Advantages with oxygen-reinforced alkaline extraction:
- Large savings of chlorine dioxide with a low charge of oxygen
- Freedom of process conditions: 65-90°C (149-194°F) in atmospheric systems
- Option: pressurising and/or addition of peroxide for further reinforcement
- Applicable in all types of sequences, classic chlorine bleaching, ECF and TCF
Addition of oxygen to the pulp in alkaline extraction is an efficient method for increasing the bleaching effect and decreasing the consumption of more expensive bleaching chemicals. Oxygen improves the dissolution of lignin. An oxygen reinforced extraction stage is designated EO. When peroxide is added, it is designated EOP.
A more powerful oxygen extraction stage is accomplished by raising the temperature in the stage, increasing the oxygen charge, pressurising the pre-retention tube and adding hydrogen peroxide. The most important factor is temperature.
| Conditions in a reinforced extraction stage* |
|---|
| Pulp consistency | % | 10-15 |
| Temperature | oC | 60-70 |
| Charge of NaOH | kg/Adt | 0.8-1.2 x Kappa No. |
| Charge of O2 | kg/Adt | 3-5 (7) |
| Charge of H2O2 | kg/Adt | 0-3 |
| Pressure | bar | 1.5 (3-4) |
| Oxygen Treatment | min | 5-15 (30-60) |
| Total retention time | min | 30-90 |
| pH (final) | | 10-11 |
*Conditions given in parenthesis refer to a more powerful extraction stage
