COMPARISON OF CHARACTERISTICS OF SOFTWOOD AND HARDWOOD SULPHITE PULPS DURING THE BLEACHING PROCESS

Abstract

Softwoods (spruce+fir, and fir only) and hardwoods (mixed hardwood, and birch only) unbleached sulphite paper grade pulps were bleached with three-stage processes (chlorination, caustic extraction, hypochlorite) and a comparison was made on behavious of softwood and hardwood sulphite pulps during the bleaching.1) In comparison with softwood unbleached sulphite of the same Sieber number, hardwood sulphite pulps were lower in the bleaching loss (Fig. 1), and higher in brightness of bleached pulp (Table 1 and 2) and consumption of caustic soda (Fig. 2). The yields of boths oftwood and hardwood bleached pulps were lower than those expected from their lignin contents calculated from Sieber number of unbleached pulps.2) The loss of xylan and mannan in pulps during bleaching process were very small and total amounts of them were about 0.2% in both softwood and hardwood unbleached pulps (Sieber number about 52) (Table 5). The dissolved carbohydrate materials were mainly xylan in the case of hardwood pulp, while considerable amount of mannan was found in addition to xylan in the case of softwood pulp. Considerable amounts of uronic acids were also removed during these treatments and the loss mounted to 20-30% of original uronic acid contents of both softwood and hardwood unbleached sulphite pulps (Table 3 and 4).3) Lignin contents of unbleached pulps were determined UV-absorptiometrically on phosphoric acid solution. It was shown that the lignin content of the herdwood unbleached sulphite pulps was about 20% less than that of softwood unbleached sulphite pulps of the same Sieber number (Fig. 5 and 6). It seems that the difference in the yield of softwood and hardwood bleached sulphite pulps was mainly due to the difference in the lignin contents.4) The amounts of lignin eliminated from unbleched sulphite pulps were determined UV-absorp-tiometrically in the bleaching process. Lignin in unbleached hardwood sulphite pulp was easier to remove than that of softwood pulp during caustic extraction after chlorination (Fig. 8) and hypochlorite bleaching. Considerable amount (about 35%) of the former was also removed even by 1% NaOH extraction at 20°C for 1 hr. (Fig. 9)