Multistage process bleaching of alkaline-process wood pulps including an acid sour between two terminal alkaline hypochlorite stages

Claim 1

THE METHOD OF BLEACHING ALKALINE-PROCESS WOOD PULP WHICH COMPRISES SUBJECTING WOOD PULP WHICH HAS BEEN DIGESTED WITH AN ALKALINE DIGESTING AGENT AT AN ELEVATED TEMPERATURE TO ADEQUATE PARTIAL BLEACHING IN PRELIMINARY BLEACHING STAGE DURING WHICH THE MAJOR PORTION OF THE TOTAL BLEACHING AGENT USED IN THE BLEACHING PROCESS IS ADDED TO THE PULP, SAID PARTIAL BLEACHING INCLUDING AT LEAST A CHLORINATION STEP, AND COMPLETING THE BLEACHING BY THE STEPS WHICH COMPRISE BLEACHING THE PULP WITH AN ALKALINE HYPOCHLORITE SOLUTION, SUBJECTING THE PULP AFTER ITS BLEACHING WITH THE ALKALINE HYPOCHLORITE SOLUTION AND AFTER THE HYPOCHLORITE HAS BEEN SUBSTANTIALLY ALL CONSUMED DURING SUCH BLEACHING AGNET TO TREATMENTS INVOLVING REDUCING FREE OF BLEACHING AGENT TO TREATMENTS INVOLVING REDUCING THE PH OF THE RESULTANT PULP SLURRY TO BETWEEN ABOUT 2.0 AND 5.0 WITH AN ACIDIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF NON-REDUCING MINERAL ACIDS AND THE FILTRATE FROM A PREVIOUS CHLORINATION BLEACHING STAGE AND WASHING THE PULP WITH WATER, AND THEREAFTER BLEACHING THE PULP WITH AN ALKALINE HYPOCHLORITE SOLUTION, THE PH AND THE DURATION OF THE BLEACHING STEPS BEING SUCH AS TO IMPART TO THE PULP A BRIGHTNESS IN EXCESS OF 75 GENERAL ELECTRIC BRIGHTNESS METER READING.

Claim 2

THE METHOD OF BLEACHING ALKALINE-PROCESS WOOD PULP WHICH COMPRISES SUBJECTING WOOD PULP WHICH HAS BEEN DIGESTED WITH AN ALKALINE DIGESTING AGENT AT AN ELEVATED TEMPERATURE TO ADEQUATE PARTIAL BLEACHING IN PRELIMINARY BLEACHING STAGE DURING WHICH THE MAJOR PORTION OF THE TOTAL BLEACHING AGENT USED IN THE BLEACHING PROCESS IS ADDED TO THE PULP, SAID PARTIAL BLEACHING INCLUDING AT LEAST A CHLORINATION STEP, AND COMPLETING THE BLEACHING BY THE STEPS WHICH COMPRISE BLEACHING THE PULP WITH AN ALKALINE HYPOCHLORITE SOLUTION, SUBJECTING THE PULP AFTER ITS BLEACHING WITH THE ALKALINE HYPOCHLORITE SOLUTION AND AFTER THE HYPOCHLORITE HAS BEEN SUBSTANTIALLY ALL CONSUMED DURING SUCH BLEACHING AGNET TO TREATMENTS INVOLVING REDUCING FREE OF BLEACHING AGENT TO TREATMENTS INVOLVING REDUCING THE PH OF THE RESULTANT PULP SLURRY TO BETWEEN ABOUT 2.0 AND 5.0 WITH AN ACIDIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF NON-REDUCING MINERAL ACIDS AND THE FILTRATE FROM A PREVIOUS CHLORINATION BLEACHING STAGE AND WASHING THE PULP WITH WATER, AND THEREAFTER BLEACHING THE PULP WITH AN ALKALINE HYPOCHLORITE SOLUTION, THE PH AND THE DURATION OF THE BLEACHING STEPS BEING SUCH AS TO IMPART TO THE PULP A BRIGHTNESS IN EXCESS OF 75 GENERAL ELECTRIC BRIGHTNESS METER READING.