Characterization of rice husk ash prepared by open air burning and furnace calcination

Rice husk (RH) is an agricultural waste, and easily available in rice growing areas. The husk is mostly burnt as a way of getting rid of it. The ash obtained after burning or calcination may have economic application, mainly dependent on its properties. These properties in turn depend on the calcination method. However, for commercial viability, and for many applications, the calcination method should not only be as simple as possible but also cheap. This study characterized the elemental composition, crystallinity, functional bonds present and morphology of rice husk ash (RHA) obtained in two ways, that is, calcination of rice husks in a muffle furnace (FRHA) at a temperature of 700°C and open air burning (ORHA) at uncontrolled temperatures. The elemental composition done by Atomic Absorption Spectroscopy showed a high percentage of silicon that is 81.01 and 79.12% for ORHA and FRHA, respectively. X-ray fluorescence showed a high percentage of silica (SiO2), 95.45 and 94.85% for ORHA and FRHA, respectively. X-ray diffractograms indicate that the FRHA was crystalline with the highest peak at 21.8°; while ORHA was amorphous in nature. Fourier Transform Infra-Red spectra confirmed the presence of –OH groups and O-Si-O bonds in the two types of ash. Scanning electron microscopy analysis showed agglomerated ORHA, which may be due to the presence of hydrogen bonding between silanol groups on the surface of rice husk ash for FRHA, and presence of –OH groups in ORHA. The study shows that ORHA is as good as FRHA in applications where crystallinity is optional.\r\n\r\n\tKey words: Rice husk ash (RHA), rice husks (RH), silica, calcination, open air burning.