Mixed solvent recrystallization for the densification and purification of soda ash

Soda, sold as light or dense soda ash, is an important bulk chemical and produced annually on a scale of several million short tons world-wide. At atmospheric conditions, soda (sodium carbonate anhydrate) does not form from its aqueous solution, as only hydrates are stable. In a mixed solvent solution, composed of an organic antisolvent (ethylene glycol) and water, the anhydrate is stable and can be crystallized by evaporative crystallization or more energy-efficiently by a double recrystallization process. Since crystallization is a highly selective separation process and since the soda is grown under controlled conditions in the mixed solvent solution, a significant purification effect was expected. The purifying effect of the double recrystallization (light soda to monohydrate, monohydrate to dense soda) was studied in this work. The effect of anionic impurities, which commonly occur in current soda production processes, was investigated: Sulfate, borate, chloride and fluoride. It was found that fluoride, chloride and borate were only incorporated in ppm levels in the solid, while sulfate was incorporated almost quantitatively.