Effects of Al2O3 and Glass on Formation of Needle-like Mullite from Kaolin

The yield of mullite produced by firing a mixture of New Zealand kaolin and alumina (Al2O3/SiO2=1.0 in molar ratio) increased rapidly above 1500°C as compared with that for kaolin alone. The mullite was recovered by treating the fired body with fluoric acid. The mullite particles obtained by firing the kaolin-alumina mixture at 1600°C for 10h were granular. However, the needle-like mullite particles were produced by firing kaolin and a kaolin-alumina mixture both containing float glass powder (10wt%) at 1400°C. Needle-like mullite particles obtained by firing kaolin, kaolin-alumina, kaolin-glass and kaolin-alumina-glass at 1600°C for 10h were 6, 3, 13.5 and 12.5μm long, respectively. The growth rate of needle-like mullite particles produced in the samples mixed with glass could be expressed as L3=1.85×1018 exp (-5.57×105/RT)t, where L is the length of needle-like mullite particles (μm), R the gas constant (J/K⋅mol), T the absolute temperature (K) and t the time (h). However, this equation was not applicable when the yield of mullite was close to 100%. The Al2O3/SiO2 molar ratios of needle-like mullites obtained by firing kaolin-glass and kaolin-alumina-glass mixtures at 1600°C for 10h were 1.59 and 1.62-1.64, respectively, larger than 1.50 for the theoretical mullite composition. Their lattice constants a0 were 7.669Å and 7.667Å, respectively. These values were also longer than 7.5456Å of JCPDS 15-776. The longer a0 values might have resulted from the alumina-rich composition of the needle-like mullite particles.