2050
R. SH. MIKHAIL
T‘ol. 67
ACTIVATIOX ASD SIXTERING BEHAVIOR OF CALCIUM OXIDE-THE EFFECT OF HYDRATIOK ON THE SURFACE AREA OF THE OXIDE PRODUCED BY THERMAL DECOMPOSITION BY R. SH. MIKHAIL Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt Received March 4, 1963 The activity of calcium oxide produced by the thermal decomposition of the hydroxide has been studied by determining the changes occurring in the specific surface. Effects of temperature of preparation, duration of heating, and presence or absence of air during the calcination process are examined. In parallel with earlier observations on the decomposition of some solids, the decomposition of calcium hydroxide results in a severalfold increase in specific surface which could be ascribed to an increase in the number of crystallites during a recrystallization mechanism folloning the decomposition of the solid. Sintering develops a t all temperatures, but increase of temperature and the presence of “non-dry” air during the decomposition enlarges its extent and larger grain size is produced under such conditions. Vapor hydration of oxide samples differing in surface activity led in all cases to a decrease in the specific area, whereas an increase is to be expected if the number of crystallites remained unchanged. The results indicate that aging or agglomeration of crystallites takes place which leads normally to a decrease in their number and consequently to a decrease in the measured specific area. The role played by the gaseous product C in reactions of the type solid A F! solid B gas C is discussed in relation t o its effect in determining the surface properties of both solid reactants.
+
Introduction Newly prepared lime is usually produced by the thermal decomposition of a parent material such as its hydroxide or carbonate. The process of decomposition often causes physical and chemical changes which result in strong variations in the activity of the product. Several factors are now known to affect the activity of the produced oxide, and previous work’ has shown that the production of an active solid by thermal decomposition is mainly controlled by such factors as the structure of the initial material, temperature and duration of calcination, and the presence or absence of air during decomposition. The reactivity of the produced oxide, particularly in lime-burning practice, is often tested by its tendency to h ~ d r a t e ,and ~ the process of hydration in itself might be expected to cause further chemical and structural changes which would invariably affect the activity of the produced solid. The present investigation is therefore conducted to study the activity of calcium oxide, as measured by the extent of its specific surface area, produced by the thermal decomposition of a parent hydroxide a t different temperatures of preparations and for various durations, in presence and in absence of air. Several produced samples of widely different activities were subjected to hydration from the saturated vapor phase and the variation of their specific area upon hydration was measured. Further decomposition of the produced hydrate was carried out occasionally, in order to throw some light upon the various effects which might play role in determining the activation and sintering of solids by thermal treatment. Experimental Materials.-The parent material was a batch of well developed crystals of calcium hydroxide. It was a part of a hardened paste of dicalcium silicate hydrated a t room temperature for 8 years. The paste was cylindrical with a hollow core in the center, and the crystals of calcium hydroxide were taken from this core. It proved t o be very pure and the total weight loss agreed (1) See e . g . , S. J. Gregg, J. Chem. floc., 3940 (1953). (2) R. I. Razouk and R. Sh. Mikhail, Actes Conur. Intern. Catelyse, Be, Parts, 1.960,2023 (1961). (3) J. W. Mellor, “Comprehensive Treatise on Theoretical and Inorganic Chemistry,” Val. 3, Longmans, London, 1923, p. 657.
within 0.27, with the theoretical loss of 24.3%, and with the ignition loss of the crystals determined in a muffle furnace a t 1050’. The specific surface of this starting material is 2.5 m.2jg. Procedure.-Separate portions of the hydroxide were calcined (a) for fixed times of heating of 5 hr. and ( b ) for various durations of calcination at different temperatures, either zn uacuo or in presence of air, and the surface areas of the products of dehydration were measured by the adsorption of nitrogen a t liquid nitrogen temperature or (occasionally) by the adsorption of cyclohexane vapor. Both ways lead t o the same area value, with a deviation of around a mean average. The thermal treatments were carried out in sztu, i.e., while the sample is connected to the adsorption apparatus, and the water contents were determined in a thermal balance a t the end of each run. To inhibit the uptake of water vapor from atmospheric air during the transfer of material to the balance, the ampoule containing the sample was sealed while being connected to the apparatus under vacuum, transferred to the thermal balance, heated to 150°, and then the neck of the ampoule was broken before heating was continued t o higher temperatures. Complete as well as partial hydration of several oxide specimens was affected by exposure to saturated vapor of n-ater a t 25’ in a silica spring balance system for various durations of exposure, and then transferred to the volumetric apparatus to measure their specific area. The hydrated samples were outgassed a t 150” for 3 hr. to remove any physically adsorbed water before the adsorption runs were carried out. At the end of each adsorption run, the water content was rechecked by the same procedure mentioned above.
Results and Discussion (i) The Thermal Decomposition of the Calcium Hydroxide Crystals.-Adsorption isotherms of both nitrogen and cyclohexane on the decomposition products of calcium hydroxide have shown that in all cases studied the isotherms belong to type I1 of the Brunauer cla~sification,~ exhibiting no hysteresis a t relative pressures below 0.3. The specific area values were obtained from the corresponding isotherms b y using % value for the cross-sectional ?rea of nitrogen of 16.2 A.2,6 and of cyclohexane of 39.0 A.2.6 In Fig. 1 the specific area values are shown for the products of calcination of calcium hydroxide obtained by heating for 5 hr. in vacuo (curve a) and in presence of air (curve b). The results indicate that the surface area of the product increases with temperature of calcination until a maximum is reached at 390’ for (4) S. Brunauer, “Physioal Adsorption of Gases and Vapors,” Oxford Preas, New York, N. Y., 1945. (5) W. D. Harkins and G. Jura, J. Am. Chem. Soc., 66, 1366 (1944). (6) N. Smith, C. Pierce, and H. Cordes, ibid., 72, 5595 (1950).
ACTIVATION AND SINTERING BEHAVIOR OF CALCIUM OXIDE
Oct., 1963
loo 80
.
-?
r 7
60
-
-? E
Q
40
20
200
400 Temp., O C .
600
800
1000
Fig. 1.-Specific area-temperature of calcination curves, for samples heated for 5 hr. Curves a and b represent areas of CaO obtained by heating in vucuo and in air, respectively. Curves c and d represent areas of the hydrated oxides; the oxides have been prepared i.n vacuo and in air, respectively.
60
. > h
ti
40
v
Y
20
20
40
60
80
100
70 Deoomposition.
Fig. 2.-Variation of the specific surface area of the product function of :percentage decomposition a t a fixed temperature: (a), i n air; (v), in vacuo.
a3 a
vacuum preparations and a t 460 O for air preparations, and then the area falls progressively with further rise of temperature. Comparison of both curves a and b in Fig. 1 clearly demonstrates the effect of presence of air during the calcination process in reducing the specific area of the product. Similar effects were observed in this Laboratory during the calcination of magnesium oxide7 and alixminum oxides in presence of air. It is worth mentioning that in all such investigations, no treatment was carried out to dry the air from its water vapor conten.t, and therefore the term "non-dry" air will be used occasionally in the present investigat'ion. As calcination for 5 hr. in air does not always bring about complete delhydration a t temperatures below 450°, and as the surface area of the oxide exceeds by several fold that of the parent hydroxide, a correction should be made to account for the presence of some percentage of the hydroxide in the products of decomposition b'elow 450'. This correction leads to an increase in the calciilated areas below 450°, but as (7) R. I. Raeouk and R. Sh. Mikhail, J . Phys. Chem., 61, 886 (1957). (8) R. I. Razouk, R. Sh. Mikhail, and G. R. Iskander, to be published.
205 1
shown by the dotted line in Fig. 1, does not lead to the disappearance of the maximum obtained, but slightly flattens the peak of the curve, without sensibly altering the common trend. The general behavior so far discussed seems t o be in harmony with the behavior of magnesium oxide,7 aluminum oxidej8 and iron oxideg upon thermal decomposition. Thus the appearance of a maximum in the specific area-temperature of decomposition curve denotes the existence of a t least two main processes, an activation process which predominates a t lower temperatures, and leads to the increase of area with temperature, and a sintering process which predominates a t temperatures higher and beyond the maximum of the curve. Plotting the change in the specific area of the product as a function of percentage decomposition in the lower range of temperatures, where the sintering forces are still insufficient t o show itself over the activation forces, have shown some interesting features which are indicative of the actual mechanism of activation. Thus Fig. 2 shows a nonlinear relation in the specific areapercentage decomposition curve, which in turn denotes that the production of surface area is not a direct consequence of the decomposition of the solid. The resulting curve being concave upwards over the later stages of decomposition denotes that the activation process is a combined effect. As previously postulated by the author,1° and based on some other evidence than surface area measurements, the most important of which are X-ray analysis and electron microscope pictures, activation is mainly due to (i) decomposition, and then (ii) recrystallization of the product to the newly formed phase. The shape of the surface areapercentage decomposition curve depends on the relative rates of both steps, the slower being the rate-determining one. Experience has shown that the specific area-percentage decomposition curve might be a straight line, as in the case of magnesium hydroxide,ll and gypsum,l2 and in such cases the process of decomposition is the rate-determining step, or it might be concave upward in the late stages, if the recrystallization of the product to the new phase lags behind decomposition and becomes rate determining. Examples for the latter case are magnesium carbonatell and lime. Similar results on lime were obtained by G1asson,13with some minor differences, which could be attributed to differences in the details of experimental procedures. I n all cases, the produced active oxide tends to sinter and to minimize its specific surface area. Increase of temperature, prolonged heating a t a fixed temperature, and presence of air,2 all enhance the mechanism of sintering. The late portion of a curve shown in Fig. 2 for the sample treated in presence of air, does not continue to be concave like the other samples treated under vacuum, but nearly flattens in its end portion probably due to an enhanced tendency for sintering in presence of air. The sintering forces in such cases (9) R. I. Razouk, R. Sh. Mikhail, and B. S. Girgis, Advances in Chemistry Series, No. 33, American Chemioal Sooiety, Washington, D. C., 1961, p. 42. (10) R. Sh. Mikhail, Ph.D. Thesis, Ain Shams University, 1957. (11) R. I. Razouk and R. Sh. Mikhail, J . Phys. Chem., 6 8 , 10.50 (1959). (12) R. I. Razouk, R. Sh. Mikhail, and A. Sh. Salem, J . Chem. U. A . R . , in prese. (13) D. R. Glasson, J . A p p l . Chem., 8 , 793 (1958).
2052
R. SH. MIKHAIL
overcome the activation forces before decomposition is complete. By analogy with previous investigations carried out in this Laboratory, the decomposition of calcium hydroxide may lead to the formation of calcium oxide having a pseudo-lattice of the hydroxide, but the formation of the pseudo-lattice does not lead to appreciable changes in surface area until it recrystallizes and collapses to bigger number of crystallites, each possessing the stable form of the oxide. Recrystallization leads to measurable increase in surfare area, but in the niean time another tendeiwy brings the sintering forces to operate at all tinies to create products of smaller surface area and larger grain size with inore perfect crystalline structure. The net effect of both mechanisms of recrystallization aiid sintering is the production of a maximum in the surface area-temperature of calcination curves. These conclusions were confirmed by studying the changes in the specific area as a function of duration of heating when the temperature is kept constant. Typical set of results are shown in Fig. 3, in which it is shown that a t comparatively low temperatures (e.g., 300°), activatioii leads to measurable increase in surface area of tlie product, while sintering forces are still weak to produce measurable effects. At the higher temperatures of calcination, activation seems to be more rapid, but with prolonged heating it becomes interrupted with the sintering forces which sooii tend to decrease the area of the product. In such cases also a maximum may appear in the specific area-time curves. The higher the temperature, the shorter the time a t which this maximum shows itself, and tlie smaller the area reached ultimately at long durations of heating. (ii) The Change of the Specific Area of CaO upon Hydration from the Saturated Vapor Phase.-- The adsorption isothernis of both nitrogen and cyclohexane on partly aiid completely hydrated oxide specimeiis are also type I1 of Brunauer's classification, exhibiting no hysteresis a t belon- 0.3 relative vapor pressure. h typical set of specific area values derived from such isotherms for some completely hydrated oxide samples is shown in Fig. 1. In this figure, curves c and d represent tlie area of the hydration products of oxides which have been prepared i n vacuo and in presence of air, respectively. The results shown in Fig. 1 clearly indicate that appreciable loss of area always accompanies tlie hydration process, and that the hydration of the oxide samples which have been prepared under vacuum teiids to decrease the area to a much larger extent than for oxides prepared in presence of air. The surface areatemperature of preparation curves show a maximum for the hydrated product almost in the same locatioii as for the corresponding parent oxide. It is more interesting, in this connection, to study the progressive changes of the specific area of the product of hydration with the degree of hydration. For this purpose, oxide samples possessing specific surface area values between 73 and 16 m2/g. were subjected to hydration from the saturated vapor phase for different durations, and the specific area of the resulting products were plotted against the percentage hydration. The results are shown & Fig. 4,and as is shown in the
Vol. G7
?
2
E:
v
vi
40
20
7
4
ii
8
10
Time, hr.
Fig. 3.-Effect of time of heating on the specific surface area ot the products obtained by decomposition in vacuo. 100
80
I
60
?
3
E:
v
vi
40
20
20
40
60
80
100
70 hydration.
Fig. $.-Variation of the specific area of the solid as a function of percentage hydration in the saturated vapor a t 25": (a), oxide prepared in air; (v), oxide prepared in vacuo.
figure, the hydration process led in each case studied to a continuous fall in area with the degree of hydration, aiid the higher the initial area of the oxide, whethcr prepared in air or in vacuo, the larger the relative loweringbf surface area.
ACTIVATION AND
Oct., 1963
SINTERIKG BEHAVIOR O F CALCIUM OXIDE
Glass0n,~4in one of his interesting papers on the reactivity of lime, carne to the conclusion that only in cases of the most active limes (area 20-100 m.”g.) does hydration cause measurable losses in surface area; limes of moderate activity (5-20 m.2/g.) may be hydrated without, changes in surface area, whereas hydration of the least active lime leads to an increase in the area. The same author considered the volume changes which accompany the hydration process, and he showed that if the number of crystallites remained constant, the formation of a Ca(OH)z-crystallite out of a CaO-crystallite is accompanied by an increase in volume and also in the specific area of the product. The hydroxide will have 1.98 times the volume and 1.54 times the surface of the oxide. Aside from any comparative study which could be made between the results of Glasson, and those obtained in the present, investigation, the significant evidence obtained from both sets of data is that mainly a lowering in the specific area takes place upon hydration and this reveals that the process of hydration is accompanied by a decrease in the number of crystallites or an increase in average particle size due to the agglomeration or rapid aging of the product. It is worth mentioning in this connection that the mechanism of aging is essentially similar to that of sintering, which usually takes place at higher temperatures. Both aging and sintering lead finally to the same goal of producing a solid of maximum grain size, minimum surface area, and minimum potential energy.’
General Discussion The results summarized in the present work can be utilized for a better understanding of the role played by a gaseous product C in modifying the surface properties of the reactants in reversible reactions of the type solid A solid B gas 12. Thus if the contact between the gas C and solid B is minimized during the thermal decomposition of A, the product B might be formed in a state of strain and consequently of maximum activity, which normally depends on both temperature and time of heating. This state of affairs is reached when decomposition takes place under vacuum, where the gas C is allowed to escape from the zone of the reaction almost with the same rate of its production, and the activity of the product B seems to reach comparatively high values. On the other hand, if air is present during the calcination process, the surface activity of solid B is much ieduced, and preliminary studies in this Laboratory have shown that for the decomposition of hydroxides, water vapor, or, in more general terms, the gas, C, is the active constituent in “air.” It is therefore, the main constituent affecting the surface properties of the product B. It has been shown that the effect of what we may call “non-dry” air, operates a t all temperatures investigated, and this might be taken as an evidence that it is associated with the mechanism of decomposition itself, and is not an after effect which starts to play after the formation of the solid B. It is therefore plausible to assume that “non-dry” air affects the mechanism of decomposition in a t least two different ways. Firstly, in presence of air, atmospheric pressure suppresses the disruptive effect of the evolved gas, lowering thus the number of cracks and vacancies than those produced in vac(14) D. R. Glasson, J . A p p l . Chem.. 8, 798 (1958).
+
-+
2053
uum decompositions. Secondly, and this seems to be more significant to the production of surface area, the recrystallization of a new phase (solid B) from the parent phase is usually preceded by a step of nucleus formation, forming nuclei of the new phase in the old one. Their number determines the number of crystallites of the new phase formed out of each parent crystal. In the presence of the product C, the number of nuclei formed seems to be diminished, and recrystallization therefore seems to proceed in ail easier manner under conditions slightly displaced from the equilibrium state, with the consequent formation of less number of crystallites, each having bigger grain size, more developed crystalline structure, and possessing minimum stresses and strains. Therefore, under identical conditions of preparation, it is to be expected that preparation in presence of air leads to bigger crystals, more perfect in habit and less in number than those prepared under vacuum. Confirmation of this postulate was obtained through electron microscope studies of some oxides produced by the decomposition of their respective hydroxides. A clear case is shown in Fig. 5a and 5b for alumina produced by the thermal decomposition of gibbsite at 950” in vacuo and in “non-dry” air, respectively. It is evident that larger grain size and better crystalline morphology are both obtained in the latter case, On the other hand, if the reaction goes in the reverse direction and a recombination process between B and C takes place, product A will be formed, but the number of crystallites will not remain unchanged. There is a tendency for agglomeration in presence of the saturated vapor phase, and the process is mainly assisted by surface adhesion forces,l5 which are among the essential forces of aging and sintering of solids, leading to a decrease in the number of crystallites and to an increase in their grain size. Similar results were obtained in this Laboratory16 during the vapor phase hydration of magnesia. To sum up the role played by the gaseous product C, it seems erident that during decomposition, the effect is mainly to control the number of nuclei of decomposition, interfering thus with the mechanism of recrystallization to the new phase, but during the recombination the main effect of the gaseous component C is to facilitate the agglomeration of the formed crystallites predominantly by surface adhesion forces. The final goal of both processes, however, is to produce SL lower number of crystallites of larger grain size. It is not, however, to be expected that further decomposition of the recombined hydroxide will lead to further reactivation of the solid. Based on experimental evidence, the surface areas of the oxides formed during the thermal decomposition of their hydrated forms do not deviate by more than 10% from each other, denoting the absence of any significant physical change taking place during the second cycle of decomposition. The hydrated oxide seems to be fully nucleated, no major physical changes will accompany the recrystallization to the oxide phase and each crystallite seems to give rise to a single crystallite of the oxide, with no significant changes in their number. I n addition, the disruptive effect of the evolved gas is also minimized in this case, and the evolution of mater vapor is facil(15) G. F. Huttig, Ko27ozd-Z., 98, 263 (1942); 99, 262 (1942). (16) Unpublished results.
R. SH. MIKHAIL
2054
(a)
VoL 67
(1,)
Fig. 5.--(it) Ient paper could be taken in favor of the role played by the first of these factors, and work is now being conducted to investigate the effect of producing lattice strains in crystalline solids on its surface activity. Acknowledgments.-Thr author thanks Professor Dr. R. I. Razouk, for hinging his attention to this type of work on solids, and Dr. S. Rrnnaurr and Dr. I,. E. Copeland of the Portland Crnirnt Association, U. S. A., for the snpply of calcium hydroxidc crystals and for taking and intcrprrting tht. el~ctroirmicroscope pictures.
DISCUSSIOS
v. 8 . RAMACHANURbN.--Tlle general ronehlsioes arrived a t in this paper are very similar to those reported by I). R. Glasson in 1958. Mikhail, however, ha8 presented an interesting theory explaining the differing surface properties of calrium liydrovide heated in air and under vacuum. The statement that the number of nuclei from the parent phase determines the mode of recrystallization needs more elaboration RS regards greatrr surface ares obtained under vacuum. L. E. COPEIANO(Portland Cement Association) (fur It. SH. MIKHarL).--The evidence as obtained from both fields of sorfiice chemistry and electron miserosropy are a11 in f w o r of the view that each crystallite of the parent miiterid decomposes to yield a bigger number of crystallites of the product. 1:nder vacuum, the number of crystallites produced is much greater thnn those obtsined in the presence of air. Assuming that each crystallite of the product is developed from a single nucleus, which seems to be B plausible assumption, then the number of nuclei developed through the stage of deeomp,sitim determines the mode of recrystallization. The number of nuclei Beems to he sensitive to the prevailing atmosphere under whirli decomposition is effected. I also believe that this point needs more elahoratinn, especially if the evidence is collected from other fields of reseiwli hesides surface chemistry and electron microscopy. the hcst being the direct optical observation on single r r y ~ t d dewrnposition under different atmospheres.
