M. K. B. DAYAND V. J. HILL
946
on the transformation of CY- into p-o-nitroaniline provides a study of a monotropic system. They studied this conversion in the temperature range 273 to 313°K. They observed an experimental energy (Arrhenius) of activation which is 17,300 cal./g. mole. and corresponds to AH* = 16,700 cal./g. mole. As they point out, this is very close to the measured heat of sublimation (19,000 cal./g. mole for the p-form) and the transformation proceeds essentially through the vapor phase. Their work also includes an instructive theoretical consideration of this process. It is instructive to examine equation 6a for the case of applied stresses. The case we shall consider is that of a pV term. This equation should then be written
$ = XXk’l exp [ - ( p
- l)AV*/RT]
{ 1 K-l exp [ ( p - l)AV/RTI 1 (8) The term AV* is the change in volume in going to the activated state and the term AV is the change in volume per mole of the products less the reactants. It should be noted that these terms may op+-
Vol. 57
pose each other. An example is the graphite diamond transition. In this case the equilibrium is displaced in favor of diamond by high pressures but the term AV* is large and positive so that the rate of the transition is reduced by increasing pressure.21-23 Another case similar to the above may be formulated about the well-known recrystallization often observed in metals. In this case, the larger grains of the metal grow at the expense of smaller ones. The driving energy is the high surface energy of the grains and the reaction goes in such a way as to yield large grains, thus minimizing the number of surface atoms and so the surface energy of the grains. Acknowledgment.-This research was supported by the Office of Naval Research under Contract N7-onr-45101, Project Number NR-032-168 with the University of Utah. We wish to express our thanks for this support. . )
(21) P. W. Bridgman, J . Chem. Phys.. 15, 92 (1947). (22) D. P. Mellor, Research. 2 , 314 (1949). (23) H. Eyring and F. Wm. Cagle, Jr., Z. Eleklrochem., 56, 480 (1962).
~
I
THE THERMAL TRANSFORMATIONS OF THE ALUMINAS AND THEIR HYDRATES BY M. K. B. DAYAND V. J. HILL Research Laboratories, The British A l u m i n u m Co., Ltd., Chalfont Park, Gerrards Cross, Bucks, England Received April 18, 1066
It has been concluded that the monohydrate bohmite, encountered in the dehydration of both gibbsite and bayerite, is produced by secondary reactions between the original dehydration products, which are virtually anhydrous aluminas, and the water which is released during the dehydration. Bayerite and bohmite yield on calcination y-AlzOa which gives rise to the 6-, e-, a-forms of anhydrous alumina. Gibbsite dehydrates primarily to x-Al,Oa which on further calcination passes to the stable a-form. In practice, however, the occurrence of a secondary reaction between x-AlzOa and through K-AIzO~ water leads to the formation of some bohmite RO that on subsequent calcination mixtures of the two series of anhydrous aluminas are obtained.
Although considerable attention has been devoted in recent years t o the study of the different forms of alumina and its hydrates, the literature is still confused, particularly in the interpretations made of the relationships existing between the hydrated and anhydrous forms. In a previous note1 we concluded that the initial product of the dehydration of either trihydrate is an anhydrous oxide, x in the case of gibbsite and y in that of bayerite, both oxides being capable of undergoing rehydration. Our conclusions were embodied in the diagram reproduced below showing an idealized alumina-water system. It will be seen from the diagram that the apparent complications associated with the calcination of gibbsite result from the formation of bohmite, by a secondary reaction, which gives rise to a separate and distinct series of anhydrous products. Brown, Clark and Elliott2 have since come to a somewhat similar conclusion relating to the origin of the two series of anhydrous products. They postulate, however, that gibbsite decomposes (1) M. K. B. D a y and V . J. Hill, Nature, 170, 639 (1962). (2) J. F. Brown, D. Clark and W. W. Elliott, J . Chsm. Soc., 84 (1963).
through two routes, either via bohmite or x-alumina, without there being a full understanding of the factors controlling such a decomposition. The purpose of this paper is to amplify the information given in our previous publication, and to demonstrate that the dehydration processes are, in themselves, fundamentally simple, confusion only arising through the occurrence of secondary reactions. It is, incidentally, shown that the hypothesis of two routes for the initial decomposition of gibbsite or of bohmite is unnecessary. A convenient and informative approach to the study of the interrelationships existing between true hydrates and their corresponding oxides is to investigate the pressure-temperature relationship during the dehydration of the hydrates. Thus Milligana obtained a dehydration isobar for a gibbsite, prepared by the Bayer process, which indicated marked instability of the trihydrate phase at about 145’ at ordinary atmospheric pressure and humidity. Dehydration proceeded smoothly until the system approached the composition of a hemihy(3) L. H. Milligan, Tvrs JOUBNAL, 26,247 (1929).
Dec., 1953
THERMAL TRANSFORMATIONS IN ALUMINA
I
947
pzq
drate. Weiser and Milligan4 reported a Gibbsite Rehydration similar curve for a synthetic gibbsite, the below 140" dehydration continuing smoothly to a Dehydration Dehydrati composition of A1&0.35-H20. In neither above 150 o Dehydratio? above 145 3. case did an inflection reveal the presence above 450 IDijhmiteI 7 r-Al,Os x - ~ ~ z ~ Rehydration y of a monohydrate. The product gave, however, an X-ray diffraction pattern charRehydration I Calcination acteristic of the monohydrate bohmite Calciiation c which persisted until the dehydration had been carried to 400'. The isomer, bayerite, gave a similar dehydration isobar, the 3. tempeiature at which decomposition occurred being somewhat lower than that c for gibbsite. The present authors have carried out dehydrations of these hydrates a-AlnOa in a vacuum and have confirmed the obFig. 1.-Idealized alumina-water system. servations of Weiser and Milligan. The presence of bohmite in the dehydration prod- a,?.,' although employing a different nomenclature, uct, and the absence of an inflection in the dehydra- confirmed that these transformations occurred in tion isobar a t the bohmite composition may be ex- the calcinations of bayerite and of bohmite, whether the latter was prepared by precipitation or as a plained in a number of ways. Weiser and Milligan attributed the absence of an product of the low temperature dehydration of inflection in the isobar to the formation of an inter- bayerite. They reported, however, that gibbsite mediate colloidal phase of bohmite which is able to on dehydration yielded a form of bohmite which on lose water below the true decomposition tempera- subsequent calcination gave rise to a new anhydrous was charture of the mass of the hydrate. A second explana- form which they called x-AIzOS; x-A~zOS tion of the absence of the inflection at the monohy- acterized by the presence of extra lines on the patdrate stage and the incompleteness of dehydration, tern previously attributed to y A I 2 0 3 . On further yielded another polymorph, desigis that the trihydrate dehydrates directly to an an- heating X-AIZO~ hydrous alumina, which adsorbs water and subse- nated ~-AlzOa,before final conversion to a-Al&. quently partially rehydrates to bohmite. Some Rooksby also confirmed that some samples of gibbsupport for this argument is available from the site have yielded x- and K - A ~ &on calcination, but work of Bentley and Feachems who demonstrated has drawn attention to the fact that the American that an anhydrous alumina was capable of rehydra- K-form is really a mixture of the true K-form with tion in steam to bohmite. If this is the mechanism O-A~ZO~. The confusing feature of this work lies in the fact of the dehydration, then the proportion of bohmite that bohmite can apparently give rise to two series in the product would be influenced by the conditions of the dehydration, i.8. those favoring adsorp- of transformationsin the anhydrous alumina region, Origin* tion would lead to a higher concentration of bohm- depending On ite in the dehydration product. Certainly the deExperimental hydration of gibbsite in a closed system leads to a Gibbsite used in these investigations was a commercial Bayer product having a loss of 34.6% on ignition and congreater yield of b i j b i t e than is obtained under taining 0.28% NarO. Bayerite was produced by the method Vacuum conditions. The results of X-ray Crystal- of Fricke and Severins8 in which the rapid decomposition lographic examination of the products of the dehy- of a sodium aluminate solution was initiated by carbon didrations and subsequent calcinations allow a dis- oxide. The product was filtered and washed until as free possible from soda; it showed, after drying at l l O o , a loss crimination between the two dehydration mecha- as of 34.6% on ignition. The bohmite used as a starting nisms just discussed. material was produced by the autoclaving of commerci$ Recent investigations of the phase transforma- gibbsite with dilute caustic soda at a temperature of 190 , 15.O % showed theoretical. a high loss on ignition, L e . , 16.7% against during the thermal decompos~tion and tions of alumina hydrates have shown that Z i series Of Small quantities of materials were used for calcination. distinct changes in crystal structure are associated Powder was spread thinly on a silica dish and calcined in a with the growth which takes place prior muffle furnace controlled potentiometrically. Rehydrations were carried out in sealed glass tubes which were heated in a to the formation of the stable a-mzoa or corundum. thermostatically controlled air oven. X-Ray diffraction photographs were taken in either a RooksbyB identified several anhydrous forms and or 19.0 cm. diameter Debye-Schemer powder camera of reported that the initial products of the dehydra- 9.0 tion of either gibbsite or bayerite, ie., bohmite and the Bradley type using Copper Ka:filtered radiation. Results Y - A I ~ O were ~ , converted entirely to r-A1203 on subThe various heat treatments employed and the sequent calcination, and this then passed through crystal forms designated &AI203 and 8-M2O3before products obtained are shown in Table I. The corultimately yielding a-A1203. Stumpf, Russell, et responding X-ray diffraction patterns are reproduced in Fig. 2 and the X-ray diffraction data for (4) H. B. Weber and W. 0.Milligan, {bid., 36, 3010 (1932); ibid., the anhydrous forms are given in Table 11. 38,1175 (1934). (5) F. J. L. Bentley and C. a. P. Feachem, J . SOC.Chem. Ind., 64, (7) H. C. Stumpf. A. S. Russell,J. W. Newsome and C. M. Tucker,
I
~
1
I
+
I
F]
I
148 (1946). (6) H.D. Rookaby, "X-Ray Identification and Crystal Structures of Clay Minerals,'' MineralogicalSociety, 1951, p. 244.
I
pi5-l
Ind. En& Chem., 42, 1398 (1950). (8) R. Fricke and H. Severins, 2.unorg. Chsm., 175,249 (1928); ibfd., 179, 287 (1929).
M. K.B. DAYAND V. J. HILL
948
VOl. 57
TABLE I SERIESOF ANHYDROUS ALUMINAS OBTAINED FROM CALCINATION OF ALTJMINIUM HYDRATES Heat treatment
Bbhmite
Gibbsite
1 5 hr. at 300' Bohmite 2 3 hr. at 500" y with 400 line split 3 Treatment 2 plus 1 hr. a t 600" y with 400 line split 4 Treatment 3 plus 1 hr. a t 700" y with 400 line split 5 Treatment 4 plus 1 hr. a t 800" 7-6 6 Treatment 5 plus 1hr. at 900" 6 7 Treatment 6 plus hr. a t 1,000" 6 8 Treatment 6 plus 1 hr. at 1,000" 6 +e 9 Treatment 8 plus 1hr. a t 1,100" a little 0 10 Treatment 9 plus 1hr. at 1,200" (Y 11 Treatment 10 plus 1 hr. a t 1,300" 12 Treatment 11 plus 2 hr. a t 1,300' a 400 line is split, and the 440 line is beginning to split and give present in the mixture is gradually being transformed into 6.
+
Bohmite
+ xa y + x0
+ major x lines
y
y 7 Y
Y Y y
+ xa
+ x" + x"
(6 +e)
~ + e
Bsyerite y
Bohmite
+
with 400 line split
7-6
+ (x +
6
K)
8+8
~ + e + a
0
+ trace a
a
+ trace
e + little
a
an extra line a t higher d-value-in
other words the
y
TABLE I1 X-RAYDIFFRACTION DATAFOR ANHYDROUS ALTJMINAS d
4.54 2.79 2.388 2.277 1.982 1.518 1.394 1.22 1.139 1.02 0.989 .910
,884 .832 .so7
6
Y
6
I
hk1
d
1
hkl
3 2 7 5 9 2 10 1 3 1 2 7 1 7 4
111 220 311 222 400 511
4.49 2.85 2.72 2.61 2.43 2.275 1.985 1.946 1.79 1.521 1.398 1.388 1.138 0.993
3 3 5 1 7 6
111
440 533 444 731 800 840 844
8 5 1
3 3 10 2 2
311 222 400 004 240 333 440 404 444 800
d
e
5.44 4.51 3.46 2.83 2.718 2.62 2.436 2.305 2.250 2.016 1.948 1.902 1.794 1.756 l.728 1.614 1.561 1.536 1.509 1.482 1.450 1.425 1.399 1.382 1.293 1.279 1.254 1.234 1.190 1.155 1.107 1.064 1.041 1.028 1.007 0.997
y-Component beginning to transform into 8.
* &Values of principal 8 lines are underlined,
K
I
2 3 1
7 10 2 9
8 7 8 2 6 4 2 3 1
2 5 1 4
5 2 5 10 1
2 2 1 ? 1 1 1 1 1 3
3
d y + x I
4.50 1 2 2.82 6 2.394 2,271 6 2.11 4 7 1.983 1.965" 3 1 1.88 2 1.52 1 1.44 1.393 10 1 1.136 2 0.992 1 .883 4 .806
f 8'
d
6.11 5.42 4.47 4.19 3.17 3.04 2.84 2.79 2.73 2.57 2.42 2.31 2.26 2.16 2.12 2.06 2.00 1.94 1.90 1.868 1.825 1.790 1.747 1.70 1.632 1.60 1.574 1.537 1.483 1.447 1.430 1.400 1,386 1.337 1.306 1.284 1.262 1.241 1.217
-
-
-
-
-
-
I
3 1 4 1 1 6 2 7 6 9 6 6 2 1 8 3 5 2 1
5 2 1 2 1 6 1
d
3.47 2.55 2.371 2.14 2.078 1.95 1.735 1.596 1.540 1.508 1.400 1.369 1.335 1.273 1.237 1.235 1.186 1.16 1.144 1.123 1.096 1.08 1.075 1.040 1.014 0.995
a I
8 9 8 1 10 1 8 10 2 4 7 10 2 3 5 4 4 1 3 3 4 1 4 6 3 5
hR1
102 104 110 006 113 202 204 116 121 108 214 300 125 208 lO(10) 119 220 306 223 312 20(10) OO(12) 314 226 402 21(10)
1
3 3 4 7 1 10 3 3 1 3 1 2
Since confusion can arise when reference is made owing t o the fact that workers in this country and to the published work on alumina and its hydrBttes America have adopted different ayatem of nomen-
Dec., 1953
THERMAL TRANSFORMATIONS
IN
ALUMIN.4
949
site at 300", however, gave a product whose Xray diffraction pattern showed, in addition to the characteristic lines of bohmite, extra haloes a t dvalues of 2.11 and 1.39. As illustrated in Fig. 2, the former is in the same position as the main characteristic x line, whereas the latter coincides TABLE m with one of the two most intense Iines of the ySYSTEMS OF NOMENCLATURE A1203pattern. The fact that the other principal Alumina hydrates Anhydrous aluminas r-Al2O3line was absent led us to conclude that the Mineral American British American British system system system system two haloes observed were the principal ones of true Bahmite a-Monohydrate 7-Monohydrate 7 Y x-A1203. At 500" all traces of bohmite had disapDiaspore &Monohydrate a-Monohydrate y 6 peared and a pattern, previously designated x-Al203 Gibbsite a-Trihydrate 7-Trihydrate 6 6 +e by Stumpf and Russell, was obtained. It will be Bayerite 8-Trihydrate a-Trihyd :ate e e a a seen that this pattern corresponds to the accepted K x + r y-A1203pattern with extra lines at d-values of 2.11, K ~ + e 1.88 and 1.44. This suggested to us that the preThe results of the rehydration experiments are viously reported x-Al203 was in fact a mixture of two oxides, ie., true x-A1203and y-Alz03,the former recorded in Tables IV and V. having its own characteristic X-ray pattern at least Discussion one line of which coincides with one of the principal It will be seen that bayerite and bohmite gave lines of the y-Al203 pattern. We interpreted the rise to the same series of aluminas as described by diffraction patterns of the products of subsequent Rooksby and Stumpf, Russell, el aE., i e . , r-A1203 calcinations in the light of this conclusion. On furwas first observed and this passed through the 6- ther calcination the y-component of the (y x) and &forms to a-AlzOa. The calcination of gibb- mixture began t o be transformed into 6-AlpOa;aubclature, and, in our opinion, some of the anhydrous aluminas identified by Stumpf and Russell are in fact mixtures of two different forms, the relationships between the two nomenclatures are shown in Table 111.
+
M. K. B. DAYAND V. J. HILL
950
Vol. 57
TABLE IV PRODUCTS OF CALCINATION AND REHYDRATION TREATMENTS Rehydration treatment
5 hr. at 120" 70 hr. a t 120" 24 hr. at 140' 5 hr. a t 170"
Aliirnina (with 400 line split) P r o L t of Product of calcination of calcination of bayerite a t 500' bohmite a t 500'
+y +x erite As above, but rather y + little bohmite + Bohmite + bayerite less y little bayerite + some (7 + x ) Bohmite + little y + y + little bohmite Bohmite + little (y trace bayerite + x ) f trace bayerite Bohmite + little y y + little bohmite Bohmite + trace (7 + x ) y
+ bohmite + bay-
Unchanged
+
+
x) Alurninas Products of calcination of gibbsite a t 500°
(y
Bohmite
Bohmite x-alumina Products of calcination of gibbsite a t 300"
Unchanged
Bohinite
Unchanged
+-
Bohmite little bay- Unchanged erite trace x Bohmite Unchanged
+
Bohmite
Unchanged
TABLE V FURTHER REHYDRATION EXPERIMENTS Material
Pure 7-alumina
7-Alumina derived from bayerite ydlumina derived from bohmite
Result of rehydrttion 24 hr. a t 130
Previous treatment
Product of calcination at 700" of hydrate obtained by hydrolysis of aluminum s-butoxide Product of calcination of aluminum sulfate a t 1,000" Product of calcination of bayerite a t 500" Product of calcination of bayerite at 500" rehydrated for 5 hr. a t 170", and subsequently calcined at 500" Product of calcination of bohmite a t 500" Product of calcination of bohmite a t 500" rehydrated for 96 hr. at 170",and subsequently calcined a t 500'
sequently a product was obtained which was a mixture of true K - A ~ ~(derived O~ from x-Alzo3) and O-AlzOa (derived from 6 and y ) , both K- and eA1203 ultimately yielding a-AI2O3. It will be seen from Table IV that the bohmite/ x-AI2Oa mixture derived from the calcination of gibbsite a t 300" yielded some bayerite on rehydration a t 120". On the other hand, above 140" bohmite only was produced. Since bohmite is unaffected by the treatments, it would seem that xA1203 is capable of rehydration to bayerite a t temperatures below 140" and to bohmite a t higher temperatures. The y-Al203 obtained by the calcination of either aluminum sulfate or the product of hydrolysis in boiling water of aluminum s-butoxide yielded only bohmite on rehydration a t 130". This is in accord with the scheme outlined in Fig. 1. However, the y-A1203derived from bayerite or commercial bohmite gave on rehydration bohmite mixed with a little beyerite. It seems likely that this apparent anomaly arises because, from the nature of their preparation, the starting materials in the last two cases could well be contaminated with a trace of gibbsite; this would yield x-A1203on calcination and bayerite on subsequent rehydration. Some support of this explanation is available from the results given in Table V. When y-A1203derived from bayerite or bohmite was rehydrated for a long time a t 170" (to convert any x-A1203present to bohmite), recalcined a t 500" and subsequently rehydrated a t 130" no bayerite could be detected. We therefore conclude that: (a) Bayerite and bohmite on calcination yield y-Al203 which is highly adsorbent and which on further calcination passes through the 6- and 8-forms before finally yielding a-AlZO3. (b) y-AloO3 is capable only of
+ trace y Bohmite + y Bohmite + bayerite + little y Bohmite + y y + little bohmite + little bayerite + little bohmite Bohmite
y
rehydration to bohmite. (c) Gibbsite, ideally, yields x-A1?03,on dehydration which is also highly adsorbent. (d) x-&o3, which has its own characteristic X-ray pattern a t least one line of which coincides with one of the principal y-&o3 lines, on further caIcination yields K-&O~before conversion to a-A120,. (e) x-Al,O, is capable of rehydration below 140" to bayerite, and above 140" to bohmite. (f) In practice, calcination of gibbsite yields a product which is a mixture of xA1203and bohmite derived from the secondary reaction of the highly active x-A1203with its adsorbed water. Subsequent calcination therefore gives rise to mixtures of the two series of anhydrous forms, i.e., y- 4 6-, 0- --t a-A1203and x- -t K- 4CYAlzOa. These conclusions are embodied in Fig. 1, from which it is apparent that the products obtained from the calcination of gibbsite are determined mainly by the conditions of dehydration. Thus, in a closed system where the water vapor is retained in contact. with the x-A1208,complete conversion to bohmite occurs, and at higher temperatures b 6 h ite subsequently gives rise t? the y- + 6- + 0- -t cr-A1203 series of transformations. Under the conditions of the present experimental work, where the bulk of the water vapor was removed rapidly from the system although some adsorbed water must have remained for an appreciable time in close contact with the active alumina, only partial rehydration occurred and mixtures of the anhydrous aluminas characteristic of the two series were obtained. Similarly, partial rehydration of the r-Al203 derived from bayerite occurs with the formation of bohmite. We are indebted to Mr. H. P. Rooksby of the Research Laboratories of The General Electric Company for his interest and helpful discussion.
