Abstract
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. Key words: Rice husk ash (RHA), rice husks (RH), silica, calcination, open air burning.
Highlights
The rice grain, commonly called a seed, consists of the true fruit or brown rice and the hull, known as the husk, which encloses the brown rice
Manganese, Silicon and Carbon had a higher percentage in ORHA; whereas iron had a higher percentage in FRHA
This is confirmed by XRF analysis that shows the levels of silica as 94% for FRHA and 95% for ORHA (Table 2)
Summary
The rice grain, commonly called a seed, consists of the true fruit or brown rice (caryopsis) and the hull, known as the husk, which encloses the brown rice. The husks are about one-fifth by weight and contain about 20-30% silica, the rest being organic lignin and cellulose (Chaudhary et al, 2004) of about 70-80% of dry hull (Mohamad, 2007). The cellulosic fiber tends to degrade at about 200°C and quickly becomes friable with loss of water (Shanks et al, 2004). These natural fibers have presence of large amounts of hydroxyl groups, which makes the properties of rice husk very much in category of hydrophilic (Mohamad, 2007)
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