Retention of amorphous silica in rice hulls at high temperatures

Abstract

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.