Method of producing bleached mechanical pulp

Claim 1

In the method of producing wood pulp in a defibrating apparatus in which wood chips, impregnated with an aqueous solution of a peroxide-containing bleaching composition and compressed to a predetermined dry content with consequent removal of bleaching solution, are introduced into a pressure vessel under superatmospheric pressure and then ground between a pair of discs which rotate relative to one another in a pressure sealed defibrating zone connected to said pressure vessel, the improvement. for minimizing the consumption of peroxide, comprising: (a) impregnating the wood chips with an aqueous solution of alkali, selected from the group consisting of sodium hydroxide, alkali silicate, alkali carbonate and alkali bicarbonate, and peroxide at a temperature ranging between 50° C. and 80° C. and for a correlated period of time ranging between 10 minutes and 2 hours to ensure presence of peroxide in the wood chips upon introduction thereof into the pressure vessel; (b) dewatering the thus impregnated wood chips by mechanical compression to a dry content between 35% and 50%; (c) introducing the thus impregnated and dewatered wood chips into said pressure vessel; and (d) introducing compressed air into said pressure vessel in an amount sufficient to maintain the impregnated and dewatered wood chips at a temperature below 90° C. and the pressure vessel at superatmospheric pressure while said impregnated and dewatered wood chips are being passed into said defibrating zone to ensure retention of peroxide during the defibration.