The effect of nickel ions on oxygen delignification

In an earlier study we found the selectivity of low consistency oxygen delignification was enhanced when Ni2+ was present, and the addition of sodium tartrate gave rise to further selectivity gains. In this paper the effects of added Ni2+ and tartrate were studied in more detail. Selectivity enhancement was mainly due to the presence of Ni2+ ions. The addition of tartrate was beneficial only at low Ni2+ concentrations. Replacement of Ni2+ by Cu2+, Mn2+ or Zn2+ was investigated, as was replacement of tartrate by citrate or oxalate. The most effective combination was Ni2+ with these dicarboxylic acids. Experiments using an EDTA washed pulp indicated the added Ni2+ and tartrate most likely increased selectivity by suppressing the carbohydrate degradation caused by trace metal ions present in the pulp. At medium consistency conditions, the presence of Ni2+ and tartrate did not significantly improve the selectivity of oxygen delignification.