Is FAS effective for colored wastepaper?

Direct Answer

Direct Answer Yes, and coloured recovered paper is often exactly where formamidinesulfinic acid (FAS) earns its place.

Why it works

• The colour in coloured recovered paper comes from dyes, inks and pigments, most of which are conjugated chromophoric systems
• Reductive bleaching breaks those chromophores down and removes or weakens the colour while keeping the fibre
• Compared with oxidative bleaching, the reductive route degrades fibre less, which suits grades where strength has to be preserved

Where it fits

• Bringing the colour of a coloured pulp down when running natural or light shades
• Acting as a colour-control lever when the share of coloured recovered paper has to be raised
• Correcting hue where brightness is already on target but the shade is off

What the result depends on

• Type of colorant: direct and acid dye types are easier to remove; pigment types such as some printing inks are harder
• Blending ratio: the higher the share of coloured paper, the higher the dose
• Target colour: the whiter the target, the higher the dose and the faster the cost rises

Realistic expectation

FAS markedly reduces colour, showing up as lower a* and b* and a shift towards a neutral hue, but it should not be expected to turn coloured pulp into virgin-grade pulp. Above about 1% the extra dose rarely pays, and the control lever should move to the furnish mix.

Bottom line

Effective on coloured recovered paper and commonly used as a colour-control tool, but limited on pigment-type colorants, so establish the achievable level by bench trial first.