Hydrothermal crystallization and crystal growth of 7-sio from rice husk silica ash in alkaline solutions

ABSTRACT

White pure and active amorphous silica was prepared from rice husk with 99.96% purity by acid leaching and pyrolysis. This silica could be crystallized into prismatic microcrystalline alpha-Sio2 [77-88nm] under hydrothermal conditions [215 °C = 20 atm. press.] from alkaline solutions. The maximum yield of alpha-Sio2 was obtained at 0.6 molar ratio of NaOH / Sio2 which reduced to 0.2 in the presence of A13+ and /or B3+ traces. This is due to the formation of silicalite crystals [considered to be a polymorph of sio2]. It has been found that transition metal cations in the reaction media affects the degree of alpha-Sio2 crystallization in the order Fe3+> Ni2+> Cr3+> amorphous Sio2> Co2+> Cu2+> Mn2+> Zn2-> Zr4+ > La3+. Both mechanism of alpha-Sio2, silicalite growth and ions incorporation were discussed