THE OXIDES AND HYDRATES OF ALUMINUM JUNIUS D. EDWARDS
AND
MARTIN TOSTERUD
Aluminum Research Laboratories, New Kensington, Pennsylvania Received February 17, lQ33
The oxides and hydrates of aluminum have been the subject of many investigations because of their commercial importance and scientific interest, The apparent multiplicity of forms in which they occur, however, has led to much confusion in their identification. A recent contribution from Weiser and Milligan (1) is indicative of this situation. After reviewing part of the literature and describing their experimental work, they reach the conclusion that there is only one crystalline monohydrate of aluminum, namely, diaspore. The form which a number of other workers have characterized as monohydrate and an isomer of diaspore, they decide is either a new form of alumina, which they call delta-alumina, or perhaps a hemihydrate. This conclusion seems likely to be untenable, once all the facts are considered. Probably Weiser and Milligan were unaware of the contribution on this subject published in “The Aluminum Industry-Aluminum and its Production” (2). The various forms of oxides and hydrates are there described with information regarding the monohydrates and methods of producing one of them. A brief review of these forms will be given before presenting additional evidence for the existence of the two forms of monohydrate. ALUMINA
Corundum is the naturally occurring form of alumina,-AI2O3, or alphaalumina, as it is commonly designated. Alpha-alumina is formed by heating any of the hydrates of aluminum to a sufficiently high temperature and is also the form commonly taken by fused alumina upon solidification. Alumina may be produced in another form known as beta-alumina, by fusing alumina with small amounts of magnesia or sodium carbonate, A third crystalline form of alumina, which is of considerable interest, is designated gamma-alumina. When aluminum monohydrate or trihydrate is dehydrated by heating, the product first formed is amorphous. With continued heating a t higher temperatures of about 5OO0C., a new crystalline phase begins to appear, which has been identified by its x-ray pattern and named gamma-alumina. Continued heating a t higher temperatures-above about 120O0C.-results in its conversion into alphaalumina. 483
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A fourth crystalline form of alumina has been reported by Barlett and designated as zeta-alumina (3). ALUMINUM HYDRATES
There are two well-recognized and ident,ifiable hydrates of aluminum, namely, the trihydrate and the monohydrat'e, and two crystal forms of each. Aluminum trihydrate occurs in nature as the mineral gibbsite, which is found also in many types of bauxites. This hydrate, designated alpha-
FIG.1. X-RAY DIFFRACTION PATTERNS OF OXIDESAND HYDRATES OF ALUMINUM 1, a-AlzOa; 2,,8-A1203; 3, y-Alz03; 4, a-AltOa.Hz0; 5, p-AlzOs.Hz0; 6,a-AlzOa.3H*O;
7, p-AlzO3.3H10.
trihydrate ( o(-Al2O3.3H20), is the product produced by auto-precipitation from sodium aluminate solutions, as in the Bayer process. Another crystalline trihydrate, designated as beta-trihydrate, can be produced by saturating sodium aluminate solutions with carbon dioxide under certain conditions, and by other precipitation methods. Betatrihydrate shows a different x-ray pattern than the alpha-trihydrate and differs also from it in solubility. It appears to be a metastable phase, and goes over to the alpha form on continued shaking or long standing in contact with alkali.
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Aluminum monohydrate, in the form known as alpha-monohydrate, is found in many bauxites, of which the French bauxites are typical. Alphamonohydrate can be produced by heating alpha-trihydrate with alkali or water under pressure. Another crystalline form of monohydrate is the mineral diaspore, and this form-carrying out the nomenclature previously employed-has been termed “beta-monohydrate.” Beta-monohydrate has not yet been made synthetically, and we have not investigated it other than to make an x-ray pattern of a typical sample. The x-ray patterns, according to the powder method, given by the various forms of alumina and the hydrates, are shown in figure 1. THE MONOHYDRATES
Weiser and Milligan’s conclusion that there is no monohydrate other than diaspore, is controverted by the fact that millions of tons of the alphamonohydrate, described above, are found in nature (4),and by its synthetic production on a large scale. Its crystal pattern, as will be seen from the figure, corresponds to the so-called delta-alumina of Weiser and Milligan. When the trihydrate is heated in water containing small amounts of sodium hydroxide, it is rapidly converted into alpha-monohydrate (5). For example, alpha-trihydrate, when heated for about twenty hours, more or less, in a solution of sodium hydroxide a t a temperature of about 170°C., is converted substantially to monohydrate. The product, after washing and drying at 105”C., has a water content of about 16 per cent (Al2O3.H2O = 15 per cent HzO). I n this method the water content is reduced from the 34.6 per cent of the trihydrate to 16 per cent by heating in water. Apparently the product contains a small amount of adsorbed water. The monohydrate also may be approached from the other direction. The trihydrate was heated in air at 600°C. for one hour and the water content was reduced to 1.6 per cent; on x-ray examination this product showed the presence of some gamma-alumina but no other x-ray pattern. This dehydrated product was then digested in water plus sodium hydroxide a t 150°C. for about fifteen hours, washed and dried a t 105°C. This product had a water content of about 16 per cent and showed only the x-ray pattern we attribute to alpha-monohydrate. These experiments offer definite evidence of the existence of the alpha-monohydrate. If a small amount of trihydrate is heated in air, most of it decomposes to an amorphous form, but if it is heated in sufficient bulk so that enough water vapor stays in contact with the material, some recrystallization to alpha-monohydrate may occur, but usually only a small percentage is so converted. The water content of the mixture can be reduced to 15 per cent or to 10 per cent, or to other values by changing the time and conditions
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of heating. Since an amorphous phase does not produce a diffraction pattern, the pattern of such a mixture is that of the monohydrate phase. Very long aging in water a t lOO"C., as shown in Weiser and Milligan's experiment of aging for 1009 hours, will produce some alpha-monohydrate from amorphous alumina (their table 7 and figure 7). Weiser and Milligan's product, treated in this manner after drying at 50"C., had a water content of 19.35 per cent, and after heating for three hours in air a t 160"C., it had a water content of 10.38 per cent. This product they considered to be either a new form of alumina with adsorbed water, or perhaps a hemihydrate. A mixture of about 70 per cent alpha-monohydrate with 30 per cent of amorphous alumina would have a water content of 19.50 per cent if the amorphous phase carried about 30 per cent adsorbed water. Upon drying such a mixture a t 160"C., the adsorbed water would probably be substantially all removed without affecting the combined water of the very stable monohydrate, and the mixture would apparently have a combined water content of about 10.5 per cent. Here again, the amorphous phase does not produce a diffraction pattern, and the pattern of such a mixture is that of the monohydrate phase. Weiser and Milligan's conclusion that their "delta-alumina" is not a monohydrate seems to have come from the assumption that they had only one phase in their mixture dried at 160°C. (water content 10.38 per cent), since they got only one crystal pattern from such a mixture. Confirmatory evidence for the existence of the alpha-monohydrate, as made by Tosterud's method, is presented by the thermal arrests observed on heating. If about 100 g. of the compound are placed in a crucible and heated in an electric furnace, an interesting series of observations is obtained from a sensitive thermocouple inserted in the mass. The timetemperature curve is smooth (see "C," figure 2) until a temperature of about 450°C.is reached, when an arrest showing heat absorption is recorded. This is the point where rapid decomposition of the monohydrate occurs. If trihydrate is heated, a similar decomposition point is observed at about 300°C. The decomposition point of the monohydrate is just as characteristic and reproducible as that for the trihydrate. Of course, the trihydrate and monohydrate will lose some water on heating a t temperatures below these points, but the decomposition becomes quite rapid at the temperature range in question. The monohydrate is more stable at elevated temperatures than the trihydrate. The evidence for the existence of the alpha-monohydrate is just as complete, and parallels that for the alpha-trihydrate. Similar thermal arrests have been recorded by others in experiments of this character. In figure 2 are also given heating curves for bauxites of two types. One, a trihydrate bauxite, gives a thermal arrest a t about 250°C., and the other, a monohydrate bauxite, gives an arrest at about 425°C. The
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arrests observed with bauxites are usually somewhat lower than those observed with the pure hydrates. Other bauxites are known which contain both monohydrate and trihydrate, and the heating curves show both arrests. Furthermore, a heating curve made with trihydrate, if carried high enough and under proper conditions, sometimes shows a slight arrest at about 450°C., indicating the formation of some monohydrate from the trihydrate during the heating, but a curve made with pure monohydrate never shows the lower arrest. The samples prepared by Weiser and Milligan according to Hiittig's procedure (K1', K1", Kz, K,, K4 and Ls, figure 6 ) have an x-ray pattern which corresponds very closely to that of beta-trihydrate given in our
Zme offieat/ng'
-
FIG. 2. HEATINGCURVES A, aluminum trihydrate; B, Arkansas bauxite; C, aluminum monohydrate; D, French bauxite.
figure 1. Additional discussion of beta-trihydrate is given in the article (2) previously referred to. While we have made no investigation of other forms of hydrate reported in the literature and reviewed by Weiser and Milligan, it seems quite likely, as they have concluded, that they are mixtures of other known hydrates, or incompletely crystallized products, Crystallization or recrystallization from a solid phase is attended with more difficulties than crystallization from a solution. In the conversion of precipitated amorphous alumina to a crystalline hydrate, there may be lines appearing in the x-ray diffraction pattern, or differences in the intensity of lines, which are caused by some incompletely crystallized product, and which can hardly be said to represent a new hydrate.
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JUNIUS D. EDWARDS .AND MARTIN TOSTERUD
The x-ray data contributed in this paper are the work of W. L. Fink and
K. R. Van Horn. SUMMARY
There is a monohydrate of aluminum, Al203-HzO,other than diaspore, and it is found widespread in nature and is being made commercially. Its existence can be demonstrated by chemical analysis, x-ray diffraction patterns, and thermal analysis. Other forms of alumina and its hydrates are briefly reviewed. REFERENCES (1) WEISER,H. B., AND MILLIQAN, W. 0.: J. Phys. Chem. 36, 3010 (1932). (2) EDWARDS, FRARY,AND JEFFRIES:The Aluminum Industry-Aluminum
and its Production, p. 164. McGraw-Hill Book Company, New York (1929). (3) BARLETT:J. Am. Ceramic SOC.16, 361 (1932). (4) HARDER,E. C.: Ores of Aluminum, Chapter IV, p. 60, in The Aluminum Industry. McGraw-Hill Book Company, New York (1929). (5) TOGTERUD, M.: Canadian patent 285,147; November 27, 1928.
