I
F. K. COLE and L. H. BROWN Chemicals Division, The Quaker Oats Co., Barrington, 111.
Solvent Extraction of Heavy Metal Chelates with Furfural H I G H solvency and selectivity of furfural suggested that it be investigated for separating mixtures of metals such as uranium-thorium, zirconium-hafnium, and niobium-tantalum. Salicylic acid ( 2 ) was chosen as a chelating agent for the first four metals. Glycolic acid was used for the niobium-tantalum separation. Furfural may be used for such separations and is adequately stable. Common ores of zirconium and hafnium contain 98 to 95 parts of zirconium and 2 to 5 parts of hafnium by weight. Zirconyl and hafnyl nitrates and chlorides and uranyl and thorium nitrates are easily prepared from ores and were chosen for study. Ammonium hexanitratocerate was studied as a middle atomic weight element.
27' C., and aliquots were withdrawn at intervals for furfural determination by the Hughes-Acree method ( I ) . For the relationship of the distribution coefficient to relative volume of the two phases, an 0.1595M zirconyl nitrate solution was extracted in the usual manner, with varying ratio of organic to aqueous phase. A 20% solution of salicylaldehyde in furfural was used to obtain the results shown in Table V. Discussion Prom the zirconium-hafnium data shown in Figure 1, a separation factor
of 22 was calculated ; this correspond3 to 150.5 grams per liter of zirconyl nitrate and 2.29 grams per liter of hafnyl nitrate. This factor may perhaps be improved by adjustment of pH or addition of other matmials to obtain a salting out effect. At a concentration of 4.22 grams per liter of uranyl nitrate and 140.8 grams per liter of thorium nitrate, a separation factor of 143 was obtained; for 62.3 grams per liter of uranyl nitrate and 3.52 grams per liter of thorium nitrate, the separation factor was 0.047. I n these examples, the uranium-thorium ratios are 1 to 30 and 19.1 to 1, respectively.
Experimental
In all cases, the salts were extracted separately and distribution coefficients and separation factors calculated. The salicylic acid-furfural solution was saturated a t 27' C. with respect to salicylic acid (1.115M). Extraction was accomplished by shaking equal volumes of furfural solution with the aqueous salt solution. Because salicylic acid precipitated niobium and tantalum, glycolic acid was substituted for the separation of these two metals. Metal chelate solutions were prepared by reaction of an excess of the hydroxide with 3.53M aqueous glycolic acid and filtering. Furfuralglycolic acid solution was prepared by equilibrating furfural with 3.53M glycolic acid. Analysis was by chemical means ( 3 ) ; niobium and tantalum were determined by evaporation and ignition of the aqueous phase; the amount in the organic phase was determined by difference. For the distribution of salicylic acid between water and furfural, a 1 to 1 volume ratio was used, the metallic salt was omitted, and the acid was titrated with sodium hydroxide. Because nitric acid may be released in the reaction of nitrates with salicylic acid, the stability of furfural to nitric acid was determined. Solutions were prepared by placing approximately 5.7 grams of furfural in a 250-ml. volumetric flask and diluting to volume with aqueous nitric acid. The flasks were stored a t
58
Figure 1. A separation factor of 22 was calculated from these data
0.002
0.01
0 .I
0.5
MOLARITY
Table 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Good Separation M a y Be Obtained in Extraction of Hafnyl and Zirconyl Chloride Mixtures (1.115.14 salicylic arid in furfural)
Salt Extracted
Zr02C12.2H20
Molarity of Orig. Soln.
0.115 0.0576 0.0287
35.4 17.9 8.93 5.42
9.45
0.160 0.0398 0.0198
8.56 10.41 5.83
79.3 11.42 5.01
HfOzC12.2H20
Ce(NH&(NOa) 6
Grams Salt /Liter Organic rlqueous 1.71 0.68 0.335
uq 20.7 26.3 26.7 0.57 0.108 0.913 1.17
4 BLANK (WATER ONLY1
Figure 2. Furfural is r e l a t i v e l y s t a b l e to aqueous nitric acid
b
0
e
I
3
4
8
6
Figure 3. Tantalum is preferentially extracted from niobium
T I M E OF R E A C T I O N I N MONTHS 0
Table I gives the results for hafnyl and zirconyl chloride and for ammonium hexanitratocerate.
Table 11. For Zirconyl Nitrate a 1 to 1 Volume Ratio Is Near Optimum Ratio, Organic/Aqueous D,“ 1:3.88 1:2.33 1: 1 2.04: 1 3.88: 1
5.94 9.70 14.7
8.02 8.47
Table 111. Distribution of Salicylic Acid between Furfural and Water Shows Some Loss of Salicylic Acid to Aqueous Phase Salicylic Acid, G. Organic Aqueous 0:: 0.950 1.891 3.79 7.59
0.013 0.034 0.060 0.110
73 56 63 69
Table IV. Separation Factors for Niobium-Tantalum Were Calculated from Data of Figure 3 Separation was reasonably good at all times
TaOt, G./L.
... 1.8 ... 8.6 ... 8.6 ... 8.6
NbOz,
G./L. 1-8
... 1.8 ... 14.3
... 8.6 ...
D: 0.32 0.9 0.32 5.2 1.2 5.2 0.9 5.2
S% 2.81 16.2 4.3 5.8
Table V. Salicylaldehyde Was an Unsatisfactory Chelating Agent Nitrate, Thorium Zirconyl Uranyl G./L. Nitrate Nitrate Nitrate Aqueous
222.5
80.0
22.8
phase
Organic phase 0::
6.34
0.847
1.31
0.0285
0.106
0.0574
The data on cerium also suggest a route to the separation of thorium from rare earths. At a n initial concentration of 0.0064M for thorium nitrate, a distribution coefficient of 7.42 is obtained. An O.16OM solution of ammonium hexanitratocerate gives a distribution coefficient of 0.108; the resultant separation factor is 68.7. p H control and salting may enhance the separations obtained. I t was found that the 1 to 1 volume ratio between solvent and feed is apparently near optimum (Table 11). With a 1 to 1 volume ratio, distribution coefficients for the extraction of 0.093M thorium nitrate ’ varied with salicylic acid concentration according to the equation Log D = 0.699 log M - 0.722
where M is the molarity of salicylic acid, and D is the distribution coefficient. I n this case, a t least, optimum extraction is obtained with a saturated solution of salicylic acid in furfural. Studp of the distribution of salicylic acid between furfural and water indicates little loss of salicylic acid to the aqueous phase (Table 111). Furthermore, furfural is relatively stable to aqueous nitric acid; there is little decomposition in 24 hours with acid concentrations up to 0.5M. T h e initial period of relatively rapid decomposition is probably due to oxidation by the nitric acid, after which the rate of decomposition is probably governed by the p H (Figure 2). While there is considerable variation in separation factor, reasonable separation of tantalum from niobium is indicated a t all times (Figure 3, Table IV). A high concentration of glycolic acid was used to diminish the tendency of the niobium and tantalum chelates to hydrolyze, but subsequent work shows lower than 3.53M concentrations may be used. The aqueous glycolic acid-chelate
2
4 0 0 1 0 WLIOHTO? 0XIOE.ORAMS P E R LITER
l
Z
1
4
solution seemed to be colloidal in nature. Salicylaldehyde was briefly investigated as a chelating agent, but was found unsatisfactory (Table V). This does not exclude the use of other chelating agents; pilot experiments show that the furoin complexes of uranium and thorium are somewhat soluble in furfural. 8-Quinolinol is unsatisfactory because of the insolubility of the uranium and thoi-ium complexes.
Conclusions Furfural is an effective selective solvent for the separation of uraniumthorium, zirconium-hafnium, and niobium-tantalum chelate mixtures. A possible thorium-rare earth separation has been pointed out. Furfural is relatively stable to aqueous nitric acid. All this points the way toward future work on the development of commercial methods for the separation of metals using inexpensive and readily available furfural as an extraction solvent.
Acknowledgment The authors wish to express their appreciation to J. W. Madden, Donna Easterly, and Patricia Evanson for the furfural analyses.
literature Cited (1) Hughes, E. E., Acree, S. F., IND.ENG. CHEM.,ANAL. ED. 6, 123 (1934). (2) Plucknett, W. K. (to United States of America), U. S. Patent 2,741,628 (April 10, 1956). (3) Scott, W. W., “Standard Methods of Chemical Analysis,” 2nd ed., rev., pp. 116, 417, 418, 461, 496, Van Nostrand, New York, 1920. RECEIVED for review Aprii 7, 1958 ACCEPTED October 31, 1958 Division of Industrial and Engineering Chemistry, Nuclear Technology Subdivision, 133rd Meeting, ACS, San Francisco, Calif., April 1958. VOL. 51, NO. 1
JANUARY 1959
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