The washed pulps still contain a certain amount of lignin, which would cause a brownish colour of the paper. To obtain white paper, the pulp must be bleached. Whereas the lignin used to be removed from the pulp with elemental chlorine, today's paper industry uses other bleaching agents such as hydrogen peroxide, oxygen, chlorine dioxide, ozone and others. Chlorine dioxide bleached pulp is also referred to as ECF (elementary chlorine free) pulp. Pulp bleached without chlorine is called TCF (totally chlorine free).
Oxygen is the basis of the environmentally friendly bleaching processes oxygen delignification and ozone-bleaching.
- Oxygen delignification:
Oxygen delignification is today a proven bleaching technology and is widely used as a bleaching stage in ECF and TCF processes, especially as a first bleaching stage. It enables a good kappa number reduction while maintaining strength and can be well integrated into the washing cycle during the Kraft process to reduce the water requirement. Furthermore, this process is characterised by its low chemical costs.
Due to its high reactivity, ozone is particularly suitable for the bleaching of high-white pulps. The reaction time is short, even at low temperatures. For the production of ozone on site ozone generators are used, which are fed for economic reasons with pure oxygen. As with oxygen bleaching, only halogen-free wastewater is produced.