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Rice hulls (or rice husks) are the hard protecting coverings of grains of
rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...
. In addition to protecting rice during the growing season, rice hulls can be put to use as
building material Building material is material used for construction. Many naturally occurring substances, such as clay, rocks, sand, wood, and even twigs and leaves, have been used to construct buildings. Apart from naturally occurring materials, many man- ...
,
fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
, insulation material, or
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy b ...
. Rice hulls are part of the
chaff Chaff (; ) is the dry, scaly protective casing of the seeds of cereal grains or similar fine, dry, scaly plant material (such as scaly parts of flowers or finely chopped straw). Chaff is indigestible by humans, but livestock can eat it. In agri ...
of the rice.


Production

Rice hulls are part of the rice seed. The hull protects the grain during the growing season from pests. The hull is formed from hard materials, including opaline
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
and
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...
. The hull is hard to eat or swallow (unless finely ground) and mostly indigestible to humans because of its enriched fibre components. However, during times of food scarcity in ancient China, a common daily meal was a pastry made from rice husks, wild vegetables, and soybean powder. This led to the idiom "meals of cereal, hulls, and vegetables for half a year," indicating poverty and
food insecurity Food security speaks to the availability of food in a country (or geography) and the ability of individuals within that country (geography) to access, afford, and source adequate foodstuffs. According to the United Nations' Committee on World ...
. Testing and commercialization of human grade anti-caking agents were done in the early 2000s. The material is approved for use in USDA Certified Organic products to replace silicon dioxide. Winnowing, used to separate the rice from hulls, is to put the whole rice into a pan and throw it into the air while the wind blows. The light hulls are blown away while the heavy rice fall back into the pan. Later pestles and a simple machine called a
rice pounder A rice pounder is an agricultural tool, a simple machine that is commonly used in Southeast Asia to dehull rice or to turn rice into rice flour. The device has similar functionality to a mortar and pestle, but with more mechanical advantage to ...
were developed to remove hulls. In 1885 the modern rice hulling machine was invented in Brazil. During the milling processes, the hulls are removed from the raw grain to reveal whole brown rice, which is then usually milled further to remove the bran layer, resulting in
white rice White rice is milled rice that has had its husk, bran, and germ removed. This alters the flavor, texture and appearance of the rice and helps prevent spoilage, extend its storage life, and makes it easier to digest. After milling ( hulling), t ...
.


Uses


Ash

Combustion of rice hulls affords rice husk ash (acronym RHA). This ash is a potential source of amorphous reactive
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
, which has a variety of applications in materials science. Most of the ash is used in the production of
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19t ...
.Otto W. Flörke, et al. "Silica" in Ullmann's Encyclopedia of Industrial Chemistry, 2008, Weinheim: Wiley-VCH, . . When burnt completely, the ash can have a Blaine number of as much as 3,600 compared to the Blaine number of cement (between 2,800 and 3,000), meaning it is finer than cement. Silica is the basic component of sand, which is used with cement for plastering and concreting. This fine silica will provide a very compact concrete. The ash also is a very good thermal insulation material. The fineness of the ash also makes it a very good candidate for sealing fine cracks in civil structures, where it can penetrate deeper than the conventional cement sand mixture. Rice husk ash has long been used in ceramic glazes in rice growing regions in the Far East, e.g. China and Japan. Being about 95% silica, it is an easy way of introducing the necessary silica into the glaze, and the small particle size helps with an early melt of the glaze. A number of possible uses for RHA include absorbents for oils and chemicals, soil ameliorants, a source of silicon, insulation powder in steel mills, as repellents in the form of "vinegar-tar" release agent in the ceramics industry, as an insulation material. More specialized applications include the use of this material as a
catalyst support In chemistry, a catalyst support is the material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the material. Cons ...
. Goodyear announced plans to use rice husk ash as a source for tire additive. Rice hulls are a low-cost material from which silicon carbide "
whiskers Vibrissae (; singular: vibrissa; ), more generally called Whiskers, are a type of stiff, functional hair used by mammals to sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are coarse ...
" can be manufactured. The SiC whiskers are then used to reinforce ceramic cutting tools, increasing their strength tenfold.


Toothpaste

In
Kerala Kerala ( ; ) is a state on the Malabar Coast of India. It was formed on 1 November 1956, following the passage of the States Reorganisation Act, by combining Malayalam-speaking regions of the erstwhile regions of Cochin, Malabar, South Ca ...
,
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
, charcoal from Rice husks (''Umikari'' in
Malayalam Malayalam (; , ) is a Dravidian language spoken in the Indian state of Kerala and the union territories of Lakshadweep and Puducherry ( Mahé district) by the Malayali people. It is one of 22 scheduled languages of India. Malayalam wa ...
) were universally used for over centuries in cleaning teeth, before toothpaste replaced it.


Rice bran oil

Rice bran oil is the oil extracted from the hard outer brown layer of rice called chaff (rice husk). It is popular as a cooking oil in the Indian subcontinent and East Asian countries, including India, Nepal, Bangladesh, Indonesia, Japan, Southern China and Malaysia.


Brewing

Rice hulls can be used in
brewing Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and fermenting the resulting sweet liquid with yeast. It may be done in a brewery by a commercial brewer ...
beer Beer is one of the oldest and the most widely consumed type of alcoholic drink in the world, and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from ce ...
to increase the
lautering Lautering () is the beer brewing process that separates the mash into clear liquid wort and residual grain. Lautering usually consists of three steps: mashout, recirculation, and sparging. Mashout Mashout is the term for raising the temperatur ...
ability of a mash. Rice husk is also used in one step of traditional preparation processes of Kaoliang (Sorghum) liquid. After fermentation, rice husk can be added into the wine tank to increase the void, which is advantageous for distillation.


Fertilizer and substrate

Rice hulls can be composted, but their high
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...
content can make this a slow process. Sometimes
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. T ...
s are used to accelerate the process. Using
vermicompost Vermicompost (vermi-compost) is the product of the decomposition process using various species of worms, usually red wigglers, white worms, and other earthworms, to create a mixture of decomposing vegetable or food waste, bedding materials, and ...
ing techniques, hulls can be converted to fertilizer in about four months. Rice hulls that are parboiled (PBH) are used as a substrate or medium for gardening, including certain
hydroculture Hydroponics is a type of horticulture and a subset of hydroculture which involves growing plants, usually crops or medicinal plants, without soil, by using water-based mineral nutrient solutions in aqueous solvents. Terrestrial or aquatic plant ...
s. The hulls decay over time. Rice hulls allow drainage, and retain less water than growstones. It has been shown that rice hulls do not affect plant growth regulation.


Fireworks

Rice hulls are coated with fine-grained
gunpowder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). T ...
and used as the main bursting charge in aerial fireworks shells.


Fuel

With proper techniques, rice hulls can be burned and used to power
steam engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
s. Some rice mills originally disposed of hulls in this way. Unfortunately the direct combustion of rice hulls produces large quantities of smoke. An alternative is
gasification Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (). This is achieved by reacting ...
. Rice hulls are easily gasified in top-lit updraft gasifiers. The combustion of this rice hull gas produces a blue flame, and rice hull biochar makes a good soil amendment. Traditional brickmaking
kilns A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...
in
Mekong Delta The Mekong Delta ( vi, Đồng bằng Sông Cửu Long, lit=Nine Dragon River Delta or simply vi, Đồng Bằng Sông Mê Kông, lit=Mekong River Delta, label=none), also known as the Western Region ( vi, Miền Tây, links=no) or South-weste ...
are using rice hulls as fuel.


Juice extraction

Rice hulls are used as a "press aid" to improve extraction efficiency of
apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...
pressing.


Pet food fiber

Rice hulls are an inexpensive
byproduct A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be consid ...
of human food processing, serving as a source of
fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
that is considered a filler
ingredient An ingredient is a substance that forms part of a mixture (in a general sense). For example, in cooking, recipes specify which ingredients are used to prepare a specific dish. Many commercial product (business), products contain secret ingredie ...
in
pet food Pet food is animal feed intended for consumption by pets. Typically sold in pet stores and supermarkets, it is usually specific to the type of animal, such as dog food or cat food. Most meat used for animals is a byproduct of the human food ind ...
s.


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.


Insulating material

Rice hulls themselves are a class A thermal insulating material because they are difficult to burn and less likely to allow moisture to propagate mold or
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
. It is also used as roofing after mixing it with mud and water.


Particle boards and cardboard

Rice hulls are also used to make particle boards and
cardboard Cardboard is a generic term for heavy paper-based products. The construction can range from a thick paper known as paperboard to corrugated fiberboard which is made of multiple plies of material. Natural cardboards can range from grey to light b ...
. The silica in rice husk make the particle boards less attractive to
termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...
s.


Geopolymers

Due to high amorphous silica content, the RHA (rice husk ash) can be used as a precursor material for geopolymer concrete.


Rice concrete

To achieve the best pozzolanic properties the combustion of the husks has to be carefully controlled by keeping the temperature below 700°C and to create conditions to minimize carbon formation by feeding sufficient air. At a given water-cement ratio, the addition of small amounts of rice hull ash (2 to 3 % of cement mass) may be useful to improve the workability of concrete mixtures by reducing the cement milk separation and segregation and increasing the strength and durability of concrete. However, the introduction of large quantities of this additive may result in poor workability of the concrete mixture if strong water reducing additives are not used.


See also

* Rice-hull bagwall construction * Winnowing barn


References

* * *{{Cite journal , last = Mitani , first = Namiki , author2=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 , pmid = 15753109 , citeseerx = 10.1.1.543.3168


External links


The Rice Hull House
where rice hulls are used for insulation
Uses for rice husk ash, or RHA
Rice products Materials Building materials Cement Concrete Pet foods