INDUSTRIAL AND ENGINEERING CHEMISTRY
1398
Frilette, V. .J., Hanle, J., and Mark. H., J . -4771. Chem.
(7)
Soc., 70,
1107 (1948). ,8)
Gardiner, K. W., and Smith, L. B., J . A m . 0 2 1 Chemzsts’ Soc., 26,
(9)
Hanson, J., Neale, S. M.,and Stringfellom, IT. A4.,Trans. Para-
194 11949). _. \ - - - - , ~
day Soc., 31, 1718 (1935). R . E., and 1-anko, \IH., . -4STM Bull. 158, 49 (1949). ( 1 1 ) Hensley, J. W., Long, -4.O., and Willard, J. E.. J . A m . Cham.
(10)
Harris, J. C., Kamp,
Soc., 70, 3146 (1948). (12) (13) (14)
(15) (16) (17)
Hermans, P. H., “Physics and Chemkry of Cellulose Fibers,” New York, Elsevier Publishing Co., Inc., 1949. Heuser, E., private communication. Heuser, E., “The Chemistry of Cellulose,” S e w Tork. John Wiley & Sons, Inc., 1944. Honnegger, E., and Schnyder, A , , J . TesiiZePnst., 34, T29 (1943). Howsmon, J. A., Testile Research J . , 19, 152 (1949). Kinsinger, R’.G., and Hock, C. IT.,IXD.EBG.CHEM.,40,1711 (1948).
(IS) Lambert, J. M., J . Colloid Sci., 2, 479 (1947). (19) (20)
Lambert, J. A t . , and hckerman, B. J., unpublished work (1946). Lambert, J. M., and Sanders. H. L., IND. ENG.CHEY.,42, 1388 (1950).
Lapp, R. E., and ilndrews, H. L., “Nuclear Radiation Physics,” Sew York, Prentice-Hall. Inc., 1948. (22) Lauer, K., Kolloid Z . , 107, 86, 93 (1944). (23) McBain, J. W.,“Advances i n Colloid Science, I,” edited by Kraemer, E. O., pp. 99-142, Sen- York. Interscience P u b lishers, Inc., 1942. (21)
(24) Mark, H., J . Phys. Chem.. 44, 564 (1940). (25) Murdison, 11. E., and R o b e r t s , J. S.,J . T e z t i l e I n s t . , 40, T5O5 (lS49I. ---, (26) Xeville, H. A., and Harim >I , J . Rrsea,ch S a t l . Bui. S t a n d a r d s , 14 765 (1935). \
Vol. 42, No. 7
Neville, H. 4., and Jeanson, C. A., J . Phys. Chem.,37,1001 (1933). Ott, E., et al., ed., “High Polymers,” Vol. V, “Ceilulose and Cellulose Derivatives,” New York, Interscience Publishem, Inc., 1943. (29) Paosu, E., Teztile Research J., 15, 354 (1945). (30) Peirce, F. T., Trans.F a r a d a y SOC., 42, 545 (1946). (31) Peters, L., and Speakman, J. B., J . Soc. dyer.^ Colouvists, 65, 6:3
(27) (28)
(1949).
Race, E:, Ibid., 65,56 (1949). Reid, A. F., “Preparation and Measurement of Isotopic Trarers,” edited by Wilson, D. TI-., et aE., pp. 83-108, Ann A r b o r , J. 1%’. Edwards, 1946. (34) Rhodes, F. H., and Brainard, S. UT., IND. ENG.CHEM.,21, 60 (32) (33)
(1929).
Schwartz, A. M.,and Perry, J. W ~“Surface , Active Agents,” pp. 316-84, New York, Interscience Publishers, Inc., 1949. (36) Sookne, A. hl.,and Harris, M., J . Research Natl. Bur. S t a n d n r d s ,
(35)
25, 47 (1940); 26, 65, 205 (1941). (37) (38)
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(39)
Tjutjunnikow, B., and Pleschkowa, S., Allam. Oel- u. Fett.-
24, 335 (1940). Ztg., 31, 59 (1934). (40) U. S. Atomic Energy Commission, Isotopes Division, Oak Ridge, Catalogue and Price List No. 3, 1949. (41) Valko, E., “Kolloidchemische Grundlagen der Textilveredlung,” Berlin, Julius Springer, 1937. (42) Walke, H., Thompson, F. C., and Halt, J.. Phys. Rec., 57, 177
(1940). (43)
Ward, K., Jr., Am. DyestuflRrptr.. 38, 122 (1949).
RECEIVEDSoveinber 21, 1949. Presented before the Division of Colloid Chemistry a t the 110th Meeting of t h e AMERICAN CHEXICAL SOCIETY. Atlantic City, h-.J.
Thermal Transfo ations of Aluminas and Alumina Hydrates J
H. C. STU31PF. ALLEN S. RUSSELL, J . W. NEWSO>IE, AND C. 31. TUCKER Aluminum Company of .4nieric*u, Vetr liensington, P a .
T h e phase transformations occurring in the thermal decomposition of the four alumina hydrates have been examined by x-ray powder diffraction analysis. A series of distinct patterns has been identified in the region which previously has been designated loosely as y-alumina. The seven crystalline modifications of the nearly anhydrous aluminas from heating pure alumina hydrates are arbitrarily designated as a-, 7-,6-, 7-, e-, K - , and X-alumina.
7-4lumina has a cubic, spinel-type structure; y-, 6-, e-, and K-aluminas are not cubic. The transformation sequences and temperatures are shown for various samples of the hydrates heated both in dr) air and in steam. 411 interesting feature of these tranbformations is that a-alumina monohydrate J ields differing phases on dehj dration when formed by direct precipitation, by dehydration of a-trihydrate, or by dehydration of p-trihydrate.
T
in t,he late trventies ( 1 ) . T h r hydrate which is produced by weding and autoprecipitation in the Bayer process is termed a-alumina trihydrate; it gives the same x-ray pat,tern as t h e mineral gibbsite. Another trihydrate produced by rapid precipitation from sodium aluminate solution is termed p-alumina trihydratr; it has been called bayerite by German writers, although it is not a product of the Bayer process. There are also two monohydrates. The one commonly occurring in certain types of bauxite ore is termed a-alumina monohydrate, and has also been given the name of boehmite. The second form is termed p-alumina monohydrate and corresponds to the mineral diaspore. In the case of the aluminas the terminology does not follow any regular pattern, inasmuch as the several forms were not discovered in any logical order and the names p- and (-alumina have been adopted for aluminas containing sodium and lithium.
HERMAI, decomposition of the various alumina hydrates yields a number of crystalline variations of alumina which are transition stages in a process eventually yielding corundum: The sequences of these transformations have been fairly well known a t these laboratories for a number of years, but recently a new investigation was conducted to verify these and to make certain that the forms of alumina which were recognized were distinct. This paper presents the results of some of the x-ray powder diffraction studies, illustrating the changes in crystalline form which occur when the hydrates are heated at temperatures up to 1200” C. Weight losses and surface areas as determined by hutane sorption for these same specimens are reported by Russell and Cochran ( 5 ) . The terminology used in referring to the alumina hydrates is essentially that adopted by Aluminum Research Laboratories
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July 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
1 eo0
c.
1100
c.
1000O C
800" C.
600° C.
400" C
PO00 C.
R
T.
Figure 1. Powder Diffraction Patterns
1399
impurity. @-Alumina trihydrate was precipitated rapidly with carbon dioxide from sodium aluminate solution a t 40" C. It had a loss on ignition of 35.5y0 and in addition to its excess water contained impurities equivalent to 0.28% sodium oxide, 0.10% silica, and a small amount of calcium oxide. a-Alumina monohydrate was prepared by digesting the CY-trihydrate in water a t 200" C. Ita loss on ignition of 16.9yG exceeded the theoretical value (15.0%). It contained no other significant impurity. The fact that the water content of the a-monohydrate was higher than the theoretical value is characteristic. It is difficult to ascribe the excess water to sorption, in view of the low surface area of the sample. One per cent of w-atei would cover an area of 30 square meters t o a monolayer thickness. The monohydrate might contain a small amount of trihydrate that is not detected b j the x-ray. The high water content of the p-trihydrate is less characteristic. This particular sample gave the same initial pattern and transformation sequence as one from the action of water on amalgamated aluminum for which the water content was nearly the theoretical and the impurities were negligible. The only &alumina monohydrate available in sufficient quantity for
Ditierences in crystalline form of products obtained when a-alumina trihydrate is heated for 1 hour at temperatures indicated
'L'he forms CY-, y-, 6-, e-, and n-alumina have been described previouely ( 2 ) . The two additional forms described here have been given the arbitrary names 7-alumina and x-alumina.
eo0 =
c.
100
c.
000
c.
MATERIALS AND PROCEDURE
The major effort in the x-ray program was devoted t o three particular samples of hydrates, selected because of relatively high purity and low surface area. Similar but not identical samples were used in experiments involving extended heating. *Alumina trihydrate was Bayer process hydrate prepared by precipitation from sodium aluminate solution so as to obtain a low iron content. Its loss on ignition (1100" C.) \$as the theoretical 34.7%, and combined sodium equivalent to 0.38% sodium oxide was its only significant
-+ Figure 2. Powder Diffraction Patterns Differen- in crystalline form of products obtained when ,¶-alumina trihydrate is heated for 1 hour at temperatures indicated
1000 C.
,000 C.
I 0 0 0 C.
!OO" C.
!. T.
1400
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c.
1eo0l o
11ocl o
c.
1000
c.
800' C.
600' C
Vol. 42, No. 7
The procedure used in preparing specimens was the following: Three-gram samples of the hydrates were tableted without any binder, and dried for 16 hours a t 110" C. This temperature is sufficient t o eliminate unbound water, but too low to permit decomposition of the hydrates. Specimens mere then placed in nickel boats and pushed into a preheated furnace containing a quartz tube 1-inch in diameter, through which the desired preheated gas (dried air 3 cu. feet per hour or steam) was passing. They were held for 1 hour a t temperatures measured with a Chromel-Alumel thermocouple in the boat. Weight losses were determined and surface areas mere measured by butane sorption, after which the samples were submitted to x-ray powder diffraction analysis. The long-period experiments involved heating in room air a t 250 300 ', 350 ",and 400 O C. until the specimen weights changed less than 0.15% in 50 hours, times ranging from 200 to 640 hours being required, O,
400' C.
Although the partially dehydrated aluminas have a strong tendency to adsorb water, the recrystallization of R. T. these materials to hydrates has never been observed here during short exposure of the samples to room air. HydraFigure 3. Powder Diffraction Patterns tion may occur on exposure to water Differencesin crystalline form of products obtained when a-alumina monohydrate is heated for 1 hour at temperatures indicated vapor at higher temperatures. Pickup of water at room temperature during preparation of the samples for x-ray examination is not believed to influence these results. this study was a sample of diaspore clay which contained Portions of the material were ground, mixed with collodion, over 10% silica and smaller amounts of potassium and titanium and extruded into rod-shaped specimens which were exposed to oxides. Its x-ray diffraction pattern showed only the lines of p-alumina monohydrate. The work with this sample was checked a t a few points by a sample of purer diaspore generously supplied by the Museum of Natural .Al.Oa History, Springfield, Mass. This material, which contained a few per cent of iron oxide, gave results on heating which were similar to those for the diaspore clay. AlrOa One sample was prepared by the controlled hydrolysis of high purity aluminum isopropoxide. It contained approximately 4 moleoules of water per moleAlLOa cule of A1203. Fine particle a-alumina trihydrate (Alcoa hydrated alumina C730) and platelet form a-alumina monohydrate (Alcoa monohydrated alumina D50) were ALOa also investigated. The former had a loss on ignition of 34.670 and combined SOdium equivalent t o 0.16Y0 sodium oxide as the only significant impurity, while the .AliOa latter had a loss on ignition of 1 5 . 4 7 ~ , nearly the theoretical value, but contained small amounts of sodium, silicon, and iron compounds. An impure "gelatinous" -A1.08 a-alumina monohydrate prepared by the neutralization of sodium aluminate containing sodium silicate with sodium bi-AI.Oa carbonate solution had a loss on ignition of 19.6% and contained other principal impurities equivalent to 4.0570 silica, Figure 4. Powder Diffraction Patterns for Seven Modifications of Alumina 0.40% calcium oxide, and 0.33% sulfur trioxide. Obtained b j thermal decomposition of relativel) pure alumina hydrates
1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
400
of the mopohydrate. The effect of prior hydrate structure persists up to the temperature a t which &alumina is formed, eAlumina is an intermediate stage in the formation of a-alumina by decomposition of a- and p-trihydrates and of a-monohydrate, but was not found in the p-monohydrate sequence. pMonohydrate transforms directly to a-alumina, and for very long heating times this change occurs between 350 O and 400 O C. The same crystalline transformations are observed for both short and long heating periods; the effect of long holding a t temperature is merely to lower the transformation temperatures. The long-period specimens were notable for a lack of the amorphous material which coexisted with the short-period decomposition products in varying amount.
200
000 800 600 400 200
0
I a - T R I l+ a
Figure 5.
1401
b
C
d
T r a n s f o r m a t i o n s on H e a t i n g a - A l u m i n a Trihydrate
Bayer process hydrate heated (a)1 hour i n dry air, (b) 1 hour i n steam, (c) fine particle hydrate heated 1 hour i n dry air, ( d ) i n room air t o constant weight A horizontal line a t any temperature shows nature and relative amounts of phases present-at 20O0 C. 75% a-trihydrate and 25% a-monohydrate are present i n (a) Arrows show temperatures for whieh patterns were made
W
E =3 I-
800
E
600
U
filtered CuK radiation in a General Electric camera of 72-mm. radius. The same camera was used for all specimens in order to avoid shift in line positions resulting from differences in camera dimensions.
W
a
3
DISCUSSION
One important result of this work has been the verification of seven nearly anhydrous crystalline alumina forms a s products from thermal decomposition of the alumina hydrates. These forms are not new, and their diffraction patterns have been recognized a t these laboratories for a number of years. The list of crystalline forms of pure aluminas is now: alpha, gamma, delta, eta, theta, kappa, and chi. Both trihydrates lose water a t low temperatures with formation of a-monohydrate. a-Trihydrate goes to a-monohydrate, to X-alumina, tu 7-alumina, to K-alumina, to &alumina, and finally to a-alumina as the temperature is increased. p-Trihydrate goes to a-monohydrate to q-, to e-, to a-alumina. aMonohydraB, if formed from solution, yields y-, then s-, &, and a-alumina. Dehydration of a-monohydrate yields differing series of aluminas, depending upon the mechanism of formation
K-MONO
200
EXPERIMENTAL RESULTS
Representative powder diffraction patterns for the products obtained on heating the hydrates in dry air for 1hour are shown in Figures 1 to 3. Examination of a number of such series has shown only the crystalline forms of alumina whose patterns are given in Figure 4. (The presence of appreciable amounts of alkali or alkaline earth metal impurities will cause additional forms such as p- or I-alumina to appear.) Varying amounts of amorphous material are also found, especially upon initial dehydration. This amorphous material is characterized by a single very broad band at 4.5 A. which is clearly distinguishable from the finely divided crystalline phases. The alumina from isopropoxide hydrolysis was initially completely amorphous, although it developed the pattern of a-monohydrate on heating or long standing. The sequences of transformations for the purer alumina hydrates, the less pure p-monohydrate, and two hydrates of particularly fine crystallite size are shown in Figures 5 to 8. A horizontal line in these diagrams a t any temperature shows the nature and relative amounts of phases present-for example, in Figure 5, a, a horizontal line a t 200" C. shows by its intercept with the phase boundary approximately 75% a-trihydrate and 25% a-monohydrate. Arrows a t the side of each diagram show temperatures for which diffraction patterns were prepared.
400
t-
P-TRI
0 a
b
C
Figure 6. T r a n s f o r m a t i o n s on H e a t i n g @-Alumina T r i h y d r a t e Precipitated hydrate heated (a)1 hour i n dry air, (b) 1 hour in steam, (c)i n room air to constant weight
The effect of steam is complex. In general, steam increases the decomposition temperature of the hydrated phases, decreases the decomposition temperature of the low temperature anhydrous phases, and has no effect on the transformations of the high temperature anhydrous phases. Steam thus seems to promote the formation of the phases with more perfect crystallinity. Transformation temperatures during activation in humid air tend to be intermediate between those for dry air and steam activation. The most striking features of the results for the fine particle hydrates are the exceptional stability of x-alumina in the case of a-trihydrate and of y- and &alumina for the a-monohydrate. This may result from more rapid removal of water during their activation in a dry air stream.
200
000 800
200 0
a Figure 7.
b
C
d
T r a n s f o r m a t i o n s on Heating a - A l u m i n a Monohydrate
Steam-digested trihydrate heated (a) 1 hour i n dry air, (b) 1 hour i n steam, ( c ) gelatinous precipitate heated 1 hour i n dry air, T d ) in room air to constant weight
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 42, No. 7
TABLE I. DEBYE PATTERX FOR ALUMINAS Interplanar spacings, d. f o r the aluminas. Relative intensities were estimated visiially, a n d are indicated by the following symbols: Strong ms Medium-strong m Medium Medium-weak mw \V Kea k T.WVery weak \.-k'\V T'ery, very weak -4 question mark indicates a n extremelv weak line for which the d-valui? could n o t b e measured accurately. A niiir;ber of such lines are reported, because t h e y were found consistently in the Debye patterns of the particular modification of alumina. Filtered CUI
