P* : One-Stage Hydrogen Peroxide Full Bleaching of Previously Delignified Softwood Kraft Pulp, 1993 Pulping Conference Proceedings

Final bleaching to 90 degrees IS0 of delignified softwood and hardwood kraft pulp can be achieved with a single alkaline hydrogen peroxide stage (P*). The results presented in this study show that two conditions must be fulfilled in order to make full bleaching possible :

- the pulp must have been sufficiently delignified (Kappa number lower than 4-5 for softwood kraft)

- the manganese content in the pulp must be lower than 3 ppm.

The optimal conditions for the P* stage have been determined: high consistency, long residence time (4 h) and high temperature (80 to 90 degrees Celsius). About 2% hydrogen peroxide o.d.p. is necessary to reach full brightness. The quantity of caustic soda applied must be optimized for the individual pulp, as well as the stabilization formula.

A P* stage has been successfully applied to softwood kraft pulp after chlorine, ECF or TCF delignification. The preferred TCF sequences are 0 Q P Paa P* and 0 Q P Ca P*, where Q is a metal sequestering stage, Paa a peracetic acid stage and Ca a Caro’s acid stage.

The mechanical properties of TCF pulps fully bleached by means of P* technology have been measured and found similar to those of conventional chlorine bleached pulps.