Cation Exchange of Metals on Kraft Pulp, 1994 Pulping Conference Proceedings

P. S. Bryant, L. L. Edwards

Successful bleaching of kraft pulp with hydrogen peroxide requires the removal or deactivation of transition metals. Removal of transition metals with an acidic stage (with or without chelating agents) prior to the bleach plant has proven effective in controlling transition metals when the acidic stage washer filtrate is sewered. However, if the bleach plant is closed by bringing the acid stage filtrate back as shower water to the brown stock washers, a portion of the metals can be redeposited onto the pulp. Under certain conditions, this recycle of metals results in a net increase of transition metals to the bleach plant. In addition, the concentration of scale inducing metals can increase throughout the fiberline and in the weak black liquor.

A mathematical model based on ion exchange and chelation theory has been developed to represent the binding of metals to kraft pulp. The model predicts the distribution of Na, Ca, Mg and Mn over a broad range of pH’s, metal concentrations and chelating agent concentrations. The model has been validated with a series of laboratory experiments investigating the removal of the metals from kraft pulp by chelation pretreatment and acid washing.