Retention of amorphous silica in rice hulls at high temperatures

Rice hulls contains around 20 wt% amorphous silica (rice hull silica). Rice hull silica containing no potassium impurity crystallized from amorphous silica to cristobalite at 1300 °C. When a small amount of MgO or a-Fe2O3 or Al2O3 were added to the rice hull silica, the rice hull silica crystallized to cristobalite at 1500°C. MgO a-Fe2O3 and Al2O3 diffused to silica as Mg2+, Fe3+ and Al3+ at high temperatures. These ions broke the SiO4 tetrahedron network, so that the amorphous state of rice hull silica was maintained. As sintering occurred at 1500°C in rice hull silica mixed with additives such as MgO, the melting of the silica particle surface by sintering made the SiO4 tetrahedrons to move easily, so that the rearrangement of SiO4 tetrahedrons occurred and amorphous silica in the rice hull silica crystallized to cristobalite at 1500°C.