Bleaching

Although cellulose fiber is white in colour, due to residual lignin traces remaining on the fibers, the pulp appears creamish. Therefore, to manufacture white paper we need to remove yellowness without physically or chemically damaging the fiber, with improvement in various properties. So the main objectives of bleaching the pulp, can be set out as follows.
To increase brightness of the pulp by removal or modification of some of the unwanted elements in the unbleached pulp. These deleterious elements are lignin traces, resins, metal ions, non-cellulosic carbohydrates etc.

Bleaching for brightness improvement should also help to keep the pulp stable without turning yellow or lose strength or reduce brilliance - due to aging.

To lower viscosity of the pulp for optimum flow, during subsequent operations.

Bleaching also should help to reduce the fiber bundles, shives and bark fragments.
Bleaching should be done with minimum mechanical action of fibers, while dissolving lignin and other unwanted residuals.

Bleaching Pulp is normally done in a step-wise sequence using different chemicals and process conditions at each stage, with washing in between stages.