Mullitisation behaviour of south Indian clays

Abstract

The sintered mullite is produced commercially, using clay and alumina as the source materials. South India has an abundant quantity of high aluminous clay. The present article reports the results of the mullitisation behavior of three south Indian clays (Neyveli, Panruti and Udayarpalayam clays) and three different alumina sources, (reactive alumina, gibbsite and boehmite) to produce stoichiometric mullite. The raw materials as per the stochiometric composition were weighed and wet milled in a planetary ball mill. The dried and powdered materials were passed through 75 μm sieve, then uniaxially pressed at 50 kN to make bars, and finally the specimens were sintered at 1600 °C for 3 h under normal atmospheric condition. By reaction-sintering the clay-alumina mixture reacts to form mullite. Physical characterization such as bulk density and water absorption and the mechanical properties, like flexural strength of sintered specimens were also carried out. The Neyveli-reactive alumina mixture showed high density (2.71 g cm −3) and high strength (82 MPa). The impure Udayarpalayam clay–reactive alumina mixture shows high strength (87 MPa), but low density (2.65 g cm −3). Microstructural studies on sintered and polished samples and X-ray diffraction studies on sintered specimens are carried out to estimate the mullitisation behavior of individual clays. Among the three clays and the three alumina sources, Neyveli clay and reactive alumina mixture gave the maximum mullitisation. The specimens with bohemite exhibited poor mechanical properties as well as poor physical properties.