Reduction of Manganese in Brown Stock - Laboratory Study and Mill Trial Results, 2001 Pulping Conference Proceedings

G. Wohlgemuth, E. H. Mannisto, I. M. Yaraskavitch, D. Jack--The Weyerhaeuser bleached kraft pulp mill in Grande Prairie, Alberta, has a limit for discharging manganese (Mn) in the treated effluent. A laboratory study and two mill trials of pH reduction and/or chelant addition were carried out to lower the Mn content of the unbleached pulp and the bleach plant effluent.

The laboratory study was a simulation of brown stock decker washing. The lab results clearly showed that the Mn content of the washed pulp decreased with decreasing pH and with the addition of a chelant (EDTA, DTPA). The first mill trial involved pH reduction in the screen room and decker vat. Mn was freed up from the pulp in the screen room at the lower pH levels but the Mn carryover to the bleach plant did not decrease significantly possibly due to the lack of pH adjustment of the decker wash water. The second mill trial involved DTPA addition to the feed of the continuous digester which resulted in a significant decrease of Mn in the unbleached pulp and the bleach plant effluent and a significant increase in Mn in the black liquor. The successful results obtained in the lab study and the second mill trial show that DTPA addition could be a technically viable solution to the problem. The Mn removed from the unbleached pulp would be sent to recovery in the black liquor and subsequently discharged to landfill with the green liquor dregs instead of to the river in the treated effluent.