Silica from Ash
Silica (SiOz ) is one of the valuable inorganic multipurpose
chemical compounds. It can exist in gel, crystalline and amorphous
forms. It is the most abundant material on the earth's crust.
However, manufacture of pure silica is energy intensive. A variety
of industrial processes, involving conventional raw materials
require high furnace temperatures (more than 700°C). In this
article, a simple chemical process is described which uses a
non-conventional raw material rice husk ash for extraction of
silica. Rice husk ash is one of the most silica rich raw materials
containing about 90-98% silica (after complete combustion) among
the family of other agro wastes. Rice husk is a popular boiler fuel
and the ash generated usually creates disposal problems. The
chemical process discussed not only provides a solution for waste
disposal but also recovers a valuable silica product, together with
certain useful associate recoveries.
The selection of ash is important as the quality of ash determines
the total amount as well as quality of silica recoverable Ash which
has undergone maximum extent of combustion is highly desirable as
it contains higher percentage of silica. It appears white-grey in
colour when compared to the black coloured ash obtained from
incomplete combustion. The carbon present in such ash hinders the
main silica digestion reaction and may change the product
characteristics (colour, etc.). This method of quality assessment
is more suitable to workers at the processing site as it does not
involve lab-scale analysis.
The initial step is extraction of silica from ash as sodium
silicate using caustic soda. This reaction is carried out at a
temperature in the range 1800-200°C and pressure ranging from 6-8
atmosphere. The reaction is:
But high reaction temperature and pressure can be avoided if ash
obtained by burning rice husk at 650°C is used. This ash is mostly
amorphous silica which is reactive around 100°C with NaOH solution
to yield sodium silicate. A viscous, transparent, colourless
sodium-silicate solution (-15%w/w) is obtained after filtration of
the reacted slurry (consisting of residue digested ash,
sodium-silicate, water and free sodium hydroxide). In the second
step of the process, silica is precipitated from sodium-silicate
using sulphuric acid. This step requires controlled conditions of
addition rate of sulphuric acid and temperature of reacting mass in
a neutralizer. The temperature is in the range of 90-100°C and
pressure is the normal atmospheric pressure.
In the second step of the process, silica is precipitated from
sodium-silicate using sulphuric acid. This step requires controlled
conditions of addition rate of sulphuric acid and temperature of
reacting mass in a neutralizer. The temperature is in the range of
90-100°C and pressure is the normal atmospheric pressure.
The reaction is:
