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Naum Aksenov
Naum Aksenov

Manufacture Of Value Added Products From Rice Husk

In this study, 13 rice husk (RH) varieties from 4 agro-ecological zones in Uganda were characterized, NaOH-pretreated, and evaluated for their potential utilization as precursors for production of bio-oil, ash, char, and activated carbon for selected applications. RH varieties were characterized through particle size analysis, bulk density, proximate and ultimate analyses, specific surface area, pore volume, as well as lignocellulosic and inorganic compositions. Selected RH varieties were subsequently pretreated at NaOH concentrations of 1-4%w/v, using pretreatment ratios of 5 g RH: 40 mL NaOH. Properties varied among RH varieties, suiting them as feedstocks for different applications. Upland rice husk varieties are more suited precursors for production of bio-oil, and activated carbon due to their relatively lower ash content, higher specific surface area, as well as higher volatile matter and fixed carbon contents. Upland rice husks could as well be employed in the preparation of electrodes for electrochemical devices, due to their relatively higher specific surface area. A high ash content (21-32% dry basis) of lowland rice husks presents good prospects for their calcination, since larger amounts of rice husk ash could be obtained, and employed in different applications. Lowland rice husk varieties could also be more suited precursors for production of char for soil amendment, due to their relatively higher ash content, which subsequently increases their char yields. However, alkaline pretreatment of rice husks using 2-4%w/v NaOH can reduce the ash content by as much as 74-93%, depending on the rice husk variety, which paves way for utilizing rice husks with a high ash content in different applications. Aside from ash reduction, the enhanced specific surface area (1.2-1.7 m2 g-1), volatile matter (68-79%db) and fixed carbon (19-24%db) contents of NaOH-pretreated rice husks suggests they are more suited feedstocks than when employed in their raw form, for production of bio-oil, as well as activated carbon.

Manufacture of Value Added Products from Rice Husk


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Rice husk ash (RHA) is the major by-product left after the burning of rice husk, which is profusely present throughout the process of the rice milling. The burnt rice husk, as RHA, in turn causes more environmental pollution and its disposal becomes a difficult problem, hence requiring serious attention from the scientific community regarding its disposal and proper reuse if possible. The major economic reason for recycling the ash is the value added products which can be generated from it. The focus is on the use of RHA as adsorbent and subsequent silica production owing to the fact that the ash is mainly composed of carbon and silica. As regards other potential applications of ash, research is still going on and some of the products, which are under development phase, have also been brought to limelight in this review. This literature review provides an effective scheme to utilize RHA and discussed process pathway for economically valuable products to provide a solution to the problem associated with its proper disposal through superior recycle of this agriculture waste.

The present work deals with the use of multiple-step procedures to obtain valuable sub-products, including nanocellulose, from rice husk. Each sub-product was characterized after every step by analyzing the chemical composition (mainly based on thermogravimetric analysis, Fourier transformed infrared spectra, and X-ray diffraction) and morphology (using visual observations and scanning electron microscopy). The results clearly showed that the selected procedure gave the possibility to separate silica in the first step and then to purify the resultant material, leading to nanocellulose production. All acquired sub-products can be used as additives and fillers in a very wide range of applications. The obtained results will be useful both from technological and academic points of view, mainly for people working in the field of biocomposites. The final material could give added value to a raw biomass material source such as rice husk.

Rice husk (RH) is a raw biomass material with high potential for manufacturing value-added products. The global rice production for 2006/07 was 420 million tons (FAO). In the course of producing the rice, approximately 80 million tons of husk residues were produced. In Asia and South America most of the rice husk is used as bedding material for animals or burned for energy generation, while the industrial applications of this material are still limited (Garcia et al. 2007; Leiva et al. 2007; Park et al. 2003, 2004; Ruseckaite et al. 2007; Stefani et al. 2005). Therefore, it will be useful to consider the use of this waste for producing value-added products, providing a clear positive effect on the environment.

The main components of rice husk are cellulose (25 to 35%), hemicelluloses (18 to 21%), lignin (26 to 31%), silica (15 to 17%), solubles (2 to 5%), and moisture ca. 7.5% (Gerardi et al. 1998; Leiva et al. 2007; Mansaray and Ghaly 1998; Stefani et al. 2005). Some of these ingredients can be recovered for further applications by suitably combining chemical and thermal treatments. Many previous studies have been focused mainly on the production of amorphous silica from rice husk; such silica can be used for multiple applications. Amorphous silica can be obtained from direct or after pre-treatment combustion of rice husk at temperatures lower than 600 ºC (Stefani et al. 2006; Sun and Gong 2001). Calcination is the most used method to obtain rice husk sub-products, nevertheless only amorphous silica can be recovered in this case, resulting in the loss of the entire organic cellulose-rich phase. Cellulose is found in the cell wall of lignocellulosic materials as microfibrils embedded in the non-cellulosic matrix, which is mainly formed by hemicellulose and lignin (Ruseckaite et al. 2007; Stefani et al. 2005; Zuluaga et al. 2009). Several routes have been used to isolate microfibrils from natural resources such as sisal (Moran et al. 2008), hemp (Wang et al. 2007), lemon and maize (Rondeau-Mouro et al. 2003), and potato (Dufresne et al. 2000). In addition, those cellulose microfibrils are composed of nanocrystalline domains and amorphous regions (Moran et al. 2008). A controlled acid hydrolysis could separate both regions, permitting isolation of crystalline domains with high elastic modulus (Samir et al. 2004). These nanofibers have been shown to be useful as new reinforcing agents in the production of nanocomposites (Faria et al. 2006; Zuluaga et al. 2007).

The rice husk (RH) used in the present work consisted of residues from the rice industries of Entre Ríos (Argentina). RH was extensively washed with distilled water to remove dust and other impurities. This operation was performed several times at room temperature and under vigorous stirring. After successive washings, RH was dried to equilibrium moisture (about 7 wt %) in an air-circulated oven at 100 2 ºC. This material was stored in hermetic plastic containers in order to prevent microbial attack (i.e. fungi).

The washed and dried rice husk samples (C0) were submitted to different types of chemical treatments: Rice husk (C0) was stirred with 3% (w/v) KOH at weight ratio of 1:12 and boiled for 30 min; then the mixture was left overnight. The filtrate was washed twice with doubly distilled water, and 10% (v/v) HCl was added (100 mL). The formed precipitate of silica was separated from organic residue (C1) (Daifullah et al. 2004). After that, the lignocellulosic residue was treated with 0.7 % (w/v) sodium chlorite at a ratio of 1:50 g solid/ ml liquor at pH 4 and kept boiling for 2 h. The remaining solid was first treated with 5% (w/v) sodium bisulphite solution at room temperature for 1 h using a solid to liquor ratio of 1 g/50 ml and then washed with distilled water and dried at 100 2 ºC in an air-circulated oven until constant weight was reached (C2). After that, the sample was treated with 17.5 % (w/v) sodium hydroxide (NaOH) solution, at room temperature for 8 hours using a solid to liquor ratio of 1 g/50 mL, washed, and dried again as in the previous step (C3). Figure 1 presents a scheme of the chemical treatments carried out on rice husk. 041b061a72


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