Extraction of Rare Earth Oxides Using Supercritical Carbon Dioxide

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Extraction of Rare Earth Oxides Using Supercritical Carbon Dioxide Modified with Tri‑n‑Butyl Phosphate−Nitric Acid Adducts Donna L. Baek,*,† Robert V. Fox,† Mary E. Case,†,‡ Laura K. Sinclair,§ Alex B. Schmidt,∥ Patrick R. McIlwain,⊥ Bruce J. Mincher,† and Chien M. Wai‡ †

Idaho National Laboratory, Idaho Falls, Idaho 83415, United States University of Idaho, Moscow, Idaho 83843, United States § Cornell University, Ithaca, New York 14853, United States ∥ Boise State University, Boise, Idaho 83725, United States ⊥ Montana State University, Bozeman, Montana 59717, United States ‡

S Supporting Information *

ABSTRACT: A new tri-n-butylphosphate−nitric acid (TBP−HNO3) adduct was prepared by combining TBP and fuming (90%) HNO3. The adduct was characterized, and its phase-equilibrium behavior in supercritical carbon dioxide is reported. Supercritical carbon dioxide (sc-CO2) was modified with this new adduct [TBP(HNO3)5.2(H2O)1.7], and the extraction efficacies of selected rare earth oxides (Y, Ce, Eu, Tb, and Dy) at 338 K and 34.5 MPa were compared with those obtained using an adduct formed from concentrated (70%) HNO3 and TBP [TBP(HNO3)1.7(H2O)0.6]. All rare earth oxides tested with both adduct species could be extracted with the exception of cerium oxide. The water and acid concentrations in the different adducts were found to play a significant role in rare earth oxide extraction efficiency.

1. INTRODUCTION Lanthanide metals and alloys are often referred to as “technology metals” because they are essential for the proper functioning of high-tech devices and engineered systems. Lanthanide-containing metals and alloys are found in miniaturized electronic devices, advanced weapons systems, electricity production devices where mechanical energy is converted to electrical energy (e.g., wind turbines), solar cells, catalysts, and electric motors where electrical energy is converted to mechanical energy (e.g., electric vehicles).1−4 Lanthanides are also present as key ingredients in photonemitting phosphors found in energy-efficient compact fluorescent lamps (CFLs) and in solid-state lighting devices such as light-emitting diodes (LEDs).2,4 Scandium and yttrium often appear in the same commercial products with lanthanides, and the combination of scandium, yttrium, and the 15 lanthanides comprise the group of elements that are commonly referred to as rare earths. Various forms of hydrometallurgy are commercially mature and have been successfully practiced on an industrial scale for decades. For the recovery of metals, the basic hydrometallurgical approach is to render an insoluble, solid form of the targeted metal into a soluble form and then extract or separate that soluble form to effect a separation. Kronholm et al.5 recently published a primer on the essential aspects of hydrometallurgical processes for rare earth separations. Several authors have recently reviewed or reported the current status of © XXXX American Chemical Society

hydrometallurgical technologies for recovering rare earths from consumer electronics, end-of-life products, and industrial waste containing lanthanides.6−15 Wu et al.6 and Tan et al.7 recently reviewed hydrometallurgical technologies used for the recovery of lanthanides from tricolor phosphors in waste fluorescent lamps. To make lanthanides present in the phosphor matrix more amenable to extraction, it was noted that phosphor powders have been either mechanically or chemically pretreated. Phosphor pretreatment schemes included caustic salt fusions and acid treatment (e.g., acid roasting). Recovery techniques for dissolved metals include chemical precipitation, solvent extraction, supercritical fluid extraction, and combined processes that involve acid leaching followed by solvent extraction. Tunsu et al.8 reported extraction of rare earths from fluorescent lamp waste using Cyanex 923. Lanthanum, cerium, europium, gadolinium, terbium, and yttrium extraction were characterized as a function of temperature, nitric acid concentration in the aqueous phase, and ligand concentration in the organic phase. The kinetic rate of extraction was measured along with the coextraction of undesirable elements (iron and mercury). Lead, zinc, iron, and mercury were found to coextract. The authors pointed to either removal of those Received: February 8, 2016 Revised: June 8, 2016 Accepted: June 14, 2016

A

DOI: 10.1021/acs.iecr.6b00554 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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mixed phosphor; terbium comprised 2.6 wt % of the mixed phosphor. When the more acidic adduct was used, the authors noted that aqueous droplets formed as a result of the reaction of TBP−HNO3 with lanthanide oxides. The authors speculated that excess water present in the adduct complex, combined with water produced upon reaction with the lanthanide, apparently led to the condensation of water and the formation of a water phase into which the lanthanides partitioned, resulting in poor recovery yields for that adduct formulation. That observation points to the importance of determining optimal reaction/ extraction conditions, having knowledge of the acid and water concentrations for different adducts, and having a keen understanding of phase-equilibrium behavior under given conditions. For the current work, we focus specifically on rare earths in their oxide form. The intent is to isolate chemical effects on extraction efficiency attributable to the adduct formulation as opposed to effects attributable to the matrix wherein rare earths are present. The solvent of choice is carbon dioxide. With slight modification of the chemistry, the supercritical fluid extraction technique used herein can be adapted to the extraction of rare earths from ores, magnets, catalysts, electrolytes, and a variety of other liquid and solid matrixes. Metals are found to be difficult to react and extract from such matrixes unless high concentrations of strong mineral acids are used. Popular belief is to make stronger and stronger acid adducts and to apply them in higher ratios to achieve successful extraction. In this work, we introduce a new TBP−HNO3 adduct that is formed using fuming (90%) nitric acid. We report characterization of the new adduct and its phase-equilibrium behavior. We also compare the rare earth extraction performance of the new adduct against those of previous TBP−HNO3 formulations in a supercritical fluid carbon dioxide solvent.

metals prior to contact with Cyanex 923 or chelation and extraction of selected contaminant metals after the fact. Selective stripping of rare earth materials from the extract was found to be possible in a single step using 4 M hydrochloric acid solution. Innocenzi et al.9 provided an extensive review of hydrometallurgical processes for the recovery of yttrium from ores, phosphor powders, and electronic equipment. Table 2 from Innocenzi et al.’s 2014 work9 contains an extensive summary of the main hydrometallurgical works regarding yttrium recovery. In a more recent study, Innocenzi et al.10 examined hydrometallurgical approaches for the recovery of lanthanum and cerium from fluid catalytic cracking catalysts. Santos et al.11 reported a hydrometallurgical method for the recovery of rare earth metals, cobalt, nickel, iron, and manganese from the anodes of spent nickel−metal hydride mobile phone batteries. The rare earth compounds were obtained by sulfate chemical precipitation at pH 1.5, with sodium cerium sulfate [NaCe(SO4)2·H2O] and lanthanum sulfate [La2(SO4)3·H2O] as the major recovered components. Iron species were recovered as Fe(OH)3 and FeO, and manganese was obtained as Mn3O4. Nickel and cobalt were both recovered as the hydroxides [Ni(OH)2 and Co(OH)2]. Yoon et al.12 revealed a hydrometallurgical process for the recovery of dysprosium and neodymium from permanent magnet scrap leach liquors using di(2-ethylhexyl)phosphoric acid extractant in kerosene diluent. The authors demonstrated >90% recovery of dysprosium using a selective acid stripping process. Borra et al.13 examined the leaching and recovery of rare earths from red mud (bauxite residue) using different acids. The greatest recoveries for scandium, yttrium, lanthanum, cerium, neodymium, and dysprosium occurred using either hydrochloric or nitric acid, with 6 M hydrochloric acid showing superior behavior. In this work, we examine supercritical fluid processing as the method of choice for the extraction and recovery of selected rare earths commonly associated with end-of-life consumer products such as fluorescent lighting phosphors. Wai and coworkers14 reported the first complexation reaction in supercritical carbon dioxide in 1992 after discovering that fluorinated complexing agents formed metal adducts having much greater solubility in supercritical carbon dioxide (sc-CO2) than their nonfluorinated analogues.15 Numerous reports of supercritical fluid extraction of rare earths from solid and liquid matrixes have since appeared in the literature.16−23 Lanthanide complexes formed in sc-CO2 using various extracting ligands have been examined.24,25 Extractions of lanthanides from their oxides using a tributyl phosphate−nitric acid adduct (TBP− HNO3) have been reported.18−20 The extraction of rare earth oxides from fluorescent lamp phosphors using supercritical carbon dioxide was previously investigated by Shimizu et al.21 In that work, the authors prepared two different TBP−HNO3 adducts to extract luminescent material from waste fluorescent lamps. Adduct formulations were prepared by contacting known volumes of concentrated nitric acid (70%) with anhydrous TBP. The adduct TBP/HNO3/H2O molecular stoichiometry was reported as 1:1.8:0.6 for complex A and 1:1.3:0.4 for complex B. Using the complex B adduct at 15 MPa, 333 K, and a static extraction time of 120 min, the authors reported >99% recoveries of the yttrium and europium, which comprised 29.6 and 2.3 wt %, respectively, of the mixed phosphor. Under those conditions, less than 7% of the lanthanum, cerium, and terbium were extracted. Lanthanum and cerium comprised 10.6 and 5.0 wt %, respectively, of the

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Dry molecular sieves were acquired from Delta Adsorbents (mSorb 3A 8 × 12 IMS, Delta Adsorbents, Roselle, IL) and dried in an oven overnight at 523 K (250 °C) prior to use. Tri-n-butyl phosphate (TBP, 97%), concentrated nitric acid (15.8 M, 70 w/w %), sodium hydroxide standard solution (0.1 N), silanized glass wool, and rare earth oxides (Y2O3, CeO2, Eu2O3, Tb2O3, and Dy2O3; >99.9%) were purchased from Sigma-Aldrich (St. Louis, MO) and utilized as received. Fuming nitric acid (ACS grade, 21.2 M, >90% w/w), sodium hydroxide standard solution (1.0 N), and phenolphthalein (indicator) were obtained from Fisher Scientific (Fair Lawn, NJ) and utilized as received. Imidazole was on hand at the Idaho National Laboratory (INL). An ultrahigh-purity liquid carbon dioxide tank with a full-length eductor tube was purchased from Matheson Tri Gas (Basking Ridge, NJ). The acid and water contents can vary from batch to batch in reagents; thus, the acid concentrations in concentrated HNO3 (70%) and fuming HNO3 (90%) and the water concentration in TBP were measured prior to use of those reagents. The acid concentrations of concentrated HNO3 (70%) and fuming HNO3 (90%) were measured by acid−base titration and found to be 15.8 and 21.2 M, respectively. The concentration of water in TBP as received from Sigma-Aldrich was measured by Karl Fischer titration (Metrohm 899 Coulometer, Metrohm USA, Riverview, FL). The as-received TBP had 1446 ppm of H2O and is referred to in this work as “wet” TBP. Wet TBP was dried for at least 24 h over dried molecular sieves to 10 M acid. In Figure S11, the H2O concentration is plotted versus the HNO3/TBP volume ratio. The H2O concentration increased quickly and reached about 50000 ppm. Compared to the values for the previous adducts, the H2O concentration is nearly 2 times greater, and the molar acid concentration is twice as high as previously reported. When fuming HNO3 was contacted with wet TBP, all of the aqueousphase acid was miscible with the organic phase, forming a single-phase solution. There was no aqueous phase to measure acid and H2O contents; thus, no acid, water, or mole ratio plots related to such conditions are reported. 3.2. Phase Equilibria of Acid Adducts in sc-CO2. Phase equilibria of various TBP−HNO3 adducts in sc-CO2 were observed visually. In Figure 6, several images were captured during the transition of the TBP−HNO3 adduct/sc-CO2 mixture from two phases to a single phase. The images were acquired by looking through the sapphire window of the equilibrium cell. In three of the images, a stir bar can be seen on the floor of the view cell. The adduct was a slight yellow color at room temperature; however, when the adduct was heated, nitronium (NO2+) ion was produced and gave a reddish-orange color.28 A known volume of the TBP−HNO3 adduct was pipetted into the view cell and heated to the desired temperature, as shown in Figure 6a. Once sc-CO2 made contact with the adduct, the adduct phase began to swell and expanded (Figure 6b). As sc-CO2 was continuously metered into the view cell, the pressure increased and the adduct phase continued to swell to the point where a “fog” of micron-sized

Figure 7. Phase diagram of TBP(HNO3)1.7(H2O)0.6 in sc-CO2. The lines are a guide for the eye.

as a function of pressure and mole fraction of the adduct. The area below each curve represents the two-phase region with respect to pressure and mole fraction at a given temperature; the area above the curve represents the single-phase region. The pressure required to reach the phase transition boundary of the various TBP−HNO3 adduct mixtures increased as the temperature increased. Each profile gradually increased up to the mole fraction of 0.01, reaching a maximum, and then decreased after a mole fraction 0.07. Enokida et al.26 reported the phase equilibrium profiles of the adduct TBPG

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The difference between the two adducts TBP(HNO3)1.7(H2O)0.6 and TBP(HNO3)5.2(H2O)1.7 is the amount of H2O present in the starting acid and TBP. TBP(HNO3)1.7(H2O)0.6 was prepared with concentrated (70%) nitric acid and dry TBP, whereas TBP(HNO3)5.2(H2O)1.7 was prepared with fuming (90%) nitric acid and wet TBP. The profiles of the phase boundary at each temperature resemble each other; however, when the empirical data are compared, the pressure required to achieve a single phase is slightly higher for TBP(HNO3)5.2(H2O)1.7. For example, 12 mL of the TBP(HNO3)5.2(H2O)1.7 adduct required 13.7 MPa (1990 psi) CO2 to achieve a single phase at 318 K compared to 12.8 MPa (1862 psi) for the TBP(HNO3)1.7(H2O)0.6 adduct at the same temperature. Typically, the pressure employed for sc-CO2 extractions is at least 13.8 MPa (2000 psi) above what is considered the single-phase limit. It is important to know the exact point at which a single phase occurs. Based on these phase boundaries, optimal parameters for the sc-CO2 extraction of rare earth oxides using acid adducts can be determined. 3.3. sc-CO2 Extraction of Selected Rare Earth Oxides. Informed by the phase equilibrium results, supercritical fluid extraction studies were performed to compare the effectiveness of two different TBP−HNO3 adducts in a head-to-head manner. The first adduct selected was the TBP(HNO3)1.7(H2O)0.6 adduct formed by mixing 1:1 v/v of dry TBP with concentrated (70%) nitric acid. The second adduct selected was the TBP(HNO3)5.2(H2O)1.7 adduct formed by mixing 1:1 v/v of wet TBP with fuming (90%) nitric acid. Each adduct was run separately in an sc-CO2 extraction experiment for the extraction of a mixture of selected rare earth oxides (Y2O3, CeO2, Eu2O3, Tb2O3, and Dy2O3) at a temperature of 338 K and a pressure of 34.5 MPa. The concentration of each adduct was also adjusted so that, in one experiment, the adduct being tested was metered in at a mole ratio value slightly to the left of the mole fraction maximum found on the phase equilibrium plot (i.e., adduct-lean conditions) and at a second condition where the adduct concentration was slightly to the right of the mole fraction maximum found on the phase equilibrium plot (i.e., adduct-rich conditions). Table 4 presents

(HNO3)1.8(H2O)0.6 in sc-CO2 at 313, 323, and 333 K with respect to pressure and mole fraction of the adduct. Although the exact values from Enokida et al.’s phase diagram were not reported, the values were estimated and compared with the data reported in this work. It was found that the values obtained from Enokida et al.’s work agreed with the values shown in Figure 7. Two additional adducts were prepared using ratios of 1:2 v/v of dry TBP/70% HNO3 [TBP(HNO3)1.9(H2O)0.7] and 1:0.5 v/v of dry TBP/70% HNO3 [TBP(HNO3)1.2(H2O)0.4]. Diagrams of their phase behaviors can be seen in Figures S12 and S13, respectively. The three adducts were made by contacting the same volume of dry TBP with different volumes of concentrated (70%) HNO3, thus giving rise to adduct mixtures having different stoichiometries and amounts of water and acid. The TBP(HNO3)1.9(H2O)0.7 adduct had the highest acid and water concentrations of 5.23 M and about 300000 ppm, respectively. The TBP(HNO3)1.7(H2O)0.6 adduct contained 4.67 M acid and about 280000 ppm water. Last, the TBP(HNO3)1.2(H2O)0.4 adduct contained 3.74 M acid and about 220000 ppm water. TBP is what makes the adducts soluble in sc-CO2; thus, adducts with less acid and water and more TBP will be more soluble in sc-CO2 at lower pressures. Comparison of Figure S12 with Figures 7 and S13 shows that the adduct having a stoichiometry of TBP(HNO3)1.2(H2O)0.4 was the most CO2-soluble of the three, meaning that the TBP(HNO3)1.2(H2O)0.4 adduct formed a single phase at lower CO2 pressures than the other two adducts. For example, at the mole fraction maximum of each phase diagram, 12 mL of the TBP(HNO3)1.9(H2O)0.7 adduct at 318 K requires 12.7 MPa (1840 psi) CO2 to achieve a single phase, whereas 12 mL of the TBP(HNO3)1.2(H2O)0.4 adduct requires only 12.2 MPa (1776 psi) to achieve a single phase. The phase-equilibrium behavior for a 1:1 v/v mixture of wet TBP and fuming (90%) HNO3 in CO2 at 318, 338, and 358 K is plotted with respect to pressure and mole fraction of the adduct [TBP(HNO3)5.2(H2O)1.7] in Figure 8. Each curve

Table 4. Extraction Efficiencies for Selected Rare Earth Oxides Using sc-CO2 Modified with a TBP−HNO3 Adduct at 338 K and 34.5 MPa Y (%)

Figure 8. Phase diagram of TBP(HNO3)5.2(H2O)1.7 in sc-CO2. The lines are a guide for the eye.

Ce (%)

>99 96.1

0.12 0.26

92.7 70.6

0.15 0.25

Eu (%)

Tb (%)

TBP(HNO3)1.7(H2O)0.6 >99 92.1 95.8 76.3 TBP(HNO3)5.2(H2O)1.7 98.7 40.0 96.7 48.0

Dy (%)

mole ratio

98.5 89.3

0.049 0.087

99.9 54.1

0.019 0.050

the sc-CO2 extraction behavior for selected rare earth oxides and the two adducts at two different concentrations. The mole ratio value of each adduct in sc-CO2 is included in the table. The top row of data for each adduct represents the lean conditions, and the bottom row represents the rich conditions. Under each sc-CO2 extraction condition, CeO2 was not extracted. CeO2 recalcitrance in this situation is attributed to its oxidation state, which can be chemically modified. At first glance, it is clear that higher acid concentration does not deliver higher extraction efficiencies. The extraction efficiencies of Y, Eu, and Tb are highest using the TBP(HNO3)1.7(H2O)0.6

represents the single-phase boundary of adduct in CO2 with respect to pressure and mole fraction at the given temperature. The area below the curve is the two-phase region; the area above the curve represents the single-phase region. Each of the curve profiles increased as the mole fraction increased up to 0.01 and then reached a maximum. The curve profiles decreased after a mole fraction of 0.08 and began to converge. H

DOI: 10.1021/acs.iecr.6b00554 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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et al.’s observation21 that “liquid droplets” formed during use of the more acidic adduct, resulting in substantially lower lanthanide extraction, is supported by information given in eq 5 and corroborated by the sc-CO2/rare earth oxide extraction results reported in Table 4.

adduct, with Dy reaching 98.5%. The TBP(HNO3)5.2(H2O)1.7 adduct, which is more acidic, produced >90% extraction for Y, Eu, and Dy but resulted in poor extraction of Tb. Adduct-rich conditions for either adduct did not yield improved results. Insight into the differences among the extraction behaviors demonstrated by the various adducts can be gained by examining the chemical reaction between rare earth oxides and TBP−HNO3. Equation 1 shows that the reaction between lanthanide oxides and HNO3 consumes 6 mol of HNO3, resulting in 2 mol of a metal nitrate salt; 3 mol of H2O are produced in the reaction. 6HNO3 + Ln2O3 → 2Ln(NO3)3 + 3H 2O

6TBP(HNO3)5.2 (H 2O)1.7 + Ln2O3 sc‐CO2

⎯⎯⎯⎯⎯⎯→ 2Ln(NO3)3 (TBP)3 (H 2O)3 + 7.2H 2O + 25.2HNO3

4. CONCLUSIONS TBP−HNO3 adducts were made and characterized. The acid, TBP, and water contents of the various adducts were measured, and the stoichiometries were determined. The phase-equilibrium behavior for each adduct was studied in sc-CO2. The phase boundary for formation of a single phase was determined for the adducts. An understanding of adduct phase behavior influenced the conditions under which sc-CO2 extractions of selected rare earth oxides were performed. This study demonstrated the importance of knowing the acid, water, and TBP contents of adducts used for sc-CO2 extractions of rare earth oxides. More acidic adducts produce excess acid and water, resulting in the formation of condensates that lower the overall rare earth extraction yield. Adding more adduct (adductrich conditions) does not necessarily improve the rare earth extraction efficiency. It was concluded that the ideal adduct might be one that is made from dehydrated components (acid, TBP) having the least amount of excess water and only enough acid to drive the formation of metal nitrate complexes. The dependence of rare earth extraction efficiencies in sc-CO2 on the acid concentration in the adduct requires further investigation.

(1)

Fox et al.24,25 demonstrated that, in sc-CO2, lanthanide nitrate hydrates will react with 4 mol of TBP to produce a 1:4 lanthanide nitrate−TBP complex (eq 2). TBP, a Lewis base, is a slightly stronger base than water; however, it is known that addition of excess water to compete against the TBP will result in a metal−ligand complex stoichiometry of 1:3 in an sc-CO2 system where water is in excess (eq 3). The 1:3 lanthanide− TBP complex stoichiometry formed when water is in excess in an sc-CO2 system is also the same as the stoichiometry found in conventional aqueous/organic solvent extraction situations.29 Note that the 1:3 lanthanide−TBP complex is less soluble in scCO2 than the 1:4 complex.24,25 The solubilities of metal−TBP complexes in sc-CO2 directly impact their extraction efficacies. Ln(NO3)3 ·6H 2O + 4TBP sc‐CO2

⎯⎯⎯⎯⎯⎯→ Ln(NO3)3 (TBP)4 (H 2O)2 + 4H 2O

(2)

Ln(NO3)3 ·6H 2O + 3TBP + 3H 2O sc‐CO2

⎯⎯⎯⎯⎯⎯→ Ln(NO3)3 (TBP)3 (H 2O)3 + 6H 2O

(3)

■

From eq 4, it can be seen that, when a TBP(HNO3)1.7(H2O)0.6 adduct is reacted in sc-CO2 with lanthanide oxides, 6 mol of adduct are needed to form 2 mol of lanthanide nitrate−TBP complex. Equation 4 is written with respect to the formation of whole metal−ligand complexes because only wholly formed complexes are soluble in sc-CO2. One can quickly surmise from eq 4 that TBP is the limiting reagent in the reaction and that acid and water are in excess. An excess of 0.6 mol of water and 4.2 mol of acid results from the reaction given in eq 4. Water is only sparingly soluble in sc-CO2; nitric acid is insoluble. TBP is required for the formation of soluble lanthanide nitrate−TBP complexes and is the molecule that forms hydrogen bonds with acid and water to keep those components in a single phase in an sc-CO2 system. If TBP is consumed through the formation of soluble metal complexes, then, in the absence of TBP, the excess water and acid will condense in CO2, forming an acidic liquid droplet into which lanthanide−nitrate complexes will partition, resulting in competing extraction equilibria and leading to an overall lowering of the lanthanide extraction efficiency.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00554. Phase equilibria, density, and water/acid concentration plots (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. Work was completed at Idaho National Laboratory under Department of Energy Idaho Operations Office Contract DE-AC07-05ID14517.

6TBP(HNO3)1.7 (H 2O)0.6 + Ln2O3

■

sc‐CO2

⎯⎯⎯⎯⎯⎯→ 2Ln(NO3)3 (TBP)3 (H 2O)3 + 0.6H 2O + 4.2HNO3

(5)

REFERENCES

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(4)

From eq 5, it can be seen that, when the more acidic adduct TBP(HNO3)5.2(H2O)1.7 is reacted with lanthanide oxides, 7.2 mol of excess water and 25.2 mol of excess acid result. Shimizu I

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DOI: 10.1021/acs.iecr.6b00554 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX