Production of high brightness high strength wood pulps

Claim 1

A STAGED PROCESS FOR THE PRODUCTION OF SUPERBLEACHED CELLULOSIC MATERIAL HAVING A BRIGHTNESS IN EXCESS OF 90 AND OF HIGH STRENGTH CHARACTERISTICS WHICH COMPRISES, IN THE SEQUENCE SPECIFIED, (A) TREATING AN AQUEOUS SUSPENSION OF UNBLEACHED CELLULOSIC MATERIAL CONTAINING A SUBSTANTIAL AMOUNT OF LIGNEOUS AND NONCELLULOSIC COLORFORMING IMPURITIES IN A FIRST STAGE WITH ELEMENTAL CHLORINE AT NOT GREATER THAN PH 6 FOR NOT IN EXCESS OF 90 MINUTES TO CHLORINATE A MAJOR PORTION OF SAID IMPURITIES WATER WASHING THE CELLULOSIC MATERIAL, (B) TREATING THE CELLULOSIC MATERIAL AT A CONSISTENCY OF ABOUT 9-17% WITH AN ALKALINE HYPOCHLORITE IN A SECOND STAGE AT GREATER THAN PH 8 FOR NOT IN EXCESS OF 240 MINUTES TO OXIDIZE A PORTION OF THE RESIDUAL LINGEOUS AND COLOR-FORMING IMPURITIES TO BRIGHTEN THE PULP WITHOUT MATERIAL DEGRADATION OF THE CELLULOSE WATER WASHING THE CELLULOSIC MATERIAL, (C) TREATING THE CELLULOSIC MATERIAL AT A CONSISTENCY OF ABOUT 9%-17% IN A THIRD STAGE WITH CHLORINE DIOXIDE AT NOT GREATER THAN PH 6.5 FOR NOT IN EXCESS OF 300 MINUTES TO OXIDIZE AN ADDITIONAL PORTION OF SAID RESIDUAL IMPURITIES WITHOUT MATERIAL DEGRADATION OF THE CELLULOSE WATER WASHING THE CELLULOSIC MATERIAL, (D) THEN ADDING ALKALI TO AN AQUEOUS SUSPENSION OF THE CELLULOSIC MATERIAL AT A CONSISTENCY OF ABOUT 9%-17% IN A FOURTH STAGE TO AN ALKALINE PH ABOVE ABOUT PH 10 TO EFFECT DISSOLUTION OF SAID OXIDIZED IMPURITIES, WATER WASHING THE CELLULOSIC MATERIAL, (E) TREATING THE CELLULOSIC MATERIAL AT A CONSISTENCY OF ABOUT 9% 17% IN A FIFTH STAGE WITH CHLORINE DIOXIDE AT NOT GREATER THAN PH 5.5 FOR NOT IN EXCESS OF 300 MINUTES TO OXIDIZE REMAINING LIGNEOUS AND COLOR-FORMING IMPURITIES WITHOUT MATERIAL DEGRADATION OF THE CELLULOSE, WATER WASHING THE CELLULOSIC MATERIAL, AND RECOVERING SUPERBLEACHED PULP HAVING A G.E. BRIGHTNESS IN EXCESS OF 90, IMPROVED COLOR STABILITY AND HIGH STRENGTH CHARACTERISTICS SUBSTANTIALLY UNCHANGED FROM THOSE OF THE UNBLEACHED CELLULOSIC MATERIAL.