COMPARISION BETWEEN KOMBELCHA AND BOMBOWHA KAOLIN DEPOSITS, ETHIOPIA

ABSTRACT

The chemical, mineralogical and textural characteristics of two kaolin deposits of diverse environmental origin had been investigated by X-ray Diffraction, Differential Thermal analysis, Scanning Electron Microscope and Chemical methods of analyses. Furthermore, attempts had also been done to study the fired prosperities of such kaolinitic materials.

Kaolinite is the predominant clay mineral of Kombolcha 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% for Bombowha kaolin with impurity elements such as iron (<1%), total alkali and titanium accounting less than 3%. On the other hand, the Kombelcha kaolin bears a relatively higher alumina, total alkali and iron, averaging 32%, 1.50% and 2.75%, respectively.

The Kombolacha kaolin shows high shrinkage and low porosity values at lower temperatures mainly due to its higher alkali and iron contents. From the firing properties verification temperature can be inferred to 1150oC for Kombolcha kaolin while the Bombowha kaolin tends to remain refractory until 1250 oC. Marked differences occur in brightness values between the two deposits but firing at higher temperatures have significantly improved the brightness value for Kombolcha kaolin.

X-ray detectable mullite begins to form at temperature of about 1150 oC. At 1250 oC, mullite is the dominant mineral phase for Bombowha kaolin with subordinate amount of cristobalite and some glassy phases. The Kombolcha kaolin also bears comparative amounts of mullite with relatively higher proportion of cristobalite. An addition of small amounts of kyanite has significantly offseted the shrinkage values but requires higher temperature treatment for complete transformation of kyanite to mullite.

Field evidences supported by granulometric and chemical analyses suggest that kaolinite is a product of in-situ weathering of granite in Kombolcha area. The Bombowha kaolin tends to be a product of both hydrothermal and in-situ weathering of pegmatites and granites.

Note: This work was done as a continuation of the LRG project "Utilization of Refractory
Raw materials in Ethiopia carried by the author and his co-researcher. The report is available at the Central Technical Library of the ESTC.

(Final Report 1995)

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