As Kraft pulp mills move toward minimum impact manufacturing, one of the most difficult challenges is the development of strategies for dealing effectively with buildup, carryover, and recovery of cationic and anionic non-process elements (NPEs). Even at low concentrations, NPEs present a serious concern due to scaling and other reactions caused by Ca, Mg, Mn, Fe, Cu, phosphates, silicates, and aluminates. The drivers behind NPE removal include environmental regulatory issues (e.g., Mn), scale formation, reduced bleaching efficiency, and corrosion. Before closure can be achieved in the bleach cycle (Figure 1), methods must be developed for efficient and cost-effective removal of NPEs from bleach filtrate streams. To be commercially viable, a highly selective, high-capacity, and regenerable media must be developed. In addition, limited prefiltration and high resistance to attrition of exchange material will significantly reduce costs, which is key to widespread commercial application. This project accurately determined the chemical composition of a Weyerhauser bleach plant effluent in the Eop, D0, and D1 stages. Due to environmental regulatory concerns, Mn was the principal target of this study. Mn was found to be present in these samples in the range of 0.16 to 3.97 ppm. The Mn was found to be in the divalent oxidation state. Other species of interest were the scale forming cations Ca (21.4 to 161 ppm) and Mg (1.0 to 20 ppm); Ba is not likely to be a significant cause of scale in these effluents and is present only in the range of 0.03 to 0.22 ppm. Various methods were evaluated for their ability to remove Mn+2 from these effluents, and carboxylate Self-Assembled Monolayers on Mesoporous Supports (SAMMS) was found to be the most effective sorbent tested. To be able to deploy SAMMS in an industrial setting, it was necessary to design a process scheme that would allow suitable flow rates with minimal back pressures. This process design was drawn up and incorporated into a truck-portable skid that could be dropped in place and plumbed into the bleach plant for pilot-scale testing and evaluation. Critical issues for the successful application of SAMMS in this up-flow fluidized bed (UFFB) system were settling velocity and particle size attrition, so these measurements were made. Attrition was found to generate fines (
Revised: September 10, 2004 |
Published: March 26, 2004