Quantitative determination of the mineral-matter content of coal by a radiofrequency -oxidation technique

Structural transformations between the different hydration states of three vermiculite samples from Sta. Olalla (Huelva, Spain), Paulistana (Piaui, Brasil) and West China, have been observed by X-ray diffraction at atmospheric pressure, P = 1.4610 ˇ2 mbar and P = 2.4610 ˇ4 mbar. The samples were studied in flake and powder forms. The effect of vacuum has been proven to be the same as that of temperature, i.e. it causes dehydration of vermiculite, but with a different evolution through the different hydration states. In fact, under vacuum, the process seems to be inhibited at a one-water layer hydration state (1-WLHS), without a further dehydration of samples to a zero-water layer hydration state (0-WLHS). Furthermore, the dehydration process has been shown to occur through different interstratified states in each vermiculite. This result has been related to the interlayer Mg-cation content, due to its affinity to water molecules. The interstratified states have been analysed by the direct Fourier-transform method. The vermiculite from Sta. Olalla exhibits the most complex process, with formation of three different interstratified phases: two phases characterized by an interstratification of interplanar

Study of the dehydration process of vermiculites by applying a vacuum pressure: formation of interstratified phases

Dehydration and rehydration of a phlogopitic Ca-vermiculite were studied by thermoanalysis (TG, DSC) and by in situ X-ray powder diffractometry. A detailed analysis of the relationship between temperature and state of hydration of the vermiculite close to equilibrium at atmospheric pressure was rendered possible by use of low heating rates in combination with a fast recording position-sensitive detector. The results reveal the existence of a number of distinct states of hydration characterized by basal spacings of 1.515, 1.493, 1.473, 1.188, 1.176 and 0.95 nm. In confirmation of results obtained earlier with Mg-vermiculite, regular interstratification occurred at dehydration during the transition between two- to one- and between one- to zero-layer hydrates showing integral series of 001 reflections with basal spacings of 2.67 and 2.11 nm, respectively.

Dehydration and rehydration of vermiculites: I. Phlogopitic Mg-vermiculite

Exfoliation of vermiculites with chemical treatment using hydrogen peroxide and thermal treatment using microwaves

Emanation thermal analysis (ETA) supplemented by thermogravimetry (TG and DTG) and evolved gas analysis (water) were used for the characterization of the thermal behavior of montmorillonite samples saturated with lithium, sodium, magnesium and aluminum cations. The ETA revealed microstructure changes that occur during the gradual heating of the clay. These changes are associated with the dehydration, dehydroxylation and formation of an amorphous meta-montmorillonite phase, and its subsequent annealing and recrystallization. On the basis of the ETA results it was demonstrated that exchangeable metallic cations have a great effect on the thermal behavior of montmorillonite.

Characterization of Montmorillonite Saturated with Various Cations

The changes in X-ray diffraction patterns of layer silicates which result from dry heat-treatments below 550~ have been studied and the results are tabulated in order of increasing temperature at which these changes are observed; the structural implications are briefly considered. In order to evaluate the effects of composition, particle size and disorder, synthetic clay minerals have been used in addition to natural clays. This summary of X-ray data may be of value in the identification of clay minerals in mixtures.

C. M. Warshaw

Papers

Changes Effected in Layer Silicates by Heating Below 550°C*