Removal of non-process elements from hardwood chips prior to kraft cooking

The major part of the intake of non-process elements (NPE's) to the pulp mill is via the wood chips. Some of the problems associated with NPE's are e.g. precipitation of sparingly soluble calcium salts in the fibre line and in the recovery department. In order to investigate the possibilities of reducing the intake of NPE's via hardwood chips to the pulp mill, laboratory studies on the acidic leaching of birch and eucalyptus chips were carried out. The results showed that potassium, magnesium and manganese were removed from both wood species at similar rates. The removal of calcium was however significantly slower from eucalyptus than from birch. Removal of NPE's from birch wood chips prior to cooking resulted in a higher rate of delignification, a higher unbleached brightness and a higher viscosity. In the case of eucalyptus, acidic leaching had no effect on the rate of delignification. The positive effect of acidic leaching of birch chips was found to be due to a lower content of calcium in the cooking stage