Improving MnO2 Catalyzed Polysulfide Generation, 2010 TAPPI PEERS Conference

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Two mills in North America are presently using polysulfide in the cooking process to preserve carbohydrate yield. One mill uses the Moxy™ process and the other uses the Paprilox™ process to generate the polysulfide. Parts of the mechanism of the Paprilox™ process are unclear. Knowing the mechanism has value because it may allow improvements in both the choice of operating conditions and the design of the system.

The questions that guided this work were: which sulfur species participate in the reaction, what is the reduced form of manganese in the catalytic system, and what is the cause of the dissolution of manganese dioxide? It was found that there are some major differences in the mechanism from that established for the stoichiometric formation of polysulfide using MnO₂: Sulfide (S^2-) rather than hydrosulfide (HS-) is the acting sulfur species; MnS rather than MnO is the reduced species that is formed; and MnS is the dissolved species that is catalytically reoxidized to MnO₂.

These findings show that high sulfidity liquor is not the most critical requirement for liquors to be used for the Paprilox™ process but high alkalinity which ensures a high S^2- concentration because of the S^2-/HS^- equilibrium. This work also shows that a soluble species is a critical intermediate which, unfortunately, then precludes the use of a reactor using MnO₂ coated surfaces as these will dissolve and not be regenerated.