- - - School of Environmental Engineering, Toyama Prefectural University Toyama ۹۳۹-۰۳۹۸, Japan
- - - Project Management Department, Vietnam National University , Hanoi, Vietnam
- - - School of Environmental Engineering, Toyama Prefectural University Toyama ۹۳۹-۰۳۹۸, Japan
- - - Faculty of Bioresources and Environmental Sciences, Ishikawa Prefectural University, Ishikawa ۹۲۱-۸۸۳۶, Japan

Purpose The solubility of silica is a key parameter affecting its suitability as a rice fertilizer. Therefore, this study determined the effect of calcination temperature and duration on the physical composition of silica derived from rice husk ash. Method Rice husks were calcined at ۱۰۰–۹۰۰ °C for ۱۵–۱۲۰ min in an electric furnace. The solubility and physical composition of the samples were measured, and the appearance of the calcined rice husk ash was determined via visual observations. Results The appearance of the rice husk ash changed drastically at a calcination temperature of ۳۰۰ °C. The husk exhibited the whitest color at ۵۰۰ °C and ۱۲۰ min of calcination. The solubility of silica in the rice husk ash increased up to a calcination temperature of ۵۰۰ °C and then started to decrease. The silica in the rice husk ash exhibited a gray zone between the amorphous and crystalline structures. Conclusion For actual field applications of silica derived from rice husk ash as a fertilizer, calcination conditions of ۴۰۰–۸۰۰ °C and ۱۵ min are proposed to improve solubility. The optimal calcination temperature should be determined based on the heat recovery efficiency.

Rice-husk ash, Silica, Fertilizer, Calcination, Recycling, Solubility