AN IMPROVED OXYGEN DELIGNIFICATION FOR MAGNESIUM BASED SULFITE PROCESS

摘要

Oxygen delignification technology has become one of the standard techniques to minimize the environmental impact of a modern bleach plant for chemical pulps. Conventionally, sodium hydroxide is used as the base during oxygen delignification. On the other hand, magnesium oxide (hydroxide) is the preferred base for oxygen delignification of magnesium sulfite pulps (the so-called O_(MgO) process). Several technical papers on this subject have been published. However, changing the base in the oxygen delignification process from sodium hydroxide to magnesium oxide or magnesium hydroxide negatively affects the lignin to carbohydrate selectivity, and thus the strength properties. In this paper, we will report an improved O_(MgO) process for sulfite pulps. The process is comprised of adding in-situ a reducing agent substantially non-reactive with oxygen to the O_(MgO) process (the so-called O_(MgO+R) process). It was found that sodium borohydride, with a catalytic quantity, serves this purpose very well. The results showed that well delignified pulps with enhanced strength properties and increased viscosity can be obtained from the O_(MgO+R) process. Another of its advantages is that the pulp yield is increased significantly compared to that of the O_(MgO) process. Good results were also obtained from the two-stage process consisting of a reducing agent treatment as the first stage, and an O_(MgO) as the second stage.