Can FAS be used to bleach CTMP mechanical pulp?

Direct answer

In high-yield mechanical pulps such as CTMP and BCTMP, brightness is limited by chromophoric groups in the lignin. Peroxide bleaching usually still leaves a need for a reductive bleaching stage to remove the remaining chromophores. FAS suits that stage: it selectively reduces carbonyl and quinone structures and damages fibre strength less than pushing the oxidative conditions harder.

1. Why mechanical pulp needs a reduction stage

• Peroxide oxidises part of the chromophores but leaves carbonyl-type structures behind, which read as a yellow cast
• A reduction stage targets those structures, and the brightness gain is usually more economical than adding more peroxide
• Mechanical pulp is high in lignin, so a reduction stage damages the fibre less than further intensifying the oxidative conditions

2. Starting conditions

3. Four points to watch

1. Wash or drain between stages: residual peroxide simply consumes FAS

2. Control metal ions: iron, copper and manganese catalyse wasteful consumption and worsen reversion

3. Check brightness and reversion together: chasing instantaneous brightness can give it back in the dryer

4. Set the dose by furnish: CTMP and BCTMP differ in lignin content and freeness, so recipes cannot be copied across

4. Common problems

• Little brightness gain: first confirm the intermediate wash is effective and the metal ions are under control
• Brightness reversion: usually residual reducing species oxidising, or metal-ion catalysis
• Strength loss: check whether temperature or time has been extended too far

Sources

Starting values are the recommended ranges from our technical materials and published literature. Mechanical pulps vary widely, so confirm by laboratory bleaching on your own furnish.