Porous mullite and mullite-based composites by chemical processing of kaolinite and aluminium metal wastes

Porous mullite and mullite-based composites have been prepared from a chemical processing route starting from a precursor obtained using an aqueous suspension of kaolinite and aluminium hydroxide. This was coprecipitated with kaolinite using ammonium hydroxide or hexamethylenediamine (HMDA), and a solution of aluminium chloride prepared from dissolved recycled wastes of aluminium metallic powders. Raw and ball-milled kaolinite during 30 minutes have been employed. The precursors and the obtained materials were characterized by X-ray diffraction (XRD) and thermal diffractometry, simultaneous differential thermal analysis (DTA) and thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), flexural strength test at room temperature and Hg intrusion porosimetry. The feasibility of using HMDA as a precipitating agent in the proposed chemical processing route for mullite preparation has been demonstrated. The advantageous use of HMDA, as compared with ammonium hydroxide, and ground kaolinite, as compared with raw kaolinite, produced single-phase mullite materials and enhanced the flexural strength of the resultant ceramic porous bodies by firing at 1550–1600 °C (porosity 50–45 vol%). These materials would have application as refractories, substrates, filters and thermal isolating materials.