181
IZDUSTRIAL AXD ENGINEERISG CHEhIISTRI-
A method has been developed for determining microscopically the effective opening of plain-weave sieves and it has been shown that this value is independent of the size distribution of the material to be tested. This method corrects for both the deviation of the average opening from the nominal and the variations between the individual openings of a single sieve. There still remain discrepancies in the case of twillweave sieves, wherefore these should be checked by comparison with calibrated plain-weare sieves if the accuracy of the results is of great importance. Further work is needed to check the applicability of the method of calibration to sieves used on niore varied materials and to improve the accuracy of the method. Such a program might well be undertahen in collahoration with qieve manufacturers.
VOL. 10, NO.4
Acknowledgment The authors are very grateful to William B. Kent for his valuable suggestions and criticism, and to the W.S. Tyler Company for t,he loan of some of t'he sieves used in this n-ork.
Literature Cited (1) Am. Soc. Testing Materials, Standards, 1936, Part 11, Nonnietallic Xaterials, p. 1413. ( 2 ) Judson. L. V.. B7u. Standurds Technol. Paper No. 321 (1926). (3) Shewhart,, W. A,,, "Economic Control of Quality of Manufactured Product, S e w York, D. Van Nostrand Co., 1931. (4) Wig, R. J., and Pearson, J. C., Bur. Standards Technol. Paper N o . 29 (1913). (5) Ibid., No. 42 (1914). RECEIVEDJanuary 17, 1938. Presented before the Division of Industrial and Engineering Chemistry a t t h e 94th Meeting of t h e American Chemical Society, Rochester, S . T., September 6 t o 10, 1937.
~~
~
Observations on the Rare Earths Quantitative Estimation of the Rare Earths by Means of Their Arc Spectra C. N. ;\IcC4RTY, L. R. SCRIBNER,
~ N M D 4RGIRET
LIWRESZ,
WITH
B. S . HOPKINS
University of Illinois, Urbana, Ill.
The determination of some individual rare earths in complex rare earth mixtures, by means of the Hilger E1 quartz-type spectrograph using an internal standard, has been studied. Magnesium, zirconium, and cerium oxides were tested as possible internal standards. Of these, zirconium oxide was found to be most suitable with respect to position and intensity of lines and rate of volatilization. 0 The region between 2300 and 3300 A. was found to be the most suitable in number and intensity of lines and degree of dispersion. The effects of other rare earths on the line intensities of an individual rare earth were found to be of similar magnitude, no matter which rare earth was added. Duplicate standard plates for lanthanuni, neodymium, samarium, gadolinium, dysprosium, ytterbium, and yttrium were prepared by photographing the arc spectrum of rare earth-zirconium mixtures of various ratios containing sufficient added lanthanum or neodymium oxides to keep the total rare earth content constant at all ratios. The arc spectra of the rare earth oxides extracted froni a number of typical rare earth ores with added zirconium oxide were photographed in a manner similar to that employed in preparing the standard plates.
The intensity ratios between the selected line pairs of rare earth and zirconium lines were determined on both the standard and the ore plates by means of a recording microphotometer. The percentages of the rare earths in the extracts of the ores were estimated by interpolating the ratios of the line pairs obtained from the ore plates on a logarithmic plot relating the intensity ratios and concentration of rare earths obtained from the standard plates. The percentages of individual rare earths present in the ores were found to vary both with the type of ore and with its geographic origin. The method was applied to the determination of the individual rare earths in an artificial mixture of rare earth oxides of known composition. The maximum error as calculated from the results was found to be * 15 per cent.
THE
selection of a suitable ore as the starting material for the preparation of a pure rare earth has always been attended by difficulty and uncertainty because of the lack of methods of analysis for the individual rare earths. This has been particularly unfortunate since, because of the lengthy and difficult processes of fractional crystallization and precipitation required to prepare a rare earth in a pure form, it is probably more important to choose as rich as possible a source in preparing the rare earths than in preparing any other elements. Early analysts, recognizing this, made such separations as were practicable and reported the rare earths in ores in percentages of cerium and yttrium groups and of cerium present. KOvery great advance in methods of analysis for
APRIL 15, 1938
ANALl-TIC.4L EDITIOS
the rare earths was made until after the introduction of quant’itative analysis by means of x-ray spectroscopy. This method has been employed by Goldschmidt and Thomassen, ITT. Koddack, I. Soddack, and K. Kimura to determine the percentages of individual rare earths present in ores, meteorites, and various artificial mixtures. Another physical method of analysis which has come into wide use for the analysis of many elements is that employing arc spectra. This method, like that of x-ray spectroscopy, rests on the fact that, all other things being constant, the intensity of the spectrum line of an element is directly related to the amount of that element present in the sample. Many improvements over the dilution methods of Hartley, Leonard and Pollock, De Gramont’, and other early spectrochemical analysts have been effected in the last few years. Gerlach was the first to introduce the idea of comparing the line intensities of the element to be determined with the intensit’ies of the lines of another substance present in nearly constant amount from sample to sample. I n this way the effects of such factors as exposure and development are eliminated, so that the relative intensities of the lines of the element being determined are more nearly a function of the concentration alone. The method of Gerlach has been modified b y Kitchie and Standen, who introduce a definite amount of a selected substance not originally present to serre as the internal st’andard, and evaluate the relative line intensities photometrically. The method as developed by Sitchie and Standen is being widely used in the determination of a large number of metals in metallurgical products, minerals, biological fluids, etc. The only application of this method to the determination of rare earths is the one made by Bauer, who quantitatively estimated lanthanum in a number of artificial mixtures containing calciuni, aluminum, and iron using zirconium oxide as the internal standard. I n 1934 LIcCarty began a study to test the possibility of applying the method to the determination of the individual rare earths in complex rare earth mixtures such as are obtained on extraction of the ores.
185
ess” plates. The arc used to excite the spectra was generated between a pointed graphite anode and a graphite cathode containing a crater in which the 0.1-cc. samples of the solutions were placed and allowed to evaporate after a preliminary arcing for the purpose of focusing the arc on the slit of the spectrograph. Thirty-second exposures were made using an arc carrying 5.5 to 6 amperes and 220 volts drop across terminals. The plates were developed in a hydroquinone developer for 2.3 minutes, fixed for 20 minutes, and washed for 35 minutes, all at 12 C. After the standard plates had been prepared, “line pairs” consisting of a rare earth line of suitable intensity and a magnesium line in fairly close proximity were selected rind marked in the spectra at each concentration. The relative intensities of the lines in the line pairs were then determined photometrically. The microphotometer used in this investigntion was one of standard design using a Ag-Bi thermocouple and recording the intensities as peaks on sensitized paper. A Photometric tracing of the region containing the line pairs was made for each rare earth concentration on each of the standard plates. The ratios between the relative intensities of the rare earth and magnesium lines were then calculated by dividing the heights of the peaks corresponding t o the magnesium lines in the line pairs. These ratios were calculated for the line pairs at each concentration and plotted as abscissas on double logarithmic paper against the rare earth Concentrations plotted as ordinates. These plots resulted in very nearly straight lines xvhich were used as standard curves relating intensit’y and concentration in the analysis of the rare earth mixtures obtained from the ores. Half-gram samples of the rare earth oxides extracted from the ores were weighed out, dissolved in nitric acid, and so diluted with standard magnesium solution that a ratio of 10 mg. of rare earth oxide to 1 mg. of magnesium oxide was obtained. The spectra of these solutions were photographed maintaining all conditions of excitation, exposure, development, etc., as nearly as possible like those existing in the photography of the spectra of the standard plates. The rare earth on magnesium line pairs in the spectra were then photometered, the intensity ratios calculated from the heights of the corresponding photometric peaks, and the rare earth concentrations corresponding t o these intensity ratios read off from the standard curves.
Selectioii of Internal Standard
The values obtained by 11cCarty for the percentages of individual rare earths present in the ores had an estimated error of 10 per cent. It seemed desirable to determine whether these values could be checked when other line pairs were used and whether the accuracy of the method could be Individual Rare Earths in Complex Mixtures increased. Further study was therefore continued by ScribAbout thirty typical rare earth ores, including gadolinites, ner. I n searching for possible improvements, particular atnionazites, euxenites, etc., were extract,ed by such suitable tention was directed a t the selection of the internal standard. standard methods as Urbain’s sulfuric acid extraction and The advisability of using magnesium oxide seemed open to Mueller and hleyer’s pyrosulfate fusion method. The perquestion, since magnesium oxide has a considerably lower centages of total rare earths present in the ores were calcuboiling point than the rare earth oxides, and one of the criteria lated on the basis of weights of rare earth oxides obtained from of a good internal standard is that it should volatilize a t a known weights of represent’ative samples of the ores. rate similar to that of the substance to be determined. ZirSpectrochemical analysis by the method of Nitchie and conium, titanium, thorium, vanadium, and magnesium oxides Standen requires the addition of some substance not originally were therefore studied by Scribner as possible internal standpresent to serve as an internal standard. For the analysis of ards for the deterniination of the rare earths. Zirconium the rare earths, McCarty chose magnesium oxide as the internal oxide was found to be the most satisfactory, since it has a standard, since it gives lines of suitable intensity without boiling point similar to the boiling points of the rare earth giving many lines which would only further complicate the oxides, gives fairly strong lines a t a low concentration, and already rich spectra of the rare earths. Standard plates of furnishes a number of suitable lines in close proximity to suitthe spectra of high-purity rare earth oxides (99.7 to 100 per able lines of the rare earths. cent pure) with magnesium oxide added were then prepared Standard plates for lanthanum, neodymium, samarium, for cerium, lanthanum, praseodymium, neodymium, samarium, yttrium, gadolinium, dysprosium, and ytterbium were prepared gadolinium, and yttrium. Oxides of the other rare earths using a method similar to that used by McCarty except that having a sufficiently high purity were not available. zirconium oxide was substituted for magnesium oxide as the internal standard. Theyave-length range photographed was In making up a standard plate,, quantities of a solution of the that from 2500 to 3400 A. Line pairs of the rare earths and pure rare earth oxide in nitric acid were mixed with the proper zirconium were then selected, particular care being taken to amounts of a magnesium nitrate solution to give solutions having the following ratios of milligrams of rare earth oxides [(RE)203] select lines which were free from the influence of other rare to magnesium oxide: 10 to 1, 5 t o 1, 4 to 1, 3 to 1, 2 to 1, 1 to 1, earth and zirconium lines. The line pairs were photometered, 0.5 to 1, 0.3 to 1, and 0.1 to 1. The spectra, of these mixtures bethe intensity ratios between rare earth and zirconium lines tween the wave lengths 3400 and 6600 A. were photographed. calculated, and the standard intensity ratio-concentration The instrument used throughout the entire investigation was a curves constructed, exactly as has been described above. Hilger E1 quartz-type spectrograph. All spectra were photoEight of the rare earth ore extracts prepared by McCarty were graphed on 25 X 10 cm. (10 x 4 inch) Cramer “spectrum proc-
186
INDUSTRIAL AND ENGIKEERING CHElIISTRY
analyzed, zirconium oxide being added as the internal standard. I n other respects the method employed was essentially the same as that used by hlcCarty. The values obtained for the indkidual rare earth contents of the ores showed some rather large discrepancies from those obtained by RIcCarty and were never in very good agreement
Possible Sources of Error The disagreement between the results of AlcCarty and Scribner indicated that further study of the problem was required. The investigation was therefore continued by Lamrenz, special attention being given to possible reasons for discrepancies in the results and for possible sources of error. Among these the following seemed to be of particular importance: unsteadiness of the arc discharge, the difference in the rates of volatihation of the two internal standards used, the smallness of the dispersion of the instrument in tlie wavelength range used by McCarty with consequent overlapping of lines, the uncertainty of the position of the base line in the photometric tracings, and the influence of the other rare earths on the intensities of the lines of a particular rare earth in the spectra of the ores. Attempts to steady the arc discharge between a pointed graphite anode and a graphite cathode impregnated with a rare earth salt solution, by varying the current and the diameter of the cathode and by withdrawing the hot vapors formed in the cathode by the application of suction to a hole bored through the anode, did not meet with success. It was found that the introduction of the rare earth salt solution into the unarced cathode, with subsequent drying in an oven, yielded a steadier arc than when the carbons Tvere first arced and the solution introduced into the hot cathode was allowed to evaporate. This improvement was ascribed to the smaller porosity of the unarced carbon cathode which served to confine tlie rare earth material to the crater of the cathode instead of allowing it to permeate the walls of the crater and deposit on the sides of the cathode and there act as a point of discharge. The influence of rare earths on the lines of other rare earths was studied by photographing the 10-mg. spectrum of a vttrium earth mixture, determining the relative intensities yttrium, ytterbium, dysprosium, and gadolinium lines by means of the microphotometer, and calculating the intensity ratios between these lines. Ten-milligram samples of a lanthanum-cerium-praseodymium mixture and of a neodymium-samarium mixture were non- added to separate 10-mg. samples of the yttrium earth mixture, the spectra photographed, the intensities of the same lines redetermined, and their intensity ratios again calculated. The intensity ratios of the lines in the yttrium mixture alone varied by 15 per cent or more from the intensity ratios of the lines in the yttrium earth mixture with added rare earths. The intensity ratios of tlie lines in the yttrium mixture v i t h lanthanum-ceriuinpraseodymium added varied by about five per cent from the intensity ratios of the same lines in the mixture t o d i i c h neodymium-samarium had been added. diniilar tests n-ith other mixtures g a m comparable results and qcemed to indicate that although a giren rare earth line is visibly affected by the presence of another rare earth, the magnitude of this eftect IS the .anie no matter idiich of the rare earths is added. The selection of the mozt suitable range for deterininations nitli the Hilger E1 spgctrograph was -tidied. The region betn-een 3300 and 4200 A. was discarded because of the prezence of catbon-oxygen and carbon-nitrogen bands, that above 4200 A. because of the small dispsrsion of the instrument in that range, and that below 2500 -4. because of the scargity of rare earth lines. The region between 2500 and 3300 A. was found to be suitable both in the number of rare earth lines which occur and in the dispersion.
of
LOL. 10, so. 4
The ideal internal standard for the determination of a rare earth by means of the arc emission spectra should be another rare earth, since the rates of volatilization of the t-ivo would be very nearly equal. Since all rare earths are present in naturally occurring mixtures, however, before a rare earth could be used as the internal standard i t was necessary to remove it completely from the mixture and then reintroduce it in definite amount,. Cerium is the only rare earth which can be removed from a rare earth mixture with any degree of ease and its removal from a monazite extract by means of oxidation with alkaline permanganate was therefore carried out. At least four repetitions of the oxidation process were found necessary to effect a complet'e separation of the cerium and other rare earths. Cerium, moreover, gave too few lines ,Of suitable intensity in the wave-length range 2500 to 3300 A. a t the concentration used for the internal etandard. Zirconium oxide was therefore used as the internal standard. Standard plates for lanthanum, neodymium, samarium, gadolinium, dysprosium, ytterbium, and yttrium mere now prepared in the following way: Solutions of the high-purity oxide of these element'sxere dissolved in nitric acid and so diluted with a standard solution of zirconium oxide in nitric acid that the following ratios of milligrams of rare earth oxides to zirconium oxide were obtained: 5 to 1 , 4 to 1, 3 t o 1, 2 to 1, 1 to 1,0.6 to 1 , O . j to 1, 0.4 to 1,0.3 t o 1,0.2 t o 1, and 0.1 t o 1. These values were also equal to the number of milligrams of rare earth oxide and zirconium oxide per 0.1-cc. samples of the solutions. The preliminary experiments had shown that the intensities of the lines of the rare earths are affected by the presence of other rare earths and since the individual rare earths nwe t o be determined in the presence of rare earth mixtures in the ores some allowance for this effect had to be made. To compensate fully for this effect it would be necessary to add a rare earth mixture identical with that present in the ore to the standards. This, of course, is not practicable since it Tvould necessitate a separate set of standard plates for each ore, even if the composition of the rare earth mixtures in the ores were known. Therefore, since the preliminary experiments had shown that each of the rare earths exercises nearly the same effect on the intensities of the lines of another rare earth, the problem was met by adding t o each of the rare earth-zirconium mixtures used in making up the standard plates that quantity of another pure rare earth which would bring the total rare earth oxide concentration up t o 10 mg. per 0.1 cc. Two standard plates, one containing lanthanum as the diluent and the other neodymium, were prepared for all rare earths list'ed except lanthanum and neodymium, for which only one standard plate each was prepared. The 0.1-cc. samples of the solutions of the standards were then measured into the craters of the cathodes and dried for 1.25 hours in an oven at 110' C. The spectra were photographed in the manner described above. The plates were developed for 4 minutes, fixed for 25 minutes, and washed for 30 minutes, all at 14" C. Suitable line pairs of rare earth and zirconium lines xere selected and photometered at each concentration. The base line of the photometric curve was determined by drawing a line through the points of maximum light transmittancy as recorded on the photometric tracing by allowing the plate light of the photometer to fall through a clear portion of the plate. The base lines xere checked for correctness by calculating the ratio between the heights of peaks due to two zirconium lines and keeping this ratio constant for all succeeding determinations on the intensity rat'io of the line pair. The intensity ratios of the line pairs v e r e then plotted against the rare earth concentrations, as has already been described. The curves obtained by plott,ing the intensity ratios from tT7-o duplicate standard plates, one containing lant~haniim as the diluent, rare earth and the other neodymium, always lay veri- close together. Txvcnty of the rare earth ores \ x r e thcn extracted, using the methods cniployed by McCarty. Particular attention was given to the complete removd or zirconium, since spectrographic traces of this elexicnt had heen found in the extracts u d by Scribner. The rare earth oxides so obtained were dissolved in nitric acid and diluted with zirconium oside solution SO as to give a rare earth oxide content of 10 mg. and a zirconium oxide content of 1mg. per 0.1 cc. of solution. The arc spectrograms of the ores mere made in triplicate following exactly the same procedure as that used in making the standard plates. The heights of the peaks corresponding t o the lines of the line pairs, obtained on photometry of the spectra, x e r e measured from a base line obtained as rlescribed under the photometry of the standard plates. The intensity ratios of the line pairs were calculated and the concentra-
APRIL 15, 1938
lShLYTICAL EDIT103
tions of the rare earths determined from the standard curves The results obtained did not agree with those of either McCarty or Scribner. The differences in the results obtained were undoubtedly due to such variations as the unequal dispersion at the wave length of the line pairs used in the various determinations, the choice of different internal standards, and the determination of the base line, as \vel1 as to the inevitable errors introduced by the inconstancy of the arc and the various factors which influence the intensity of the lines upon the plate.
In order to determine the accuracy which would be expected by the use of a standardized procedure, several control d e terminations were made upon artificial rare earth mixtures, prepared from materials of known composition. The individuals selected for this work were yttrium, neodymium, samarium, gadolinium, dysprosium, and ytterbium. These solutions were diluted with a standard zirconium solution and triplicate determinations -sere made according to the method employed in the analysis of the ores. The results are shown in Table I . TABLE I. ARTIFICIALR.1 IXTUREE
-I
% T
Nd Sm Gd
I”:
35 22 8 8 10
12.5
Found-I1 111
Av.
Error Individual A v . comdetermi- pared with Present nations theoretical
%
%
%
%
%
38.5
42 22 10 8 12 11
38.6 22.17 8.66 8.5 11.33 11.17
40 20 10
17.5
2.5 20.0
10 10
20.0
22.5 8 9.5 12 10
10
15.0
26.0
%
- 3.76
+10.35
-13.4
-15.0 +13.3 +11.7
N e t error +3.20
While the net error in all determinations is not large, considering the difficulties which are inevitable, a glance at the fluctuations of individual determinations shows that the favorable net error is due to a series of compensating errors. The variations in the triplicate analyses are too large for a successful analytical method. But it must be borne in mind that there are no methods for complete analysis of rare earth ores and that the problem presents a good many serious difficulties. The authors’ work has progressed far enough to convince them that these difficulties are not entirely insurmountable and that further refinements will develop a method by which the composition of the unfractionated extracts of rare earth ores may be made with a reasonable degree of accuracy. Since this method of analysis depends upon the intensity of the lines in the arc spectra, it is evident that the length of exposure, the volatility of the internal standard, the dispersion of the spectroscope, the treatment of the plate, and the dependability of the microphotometer d l all influence the accuracy of the method. I t is believed that the precautions taken have largely eliminated mozt of these sources of error. The authors recognize that zirconia is not an ideal internal standard, but they believe that the major error is still to be found in t,he fluctuation5 of the arc during exposure. If it were possible to devise a method of producing a steady arc, they believe that this analytical process ~vouldbe reasonalily satisfactory. One of the controversial points with respect to rare earth ore5 has to do with the percentage of the individual members of the group in various samples of the same ore. Do samples of monazite sand from various localities contain the same perrentage of samarium? There have been decided differences of opinion in regard t’o the correct answer to such a question. If crrie is interested in the study of snmarium, an accurate anh w r to the question is of prime importance. Sonie light can /)e tlirown upon t h e coniposition of a11 ore by means of x-ray niialysis, but evidently these nietliods h a w never been applied except to material after coiisiderable fractionation. These attempts partially to separate the complex mixtures which are
187
obtained directly from rare earth ores are never quantitative and must always result in more or less serious disturbances of the quantities present. While the method of analysis by arc spectra is not yet ideal from the standpoint of accuracy, it is possible to get approximate results which are sufficiently refined to give some idea concerning the composition of some similar ores from varying localities. The results obtained in the present study are tabulated in Table 11. The values given are the average of two or more determinations of the intensity ratios obtained with the use of zirconium as a n internal standard. The procedure was the same as was employed in making up the standard plates. A special effort was made to maintain uniform conditions of exposure, development, etc. Three arc spectrograms of each ore were taken to allow for inaccuracies resulting from the flickering of the arc. The selected line pairs were marked and photometered in exactly the same way as in the case of the standard plates. The base line was determined by the points of maximum light and checked for correctness by using the intensity ratios between zirconium lines determined in the standard plates. 1111 photometric determinations were made in duplicate. The intensity ratios between the members of the various line pairs were calculated from the heights of the peaks representing zirconium and rare earth lines and the percentages corresponding to these intensity ratios read from the graphs made from the standard plates. It was found that lanthanum could not be successfully determined in the yttrium earth ores because of the presence of several extremely strong yttrium lines a few angstrom units from the selected lanthanum and zirconium lines. Since no other suitable lanthanum line could be found, this element was not determined in the yttrium group ores. T.4BLE
Ore Gadolinite
Samarskite
11.
COMPOsITION OF
Nd
ORES
Percenta e Found Sm $d Dy 3 . 5 4.96 2.6
‘ Y 32.25
5.5
36.00
2.0
2.15 3.0
2.0
5 0
23.25
7 4
3.6
3.45
2 90
6.0
Unknown 23.5 American 23.25 Mitchel C o . , N.C. 21.8
6.0 6.2
4.9 5.1
6.7
5.6 1.17
, ,
4.82
5.9 6.2
6.8
4.9
4.2
6.4
1.5
...
28.0 29.0
8.2 7.4
4.25 4.55
4.9 4.1
3.7 3.3
5.75 6.05
23.0
8.6
4.5
3.75
2 75
5.5
25.0
7.4
3 3
3.4
2.4
6.2
28.4 32.8
1?.?5 4 . 0 J.O 2.5
4.1 4.2
3 . 8 5 5.85 5,0 5.7
28,i 10.25
5.5 2.5 19.5 6.0 18.0 4.5 7.4 2.95 2.2 5 5.0 13.254.8
4.2 4.0 2.6
5.0 3.3 1.0 2.15
Source Arizona Probably European Hittero, Norway
Euxenite
Arendal, Norway Xorway
Fergusonite
Norway Llano Co., Texas drendal, Norway Ceylon
Cyrtolite .. .. . . . . Xenotime . . . . .. . Tschefkinite .. . . .. . heschynite . . . .. . . . Allanite ........ Orthite ........ Cerite Bastnas , Sweden Ampangaheite Madagascar
..
2.0 24.25 7.0
20.5 2 0 95 0
20.00 5 3;
3.3 3.75 3.6
Yb 5.0
La
. ,
5.7 . 1 . 7 10.5 1.0 4.55 7.1 ,. , 2.0 1.02 , . 2.462.95 . .
4.25
1.75
1.0
1.0
9.9
3 3
3 4
2.7i
5 85
.
,
The results of these analyses must be accepted as tentative, but it is believed that they are suffii:iently accurate to justify the concIusion that xeiiotine and allanite are better sources of neodymium tliaii gadolinite n-ould be. They also seem to indicate tlirit similar rare earth ores from different localities shox considerable variation in the perceiLtapes of the indid u a l rare earths v-liich they contniii.
