SOLVENT EXTRACTION STUDIES ON LANTHANIDES AND SOME ASSOCIATED ELEMENTS

Abstract

Lanthanides exhibit characteristic properties that find many technological applications in the manufacture of magnets and advanced alloys, as polymerization initiators and in steel and optical industries. They are invariably associated in nature with other metal ions such as uranium, thorium, zirconium, vanadium, titanium and yttrium. Lanthanides being intricately similar in their physical and chemical properties pose a serious problem in their separation. Their separation from the above said associated metals has also assumed almost an equal importance. Needless to say that these associated elements in their own right have important industrial applications and therefore their mutual separationhas always been viewed as a problem of topical interest. Among the various methods available for the separations, liquid-liquid extraction is a premier separation technique and commands a special place in separation science and technology. Its prominence over other techniques can be attributed to its simplicity, versatility, rapidity and less operational cost. Moreover, the procedures developed using this technique can be conveniently extended from micro levels to macro concentrations. To meet an ever-increasing popularity of solvent extraction many extractants have been explored for the extraction and separation of lanthanides and associated elements but search for more efficient and convenient extractants still continues. Over the years, the important classes of extractants which have come up on the forefront are chelating agents, high molecular weight amines, carboxylic acids and alkylphosphorus compounds. A survey of literature on high molecular weight amines, various carboxylic acids and chelating reagents show a definite increase of interest in the study of synergism in extraction. Avariety of chelating agents, high molecular weight amines and carboxylic acid systems have been investigated for a large number of bench level separations. However, their industrial applications for the said separations are still on a limited scale. This may probably be due to some of the drawbacks like extractant loss, emulsion formation, poor selectivity or the requirement of strict control of phase variables. With the advent of transplutonium era a variety of organophosphorus compounds emerged out as potential extractants. Among these reagents tri-n-butyl phosphate (TBP), di-(2- ethylhexyl) phosphoric acid (DEHPA) and tri-n-octyl phosphine oxide (TOPO) have been studied far more thoroughly than many others of this class. In the eighties American Cyanamid Company marketed some new organophosphine derivatives under the trade name of 'CYANEX'. The list includes a phosphinic acid and phosphine oxides and their sulphur analogues. All of these reagents differ from other organophosphorus extractants in that the alkyl groups are bonded directly to the phosphorus atom through P-C bonds rather than P-O-C bonds which exist for example, in TBP and DEHPA. This tends to make these phosphine derivatives more resistant to hydrolysis and less water soluble than other reagents. Preliminary investigations and literature survey revealed that II Cyanex 923 (a mixture of four trialkyl phosphine oxides namely R3P=0, R'3P=0, R2R'P=0, R'R2P=0; Ris n-ocryl and R' is n-hexyl chain) has the potential to be usefully employed as an extractant for the extraction and separation of lanthanides and the associated elements. The present work is a systematic study on the extraction behaviour of lanthanides and some other metal ions like U(VI), Th(IV), Zr(IV), V(IV), Ti(IV) and Y(III) in toluene solution of Cyanex 923. The effect of various parameters such as equilibration time, the nature of diluent, temperature, concentration of the acid, metal ion and extractant on the partition has been studied. The composition of the extracted species has been proposed. The loading and recycling capacity of Cyanex 923 for different metal ions have been assessed. Based on the extraction trends optimum conditions for various binary and ternary separations of analytical interest have been worked out. Partition data have been utilized for the separation of light, middle and heavy lanthanides. The lanthanide metal ions have also been separated from other commonly associated metal ions like U(VI), Th(IV), Zr(IV), V(IV), Ti(IV) and Y(III). Also the possibilities of separating uranium, thorium, zirconium, vanadium, titanium and yttrium among themselves have been worked out. The developed separation procedures have been extended for the recovery of different metal ions of interest from synthetic mixtures or ores such as monazite sand, biotite-chloride schist with uraninite encrustation, quartzite with pitchblende alteration, allanite and zircon. Ill The workembodied in the thesis has been divided into the following chapters I. General Introduction II. Materials and Methodology III. Studies on the Extraction of Some Lanthanides Using Cyanex 923 IV. Studies on the Extraction of U(VI), Th(IV), Zr(IV), V(IV), Ti(IV) and Y(III) Using Cyanex 923 V. Separations of Lanthanides and Associated Metals and Their Recovery from Different Minerals Chapter I embodies a general introduction of liquid-liquid extraction technique. Aclassification of different types of extraction systems with an overall review of the available literature on different kinds of extractants is presented. The aims and objectives of the present study are defined and an outline of the research work executed is given. Chapter II gives the details of materials and equipments used during the course of present investigations. The experimental details along with the methods followed for the computation of data have beendescribed. Chapter III presents partition data on La(III), Ce(III), Nd(III), Eu(III), Gd(III) and Yb(III) from nitric, hydrochloric, sulphuric and phosphoric acid media using Cyanex 923 solution in toluene. Detailed investigations on the extraction behaviour of lanthanide metal ions were carried out in nitric acid medium. Effect of equilibration time has been studied. Adecrease in percent extraction of the metal ions is observed with an increasing polarity of the diluent rv used. However, in all the studies toluene was employed as a diluent. It is observed that kerosene (160-200°C fraction) can replace toluene as an economical diluent without any significant change in the extraction. The extraction of investigated lanthanide metal ions increase linearly with an increasing Cyanex 923 concentration. The log-log plots of the extractant concentration vs distribution ratio give straight line with a slope of around two indicating a 1 : 2 stoichiometric ratio of metal to extractant. The dependence of partition on nitrate ion concentration has also been evaluated. The results suggest the extracted species to be M(NOa)n. 2R where R refers to Cyanex 923 and n is charge on the metal ion. The results of loading of lanthanides also support the proposed composition of the extracted species. The results of effect of temperature on the extraction indicate the extraction process to be exothermic. A number of stripping reagents have been attempted for the recovery of extracted lanthanides from the organic phase. The stability of the extractant towards nitric acid has been examined. No appreciable change in the percent extraction/recovery of the investigated metal ions was observed even after fifty days contact of the extractant with 5 molL-1 HNO3. Cyanex 923 was tested for recycling up to fifteen cycles and no significant change in its extraction efficiency was observed. On the basis of separation factors calculated from distribution data, the conditions for mutual separation of lanthanides from different acid media have been proposed. It is concluded that separation of light and heavy lanthanides can be conveniently achieved from all the four acid media studied. The separation of light lanthanides from middle ones is only possible in nitric acid medium. However, the separation factors for the pairs containing a middle and Ho/Yb are relatively high from sulphuric acid medium. The utility of extraction data has been explored by attaining separations of the lanthanides into different fractions by processing a synthetic mixture. The extraction behaviour of U(VI), Th(IV), Zr(IV), V(IV), Ti(IV) and Y(III). along with Fe(III), Al(III) and Mn(II) from nitric, hydrochloric, sulphuric and phosphoric acid media in Cyanex 923 is described in Chapter IV. The effect of equilibration time and temperature on the extraction has been examined. A variety of diluents were employed in search of a better system and toluene was used for all the studies. The extraction of metal ions increases with increasing Cyanex 923 concentration and the log-log plots between the extractant concentration and distribution ratio give straight lines with slope around one for Zr(IV), two for U(VI), V(IV), Ti(IV) and Y(III) and three for Th(IV). The loading capacity of Cyanex 923 solution for U(VI), Th(IV), Zr(IV), V(IV), Ti(IV) and Y(III) has been evaluated. A number of aqueous solutions have been tried for the stripping of metal ions from the organic phase. The mutual separations of these metal ions have been carried out by varying acid/extractant concentration or by using different stripping solutions. The regeneration and recycling experiments were conducted up to fifteen cycles and the results reveal practically insignificant change in the efficiency of the extractant. VI Chapter V reports separations and recovery of lanthanides and the associated metal ions. The separations have been achieved either by selective extraction and or selective stripping. Based on the extraction data optimum conditions for the separation of rare earth fraction from U(VI), Th(IV), Zr(IV), V(IV), Ti(IV), Fe(III), Y(III), Al(III) and Mn(II) have been worked out. To demonstrate the practical utility the reported conditions of separation have been tested on synthetic binary mixtures. From the synthetic mixtures the metal ions have been recovered almost quantitatively and in a relatively pure form (97-99%). Based on the results of separations, an elaborate separation scheme has been designed for complex matrices. The validity of the scheme has been checked by applying it to some natural minerals or ores. The thesis concludes with a brief discussion on the findings of the present investigations in the form of Conclusions. Lanthanides and associated elements can be extracted quantitatively in Cyanex 923 from different acid media. Cyanex 923 exhibits high loading capacity for lanthanides and associated elements and stability towards nitric acid contact even after fifty days. The good recycling capacity and quicker phase separation of Cyanex 923 further increases its commercial potential as an extractant. Separations of uranium and thorium from lanthanide fraction and some other metal ions highlight the prospects of the use of the extractant in nuclear fuel processing. The possibilities of separation of lanthanides in to light, middle and heavy fractions have been explored. Light lanthanides can be separated with a high separation factor from the heavy ones

in one or two extraction steps from all the investigated acid media. While the separation of light and middle lanthanide fraction and that of middle fraction from heavy ones require nitric or sulphuric acid medium. Similarly mutual separations of associated metal ions were achieved in different mineral acid media. The recovery of lanthanides and associated elements is more than 90% with purity around 95-99%. The stripping reagents used are simple in composition and yield quantitative recovery. It may be pertinent to point out that the present studies have been conducted on bench level but the results indicate that the process of scaling up of proposed procedure may not pose any serious problems. The present investigations reflect upon the utility of Cyanex 923 for the separations involving lanthanides and some of the commonly associated elements. Selectivity coupled with good recycling capacity and quicker phase separation makes this extractant possibly a better choice than many of those in vogue for the said purpose.