0.3000 gram of the airdried coal were weighed into platinum crucibles and thoroughly mixed with 0.15 gram of sodium carbonate. The samples were then ashed a t 450" C. with occasional stirring. A final heating a t about 1000° C. completed the ashing process. After cooling, the fusion was dissolved in excess 6M hydrochloric acid. To the resulting solution, standard germanium solution equivalent to 0.300 mg. of germanium was added. After filtration the pH was adjusted to 2.0 by addition of sodium hydroxide solution. The sample was then admitted to the ion exchange column and the column rinsed with water. The collected effluent was analyzed for germanium by the procedure described above. Typical results are shown in Table 11. Qualitative spectrographic analysis of the coal samplcs revealed the presence of lead, silicon, magnesium, iron, aluminum, copper calcium, titanium, chromium, and sodium, but gave no evidence of the ,presence of germanium. Blank determinAions gave further evidence of the absence of germanium in the original coal samples. The efficiency of the separation was dependent on the acidity of the sample solutions and upon the quantity of sodium carbonate used. At pH values below 1.0 the column failed to retain the interfering ions completely. A t pH values much above 2.0 precipita-
Effect of Various Ions on Determination of Germanium (Each sample contained 0.300 mg. of Ge) Relative No. Error, Mg. Std.Dev., Mg. Interfering Ions Added Detns. Average Maximum % 0.~ .2 +0.002 5 +o .001 Sb(II1) 0.5 -0.006 -0.003 9 As(II1) 1.1 +0.013 +O. 008 5 each Mo(VI), Ca, Mg 0.6 6 each +O ,004 +0.009 1.4 +O. 004 rt0.018 Nil Bi plus 6 each 5 each Sn(IV), Zn, Fe(II1) Table
I.
E!&,
Table II. Analysis of Coal (All samdes contained 0.10% Ge) . I "
Sample Weight, Gram
Ash Content, yo
0,3000 0.3000
17.29 4.80
No. Detns. 5
Relative Std. Dev.,
Absolute Error, yo Average Maximum -0.001 -0.001
3
%
0.03 0.001
-0.004 -0.001
LITERATURE CITED
tion of hydroxides begins to occur and low results are obtained, probably because of coprecipitation of germanium. A p H of 2.0 was found to be satisfactory for the separation. Care must be exercised in the use of sodium carbonate to ensure that the capacity of the resin will not be exhausted. For the samples used here and the amount of resin employed, 0.15 gram of sodium carbonate was found to be satisfactory.
(1) Cluley, H. J., Analyst 76, 523 (1951). (2) Ibid., p. 535. (3) Klement, R., Sandmann, H., 2. anal. chem. 145. ~~- 325 . - 11955). ,- --, (4) Luke, C'. L., Campbell, M. E., A N A L . CHEM.28, 1273 (1956). (5) Payne, S. T., Analyst 77,278 (1952). (6) Schneider, W. A., Sandell, E. B., Miktochim. Acta 1954, 263.
RECEIVEDMay August 22, 1960.
16,
1960. Accepted
Radiochemical Precipitation Studies of Ra re-Ea rth Oxa Iates KENNETH G. BROADHEAD and HOWARD H. HEADY Reno Metallurgy Research Center, Bureau o f Mines,
b Radiochemical techniques are used to determine quantitatively the effect of temperature, digestion time, pH, rare-earth concentration, oxalic acid concentration, stirring, and the presence of other rare-earth ions on the oxalate precipitation of lanthanum, samarium, and yttrium. A Latin Square experi.mental design is used to evaluate some of the variables statistically. Precipitation losses range from near zero to about 7%, and mineral acid and rare-earth concentrations are the predominant factors affecting completeness of precipitation. Optimum conditions are given for precipitating rare-earth oxalates.
D
the past 20 years research aimed at developing more efficient separation and purification techniques applicable to the rare-earth URING
U. S.
Department o f the Inferior, Reno, Nev.
elements and yttrium has been extensive. Common to most of these procedures has been the use of oxalic acid to precipitate this group of elements selectively. However, this particular use of oxalic acid has a much earlier origin; its application in precipitating yttrium was first reported by Mosander (8) in 1843. Despite long and popular employment of this technique, only limited and somewhat conflicting data are available regarding the many factors affecting the degree of rare-earth oxalate precipitations. In comprehensive studies of the rareearth elements, Spencer (12) and Vickery (13) emphasized the advantages of using oxalic acid. Vickery also pointed out the depressant effect of excess oxalic acid upon the solubility of rare-earth oxalates in mineral acids. Sarver and Brinton (10) made a careful study of rare-earth oxalate solubilities
and observed that excess mineral acid is to be avoided, as the oxalate solubility is increased. In their discussion of separation methods, Quill and Rodden (9) suggest that rare-earth solutions should be precipitated with saturated oxalic acid and digested at 90' C., then digested several hours a t room temperature. The somewhat different procedure described by Schoeller and Powell (11) recommends that the oxalates are best precipitated from a cold solution; after an hour the solution is stirred vigorously while being heated to boiling, then cooled overnight. According to Boyd and, Hume ( I ) , Myers applied tracer techniques in attempting to determine the most suitable conditions for precipitating rareearth oxalates. Loss was minimum when the precipitation was performed a t room temperature. However, it was VOL. 32, NO. 12, NOVEMBER 1960
1603
Table 1.
Factors Stirring time, min. Od
2 ' 60d 120d Pptn. and digestion temp., O
Effect of Various Factors on Rare-Earth Oxalate Precipitation
Rare-Earth Oxalate Lost, %a Digestion Time, Hours* Digestion Time, Hoursc 2 20 2 20 2 20 2 20 2 20 2 20 Lanthanum Samarium Yttrium Lanthanum Samarium Yttrium 0.81 0.24 0.11 0.06 5.00 1.59 0.19 0.08 0.02 0.03 0.39 0.25 0.57 0.24 0.08 0.07 1.43 1.51 0.02 0.24 0.19 0.08 0.07 0.03 0.65 0.29 0.08 0.08 1.47 1.40 0.13 0.10 0.03 0.02 0.26 0.24 0.61 0.27 0.07 0.06 1.53 1.48 0.11 0.08 0.03 0.02 0.28 0.25
c.
0 0.18 0.08 1.47 0.57 0.24 0.08 0.07 25 1.43 1.51 906 0.57 0.26 0.08 0.09 2.12 1.34 Boiling' 0.78 0.32 0.13 0.09 4.96 3.42 Fivefold stoichiometric amount of oxalic acid, pH range of 3.0 to 4.0. b &re-earth concn., 0.002M. Rare-earth concn., 0.02M. d Digested a t room temperature. e Digested for 5 minutes a t 90" C. and allowed to cool t o 25" C. Boiled €or 5 minutes, digested at 90" C. €or 1 hour, and allowed t o cool t o 25'
observed that precipitates formed in hot solutions were coarser and hence more readily filtered. A recommended additional procedure was to cool the mixture to room temperature before filtering. The work of Crouthaniel and Martin ( 2 ) shows the importance of oxalate ion activity, and thus indirectly the hydrogen ion activity to rare-earth solubility a t equilibrium. Using their method and the yttriuni oxalate solubility data of Feibush, Ronley, and Gordon (4). the yttrium precipitation losses a t equilibrium can be estimated by successive approximations, providing t,he final pH of the solutions are known. Experimentally, the precipitation losses after digesting 0.002 and 0.02111 yttrium chloride solutions for 20 hours with a fivefold stoichiometric amount of oxalic acid closely approached the equilibrium value. However, the precipitation losses of 0.1M yttrium chloride solutions under nearly identical conditions exceeded the equilibrium losses by a factor of 10. Unfortunately, a knowledge of the equilibria gives no indication of the effects of time, stirring, or temperature on the completenrss of precipitation. The objective of this rare-earth oxalate study was to determine systematically by radiochemical techniques what effects, if any, could be attributed to the several variables-temperature, digestion time, acid concentration, rareearth concentration, oxalic acid concentration, stirring, and the presence of other rare-earth ions. As the purpose was to determine the niost practical and efficient conditions for precipitating the rare-earth elements, no attempt was made tQevaluate any of the variables under equilibria conditions. EXPERIMENlAL
Materials. Three radioactive rareearth isotopes-Yw, La140, and Sm15were obtained in carrier-free hydro1604
ANALYTICAL CHEMISTRY
0.05 0.08 0.09 0.09
0.07 0.06 0.06
0.02 0.02 0.02 0.03
0.03 0.02 0.03
0.20 0.24 0.33 0.52
0.19
0.16 0.44
C.
chloric acid solution from the Oak Ridge National Laboratory. The radioisotopes were checked for radiochemical purity by measuring half life and plotting absorption curves. Gamma spectra for Lala and Sm153 compared favorably with spectra reported in the literature ( 7 ) and gave no indication of impurities. The YN was eontaminated with a slight amount of Srw, which was removed by addition of strontium carrier and prccipitation with fuming nitric acid (15). The carrier-free Y N was converted to chloride form by several evaporations t o near d r y n e s with addition of hydrochloric acid. "he absorption and half-life nicasurements subsequently showed that essentially all of the Sra had been removed. Radioactive samples were p-counted with a mica end-window Geiger-llueller tube (1.6 mg. per sq. cm.) connected to a decade-type scaler. A Tracerlab single-channel continuous scanning gamma spectrometer was used to check the gamma spectra. A Beckman Model 0 p H meter, a Burrell Wrist-Action shaker, and magnetic stirrers a w e employed in the oxalate precipitations. Reagent-grade oxalic and mineral acids and 99.9+% pure rare-earth oxides were used to prepare the required solutions. Procedure. The weighed rareearth oxide was dissolved in hydrochloric acid, evaporated to incipient dryness t o remove the exeess acid, and dissolved in water. After the tracer was added, the solution was boiled to ensure complete exchange between the inert and radioactive rare-earth isotopes. Aliquots of the solution were withdrawn for precipitation with oxalic acid solution under various conditions of stirring, temperature, digestion t)ime, rare-earth element concentration, oxalic acid concentration, and pH. ,411 tests were run in duplicate. Thr rare-earth oxalate precipitates, obtaincil undfsr prescribed t w t condition.. were filtered. washed with a small amount of 0.5y0oxalic acid, and
discarded. The combined filtrate and wash containing the sought-after unprecipitated rare-earth ions was retained. A solution containing 20 mg. of the respective rare-earth chloride was added to the filtrate to act as a carrier for the small quantity of rare-earth ions and precipitated with more oxalic acid. The precipitate was filtered and CODverted to oxide by ignition for 1 hour a t 800" C. The oxide was then slurried and ground in alcohol, t,ransferred into a cylindriral stainless steel filtering tower, and collected under suction on a 2.4-cm. filter paper disk. The samples were dried a t 110' C.. mounted on planchets, and covered with a 0.4 mg. per sq. cm. rubber hydrochloride film. Snmples were counted for enough time, usually 1 to 5 minutes, to accumulate a t least 4000 counts in order to ensure a standard deviation of on1)about 1.6yc. Scccssary corrections were made for decay, background, precipitate thickness, and coincidence loss. KOcorrections for counting geometry were required, as all samples and respective standards were prepared and counted in the same manner. Standards were prepared in duplicate by precipitating aliquots of the original rare-earth solution containing the radioactivity. The oxalate precipitate? n-ere filtered, ignited to the oxides, and prep a r d for counting as previouslj. dexribed. The average count of the two standard samples, when corrected for thickness and precipitate loss, was as-, sumed to represent 100% of the particular rare-earth element present in the sample. The percmtage of rare-earth element in each carrier sample, representing the amount lost during the precipitation under the test conditions, was calculated by taking the ratio of the carrier sample count to the count of the standard sample. RESULTS AND DISCUSSION
Experiments to determine the effect of stirring o n oxalate precipitation of La,
~
Sm, and Y were done at room temperature using a fivefold stoichiometric amount of oxalic acid. As differences were negligible for equal periods of manual stirring vs. mechanical shaking, all data were taken by the latter method. Precipitation losses resulting from various combinations of digestion time, shaking, and rare-earth concentration are compared in Table I. These data show that in most instances precipitation loss can be reduced by stirring or shaking for 2 minutes immediately after adding oxalic acid. Little or no improvement was achieved by shaking for longer periods. All subsequent experiments were conducted with 2 minutes of stirring. These data also indicate that the rare-earth losses are higher a t the lower rare-earth concentrations and somewhat higher a t the shorter digestion times. Tests were conducted to determine the effects of different precipitation and digestion temperatures. The results, summarized in Table I, show that boiling or hot solutions are to be avoided, particularly when using dilute rareearth solutions. Although precipitation from boiling solution enhances crystal growth, the authors found that even the most dilute solutions could be filtered readily. Precipitation in an ice bath for 2 hours proved preferable to precipitation at room temperature for 20 hours. Experiments involving hydrogen ion as a variable had a dual purpose: to determine the pH range in which it was possible to work without introducing appreciable errors, and to investigate the effects of concentrations of various mineral acids. Results were reproduc-
Table
It.
Acid
Effect of Acid Concentration on Rare-Earth Oxalate Precipitation
Rare-Earth Oxalate Lost. % Lanthanum Samarium Yttrium Concentration0 0.002M 0.02M 0.002M 0.02M 0.002M 0.02M
HCl HC1 HCl HCl HCI HNOs HNOs HzSOi HISO‘ EDTAb
pH 4 . 0 2.5 1.5 O.1M 0.5M 0.124 0.5M O.1M 0.5M 0.2M
0.24 0.33 0.82 0.71 2.66 0.63 6.10 0.74 7.33
0.07 0.07 0.06 0.16 0.30 0.18 0.54 0.14
0.08 0.06 0.07 0.09 0.43 0.13 0.83 0.14 0.66 2.32
0.50
0.02 0.02 0.02 0.03 0.05 0.03 0.08 0.07 0.08 1.38
1.25 1.35 1.40 2.83 4.90 3.68 4.69 2.99 5.41
0.15 0.13 0.13 0.33 0.58 0.31 0.55 0.41 0.61 1.36
Determinations in pH range were at fivefold stoichiometric amount of oxalic acid and Others were a t 0.4M oxalic acid and 20 hours’ digestion at room temperature. * EDTA solutions were a t a pH of 3.5 prior to precipitation in 0.4M oxalic acid. 0
20 hours’ digestion at room temperature.
ible within about 5% provided the p H of the solutions was maintained above 3.0 before the oxalic acid was added. Table I1 shows that excess mineral acid is to be avoided if quantitative results are desired. Even in the p H range, the losses with the dilute rareearth concentrations tended to increase with increasing acidity. This result corroborates the data submitted by Boyd and Hume (1). Under p H conditions a fivefold stoichiometric excess of oxalic acid was adequate, but a t higher acid concentrations a considerably larger amount of oxalic acid was required to precipitate the dilute rare-earth solutions. Of the three mineral acids tested, hydrochloric acid proved to be the most desirable for minimizing oxalate losses. As in the
Table 111.
tests with mineral acids the presence of excess diammonium salt of (ethylenedinitri1o)tetraacetic acid (EDTA) increased the precipitation losses. The effects of rare-earth concentration, digestion time, and oxalic acid to rare-earth ratio were investigated simultaneously, using a Latin Square experiment (3, 6). This method w a ~ chosen as the most expedient means of evaluating these variables, which were assumed to be noninteracting. The tests were designed as shown in Table 111. Table IV summarizes the analysis of variance for the three rare-earth elements tested. In this analysis, F ratios calculated from experimental data are compared with an F value of 4.76 listed in statistical tables (3). A ratio larger than this value indicates
Latin Square Experimental Design and Data
Stoichiometric Ratio, Oxalic Acid/Rare-Earth Concn. 1.2
1.6
2.0
c
D
A
B
c
D
5.0
3
2
0.002 0.005
c
~1
5
0.020
w&
0.100
i
A
~
0.14 0.15
0.04
I
02 . 73 68
0.78 1.42
01..5294
01. 5493
0.31
0.21
0.25
0.26
0.20
0.14
0.10
0.14
’
0.10
0.10
0.06
0.06
0.04
0.03
0.06
0.04
0.04
0.03
0.02
0.04
0.04
0.03
0.03
0.02
VOL. 32, NO. 12, NOVEMBER 1960
1605
Table IV. Analysis of Variance Summary Based on Latin Square Experiments
Table
F Rstiosa
Element
Oxalic acid
Digeetion time, hours
Lanthanum Samarium Yttrium
13.3 9.0 38.6
4.0 1.0 2.44
2.0 2.5 1.92
0 F = 4.76 for 95% confidence with (3, 6) degrees of freedom.
La-trace Y La5 Y La- 50 Y La- 95 Y Trace La-100 Y 100 95 50 5
100 95 50 5
Y -trace Y - 5 Y - 50 Y - 95 Trace Y -100
Sm Sm Sm Sm Sm
Sm-trace Sm5 Sm- 50 Sm- 95 Trace Sm-100
La La
100 95 50 5
that the particular variable significantly d e c t s the results. The only variable that appreciably affected the precipitation was shown to be the rareearth concentration; the effects of both digestion time and oxalic acid to rareearth ratio were negligible compared with the over-all effect of the three variables. No gross losses due to large excesses of oxalic acid were noted in the series of tests. Tests to determine the effects of coprecipitation of one rare-earth element on others were conducted at constant rare-earth molarity. Weaver (14) and Feibush, Rowley, and Gordon (6) have made coprecipitation studies of the rare-earth oxalates, but their work waa based on precipitation from homogeneous solutions and did not include determination of the completeness of the oxalate precipitation. Table V shows that the precipitation losses of lanthanum and yttrium, when coprecipitated with samarium, decrertsed M their respective concentration decreased. This effect is predictable on the basis of solubility product data. However, the anomalous behavior of h t h a n u m and yttrium in the presence of each other cannot be explained. No trends can be definitely established, as some of the differences between the oxalate losses might be attributed to expected experimental variation. To demonstrate the reproducibility of the general precipitation, mounting and counting procedure on a day-today basis, 12 samples of each of the three rare-earth solutions containing the respective radioisotopes were precipitated under identical conditions over a 3day period. A 0.002M rare-earth solution was chosen for this test as indicative of the poorest reproducibility obtained in the entire series of experiments. The results showed an average standard deviation of *7.9%. The average standard deviation obtained from evaluating more than 300 pairs of determinations was rt5.4%. Of the variables and conditions investigated, the most important proved ANALYTICAL CHEMISTRY
Coprecipitation Effects of Selected Rare-Earth Oxalates
Rare-Earth Mixture, Mole % ’
Rareearth element concn.
1606
V.
La La La
Rare-Earth Oxalate Lost, %” Lanthanum Yttrium Samarium 0.24 0.30 0.33 0.45 0.29
1.56 1.51 1.84 1.45 1.43 1.43 1.36 1.05 0.85 0.63
0.23 0.09 0.07 0.06 0.07
0.18 0.18 0.23 0.25 0.24
0.07 0.06 0.06 0.09 0.12
a 0.002M rare-earth concentration, fivefold stoichiometric amount of oxalic acid, and 20 hours’ digestion.
to be mineral-acid and rare-earth concentration. This is particularly evidenced by the fact that a combination of 0.5N HC1, 0.002M lanthanum, and a fivefold stoichiometric amount of oxalic acid produced no precipitate within 24 hours. This effect is due to the suppression of the oxalate ion activity to the point where no rare-earth oxalate precipitate can form. The Latin Square tests indicated negligible main effects due to digestion time and oxalic acid concentration, but it was not feasible to study the interaction of these variables with rareearth concentration in the same design. However, even without use of a complete factorial design, experimental data showed that at low rare-earth concentrations both digestion time and oxalic acid concentration did influence the rare-earth oxalate loss. The presence of excess EDTA, as might be found in ion exchange effluents, definitely hindered the rareearth oxalate precipitation. Consequently, in ion exchange separations of the rare-earth elements, consideration should be given to the effect of oxalate precipitation in the presence of the complexing eluent. General recommendations for minimizing rare-earth oxalate precipitation losses are as follows: Maintain rareearth concentrations above 0.01M; stir the mixture for several minutes after precipitation; use an adequate (twoto fivefold) excess of oxalic acid; precipitate at room or iced temperatures and digest for a t least 1 hour; avoid an excess of acid and if possible maintain the p H above 2.0; and avoid excess complexing agents such as EDTA. The conditions as stated in the third and fifth recommendations are those
which might be expected from a knowledge of the solubility products of the rare-earth oxalates. LITERATURE CITED
(1) Boyd G. E., Hume, D. N., “Analyt-
ical dhemistry of the Manhcttan Project,” 1st ed., Chap. 28, pp. 677-8, McGraw-Hill, New York, 1950. (2) h u t h a m e l , C., Martin, D. L., Jr., J . Am. Chem. SOC.72,1382 (1950). (3) Dixon, W. J., Masmy, F. J., Jr;l “Introduction to Statistical Analysis pp. 139-41, McGraw-Hill, New York, 1951. (4) Feibush, A. M., Rowley, K., Gordon, L.,ANAL.CHEM.30,1610 (1958). (5 Zbid., p. 1605. (6j Gore, W. L., “Statistical Methods for Chemical Ex erimentation,” pp. 83-94, Interscience, !Jew York, 1952. (7) Heath, R. L., “Scintillation Spectronf;
etry Gamma Ray Spectrum Catalogue, U. S. Dept. of Commerce IDO-16408 f 1957).
\_.__,.
(8) Mosander, C. G. Phil Mug. 23, 241 (1843); J. ptakt. dhem. 30,276 (1843). (9) Qui]!, L. L., Rodden, C. J., “Analytic! Chemistry of the Manhattan Proiect, 1st ed., -Chap. 22, p. 495, McGrawHill, New York, 1950. (10) Sarver, L. A., Brinton, P. H. M., J.Am. Chem. SOC.49,943 (1927). (11) Schoeller, W. R., Powell, A. R., “The Analysis of Miyrals and Ores of the Rarer Elements, 3rd ed., p. 103, Hafner Publishing Co.,New York, 1955. (12 Spencer, J;,F., “The Metals of ti &re Earths, Longmans, Green ana Co., London,’En l a d , 1919. (13) Vickery, R. “Chemistry of the Lanthanons,” pp. .92-7, 221, Academic Press, New York, 1953. (14) Weaver, B., ANAL. CHEM.26, 479
8,
(1956). (15) Zumwalt, L. R., et a1 “Handbook of Radiochemical Analy&,” Vol. 11, Radiochemical Procedures, p. 122, Pb121689, United States Dept. of Com-
merce, Office of Technical Services, 1952.
RECEIVEDfor review February 2, 1960. Accepted August 15, 1960.
