NO744609L

1452710 Bleaching wood pulp A AHLSTROM OSAKEYHTIO 13 Dec 1974 [20 Dec 1973] 54038/74 Heading DIP A foamed suspension of wood pulp is bleached to remove lignine by treating with alkali solution and oxidising gas at elevated temperature and pressure. The foam is stabilised with 1 kg/ton of pulp of a foaming agent (a polyoxyalkylene-condensate, dodecylbenzene sulphonate, octylphenoxypolyethoxy ethanol or black liquor. The oxidising gas may be oxygen, air, ozone, chlorine, chlorine dioxide or mixtures thereof. Pulp is digested at 1 and washed to remove most of the black liquor. The pulp, oxygen gas and foaming agent are mixed at 3 and sodium hydroxide solution, oxygen and steam added to reactor 8. A pressure reducing valve may be situated between mixer 3 and reactor 8 to expand the bubbles in the foam. The amount of pulp in the foam is up to 15% by wt., e.g. 8 or 10%. The sodium hydroxide may comprise 6% by weight. Specified reactor conditions are temperature 90 or 110‹C, pressure 4 atmospheres for 10 to 30 minutes. The bleached pulp is degassified at 11 and washed at 10. Kappa valves of 14 to 16.5 are obtained. In the embodiment of Fig. 2, not shown the sodium hydroxide and the foaming agent are applied to the pulp simultaneously and then the oxygen is applied.