Sulfarnic Acid in the Separation of the Rare Earths J. KLEINBERG, W. A. TAEBEL, AND L. F. AUDRIETH University of Illinois, Urbana, Ill.
Procedure
Separation of cerium-free rare earth oxide mixtures into lanthanum and yttrium subgroups is effectively accomplished by conversion to sulfamates and the subsequent treatment of an acidic solution of the sulfamates w-ith sodium nitrite. This method compares favorably with the classical alkali double sulfate procedure. The sulfamates of lanthanum, neodymium, samarium, and yttrium have been prepared.
One hundred grams of a cerium-free rare earth oxide mixture, composed largely of the lanthanum earths, was treated with a slight excess of sulfamic acid. The solution was diluted t o 1500 cc. and the small amount of insoluble material was removed by filtration. The solution of rare earth sulfamates was cooled in an ice bath and solid sodium nitrite was added slowly with constant stirring. A precipitate began to form almost immediately. Sodium nitrite was added until examination of the supernatant liquid with a Hilger constant-deviation spectroscope showed that the 6370 A. neodymium absorption line had dropped out. The precipitate was removed by filtration. The filtrate, presumably containing the yttrium earths, was treated with an excess of saturated oxalic acid solution. The precipitated oxalates were filtered, washed with warm water until the filtrate gave no test for sulfate ion, and ignited in an electric muffle furnace, The oxides thus formed were redissolved in nitric acid and the oxalates reprecipitated and reignited. The precipitate of the insoluble fraction was dissolved in a solution of ammonium acetate (150 grams of NH4C2H302per 1500 cc. of solution, 4). The oxalates were precipitated and treated in the same manner as those of the soluble fraction. One hundred grams of the same oxide mixture were fractionated a t approximately 90" in the manner described above. Comparative fractionations by the double sodium sulfate method were made on 100-gram samples of the same rare earth oxide mixture. A synthetic mixture rich in yttrium members was prepared by mixing intimately 130 grams of yttrium group oxides with 80 grams of cerium-free lanthanum group oxides. One hundred grams of this mixture were fractionated at room temperature by the sulfamic acid procedure. The mean atomic weights of the original materials and the various fractions were determined from the oxide-oxalate ratio, the oxalate being determined by permanganate titration (5).
T
HE classical alkali sulfate method is one of the most
effective procedures for separating the rare earth members into subgroups. First proposed by Berzelius @), this method and its numerous modifications are dependent upon the fact that the alkali double sulfates of the lanthanum group-lanthanum, praseodymium, neodymium, and samarium-are almost wholly insoluble in a saturated solution of alkali sulfate, whereas those of the yttrium groupyttrium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutecium-are soluble. The double alkali sulfates of europium, gadolinium, and terbium (the terbium group) are slightly soluble. Usually the double sulfate separation is employed to divide the rare earths into two fractions containing, respectively, the lanthanum and the yttrium earths. When this is done, the separation takes place at gadolinium, which is found in both subgroups. The separation into two groups is best accomplished by sifting solid potassium or sodium sulfate into a solution of t h e rare earth sulfates, nitrates, or chlorides until the neodymium absorption lines become very faint or disappear entirely. T h e precipitated double sulfates are then composed of fairly pure lanthanum group material, whereas the soluble fraction contains the yttrium earths. However, the separation into the two groups is not sharp, each group invariably being contaminated by the other. Sulfamates undergo the following reaction when treated with alkali nitrite in acid solution: H+ SOsNHnNOn- +Nz SO4-HzO
+
+
The results of the fractions are recorded in Tables I and
11. Discussion From the differences in mean atomic weights of the soluble and insoluble fractions, it is readily apparent that the separation of rare earth oxide mixtures into lanthanum and yttrium groups may be carried out as efficiently by the action of nitrous acid upon a solution of sulfamates as by the old classical method, and that the alkali double sulfates formed are less soluble a t higher temperatures. The quantity of materials necessary for at least the same degree of separation is much less by the sulfamate method.
+
TABLE I. COMPARATIVE FRACTIONATIONS OF MIXTURERICH IN LANTHANUM GROUP
An acid solution of rare earth sulfamates upon treatment with sodium nitrite will consequently contain all those ions necessary for t h e formation of the double alkali sulfates. I n other words, the sulfamate ion is a potential source of sulfate ion, which by means of this reaction can be produced in situ and a t any desired rate. It should, therefore, be possible to effect a separation of the rare earth elements into two groups by means of the above reaction. The experimental work outlined below deals with a study of this possible method for the fractionation of the rare earths. Comparative experiments using the classical method were also carried out. Since cerium is easily removed from a rare earth mixture b y oxidation to the tetravalent state, ceriumfree materials were used in this study.
(Using 100-gram samples of rare earth oxide. Mean Mean Atomic Oxides Weight from of InFractionation Reagent Insoluble soluble Method Used Fraotion Fraction Grams Grams NaNOz Sulfamate 82.7 81.4 145.9 (oold) 82.7 89.4 142.3 Sulfamate (hot) NaaSOd 84 Double sulfate 450.6 142 5 (cold) 90.4 Double sulfate 289 9 142.7 (hot)
368
atomic weight = 141.8) Mean Oxides Atomio from Weight of Soluble Soluble Fraction Fraotion Grams 13.5
113.0
5.4
104.3
7.8
106.3
3.5
104 8
ANALYTICAL EDITION
JULY 15, 1939
TABLE11. FRACTIONATION OF MIXTURERICH IN YTTRIUM GROUP Weight
Mean Atomic Weight
Grams
Original Insoluble fraction Soluble fraction
100 55.1 35.2
117.1 132.4 104.4
Furthermore, in the classical double sulfate method, a larger amount of sodium sulfate is present in the insoluble fraction and, therefore, this fraction requires more treatment than is necessary when the sulfamate method of separation is used. These are important factors where large quantities of rare earth materials are to be separated into lanthanum and yttrium groups. This new procedure is also of distinct technical interest because of the availability of sulfamic acid (1) as a new industrial chemical a t a moderate cost. PREPARATION OF SOME RAREEARTH SULFA MATE^. I n view of the fact that the method of fractionation described above involves the use of rare earth sulfamates in solution, it was considered desirable to isolate the sulfamates of several typical rare earth metals. The following procedure was used.
'
Sulfamic acid solution was added t o an excess of the rare earth oxide. The mixture was stirred for approximately 0.5 hour and then heated on a steam bath for 1 hour. The reaction mixture was then filtered and concentrated to a small volume on a steam bath. Crystallization of the sulfamates from aqueous solution was ineffective because of their abnormally high solubility in water. Consequently, the concentrated aqueous solutions were gradually dehydrated by shaking with absolute ethanol. Continued treatment of the alcohol-insoluble solution (gummy mass, or solid) with fresh alcohol eventually yielded a powdery material. The sulfamates of lanthanum, neodymium, samarium, and yttrium were prepared in this manner. Obviously, the amount of water of crystallization in the compounds could have varied considerably. Consequently,
the products were analyzed for their sulfur and rare earth metal content and the ratio of per cent of rare earths to per cent of sulfur was calculated in each case and compared with the theoretical ratio, assuming the starting materials to be 100 per cent pure. The analytical data given in Table I11 indicate satisfactory concordance between found and calculated values. The rare earth sulfamates listed in Table I11 are insoluble in absolute ethyl alcohol, methyl alcohol, acetone, dioxane, pyridine, and liquid ammonia. They decompose slowly a t 100" C. to give the corresponding sulfates. In hot aqueous solution the rare earth sulfamates seem to undergo slow hydrolysis. TABLE 111. SULFAMATE VALUES Per Cent SUIFormula fur Found La(S0aNHz)a.XHzO 20.26, 2 0 . 1 4 Nd(SOaNH*)3.XHzO 20.14, 2 0 . 0 4 Sm(S03NHz)a.XHzO 19.71, 1 9 . 7 6 23.49, 2 3 . 5 4 Y(80&"1)9.XHzO
Rare Earthper c e n t Sulfur Ratio x = Rare Earth Ob- Calcu- Moles Metal Found tained lated H20 2 9 . 1 4 , 2 9 . 3 2 1 . 4 4 1 . 4 4 2-3 3 1 . 0 5 , 3 0 . 9 6 1 . 5 4 1 . 5 0 1-3 3 1 . 4 9 , 3 1 . 6 6 1 . 5 9 1 . 5 6 2-3 21.21, 2 1 . 1 6 0 . 9 0 0 . 9 2 1-3
Acknowledgment The authors wish to acknowledge their indebtedness to the Grasselli Chemicals Department of E. I. d u Pont de Nemours & Company, Inc., for technical sulfamic acid and to B. S. Hopkins for use of the rare earth materials.
Literature Cited (1) Cupery, IND. ESG.CHEX.,30, 627 (1938). ( 2 ) Gahn and Berzelius, Schweigger's J . , 16, 250 (1816). (3) Gibbs, Am. Chrm. J . , 15, 546 (1893). (4) Urbain, Bull. SOC. chim., 15, 347 (1896).
A Continuous Extraction DiEusion Device MARTIN MEYER Brooklyn College, Brooklyn, N. Y.
A
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S I M P L E and continuous diffusion extraction apparatus, for use where complete extraction is unimportant, is shown in the diagram. It requires a 12-inch section of 3- or 4-inch glass tubing, a Gooch crucible or similar device, a test tube equipped with an internal siphon (which leads into the outside solution, not into the Gooch crucible), a calcium chloride tube packed with cotton, glass tubing, stoppers, a test tube, connectors, and stopcocks as shown. The solvent is placed in the suction flask and the material to be extracted in the crucible. The apparatus is then closed and the vacuum from an ordinary aspirator is turned on. Operation is controlled and may be adjusted by changing the level of the right-hand tube a t A . The effectiveness depends upon the difference in levels a t this point. This device may be made to operate a t any desired temperature by removing the vacuum and heating the suction flask. It will then work on pressure instead of vacuum and breathing can be obtained a t the top. The water test tube is designed to compensate for evaporation where the solvent is aqueous, and the cotton removes dirt from the air. The test tube with the siphon in the glass column prevents splashing as the liquid sucks up.
VACULIM f-
