640
G. F. ASSELIN, L. F. AUDRIETH, AND E. W. COMINGS
T H E SEPARATION OF THORIUM AND RARE EARTH SALTS BY SOLVENT EXTRACTION'G. F. ASSELIN, L. F. AUDRIETH, AND E. W. COMINGS W. A . Noyes Laboratory of Chemistry, University of Illinoie, Urbana, Illinois Received June 47, 1940
The separation of the rare earth elements and thorium from the natural mixtures has long taxed the ingenuity of chemists. Even today separation procedures are unduly tedious, time consuming, and wasteful of materials. An attempt was therefore made to develop a fractionation scheme based on selective solvent extraction, which would possess all of the inherent advantages of a continuous counter-current method. Remarkable success was achieved in separating thorium from the rare earth group by the preferential extraction of the former in 1-pentanol from an aqueous solution of the mixed nitrates and ammonium thiocyanate. HISTORICAL
The literature dealing with the solvent fractionation of thorium and rare earths is quite meager. Fischer et a2. (2) report briefly on the distribution of rare earth halides between water and such organic solvents as alcohols, ketones, and ethers. Selwood and Appleton (5) present preliminary data on the solubility of a variety of rare earth salts in a large number of organic solvents. They also carried out a few batch extractions to separate neodymium from praseodymium. Leventhal (3) determined equilibrium distribution ratios for a number of rare earth salts between water and I-butanol. Fischer and Bock (1) studied the purification of scandium by ether extraction from aqueous solutions of various metal salts containing ammonium thiocyanate and extended their work in a preliminary fashion to the rare earths and thorium. The few data which they present indicate that thorium should be extracted preferentially from the rare earths under these conditions and that very large quantities of ether would be required to purify any reasonable amount of thorium. More recently, Templeton and Hall (6) determined the solubility of thorium nitrate tetrahydrate in some sixty-five organic solvents. They then extended (4) the investigation to cover the separation of thorium nitrate from rare earth nitrates. Equilibrium distribution data and preliminary batch extractions of thorium-rare earth mixtures indicate that it should be feasible t o effect such separations by preferential solvent extraction. THEORETICAL
The present investigation was guided by two theoretical postulates based on expected differences in behavior among the rare earths and thorium. Neither 1 This paper was abstracted from a thesis submitted b y G. F. Asselin t o t h e Graduate College of the University of Illinois in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering, June, 1947. 2 Publication of this paper has been approved b y the Chemistry Branch of the Office of Naval Reserve, Washington, D. C., and b y the United States Atomic Energy Commission.
SOLVEST EXTRACTION OF RARE EARTH SALTS
641
working hypothesis proved to be entirely correct, but each will be mentioned briefly. First, since the ionic radii of the rare earths decrease gradually ~ i t inh creasing mass (atomic number), it was anticipated that salts of the heavier rare earths should be less ionic in character and hence more soluble in organic solvents than the lighter ones. It should then be possible to extract the heavier earths preferentially from an aqueous solution by employing an immiscible organic solvent. Secondly, with increasing mass and nuclear charge, the rare earths might be expected to display an increasingly greater tendency toward the formation of both cationic and anionic complexes. Since such complexes are generally more soluble in organic liquids than simple ionic compounds, it was expected that the differences in solubility among the rare earths anticipated by the first hypothesis would thereby be augmented. EXPERIMENTAL
Data were obtained for the equilibrium distribution of various rare earth and thorium salts between aqueous and solvent phases by standard techniques. Measured volumes of solvent and of aqueous solutions of solutes were placed in separatory funnels at room temperature and agitated until equilibrium had been attained. The phases were then allowed to settle, were separated, and were subsequently analyzed for solute content. The organic solvent was removed from all samples by steam distillation prior to analysis. Rare earth determinations were generally made by precipitation with oxalic acid and ignition to the oxide, although a few samples were checked also by precipitation with ammonia and subsequent ignition. Thorium was usually determined by precipitation with ammonia and ignition to the oxide, but a few samples were checked by precipitation with oxalic acid. In all cases the agreement between the two methods mas entirely satisfactory. Mixtures of thorium and neodymium were analyzed by determining the neodymium contept of the solution spectrophotometrically after removal of thiocyanate by silver nitrate, since the thiocyanate affects the absorption spectrum of neodymium. The thorium content was then obtained by difference after precipitation of both thorium and neodymium as hydroxides by addition of ammonia and ignition to the mixed oxides. Thiocyanate-ion concentrations were checked by use of the standard Volhard procedure. All rare earths were obtained from the University of Illinois stocks and were of high purity, with the exception of the erbium sample which had an apparent atomic \\-eight of approximately 150. The oxides were dissolved in a slight excess of the acid whose salt it was desired to prepare, and the excess acid was removed by evaporating the solution to dryness on a steam bath. The dry material mas then redissolved in sufficient mater to yield a stock solution of the desired strength from which test samples of various concentrations were prepared by dilution. All stock solutions had a pH lying in the range between 3 and 4. Thorium nitrate was supplied by the Lindsay Light and Chemical Company and was dissolved in water to give a stock solution with a pH of 1.8. Cerous chloride was used as obtained from the G. F. Smith Chemical Company.
642
Ct. F. ASSELIN, L. F. AUDRIETH, AND E. W. COMINGS
PREBENTATION AND DISCUSSION OF DATA
General With the objective of developing an improved method for isolating salts of pure individual rare earths and thorium from mixtures of these by liquid-liquid extraction, experiments were directed primarily toward the fractionation of mixtures readily obtainable from natural rare earth ores. The design of equipment for such a commercial process requires phase equilibrium data for a system consisting of two solvents and two or more solutes. The determination of complete equilibrium data for such a complex system is an almost prohibitive task and certainly would not be undertaken without preliminary assurance that separations could be effected. I t was considered probable that data on the distribution of individual solutes as a function of concentration would be indicative of the possible success of such a method, provided no interaction occurs between the solutes when they are present in a mixture. If the distribution curves for the pure salts of two rare earths which are widely separated in position in the series coincide for a given set of conditions, then it is quite unlikely that any separation can be achieved by extracting a solution containing a mixture of these salts under these conditions. On the other hand, if such equilibrium curves are found to be markedly different under the same conditions, then further investigation is warranted to ascertain whether or not the distribution of one pure component is affected by the presence of the other in a mixture of the two. It was anticipated that such an effect would be found, but even so it was felt that the distribution curves for the pure salts might be used as first, approximations to estimate the feasibility of achieving a separation. Water was naturally selected as one solvent. 1-Butanol was considered as the second solvent, as Leventhal(3) had found that it extracts appreciable quantities of rare earth salts from aqueous solutions and posesses some degree of selectivity. A number of other solvents were tested qualitatively for extraction capacity to find one less miscible with water than I-butanol, because it was believed that greater selectivity would be realized from such a solvent. Those tested included 1-butanol, 1-pentanol, diethyl ether, dibutyl ether, chloroform, nitromethane, nitrobenzene, ethyl acetate, and amyl acetate. Only 1-butanol and 1-pentanol extracted satisfactory amounts of rare earth salts. 1-Butanol extracted larger quantities than 1-pentanol with apparently no great difference in selectivity and it was chosen as a suitable, although not necessarily the best, solvent for the initial work. 1-Butanol as solvent Data for the equilibrium distribution of neodymium chloride and of the chloride of an erbium-rich mixture between water and 1-butanol are represented graphically in figure 1. In this and all subsequent graphs, concentrations are expressed as grams of metal per liter. I t will be noted from figure 1 that very little of either material is extracted by the alcohol and that the difference between the
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urable differencesin extractability among the various materials. Contrary to the original hypothesis, the lighter rare earths are extracted more readily than the heavier ones. To determine the possible effect of complex formation, batch extractions were made with 1-butanol and aqueous solutions of rare earth chlorides containing ammonium thiocyanate in the amount of 6 moles of thiocyanate to 1 mole of rare earth. This ratio \vas selected arbitrarily as corresponding to a likely coordination number for any complex which might form. The equilibrium distribution of both neodymium and an erbium-rich mixture was determined under these conditions. Erbium was found to be measurably but not significantly more extractable than neodymium in the presence of thiocyanate, as may be seen
644
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from figure 3. Water and I-butanol were later discovered to be completely miscible in the presence of a sufficiently high thiocyanate concentration and appreciably miscible at lower concentrations. This behavior limits the practical usefulness of 1-butanol in a process employing thiocyanate.
I-Pentanol as solvent Preliminary qualitative extractions indicated that 1-pentanol extracts slight amounts of rare earth salts from aqueous solutions. Its miscibility with mater in 0
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FIG.3. Distribution of rare earth chlorides between water and 1-butanol in t h e presence of ammonium thiocyanate. Conditions of t e s t : 50 ml. of aqueous solution containing rare e a r t h chioride and 6 moles of ammonium thiocyanate per mole of rare earth in contact with 50 ml. of 1-butanol. FIG.4. Distribution of rare earth nitrates between water and 1-pentanol in the presence of ammonium thiocyanate. Conditions of t e s t : 40 ml. of aqueous solution containing rare earth nitrate and0.03i6 mole of ammonium thiocyanate in contact with 76 ml. of 1-pentanol.
the presence of high thiocyanate concentrations is low; consequently, further tests were made with it. Equilibrium distribution data for the nitrates of neodymium, yttrium, an erbium-rich mixture, ytterbium, and thorium in the presence of fixed amounts of ammonium thiocyanate were determined as a function of concentration and are presented in figure 4.The concentration of thiocyanate in the aqueous phase when equilibrium is established varies somewhat, since only the amount of thiocyanate in the aqueous phase before extraction was maintained constant, while its distributition bet'ween the phases is affected by the concentration of the other
SOLVEST EXTRhC7ION O F RARE EARTH SALTS
645
salts present. This variation is shown in figure 6, which presents data on the distribution of thiocyanate between water and 1-pentanol in the presence of varying amounts of thorium and neodymium nitrates and mixtures of these at the overall concentration employed in the above series of tests and at a higher overall concentration used in tests which will be described later. It will be noted from figure 4 that the differences in extractability among the various rare earths are not sufficient to afford an easy means of separation. Further, the order of extractability by l-pentanol in the presence of thiocyanate is essentially the same as that observed for 1-butanol in its absence, so that no definite conclusions can be drawn from these data as to whether or not the heavier rare earths form complexes more readily than the lighter ones. The distribution of thorium is much more interesting. It will be recalled that thorium is extracted by 1-butanol to a lesser extent than any rare earth and presumably this would also be the situation with 1-pentanol. However, in the presence of thiocyanate, thorium is extracted by 1-pentanol three or four times as readily as neodymium, one of the most extractable of the rare earths. This behavior is considered strong evidence of the formation of some type of thiocyanate complex by thorium. The data presented in figure 4 indicate that it should be easy to separate thorium from the rare earths by preferential extraction of t'he former with 1pentanol from an aqueous solution of the mixed nitrates containing thiocyanate, provided no interaction iyhich would affect their distribution occurs between the salts Jvhen they are present together in a mixture. Because these data appeared promising, further tests were made to verify the expected separation of thorium from the rare earths under these condit,ions.Neodymium was selected as a represent,ative rare earth which should be one of the most difficult to separate from thorium. Seodymium is also fairly abundant in purified form and is easy to det,ermine spectrophotometrically. The distribution of neodymium and t,horium nitrates and of cerous chloride between water and 1-pentanol in the presence of a greater amount of ammonium thiocyanate than was used in obtaining the data in figure 4 Tyas investigated and these data are presented graphically in figure 5 . h comparison of figures 4 and 5 shom that the extractability of neodymium and thorium increases with increasing thiocyanate concentration. The distribution of cerium, the lightest rare earth, ]\-as investigated to justify the use of neodymium to represent the rare earth most difficult to separate from thorium. Figure 5 shows that the distribution of cerous chloride is very nearly the same as the other rare earth nitrates in the presence of comparable amounts of thiocyanate; hence, cerium should not be more difficult to separate from thorium than neodymium. Although it was shown previously that chlorides are generally less extractable than nitrates, it is felt that the comparison of chlorides and nitrates in the presence of thiocyanate is valid because the effect of thiocyanate far overshadows any differences which would otherwise exist betwen the extractabilities of chlorides and nitrates alone. Cerium is present to an appreciable extent in the tetravalent state in natural rare earth mixtures and must be reduced to the cerous state prior to extraction
646
G . F. ASSELIN, L. F. AUDRIETH, .4ND E. W. COMINGS
in the presence of thiocyanate. Ceric cerium is known to oxidize the thiocyanate radical to free thiocyanogen, which subsequently polymerizes and yields a yellow to orange precipitate of what is presumed to be the polymeric modification. Free thiocyanogen also undergoes rapid hydrolysis to yield sulfuric acid and hydrogen cyanide. An attempt was made t o extract a natural mixture of thorium and the rare earths, but the addition of thiocyanate t o this mixture resulted in the immediate evolution of hydrogen cyanide gas and the prompt formation of a yellow 02
FIG. 5. Distribution of neodymium and thorium nitrates and cerous chloride between water and 1-pentand in the presence of ammonium thiocyanate. Conditions of t e s t : 40 ml. of aqueous solution containing rare earth salt and 0.0594 mole of ammonium thiocyanate in contact with 75 ml. of 1-pentanol. FIG.6. Effect of neodymium and thorium nitrates on the distribution of ammonium thiocyanate between water and l-pentanol. Conditions of test: 40 ml. of aqueous solution containing 0.0376 or 0.0594 mole of ammonium thiocyanate and mixtures of thorium and neodymium nitrates in contact with 75 ml. of I-pentanol.
precipitate. This difficulty is readily obviated by reducing cerium to the cerous state with hydrogen peroxide and removing the excess peroxide by acidification and boiling. Separation of thoriumneodymium mixtures
To verify the expectation based on the distribution of pure salts that thorium could be separated easily from the rare earths in the presence of thiocyanate, synthetic mixtures of thorium and neodymium nitrates of varying concentrations and proportions of thorium and neodymium were prepared and extracted by I-pentanol in the presence of two different fixed amounts of ammonium thio-
647
SOLVEST E X T R A C T I O S O F R A R E E A R T H SALTS
cyanate. The data are presented in table 1. The distribution of thiocyanate between the phases was determined for some of these samples and the data are included in figure 6, from which it is evident that the distribution of thiocyanate between the phases is affected markedly by the amounts of thorium and neodymium present. The data in table 1 definitely prove the feasibility of the method for separating thorium from the rare earths. This is most readily apparent from a comparison of the first and the last' two columns, which shows clearly the separations obtained in single batch extractions. I n view of the excellent separations obtained in single steps, relatively few counter-current extraction stages would be required TABLE 1 Extraction of 40-tnl. portions o j aqueous solutions of neodymium and thorium nitrate miztures containing ammonium thiocyanate by 76 ntl. of 1-pentanol I
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