Observations on the Rare Earths Double Sodium Sulfate Precipitation for Separation of the Terbium and Yttrium Earths THERALD MOELLER
T
AND
HOWARD E. KREMERS,
Noyes Chemical Laboratory, University of Illinois, Urbana,
fractionations had been made. Fractionation was effected by precipitating about one third of the rare earths with powdered sodium sulfate from cold 10% nitrate solutions and then warming the mother liquors to obtain a second precipitate. After several repetitions of this procedure, little alteration in separation as judged by changes in oxide color was noted. so fractionation was continued from warm solutions. Cornkinations of fractions were made on the basis of analyses. Results of these fractionations are summarized in Table 11, where a portion of the data obtained is given. Fractions were removed from the insoluble end when the concentration of yttrium, as oxide, dropped below 15% and from the soluble end when nearly colorless oxides were obtained. Rare earths were recovered from double sulfate precipitates by dissolution in 10% ammonium acetate (8) followed by oxalate precipitation, and from mother liquors by oxalate precipitation. Analyses were based upon the werage atomic weight, R, as determined from the oxalate to oxide ratio (I), and absorptjon spectra data ( 6 ) , as obtained with a General Electric recording spectrophotometer in the range 400 to 700 mp using nitrate solutions containing theequivalent of 2% rare earth oxide in I-cm. cells a t a slit width of 10 mp. This instrument had been calibrated against pure samples of praseodymium, neodymium, samarium, and erbium materials, and these elements were estimated directly from absorption data a t selected wave len hs ( 6 ) . Since pure holmium material was not available for Cali ration, this element was estimated from a comparison of spectrophotometric data with the data of Rodden (6). Such a comparison is admittedly inaccurate; so the holmium analyses are only approximate and serve merely to indicate a trend. Cerium was determined by oxidation with ammonium peroxydisulfate and titration with standard ferrous sulfate. Yttrium was estimated from the average atomic weight and a knowledge of the quantities of the other elements present. The Table I. Preliminary Double Sodium Sulfate Separation amounts of europium, terbium, gadolinium, Probable Composition as Per Cent Oxide Grama and dysprosium (reported together) were RZOI R" La908 CeOt Pro011 NdrOa SmrOI YIOI Hot01 ErtOa estimated in a similar fashion. Lanthanum 1710 142 30 22 10 20 10 ca. 6 ., was obtained by difference. 1230 148 ,. 22 10 33 12 Nil *.
HE most widely employed method for effecting a preliminary separation of crude rare earth mixtures is precipitation by means of alkali sulfates. By this means, the rare earths can be divided roughly into three groups-namely, the cerium group, consisting of lanthanum, cerium, praseodymium, neodymium, and samarium, the double alkali sulfates of which are relatively insoluble; the terbium group, consisting of europium, gadolinium, and terbium, the double alkali sulfatea of which are moderately soluble; and the yttrium group, consisting of dysprosium, yttrium, holmium, erbium, thulium, ytterbium, and lutecium, the double alkali sulfates of which are comparatively soluble (4, 7). I n practice, the separation is never sharp because of a gradual change of solubilities in the series, and the method is always only fractional in character (5). In spite of early attempts to use double alkali sulfate precipitation as a means of isolating the terbium earths or fractionating rare earth mixtures @),the method is apparently used a t present only for division of the earths into cerium and yttrium groups, the terbium earths being distributed between these two groups. Because many of the early data were accumulated before the identities of certain of the rare earth elements were established, a further investigation of the method as a means of separation appeared desirable. The results presented here illustrate the utility of double sulfate precipitation for the separation of the yttrium and terbium earths and the concentration of yttrium.
I?
Fraction Crudeoxide DSS-1 DSS-2 208 128 .. .. 3 6 41 55 99 .. . 83 8 DSSF-2 a Average atomic weight. b Present, but could not be determined owing t o interferences in absorption apectra.
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EXPERIMENTAL
1
DISCUSSION
Data in Table I indicate clearly that a separation of the crude earths into the cerium and yttrium groups is readily effected with sodium sulfate. The formation of a second precipitate as a result of heating has the desirable effect of splitting the normal yttrium group fraction and thereby yielding a concentrate high in yttrium itself. The cerium earths up to samarium sppear in the insoluble fractions, but samarium is not so readily removed. "he fractional character of the process is apparent. Cerium appears to distribute itself through the fractionation. Fractionation by double sodium sulfate precipitation rapidly concentrates yttrium in the soluble fractions, as the data in Table I1 indicate. From these concentrates, yttrium oxide of 90 to 94% purity was obtained. The ultimately attainable concentration of yttrium appears to depend upon the ratio of yttrium to the other yttrium earths in the starting material, since holmium and erbium, and to a lesser extent dysprosium, concentrate with yttrium. Continued double sulfate fractionation of the yttrium group concentrates is not recommended for the isolation of particular elements, for analyses indicate only very slowly changing ratios of the yttrium group elements as fractionations proceed. However, this method is recommended for the ready isolation of the yttrium and terbium groups and for a preliminary rapid concentration of yttrium itself. Not more than five to ten fractional
Fbre earth oxides (1710 grams) prepared from monazite residues by the Lindsay Light and khemical Com any and containing about 5% yttrium earths, were added to got, 70% nitric acid. The resulting concentrated solution was diluted to contain the equivalent of 8.5% oxides, and approximately one third of the cerium present was removed as an insoluble basic salt. Powdered sodium sulfate was added slowly to the cold filtered solution with stirring until the neodymium absorption bands in the mother liquor became almost invisible (5). After 22 hours, the double sulfate precipitate (DSS-I) was removed by filtration. The filtrate was heated by steam injection, ana a second precipitate (DSSZ), which formed, was removed. The rare earths were recovered from the mother liquor (DSSF-2) from the second precipitation by oxalate precipitation. Results of this preliminary separation are summarized in Table I. Partial removal of cerium by hydrolysis and the incomplete precipitation of oxalates from the double sulfate liquors account for the ap arent losses shown In this table. &nce the first double sulfate precipitate contained only insignificant uantities of the yttrium earths, i t was not treated further. Tge second precipitate and the material recovered from the mother liquor, containing, respectively, 41 and 83% yttrium oxide, were further fractionated as double sodium sulfates in series of two to five fractions until'a total of sixty-five
m.
111.
Present addresa, Lindsay Light and Chemical Company, West Chicago,
44
ANALYTICAL EDITION
January, 1945
Table
II.
terbium, and yttrium groups. Systematic fractional precipitation of the soluble fractions from a preliminary separation of the crude earths effects rapid separation of the terbiumandyttriumgroups and a rapid concentration of yttrium. The method does not separate any individual rare earth element and can be recommended only as an excellent means of preparing concentrates for further systematic separations. Double sulfate mother liquors from a precipitation carried out a t room temperature can be conveniently fractionated by warming to produce further precipitates. After preliminary separation into yttrium and cerium groups, the double sulfate fractionation can be performed in either hot or cold solutions with but little difference in the course of the separation. For small quantities of material, hot solutions are the more convenient. LITERATURE CITED (1) Barthauer, G.L., Russell, R. G., and Pearce, D. Pi., IND.ENG.CBEM., ANAL.ED.,15,548
Fractionation b y Double Sodium Sulfate Precipitation
Probable Composition as Per Cent Oxide Grams RrO; Rb NdzO; SmzOi (Eu-DyhO; YzOi HoaOa ErzOa Trace Trace 70 16 144 4 8 47 2 2 2 8 47 39 47 132 4 4 ... 4 30 62 34 117 27 96 ... 2 1 89 4 4 38 99 ... ... 1 83 8 8 22 148-153 4-8 6-9 76-80
