Process for purifying ash which principally consists of sodium sulphate from a recovery boiler

摘要

Ash from a recovery boiler is purified by leaching or evaporation- crystallizing contaminants such as, chlorides and potassium salts in water. The leaching or evaporation-crystallizing takes place in an aqueous solution at a temperature that is in the vicinity of the boiling point of the aqueous solution. The leached or evaporation- crystallized ash is returned to the black liquor or to the recovery boiler while the leaching water or the evaporation-crystallization water is then cooled to less than 32° C., preferably 10-15° C. Sodium sulphate containing crystallization water, such as sodium sulphate heptahyfdrate or sodium sulphate decahydrate, is crystallized out and then separated to be reintroduced into the black liquor, directly into the recovery boiler or into the leaching solution or evaporation- crystallization solution.