Multi-stage crystallization of sodium carbonate

Abstract

Sodium carbonate is crystallized in multi-stage evaporative crystallizers from a pregnant liquor process solution typically obtained by dissolving calcined crude trona in water, separating insoluble impurities, and removing the majority of organic matter with activated carbon. After separation from the sodium carbonate crystals, a portion of the mother liquor usually is recycled to the crystallizers, and the remainder is purged to prevent buildup of impurities. Product quality is improved, and the amount of purged mother liquor is reduced by feeding the pregnant liquor predominantly to the initial stage crystallizers, directing the majority of the recycled mother liquor to a final stage crystallizer, recovering sodium carbonate crystals from slurries from at least the initial stages, and withdrawing purge liquor only from the slurry from the final stage crystallizer. Automatic compensation for variations in feed rates is obtained by introducing fresh feed into one end of a common crystallizer feed pipe adjacent to the first stage and directing recycled mother liquor to the other end of the feed pipe adjacent to the final stage. Preferably the stages are arranged in order of decreasing temperature so that the purge liquor is taken from the lowest temperature stage.