Bleaching

Bleaching is performed in stages.

The early stages remove remaining lignin; final stages brighten the pulp

Pulp is usually washed between stages to remove any soluble organic material

• Classes of bleaching chemicals

• Strong oxidizing agents

• Alkali

• Metal removal agents

Elemental chlorine (Cl 2) is an effective delignifying agent. As it breaks lignin bonds, it adds chlorine atoms to the lignin degradation products, thus producing significant amounts of chlorinated organic material.

Ozone (O 3) is also an effective delignifying agent. It also brightens the pulp as well. Ozone has not been used in the past because mills have not been able to improve its selectivity - ozone attacks the cellulose fibre as well as the lignin. Recent technological developments, however, have solved this problem and have allowed mills to take advantage of this cost-effective bleaching agent.

Chlorine dioxide (ClO 2) is a highly selective chemical that can both delignify and brighten pulp. It oxidizes lignin, but does not add chlorine atoms onto lignin fragments; however, small amounts of elemental chlorine and other chlorine compounds formed during the chlorine dioxide bleaching process react with degraded lignin to form chlorinated organic compounds.

Oxygen (O 2) is an inexpensive, highly effective delignifying agent that is usually used at the beginning of the bleaching process. It has intermediate selectivity.

Sodium hypochlorite (NaOCl) is an inexpensive delignifying agent formed by mixing elemental chlorine with alkali at the mill. Mills are phasing out the use of hypochlorite because it generates large quantities of chloroform when it is used to bleach pulp.

Hydrogen peroxide (H 2O 2) is mainly used to brighten pulps in the final bleaching stages. Peroxide is often used at the end of a conventional bleaching sequence to prevent the pulp from losing brightness over time. Researchers have found operating conditions under which peroxide will delignify pulp, and are working on technologies that will consume less.

More