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Rice hulls (or rice husks) are the hard protecting coverings of grains of rice. In addition to protecting rice during the growing season, rice hulls can be put to use as building material, fertilizer, insulation material, or fuel.

Production

Rice hulls are the coating for the seeds, or grains, of the rice plant. To protect the seed during the growing season, the hull is made of hard materials, including opaline silica and lignin. The hull is mostly indigestible to humans. During the milling process, the hulls are removed from the grain to create brown rice, the brown rice is then milled further to remove the bran layer to become white rice. The very high content in amorphous silica of the hulls confer to them and to their ash (SiO2 ~ 20 wt.%) after combustion very valuable properties.

Use

A number of rice-producing countries, (e.g. Thailand), are currently conducting research on industrial uses of rice hulls. Some of the current and potential applications are listed below.

Chemistry

Rice hulk can be used to produce mesoporous molecular sieves (e.g., MCM), which are applied as catalysts for various chemical reactions, as a support for drug delivery system and as adsorbent in waste water treatment.

Pet food fiber

Rice hulls are the outermost covering of the rice and come as organic rice hulls and natural rice hulls. Rice hulls are an inexpensive byproduct of human food processing, serving as a source of fiber that is considered a filler ingredient in cheap pet foods. [173097]

Building material

Rice hulls are a class A insulating material because they are difficult to burn and less likely to allow moisture to propagate mold or fungi. It was found out that rice hull when burned produced amounts of silica. For these reasons it provides excellent thermal insulation.

Pillow stuffing

Rice hulls are used as pillow stuffing. The pillows are loosely stuffed and considered therapeutic as they retain the shape of the head. In Chinamarker, in 2009, where these pillows have become popular, one pillow costs approximately 40 Reminbi (almost USD $6), and is considered a luxury item. The high price is testament to its trendiness, as one ton of rice hulls, even in the US, costs approximately $5 US.

Fertilizer

Rice hulls are organic material and can be composted. However, their high lignin content can make this a slow process. Sometimes earthworms are used to accelerate the process. Using vermicomposting techniques, the hulls can be converted to fertilizer in about four months.

SiC production

Rice hulls are a low-cost material from which silicon carbide "whiskers" can be manufactured. The SiC whiskers are then used to reinforce ceramic cutting tools, increasing their strength tenfold.

Fuel

With proper techniques, rice hulls can be burned and used to power steam engines. Some rice mills originally dispose off the hulls in this way.

Brewing

Rice hulls can be used in brewing beer to increase the lautering ability of a mash.

Juice extraction

Rice hulls are used as a "press aid" to improve extraction efficiency of apple pressing.

Rice husk ash

The ash produced after the husks have been burned, (abbreviated to RHA), is high in silica. A number of possible uses are being investigated for this. These uses include
  • aggregates and fillers for concrete and board production
  • economical substitute for microsilica / silica fumes
  • absorbents for oils and chemicals
  • soil ameliorants
  • as a source of silicon
  • as insulation powder in steel mills
  • as repellents in the form of "vinegar-tar"
  • as a release agent in the ceramics industry
  • as an insulation material for homes and refrigerants


See also



External links



References

  1. S. Chiarakorn et al. "Influence of functional silanes on hydrophobicity of MCM-41 synthesized from rice husk" Sci. Technol. Adv. Mater. 8 (2007) 110 free download
  2. J. Chumee et al. "Characterization of platinum–iron catalysts supported on MCM-41 synthesized with rice husk silica and their performance for phenol hydroxylation" Sci. Technol. Adv. Mater. 9 (2008) 015006 free download
  3. http://www.ms.ornl.gov/researchgroups/process/cpg/sic.htm
  4. Press aids







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