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Direct in situ supercritical fluid extraction of neodymium ion from its
oxide using thenoyl tri fluoro acetone–tri butyl phosphate–methanol
in carbon dioxide
Tessy Vincent a, Mamata Mukhopadhyay b , P.K. Wattal a
a
b
Back End Technology Development Division, Nuclear Recycle Group, BARC, Trombay, Mumbai 400085, India
Department of Chemical Engineering, IIT Bombay, Powai, Mumbai 400076, India
a b s t r a c t
Keywords:
Supercritical CO2
Neodymium
Methanol
Extraction
Mechanism: TTA–TBP
A novel process has been developed for the direct extraction of heavy metal ions from their oxides using
ligand assisted supercritical carbon dioxide (SC CO2 ) without and with a cosolvent. A systematic investigation has been carried out to ascertain the mechanism of direct in situ supercritical fluid extraction,
which involves three essential steps, namely ionisation, complexation and extraction. The present work
involves direct extraction of metal ions from their oxides using neodymium (Nd) as candidate metal utilizing a synergy of thenoyl tri fluoroacetone (TTA) and tri butyl phosphate (TBP). The mechanism has been
established by analysing the conversion of neodymium oxide to Nd3+ and metal complex. By the reaction
with carbonic acid, the metal oxide is converted into metal cation. SC CO2 facilitates tautomerisation and
dissolution of TTA required for the chelation. Addition of methanol to SC CO2 enhances the formation of
metal complex which is being extracted in the SC fluid phase. Effects of process parameters have been
evaluated on the performance of the process to ascertain the optimum conditions.
1. Introduction
Reprocessing and recycling of fissile and fertile materials is an
integral part of nuclear energy programme. The success of the
overall nuclear energy programme, therefore, hinges on a safe,
cost effective and efficient reprocessing technology. A conventional solvent extraction process called PUREX, using tri butyl
phosphate (TBP) as extractant and dodecane as diluent is being
carried out for the extraction of Pu and U from the nuclear spent
fuel. Though it is an excellent processing method, it involves
five cycles of solvent extraction using pulse column as mass
transfer contacting device and therefore requires a massive cell
volume. In addition, it generates considerable volume of highly
active aqueous nuclear waste. In order to reduce the reprocessing
steps and improve the subsequent waste management opera-
Abbreviations: CO2 , carbon dioxide; DISCFE, direct in situ supercritical fluid
extraction; ICP-AES, inductively coupled plasma-atomic emission spectroscopy;
NMR, nuclear magnetic resonance; Nd, neodymium; Nd(NO3 )3 , neodymium nitrate;
Nd2 O3 , neodymium oxide; Pu, plutonium; PUREX, plutonium reduction extraction;
SC, supercritical; SCF, supercritical fluid; SCFE, supercritical fluid extraction; SuperDIREX, supercritical direct extraction; TTA, thenoyl tri fluoro acetone; TBP, tri butyl
phosphate; U, uranium.
tions, various aqueous/non-aqueous methods are being explored
[1–5].
Recently Super-DIREX process was suggested as an alternative
using a SC mixture of CO2 and TBP–HNO3 complex [6]. However,
this method makes use of concentrated nitric acid. The usage of
TBP–HNO3 complex at high pressures could lead to enhanced corrosion. Therefore, there is a need to develop a new supercritical
fluid extraction process, obviating the usage of HNO3 .
Since supercritical carbon dioxide (SC CO2 ) is a non-solvent for
ionic species, suitable modifiers or coordinating ligands need to be
added to SC CO2 to facilitate the dissolution of metal complex in SCF
phase. The ␤-diketone class of chelating agent has been chosen for
the present study as it easily chelates with most of the elements.
Among the ␤-diketones, thenoyl tri fluoroacetone (TTA) has been
selected since it is solid at room temperature. Several studies have
been reported for the SC CO2 extraction of lanthanides/actinides
from their oxides using synergy of ligands [7–9]. However, no
attempt has been made to understand the mechanism of direct in
situ supercritical fluid extraction in detail.
Accordingly, the aim of the present work is to develop a novel
method, and to establish the process mechanism and the role of
SC CO2 in metal extraction starting from its oxide considering Nd
as a candidate metal, which involves three essential steps, namely
ionisation of the metal oxide into metal cation, complexation of
the cation with the ligand and extraction of the formed metal com-
231
Fig. 1. Experimental set-up for supercritical fluid extraction studies.
plex into the SCF phase. This has been achieved in three stages.
The first stage involves performing a set of individual experiments
separately on ionisation, chelation and dissolution. In the second
stage, the percent conversion achieved by carrying out the steps
individually and in sequence of two or three consecutive steps is
compared. The third stage involves a systematic parametric study
to optimise the performance of the process in the light of proposed
mechanism.
2. Experimental setup and procedures
2.1. Materials
For experimental measurement of dissolution in SC CO2 , TTA
(99.0% purity, M/s Sigma–Aldrich Co. Ltd., Dorsel, UK), TBP (99.88%
purity, Heavy Water Board, Tulcher, India), Nd(NO3 )3 ·6H2 O (99.9%
purity, M/s Indian Rare Earths, Alwaye, India) and dodecane (99+%
purity, Sigma–Aldrich Chemie GmbH, Germany) were used. Chelate
of Nd–TTA was made from TTA and Nd(NO3 )3 ·6H2 O by conventional
method and adduct of Nd–TTA–TBP was prepared from TTA, TBP
and Nd(NO3 )3 ·6H2 O. For direct extraction studies, Nd2 O3 (99.9%
purity, M/s Indian Rare earth, Alwaye, India) was used. Analytical
grade methanol of purity 99.8% was used as cosolvent to SC CO2 in
the present study and was purchased from SISCO Research Laboratories Pvt. Ltd. (Mumbai, India). CO2 (99% purity) was supplied by
the Sicgil Industrial Gases (Mumbai, India). In all the direct extraction studies starting from oxide, distilled water was used for the
ionisation of metal oxide.
2.2. Experimental setup
For all the experiments on ionisation, chelation and extraction, the same experimental setup was used and the schematic is
shown in Fig. 1. SCFE system consists of a CO2 pump, extractor,
temperature/pressure-control system, isolation valves, backpressure valve and a flow meter. Liquid CO2 nearly at 6.0 MPa is used
for the experiments. It is first cooled and compressed using a highpressure liquid CO2 pump (Haskel, USA). Extractor designed for
50 MPa and 373 K is of 5 × 10−4 m3 capacity and is made from SS 316.
For accurate measurement of CO2 flow at low pressures and low
flow rates, a wet gas flow meter of INSREF make (Model No: 06B)
is used. Pressure transmitter for accurate pressure monitoring and
control over a wide range of operating conditions is provided. Temperature is measured by thermocouple, is controlled and maintains
the temperature within ±0.1 ◦ C.
2.3. Procedure adopted for ionisation of neodymium oxide
The extractor is first charged with finely ground oxide particle
of 3–5 ␮m mean diameter and mixed with requisite quantity of
water and glass beads. Both ends of the extractor are plugged with
glass wools to avoid trickling. Desired values of temperature and
pressure are set in the temperature indicator controller (TIC) and
pressure indicator and controller (PIC) respectively. After closing
the backpressure valve completely, CO2 cylinder is opened. When
the pressure indicator reads full cylinder pressure, high-pressure
CO2 pump is kept on. Preset values of SC conditions are achieved
by operating the pump and heater. The entire system is kept under
static conditions for the required duration. Upon completion of the
run, the pump and heater are kept off and liquid CO2 cylinder valve
to pump suction is closed. Then the metering valve is opened slowly
and the system is depressurised. The oxide undergoes dissociation
into metal cations. The contents of the extractor are washed with
distilled water and analysed for metal by ICP-AES.
2.4. Procedure adopted for complexation/direct in situ
complexation
When complex formation is performed starting from nitrate,
it is called complexation. When complexation is carried out using
oxide as feed, it involves ionisation in addition to complexation and
is referred as direct in situ complexation. The procedure for dissolution of TTA is established first as SC CO2 facilitates tautomerisation
of the ligand required for the complexation.
After charging the extractor with TTA and glass beads, preset
values of SC conditions are achieved. The system is kept under
static conditions for a minimum duration of half-an-hour. Then the
metering valve is opened slowly. In order to avoid plugging and
to maintain a smooth flow of the metal chelate, metering valve
is always heated during dynamic extraction. 1/3rd volume of the
extractor is released through the wet test flow meter via a separator which contains methanol. Upon evaporation of methanol, the
dried material is weighed and concentration is determined.
For direct in situ chelation studies neodymium oxide and water
were mixed with the glass beads whereas neodymium nitrate was
mixed with glass beads in chelation studies. TTA was also mixed
with glass beads and charged at the lower portion of the extractor
to ensure that SC CO2 is saturated with ligand under static/dynamic
conditions. For the studies involving mixed ligands, two ligands
were mixed and the solution was mixed with glass beads and
charged into the extractor. In the chelation/direct in situ chelation studies involving methanol as cosolvent, bottom portion of the
extractor was loaded with methanol, followed by ligands and then
the substrate.
After loading the extractor with oxide/nitrate feed and requisite
quantity of ligands/modifier, static conditions are maintained as
explained before. Then the metering valve is opened and the system
is depressurised. The complexed metal in the extractor is collected
in dodecane. After the back extraction of the complex with 4N HNO3
into aqueous, neodymium is analysed by ICP-AES.
232
2.5. Procedure adopted for dissolution of TTA–TBP adducts of Nd
All the three steps involved in DISCFE occur simultaneously and
ultimately the extent of extraction is determined by the rate of dissolution of the adduct in SC CO2 , formation of which in turn depends
on ionisation.
For the dissolution studies, the chelates of Nd–TTA have been
prepared off-line as follows.
Neodymium nitrate and TTA dissolved in acetone were mixed
and stirred for some time at room temperature and allowed the
solvent to evaporate. The coloured chelate was separated from the
aqueous portion and dried. It was kept in a desiccator for removing the moisture content. TTA–TBP adducts of Nd was prepared by
replacing the coordinated water molecule present in the prepared
TTA chelates.
The extractor is charged with the chelate/adduct and glass
beads. After keeping the contents in SC conditions for the required
duration in static mode, 1/3rd volume of the extractor is released
through the wet test flow meter via a separator, which contains
dodecane. CO2 becomes gas and escapes from the separator leaving the metal chelate/adduct. Since the dissolution of the metal
complex in gaseous CO2 is negligible, almost all the chelate/adduct
is separated. The entire system is depressurised from the set value
to the atmospheric conditions. The components of the system are
dismantled and cleaned twice with acetone to avoid cross contamination. The extracted complex in the separator is back stripped
with 4N HNO3 to enable the analysis of neodymium by ICP-AES.
For the dissolution studies of the Nd–TTA–TBP adduct in SC
CO2 –methanol system, methanol was added in the lower portion of
the extractor followed by the complex. Packing of glass wool/glass
beads was provided in between.
2.6. Procedure adopted for complexation/direct in situ
complexation-cum extraction
After charging the extractor with oxide/nitrate feed and requisite quantity of ligand/modifier along with glass beads, SC
conditions are maintained in static conditions. for ionisation and
chelation. Subsequently, dynamic extraction is performed and the
metal dissolved in SC CO2 phase in the form of adduct is collected in
a separator containing dodecane. After depressurisation, the contents of the extractor are taken out and the metal that is complexed
but inextracted is washed with dodecane. Samples of the complex
both extracted and inextracted are back extracted with nitric acid
and analysed for metal content by ICP-AES. Methanol was added
to the extractor to study the cosolvent effect during complexation/direct in situ complexation-cum extraction.
Percent extracted and percent complexed are calculated as per
the following equations:
% extraction =
calibration curve is prepared for a standard solution with different
concentration and the same is used to determine the concentration
of the given sample [10].
2.7.2. Nuclear magnetic resonance spectroscopy
NMR spectroscopy is a powerful analytical tool in studying
keto–enol tautomerisation. The actual keto–enol tautomerisation
is the exchange between the vinylic CH and enol OH sites [11]. By
keeping the nuclei in a radio frequency (RF) (1–600 MHz) field of
appropriate frequency, two different energy states are produced
due to the alignment of the nuclear magnetic moments relative to
the applied field and a transition between these energy states takes
place. The energy absorbed in this process produces signal at the
detector and the signal is amplified and records as a band in the
spectrum. A plot of the absorption frequency verses the intensity
of the absorption constitutes the NMR spectrum.
In pulse NMR, a strong RF pulse is applied for a short duration.
The spin excited in this way are allowed to process freely and come
back to their equilibrium positions. During this process, an electric
signal is induced in a suitably placed RF coil. This signal that is
monitored with respect to time is called free induction decay (FID).
The FID, which is in time domain gives its equivalent frequency
domain spectrum on Fourier transformation. The instrument used
in the present study is of VARIAN, USA make and Model, Mercury
Plus 300 NMR Spectrophotometer.
3. Proposed mechanism
In order to study the mechanism of DISCFE using ligand assisted
SC CO2 , the following sequential steps are considered essential:
I Conversion/ionisation of metal oxide to metal cations.
II Complexation of metal cations with ligands.
III Extraction of metal chelate/adduct by SC CO2 .
It is hypothesisd that DISCFE for the extraction of metal ions
directly from their oxides involves several sequential phenomena
of mass transfer and reaction kinetics governed by their respective
phase and reaction equilibria. When SC CO2 is passed through a bed
of powdered metal oxide mixed with water, carbonic acid is formed
thereby lowering its pH in the liquid film around the solid particles.
This carbonic acid is presumed to facilitate the ionisation of metal
oxide to metal cations. This phenomenon is explained as:
CO2 (scf) CO2 (l)
(3)
−
H2 O(l) + CO2 (l) H2 CO3 (l) HCO3 (l) + H (l)
+
M2 On (s) + 2nH (l) → 2M
n+
2.7.1. Inductively coupled plasma-atomic emission spectroscopy
In all the experiments, ICP-AES is used as the instrument tool
for the analysis of metal content present in the aqueous sample.
An error percent of ±5 is considered for the analysis of Nd by ICPAES. Error on dilution effects is normally neglected. This method
is based on the emission of radiant energy using ICP. The intensity
of light emitted by different elements at respective wavelengths is
measured. From the intensity, concentration can be determined. A
(5)
(1)
Quantity of metal in SC CO2 phase + Quantity of metal remained as adduct in the extractor
Quantity of metal in the feed before extraction
2.7. Analytical methods
(4)
(l) + nH2 O(l)
Quantity of metal in SC CO2 phase
Quantity of metal in the feed before extraction
% complexation =
+
(2)
where (s) represents solid phase, (l) represents liquid phase, (scf)
represents supercritical fluid phase, M2 On represents metal oxide
where valency of metal is ‘n’.
Thus, Step I, i.e., conversion of metal oxide to cations in aqueous
film surrounding the solid particle involves
• Dissolution of SC CO2 in water.
• Formation of carbonic acid.
• Diffusion of carbonic acid and its reaction with oxide at the
solid–liquid interface.
233
Fig. 2. Schematic of the proposed mechanism of direct in situ supercritical fluid extraction.
• Diffusion of the cation into the aqueous phase.
Step II involves chelation of metal cations with a ligand for which
the ligand is dissolved in SC CO2 . The amount of dissolution of ligand will depend upon the temperature, pressure, time of contact
and nature of the ligand and the maximum dissolution will occur
at vapor–liquid equilibrium or solid–fluid equilibrium (in case of a
solid ligand) or if a single phase exists (if the conditions are above
the mixture critical pressure of a liquid ligand and CO2 system).
The distribution of the ligand in three phases at equilibrium is
represented as:
L(s) L(l) L(scf)
(6)
Further, when the ligand is a ␤-diketone, it also undergoes a
keto–enol tautomerisation and is represented as:
L(keto) → L(enol)orHL
(7)
around 3 due to the formation and dissociation of carbonic acid.
The concentration of deprotonated TTA would be less due to the fact
that the carbonic ion is less basic than the conjugate base of TTA.
However, when TBP is added, it works as a strong base, converting
TTA to a depronated form which results in an easy formation of
metal adduct and enhanced extraction rate of metal ion.
The addition of methanol as a cosolvent to SC CO2 would affect
the chelation as well. The addition of methanol increases the dissolution of TTA and its tautomerisation to enol form. By involving a
direct coordination between the ligand and methanol, it accelerates
the kinetics of chelation.
The metal complex is removed from the aqueous film in Step III
by dynamic extraction with continuous flow of SC CO2 to the system. The rate of extraction depends on the solubility of the formed
complex in the SCF phase at that particular supercritical condition
and the flow rate of SC CO2 as represented as:
where HL represents the enolic form of ␤-diketone ligand, which is
believed to chelate with the metal ion. HL diluted in SC CO2 diffuses
to the aqueous film and becomes ionised as represented as:
MLn(aqueous) MLn(scf)
HL H+ + L −
• Dissolution of the complex in SC CO2 .
• Convective transport of the complex to the SCF phase.
(8)
The reaction of HL with metal cation to form neutral chelate is
represented as:
nH+ + nL − (l) + Mn+ (l) → ML n (l) + nH+ (l)
(9)
Thus, Step II, i.e., chelation of metal cations with ligand involves
• Conversion of TTA into enol form in SC CO2 .
• Diffusion of TTA dissolved in SC CO2 into the aqueous film around
the solid particles.
• Ionisation of enolic TTA in aqueous film.
• Reaction of cations with ionised enolic TTA to form metal chelate
at the aqueous–SCF interface.
The formed chelate includes hydration molecules also, i.e., the
chelate will be MLn (H2 O)y , where y is the number of hydrated water
molecule. During the synergistic complexation, the solvation shell
around the solute is subsequently replaced by MLn (S)y where S
stands for neutral ligand.
The reaction of enolic TTA to form the chelate with the metal ion
should accompany the deprotonation of the ligand. When water is
in equilibrium with CO2 under SC conditions, the pH of water is
(10)
Thus, Step III, i.e., extraction of metal complex by SC CO2 involves
The schematic of entire mechanism is illustrated in Fig. 2.
It is thus clear that the in situ extraction of metal oxide by SC CO2
involves many simultaneous and sequential paths and accordingly
the analysis of the mechanism for finding the rate-controlling step
is intricate and complicated. Therefore, the strategy adopted has
been to study each step individually and in sequence of two or three
consecutive steps.
4. Ionisation of Nd2 O3 at static conditions
Experiments on ionisation have been performed and details are
summarised in Table 1. As it was not possible to measure pH under
the SC condition using an on-line technique, the pH values estimated [12] after extrapolation are also listed in Table 1. From the
experiments carried out by keeping duration of half-an-hour, it
appears that the amount ionised is more at higher pressures and
higher temperatures. This is attributed to the higher rate of CO2
dissolution in water as the solubility of CO2 in water increases with
pressure and the diffusivity increases with temperature.
234
Table 1
Ionisation of Nd2 O3 with water in SC CO2 (Mole ratio of water to Nd2 O3 = 40).
Table 2
Comparison of keto–enol conversion of TTA in SC CO2 .
Conditions
Conditions
Enol/keto
Enol%
TTA at ambient conditions
TTA at 25 MPa and 338 K
TTA at 35 MPa and 338 K
TTA at 35 MPa and 323 K
1.28
3.29
11.01
18.67
56.18
76.70
91.66
94.92
pH
% ionised in
0.5 h
1.5 h
3h
0.003
0.031
0.037
0.120
0.062
0.154
0.147
0.157
0.263
0.164
15 MPa
338 K
2.88
0.005
25 MPa
318 K
338 K
2.84
2.85
35 MPa
318 K
338 K
2.78
2.81
Fig. 4. Dissolution of TTA as a function of SC CO2 density.
Fig. 3. Effect of pH on conversion after 3 h static time.
Effect of pH has been analysed from the conversion studies carried out in 3 h so that enough time is provided for equilibrium to
be reached between CO2 and water. The results are interpreted in
terms of the effect of pH on ionisation of the metal oxide. The aqueous phase is expected to be acidic due to the dissolution of CO2
in water. It is reported that the solubility increases with increase
in pressure and decreases with increase in temperature. Either
increasing the temperature or decreasing the pressure tends to
lower the solubility of CO2 in water and hence would raise pH.
Table 1 confirms that at a lower pH or at a higher acidic condition
(corresponding to CO2 at 35 MPa and 318 K) the maximum conversion of Nd2 O3 is obtained. A plot of anticipated pH vs. the percent
conversion is given in Fig. 3.
The ionisation studies indicate that by the dissolution of SC
CO2 in water forming carbonic acid, conversion of metal oxide into
cations is possible and thus the first step in the mechanism is established.
5. Dissolution and tautomerisation of TTA in SC CO2
The behavior of dissolution of TTA in SC CO2 has been
investigated to evaluate the extent of keto/enol conversion at
various conditions of SC CO2 . The extracted TTA at various
temperature–pressure conditions has been analysed by NMR. The
enolic as well as keto amount present in the dissolved TTA has been
determined and is presented in Table 2.
It is seen that enol population increases with increase in pressure and keto population increases with increase in temperature. At
high pressures of SC CO2 , a shift of keto–enol equilibrium towards
the more non-polar form of the ␤-diketone (i.e., enol) is observed.
The reaction rate constant for the tautomerisation changes with
pressure according to transition state theory and high-pressure
thermodynamics. At high pressures, an increase in tautomerisation
reaction rate constant is observed due to a large negative activation
volume. It can be noticed from Fig. 4 that an increase in SC CO2
density or a lower temperature enhances the conversion to enolic
TTA.
Dissolution studies confirm that sufficient enolic TTA is available in SC CO2 at 35 MPa and 323 K in half-an-hour, which could
be transferred to aqueous phase for chelation at the interface of
aqueous and SCF phases. By this study, it is validated that prior to
chelation, dissolution of TTA and its tautomerisation to enolic form
occur in SC CO2 .
6. Dissolution of Nd–TTA–TBP adduct in SC CO2
Dissolution behavior of TTA–TBP-adduct of Nd has been investigated at various temperatures and pressures by varying the static
time. The TTA–TBP adduct of Nd has been made by adding TBP
to the prepared Nd–TTA chelate. It is observed from Table 3 that
higher pressures, lower temperatures and longer duration enhance
the dissolution.
In order to find the effect of methanol on dissolution of TTA–TBP
adduct of Nd, dissolution has been performed at 333 K at 35 MPa for
a static time of 3 h with the addition of 2 mole% of methanol. Results
are included in Table 3. It is noticed that the addition of methanol
Table 3
Dissolution data of Nd–TTA–TBP adduct in SC CO2 .
Temp (K)
Pressure
(MPa)
Density of SC
CO2 (kg/m3 )
Dissolution of Nd
(TTA)3 ·3TBP (% wt)
Static
time (h)
333
333
333
333
333
313
35
35
35
35
25
35
875
875
875
875
772
963
0.82
1.04
2.88
4.03a
0.60
1.82
0.5
1
3
3
1
1
a
Addition of methanol.
235
enhances the dissolution from 2.88 to 4.03 wt% as it modifies the
polarity of the SCF phase.
7. Optimal conditions selected for DISCFE studies
As ionisation, tautomerisation and dissolution of the metal complex are favored by higher pressures, a pressure of 35 MPa is chosen
for DISCFE studies. Since lower temperature favors ionisation and
tautomerisation and at the same time, the rate of ionisation is more
at a higher temperature, for the DISCFE studies (which involve a
combination of the static and dynamic modes), a temperature of
333 K is selected for the static mode (ionisation and chelation studies). Though lower temperatures favor the dissolution of extracted
Nd–TTA–TBP adduct, a temperature of 338 K will be considered to
mitigate the loss of TBP along with the extracted adduct during
DISCFE in which TBP is added in batch mode.
8. Parametric studies on adduct formation of solid Nd
(NO3 )3 ·6H2 O using TTA–TBP
This study has been taken up to investigate the chelate formation, i.e., Step II as an independent step with TTA–TBP starting from
metal ions so that the chelation step is not constrained by the ionisation step, i.e., Step I.
Neodymium nitrate reacts with ionised enolic TTA to form
Nd(TTA)3 ·3H2 O and three molecules of water are released for each
molecule of the hydrated chelate formed.
Nd(NO3 )3 ·6H2 O + 3HTTA → Nd(TTA)3 ·3H2 O + 3HNO3 + 3H2 O
(11)
In the mixed ligand system, the charge neutralisation of cation
is achieved by chelation with TTA and solvation by the adduct formation, represented as:
Nd(NO3 )3 ·6H2 O + 3HTTA + 3TBP
→ Nd(TTA)3 ·(TBP)3 + 6H2 O + 3HNO3
(12)
Nd(TTA)3 ·(TBP)3 complex is more stable and soluble in SC CO2
than Nd(TTA)3 ·(H2 O)3 and thus this option is envisaged as a preferred path for faster chelation required for the DISCFE technique.
In order to optimise the process parameters for maximum conversion, studies have been performed with neodymium nitrate
using TTA–TBP combination and the results are presented in Table 4.
8.1. Effect of temperature
It can be noticed from Table 4 that an increase in temperature
from 323 to 338 K, lowers the conversion into adduct. Though pH
is higher at 338 K than at 323 K, it is quite likely that due to the
higher solubility of TBP in SC CO2 at 323 K, than at 338 K, more
TBP is exposed to the already formed hydrated chelate of Nd–TTA,
leading to a higher conversion.
8.2. Effect of static time
The effect of static time on the formation of adduct formation
is analysed by varying the duration of static time. It is found that
upon increasing static time from 1 to 3 h, the degree of adduct formation is increased from 50.08 to 65.06% due to higher amounts of
dissolution of TTA and CO2 in aqueous layer.
8.3. Effect of pressure
From the experiments performed at two different pressures, it
can be noticed that the conversion increases from 58.14 to 65.06%
with an increase in pressure from 25 to 35 MPa due to an increase
in SC CO2 density which in turn enhances the TTA solubility in SC
CO2 and then in water.
8.4. Effect of mole ratio of TTA to Nd
The effect of mole ratio of TTA:Nd is also investigated by varying
it as 10:1 and 8:1 while maintaining the process parameters pressure, temperature and static time constant. It is seen that lowering
the mole ratio from 10:1 to 8:1 lowers the conversion. Therefore, a
mole ratio of TTA:Nd of 10:1 is selected for further studies.
Table 4
Parametric studies on adduct formation of neodymium nitrate with TTA–TBP.
To study
Conditions
Feed molar ratio
35 MPa, 338 K
Time: 1 h
pH: 2.81
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 6
% adduct formation of Nd
47.14 ± 2.4
Effect of lower temp
35 MPa, 323 K
Time: 1 h
pH: 2.78
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 6
50.08 ± 2.5
Effect of static time
35 MPa, 323 K
Time: 3 h
pH: 2.78
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 6
65.06 ± 3.3
Effect of lower pressure
25 MPa, 323 K
Time: 3 h
pH: 2.79
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 6
58.14 ± 2.9
Effect of low mole ratio
35 MPa, 323 K
Time: 3 h
pH: 2.78
TTA:Nd = 8
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 6
59.56 ± 2.9
Effect of excess water
35 MPa, 323 K
Time: 3 h
pH: 2.78
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 42
18.12 ± 0.9
Effect of methanol
35 MPa
323 K
Time: 3 h
pH: 2.78
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 42
Methanol: 2 mol%
67.51 ± 3.4
236
8.5. Effect of water
The effect of water is studied by keeping the parameters namely
pressure, temperature, ratio of TTA:Nd constant with and without
addition of water into the system. It is interesting to notice that the
amount of Nd adducted is drastically reduced from 65.06 to 18.12%
at 35 MPa and 323 K by increasing the H2 O:Nd(NO3 )3 ratio from 6
to 42. This is attributed to the fact that water is a product of chelation and that TTA would undergo hydration reaction with excess
water and correspondingly the enolic content is reduced. In addition, water is a product for adduct formation as well. Consequently,
a lower conversion is obtained with excess water.
It is observed that the degree of ionisation increases from 0.154
to 0.84% using TTA and TBP by performing Step I and Step II together.
It indicates that ionisation is carried to the forward direction by in
situ chelation of cations formed. Experiments have been carried out
by enhancing the contact time from 0.5 to 1.5 h. It is seen that the
degree of conversion increases from 0.84 at 338 K to 32.02% at 333 K.
With the addition of methanol it is further enhanced up to 41.44%.
The result shows that the addition of methanol enhances the complexation due to an increase in dissolution and tautomerisation of
TTA to the enolic form. By involving a direct coordination between
the ligand and methanol, it accelerates the kinetics of chelation.
10. Complexation and extraction of Nd using TTA–TBP and
TTA–TBP–methanol
8.6. Effect of cosolvent
As seen from Table 4, a marginal increase from 65.06 to 67.51%
is observed by the addition of 2 mole% methanol to SC CO2 . This
is attributed to the increase in TTA dissolution in SC CO2 and enol
conversion of TTA in presence of CO2 and methanol, which subsequently increases the extent of adduct formation.
9. In situ complexation of Nd2 O3
It involves ionisation of oxide in addition to its complexation, i.e.,
Step I and Step II together. In situ complexation has been performed
at 35 MPa and at 333 and 338 K. The details are given in Table 5.
Table 5
In situ of complexation Nd2 O3 in static SC CO2 medium.
Conditions
Feed molar ratio
35 MPa
338 K
Time: 0.5 h
TTA:Nd2 O3 = 10
TBP:Nd2 O3 = 7
H2 O:Nd2 O3 = 90
Nd converted into complexes (%)
35 MPa
333 K
Time: 1.5 h
TTA:Nd2 O3 = 10
TBP:Nd2 O3 = 7
H2 O:Nd2 O3 = 90
32.02 ± 1.5
TTA:Nd2 O3 = 10
TBP:Nd2 O3 = 7
H2 O:Nd2 O3 = 90
Methanol: 2 mol%
41.44 ± 2.0
0.84 ± 0.04
The synergistic effect on complexation and extraction has been
evaluated and the details are summarised in Table 6. It is noticed
that by performing Steps II and III together, the performance of Step
II in terms of the degree of conversion is increased to 61 from 47%.
The results show that the percent conversion into adducts has
been increased from 61 to 69% by addition of 2 mole% methanol
as modifier. With the addition of methanol both complexation and
extraction are enhanced due to a marginal increase in dissolution
of ligands TTA and TBP and replacement of TTA-chelated methanol
molecule with TBP.
11. DISCFE of Nd in SC CO2 and SC CO2 –methanol
After maintaining the static condition for complexation with
TTA–TBP in SC CO2 at 35 MPa and 333 K for 1 h, dynamic extraction
is carried out at the same conditions at an average CO2 flow rate of
1.20 × 10−1 m3 /h. To enhance the efficiency of recovery, duration of
dynamic extraction has been extended and the extracted adduct
was collected at different time intervals. Details of the results
are presented in Table 7. It can be seen that cumulative percent
extraction of Nd increases with time and the volume of CO2 . After
the dynamic extraction, 5.6% of Nd remained in the extractor as
adduct.
DISCFE studies have been carried out using TTA–TBP–methanol
system also. SC conditions of 35 MPa and 333 K were kept for 1 h
static run followed by dynamic extraction at an average CO2 flow
rate of 2.33 × 10−1 m3 /h. Details of the results are presented in
Table 6
Synergistic complexation and extraction of neodymium nitrate.
Conditions
Static (333 K)
Dynamic (338 K)
35 MPa
1h
35 MPa
1h
CO2 :0.12 m3
Feed molar ratio
% extracted
% remained in the extractor (as complex)
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 6
36.50 ± 1.8
24.50 ± 1.2
TTA:Nd = 10
TBP:Nd = 5
H2 O:Nd(NO3 )3 = 6
Methanol: 2 mol%
41.00 ± 2.0
28.00 ± 1.4
Table 7
DISCFE of Nd at 35 MPa at an ave. flow rate of 1.2 × 10−1 m3 /h.
Duration (h)
0–0.50
0.50–0.92
0.92–1.33
1.33–1.72
1.72–2.10
2.10–3.02
Flow rate (m3 /h)
% of Nd extracted
Cumulative extrn of Nd
9.6 × 10−2
1.14 × 10−1
1.14 × 10−1
1.2 × 10−1
1.2 × 10−1
1.56 × 10−1
10.1
10.9
10.5
9.6
7.4
11.2
10.1
19.9
28.3
35.2
40.0
47.2
±
±
±
±
±
±
0.5
1.0
1.4
1.8
2.0
2.4
Conditions and feed molar ratio
35 MPa
333 K
Static 1 h
TTA:Nd2 O3 = 10
TBP:Nd2 O3 = 7
H2 O:Nd2 O3 = 90
237
Table 8
Percent extraction of Nd in CO2 –methanol system at 35 MPa at an average CO2 flow
rate of 2.334 × 10−1 m3 /h.
Duration (h)
0–0.33
0.33–0.66
0.66–1.0
1.0–1.33
1.33–1.67
1.67–2.0
Flow rate (m3 /h)
% of Nd extracted
1.99 × 10−1
2.16 × 10−1
2.43 × 10−1
2.70 × 10−1
2.79 × 10−1
1.94 × 10−1
16.04
14.70
10.02
2.54
0.84
0.18
±
±
±
±
±
±
0.8
0.7
0.5
0.13
.007
.009
Conditions and feed
molar ratio
35 MPa
333 K
Static 1 h
TTA:Nd2 O3 = 10
TBP:Nd2 O3 = 7
H2 O:Nd2 O3 = 90
Methanol: 3 mole%
the conversion of keto–enol form is favored by higher pressures and
lower temperatures. Parametric studies on chelation with TTA–TBP
show that the amount of adduct formed is a function of temperature, pressure and contacting time. Dissolution of Nd–TTA–TBP
adduct is favored by high pressure and low temperatures.
The conversion is always found to be better in two or three consecutive steps due to the shifting of equilibrium in the forward
direction. Addition of methanol to SC CO2 enhances the chelation
as well as extractability of the metal ion.
It is established that SC CO2 plays an important role in all three
steps viz. ionisation, complexation and supercritical fluid extraction.
References
Fig. 5. Extraction behavior of Nd in SC CO2 –methanol.
Table 8. The rate of extraction is plotted against the extraction time
in Fig. 5.
It is seen that in the initial period, rate of extraction is enhanced
with the addition of methanol and then it is rapidly reduced and
after 1.67 h, extraction is found to be almost negligible. This is
attributed to the loss of ligands TTA and TBP from the system due to
the increased solubility of these ligands in SC CO2 –methanol system. It also indicates that continuous addition of methanol could
enhance the percent extraction.
12. Conclusions
Ionisation studies under supercritical conditions reveal that ionisation is a pH-dependent process and the conversion of Nd into
ions increases with an increase in pressure and decreases with an
increase in temperature. Dissolution studies of TTA indicate that
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