TCF BLEACHING OF SOFTWOOD KRAFT PULP - PART 4 - REMOVAL OF MANGANESE FROM WOOD SHAVINGS PRIOR TO COOKING

The removal of transition metals, especially manganese, from spruce wo od shavings prior to kraft cooking has been investigated. The effects on the different stages in the sequence Pretreatment Cooking O Q P hav e also been studied. The manganese-removing pretreatments investigated were EDTA-extraction (Q(wood)), acid pretreatment (A), an ion exchang e with magnesium sulfate (Mg) and a mixture of magnesium and calcium a cetate (Mg+Ca). The results show that it is possible to remove more th an 90% of the manganese from the wood shavings by using these pretreat ments. In trials where no or relatively low manganese-removal from woo d shavings was achieved in the pretreatment prior to cooking, a decrea se in the manganese content could be noted during the cooking. When a high degree of manganese-removal from wood shavings prior to cooking h ad been accomplished using the pretreatments Q(wood), Mg and (Mg+Ca), the content (mg/kg pulp) of manganese was higher in the pulp than in t he pretreated wood shavings (mg/kg wood). These results suggest that a small part of the manganese is very strongly attached to the wood mat erial and that this part has a much higher affinity for the kraft pulp fiber than for the black liquor. However, after the acid pretreatment of wood shavings (A), the manganese content was substantially lower i n the pulp than in the pretreated wood shavings. The higher degree of manganese removal obtained during cooking after acid pretreatment can be explained by a low content of calcium and magnesium in the wood sha vings after this pretreatment. During cooking the formation and dissol ution of substances with chelating properties (hydroxy acids and ligni n-related compounds) take place and the low concentration of magnesium and calcium ions could increase the potential for the formation of so luble manganese complexes. Positive effects of the different pretreatm ents on the brightness and the viscosity/kappa relation could be seen after oxygen delignification. After peroxide bleaching without a prece ding chelation stage, the benefit of the manganese-removing pretreatme nts on brightness was clearly shown. The pretreatments do not, however , completely exclude the demand for a chelating stage immediately prio r to peroxide bleaching.