Purification of Diatomaceous Earth using Acid Leaching Process to Produce High Purity Silica for Solar-Grade Silicon

The core focus of this research study was preparation of high-purity silica by the wet-chemical process for solar grade silicon (SOG-Si) using diatomaceous earth expecting a huge amount of resources in the earth crust. Our purification method involved dissolution and precipitation of silica in sodium hydroxide (NaOH) and tetramethylammonium hydroxide (TMAH) aqueous solution followed by acid leaching with HCl solution. By decreasing the pH, more than 90% of the impurities like Al and Fe were precipitated. A large amount of Na impurities were noticed in the final silica product as NaOH was used for the preparation of aqueous alkaline solution. To overcome this serious issue deteriorating the semiconductor characteristics, the mixture of NaOH and TMAH was introduced in order to lower the Na levels during the preparation of alkaline solution. Experimental results suggested that the Na⁺ ions were needed to have a better dissolution rate of silica in aqueous alkaline solution. This also resulted in the optimum concentration of 0.05 M NaOH and 1.40 M TMAH for the dissolution and precipitation process in order to help the purification of silica. The 3 stages of leaching process with the above mentioned concentration helped us in achieving the 5 N (99.999 wt%) level of silica purity.