THE INFLUENCE FACTOR OF STRUCTURAL CHANGE IN KAOLINITE SUSPENSIONS DURING A FREEZE-DRYING PROCESS

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Structural changes and thermal conductions in kaolinite suspensions were examined as a function of the solid fraction and freezing temperatures. Solid fractions ranging from 20 to 50% were tested, and each sample was prepared with freezing temperatures of -30℃, -50℃p (in a freezer) and -196℃ (with liquid nitrogen). Effective thermal conductivities were calculated using the equation of heat conduction. As a result, it was found that the effective thermal conductivity ratio of kaolinite suspension slightly decreases with an increasing solid fraction. Applying Fricke's model to these results, we predict that an increasing solid fraction promotes orientation of platy particles in kaolinite suspensions. Particle arrangement in freeze-dried suspensions was examined with SEM. Consequently, the particle arrangement in the kaolinite suspension with a 20% solid fraction and a freezing temperature of -30℃ takes on a honeycomb structure, and the other samples prepared in a freezer have a halfpipe-like structure. On the other hand, the particle arrangement of samples lyophilized by liquid nitrogen is similar to a sponge structure. According to an additional observation with an ESEM, namely an environmental scanning electron microscope, it becomes evident that the freeze-drying method with liquid nitrogen is one of the best sample preparations to keep particle structure in kaolinite suspensions for SEM observation.
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