ASA and kraft pulping of eucalyptus globulus and eucalyptus nitens

Eucalyptus globulus and Eucalyptus nitens from Chilean plantations were pulped in a modified alkaline sulfite-AQ (ASA) process. Different parameters such as total chemical charge, Na2SO3 to NaOH ratio, time of NaOH charging during the cooking process, and maximum cooking temperature were investigated. The ASA pulps were bleached in an O-Q-OP-Q-P sequence. Kraft reference pulps were produced and bleached in the same sequence. E. nitens contains more lignin and extractives than E. globulus and requires more drastic cooking conditions. Thus, in ASA pulping of E. globulus 20-22.5% total chemicals, calculated as NaOH, were sufficient, whereas 25% were needed for E. nitens. The optimal Na2SO3 to NaOH ratio ranged between 30:70 and 40:60. The ASA cooks are preferably started off with a pure sodium sulfite charge adding the NaOH after 90 min when the maximum temperature (T max) is reached. For E. globulus a Tmax of 155°C, for E. nitens 160°Chas proven as optimal. Cooking of E. globulus under optimal conditions resulted in a kappa number of 15 at a yield of 58%. For E. nitens a higher kappa number of 18 and a lower yield of 53.6% were attained. Compared to the kraft reference, ASA cooking led to a 2% higher yield at the same kappa number. All pulps could be bleached to high brightness but the E. nitens pulps required more severe conditions. As compared to E. globulus, the E. nitens pulps had remarkably higher tensile strength, but the tear index and the specific volume were significantly lower.