The present invention provides preparation of nano-silica from a cost competitive spent foundry sand by amenable depolymerization followed by sol-gel method (precipitation). The prepared nano-silica was characterized using different modern analytical techniques. In the present invention the prepared silica nanoparticles delivers particle size in the range of 8 nm to 100 nm. Further, the average particle size of the obtained nano-silica from sodium silicate of waste foundry sand was about 25 nm. The pore size and surface area of the synthesized nano-silica were found to be 5.5 nm and 345 m2/g respectively. The synthetic process using spent found sand developed in the present work for the preparation of nano-silica offers number of special features with regard to concentration of alkali silicate used, pH of the precipitation reaction, time, temperature and condition of aging and percentage yield. The yield of the nano-silica obtained was about 90 %. The value added nano-silica obtained in the present work from spent foundry sand can be used in different industrial applications such as fillers in concrete, desiccants. adsorbenL sealants, and reinforcements Thus, the present invention is considered to be a cost competitive and breakthrough route that facilitates value addition to the solid waste.