Abstract
Rice industrialization worldwide generates significant amounts of rice husk as a by-product. When rice husk is burned to obtain energy, a relatively common practice, a substantial portion of the husk turns into ash, and both constitute environmental liabilities. Using rice husk and ash as starting materials to produce high-value products could help in mitigating the environmental impact while providing economic revenue. Rice husk and rice husk ash as produced in a local cogeneration plant without any pretreatments were evaluated as feasible sources for silicon nitride (Si3N4) and silicon oxynitride (Si2N2O) whiskers by carbothermal reduction and nitridation. Rice husk and the ash were held at temperatures between 1200 and 1400 °C for 3 h under flowing nitrogen. Increasing soaking temperature values led to higher whisker development for both starting materials, with the best results observed at 1400 °C. Whereas α-silicon nitride whiskers were obtained when rice husk was employed, the graphite surface-to-ash ratio dictated whisker composition for the ash. Treatment of the ash at the soaking temperature value of 1400 °C led to silicon oxynitride for lower graphite surface-to-ash ratios, but when this ratio was increased, α-silicon nitride predominated. α-silicon nitride whiskers had cross sections ranging from about 100 nm to 1 µm in width, whereas the silicon oxynitride whiskers had cross sections ranging from approx. 100 to 500 nm in diameter. Both types of whiskers were observed to be in the millimeter length range.
Highlights
Rice husk accounts for nearly 20% of the weight of paddy grain and is the main waste associated with the rice mill industry
The as-received rice husk ash, more fragmented, retains somewhat a similar structure to the one found in the husk, and some insufficiently burned husk portions can be observed in the ash as well
Both XRD patterns exhibit a peak at 2θ = 22° indicating the presence of relatively amorphous cristobalite, where a sharper peak can be observed in the industrial ash suggesting that the silica present in the ash could have a higher degree of crystallinity when compared to the husk
Summary
Rice husk accounts for nearly 20% of the weight of paddy grain and is the main waste associated with the rice mill industry. The inorganic fraction, mainly silica, accounts for roughly 20% of the husk mass [1]. When rice husk is burned, usually to generate energy, large amounts of ash are produced. One option to mitigate the environmental impact of the waste disposal and at the same time add value to the husk and the ash could be found by using these to produce high-value ceramic materials. Rice husk and rice husk ash can be used as an alternative source of silica in both traditional and advanced ceramics [2,3,4,5]. Carbothermal reduction of silica leads to silicon carbide (SiC) [6,7,8], while carbothermal reduction of silica followed by nitridation results in silicon nitride [7], and some reports indicate silicon oxynitride as well [9]
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