In Situ Formation of Formamidine Sulfinic Acid: An Oxidative/Reductive Bleaching Process for Recycled Fiber, 1994 Pulping Conference Proceedings

Cynthia K. Fallon

Formamidine sulfinic acid (FAS) is recognized as an effective reagent for dye destruction, and sequences involving an oxidative hydrogen peroxide (P) stage followed by a reductive FAS stage can yield impressive bleaching results on recycled fiber. Formamidine sulfinic acid is formed via the reaction of peroxide with thiourea (TU) at low temperature in a slightly acidic medium, whereas bleaching with FAS is most effective at high temperature under alkaline conditions. Since the major drawback to the use of FAS is its cost, an investigation was initiated to see if FAS could be formed in the presence of pulp by consuming the residual peroxide from a P-stage. This could potentially reduce costs because 1) residual peroxide would not have to be destroyed prior to a reductive stage, and 2) thiourea is less expensive than FAS. The viability of such an in situ FAS, oxidative/reductive pulp bleaching process was demonstrated and yielded outstanding bleaching results.