Physicochemical study of extractants for extraction of rare earth element
© The Author(s). 2016
Received: 22 March 2016
Accepted: 21 September 2016
Published: 29 September 2016
In nuclear technology, the solvent extraction is an important step for recovery of rare earth materials, purification, radionuclide production, and preparation of nuclear reactor materials. Tributyl phosphate (TBP) and toluene are taken as extractant and diluent, respectively, for study of extraction efficiency for rare earth element like CeO2.
In the present paper, idea of ultrasonic sound theory is intensively applied to study the physicochemical property of extractants and diluent pair.
The experimental value of ultrasonic velocity, measured density, molar volume, and viscosity are used for computation of acoustical parameters and their excess values. The variations of physical and acoustical parameter are discussed in terms of molecular interactions. The prepared samples are treated for extraction of CeO2 by separating the organo-aqueous phase. The recovery of this reactor material with the help of this ultrasonic technique has been explained in terms of nature and extent of intermolecular interactions present in the binary mixture.
The ultrasonic treatment for the recovery of this material provides an optimum value of the binary mixture for recovery of CeO2.
The reprocessing of spent nuclear fuels is one of the key processes in the nuclear fuel cycle. As the conventional method suffers from many drawbacks in reprocessing of nuclear fuel from the view point of cost and minimization of waste, the development of a new process involving cost-effectiveness coupled with minimizing waste amount is a great challenge for the next generation reprocessing. Liquid-liquid extraction was one of the most promising methods of separating REE’s material. The repeated use of different organic solvents for this process results in severe corrosion to the equipment, and usage of lots of volatile organic solvents may lead to severe environmental pollution. The real picturisation of the role of extractant-diluent pair (EDP) is not still well defined in spite of work by many workers in this field (Laxmi et al. 2015; Thirumaran and Jayakumar 2009; Liu et al. 2016; Mahapatra et al. 2014; Mastumiya et al. 2014; Bhatanagar et al. 2010; Joshi et al. 2010; Patel and Parsania 2010; Palani and Kalavathy 2011; Sakthipandi et al. 2012; Srivastava et al. 2014; Aswar and Chudhary 2014; Giri and Nath 2015). Thus, an optimal concentration or range of concentration of extractants with particular diluents is a serious concern in the solvent extraction or purex process. Thus, the aim of the present investigation is to make an attempt to find an optimum concentration of EDP used for extraction process. The accurate concentration of EDP and their variation with different physical factors can be well studied by the propagation of ultrasonic wave in the concerned medium.
To understand the basic process with the help of some fundamental parameter with easier way, another method called ultrasonic irradiation is more effective and efficient in this regard. The high frequency and short wavelength of ultrasonic wave makes it possible to interact with the atoms and materials of the concerned medium without any destruction of the medium or individual property of component. Tributyl phosphate (TBP) has been extensively used as a solvent in nuclear industry for fuel reprocessing due to its excellent chemical resistance and physical properties which results in better separation than other solvents. The extracting power of TBP is mainly due to presence of phosphoryl group which form solvates with the metal ions. The diluent-like toluene improves the physical properties of TBP by lowering its density and viscosity for better phase separation. Hence, it is important to study various physical properties of TBP in presence of diluent. In applications of liquid-liquid extraction processes for recycling of used nuclear fuel, the aqueous phase co-exists with an organic extracting phase which consists of a mixture of an extracting agent and a diluent. Cerium is a member of the lanthanide series of metals and is the most abundant of the rare earth elements in the earth’s crust. When present in compounds, cerium exists in both the trivalent (Ce3+) and the tetravalent (Ce4+) state. Cerium is found in nature along with other lanthanide elements in the minerals like alanite, bastnasite, monazite, cerite, and samarskite; however, only bastnasite and monazite are important sources commercially. Because of its unique stability in the tetravalent state, cerium can be separated out from the other rare earth elements through oxidation (forming CeO2) followed by variable solubility filtration.
Materials and instruments
High purity and analytical grade samples of TBP (AR > 98 %), toluene (AR > 99 %), HNO3 (AR > 15.5 mol/L), and CeO2 (AR > 99 %) procured from CDH chemicals were used as received. The binary mixture were prepared on percentage basis (w/w) by mixing known mass of toluene in appropriate masses of TBP and measuring their masses with the help of a high-precision electronic balance of (WENSAR, PGB 100, with accuracy ±0.001 g). The densities of all mixture as well as pure liquid were measured by a specific gravity bottle calibrated with deionized double-distilled water of density 0.9960 × 103kg/m3 at 303.15 K. The precision of density measurement was within ± 0.0001 kg/m3. The ultrasonic velocity in the mixtures as well as in the component liquids were measured at 303.15 K (calibrated up to ±0.01 m/s) by a single-crystal variable-path multifrequency ultrasonic interferometer operating at different frequencies 1–4 MHz (Mittal Enterprises, New Delhi, Model-M-81S). The temperature of the mixture was maintained constant within ±0.01 K by circulation of water from thermostatically regulated constant temperature water bath (B-206) through the water-jacketed cell. Viscosities of the mixtures were measured by Redwood apparatus (MAC, #RWV-5271 was precise up to ±0.0001 Nsm−2).
Different concentrations of extractant were prepared by dissolving various amounts of TBP in toluene. All samples were stored in ground-glass stopper bottles to prevent the evaporation. The concentrations of extractant were studied and optimized by ultrasonic method in terms of existence of different intermolecular interaction explaining the various acoustic parameters with their deviated values. The ultrasonic velocity of the pure liquids and their freshly prepared mixtures of (TBP-toluene) were measured using multifrequency ultrasonic interferometer operating at different frequencies (1–4 MHz). The working principle used in the measurement of velocity of sound through medium was based on the accurate determination of the wavelength of ultrasonic waves of known frequency produced by quartz crystal in the measuring cell. The temperature of the solution was controlled by circulating water at a desired temperature through the jacket of a double-walled cell.
Results and discussion
Experimental density (ρ) and viscosity (η) values for pure liquids with literature values
η (Nsm−2) × 10−3
Experimental values of density (ρ),viscosity (η), and molar volume at temperature 303.15 K
Mole fraction of TBP
Viscosity (Nsm−2) × 10−3
Ultrasonic velocity (C),excess acoustic impedance (ZE), excess isentropic compressibility (Δβs) of pure tributyl phosphate (TBP), toluene, and binary mixture of TBP and toluene at different mole fraction with different frequencies (1–4 MHz) for temperature 303.15 K
Mole fraction of TBP
Excess intermolecular free length (Lf E), excess surface tension (σE), excess molar volume (VE m) and excess of pure tributyl phosphate (TBP), toluene, and binary mixture of TBP and toluene at different mole fractions with different frequencies (1–4 MHz) for temperature 303.15 K
Mole fraction of TBP
Lf E (m) × 10−7
VE m (m3mol−1) × 10−7
ηE (Nsm−2) × 10−3
Percent extraction of cerium from CeO2 with concentration of extractant diluent pair
Mole fraction of EDP
% extraction of cerium
The ultrasonic study of TBP and toluene is a nondestructive investigation used for probing the nature of the acoustical and molecular interaction in solvent mixture. The acoustic data of ultrasonic velocity, density, viscosity, molar volume, and acoustic parameters with their excess values of TBP with toluene over the different concentration range suggest the existence of a strong molecular interaction like dipole-induced dipole, dipole-dipole, and hydrogen bonding type. The frequency of the ultrasonic wave also influences the intermolecular interaction as all the parameters are based on the computation of ultrasonic velocity. The change of deviated and excess physicochemical parameter from certain concentration hints the presence of compatibility of the solvent mixture. Again, the extraction of cerium for each concentration of TBP and toluene indicates the maximum efficiency of the TBP and toluene is the same as demonstrated by each physicochemical parameter. The nature of interaction present in the TBP and toluene mixture provides an optimized value for the extraction process. As such, toluene with TBP may be used as effective diluents/modifiers in the extraction of cerium from cerium oxide material.
The authors are thankful to Hon’ble Vice chancellor and Dean (PGS & R) for providing the financial support and laboratory facilities to carry out the research work.
RG carried out preparation of sample for the experimental work, measured and computed different experimental datas under the guidance of GN also participate in the sequene alignment, scientific analysis and discussion of the different results. All authors read and approved the final manuscript.
The author declare that they have no competing interests.
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