Extraction of Silica from Rice Husk
The plants belonging to graminae family, namely, rice, wheat,
barley, oats, maize are the main sources of biogenic the roots as
dissolved monosilicic acid, Si(OH)4& precipitated within around
cells as hydrated deposits.The monosilicic and moves by evaporation
of polymerized to force a cellulose silica membrane. It has been
formal that silica is highly concentrated in the inner and
outermost surfaces of the epidermal twice. As a result, rice husks
are relatively friable, brittle and abrasive. The term white ash
refers to residue obtained from complete combustion of rice husk.
It if chiefly composed of silica (86-97%). This silica has
following structure, in which the atom is bonded to four
surrounding O atoms by Si-O-Si bonds. Above fig. shows that each Si
atom is surrounded by four O atoms and O atom serves as a neighbor
to two Si atoms there by leading to a condensation product with a
formula SiO2.Silica occurs in nature mainly in three crystalline
fours namely quartz (hexagonal), cristobolite (white) and tridymite
(hexagonal) and also in amorphous form like opal and silica in rice
husk being of bioquenic origin is essentially of amorphous nature.
But it can be connected into any of above three crystalline forms
by heat treatment at different temperature.
The precipitated silica market can be broadly classified as rubber
grade and non- rubber grade. The total size of the silica market in
India is about 16000 tones. Out of this rubber grade silica market
is 11000 tones and non-rubber grade market is 5000 tones. Tires,
rice roller mills, adhesive, toothpastes, free-flowing salt and
certain food products are major users of silica in the country. The
market is currently growing at the rate of 8% per annum. Some uses
are as listed below
Reinforcement of elastomeric products like shoe soles.
Reinforcement of silicone rubber.
Reinforcing material in tires.
In sheathing compounds for cables.
Constituent of adhesives in for bonding of unvulcanized rubber to
textiles or steel tire cord.
In thermoplastics used to act as anti-blocking agents and to
prevent plate out effects in films and film production.
To improve mechanical properties of PVC flooring.
As carrier silica for materials and as free flow agents for
powder formulation, particularly of hygroscopic and adhesive
substances.
As adsorbent.
In toothpaste to control rheological properties and as a
cleansing agent.
Hydrophobic precipitated silica is used in mineral oil and
silicone oil antifoaming effect.
Purification and stabilization of beer.
Analysis of blood.
Cosmetics.
Food industry as an anti-caking agent.
Specially prepared silica gels from silica are used for making
thermal insulation material.
As a dehumidifying agent for air and other gases.
As a filtering agent to clarify of juices.
