Studies in liquid-liquid systems

Liquid- liquid extraction commonly known as solvent extraction is used for separating

the components of a solution by distributing them between two liquid phases. Its applications

cover a wide range of industries, e.g. chemical, petroleum, pharmaceutical, metallurgy,

nuclear industries, food processing and biotechnology. Tri-n-butyl phosphate (TBP) is the

most frequently used solvent in liquid – liquid extraction for nuclear fuel reprocessing. 30%

TBP in n-dodecane is used as the solvent for recovering plutonium and uranium from spent

nuclear fuel in PUREX process (Schulz et al., 1990). The mutual solubility of TBP and nitric

acid leads to transfer of some finite amount of TBP into the aqueous phase. The dissolved

TBP in aqueous stream in presence of nitric acid and heavy metal nitrate salts at elevated

temperatures can lead to exothermic reactions of explosive violence. Many accidents have

been reported due to improper disposal of aqueous waste contaminated with TBP. Thus, the

removal of dissolved TBP is important for safe disposal of this aqueous waste.

The main objective of the present work is to study the removal of dissolved TBP from

acidic solutions by solvent extraction technique. Thus, various physiochemical properties of

TBP have been studied. The kinetics study has been performed to determine the effect of

various parameters on extraction time of TBP. Equilibrium data has been generated for

different concentrations of TBP and nitric acid which would help in calculating the number of

theoretical stages for designing of the extraction column to remove dissolved TBP from the

aqueous waste. The effect of metal nitrates on the solubility and equilibrium data for TBP –

diluent - nitric acid system has been determined. Batch studies have been followed with

continuous runs on mixer-settler for extraction of TBP from the aqueous waste.

Physical properties study for TBP-diluent-nitric acid system

TBP has been extensively used as a solvent in nuclear chemistry like PUREX process

due to its excellent chemical resistance and physical properties which results in better

separation than other solvents (Chang et al., 2000). The physical properties like viscosity,

density, solubility and interfacial tension play a very important role in solvent extraction

studies. Drop size, drop formation and extracting power of TBP depend on all these physical

properties. The effect of TBP on physical properties of dodecane- nitric acid system has been

successfully studied. The physical properties like density, viscosity, interfacial tension and

solubility have been measured for TBP-nitric acid-dodecane system using pycnometer,

viscometer, pendant drop method and High Performance Liquid Chromatography (HPLC)

respectively. It has been observed that density and viscosity increases but interfacial tension

and solubility decreases with the concentration of TBP in dodecane-nitric acid system. The

complete study of physical properties for 30% TBP in dodecane - nitric acid system which is

useful in PUREX process has also been done in the present work.

Kinetics and equilibrium study for TBP-diluent-nitric acid system

The kinetics study for the removal of dissolved TBP from the aqueous nitric acid

solutions has been done. The effect of solvent concentration, nitric acid concentration, type of

solvent and A/O ratio on the extraction of TBP from the aqueous solutions has been studied.

It has been found that extraction of TBP is maximum using Normal paraffin hydrocarbon

(NPH) as the solvent.

The equilibrium data has been generated for TBP-NPH-nitric acid system for different

concentrations of nitric acid ranging from 0.3M to 3M. The distribution co-efficient (Kd) of

TBP has also been calculated by measuring the concentration of associated TBP in the

aqueous and the organic phase. The Kd values have been found to be dependent on the

concentration of TBP, nitric acid and NPH in the system. It has also been proved that the

concentration of nitric acid varies during this equilibrium study due to complex formation

with TBP (Alcock et al., 1956). This study will be useful in nuclear waste management

generated by spent fuels of reprocessing origin.

Effect of metal nitrates on solubility and equilibrium data of TBP

Various metal elements are present in the highly radioactive liquid waste solution

(HRLW) in nitrate form. The solubility of TBP in the aqueous solution decreases when

inorganic solutes are added to these solutions. The effect of these inextractable metal nitrates

on the solubility of TBP in the aqueous solution and distribution co-efficient of TBP has been

studied. The solubility of TBP in water in presence of different concentrations of sodium

nitrate has been determined and salting co-efficient of TBP is calculated using Setschenow

equation. It has been found that the salting co-efficient and activity co-efficient values

increases with the concentration of sodium nitrate in water. Sodium nitrate is an electrolyte

which acts as a salting agent by limiting the solubility of TBP in water (Higgins et al., 1959).

The effect of different concentrations of sodium nitrate and calcium nitrate on the solubility

of TBP in 3M HNO3 is also studied. It has been observed that the solubility of TBP in 3M

HNO3 decreases in presence of these inextractable nitrates. As sodium nitrate has smaller

ionic radius and lower ionic strength compared to calcium nitrate, it is easily soluble in nitric

acid and exerts greater pressure on nitric acid thereby, reducing solubility of TBP in nitric

acid. Hence, TBP salted out more in sodium nitrate as compared to calcium nitrate.

Equilibrium curves have been generated for TBP-NPH-nitric acid system in presence

of inextractable metal nitrates. The effect of TBP, NPH, and nitric acid on the distribution of

TBP between the two phases in presence of inextractable metal nitrates like sodium nitrate

and calcium nitrate has been studied. The distribution co-efficient value of TBP is greater in

presence of sodium nitrate as compared to calcium nitrate due to more salting out of TBP.

The variation in the concentration of nitric acid at equilibrium is also larger in presence of

sodium nitrate than calcium nitrate. The presence of both metal nitrates in nitric acid

restricted the solubility of TBP to maximum extent. All the obtained results have

conclusively demonstrated that sodium nitrate is better salting agent for TBP compared to

calcium nitrate.

Mixer-settler studies for removal of dissolved TBP

The 10 stages combine air lift mixer-settler unit (CALMSU) has been used to

remove dissolved TBP from aqueous acidic solutions. The CALMSU was operated smoothly

in a countercurrent mode for about 10-13 hrs continuously with aqueous as the continuous

phase during the study. The CALMSU was observed for stability and concentration profile.

From the stability profile it can be concluded that at the given operating conditions the

CALMSU stabilization time is four hours. The maximum extraction of TBP from aqueous

nitric acid was achieved in only 3 stages with pure n-dodecane as solvent because of high

concentration gradient of TBP. The dissolved TBP in aqueous solutions can be brought down

from 160 ppm to 15 ppm at A:O ratio of 1:1 as compared to 6:1 as a consequence of

accessibility of adequate amount of solvent. Although the runs with A:O ratio of 6:1 with

3.6% TBP in n-dodecane requires 5 to 6 stages in CALMSU, they are preferable in view of

reusability of n-dodecane solvent. The results obtained reveal a safer method for removing

dissolved TBP from spent nuclear fuel with organic waste minimization. These studies have

conclusively demonstrated the applicability and reusability of dodecane as solvent in

removing dissolved TBP from reprocessing aqueous solutions.

References:

Schulz, W.W and Navratil, J.D., 1984. Science and Technology of Tributyl phosphate: Vol.

1, CRC Press, USA.

Chang, Z.D., Liu, H.Z. and Chen, J.Y., 2000. Foam separation of tributyl phosphate from

aqueous solutions - Part I. Experiment. Sep. Purif. Technol., 19:131–136.

Alcock, K., Grimley, S.S., Helay, T.V.,Kennedy, J. and Mckay, H.A.C.,1956. The extraction

of nitrates by tri-n-butyl phosphate (TBP) Part 1- The systemTBP + Diluent + H2O +

HNO3

Higgins, C.E., Baldwin, W.H. and Soldano, B.A., 1959. Effects of electrolytes and

temperature on the solubility of Tributyl Phosphate in water, J. Phys. Chem.,

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