Why can FAS be used for deinking pulp?

Direct Answer

Direct Answer Because the brightness ceiling in deinked pulp is set by chromophoric groups rather than by lignin, and chromophoric groups are exactly what reductive bleaching addresses.

The principle

• The colour of recovered pulp comes from residual ink, dyes and chromophoric groups formed by oxidation during storage
• Reductive bleaching reduces conjugated structures, azo bonds and carbonyl groups in those chromophores to non-coloured forms, and brightness rises
• It does not delignify, so fibre yield loss is minimal, which matches a high-yield recovered-fibre system

What formamidinesulfinic acid (FAS) contributes

• It releases sulfoxylate under alkaline conditions as the active reductive species
• Release is gentler than with sodium dithionite and easier to control at normal consistency and temperature
• The end products are urea and sodium sulphate, with no heavy metals and no chlorinated compounds

Where it sits in the sequence

FAS is normally placed after hydrogen peroxide bleaching as a reductive post-bleach stage: the oxidative stage lifts brightness and the reductive stage pulls down the residual colour left by oxidation and improves the shade by lowering b*.

What the result depends on

Furnish mix, ink system, consistency, pH, temperature, reaction time, metal ion content and the FAS dose together determine the brightness gain actually achieved.

Bottom line

Deinked pulp colour comes mainly from chromophoric groups. FAS reduces them under alkaline conditions without costing yield, which is why it suits reductive post-bleaching.

Sources

1. US5958184A, Process for producing thiourea dioxide（还原当量质量比、碱中水解式、木浆与脱墨浆等当量漂白实施例），https://patents.google.com/patent/US5958184A/en