Effect of cooking temperature on kraft pulping of hardwood.

Application: This study shows how changes in pulp cooking temperature affects pulp yield, pulp quality, and residual lignins. It may help some mills to modify their hardwood continuous digester operations for specific results.

This study investigates the effect of decreasing cooking temperature from 170[degrees]C to 165[degrees]C, 160[degrees]C, and 155[degrees]C on the kraft pulping of hardwood and the characteristics of the residual lignin in unbleached pulp. In order to maintain the same output, the active alkali charge was increased while the kappa number and the cooking time were held constant at 21 [+ or -] 1 hours and 1 hour, respectively. Results on the cooking of rejects gave the estimation of total pulp yield as the sum of accepts and 65% of rejects. Thus, cooking at 165[degrees]C produced a pulp with the highest total yield, the lowest tear index, and the highest tensile index among the cooking temperatures studied. As the cooking temperature decreased, both the degree of oxygen delignification and the total kappa factor (total active chlorine charge / kappa number) of an elemental chlorine free (ECF) bleaching sequence (D100ED1D2) decreased. That, in turn, correlated with the ratios of optical densities of the 1330 cm-1 to 1270 cm-1 bands in the infrared spectra of residual lignins. The content of total phenolic hydroxyl groups in residual lignins, which decreased with decreasing cooking temperature, also correlated well with the bleachability, as measured by the total kappa factor, and functional groups such as copper number and carboxyl group of the bleached pulp.

Tran is principal research scientist, Technical Department, Daio Paper Corporation, 5-1 Kamiya-cho, lyomishima, Ehime, Japan 799-0492,. email: [e-mail]