Improving the Soda-Sulfur Chemical Make-up Balance: Modifications to the Chlorine Dioxide Plant to Make Neutral Saltcake, 2010 TAPPI/PAPTAC International Chemical Recovery Conference

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Most bleached kraft pulp mills use the by-product sodium sesquisulfate from their chlorine dioxide plant as a source of make-up sodium and sulfur in the kraft liquor cycle. However, sodium sesquisulfate has a relatively low sodium-to-sulfur ratio — much lower than what is needed to maintain soda inventories. A better make-up chemical would be sodium sulfate, or neutral saltcake, which provides 33% more sodium for the same amount of recovered sulfur. Neutral saltcake cannot be made directly in a methanol-based sub-atmospheric chlorine dioxide generator, but it can be produced from the sesquisulfate using a simple add-on process which takes advantage of the different solubilities of the component salts. This paper will review the chemistry behind this process and the equipment required, and will present mill data demonstrating the chemical savings available for bleached kraft pulp mills.