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

Abstract

Hardwood unbleached sulphite pulps consumed more caustic soda than softwood pulps during caustic extraction in bleaching process. This investigation concerns this problem.The results of conducto-and potentiometric titrations of spent caustic extraction liquors (Fig. 1), and gas-chromatographic analysis of the same liquors (Fig. 7) and hydorolysates of these pulps show that the difference in the consumption of caustic soda is due to the difference of acetyl group contents in the both unbleached pulps. Acetyl group contents of hardwood unbleached sulphite pulps (47-55% of total cooking yield) are 0.6-1.0%, while only about 0.1% in softwood unbleached pulps.In the bleaching process, these acetyl groups in unbleached sulphite pulps were not or little removed by chlorination (Fig. 5 and 6), but most parts were removed by caustic extraction. Therefore, acetyl groups cannot be found in bleached sulphite pulps.The acetyl groups in unbleached sulphite pulps were nearly completely removed by 1% NaOH extraction (20°C, 1 hr.) or bisulphite-soda cooking. By those processes the amotnts of caustic soda consumed in bleaching of hardwood unbleached sulphite pulps decreased greatly to that of softwood unbleached sulphite pulps of the same Sieber number (Table 1, Fig. 3, 4 and 8).It seems that the acetyl groups combine with xylan existing in amorphous region of unbleached sulphite pulps.