BLACK LIQUOR PRETREATMENT IN KRAFT COOKING

The effect of black liquor pretreatment of pine chips was studied in k raft cooking. The influence of the following factors on delignificatio n rate and kraft pulp properties was studied: a) alkalinity of black l iquor used in pretreatment, b) pretreatment temperature, c) molecular mass distribution/viscosity of black liquor used in pretreatment, and d) recycling of black liquor in pretreatment as a laboratory simulatio n of displacement cooking. With a black liquor pretreatment at 90-degr ees-C in 60 min before kraft cooking, the kappa number of pulp was red uced by about 10 units from the kappa number of 33 in a conventional k raft cook. Increasing the alkalinity in the black liquor treatment imp roved the delignification rate in the cook. Lowering the alkalinity to around neutral pH resulted in a pulp kappa number eve higher than in normal kraft cooking. The change of alkalinity in the pretreatment did not affect the pulp yield, which was about the same as in normal kraf t cooking at the same kappa number. Black liquor pretreatment of chips gave a better result than a purely inorganic sulphide treatment at th e same alkalinity. At high pretreatment temperatures, the pH of the bl ack liquor dropped drastically. This possibly caused precipitation of lignin during the pretreatment and an increase in kappa number, compar ed with pulps pretreated at lower temperatures. Storing black liquor a t elevated temperatures caused a decrease in the liquor's high molecul ar lignin fractions and viscosity. It also had an adverse effect on co oking since the reactions taking place at the high temperature of stor age consumed alkali in the black liquor. The strength properties of kr aft pulps from chips pretreated with black liquor were better than tho se of conventional kraft pulps. Increasing the alkalinity of the pretr eatment improved the tear strength of the pulp. Pulp properties were n ot improved when black liquor was recycled in several cooks. It is, ho wever, difficult to simulate commercial displacement cooks in laborato ry. The better strength properties were maintained during subsequent b leaching. Kraft pulps from preatreated chips had a 10% higher tear ind ex after conventional and oxygen-peroxide bleachings than the referenc e kraft pulps.