Production of Neutral Saltcake in Sub-Atmospheric Chlorine Dioxide Plants, 2010 TAPPI PEERS Conference

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The predominant chlorine dioxide process in North America uses a sub-atmospheric generator, operating at high acidity and using methanol to reduce the sodium chlorate to chlorine dioxide (i.e. ERCO R8 and Eka SVP-Lite). Because of the high acidity needed to maintain an acceptable reaction rate, these processes make acidic saltcake (sodium sesquisulfate, Na3H(SO4)2) as a by-product. While sodium sesquisulfate is a good source of both sodium and sulfur for kraft liquor cycle make-up, neutral saltcake (sodium sulfate, Na2SO4) would be even better, because it provides two units of sodium for every unit of sulfur recovered. A methanol-based sub-atmospheric chlorine dioxide generator cannot be effectively operated at an acid concentration low enough to produce neutral saltcake. However, R8 and SVP-Lite chlorine dioxide plants can be modified with an add-on process (ERCO R10 and Eka SVP-SCW, respectively) that metathesizes (or re-crystallizes) neutral sodium sulfate from an acidic solution of sodium sesquisulfate. The neutral saltcake can be safely dissolved directly in black liquor, providing the recovery cycle with more sodium for a given amount of sulfur, thus decreasing the need for purchased make-up caustic. The acidic sodium bisulfate still in solution can be recycled to the chlorine dioxide generator, displacing purchased acid.