THE MECHANISM OF THE DEHYDRATION O F CRYSTALLINE ALUMINUM HYDROXIDE AND O F THE ADSORPTION OF WATER BY THE RESULTING ALUMINA1 BY LOWELL H. MILLIGAN
1.
Introduction
Ramsay2 investigated the rate at which amorphous, ammonia-precipitated aluminum hydroxide loses water when heated to different temperatures, and concluded that under these conditions, either there are no definite hydrates, or a very large number exist, the vapor tensions of which are only slightly different from each other. Carnelley and Walker3 worked with similarly prepared material and confirmed Ramsay's conclusions. J. M. van Bemmelen4 investigated both the colloidal aluminum hydroxide and the crystalline deposit prepared from sodium aluminate solution. For the colloidal material he concluded that between 15" and 300" C the rate of water loss is uniform and hence no definite hydrates are formed. After drying at 300" C the remaining material had approximately the composition A1203. H2O. However, the precipitate from an alkaline aluminate solution was found t o be a true hydrate, a definitely micro-crystalline, chemical compound, Al2O3,3H20. It remained practically unchanged in a stream of dry air at 160" C, but at higher'temperatures it was decomposed. The results obtained by E. Martin5 were very similar to those of the previous investigators and clearly indicated the difference between the amorphous, ammoniaprecipitated, hydrated alumina, and the crystalline aluminum hydroxide prepared by the Bayer process. Contribution from the Research Bureau of the Aluminum Cor.-qany of America, New Kensington, Pa. *Jour. Chem. SOC.,32,395 (1877). Ibid., 53, 59 (1888). Rec. Trav. chim. Pays-Bas, 7, 75 (1888). Mon. Sci., (5)5,225 (1915); through Chemical Abstracts, 10,571 (19ib)
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Alumina prepared by the ignition of hydrated aluminum oxide a t low temperature, has been used as a desiccating agent, and its relative value compared with other common desiccants has been studied by various authors.1 Hydrated ferric oxides, which are in many respects similar to the corresponding aluminum compounds, have recently been investigated by Posnjak and Merwin.2 The conclusions which they drew were based chiefly on dehydration curves obtained for the original material and also for the dried or partially dried product which had been allowed to readsorb water, each point on the curves representing the amount of water contained by the sample when dried to constant weight at a given temperature. I n the work described in the present paper this method was applied to the study of the mechanism of the dehydration of crystalline aluminum hydroxide, and of the adsorption of water by the resulting alumina; and the results have been plotted as equilibrium drying curves, between 20 O and 275' C. 2.
Experiments
The aluminum hydroxide used was prepared by the Bayer process, was washed thoroughly and dried. It consisted of white granular particles which would pass through a 100 mesh screen, the particles themselves being microscopic aggregates of anisotropic crystals. It had the following analysis : , Al(0H)s. . . . . . . . . . . . .99.82% Fe203.. . . . . . . . . . . . . . 0.09 SiOa.. . . . . . . . . . . . . . . . 0 . 0 4 N a 2 0 . .. . . . . . . . . . . . . . 0.05 "io2. . . . . . . . . . . . . . . . .none. A portion of this was heated in an electric muffle for 30 minutes at 810" C and then cooled in a desiccator over concentrated sulfuric acid, and marked alumina "A." A 1 F. M. G. Johnson: Jour. Am. Chem. SOC., 34, 911 (1912); Marden and Elliott: Jour. Ind. Eng. Chem., 7, 320 (1915); Dover and Marden: Jour. Am. Chem. SOC., 39, 1609 (1917). ZAm. Jour. Sci., (4) 47, 311 (1919).
,
Dehydration of Alacnzinum Hydroxide
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portion, heated for 1 hour a t 985" C and similarly cooled,, was marked alumina "B." A few experiments were made in order to show in a preliminary way that the water content of the alumina will come t o equilibrium with the water vapor in the air a t a given temperature. Check samples of alumina "A" were taken directly after the original ignition, and placed in an oven at 106" C, where they were allowed to come to practically constant weight. Under these conditions water was adsorbed, even though the temperature was above 100" C. The water in the samples was then determined by ignition. Check samples of alumina "A" in a thin layer were exposed to 50 percent saturated air a t room temperature for two hours; and following this treatment, they were dried to constant weight at 106" C and determinations made of the water which was thus driven off and of the water still remaining. Two other samples were wet with water and then dried to constant weight at 106" C, and their water content determined after this treatment. The results are given in Table I. TABLE I The Adsorption of Water by Alumina "A" Procedure
Alumina held a t 106" C immediately after ignition. Alumina exposed to 50% saturated air and then dried a t 106" C. Alumina wet with water and then dried a t 106" C.
0.00 0.00 4.30 4.43 not determined not determined
2.22* 2.39"
2.GG 2.Gl 2.92 2.8'9
These results show that alumina tends to come to equilibrium with water vapor at a given temperature, and that the point of equilibrium is roughly independent of the direction from which it is approached, whether i t be reached by allowing the dry alumina to adsorb water at a given temperature, or by wetting the alumina and then drying i t at thattemperature. The fact that the adjustment at or near the equilibrium point
* This represents
water adsorbed by the sample at 106" C.
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is very slow helps to account for the fact that these results are not identical. The drying curves for the crystalline aluminum hydroxide and for the alumina prepared from it, were made by placing about 2-gram samples in weighing bottles, 7/8’’ diameter 1.5” high, and allowing the water content of these to come practically to equilibrium with a slowly moving current of air of constant moisture content, at a given temperature. These were weighed at intervals after stoppering and cooling them in a desiccator, in order t o determine the point of equilibrium. At the lower temperatures, the humidity of the air which was passed over the samples was controlled. The open weighing bottles were placed in a covered aluminum pan, which in turn was placed inside a Freas oven, the temperature of which could be maintained constant within at least 2’ C. Through the pan was circulated a current of air brought to a known humidity outside the oven and heated to oven temperature by passing it through a small aluminum coil just before it was conveyed to the pan containing the samples. The temperature inside the pan was measured by a thermocouple inserted in an aluminum well in the pan lid. The humidity adjustments were obtained by passing the slow current of air through bottles containing either water, or 56% sulfuric acid (which gives 25% saturated air at room temperature), maintained at constant temperature. When air containing 51 rng of water vapor per liter (air saturated at 40” C), was used, the inlet tube between the saturator and the oven was electrically heated with nichrome wire to prevent condensation in it. At temperatures above 196” C the samples were placed directly in the oven and no attempt was made to control the humidity of the air. All the experiments were made a t approximately atmospheric pressure. Duplicate samples were run in each case. After the samples had been brought to equilibrium at 275” C, they were exposed for 54 hours in 50% saturated air at room temperature. This procedure served to “rehydrate” them, after which they were again brought to equilibrium at various temperatures.
Dehydration of A Lunzinunz Hydroxide
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Finally the samples were brushed as completely as possible into weighed, covered, platinum crucibles and ignited at about l l O O o C to constant weight. The weight of the ignited alumina (,when corrected for the amount which stuck in the 1 mg) weighing bottle and could not be removed-about was used in the case of the alumina samples as the basis for
Fig. 1 Equilibrium Drying Curves in Air containing about 5 Milligrams of Water Vapor per Liter. (A). Crystalline Aluminum Hydroxide. (B). Crystalline Aluminum Hydroxide after drying a t 270” C followed by Rehydration. (C). Alumina “A” (ignited a t S10” C for 30 minutes). (D). Alumina “A” after drying a t 270” C followed by Rehydration. (E). Alumina “B” (ignited a t 985” C for 1 hour); and Alumina “B” after drying a t 275” C followed by Rehydration.
the calculation of the final results. The hydrate samples, however, contained sufficient water even a t the final drying temperature, to cause them t o boil up when they were ignited, and consequently a small quantity of these samples was lost. Accordingly the theoretical figure of 34.60y0 water in A1(OH)3 was used for the calculations in these cases.
Lowell
252
H.illilligaiz
The effect which a variable amount of water vapor in the air had on the point of equilibrium at constant temperature is best shown by the results at 100" C for alumina "A," which are given in Table 11. TABLE I1 I
Change in the Water Content of Alumina "A" a t 100" C with Various Amounts of Water Vapor in the Air Water vapor per liter of air (mg)
Water content of alumina a t equilibrium (%)
n
2.69 2.i2 2.92*
9
17 31
24
16
8
0 IIRYlNG TIME IN HOURS
Fig. 2 Curves showing the Experimental Rate of Drying of Crystalline Aluminum Hydroxide in Air containing about 5 hIilligrams of Water Vapor per Liter. ( A ) . Temperature elevated from 145' C and maintained a t 157" C. (E) Temperature elevated from 178" C and maintained a t 196" C.
* When the alumina was again brought to equilibrium in air containing 17 mg of H20 per liter, the water content dropped back t o its former value.
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The complete drying curves for the crystalline aluminum hydroxide and for the alumina samples (both before and after "rehydration") in air containing about 5 mg of water vapor per liter, are given in Fig. 1. Absolute equilibrium was not necessarily reached at each temperature, but the experiments were continued until the weight was approximately constant, and the curves of Fig. 2 will give an idea of the rate at which the drying of the hydroxide took place and the duration of the experiments at two given temperatures. As would be expected, the alumina reached equilibrium much more rapidly than did the hydroxide.
3. Conclusions 1. Crystalline aluminum hydroxide in air at ordinary atmospheric pressure and humidity, is not affected by temperatures below 145" C, and remains constant in composition as the trihydrate, A1(OH)3, up t o this temperature. The decomposition and evolution of water starts just above 145" C, and as far as the evolution of the chemically combined water of the trihydrate is concerned, is practically complete a t 200" C. (It is not known whether the slight irregularities in the drying curve between 145" C and 196" C have any particular significance, but it is thought that they are due t o the very slow rate a t which equilibrium is reached at these particular temperatures, and to the resultant experimenta1 errors.) All the water is not driven off at 200" C, but an amount equal t o about 8% of the original A1(OH)3is retained at this temperature, and is driven off slowly as the ignition temperature is increased, very much higher temperatures being required for complete dehydration. However, above 200" C the curve has the general form of an adsorption curve, as far as the experiments were carried, and the 8% of water which is retained a t 200 " C seems, therefore, to be practically adsorbed by the highly porous Al2O3resulting from the decomposition of the original A1(OH)3. The evidence does not show the existence of any series of hydrates formed by stepwise dehydration. The dehydration corresponding to a given
'
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temperature, as found in this work, is somewhat greater than that obtained by previous investigators who did not continue their experiments for nearly so long a time at one temperature. 2. When alumina, produced by drying crystalline Al(OH), a t a temperature as low as even 275” C, is allowed to take up water, this water is simply adsorbed, and does not recombine chemically with the alumina. The drying curve after “rehydration” shows that this is the case, and bears no resemblance whatever to the original dehydration curve of the A1(OH)3. Hence, for a given sample of alumina the amount of water contained at equilibrium a t any temperature will depend on the temperature and the amount of water vapor in the air (neglecting differences due to the sluggish equilibrium, and working at temperatures far enough away from the temperature a t which the alumina was originally ignited, so that the original adsorptive capacity of the alumina is not altered). In general, the higher the ignition temperature, the smaller the amount of water which the alumina can adsorb under a given set of conditions. No experiments were made with gelatinous, ammoniaprecipitated, hydrated alumina. Previous investigations, however, have shown that this material is not a well-defined compound, but that its composition varies according to the conditions of precipitation; and it does not behave like a definite hydrate during drying. Should this material be ignited a t a dull red heat and then “rehydrated,” drying curves of the adsorption type, similar to those found in this work, would undoubtedly be obtained. Prof. W. J. Mead of the University of Wisconsin, made an X-ray examination of the crystalline aluminum hydroxide of alumina prepared from it by ignition, and of this alumina after “rehydration.” His results are in accord with the conclusions of this paper, and may be summarized as follows: The X-ray pattern of the original crystalline aluminum hydroxide is identical with that of the mineral gibbsite, which is the trihydrate Al2O3.3 H z 0 (or AI(OH),), and is a definitely crystalline, chemical compound. Alumina, prepared by cal-
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cination of the crystalline hydroxide at 325" C gives no trace of the original trihydrate structure; and it is similar, neither t o the mineral diaspore which is crystalline mono-hydrate A1203.HzO, nor to corundum which is crystalline anhydrous A1203. It does, however, show a distinct set of lines which are indicative of a crystalline condition. The adsorption of water does not alter its structure, and none of the trihydrate is produced. Alumina prepared by calcination a t 600" shows a faint pattern similar t o the preceding sample, but the bulk of the material is probably amorphous. When calcination temperatures somewhat above 1000O are employed, the product gives the pattern of .corundum faintly, and still higher ignition temperatures increase the intensity of this pattern until it equals that of pure corundum.
