Abstract: The incinerator product obtained from the incineration of black liquor and other spent liquors from a soda pulping process is treated to remove contaminants including chlorides, sulphates and potassium compounds and to recover a high proportion of the sodium carbonate. The incinerator product is treated with water to form a slurry in which substantially pure sodium carbonate monohydrate is crystallized and a major proportion of the contaminants are dissolved, the sodium carbonate monohydrate is then separated and dissolved sodium carbonate is recovered as a bicarbonate or sesquicarbonate precipitate by carbonation of the solution. The preferred process involves an initial flue gas carbonation of the separated mother liquor after removal of sodium carbonate monohydrate to produce a sodium sesquicarbonate precipitate and a subsequent carbonation of the mother liquor, after separation of the sodium sesquicarbonate therefrom, with pure carbon dioxide to precipitate sodium bicarbonate.