Should FAS bleaching be combined with a chelating agent? Can sodium silicate be used?

Direct answer

Reductive bleaching of pulp is sensitive to metal ions: iron, copper and manganese catalyse peroxide decomposition, accelerate wasteful consumption of FAS and cause brightness reversion. In practice the FAS stage therefore works together with chelating treatment, while sodium silicate belongs to the peroxide stage, not the reduction stage.

1. What each auxiliary does

The conclusion: a proper chelating wash before the reduction stage is often more effective than adding more FAS to it.

2. Why metal ions matter so much to FAS

• Iron, copper and manganese ions catalyse wasteful oxidation of the reducing agent, consuming reducing power that should be doing the bleaching
• They also form coloured complexes with chromophoric groups in the stock, lowering brightness directly and worsening reversion
• Controlling metal ions is therefore one of the most direct ways to raise FAS utilisation

3. Recommended sequence

1. Before the oxidative stage, run a chelating treatment or chelating wash to carry metal ions out of the system

2. Between oxidation and reduction, wash or drain thoroughly so residual peroxide and silicate do not enter the reduction stage

3. In the reduction stage, add only FAS and alkali, keeping the system simple and limiting side reactions

4. After reduction, wash once more to reduce the residual load on the paper machine white-water loop

4. Three cautions

• More chelating agent is not better: excess adds cost and effluent load, so set the dose from the metal-ion content
• Residual silicate scales: if the reduction stage immediately follows the peroxide stage, the intermediate washing must be effective
• Measure the metals first: iron and copper levels differ greatly between furnishes, so design from measured data

Sources

The division of labour between auxiliaries and the effect of metal ions come from our technical materials and published literature. Actual doses must be confirmed on a bench trial with your own furnish and water.