TWO-STAGE NONCHLORINATION BLEACHING OF SECERAL HARDWOOD NEUTRAL SULFITE SEMICHEMICAL PULPS

Several hardwood neutral sulfite semichemical pulps were bleached to about 85 percent brightness with a two-stage application of sodium hypochlorite, using a total chlorine equivalent of 12 to 25 percent, depending on the yield of the unbleached pulp. Brightness gains decreased with increasing brightness of unbleached pulp. Yields of bleached pulps were from 3 to 5 percent higher with two-stage bleaching than they were with three-stage chlorination hypochlorite bleaching. With this process, however, 88 percent brightness was obtained with equal or lesser amounts of chlorine equivalent. Except for the highest yield pulp, hydrogen peroxide was as effective as hypochlorite in the second stage, but chlorine dioxide was about 10 percent less effective. Effect on strength varied from negligible to increases as much as 80 percent for the average overall strength. Author