The advancement of oxygen bleaching by using chemical additives to improve pulp properties

The purpose of this research was to evaluate the addition of HEDP (1-Hydroxyethylidene-1,1-diphosphonic acid) and magnesium gluconate as additives to improve the selectivity (the ratio of delignification to depolymerization) of the oxygen bleaching process. Through examination of levels of delignification, depolymerization, brightness, strength, radical generation, metal content, and fiber structure it was concluded that both additives are effective at increasing the selectivity of oxygen bleaching, but their mechanisms of action are very different.; HEDP improves selectivity by increasing the rate of delignification, while improving bonding strength. HEDP functions as a chelant, deactivating metal ions that can catalyze the formation of radicals, which attack both lignin and cellulose. Surprisingly, the ability of HEDP to function is slowed by the removal of metal ions, indicating the presence of metals is important to the delignification mechanism. HEDP addition improves fiber flexibility and fiber bonding. Moreover, HEDP addition allows the fibers to be refined more quickly, resulting in an increased amount of external fibrillation, which contributes to fiber bonding. Faster refining might also lead to easier access of bleaching chemicals to cell components, allowing for equivalent delignification levels to be reached in a shorter amount of time. The optimum level of HEDP application for these experiments was determined to be 1%.; Magnesium gluconate also improves reaction selectivity. It functions as a radical scavenger, decreasing the amount of hydroxyl radicals present in the oxygen stage. In the presence of large amounts of manganese, delignification and depolymerization are both severely impaired. When metals are removed, the delignification rate is comparable to the control, while cellulose depolymerization is still slowed, increasing selectivity. Magnesium gluconate appears to function to protect hemicellulose and cellulose components against degradation. While fiber flexibility and bonding are improved over the control, external fibrillation from refining does not appear to be significantly increased. To function optimally, this additive requires a metal removal stage before the oxygen bleaching. The addition of 0.5% magnesium gluconate is the lowest amount necessary to see improvements in reaction selectivity.