Abstract
In this study, the effect of dry milling of kaolin (92 mass% of kaolinite) on its physical properties and microstructure development during firing was investigated using thermal analyses, X-ray diffraction, scanning electron microscopy, and DC conductivity measurements. X-ray diffraction showed a decrease in the intensity of reflections of kaolinite with rising milling time.
Moreover, formation of agglomerates from kaolinite particles was observed. A longer milling time led to a lower dehydroxylation temperature and contributed to a more substantial overall contraction of samples.
After dehydroxylation, the relative bulk density increased with an increasing milling time. Below the temperature 450 degrees C (start of dehydroxylation), the highest values of the DC conductivity of raw samples were observed for the longest milling times.
The main charge carriers are the H+ and OH- ions, originated from dissociation of the adsorbed water and from the coordinated water that was formed during mechanical dehydroxylation of kaolinite, complemented with alkali ions Na+ and K+ which are present as impurities in kaolin. The presence of coordinated water was proven by increasing values of the DC conductivity and by decreasing values of conduction activation energy from 1.73 eV to 0.85 eV with increasing milling time.
A similar trend of conduction activation energy was observed in the temperature range 650-750 degrees C, where the values of conduction activation energy changed from 0.79 eV to 0.52 eV with increasing milling time. After dehydroxylation (above 750 degrees C), the DC conductivity of raw samples slightly decreased with increasing milling time.