COOKING OF PINE IN THE SODA AQ/METHANOL S YSTEM/

During start-up the Bayerische Zeilstoff mill in Kelheim produced pulp with the one-stage Organosolv process, which did not meet the market requirements. In particular, it was by far not possible, to reach the strength properties of those pulps produced in the two-stage process a pplied in the demonstration plant. Our investigations on the one-stage cooking of pine in the system soda/AQ/methanol shows a serious lack o f selectivity in delignification. An optimisation of the important coo king parameters, considering the installed technology at the Kelheim m ill, did not result in a significant improvement of the cooking result s. Due to strong cellulose degradation the cooks had to be stopped at still high kappa number clearly above 30 to attain acceptable pulp pro perties. A substantial improvement of the cooking results could be ach ieved using a process modification. By adding the caustic soda to the digester at a later time of cooking, the pulping process proceeds in t wo phases. At first a prehydrolysis in methanol/water at 160 degrees C , then alkaline cooking at the same temperature is conducted. The preh ydrolysis facilitates the alkaline cooking in such a way that it can b e carried out at lower temperature and the cellulose is degraded to a much lower extent. The result is a remarkably improved pulp quality. T he bleaching trials showed that the pulp produced in the modified soda /AQ/methanol process can be bleached in an OQ(OP) bleaching sequence t o 85 % ISO brightness, but this requires high charges of peroxide and alkali as well as the use of a peroxide activator in the final bleachi ng stage. The strength of the bleached pulp is on the level of TCF sof twood kraft pulps.