26
F. TOUSSAIST, J. J. FRIPIAT, AND M. C. GASTUCHE
gous to those obtained with CdSe, and CdS synthesis can be effected completely in a single stage by employing the exact procedure defined for CdSe, via, heating at O.Zo/min. or less to 450". 2 . The reaction Cd-Te, unlike any of the others, proceeds to some extent even with pellets. The results are shown in Fig. 3. Trace 3A shows a reaction exotherm at 429,' while in 3B and 3C the subsequent cool.. ing and heating cycles still show the presence of large quantities of unreacted elements. The powder reactions are shown in 3D-3F, frommhich it is discerned that these reactions are much more complete. The single stage synthesis from powders is accomplished using the exact procedure defined for CdSe. What is evident from these results is that the solubility curves in the systems CdTe-Cd and CdTe-Te move away from the end member axes a t low temperatures, which results are in agreement with the published information on phase equilibria in the system.6 From these results it appears also that the controlling factor (6) D. deNobel, Ph.D. Thesis, Univ. of Leiden, 1958.
VOl. 67
in the powder reactions is the higher melting elemental constituent. 3. The reaction Z n S is very similar to that observed for Cd-Se and Cd-S. Preparation of this material in a one-stage synthesis also follows the procedure defined for CdSe. 4. The reaction Zn-Se also closely resembles the Cd-Se reaction, with the notable difference that a single stage complete powder synthesis can be effected a t rates as high as 3O/min. 5 . The Zn-Te reaction is shown in Fig. 4 and, as can be seen, its behavior resembles that for the other binary reactions excluding the Cd-Te reaction. From the data it is observed that in the system Zn-Te, the solubility of ZnTe in melts rich in either Zn or Te becomes appreciable at around 600-700'. The method of synthesis is being studied for other reactions whose products melt a t appreciably higher temperatures than their elemental constituents. These include a number of 111-V, 111-VI, and 11-V compounds.
DEHYDROXYLATION OF KAOLINITE. I. KINETICS BY F. TOCSSAIKT, J. J. FRIPIAT, AND AI. C. GASTUCHE Laboratoire des colloides (IATEAC)et de chimie wzin6rale, Institut Agronomipue, University of Louvain, Hdverld-LouvairL (Belgium) Received April SO, 1961 The kinetics of kaolinite dehydroxylation has been studied by thermogravimetry. Two diffusion processes must be taken into consideration: gross diffusion of water vapor from the clay surface toward the gas phase, and inner diffusion of water molecules, nucleated in the lattice, outward from the crystal. The first process may be controlled by working a t relatively low temperature (430'). When this condition is fulfilled, the order of the rate process with respect to the solid phase is unity but no definite order exists with respect to the vapor phase. This is explained by the existence of a water film operating as a diffusion barrier a t the reaction interface. Dehydroxylation occurs by removal of constitution water from a complete octahedral layer and not by random nucleation in the lattice. This conclusion agrees with previous deductions from n.m.r.
I. Introduction The object of this paper and of a following one is the study of t'he dehydroxylation process of kaolinite, the properties of samples t,aken at intermediate st,ages between kaolinit'e and met'akaolin having been described elsewhere. Although numerous papers already have been devoted to this subject,* the dehydroxylat'ioii mechanism never has been completely explained. For the kinetic studies the two methods of differential thermal analysis and thermogravimetry were used in an attempt to distinguish between the two different diffusion processes which must be considered. These are: (1) the diffusion of water molecules, nucleated inside the lattice, toward the crystal surface ("inner" diffusion process); and ( 2 ) the diffusion of mater molecules in the powder mass ("grossJ' diffusion process). The first diffusion mechanism, nucleat'ion and growth of nuclei, cannot be distinguished, but the second one can be controlled by careful adjustment of the experiment'al p aramet ers. The influence of particles size distribution upon t'he dehydroxylation process, as emphasized by Eyraud, , ~ NieuwenPret'tre, et al.,3 Schmidt and H e ~ k r o o d tVan (1) M. C. Gastuche, F. Toussaint, J. J. Fripiat, R. Touillaux, and M. Van Meersche, Clay Minerals Bull., 1962, t o be published. (2) E. B. Allison, ibid., 2, 242 (1955); H. E. Kissinger, Anal. Chem., 29, 1702 (1957); E. C. Sewell, Clay Minerals Bull., 2, 233 (1955); T. Jacobs, Nature, 132, 1086 (1958); R. Guennelon, Bull. G r o u p e Frang. Argiles, 6, 27 (1959).
berg and Pieters,6 Laws and Page,6 and Robert,sonBrindley, and Mackenzie,' can be accounted for by the adoption of both diffusion mechanisms. As shown by Grims and Caillhre and HBninJQthe first diffusion process is determined by crystalline properties while the partial vapor pressure of water mainly affects diffusion through the sample ma^^.^^*^^ References 3-1 1 are mainly concerned with dehydroxylat,ion studies by d.t.a., that is to say under non-isothermal conditions. The requirement that the diffusion mechanisms shall be controlled as well as possible limits the value of d.t.a. I n general, isothermal thermogravimet'ry is better adapted to kinetic investigations. Murray and White12 made a thorough study of the rate process by this method and showed that it obeys a (3) C. Eyraud, R. Goton, Y. Trambouze, T. TI. The, and AI. Prettre, Compt. Rend., 240, 862 (1955). (4) E. R. Schmidt and R. 0. Heckroodt, Mineral. Xag., 32 [2471, 814 (1958). ( 6 ) C. J. Van Nieuwenberg a n d H. A. Pieters, Rec. t7au. chirn., 48, 27 (1929). (6) W. Laws and J. B. Page, Soil Sei., 62, 319 (1936). (7) R. H. S. Robertson, G. W.Brindley. and R. C. Mackenaie, Am. -Wineralopist, 39, 118 (1954). (8) R. E. Grim, ibid., 32, 493 (1947). (9) S. CaillAre and S. HBnin, Actes. Conpr. Ceram. Intern., 137 (1948). (IO) R . L. Stone, J . Am. Ceram. Soc., 38, 50 (1952). (11) jV. Soliramli and F. Becker, Ber. Deut. Keram. Oes., 37 [.;I, !!27 (1960). (12) P. Murray and J. White, C l a y Minerals Bull., 1, 84 (1949): Trans. Brit. Ceram. SOC.,48, 151 (1949); 54, 187 (1955).
KINETICS O F DEHYUROXYLdTION
Jan., 1963
first-order law for samples of different origins and for degrees of transformat.ion less than. 75%. Unfortunately, they did not investigate the diffusion phenomena specifically and from this viewpoint their results are open to criticism. Brindley and Nakahira13 surveyed t)he influence of size and shape of the sample and clarified the gross diffusion mechanism report.ed above. They also found a first-order law, below the limit established by Murray and White. The process of dehydroxylation belongs to the class of heterogeneous reactions, t,he rate of which can be expressed.as
v
=
1CCS"~CV"V
27
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