Observation of Physio-Chemical Differences of Rice Husk Silica under Different Calcination Temperatures

Aims: Amorphous silica in rice husk ash is a reliable and sustainable resource, although rice husks are currently an issue for farmers. Amorphous silica is a resource in various industrial applications and can easily be obtained from rice husks if they are treated correctly. If the treatment is not conducted properly, amorphous silica becomes crystalline, which is carcinogenic. In this study, physio-chemical observations of the transformation of amorphous to crystalline silica were conducted under “no-fixed carbon” conditions.

Study Design: Rice husks were burned at different temperatures in an electric furnace and their ash was analyzed for solubility, fixed carbon, volatile matters, ash, and moisture. XRD, WDX, and SEM were used for physical state analysis.

Location and Duration of Study: The study was conducted in a laboratory at Toyama Prefectural University from April 2015 to March 2017.

Methodology: Rice husks from Koshihikari (Oryza sative L.) were used in this study. Rice husks were calcinated in a laboratory-scale electric furnace (KBF794N1, Koyo for 500-1,100°C, Lindberg, Koyo for 1,500°C) for two hours at 500-1,500°C after being washed with acetic acid (5%). The physical state of rice husk silica was determined by X-ray diffraction analysis (XRD: MultiFlex 40 kV, 30 mA, CuKα, 2θ: 5-80°, Rigaku). The rice husks were ground into a powder and analyzed by XRD.

Results: It was found that amorphous silica in rice husks transformed into crystalline silica at burning temperatures of over 1,000°C. Total silica (T-silica) in the ash burned at 500°C—800°C, almost equal to the burning temperature of soluble silica (S-silica), and beyond 900°C, the non-S-silica portion of T-silica increased. From the morphological observations, at low temperatures the rice husk ash was fragile and did not hold its original shape; in contrast, the rice husk ash held its original shape at high temperatures. At extremely high temperatures, such as 1,000 and 1,500°C, the original shapes seemed to shrink by melting on the ash’s surface of the ash.

Conclusions: Under no influence of fixed carbon, the amorphous silica in rice husks transformed to crystalline silica at temperatures over a 1,000°C burning temperature. The T-silica in the ash that burned at 500°C—800°C were almost entirely S-silica, and then over 900°C, the non-S-silica portion increased in T-silica.