho experience less dissolution than solid nitrate salts. Computational Work. We are aware of only one computational study of lanthanide behavior in the TBP/ supercritical CO2 system.58 It identified the importance of interfacial phenomena in mass transfer between liquid and supercritical phases. There has also been a computational study of actinides in TBP/supercritical CO2 systems, which provided important insights including illustrating the formation of multiple complexes with different numbers of TBP coordinated around the central metal and side reactions including the complexation of TBP with water and nitric acid.40

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SELECTIVITY Some flow-through studies have shown that heavier lanthanides are preferentially extracted over lighter lanthanides by TBP in supercritical systems, just as they are in liquid/liquid solvent extraction systems. (See, for example, the results of Sinclair 2017 in Table 1.) Several equilibrium studies have also shown that heavier lanthanides have a higher equilibrium extraction extent.37,39,43,57 Interestingly, the separation of lanthanides appears to be greatest when water is minimized; it is postulated that this is mainly due to the divergence of Nd and Ho aqueous activity coefficients.57 A small amount of work has also aimed at evaluating selectivity for lanthanides over impurity elements, although more work is required in this area.47,50,53,54 In general, these 9207


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Industrial & Engineering Chemistry Research studies have found that some selectivity can be achieved. (See, for example, Tomioka 2002 and Sinclair 2017 in Table 1.) Strategies have been suggested for maintaining high selectivity for lanthanides. For example, one study54 found that the selectivity for lanthanides over ferrous iron was higher than the selectivity over ferric iron. Questions remain about whether the selection mechanism fundamentally differs from a liquid/liquid system. No study has yet directly compared extraction with supercritical CO2 with liquid/liquid extraction under the same conditions.

ln(S) = k × ln(ρ) +

A +B T

(4)

where S is the solubility of the solute, ρ is the supercritical CO2 phase density, k is the number of CO2 molecules associated with each complex, T is the temperature in Kelvin, A is an empirical constant (theoretically A = (ΔH solvation + ΔHvaporization)/R), and B is an empirical constant (theoretically B = ln(Msolute + kMCO2) + (ΔSsolvation + ΔSvaporization)/R − k(MCO2), where Ms is the molar mass of species s).43,61 The parameter k can be calculated from fitting the Chrastil model to experimental data62 or from simulations.63 Other empirical solubility models have been attempted but can be challenging with organometallic complexes in supercritical CO2 systems. For example, regular solution theory employing solubility parameters has not succeeded in replicating the supercritical CO2 solubility of organometallic complexes, likely because of the volume change associated with clustering of CO2 or the significant size and polarity differences between solute and solvent.1,3 Models employing the Peng− Robinson equation of state with simple mixing rules have been attempted with limited success for organometallic complexes, likely due to the lack of information about the critical temperature and pressure of the solutes as well as the dissimilarity between CO2 and the solute.1 Nitrate Solution/CO2 Phase Equilibrium. Whereas extraction has been shown to increase with pressure in solid nitrate/ supercritical CO2 systems as described above, phase equilibria in the presence of an aqueous phase appear to be more complicated. It is first necessary to understand the behavior of water in the presence of a nonpolar phase containing TBP. In a supercritical system containing TBP, higher pressure will increase water dissolution in the supercritical phase, thus increasing the supercritical phase polarity.57,59 Evidence from liquid/liquid systems suggest that when organic phase polarity increases, metal extraction is reduced. (For example, when aqueous acidity reaches ∼4 M HNO3, the organic phase experiences a simultaneous increase in water content and decrease in lanthanide content.36,64) Therefore, it is logical to expect that the increased dissolution of water as pressure increases would lead to a simultaneous drop in metal extraction. Indeed, lanthanide (and actinide) extraction from a dilute aqueous solutions consistently shows decreased extraction with higher pressure in the same range,7−9,42 exactly the opposite of the aforementioned trend in dry systems. Therefore, when evaluating the effect of pressure on extraction, the evidence suggests that the amount of water is a critical factor. A similar argument can be made regarding the effect of temperature. When the temperature increases, the water solubility increases in the supercritical phase.59 Whereas extraction from solid nitrates changes little in the 45−60 °C range (see Zhu 2016 in Table 1),43 extraction from dilute solutions decreases with temperature in the same range (although it should be noted that these were actinide studies).7,8 Note that another dilute aqueous study showed little effect from temperature in this range.42 Effect on Kinetics and Mass Transfer Rates. In addition to its effects on equilibria, temperature and pressure could also affect the rate of extraction from solids and liquids due to changes in complexation kinetics and mass transfer properties including viscosity and diffusivity. It is challenging to study the effect of temperature and pressure specifically on kinetics

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EFFECT OF TEMPERATURE AND PRESSURE Researchers have found a variety of trends with temperature and pressure. Temperature and pressure could theoretically have a range of effects on complexation equilibria, phase equilibria, complexation kinetics, and mass transfer rates. Isolating these individual effects can be challenging, but some insight can be gained by looking specifically at equilibrium-type studies because equilibrium distributions should be independent of kinetic and mass transfer effects. Effect on Complexation Equilibria. Changes in temperature and pressure can shift the equilibrium of TBP complexation with the metal nitrate. For example, lower temperatures can favor the complexation of the metal, consistent with a shift to a lower entropy state.24 Increasing pressure has been shown to decrease the formation of complexes in aqueous systems, consistent with a shift toward a lower volume state.41 However, it is often difficult to study the temperature or pressure effects on complexation equilibria in a two-phase system because both complexation equilibria and phase equilibria will be affected simultaneously. Effect on Phase Equilibria. Phase equilibria can include the partitioning of lanthanides between a solid nitrate and supercritical CO2 or between a lanthanide nitrate solution and supercritical CO2. Solid Nitrate/CO2 Phase Equilibrium. It is clear from equilibrium studies with supercritical CO2 and solid lanthanide nitrates that extraction increases with pressure, leveling off above ∼15 MPa.43,57 Increased solubility with higher CO2 pressure has been observed for a wide range of solutes.59 This is generally thought to be due to CO2 density increasing with pressure, which increases the clustering of CO2 molecules around the solute. This clustering effect has been shown to enhance solubility two to five orders of magnitude above the solubility predicted by an ideal gas mixture model based on the solute’s vapor pressure.1 It should be noted that if CO2 density is indeed a major driver of solubility, then this should also be reflected in the effect of temperature on lanthanide extraction from solid nitrates. Increased temperature reduces the density of the CO2 phase, which, in theory, should impact solubility. (As an additional complication, temperature will also affect the vapor pressure of the lanthanide/TBP complexes.) To date, temperature has only been investigated in one equilibrium study of solid lanthanide nitrates, and it was found to have a negligible effect on extraction in the range of 45−60 °C.43 Some researchers have used models, ranging from highly theoretical to highly empirical, to model the solubility of lanthanide/TBP complexes. The Chrastil model is a largely empirical model of solubility that has been applied in several studies to model increased solubility with density60 9208


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with wide ranges of water content. The evidence also suggests that excessive water content in TBP/HNO3 adducts can lead to the formation of droplets that segregate lanthanides and limit recovery. Commercial adoption of this technology to lanthanide extraction from ores or recycled materials requires further work. Further studies are required to demonstrate selectivity for lanthanides over impurities. The economic feasibility of such a process must be determined and evaluated relative to other processing options.

because of the interconnected nature between kinetics and phase equilibria in these systems. In addition, drawing comparisons between various flow-through type experiments is difficult because these studies have used a range of flow rates, residence times, adducts, and starting materials. Various flowthrough studies have claimed to find decreasing46 or increasing51 extraction with pressure in the 20−30 MPa range. However, a close look at the data reveals that the effect of pressure in these studies was very minimal. This lack of dependence on pressure within this range is in agreement with equilibrium data.43,57 Only two studies have examined the effect of temperature on extraction kinetics in a flow-through system. The temperature ranges were 40−60 °C46 and 35−55 °C.51 Both showed little change in extraction over this temperature range. This is corroborated by equilibrium measurements43 that showed no significant change in solubility over a similar temperature range.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

L. K. Sinclair: 0000-0003-0143-5420 R. V. Fox: 0000-0003-0745-186X

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Notes

FUTURE DIRECTIONS Mechanisms. Whereas the ability to extract lanthanides from solutions and solids has been well established, some fundamental questions remain about the mechanisms at play. There is potentially a large number of different complexes that form in these systems, including TBP/H2O, TBP/HNO3, TBP/H2O/HNO3, and TBP/H2O/lanthanide nitrate complexes with various stoichiometries. Whereas the use of in situ spectroscopy has allowed for measurements of lanthanide and water concentrations in the supercritical phase, no study has demonstrated a method to measure the concentrations of each specific complex. Such a method would aid in answering some of the outstanding questions in this field, including the mechanism behind extraction selectivity for heavier lanthanides and the role of temperature and pressure on extraction equilibria. Simulations could also play a role in elucidating which complexes are formed under various conditions. Demonstration of Commercial Viability. Whereas the extraction and separation of lanthanides has been demonstrated, questions remain about how this would be applied in practice to extraction from ores or recycled sources. As discussed, commercial adoption for primary extraction or recycling will require a demonstrated advantage relative to conventional processing. This could include fewer equilibrium stages required to achieve a given purity (possibly aided by solubility tuning between stages using temperature or pressure adjustment) or faster extraction rates allowing for smaller extraction and stripping vessels. The capital costs associated with high-pressure equipment would be one major factor to weigh against these potential benefits.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the help of Donna Baek, Dan Ginosaur, Bruce Mincher, Mary Case, Arna Pálsdóttir, and the entire team at CF Technologies.

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REFERENCES

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CONCLUSIONS This Review has documented the past 25 years of research regarding the lanthanide/TBP/supercritical CO2 system. Studies have included flow-through extractions of lanthanidecontaining materials and equilibrium-type experiments. Whereas it is clear that lanthanide nitrates, oxides, hydroxides, and nitrate solutions can be easily extracted with supercritical CO2, further work is required to firmly establish the effect of temperature and pressure on extraction equilibria of the metals and more specifically on the role of water. The evidence suggests that the phase partitioning of water may explain the contradictory effects of pressure and temperature in studies 9209


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