Comparison of Kombelcha and Bombowha kaolins of Ethiopia

The chemical, mineralogical and textural characteristics of two kaolin deposits of diverse origin were investigated by X-ray diffraction (XRD), Differential Thermal Analysis (DTA), Scanning Electron Microscope (SEM) and Chemical Methods of Analysis. Furthermore, attempts were made to study the fired properties of such kaolinitic clays. Kaolinite is the predominant clay mineral of the Kombelcha and Bombowha areas with quartz, feldspar and illite/muscovite occurring as subordinate minerals. Other than kaolinite, halloysite and gibbsite characterize the Bombowha kaolin deposit. Alumina is generally above 35% in the Bombowha kaolin with impurity elements such as iron (<1%) and total alkali and titanium accounting for less than 3%. On the other hand, the Kombelcha kaolin bears a relatively lower alumina (33.24%), and higher total alkali and iron, averaging 2.54 and 2.63%, respectively. The Kombelcha kaolin shows high shrinkage and low porosity values at lower temperatures mainly due to its higher alkali and iron contents. From the firing properties vitrification temperature can be inferred to as 1150°C for the Kombelcha kaolin while the Bombowha kaolin tends to remain refractory until 1250°C. Marked differences occur in brightness values between the two kaolinitic clays but firing at higher temperature significantly improved the brightness value for the Kombelcha kaolin. Field evidence supported by granulometric and chemical analyses suggest that kaolinite is a product of in situ weathering of granite in the Kombelcha area. The Bombowha kaolin tends to be a product of both hydrothermal and in situ weathering of pegmatites and granites.