Cost-effective synthesis of high purity precipitated silica from locally available rice husk ash

Abstract

Revolutionary bio-material Rice Husk Ash (RHA) has high potential to help the tire and rubber industry to become more sustainable. This material has also a good candidate to reduce energy consumption and CO2 emission. The conventional process of manufacturing precipitated silica by sand fusion with soda ash followed by acidification, is highly energy intensive and expensive as the process requires the reactants to be heated to high temperatures of around 1400ºC. The rising cost of raw materials such as sodium silicate, soda ash and fuel in international market is also the reason to seek the alternative manufacturing route of silica production. In contrast, rice husk ash, an environmental burden made by burning rice husk (RH) as fuel to generate thermal energy, would be an important and potential alternative cheaper silica source for producing silica where the silica content can be as high as 97%. In this study, a cost-effective synthesis method of commercial grade precipitated silica (≥ 99%) has been developed based on alkaline extraction method of silica from locally available rice husk ash.

An integrated process flow sheet was developed and practiced that included initial leaching, alkali extraction of rice husk ash, acid precipitation of sodium silicate and finally drying and pulverizing of as produced precipitated silica from rice husk ash. Alkali extraction of purified rice husk ash was investigated within the temperature range of 120-170ºC. The extracted sodium silicate solution was then acidified by sulphuric acid to reduce pH of the solution to ~7. At around pH 7, precipitation process of silica was completed. After washed, precipitated silica was aged, dried and pulverized to get fine powder. Analysis of rice husk ash and final precipitated silica powder by X-Ray Fluorescence (XRF) showed amorphous silica content 88-97% and 97-98% respectively. X-ray diffraction results confirmed presence of amorphous silica due to the broad peak zone and presence of trace amount crystalline silica due to the presence of cristobalite. The effects of temperature, pH, leaching agents, acid concentrations, alkali and acid amount optimized in this study.

A production cost analysis was also carried out, which shows the developed synthesis route of high purity precipitated silica from rice husk ash cost ~43 BDT/kg. Whereas, other existing commercial process cost >46 BDT/kg for producing precipitated silica, which indicates the suitability and exploitability of the cost-effective synthesis process being developed in this research work for producing high purity precipitated silica.