Crystallization of sodium carbonate from green liquor by cooling

Cooling crystallization of green liquor has been proposed for separating so dium carbonate for use in split-sulfidity kraft pulping. However most publi shed information on the crystallizing process is for synthetic mixtures of sodium carbonate and sulfide. Laboratory experiments on green liquor from a pulp mill showed that solubility of sodium carbonate at low temperatures i s similar to that in synthetic mixtures, but that the dynamics of the cryst allization of green liquor are much more complicated than equilibrium tests suggest. A series of metastable conditions are found on cooling the liquor . Although some crystallization occurs between c. 20 and c, 15 degrees C th e apparent solubility of sodium carbonate is considerably higher than the e quilibrium value. Below 15 degrees C, crystallization is rapid and the meas ured solubility is similar to equilibrium values found for synthetic mixtur es. Industrially, these findings are useful for process purposes as they wo uld permit most of the cooling of green liquor to be performed rapidly in o rdinary heat exchangers, and the more expensive crystallizing coolers can t herefore be comparatively small.