CATION-EXCHANGE OF METALS ON KRAFT PULP

Successful bleaching of kraft pulp with hydrogen peroxide requires the removal or deactivation of transition metals. Removal of transition m etals with an acidic stage (with or without chelating agents) prior to the bleach plant has proven effective in controlling transition metal s when the acidic stage washer filtrate is sewered. However, if the bl each plant is closed by bringing the acid stage filtrate back as showe r water to the brown stock washers, a portion of the metals can be red eposited onto the pulp. Under certain conditions, this recycle of meta ls results in a net increase of transition metals to the bleach plant. In addition, the concentration of scale-inducing metals can increase throughout the fibreline 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 pHs, metal co ncentrations and chelating agent concentrations. The model has been va lidated with a series of laboratory experiments investigating the remo val of the metals from kraft pulp by chelation pretreatment and acid w ashing.