Keywords (Extracted from title, table of contents and
abstract of thesis)
Rare, Atoms, Earth, Oxygen, Neutral,
Extraction, Cations,anions, Solution, Synergic, Chemical, Processes,
Series, Molecule, Acid Metal, Oxo, Donors, Elements, Picrolonic |
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Abstract
Extensive work is
being carried on the extraction of rare earth elements due to their
special chemical, metallurgical, optical, magnetic and nuclear
properties and their use in advanced technologies as well as in
nuclear industry.Different chemical processes are being applied for
the extraction of rare earth elements from their ores and their
mutual separation on laboratory scale as well as on commercial
basis. However, these processes are facing problems such as large
number of stages due to low separation factor, low efficiency and
waste management. Keeping in view of these problems, in the present
research work, a synergic extraction system comprising of picrolonic
acid as an acidic chelating agent and oxygen based neutral donors,
for the extraction / separation of rare earth elements has been
studied.
Synergic extraction of Ce(III), Nd(III), Eu(III), Tb(III), Tm(III)
and Lu(III) as representative of trivalent lanthanides, using
picrlonic acid (1-p-nitrophenyl-3-methyl-4- nitro-5-pyrazolone, HPA,
pKa = 2.52) as acidic chelating agent with crown ether such as
18-crown-6 (18C6), Benzo-15crown-5 (B15C5), 12crown4 (12C4) as
neutral oxo-donors in chloroform from aqueous buffer solution of pH
1-2 having ionic strength 0.1 mol L-1 (K+/H+,Cl-) has been studied.
Radiotracer technique using their appropriate radio-isotopes
prepared in the research reactor of PINSTECH such as Ce141, Nd147,
Sm151, Eu152/154, Tb160, Tm170, Lu177, Hg 203, Fe59 etc., were used
for the quantification of metal ions in the aqueous and organic
phases. Quantitative extraction (>98%) of these metal ions was
observed only using HPA and B15C5 synergic mixture at pH 2 within
five minutes and the extraction was increased with the increase in
ionic radii of lanthanide ions. Composition of the extracted species
was determined by slope analysis method and found to be Ln(PA)3.nS,
where Ln represent lanthanide ion, PA conjugate base of HPA molecule
and S as neutral oxo-donor.The value of n is 1 and 2. Among the
various cations and anions tested for their influence on the
extraction these lanthanides only Fluoride, oxalate, Cu(II) , Fe(II)
and Zn(II) had some deleterious effect.The proposed synergic system
presented clean separation of lanthanide ions from mono, and various
divalent metal ions especially alkali and alkaline earth metal ions.
The effect of other neutral donors such as trioctylphosphineoxide (TOPO),
triphenylphosphineoxide (TPPO), tributylphosphate (TBP) and
triphenylphosphate (TPP) was also studied on the extraction of
Eu(III). Quantitative extraction of Eu(III) was observed with TOPO,
TPPO and TBP from aqueous phase of pH2. Synergic adduct composition
was found to be Eu(PA)3TBP, Eu(PA)3.2TOPO and Eu(PA)3.2TPPO by slope
analysis method. On the basis of the estimated values of the
synergic coefficient, and extraction constants (log Kex), the oxo-donor
effect was found in the order of TOPO>TPPO>TBP.
The effect of various diluents such as 1-octanol (ONL), 1-hexanol (HNL),
1-butanol (nBNL), 2-butanol (2-BNL), n-butylether (BE),
dichloroethylether (DCEE), acetylacetone (ACAC), diisobutylketone (DIBK),
cyclohexanone (CHN), benzene, toluene on the extraction of Eu(III)
from aqueous solution of pH 1-2 using HPA as extractant has been
studied. The extraction of Eu(III) using benzene and toluene was
found to be negligible, with 1 & 2-butanol it was low (< 50%), where
as with the other diluents studied, the extraction was quantitative
at pH 2. On the basis of log Kex, the solvents can be arranged with
respect to their extractability in the order ACAC > DIBK > BE > DCEE
> ONL > HNL > CHN.
To find the trend of lanthanide extraction within the series, three
solvents CHN, ONL and DCEE as representative of ketones, alcohols
and ethers, respectively, were selected for the extraction of Ce(III),
Tb(III) and Lu(III) using HPA as chelating agent from aqueous
solutions of pH 1-2, quantitative extraction was observed at pH2 and
their extraction order was found to be Ce(III)>Tb(III) >Lu(III).The
composition of the extracted adduct was found to be M(PA)3 in CHN,
ONL and M(PA)3.HPA in DCEE by slope analysis method.
The synergic mixture comprising HPA and B15C5 in benzene and toluene
separately were studied for the extraction of Eu(III) from aqueous
solution of pH 1-2 and quantitative extraction was observed at pH 1
with both the solvents. On the basis of their estimated values of
synergic coefficient and log Kex, benzene was found to be better
solvent than toluene. The composition of the synergic adduct was
found to be Eu(PA)3.2B15C5 and proposed to be a sandwich type
complex having one crown ether molecule on either side of the metal
chelate bound to the central metal only through three oxygen atoms.
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