Can FAS be used together with enzymatic deinking?

Direct answer

FAS and enzymatic deinking can be combined, but in separate stages. Enzyme treatment normally runs at neutral to weakly acidic pH and 40 to 60 C, while FAS reductive bleaching needs alkaline conditions and a higher temperature. The two sets of conditions pull against each other, so the right approach is staged treatment with a wash in between.

1. A typical stage sequence

1. Pulping and pre-cleaning: remove coarse contaminants

2. Enzyme stage: at the pH and temperature the enzyme requires, typically neutral to weakly acidic, 40 to 60 C

3. Inactivation and washing: the enzyme must be inactivated or it keeps acting into later stages

4. Oxidative stage (if needed): peroxide bleaching

5. Intermediate wash: remove residual oxidant

6. FAS reduction stage: alkaline conditions to lift brightness and correct hue

2. Why they cannot share a bath

The conclusion: putting the two in one bath most likely means neither works properly.

3. Four operating points

1. Inactivate the enzyme at the end of its stage, by raising the temperature or shifting the pH, as the enzyme requires

2. Wash thoroughly in between so enzyme and surfactant residues do not carry into the reduction stage

3. Set the two doses separately by bench trial; do not offset one against the other

4. Record the two sets of results separately: ink detachment and freeness for the enzyme stage, brightness and reversion for the reduction stage

4. When enzymatic deinking is worth using

• Wastepaper pulp with demanding dirt count and ink-detachment requirements
• Lines aiming to reduce total chemical consumption
• Mills that can support staged treatment with intermediate washing

Sources

The enzyme treatment conditions are the common ranges in published literature. The specific enzyme, pH and temperature must follow the enzyme supplier's instructions.