Patent ID: 7384502

Claim:
A process for preparing bleached mechanical pulp having high brightness from wood chips consisting essentially of the steps, in order, of impregnating wood chips having low bleachability with a chemical liquor consisting essentially of an aqueous solution of an alkaline inorganic compound and a chelating agent at a pH range of 7-12 and then removing said impregnating chemical liquor from said chips, next followed by the sequential steps, in order, of (a) defibration by primary refining, bleaching, and beating by secondary refining, or (b) defibration by primary refining, beating by secondary refining and bleaching, wherein the step of impregnating comprises compressing the chips, immersing the chips under compression or after compression in said chemical liquor and releasing pressure to impregnate them with said chemical liquor, and wherein the step of removing the impregnated chemical liquor comprises compressing the chips impregnated with said chemical liquor to drain the impregnating chemical liquor from the chips.