Rice hulls

Rice hulls

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 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 as a 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. [http://www.dogfoodproject.com/index.php?page=badingredients]

Use as 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.

In 1987, a house was built in Quezon City using Portland cement mixed with rice hull ash and lime as the main building material. The house was still standing in 2002.

Use as 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.

Use in industry

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.

Use as 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.

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 repellants in the form of "vinegar-tar"
*as a release agent in the ceramics industry

ee also

*Rice-hull bagwall construction

References

*Cite journal
last = Ma
first = Jian Feng
coauthors = Kazunori Tamai, Naoki Yamaji, Namiki Mitani, Saeko Konishi, Maki Katsuhara, Masaji Ishiguro, Yoshiko Murata, Masahiro Yano
date = 2006
title = A silicon transporter in rice
journal = Nature
volume = 440
issue = 7084
pages = 688–691
doi = 10.1038/nature04590
issn = 0028-0836
accessdate =

*Cite journal
last = Mitani
first = Namiki
coauthors = Jian Feng Ma, Takashi Iwashita
date = 2005
title = Identification of the silicon form in xylem sap of rice (Oryza sativa L.)
journal = Plant Cell Physiol.
volume = 46
issue = 2
pages = 279–283
doi = 10.1093/pcp/pci018
url = http://pcp.oxfordjournals.org/cgi/content/abstract/46/2/279
accessdate = 2008-05-05
pmid = 15695469

*Cite journal
last = Mitani
first = Namiki
coauthors = Jian Feng Ma
date = 2005
title = Uptake system of silicon in different plant species
journal = J. Exp. Bot.
volume = 56
issue = 414
pages = 1255–1261
doi = 10.1093/jxb/eri121
url = http://jxb.oxfordjournals.org/cgi/content/abstract/56/414/1255
accessdate = 2008-05-05
pmid = 15753109

* [http://www.esrla.com/pdf/The-Rice-Hull-House2.pdf The Rice Hull House] where rice hulls are used for insulation
* [http://www.berr.gov.uk/files/file15138.pdf Uses for rice husk ash, or RHA]
* [http://www.ms.ornl.gov/researchgroups/process/cpg/sic.htm Rice hulls used in cutting tool industry]


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