Authors:S. Shoval, K. Michaelian, M. Boudeulle, G. Panczer, I. Lapides, and S. Yariv
The products of dickite heated in air at 1000 to 1300°C were studied using curve-fitting of transmission and photoacoustic infrared and micro-Raman spectra. The spectra were compared with those of mullite, Al-spinel, corundum, cristobalite, amorphous silica and meta-dickite. Bands that characterize crystalline phases appeared at 1100°C and became stronger with increasing temperature. Mullite, Al-spinel, corundum and amorphous silica were identified by their characteristic bands. The characteristic IR bands of cristobalite overlap those of mullite and amorphous silica, and its presence was therefore established from intensity ratios of the appropriate bands. The research clearly demonstrated the advantage of using curve-fitting for the identification of high temperature phases in the study of the thermal treatment of kaolin-like minerals by infrared and Raman spectroscopy. This technique seems to be a useful method for materials analysis in the ceramic industry.
Authors:V. Balek, M. Beneš, Z. Málek, G. Matuschek, A. Kettrup, and S. Yariv
Emanation thermal analysis
(ETA) and thermogravimetry measured in the range 20–1000C was used
to characterize the thermal behaviour of Na-montmorillonite (Upton Wyoming,
USA) and homoionic montmorillonite samples prepared by saturation with cations
Li+ , Mg2+ , Al3+
, respectively. It was confirmed that the presence of cations used for montmorillonite
saturation (Li+ , Mg2+
, Al3+ ) influenced the thermal behaviour of the
samples. The results that indicated the decrease of radon release rate corresponding
to a collapse of the interlayer space between the silicate sheets after water
release and the crystallization of meta-montmorillonite in the respective
temperature intervals were compared. From the ETA results it followed that
the thermal stability of intermediate microstructure depends on the type of
exchanged cation. A mathematical model was used to evaluate the ETA data.
Authors:N. Lahav, D. Ovadyahu, A. Gutkin, E. Mastov, T. Menjeritzki, A. Adin, L. Rubinstein, D. Tropp, and S. Yariv
A device was constructed in which a clay suspension is hermetically heated at 220°C for a few minutes. This thermal treatment
is accompanied by a pressure increase in the cell. Once the valve is opened, there is a fast release of the pressure inside
the cell and a sudden evolution of the interparticle water. This shock leads to a quasi explosion of the clay particle. This
technique was named thermal vapour pressure shock explosion (TSE). The effect of TSE treatment on the properties of palygorskite
suspensions was investigated. Palygorskite suspensions in water are rather unstable and particles smaller than 3 μm in size
are not found before a TSE treatment. Stabilization of the suspension can be obtained by TSE treatments and/or by using a
dispersing agent such as pyrophosphate, or both. As a result of TSE treatments smaller particles are obtained, the dispersiveness
of the particles is improved and electrophoretic mobility is increased. Electron microscopy scans showed that the aggregates
of needles which form the palygorskite fibres, disintegrate to separated thin needles as a result of the TSE treatment.
Authors:E. T. Stepkowska, S. Yariv, J. L. Pérez-Rodríguez, A. Justo, A. Ruiz-Conde, and P. Sánchez-Soto
A dredged sludge was studied to investigate the influence of ageing and of pretreatment on its drying rate, water sorption/retention, thermal mass loss, XRD and microstructure (SEM).
Ageing caused change in particle thickness and specific surface, a gradual aggregation to form units of the size 10–50 μm, formation of macropores of similar size, unhomogeneity and fissures between aggregates and “super-aggregates”. Macropores were detectable by the initial drying rate especially at 45°C. They indicated a tendency of collapsing at a lower drying rate at 30°C. This is consistent with SEM observations. With ageing the aggregates were more compact and less sensitive to drying.
The aggregated system indicated a higher initial drying rate (higher permeability), whereas stirring induced a lower drying rate, favouring the formation of compact laminar structure.
XRD peak intensity was lowered with ageing due to decrease in crystallinity (stacking faults and/or decrease in crystallite size). The content of amorphous material was lowered as well, reducing water sorption/desorption, which indicated that the specific surface is lower.
From the suitable microstructure induced by ageing some new phases may form (feldspar, zeolites), preferably in the coarser fraction of the sludge. This is disturbed by stirring which operation expels also carbonates from the particle edges and this may reduce the structural strength of the sludge. In aged bentonite suspension a similar tendency was observed of formation of specific microstructures capable of phase transformation, e.g. to feldspar.