SOLVENT EXTRACTION AND REVERSE-PHASE TLC STUDIES OF LANTHANIDES USING ALKYLPHOSPHORUS ACIDS AND HIGH MOLECULAR WEIGHT AMINES

Abstract

Solvent extraction is one of the premier separation techniques and commands a special place in separation sceince and technology. Its pre-eminence over the other techniques can be attributed to its simplicity, versatility, rapidity and less operational cost. Apart from achieving a wide ranging inorganic and organic separations, solvent extraction data provide useful guidelines for reverse-phase or extraction chromatographic studies. On the top of it, this technique can generally be extended from micro levels to macro concentrations. The solvent extraction studies of lanthanides have always been viewed by the researchers in the field as a useful analytical exercise. It is mainly because lanthanides, being intricately similar in their physical and chemical properties, pose a serious problem in their separations. Thus, their separation amongst themselves and from other associated elements stands as a measure for the finesse of the technique and has a direct bearing on the efficiency of the extractant. All the more, the separation of lanthanides from titanium, zirconium, thorium, uranium which occur together with them in their ores has always been a topical problem. To suit the ever increasing popularity of the technique many extraction systems have been explored and the search for better extractants still continues. Over the years, alkylphosphorus acids and high molecular weight amines have come up to the forefront with reportedly high 11 extraction potential, especially for lanthanides. Alkylphosphorus acids have come into the limelight with the advent of transplutonium chemistry. The development of these reagents was closely connected with the object of finding organophosphorus acids with considerable promise for practicable solvent extraction applications in the field of processing spent nuclear fuel. Earlier, during the use of tributylphosphate (TBP) for nuclear fuel processing it was observed that its hydrolysis products were themselves effective metal extractants. This observation prompted systematic investigations on phosphoric acid derivatives as solvent extraction reagents. Since then scores of alkylphosphorus acids have been tried. But out of them di-2-ethylhexyl phosphoric acid (DEHPA) has by far singularly received the maximum attention. Many of the commonly available acids were not put to much use inspite of their reportedly higher extraction potential. Based on the preliminary investigations octylphenyl acid phosphate (OPAP), a mixture of mono- and di-octylphenyl acid phosphates, was chosen to study the extraction of some lanthanides and other associated elements like Y(III), Ti(IV), V(IV), Zr(IV), Th(IV) and U(VI). Another class of extractants which has gained prominence during the last few decades is high molecular weight amines (HMWA) popularly known as liquid anion exchangers. Extensive studies have been carried out on the Ill extraction of metal ions in these extractants from different mineral acids. Scanty references are available on extraction from organic acid media. The anionic oxalato complexes of lanthanides are known but no data is available on their extraction in HMWA. It may be interesting to study these systems and explore them for separations. The present work aims at systematically studying the extraction behaviour of lanthanides and other elements using OPAP and some of the commercially available high molecular weight amines as extractants. The effect of various variables on the extent of extraction has been studied with a view to achieve the optimum conditions for separations. The extracting species have been identified. It has been possible to separate lanthanides and yttrium from some of the associated elements using these extractants. Reverse-phase chromatographic studies, using OPAP as impregnant, have also been carried out. The work embodied in the thesis is divided into following five chapters : I. GENERAL INTRODUCTION II. MATERIALS AND EQUIPMENT III. STUDIES ON THE EXTRACTION OF LANTHANIDES USING OPAP IV STUDIES ON THE EXTRACTION OF ANIONIC OXALATO COMPLEXES OF LANTHANIDES USING HMWA V. REVERSE-PHASE PARTITION CHROMATOGRAPHIC STUDIES USING OPAP AS IMPREGNANT. IV In Chapter I a brief introduction about the characteristic features and the importance of alkylphosphorus acids and high molecular weight amines has been included. An up-to-date literature survey report on these two classes of extractants with special reference to the extraction of lanthanides has also been presented. Chapter II deals with the materials and equipment used for various studies. Partition studies were carried out by using radiotracers or Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Chapter III includes the systematic studies on the extraction behaviour of some lanthanides and other metals which commonly occur together with them in their ores. By exploiting the difference in their solubilities in benzene, the mono- and di- components of OPAP have been separated. Physical characteristics of these two components like melting point, solubility and pK& values were determined. The identity of the two components was established by I.R. studies and C, H analysis data. Distribution studies revealed that MOPAP exists as a dimer in toluene. In order to ascertain as to which component is contributing to the higher extraction efficiency of OPAP, comparative studies have been carried out with mono- and di- octylphenyl acid phosphates. It has been observed that mono-octylphenyl acid phosphate (MOPAP) is the major contributing component and its di-counterpart shows virtually negligible extraction. For the same reason, systematic studies were carried out on the three representative lanthanides, namely, Ce(III), Gd(III) and Yb(III) using MOPAP as extractant. The effect of various variables like the concentration of hydrogen ion, metal and extractant and nature of diluent has been investigated. Basing on the extraction data, relevant equilibria have been proposed. Extracting species has been identified as M(HR) where M is a trivalent lanthanide ion and HR is the anion of MOPAP. The extraction constants were also calculated. It has been found that the extraction constants increase, though slightly, with the increase in the atomic number. With a view to explore the utility of the commercially available product OPAP the extraction behaviour of some of the lanthanides and other associated elements like Y(III), Ti(IV), V(IV), Zr(IV), Th(IV) and U(VI) has been studied. The effect of the concentration of nitric acid, extractant and nature of diluent on the extraction has been investigated. The extraction increases with the increase in the concentration of extractant and decreases with that of nitric acid. There is no clear correlation between the dielectric constant of the diluent and the extent of extraction. Comparative studies with other commonly available alkylphosphorus acids established the higher extraction efficiency of OPAP than other acids for lanthanides. It has been observed that the extraction of lanthanides increases, though slightly, with the increase in the atomic number. The extraction of Ce(IV) is higher than that of Ce(III). Basing on the above extraction VI data, it has been possible to propose optimum conditions for the separation of Y(III) and lanthanides(III) ions from Ti(IV), Zr(IV), Th(IV) and U(VI). The feasibility of the suggested separation conditions has been checked by carrying out separate experiments with binary mixtures of these above mentioned elements. The detailed investigations on the extraction of lanthanide oxalates using different high molecular weight amines like Primene JM-T (a primary amine), Amberlite LA- .2 (a secondary amine), Alamine 336S (a tertiary amine), Aliquat 336S (a quaternary ammonium salt) constitute the Chapter IV. The extraction behaviour of the three representative anionic lanthanide oxalato complexes, namely, those of Ce(III), Gd(III) and Yb(III) has been studied. The effect of various parameters on the extraction like, the pretreating acid, type of amine and diluent and the concentrations of amine, metal and oxalic acid has been investigated. It has been observed that sulphuric acid pretreated amines show higher extraction efficiency. The extraction efficiency of Primene JM-T has been found to be highest. The concentrations of hydrogen ions and metal have no effect on the extraction in certain investigated range. Even the nature of diluent has no discernable effect on extraction. But the concentration of amine and oxalic acid have shown considerable effect on the extraction; that the extractions increased with the increase in their concentrations in a certain investigated range. Extractions VII in all the three cases have shown third power dependence of amine and oxalic acid concentrations. The extracting species in Primene JM-T for all the three investigated 3- + elements has been determined as [M(Ox),] [(RNHjJjJ where -3 • 2 — M is a trivalent lanthanide ion and Ox is oxalato ion and RNH* is the protonated Primene JM-T. The extraction behaviour of some other elements like Y(III), Ti(IV), Zr(IV) and U(VI) has also been studied and it has been possible to separate Y(III) and lanthanides from Ti(IV)and Zr(IV). By taking the binary mixtures of Ti(IV) or Zr(IV) and lanthanides, the suitability of these proposed conditions for separation has been checked. Chapter V comprises of the reverse-phase TLC studies of lanthanides and other elements using OPAP as impregnant. Nitric acid has been used as the eluent. The effect of concentrations of nitric acid and OPAP on Rf values has been studied. The chromatographic data thus obtained has been found to be in agreement with that of solvent extraction. Rf value and percentage extraction more or less bear an inverse proportionality. It has been possible to achieve some binary separations of lanthanides from other elements, like Ti(IV), Zr(IV), Th(IV) and U(VI).