Stability of Sodium Sulfate Dicarbonate (~2Na2CO3.Na2SO4) Crystals Obtained from Evaporation of Aqueous Solutions of Na2CO3 and Na2SO4

Crystallization of sodium sulfate dicarbonate (~2Na2CO3·Na2SO4) is known to be a primary contributor to the fouling of heat-transfer equipment in spent-liquor concentrators used in the pulp and paper industry. Therefore, understanding the conditions leading to the formation and in-process stability of this double salt and the related burkeite salt is crucial to the elimination or reduction of industrial problems. In this work, double salts were generated in a batch crystallizer at close-to-industrial process conditions. X-ray diffraction, calorimetry, and microscopic observation were used to investigate the stability of the salts to in-process aging, isolation and storage, and exposure to high temperature. The results verify that sodium sulfate dicarbonate exists as a unique phase in this system and that it remains stable at process conditions.