Study of the Dehydroxylation–Rehydroxylation of Pyrophyllite (original) (raw)
Related papers
Clays and clay …, 2002
The thermally-induced dehydroxylation and transformations of the 2:1 phyllosilicate pyrophyllite have been studied using infrared spectroscopy in the frequency range 350À11000 cm À1 and the temperature range 200À15008C. The dehydroxylation of pyrophyllite to pyrophyllite dehydroxylate occurs between 500 and 9008C. It is characterized by a decrease in the intensity of the OH signals and phonon bands of pyrophyllite and the eventual disappearance of these features as well as the appearance of extra signals related to pyrophyllite dehydroxylate and an intermediate phase. Our results are consistent with previous observations that the SiO 4 tetrahedral sheet structure still exists in pyrophyllite dehydroxylate, that the SiÀOÀAl linkages and 2:1 structure remain in the pyrophyllite dehydroxylate, and that AlO 5 trigonal bipyramids form.
SEM study of pyrophyllite high-temperature transformations
Journal of Materials Science, 1989
The changes taking place during the dynamic heating of pyrophyllite up to 1430° are investigated by means of scanning electron microscopy. X-ray diffraction was used as a complementary technique for phase identification, being pyrophyllite and its dehydroxylate, mullite and cristobalite. The different morphologies developed when pyrophyllite is progressively transformed by heating into new phases such as mullite and cristobalite are revealed and presented in this study. It is established that pyrophyllite dehydroxylate shows the same shape as the unheated material. However, upon further heating, the formation of mullite and later cristobalite, as well as the increase of the glassy phase, contribute to the observed microstructures.
Influence of mechanical and thermal treatments on raw materials containing pyrophyllite
Boletín de la Sociedad Española de Cerámica y Vidrio, 2000
In the present work results obtained in a study on the influence of thermal, mechanical by dry grinding, and their combination, in raw materials containing pyrophyllite, are discussed. First of all, it is studied the influence of thermal treatment concerning the development and evolution of crystalline phases (mullite and cristobalite) from dehydroxylated pyrophyllite. On the basis of these results, it is analyzed what happens in a natural raw mixture of pyrophyllite with kaolinite and mica (sericite) submitted to thermal treatments. The raw pyrophyllite materials are altered under laboratory conditions using mechanical treatments by dry grinding. It is noted that the increase of surface area and particle size reduction is produced by grinding, but other effects are produced on the structure and properties of the solid submitted to grinding. In general, grinding leads to a progressive destruction of the original crystal structure of the present layered silicates, but it is preferentially produced along the "c" axis. In other words, mechanochemical reactions are induced by dry grinding due to the increase of reactivity of the system. Between these reactions, it is enhanced the reagglomeration process that occurs above a determinate limit of grinding time. The grinding treatment can be combined with a subsequent thermal treatment that enhances the increase of reactivity, producing the formation of crystalline phases (mullite and cristobalite) at lower temperatures that in unground samples with energy saving. The results are compared taking into account the crystal structures of both kaolinite and pyrophyllite, the thermal transformation of kaolinite to mullite, and the process of grinding kaolinite because this layer silicate is present in the raw materials containing pyrophyllite.
Pyrophyllite: An Economic Mineral for Different Industrial Applications
Applied Sciences
Pyrophyllite (Al2Si4O10(OH)2) is a phyllosilicate often associated with quartz, mica, kaolinite, epidote, and rutile minerals. In its pure state, pyrophyllite exhibits unique properties such as low thermal and electrical conductivity, high refractive behavior, low expansion coefficient, chemical inertness, and high resistance to corrosion by molten metals and gases. These properties make it desirable in different industries such as refractory; ceramic, fiberglass, and cosmetic industries; as filler in the paper, plastic, paint, and pesticide industries; as soil conditioner in the fertilizer industry; and as a dusting agent in the rubber and roofing industries. Pyrophyllite can also serve as an economical alternative in many industrial applications to different minerals as kaolinite, talc, and feldspar. To increase its market value, pyrophyllite must have high alumina (Al2O3) content, remain free of any impurities, and possess as much whiteness as possible. This paper presented a rev...
Exploring the Rehydroxylation Reaction of Pyrophyllite by Ab Initio Molecular Dynamics
The Journal of Physical Chemistry B, 2010
We have investigated the process of rehydroxylation of pyrophyllite as a limiting factor to the dehydroxylation upon thermal treatment. Car-Parrinello molecular dynamics simulations based on density functional theory have been used along with the metadynamics algorithm. Two possible rehydroxylation mechanisms reaction have been characterized, related to two possible intermediate structures along the rehydroxylation paths, and both involve the interaction of the apical oxygen atoms. At high temperature, the rehydroxylation reaction is highly competitive (free energy barrier (∆F) ) 1.5 kcal/mol) and inhibits the progress of the dehydroxylation reaction (∆F ) 40 kcal/mol). In addition to the rehydroxylation of the dehydroxylated structure, the water molecule supports the interconversion of the cross and on-site intermediates as well. Thus, rehydroxylation and interconversion among intermediates can justify the wide range of transformations as a function of the temperature observed experimentally.
The Journal of Physical Chemistry A, 2008
We delineate the dehydroxylation reaction of pyrophyllite in detail by localizing the complete reaction path on the free energy surface obtained previously by Car-Parrinello molecular dynamics and the implemented metadynamics algorithm (Molina-Montes et al. J. Phys. Chem. B 2008, 112, 7051). All intermediates were identified, and a transition state search was also undertaken with the PRFO algorithm. The characterization of this reaction and the atomic rearrangement in the intermediates and products at quantum mechanical level were performed for the two reaction paths found previously: (i) direct dehydroxylation through the octahedral hole (cross mechanism) or between contiguous hydroxyl groups (on-site mechanism) and (ii) two-step dehydroxylation assisted by apical oxygens for each of the two steps. New intermediates were found and determined structurally. The structural variations found for all intermediates and transition states are in agreement with experimental results. The formation of these structures indicates that the dehydroxylation process is much more complex than a first-order reaction and can explain the wide range of temperatures for completing the reaction, and these results can be extrapolated to the dehydroxylation of other dioctahedral 2:1 phyllosilicates.
Infrared study of CO2 incorporation into pyrophyllite [Al2Si4O10 (OH) 2] during dehydroxylation
Clays and Clay Minerals, 2003
We report infrared spectroscopic observations of the incorporation of CO 2 into pyrophyllite that has been heated between 200ºC and 1250ºC for periods of 15 min, 1 h and 5 days. The presence of CO 2 is characterized by the n 3 band of CO 2 near 2347 cm -1 , detectable in samples in which dehydroxylation has commenced after heating above 450ºC. With increasing temperature, the CO 2 signal becomes more intense. The signal reaches its maximum intensity near 800ºC with an annealing time of 15 min. Further heating leads to a decrease in the CO 2 signal and the occurrence of an extra signal near 2156 cm -1 that implies the presence of CO. The process is characterized by significant time-dependence, indicating its kinetic nature. The peak positions of CO 2 signals show systematic variations with temperature. Our results suggest that the CO 2 molecule is associated with the local structure rather than being present as free gaseous CO 2 , and that the local structure of pyrophyllite is gradually modified during high-temperature treatments. However, no signals related to carbonate molecules (CO 3 2 -) were detected. The results suggest that CO 2 or other carbon-based molecules may diffuse into some clay minerals during dehydroxylation and may become altered due to structural modifications at high temperatures. This may have significance for possible CO 2 sequestration in shales and clay formations.
A mineralogical study of a natural pyrophyllite
Powder Diffraction, 2000
A chemical and mineralogical investigation of a Moroccan pyrophyllite is presented. X-ray powder diffraction has been largely used for phase identification and crystal symmetry determination. It is shown that this mineral has a triclinic symmetry with cell parameters: a = 5.160 A, b = 8.993 A, c = 9.360A, a=90.77°, f3= 100.57°, and y=89.71°.