Flash calcination is a process whereby a solid, usually in finely divided form, is heated rapidly, held at temperature for a short time, and then cooled rapidly.

Although the kinetics of many physical and chemical transformations within a solid at high temperature are quite rapid, flash-calcination conditions are such that the transformations may not be complete before the temperature is reduced to the level that further changes are kinetically frozen.

When kaolinite is heated at normal rates (soak calcined), complete dehydroxylation occurs near 600°C with the resultant solid, known as metakaolin, maintaining a semblance of the kaolinite structure. Further heating to near 1000 °C results in the transformation of metakaolin to an aluminum-silicon spinel plus free silica. At about 1200 °C Mullite and Christobalite are formed. The density of the kaolinite starting material also changes during the heating process. Thus, flash calcination was observed to decrease in density of the final product, accompanied by an increase in volume (particle swelling is expected). The rapid rate of heating causes dehydroxylation to occur much faster than the product water can diffuse from the particle. This causes a build-up in steam pressure within the particle which, at the proper temperature, became elastic permitting bubble formation. The high quench rates then froze the bubbles into the product.

In the absence of structural collapse, an idealized kaolinite would lose 13.95% of its mass on complete dehydroxylation and its density would decrease from 2.64 to 2.27 g /cm3. Flash calcines are partially dehydroxylated (as opposed to fully dehydroxylated soak calcines) and have different properties from those of soak calcine, e.g. lower densities than kaolinite (soak calcines are denser than kaolinite) and internal voids (absent from soak calcines) with diameters similar to the wavelengths for visible light (causing light scattering and so imparting opacity to the material). During flash calcination kaolinite particles are heated at such a speed that the steam generated within them is generated faster than it can escape by diffusion, and structural disruption is a consequence.

The initial low-temperature mass decrease corresponds to loss of non-constitutional (sorbed) water. Dehydroxylation of kaolinites takes place at T>400 C, the onset temperature being affected by particle size, crystallinity, atmosphere and mass of sample. Kaolinites with the lowest particle sizes and poorest crystallinities have the lowest onset temperature.

In flash dehydroxylation, layers near particle surfaces are dehydroxylated much faster than others, generating a crustal barrier to diffusion of H20 from the bulk. This situation is likely to be exacerbated at the higher heating rate, leading to greater disruption.

Dehydroxylation levels >70% are achieved at T>900 C. This could correspond to removal of all interlamellar hydroxyls, with intralamellar hydroxyls being more difficult to remove.

According to [28], most highly dehydroxylated materials are obtained at T> 900 C with high heating rates and longer residence times. Such materials have the lowest densities, this arising from the highest degree of structural disruption. Such disruption provides more facile routes for the exit of 'water' from the structure.

Up to 60% of dehydroxylation, a proportion of residual hydroxyls are lost relatively easily. During flash calcination the kaolinite particles are raised from ambient to high temperature very rapidly (heating rate ≈105 K/s). The dehydroxylation almost all occurs at high temperature, this being in marked contrast to dehydroxylation during slow heating (soak calcination and TG). It follows that the course of dehydroxylation during flash calcination is not necessarily the same as that characteristic of slower heating; one clear difference is the production of voids in the calcine particles during flash calcination only. Both initial heating rate and calciner temperature have profound effects on the product, increases in either variable leading to increasingly dehydroxylated (higher ) calcines. The lowest density calcines are also those which are the most dehydroxylated. This decrease in density is attributed to the presence of voids.

The key to producing high-quality MK is to achieve a near-complete dehydroxylation, without overheating. The thermal treatment beyond a defined point results in sintering and the formation of mullite, which is nonreactive. In other words, optimally altering kaolinite to MK state requires that it is thoroughly roasted but never burnt. The temperature of thermal activation in the processor should correspond to the range between the endothermal dehydration of the clay and the exothermal recrystallization of high temperature phases. This range can be determined using the differential thermal analysis (DTA) technique. The temperature in the processor is typically maintained in the range of 650 °C–900 °C.The calcination time is reduced to a few seconds, using flash bed calcination process on finely ground raw kaolin. It consists of very rapid heating, calcining, and cooling of powdered material suspended in a gas [24]. The calcined product may not necessarily be ground further. Thus, flash calcination can simplify industrial production installations and decrease the energy cost of grinding. In the heat treatment processor, as the residence (calcination) time reduces, the processing temperature increases to achieve higher calcination rate. While longer residence time and lower temperature is generally quite effective at dehydroxylation of kaolin, it is seen that higher calcining rate affects the pozzolanic reactivity of MK. Salvador [24] found that flash-calcined kaolin had higher initial reactivity and gave better compressive strength in comparison to that treated in a fixed bed.

The findings reported by Shvarzman [5] found that MK containing less than 20% of amorphous phase can be considered as an inert material from the standpoint of pozzolanic activity. The authors suggested that even with the partial dehydroxylation of kaolinite accompanied with about 55% amorphization, the material may be considered as a very active pozzolanic admixture. The findings direct toward the possibility of reducing energy consumption during the large-scale manufacture of MK by heat treatment.

It should be noted that slightly increasing densities at the longest residence times could be evidence for shrinkage of internal voids (by atom migrations in the transformed material at the calciner temperature), this process being accompanied by a slightly increasing proportion of metakaolin-like Si environments.

Dehydroxylation processes can be affected by crystal structure, particle size and size distribution, as well as temperature and water pressure in the vicinity of the powder.

Most dehydroxylation takes place between 400 C and 600C, the rate increases as the temperature increases. Ofcourse, the rate depends on particle size and the extent of disorder in the crystals.

In flash calcination, owing to the fast heat transfer, water may be produced at a greater rate than it diffuses through the particle body, leading to a very high vapour pressure (450 atm) with consequent disruption of the structure. Transmission electron micrographs of flash calcines have revealed an expanded structure with bubble-like formations which are suspected to be responsible for the lower density. This new physical structure promotes, to some extent, dehydroxylation of flash calcined kaolinite, as chemically free water retained in the structure may escape rather easily through the disrupted crystal, in directions other than those of the interlamellar space.

The interlayer spaces are expected to play an important role in the thermal dehydroxylation of kaolinite during soak calcination, providing low-resistance channels for diffusion of water out of the crystal. In flash calcination additional channels may be created by explosive dehydroxylation, while the interlamellar spaces preserve their function. However, as the calcination proceeds, the interlayer distance decreases and consequently the structure partially collapses after a certain degree of dehydroxylation. Thus the loss of the diffusion channels provided by the interlamellar space would explain the suddenly increased apparent activation energy for isothermal dehydroxylation in the flash calcine with an initial dehydroxylation (o) of ca. 70%, together with the slowing of the reaction rate of kaolinite after 60-70% dehydroxylation.

The slower loss of water at high dehydroxylation levels is probably due to collapse of the interlamellar space rather than preferential loss of the interlamellar hydroxyls initially, and could also be dependent on the range of particle sizes.

The apparent activation energy for dehydroxylation of the parent kaolinite, calculated from the Arrhenius equation, is ca. 213 kJ /mol. The activation energy of calcines decreased as the extent of dehydroxylation increased, although there was a sharp increase in E to 266 kJ/ mol for o =69%, before decreasing to E = 149 kJ/ mol. The increase in activation energy at 69% dehydroxylation may be due to collapse of the interlamellar space, while at higher levels of dehydroxylation explosive dehydroxylation might provide additional channels for diffusion, thereby lowering the activation energy.

Variations in rate constants and activation energies are explicable if the rate-determining step is diffusion of water out of crystallites, which is affected by changes in structure such as collapse of the interlamellar space or formation of spinel phase.In flash calcination rapid heating and high temperatures lead to build up of water vapor pressure in the crystal, at a faster rate than it can escape, until the structure is disrupted by the high pressures, thereby rendering subsequent dehydroxylation easier.

References1. D. Bridson, T.W. Davies and D.P. Harrison. PROPERTIES OF FLASH-CALCINED KAOLINITE,

Clays and Clay Minerals, Vol. 33, No. 3, 258-260, 19852. Richard H. Meinhold, Husnu Atakul, Thomas W. Davies and Robert C. T. Slade. Flash

Calcines of Kaolinite: Kinetics of Isothermal Dehydroxylation of Partially Dehydroxylated Flash Calcines and of Flash Calcination Itself, J. MATER. CHEM., 1992, 2(9), 913-921