FAS (Formamidinesulfinic Acid) is an odorless, stable, alkaline-activated reductive bleaching agent for the pulp and paper industry. It is especially suitable for reductive post-bleaching of deinked pulp to increase final brightness and remove color shades.

The extent to which deinked fibers are bleached depends on the raw material and the brightness requirement for the final product. Bleaching is conducted by either the oxidation or reduction of the chromophores in the fibers. Chemical pulps are bleached in delignifying multi-stage processes with strong oxidizing agents like oxygen, chlorine dioxide and hydrogen peroxide. Delignifying bleaching steps cannot be applied on mechanical pulps because of their high lignin content. Generally, the recycling of wastepaper for printing, writing and tissue grades, includes an oxidative bleaching step with hydrogen peroxide. The additional application of formamidine sulfinic acid (FAS), a strong reductive bleaching agent, can produce a further increase in brightness and thereby improve the quality of the final product.

Bleaching with FAS (Formamidinesulfinic Acid) yields the best results under alkaline conditions. FAS is only poorly soluble in water (~27g/l). under alkaline conditions up to 100g/l are soluble as sulfinate. While the sodium salt of FAS is highly soluble, it is also very unstable. Consequently the solution cannot be stored for a prolonged period. Therefore, alkaline bleaching solutions of FAS should be prepared continuously on-site and feed directly to the bleaching process.