Preparation of mullite ceramics from coprecipitated aluminum hydroxide andkaolinite using hexamethylenediamine

Mullite ceramics have been prepared from an aqueous suspension of kaolinite (raw or ground) and aluminum hydroxide, The precursor was coprecipitated i n the mixture using hexamethylenediamine (HMDA) or ammonium hydroxide, and a solution of aluminum chloride from acid dissolution of wastes of aluminum metal. The precursor and the resulting materials were characterized and st udied by X-ray diffraction, thermal methods, and mechanical strength and po rosity measurements. The feasibility of the proposed chemical processing ro ute for mullite preparation was demonstrated, in particular using HMDA as a precipitating agent for aluminum hydroxide instead of ammonium hydroxide, which adversely affects the system reactivity. The use of HMDA, as compared with ammonium hydroxide, and ground kaolinite produces single-phase mullit e and enhances the flexural strength (maximum of 49 MPa) of the resultant c eramic porous bodies (porosity ca. 52-45 vol%) fired at 1550-1600 degreesC for 30 min.