THE ROLE OF WATER VAPOUR IN THE DEHYDROXYLATION OF CLAY MINERALS

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THE ROLE OF WATER VAPOUR IN THE DEHYDROXYLATION OF CLAY MINERALS By G. W. BRI~qDLE~C and M. NAtOa~RA Department of Ceramic Technology, The Pennsylvania State University, University Park, Pa., U.S.A. [MS.

1 THE ROLE OF WATER VAPOUR IN THE DEHYDROXYLATION OF CLAY MINERALS By G. W. BRI~qDLE~C and M. NAtOa~RA Department of Ceramic Technology, The Pennsylvania State University, University Park, Pa., U.S.A. [MS. Received 19th March, 1957.] ABSTRACT Water vapour atmospheres are known to raise the temperature of the endothermic reaction of clay minerals in differential thermal analyses. In isothermal kinetic studies of dehydroxylation, it is shown that a stationary state is attained depending on the operating temperature. The rate of dehydroxylation is a function of specimen size and shape, which suggests a controlling influence by entrapped water vapour. Even at constant reaction temperatures, the rate of dehydroxylation varies greatly between the interior and the exterior of a specimen. The bearing of these results on thermal analyses and on kinetic studies generally is discussed. INTRODUCTION In this paper we wish to report briefly on certain aspects of the dehydroxylation of clay minerals, namely, the effect of the water vapour atmosphere in the neighbourhood of the clay mineral particles. Dehydroxylation is the loss of the so-called "structural water" which takes place as an endothermic reaction from kaolin clays at temperatures around 450~176 This reaction may be represented by the relation OH + OH~H20 j~ + O (1) which indicates that two hydroxyl groups react together to produce one HzO molecule and an oxygen atom. The departure of the H20 molecule produces a lattice vacancy and when sufficient vacancies are created the structure deforms into metakaolin. The reversible arrows express the view that the reaction is reversible. Ordinarily this reversibility is not considered because the reverse reaction from metakaolin to crystalline kaolinite requires considerable water vapour pressures at temperatures not exceeding about 40Y'C (see Roy and Osborn (1954); Roy and Brindley (1956)). Since the forward reaction is carried out at temperatures higher than this value and at water vapour pressures seldom exceeding about 1 atm., it would seem reasonable to suppose that a reverse reaction (if it exists at all) is quite negligible. 114

WATER VAPOUR IN DEHYDROXYLATION OF CLAY MINERALS 1 15 However, there is very clear, direct evidence from the experiments of Stone (1952, 1954) and of Stone and Rowland (1955) and also from the much

2 WATER VAPOUR IN DEHYDROXYLATION OF CLAY MINERALS 1 15 However, there is very clear, direct evidence from the experiments of Stone (1952, 1954) and of Stone and Rowland (1955) and also from the much earlier work of Pieters (1928) on the differential thermal analysis of clays in controlled water vapour atmospheres, that pressures as low as a few atmospheres suffice to retard considerably the endothermic reaction. For example, the starting temperature of the endothermic reaction of kaolinite is raised by 100~ or even more, while the peak temperature of the reaction is raised by about 50~ for a water vapour pressure of 6 atm. Processes occurring in differential thermal analyses are essentially dynamic and are related, therefore, more to reaction kinetics than to equilibrium states. The importance of the water vapour atmosphere has been shown also in kinetic studies of clay mineral reactions which are under investigation by the present writers (Brindley and Nakahira, 1957), EXPERIMENTAL EVIDENCE Dehydroxylation is a temperature-activated reaction and is dependent on many factors among which time and temperature are major variables. Since the temperature dependence may be assumed to vary according to an Arrhenius factor exp (-E/RT), it is advantageous to carry out the experiments under isothermal conditions and thus to separate the time and temperature variables. Moreover, if the temperature is kept sufficiently low so that the reaction proceeds slowly, then self-cooling becomes unimportant. This is an essential condition for isothermal operations. Under these conditions, it would be expected that if the reverse reaction in equation (1) is negligible, the dehydroxylation process would proceed to completion. As Murray and White (1949, 1955) especially have shown, and as the present writers (Brindley and Nakahira, 1957) have confirmed in greater detail, the dehydroxylation of kaolinite takes place according to first order kinetics. If w t is the effective weight of the reacting material at time t, and Wo the initial weight, then w,= Wo exp( - kt) or log (wt/wo) = - kt (2) In other words, log (wt/wo) versus time should be a linear relation, continuing to ever smaller values of (wjwo) as t-+~. In fact, this is only true for infinitely thin specimens and for all actual specimens, the curves of log (wt/wo) swing round to a constant or practically constant value when the experiment is sufficiently prolonged. This in itself suggests a balance between a forward and a back reaction.

3 116 G.W. BRINDLEY AND M. NAKAHIRA If, after attaining such a stationary state, the temperature is raised only a few degrees centigrade, a new stationary state is attained. The higher the temperature, the more nearly the reaction goes to completion in the forward direction. Fig. 1 shows the kind of results obtained for kaolinite. At 467~ heating for 6 days was required to attain a stationary state. At 10~ higher temperature, 4 days were required to attain a new stationary state, and so on. I "~:( ~ o FIG. l--log (W]Wo) plotted against { time for kaolinite ( micron size). The ordinates express the per- o centage of unreacted material. ~ I I 1 I I I I 1 I ] 6 10 ~0 24 Time, days The part played by water vapour is shown indirectly in a second type of experiment. In this case a series of disc shaped specimens were prepared ranging from about 0.38 to 2.68 mm in thickness. These were obtained by pressing a suitable quantity of kaolinite powder at 200 p.s.i, for 5 minutes. These conditions were kept standard in order that all the discs would have the same apparent density (porosity) and permeability. The apparent density was, in fact, constant and the porosity was 33%. The log (wt/wo) versus time curves were then measured for a series of discs at the same temperature. Clearly if temperature is the sole controlling variable for the dehydroxytation process of a given clay, then the log (wt/wo) curves should be coincident for a given temperature and should be independent of the size of the specimen. Experiment shows (see Fig. 2) that the reaction rate is very dependent on the thickness of the disc. In fact, the reaction constant k in equation (2) is related to the disc thickness, x, by the equation 1/k=a + bx (3)

4 WATER VAPOUR IN DEHYDROXYLATION OF CLAY MINERALS 117 IOO I 1 I I I t3 30 2O J ',~o" ' ' t 50 I Time. minutes FIG. 2--Log (w/wo) plotted against time, in minutes, for kaolinite discs heated at a constant temperature of 497~ Curves a, b, c, d correspond to discs of 0-38, 0.83, 1 '55 and 2-68 mm respectively. Curves 1 and 2 correspond to an infinitely thin disc and to a thin layer of uncompacted powder respectively. From this equation it follows that 1/a is the reaction constant for the infinitely thin disc, x=0. The dependence of the reaction rate on the disc thickness can be understood if it is supposed that water vapour becomes increasingly entrapped as the thickness increases. This supposition has been tested experimentally in the following way. If the hypothesis is correct that the forward reaction is retarded by water vapour, then it would be expected that the water vapour pressure would vary from the outside to the centre of the disc and that the progress of the reaction would diminish correspondingly from the exterior to the centre. This has been tested by firing a disc of kaolinite of about 3 mm thickness until the overall dehydroxylation, as shown by weight loss, was about 50%. The specimen was then cooled and layers were scraped carefully from the disc so that it could be sampled layer by layer. The extent of the dehydroxylation was then measured by quantitative X-ray intensity measurements using a Geiger counter diffractometer. The results (see Fig. 3) showed that the dehydroxylation was about 95% complete at the surface, but no more than about 40~ at the centre of the disc. Again it is seen that although the whole specimen was maintained at the same temperature, there

5 118 G. W. BRINDLEY AND M. NAKAHIRA I I I I! I I ~ 6O,~ 50 c 0 o 40 "o o! ~ 2O FIG. 3--The percentage of unreacted kaolinite at different depths within a disc about 3 mm thick heated at 517~ until 50~ dehydroxylated. The diagram represents the variation between the outer layer and the centre of the specimen. IO P I t I I = I 0 I-O 1.6 Depth in specimen, ram. was a very large difference between the extent of the dehydroxylation at the outside and the inside of the specimen. DISCUSSION The observations described here for kaolinite have been repeated for halloysite, and measurements for other hydrated materials are in progress. When these experiments are considered in conjunction with the differential thermal analysis data using controlled water vapour atmospheres, there seems no doubt whatever that the dehydroxylation process is very sensitive to relatively low water vapour pressures. The question which then arises is how this conclusion can be reconciled with the high water vapour pressures required to convert metakaolin back to crystalline kaolinite in a time of hours or days. This is a problem not yet fully solved, but the present suggestion is that the reaction is less simple than equation (1) suggests. It is possible that the hydroxyl groups must be raised to activated states before their conversion to H20 and oxygen takes place, and that the presence of only low water vapour atmospheres has a marked influence in the first stage of this process. This mechanism can be represented as follows :- OH + OHm(OH)* + (OH)*-+HzO 1" + O where (OH)* indicates an activated state. If the reversibility is confined essentially to the first stage, and if this is highly sensitive to

WATER VAPOUR IN DEHYDROXYLATION OF CLAY MINERALS 119 the presence of water vapour, then it follows that the overall process is also sensitive to water vapour.

6 WATER VAPOUR IN DEHYDROXYLATION OF CLAY MINERALS 119 the presence of water vapour, then it follows that the overall process is also sensitive to water vapour. The second process is shown as a forward reaction only. These investigations have an immediate bearing on the interpretation of differential thermal analyses. If, for example, two similar, but not identical, kaolin clays are shown to give minor differences in their differential thermal curves, then it is easy to think that these differences are indicative of differences in the clay minerals themselves, when actually the thermal behaviour may be simply a consequence of a slightly different packing of the materials. If the two samples have different morphologies, then it is virtually impossible to control the vapour permeability of the materials. The kinetic studies have revealed that the reaction rates are extremely sensitive to the packing of the material and this must have a direct bearing on the finer points of d.t.a, recordings. A second important consequence of these results is that kinetic data cannot be related immediately to activation energies until the influence of specimen size, shape, compaction, etc., is taken into account. The approximate first order character of the reactions shown in Fig. 2 and the non-linear Arrhenius plots of log k versus 1/T given by Murray and White (1955) are directly related to the effects discussed in this paper. Elsewhere we show that strictly linear Arrhenius plots are obtained when data for infinitely thin specimens are used. Acknowledgement.--We are indebted to the Gulf Research and Development Company for a grant-in-aid which has made possible the programme of research of which this investigation forms a part. REFERENCES Brindley, G. W. and Nakahira, M Clays and Clay Minerals (Fifth National Conference) in press. Murray, P. and White, J Clay Min. Bull., 1, Trans. Brit. Ceram. Soc., 4g, Trans. Brit. Ceram. Soc., 54, Clay Min. Ball., 2, Pieters, H. A Thesis (Delft, Netherlands). Roy, R. and Osborn, E. F Amer. Min., 39, Roy, R. and Brindley, G. W Clays and Clay Minerals (Fourth National Conference), Stone, R. L d. Amer. ceram. Soc., 35, Stone, R. L Clays and Clay Minerals (Second National Conference), Stone, R. L. and Rowland, R. A Clays and Clay Minerals (Third Nation al Conference),

THE CONSEQUENCES FOR DIFFERENTIAL THERMAL ANALYSIS OF ASSUMING A REACTION TO BE FIRST-ORDER

THE CONSEQUENCES FOR DIFFERENTIAL THERMAL ANALYSIS OF ASSUMING A REACTION TO BE FIRST-ORDER By E. C. SEWELL Building Research Station, Watford [Read 5th November, 1954] AKt;TRACT From their isothermal