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: