Effect of the particle size of magnesium hydroxide on the cellulose polymerization during the oxygen delignification of radiata pine kraft pulp

Abstract

Magnesium salt is a known cellulose protector during the oxygen delignification process and it has been widely used in commercial practice, in particular, for feedstocks with a low magnesium presence. Minimizing the negative impact of transition metals by magnesium is one of the main mechanisms. A key parameter affecting the magnesium salt performance for this purpose is its particle size. In this study, different particle sizes of magnesium hydroxide were used as additives to the oxygen delignification system, and various transition metal ions (Cu2+, Mn2+ and Fe3+) were added to the radiata pine Kraft pulp. The results show that the magnesium hydroxide particle sizes had negligible effects on the pulp yield, Kappa number and brightness; however, when large size magnesium hydroxide was used, handsheet physical properties suffered a great loss, which was accompanied with much lowered viscosity and degree of polymerization in the resultant pulp. The explanation is that small sized magnesium hydroxide particles have large specific surface area and can adsorb more transition metal ions than the magnesium hydroxide with larger particle size, consequently, the transition metal contents of resultant pulp in the former are significantly lowered, which then decreases the negative impact of transition metals on the cellulose degradation in the process. It is thus concluded that smaller particle size of magnesium hydroxide is favorable as a cellulose protector during the oxygen delignification process.