Article
pubs.acs.org/IECR
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
© 2016 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:
Revised:
Accepted:
Published:
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February 8, 2016
June 8, 2016
June 14, 2016
June 14, 2016
DOI: 10.1021/acs.iecr.6b00554
Ind. Eng. Chem. Res. 2016, 55, 7154−7163
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Industrial & Engineering Chemistry Research
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 <350 ppm
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Figure 1. Schematic of the sc-CO2 system used for the extraction of rare earth oxides.
help maintain temperature control. An inline filter with a 5- and
10-μm frit assembly (Autoclave Engineering, Erie, PA) was
placed after the extraction column to further prevent any
powder from being carried over to the trap solution. A heated
micrometering valve (Autoclave Engineering, Erie, PA) was
used to assist in controlling CO2 flow exiting to the trap
solution. After extraction, the contents of the trap solution were
analyzed by ICP-MS for metal content. All results are reported
as percentages of metal extracted. All experiments were
performed in triplicate.
2.2.2. Phase-Equilibrium Studies. For phase-equilibrium
studies, the system setup consisted of the same components as
mentioned above albeit in a slightly modified configuration.
Phase-equilibrium measurements were acquired using the two
CO2 pumps and a single view cell (equilibrium cell) mounted
on a magnetic stirrer. Temperature and pressure process
monitoring and controls were the same as those employed in
the extraction system. For the phase-equilibrium studies the
view cell contents were carefully pipetted into the cell, and the
cell was sealed and then charged with CO2 at the desired
temperature and pressure. Phase behavior as a function of
temperature, pressure, and added materials was observed by
eye. Photographic evidence was also acquired.
2.2.3. Inductively Coupled Plasma Mass Spectrometry
(ICP-MS) Analysis. Samples for ICP-MS were analyzed using a
Thermo Scientific (Waltham, MA) XSERIES 2 ICP-MS
instrument equipped with the Organic Matrix Kit. The
manufacturer-supplied kit allows for the routine analysis of
metals in organic/hydro-organic matrixes.
2.3. Procedure. 2.3.1. Characterization of the Various
TBP−HNO3 Adducts. Solutions of adduct were prepared fresh
by mixing a volume-to-volume ratio of either concentrated
(70%) nitric acid (0.05:1−8:1) with TBP (dry or wet) or
fuming (90%) nitric acid (0.05:1−1:1 volume-to-volume ratio)
with wet TBP in a vial. The contents of the vial were mixed for
5 min and then centrifuged for 5 min at 5500 rpm. After
centrifugation, the adduct (organic layer) and the dilute acid
(aqueous layer) were clearly identifiable and were independently harvested using a Pasteur pipet for characterization.
After the organic phase had been harvested, the amount of
water present in the organic phase was determined by Karl
Fischer titration (Metrohm 831 Coulometer, Metrohm USA,
Houston, TX). To determine the acidity of the organic phase,
an aliquot of adduct was washed four times with nanopure H2O
to break up the complex and remove HNO3. The HNO3
concentration in the organic phase was determined by acid−
base titration of the wash water. The H2O concentration in the
of H2O (as measured by Karl Fischer titration) prior to use.
TBP with a H2O concentration of <350 ppm is referred to as
“dry” TBP in this work.
2.2. Experimental Setup and Instruments. 2.2.1. Extraction Studies. The supercritical fluid apparatus used for the
sc-CO2 extraction of rare earth oxides was constructed inside a
fume hood and is illustrated in Figure 1. It consists of a liquid
CO2 tank connected to an ISCO syringe pump (Teledyne
ISCO model 260D, Lincoln, NE) with a series D pump
controller. A number of ON/OFF valves (Autoclave Engineering, Erie, PA) and tees (Valco Instruments Co., Houston, TX)
were used to connect and direct CO2 flow downstream. A
check valve (High Pressure Equipment Company, Erie, PA)
was placed after the CO2 pumps to protect those pumps from
backflow, and a relief valve (Swaglok, Idaho Valve & Fitting
Co., Idaho Falls, ID) was used to release excess pressure above
34.5 MPa (5000 psi). A high-performance liquid chromatography (HPLC) pump (Alltech 426) was used to meter the
ligand or adduct into the sc-CO2. A view cell (Idaho National
Laboratory) was constructed from type 316 stainless steel and
1-in. sapphire windows (Esco Products, Oak Ridge, NJ). For
extractions, the view cell was employed as a stirred equilibrium
vessel where sc-CO2 and adduct were mixed and equilibrated
before entering the extraction column. The view cell was
equipped with BT-style (bored-through) fittings so the inlet
tube could protrude through the cell wall and into the vessel.
The terminus of the inlet tube was positioned near the bottom
of the cell to allow for better mixing between the adduct and
the sc-CO2. The extraction column was constructed of type 316
stainless steel tubing approximately 10.2 cm (4 in.) long and
having a 9.52-mm (3/8-in.) outer diameter (o.d.) and a 6.35mm (1/4-in.) internal diameter (i.d.). Swagelok fittings (Idaho
Valve & Fitting Co., Idaho Falls, ID) were used for column end
fittings and for connectivity to the rest of the pressure system.
Each extraction column inlet and outlet was fitted with a 2-μm
circular porous frit that fit the internal diameter of the column
end fittings and was snugly held in place. Cartridge heaters,
heating tape, an RTD thermocouple, and a proportional−
integral−derivative (PID) controller (Omega Engineering,
Stamford, CT) were used to heat and monitor the temperature
of the view cell and extraction column. A variac (Staco Energy
Products Co., Dayton, OH) was used in conjunction with the
PID controller to assist in fine control of the power distribution
to the cartridge heaters. System pressure was monitored with a
Heise digital pressure indicator (Stratford, CT). The view cell
was placed on a stir plate to allow mixing with a magnetic stir
bar during experiments and was wrapped with insulation to
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with an HPLC pump at a flow rate dictated by the experimental
parameters for each extraction. After the dynamic extraction,
fresh sc-CO2 was allowed to flush the system for a period of 45
min at a flow rate of approximately 3 mL/min. Dodecane (20
mL) was used as a trap solution. An aliquot of each trap
solution was acquired after each experiment and subjected to
ICP-MS analysis.
aqueous acid phase was measured by Karl Fischer titration. The
acidity of the aqueous phase was determined by acid−base
titration. Acid−base titrations were manually performed ( by
Class A buret) using either 1.0 or 0.1 N standardized NaOH as
the titrant and phenolphthalein as the indicator.
A note of caution should be given with regard to the mixing,
handling, use in experiments, and storage of TBP solutions
containing nitric acid. Mixtures of concentrated (70%) nitric
acid and TBP have previously been characterized and widely
reported in the literature without mishap.17−23,26 In this work,
it was found that HNO3 mixtures with TBP approaching acid
concentrations in the organic phase greater than approximately
10 M HNO3 are unstable and can undergo exothermic
decomposition (hydrolysis). Care should be taken not to
formulate mixtures of TBP and fuming nitric acid that would
result in greater than approximately 10 M HNO3 in the organic
phase. Careful titration of acid reagents prior to use and
knowledge of the acid concentration are important for safe
practice. Mixtures of fuming nitric with TBP approaching 10 M
should be made in a small test quantity (i.e., less than 10 mL)
first prior to the creation of larger-volume mixtures. Care
should be exercised after TBP−HNO3 mixtures are made to
ensure that they are stable and not emitting NOx vapors, which
could build up and overpressurize a storage container or a
centrifuge tube. Fresh solutions should be made and used and
not stored for periods greater than 72 h. Solutions should be
stored in chemically compatible containers and either kept in
the hood for immediate use or stored in an approved acid
cabinet until use. If TBP−HNO3 solutions with >6 M acid in
the organic phase cannot be used in the same day, then they
should be neutralized and discarded. Extracts arising from a
supercritical fluid extraction process that contain metal, acid,
and TBP should be handled in the manner described above.
TBP−HNO3 mixtures should not be mixed with other organic
reagents such as short-chain alcohols, aldehydes, ketones, and
other common organic solvents that are incompatible with
nitric acid. TBP that has come into contact with nitric acid
should be neutralized first prior to being discarded as waste.
2.3.2. Phase-Equilibria Experiments. Solutions of TBP−
HNO3 adduct were prepared fresh by contacting a specific
volume-to-volume ratio (e.g., 1:1) of either wet or dry TBP
with concentrated (70%) nitric acid or wet TBP with fuming
(90%) nitric acid. The solutions were contacted for 2 min (with
shaking) and then centrifuged for 4 min at 5500 rpm. After
centrifugation, a known volume (1−50 mL) of solution was
pipetted into the view cell. The view cell was sealed and then
heated to either 318, 338, or 358 K and charged with CO2 up to
6.89 MPa (1000 psi) with stirring. CO2 was then slowly added
until a single-phase condition was visually confirmed. Once a
single phase was observed, the final pressure and volume of
CO2 dispensed were recorded. Visual observations and
photographic evidence were obtained.
2.3.3. Rare Earth Oxide Extraction Studies Using Acid
Adducts in sc-CO2. The rare earth oxides Y2O3, CeO2, Eu2O3,
Tb2O3, and Dy2O3 were massed to 14.56, 11.10, 22.70, 23.60,
and 24.06 mg, respectively, or 1.29 × 10−4 mol of each. Rare
earth oxide powders were placed between silanized glass wool
plugs in the extraction column. The glass wool served to keep
the powder in place during extraction. The sc-CO2 extractions
were performed at a temperature and pressure of 338 K (65
°C) and 34.5 MPa (5000 psi), respectively. Dynamic extraction
occurred for 90 min with an sc-CO2 flow rate averaging 3 mL/
min. The TBP−HNO3 adduct was metered into the sc-CO2
3. RESULTS AND DISCUSSION
3.1. Characterization of the TBP−HNO3 Adducts.
3.1.1. Wet TBP−70% HNO3 Adducts. To determine the
stoichiometry of the TBP−HNO3 complex, the acid and H2O
concentrations and the density of the complex were determined
for each mixture. The density of each (70%) HNO3−wet TBP
complex solution was determined by weighing a known volume
of the complex. A plot of the density with respect to the starting
volume ratio of 70% HNO3 to TBP is shown in Figure S1. As
the HNO3/TBP volume ratio increased, the density increased
and then became constant. At the point at which the mixture
reaches constant density, it is believed that the adduct has
achieved its most stable form. Adduct density was used as an
aid in determining the concentration of TBP and, thus, the
stoichiometries of the various adducts. Table 1 lists all of the
Table 1. Stoichiometry of TBP(HNO3)x(H2O)y Adduct
When Concentrated (70%) HNO3 Was Contacted with Wet
TBP
starting
aqueous/organic
volume ratio
x
0.05
0.1
0.15
0.2
0.25
0.3
0.4
0.03
0.41
0.58
0.83
0.91
1.01
1.16
y
starting
aqueous/organic
volume ratio
x
y
0.33
0.68
0.73
0.58
0.47
0.41
0.4
0.5
0.75
1
2
3
5
8
1.28
1.5
1.55
1.93
2.03
2.13
2.15
0.43
0.52
0.57
0.67
0.73
0.73
0.73
adducts formed when 70% HNO3 was contacted with wet TBP
at different volume ratios. Based on the stoichiometry shown in
Table 1, the numbers of moles of HNO3 and H2O increased
and reached a constant value. This trend supports what was
observed in the density data.
In the Supporting Information, plots of the acidities of the
organic and aqueous phases as functions of the volume ratio of
the concentrated (70%) HNO3 and wet TBP are shown
(Figure S2). The molar acid concentration in the organic phase
(open circles) increased until the HNO3-to-TBP volume ratio
reached 2. When the HNO3/TBP volume ratio exceeded 2, the
molar acid concentration increased slightly to 6 M. The molar
acid concentration in the aqueous phase (solid circles) followed
the same trend as in the organic phase; however, the molar
concentration approached 15 M as the HNO3/TBP volume
ratios increased. In Figures S3 and S4, plots of the H2O
concentration in the aqueous and organic phases are shown.
The H2O concentration in the aqueous phase decreased
exponentially as the HNO3/TBP volume ratio increased;
however, the profile of the H2O concentration in the organic
phase was unique. The H2O concentration increased up to a
HNO3/TBP volume ratio of 0.15 and then decreased to a
HNO3/TBP volume ratio of 0.4. The H2O concentration began
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adduct. As additional HNO3 is added, further hydrogen
bonding between the hydrogen on the HNO3 and the adduct
occurs. Ferraro et al.27 investigated the hydrogen bonding
between TBP and HNO3 in octane by Fourier transform
infrared (FTIR) spectroscopy. A solution of 20% v/v TBP in
octane was contacted with 1−16 M HNO3 solution. Solutions
were analyzed by FTIR spectroscopy. The νasNOO band at
1650 cm−1 was monitored and is attributed to the inner-sphere
interaction of HNO3 hydrogen bonded to the phosphoryl
group of TBP. When HNO3 was present, the νasNOO band
appeared and became more intense as the HNO3/TBP
approached 1. Once the HNO3/TBP mole ratio exceeded 1,
a second band appeared at 1670 cm−1, and the band at 1650
cm−1 became less intense. This observation is indicative of a
second HNO3 molecule disrupting the hydrogen bonding
within the inner sphere and supports the interpretation of the
mole ratio plot shown in Figure 2.
In the mole ratio plot of the [HNO3]org/[H2O]org with
respect to the [HNO3]aq/[H2O]aq, there are two distinct
regions (Figure 3). The first region shows an increasing steep
slope and an intersection at a [HNO3]org/[H2O]org mole ratio
of 2.8. The second region showed the [HNO3]org/[H2O]org
mole ratio to be slightly decreased. Enokida et al.26 reported the
mole ratio plots of the TBP−HNO3 adduct produced with
concentrated (70%) HNO3 and wet TBP as well. The
[HNO3]org/[TBP]org versus [HNO3]aq/[H2O]aq mole ratio
plot reported by Enokida et al. showed two distinct regions,
whereas three distinct regions were observed in this work. It is
possible that the difference in H2O concnetrations in TBP and
HNO3 between this work and Enokida et al.’s work and the
higher data density in this work allowed for observation of fine
structure to appear in the mole ratio plot. Enokida et al. did not
report H2O concentration for any of the reagents used in that
work. Both the [HNO3]org/[H2O]org versus [HNO3]aq/
[H2O]aq mole ratio plot reported by Enokida et al. and the
same plot shown in this work are nearly identical in all other
aspects. Based on a mole ratio plot of [HNO3]org/[H2O]org
versus [HNO3]aq/[H2O]aq, Enokida et al.26 reported the
second complex to have the general formula of 3HNO3·H2O.
3.1.2. Dry TBP−70% HNO3 Adducts. From the results of
various investigations by other researchers, we hypothesized
that the water concentration is a critical component in the
TBP−HNO3 adduct used to dissolve and extract metal oxides.
It is known that H2O disrupts hydrogen bonding between TBP
and HNO3 when it is in excess. It was also seen by Shimizu et
al.21 that the presence of excessive water could potentially
condense and adversely impact the lanthanide extraction
efficiency. With that in mind, TBP−HNO3 adducts were
prepared using dry TBP and concentrated (70%) HNO3 to
demonstrate the importance in minimizing the H2O introduced
into the system. The stoichiometries of the various TBP−
HNO3 adducts formed are listed in Table 2. The HNO3
concentration increased as the HNO3/TBP volume ratio
increased up to about a volume ratio of 2 and then started to
remain constant when the volume ratio exceeded 2. The H2O
concentration fluctuated and then reached a constant value
after a volume ratio of 2, similarly to the behavior observed with
the wet TBP−70% HNO3 mixtures. The density of each adduct
mixture was plotted as a function of starting volume ratio
(Figure S5) and supports the stoichiometries reported in Table
2. After a volume ratio of 2 was exceeded, the density remained
fairly constant as additional HNO3 was contacted with the dry
TBP. The acid and water concentrations in the organic and
to increase and remained fairly constant after a HNO3/TBP
volume ratio of 3.
Based on the density, acid concentration, and H2O
concentration, mole ratio plots were configured and are
shown in Figures 2 and 3. From the mole ratio plot of
Figure 2. Mole ratio plot of [HNO3] in the adduct versus [HNO3]
remaining in the aqueous phase after concentrated (70%) HNO3 was
contacted with wet TBP and the mixture reached equilibrium. Lines
were derived from linear-regression analysis of the data between
selected points.
Figure 3. Mole ratio plot of [HNO3]/[H2O] in the organic phase as a
function of [HNO3]/[H2O] in the aqueous phase after concentrated
(70%) HNO3 was contacted with wet TBP and the mixture reached
equilibrium. Lines were derived from linear-regression analysis of the
data between selected points.
[HNO3]org/[TBP]org as a function of [HNO3]aq/[H2O]aq in
Figure 2, there are three distinct regions that are broken up at
the intersections of the regression lines. The first intersection
occurred at [HNO3]org/[TBP]org = 1.47, and the second was at
[HNO3]org/[TBP]org = 1.53. It is plausible to assume there are
two types of complexes being formed, for example, first
TBP(HNO3)x(H2O)y and then TBP(HNO3)x(H2O)y·zHNO3.
Initially, when HNO3 is transferred into the organic TBP phase,
the first adduct type forms (region I) and reaches a single stable
complex (region II). In region III, more HNO3 is transferred
into the organic TBP phase to form the second adduct type. It
is known that hydrogen bonding between the phosphoryl
oxygen on TBP and hydrogen from HNO3 occurs to form an
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Table 2. Stoichiometry of TBP(HNO3)x(H2O)y Adduct
When Concentrated (70%) HNO3 Was Contacted with Dry
TBP
starting
aqueous/organic
volume ratio
x
0.05
0.1
0.15
0.2
0.25
0.3
0.4
0.2
0.39
0.6
0.76
1.04
0.99
1.29
y
starting
aqueous/organic
volume ratio
x
y
0.33
0.66
0.73
0.59
0.5
0.41
0.4
0.5
0.75
1
2
3
5
8
1.25
1.51
1.67
1.92
2.01
2.03
2.29
0.43
0.53
0.59
0.67
0.71
0.74
0.77
aqueous phases were measured and are shown in Figures S6−
S8. As for the previous adduct, the acid concentration in the
organic phase (open circles) increased and approached 6 M,
and the acid concentration in the aqueous phase (solid circles)
reached 15 M. In Figure S7, the water concentrations in the
aqueous phase are plotted versus the HNO3/TBP volume ratio.
The H2O concentration decreased exponentially and leveled
out at about 310000 ppm as HNO3 increased. The H2O
concentration in the organic phase was plotted with respect to
the HNO3/TBP volume ratio and is shown in Figure S8. The
profile is similar to the trend produced by the H2O
concentration from the previous adduct. The H2O concentration increased up to the volume ratio 0.15 and then
decreased to 0.40. The H2O concentration increased after 0.4
and reached a constant value after a volume ratio of 2. If the
acid adducts prepared with dry TBP are compared to those
prepared with wet TBP, then based on the stoichiometry and
the acid concentration in the organic phase, it can be concluded
that more HNO3 is able to be incorporated into the complex
because of the lower competition with H2O. This finding points
again to the importance of minimizing water in the system.
Mole ratio plots were constructed and are shown in Figures 4
and 5. Once again, three regions and two intersections are
clearly defined in the plot of [HNO3]org/[TBP]org versus
[HNO3]aq/[H2O]aq in Figure 4. The intersections occur when
the [HNO3]org/[TBP]org mole ratio is 1.21 and 1.24, about 0.25
less than what is seen in the mole ratio plot for wet TBP−70%
Figure 5. Mole ratio plot of [HNO3]/[H2O] in the organic phase as a
function of [HNO3]/[H2O] in the aqueous phase after concentrated
(70%) HNO3 was contacted with dry TBP and the mixture reached
equilibrium. Lines were derived from linear-regression analysis of
selected data points.
HNO3. The transition from the first complex to the second
complex is more congruous with what was observed by Ferrero
et al.27 in the FTIR spectra described above and in the Enokida
et al.26 mole ratio plot. It shuold be noted that the transition
also occurs at a lower [HNO3]aq/[H2O]aq mole ratio compared
to that for wet TBP−70% HNO3 mixtures. This is likely due to
the lower concentration of H2O. The mole ratio plot shown in
Figure 5 resembles the mole ratio plot in Figure 3 and that
reported by Enokida et al.26 Based on the mole ratio plot
shown in Figure 5, it is possible for up to three HNO3
molecules to be associated with the TBP(HNO3)x(H2O)y·
zHNO3 complex.
3.1.3. Wet TBP−Fuming (90%) HNO3 Adducts. A new series
of adducts were prepared by contacting reagent-grade fuming
(90%) HNO3 with wet TBP. The stoichiometries of the various
mixtures are listed in Table 3. The initial molar acid
Table 3. Stoichiometry of TBP(HNO3)x(H2O)y Adduct
When Fuming (90%) HNO3 Was Contacted with Wet TBP
starting
aqueous/organic
volume ratio
x
0.05
0.1
0.15
0.2
0.25
0.3
0.68
0.86
1.17
1.57
y
starting
aqueous/organic
volume ratio
x
y
0.13
0.27
0.34
0.4
0.5
0.3
0.4
0.5
0.75
1
1.73
2.28
2.68
3.95
5.19
0.6
0.8
0.93
1.3
1.66
concentration is large compared to that in concentrated nitric
acid (70% HNO3) mixtures with TBP, which are able to reach a
maximum acidity of approximately 5.6−5.7 M HNO3 in the
organic phase (Figures S2 and S6). The number of moles of
HNO3 in the adduct increased quickly to 5.19 when the volume
ratio of fuming HNO3/TBP was 1:1. For this study, adducts
used for examination were not prepared beyond a volume ratio
of 1:1 with fuming HNO3 due to the fact that TBP would
rapidly hydrolyze.
It can be seen that the initial H2O concentration in the
organic phase is less than that in the case of using 70% HNO3
and either wet or dry TBP mixtures. However, the H2O
concentration in the organic phase quickly increased as more
Figure 4. Mole ratio plot of [HNO3] in the adduct vs [HNO3]
remaining in the aqueous phase after concentrated (70%) HNO3 was
contacted with dry TBP and the mixture reached equilibrium. Lines
were derived from linear-regression analysis of selected data points.
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Figure 6. TBP−HNO3/sc-CO2 transition from two phases to a single phase as sc-CO2 is added to the view cell.
droplets appeared (Figure 6c). The remaining droplets of the
liquid-phase adduct slowly dissolved into the sc-CO2 (Figure
6d) and became a single phase (Figure 6e). Once a single phase
was reached, the phase appeared crystal clear. No bubbles or
other phase inhomogeneity was observed. When the pressure
was decreased, the single phase separated and became two
phases again (Figure 6f). From this experiment and through
careful control of temperature and pressure and measurement
of the volume of CO2 dispensed into the cell, the phase
boundaries were determined for the different TBP−HNO3
adducts.
Figure 7 shows the phase boundaries of the TBP(HNO3)1.7(H2O)0.6 adduct in sc-CO2 at 318, 338, and 358 K
fuming HNO3 was added. Unlike adducts prepared with 70%
HNO3, the stoichiometry of the 90% HNO3 and wet TBP
adducts did not reach a constant value. This is likely due to the
low amount of water present and the high acid concentration of
fuming HNO3.
The densities of the various adduct mixtures were measured
and are shown in Figure S9 as a function of the HNO3/TBP
volume ratio. The density increased as the HNO3/TBP volume
ratio increased, but did not level off as observed before in the
case of 70% HNO3 adducts. The acidity of the organic phase
was determined using acid−base titration and plotted with
respect to the HNO3/TBP volume ratio (Figure S10). The
molar acid concentration increased, but solutions became
unstable at >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 TBP7160
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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
Figure 8. Phase diagram of TBP(HNO3)5.2(H2O)1.7 in sc-CO2. The
lines are a guide for the eye.
Y (%)
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.
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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)
24,25
Fox et al.
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)
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