Phase diagram of SiO₂ crystallization upon rice husk combustion to control silica ash quality

Rice husk (RH), which is an abundant agricultural waste, consists of ca. 20 % silica (SiO₂·nH₂O). Upon RH combustion, a large amount of silica ash is generated. RH silica is originally amorphous; however, the ash is crystalized depending on the conditions of the combustion. Crystallization of RH occurs at a much lower temperature than that of pure SiO₂ due to the eutectic effects of minerals, such as Na and K, that are initially present in RH. Controlling for anti-crystallization is required for expanding the possibility of using RH ash that is abundantly generated by combustion. Here, RH is combusted, followed by a highly thermocontrolled investigation, and the time and temperature dependency of RH ash crystallization are studied. Crystallization is avoidable when the ash is rapidly cooled; for instance, 20 °C/min cooling can avoid crystallization even if the combustion temperature reaches 950 °C. Various pathway patterns for achieving temperature and cooling ratio are plotted on a uniform diagram of temperature vs. time. Furthermore, a border zone, indicating the regions in which RH ash becomes amorphous and crystals, is successfully drawn into the diagram by using the data maintained at a constant temperature. A comparison with a few different types of RH showed that the border zone on the diagram can move depending on their ratios of inorganic elements, causing a eutectic effect.