Characterization of Hydrated Aluminas by Infrared Spectroscopy Application to Study of Bauxite Ores L E O D. FREDERICKSON, JR. Aluminum Research Laboratories, Aluminum Co. o f America, New Kenrington, Pa.
Infrared spectroscopic examination has been made of the four distinct alumina hydrate phases. or-Monohydrate (bKhmite), a-trihydrate (gibbsite), &monohydrate (diaspore), and P-trihydrate (bayerite) possess hydroxyl groups and these produce sufficiently different absorption patterns in the 0 - H stretching and deformation regions to permit their identification by this means. The infrared 0 - H absorption patterns observed with the pure hydrates have been used t o identify gibbsite and biihmite in a variety of bauxite ores and diaspore in Missouri diaspore clays.
NACI
PRISM
R"""
N T interest in the application of infrared spectroscopy to the study of inorganic compounds is illustrated by the publications of Hunt et al. (7) and Miller and Wilkins ( I S ) . Vibrational assignments and structure correlations have also been reported in the studies of silicate minerals (9, 11, 16) and clay minerals ( 1 , 8, 1%'). Kendall (10)has described the identification of polymorphic crystal forms, both organic and inorganic, by infrared. Heretofore, the various alumina hydrate phases have been identified, both in their pure state and in ores, by characteristic x-ray powder diffraction patterns (3, 6, 18). This technique has helped to establish the identity of the polymorphic forms, and has been applied, on a quantitative basis, to the analysis of bauxite exploration samples ( 2 ) . Orsini and Petitjean (14) have reported a n infrared study of bohmite in the 6- to 14-micron region. Other infrared examination of alumina has been limited to observation of anodically formed films (15), although certain of the phases have bee; included in studies of hydrated inorganic salts (4). This paper presents the first infrared characterization of all alumina hydrate forms, by means of the "hydroxyl" 0-H stretching and deformation vibrations. Indications of the usefulness of this differentiation are given in survey spectra of bauxite ores and diaspore clays. Although alumina is a major constituent of the earth's crust, bauxite ores are the principal source of the material useful for reduction to metallic aluminum, and deposits of ore throughout the world differ in the type of hydrate they contain. Most United States and South American ores contain a trihydrate phase whereas European bauxites are predominantly a monohydrated form. The terminology with regard to the designation of the various alumina hydrate phases is the following (17): a-alumina trihydrate (gibbsite, hydrargillite), a naturally occurring phase in South American and United States bauxites; a-alumina monohydrate (bohmite), naturally occurring in European bauxites; &alumina trihydrate (bayerite), not identified in nature but formed by precipitation from sodium aluminate solution; and &alumina monohydrate (diaspore), found in nearly pure deposits in Massachusetts and widely distributed in clays throughout the n-orld.
Figure 1. 0 - H Spectra, Pure Alumina Hydrates, Stretching Region A . 8-Monohydrate B. WMonohydrate C. a-Trihydrate
oudy described (18). Bauxite ore and diaspore clay samples were originally ground to pass 100-mesh screen and dried at 108' C. to remove adsorbed moisture. All ores had been previously studied by x-ray diffraction, emission spectroscopy, and wet chemical means. Most of the samples were examined as mulls after further grinding to assure a particle size less than 2 to 3 microns. Fluorolube oil (polytrifluorovinyi chloride, Hooker Electrochemical Co.) was used to mull samples for observation in the 2- to 3-micron
Table I.
0 - H Vibrations in Stretching and Rending Regions
Freq., Phase L c1 Kaysers ( C m . 3 ) 0-H Stretching Vibrations Observed, (2-3-Micron Region) LiF Prism u-Alz08 monohydrate 3.247 3079 3.065
3262
u-AlzOr trihydrate
2.975 2.960 2.917 2.842 2.765
3301 3378 3428 3518 3616
8-AlzOa monohydrate
3.42a
2900 f 2oa
8-AlzOa trihydrate
2.940 2.895 2.842 2.830
3401 3454 3518 3533
0-H Bending Vibrations Observed (8-11-Micron Region) NaCl Prisd 8.73 a-AlnOa monohydrate u-AhOa trihydrate
EXPERIMENTAL
All spectra were obtained using a Perkin-Elmer Model 112 infrared spectrometer. Both sodium chloride and lithium fluoride prisms were used, wave-length calibration being made using known absorption bands of water vapor, carbon dioxide, ammonia, and pure liquid benzene. Preparation of the pure alumina hydrates used has been previ-
1883
8-Alios monohydrate 8-AlzOa trihydrate 0
Measured with NaCl prism.
9.32
1145 1073
9.80 10.34
1020 967
(not examined in this region)
lt:;:}broad
1024 975
A N A L Y T I C A L CHEMISTRY
1884 region (0-H stretching) whereas Nujol (mineral oil) was used in the 8- to 11-micron region (0-H deformation or bending). Mulled samples were examined as a film on a single rock salt plate. Pressed potassium bromide plates were used for examination of the diaspore clays. Spectral slit widths employed in measurements with the sodium chloride prism were 0.005 and 0.019 micron, respectively, for the 0-H stretching and bending regions. For the lithium fluoride prjsms, spectral slit width in the 0-H stretching region was 0.002 micron. It was found desirable to suppress atmos heric carbon dioxide spectral absorptions in the vicinity of the 8-H stretching vibrations, particularly when using the lithium fluoride prism. This was accomplished by passing carbon dioxide-free air through the spectrometer and associated evternal optics.
I
NACI PRISM
PURE ALUMINA HYDRATES
Figure 1 shows the absorption spectra of the hydroxyl vibrations for pure 8-alumina monohydrate, a-alumina monohydrate, and a-alumina trihydrate. The ordinate in this and subsequent figures has only qualitative dimensions of energy, as no precise control was exercised over mull thickness. It is appaient from this figure that the sodium chloride prism provides adequate separation of the 0-H stretching vibrations for these three naturally occurring phases. Figure 2 shows the fine structure of the 0-H bands for three of the phases a s obtained with the lithium fluoride prism. The broad nature of the 0-H vibration pattern in the 8-alumina trihydrate is suggedtive of hydrogen bonding In the crystal. As yet, specific details of the structure for this form have not k e n pstahlished ( 1 7 ) .
I
WAVE LENGTH
Figure 3. 0 - H Spectra, Pure Alumina Hydrates, Bending Region A . a-Monohydrate
B . a-Trihydrate C. 0-Trihydrate
Table 11. Infrared Identification of Bauxite Ores for Alumina Hydrate Phase A1103 Hydrate Form Found a-Trihydrate a-Monohydrate a-Trihydrate a-Monohydrate a-Trihydrate a-Trihydrate a-Trihydrate a-Trihydrate a-Trihydrate a-Mono- and a-trihydrate
Bauxite 4 typical Surinam ( S . America) 4’ typical France 2: typical Arkansas Iatria No. 1 (Italy) GeoTgia Nos. 1, 2 (U.S.A.) India Nos. 1 , 2 Dalmatian No. 1 Yugoslavia) America) Demerara No. 1 Eufaula No. 1 (Alabama, V.S..4.) Arkansas No. 3
(8.
~____
-~~
_.____
BAUXITE O R E S .
Kineteen representative bauxite ores were studied in the same manner employed for the pure alumina hydrates. Figures 4, 5, and 6 show the absorption spectra for several of these materials. NACI PRISM 2 917
--
WAVE
LENGTH
Figure 2. 0 - H Spectra, Pure Alumina Hydrates, Stretching Region, Lithium Fluoride Prism A . a-Monohydrate B . a-Trihydrate C. 8-Trihydrate
The 0-H stretching vibration observed for diaspore occurs a t a lower frequency than those of the other hydrates. Ewing (6)postulates that the atoms in diaspore are arranged in a closepacked hexagonal structure, each oxygen atom equivalent to every other and joined together through a hydrogen atom. This type of structure agrees well with the observed low frequency and broad weak nature of the 0-H stretching band, indicitive of a strong hydrogen bond. Figure 3 shows spectra of three of the pure hydrates in the deformation or bending region. Enough pure diaspore was not available for observation of the deformation frequency region. Table I is a tabulation of the 0-H vibrations obscrved in tmth thp stretching and bending regions.
W [r
W Z W
WAVE LENGTH
Figure 4. 0 - H Spectra, Aluminas in Bauxite Ores, Stretching Region A . France No. 1 B. France No. 2 C. Istria No. 1
D . Surinam No. 1 E. Georgia No. 1
V O L U M E 2 6 , NO. 1 2 , D E C E M B E R 1 9 5 4
1885 interference upon the type of 0-H vibrat>ionpattern observed. The presence of diaspore could not be established in those bauxites examined, although it is possibly present in minor amount. The broad weak nature of the 0-H hand in diaspore wems to preclude its infrared detection except in major amounts. SUMMARY
Infrared spectra have been obt,ained for the known alunlina hydrate phases as produced by the hydroxyl vibrations. The resulting patterns have been sucnessfullg. used to characterize the contained hydrate phases in various bausit.e ores and Missouri diaspore clays. This technique is rapid, as an identification usually requires no more t,han 5 minutes for completion which includes grinding to required particle size. I t should prove to be a valuable adjunct to the s-ray method in bhe study of other sg.st,emeof a similar nature. LITERATURE CITED
I
WAVE
LENGTH
Figure 5. 0 - H Spectra, Aluminas in Bauxite Ore, Stretching Region, Lithium Fluoride Prism A . Arkansas No. 3 B. Arkansas No. 2 C. Arkansas No. 1
Of particular interest is the spectrum of Arkansas No. 3, an unusual case which contained both a-monohydrate and a-trihydrate phases. Typical Arkansas ores normally contain only the a-trihydrate form. Table I1 is a compilation of the various hauxites studied together with the infrared identification a s to the type of alumina hydrate phase found. Without exception, infrared ahsorption characterizations of the contained hydrated aluminas were identical with those obtained by x-ray powder diffraction (6). DIASPORE CLAYS
(1) Adler. H. H.. Am. Petroleum Inst., Project 49, Rept. 8, Section I(1950). (2) Black, R. H., ANAL.C H E M .25, , 743 (1953). '. J., J . Phys. Chem., 57, 946 (1953). (3) Day, M. K. B., and Hill, 1 (4) Duval, C . , and Lecomte, J., Bull. soc. chim. France, 8, 713 (1941). (5) Ewing, F. J., J . Chem. Phys., 3, 203 (1935).
NAG1 PRISM
/I
3.42
-
WAVE LENGTH
Figure 7 . 0 - H Spectra, Diaspore Clays, Figure 7 shows spectra for several Stretching Region Missouri diaspore clays in the 0-H A . Pure diaspore, Chester, Mass. stretching region. The position and 0 H Spectra, Aluminas in Figure 6. B Belle Mo. Bauxite Ore, Bending Region C: Red Bud, Mo. contour of the band in these materials D , E . Gasconade County, Mo. are identical with those s h o m for pure A . France No. 1 B. Arkansas No. 3 diaspore. The low-frequency (high C. Surinam No. 1 xave-length) position for this band makes it appear very unlikely that any (6) Foster. L. &I., -4luiiiiiiiitii ( *o. of . h c r i c a , unpublished report, hydrate other than diaspore is contributing to the 0-H stretch1950. ing pattern in these clays. (7) Hunt, J. M., Wisherd, 11. P., and Bonham, L. C., AXAL. DISCUSSION
It is possible, in ores of this kind, that the 0 - H deforination vibration region may be overlaid by absorptions caused by other infrared-active groups such as silica, etc., which suggests that a better characterization mav bc effected in the stretching region a t 2 to 3 microns here such interferences would not occur. However, no serious difficultv has arisen from this possibility with the ores reportrd here. Quantitative determination of hydrated aluminas in ores presents difficulties analogous to those encountered with the x-ray diffraction method. Other hydrated salts present in the ores to any large degree could also contrihute to the magnitude or contour of the 0-H absorption. Their presence in bauxite is usually only in small amount and seems to present no major
CHEM.,22, 1478 (1950). (8) Keller, W. D., and Pickett, E. E., Am. J . Sci., 248, 264 (1950). (9) Keller, W. D., and Pickett. E. E., Am. Mineralogist, 34, 855 (1949). (10) Kendall, D. N.. ANAL. C H E M .25, . 382 (1953). (11) Launer, P. J., Ana. Min~ralogisl.37, 764 (1952). (12) Lecomte, J., A n a l . Chim. Acta, 2, 727 (1948). . 24, 1253 (1952). (13) Miller, F. A, and Wilkins, C. H., A . v . 4 ~CHEM., (14) Orsini, L., and Petitjean, M., Compt. Tend.. 237, 326 (1953). (15) Parodi, lI.,Ibid., 205, 906 (1937). (16) Roy, R.. and Francs, E. E., A m . Mineralogist, 38, 725 (1953). (17) Russell, A. S.,.Uuminum Co. of rlmerica, Tech. Paper 10 (1953). (18) Stumpf, H. C., Russell, 9. S.,Newsome, J. W., and Tucker, C . ll.,Ind. Eng. Chem., 42, 1398 (1950).
RECEIVED for review July 1, 1954. Accepted August 25, 1954. Presented before the Division of Analytical Chemistry at the 126th Meeting of the AMERICAN CHEMICAL SOCIETY, New York. N. Y .
