Alkaline sulfite digestion followed by acid digestion with bisulfite

Claim 1

A MULTISTAGE PULPING PROCESS FOR THE PRODUCTION OF HIGH YIELD PULP HAVING A LIGNIN CONTENT LESS THAN 20 PERCENT BY WEIGHT BASED UPON THE WEIGHT OF THE PULP COMPRISING, IN COMBINATION, THE CONSECUTIVE STEPS OF, (A) AT LEAST ONE ALKALI DIGESTING, AND THEN BLOWING FROM SAID ALKALI DIGESTING, PARTICLES OF FIBROUS CELLULOSIC MATERIAL, SAID ALKALI DIGESTING BEING BY CONTACTING SAID PARTICLES OF FIBROUS CELLULOSIC MATERIAL FOR A PERIOD OF ABOUT 10 TO ABOUT 60 MINUTES WITH AN AQUEOUS SOLUTION OF A SULFITE COMPOUND HAVING AN INITIAL PH OF ABOUT 8.0 TO ABOUT 11.0 AT A TEMPERATURE OF ABOUT 280*F. TO ABOUT 400*F. AND A PRESSURE OF ABOUT 50 TO ABOUT 150 P.S.I.A., SAID AQUEOUS SOLUTION OF A SULFITE COMPOUND BEING PRESENT IN A VOLUME SUFFICIENT TO PROVIDE A SOLUTION TO PARTICLES VOLUME RATIO OF ABOUT 3 TO 1 TO ABOUT 6 TO 1 AND SAID SULFITE COMPOUND BEING PRESENT IN A CONCENTRATION SUFFICIENT TO PROVIDE AT LEAST ABOUT 5 PERCENT BY WEIGHT BASED UPON THE DRY WEIGHT OF SAID PARTICLES, AND THEN (B) AT LEAST ONCE, ACID DIGESTING, AND THEN BLOWING FROM SAID ACID DIGESTING, THE ALKALI DIGESTED AND BLOWN PARTICLES OF FIBROUS CELLULOSIC MATERIAL TO RECOVER SAID HIGH YIELD PULP, SAID ACID DIGESTING BEING BY CONTACTING SAID ALKALI DIGESTED AND BLOWN PARTICLES OF FIBROUS CELLULOSIC MATERIAL FOR A PERIOD OF ABOUT 10 TO ABOUT 60 MINUTES WITH AN AQUEOUS SOLUTION OF A BISULFITE COMPOUND HAVING AN INTITIAL PH OF ABOUT 4.3 TO ABOUT 6.9 AT A TEMPERATURE OF ABOUT 280*F. TO ABOUT 400*F. AND A PRESSURE OF ABOUT 50 TO ABOUT 150 P.S.I.A., SAID AQUEOUS SOLUTION OF A BISULFITE COMPOUND BEING PRESENT IN A VOLUME SUFFICIENT TO PROVIDE A SOLUTION TO PARTICLES VOLUME RATIO OF ABOUT 3 TO 1 TO ABOUT 6 TO 1 AND SAID BISULFITE COMPOUND BEING PRESENT IN A CONCENTRATION SUFFICIENT TO PROVIDE AT LEAST ABOUT 5 PERCENT BY WEIGHT BASED UPON THE DRY WEIGHT OF SAID PARTICLES.