Advantages with oxygen delignification:
- Additional delignification after cooking at higher selectivity than continued cooking
- Reduced emissions from the bleach plant
- Reduced consumption of bleaching chemicals
- Higher brightness ceiling in a given bleaching sequence
- Lower shives content, reduced extractives content, maintained pulp strength
- Facilitates system closure
- Well proven technology, suitable for future process changes.
Oxygen delignification can be regarded partly as a continuation of cooking and partly as the first stage of bleaching. Today it is a standard stage in the production of bleached chemical pulp. Production cost benefits is apparent in both ECF and TCF bleaching.
In the oxygen delignification stage, the pulp is treated with oxygen in a pressurised vessel at elevated temperatures in an alkaline environment. The delignification may vary in the range of 40-70% depending on the wood raw material and whether one or two reactors in series are used. Unbleached kraft (sulphate) pulp has a lignin content of 3-5%, which after oxygen delignification, can be decreased to about 1.5% or a kappa number of 8-10.
The balance between cooking and oxygen delignification:
- From a bleachability, selectivity and yield point of view, it is better to extend the oxygen delignification instead of extending the cook
- The cooking kappa should be optimised together with the oxygen delignification
- Extend the cook, but not too far, let the oxygen delignification do its job.
The Importance of good washing:
- Poor washing before oxygen delignification increases alkali and oxygen and impairs selectivity
- Carry over COD into the reactor should be below 100kg/ton.
- COD from oxygen delignification consumes the bleaching chemicals. Removal/thorough washing is important.
|Conditions in a medium consistency oxygen delignification stage:|
|Degree of delignification||%||40-50|