Date and place of thesis defense:
06.03.2009, Faculty of Metallurgy and Materials Science
mr. sc. Marina Jovanović
prof. dr. Sulejman Drljević
Structure and phases transformations in clay during sintering
In function of choosing clay for studying structure and phase transformations during sintering five clays from deposits in Sarajevo-Zenica basin were investigated. The characterization of clays has carried out by chemical, granular and mineralogical composition, refractoriness, physical, technological and ceramic properties. All investigated clays are classified to illite-kaolinite type. “Čavka crvena” clay is brick clay, “Klokoti” clay is refractory clay and other clays can be used during the production of some ceramic products in traditional ceramic assortment.
The beneficiation of clays was done by wet separation with extract of fraction over 20 µm. The results of chemical analysis, heating behavior monitored by thermal microscope and differential thermal analysis (DTA) and X-ray diffraction (XRD) have shown decreasing of quartz content and increasing of clay mineral content and fluxing materials. The biggest effect of beneficiation is on clay “Klokoti” and the smallest is on clay “Kiseljak”.
The sintering, at 1100oC, 1250oC, 1320oC and 1400oC with two hours holding time at the highest temperature, were performed on the raw and beneficiated clay “Klokoti”. The microstructure on scanning electron microscope (SEM) was observed besides porosity and phase composition. These experimental results indicate to the big content of coarse quartz in raw clay and gradually increase of crystalline mullite content with increasing temperature of sintering. The beneficiated clay at higher temperature of sintering produces larger glassy phase content, which is result of larger fluxing content in its composition. It is also established that from this clay by sintering at 1250°C crystallize the most mullite along with minimum porosity and the lowest glassy phase content.
Specimens for synthetic mullite were prepared by sintering at 1580oC, 1635oC and 1700oC from the mixtures of raw and beneficiated clay “Klokoti” with electro filtering and milling metallurgical alumina. It is ratified that at selected temperatures of sintering clay beneficiation has much stronger effect on mullitization process than type and granulation of alumina. There are two classes of specimens: (1) specimens with raw clay characterized satisfactory mullitization degree and insufficient density and (2) specimens with beneficiated clay containing some mullite, much more corundum and increased glassy phase content. A little mullitization degree in specimens with beneficiated clay is result of too high temperatures of sintering and increased impurity.
Key words: clay, beneficiation, sintering, phase composition, synthetic mullite