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Electrically Switched Ion Exchange (ESIX) for the Separation of Potassium and Chloride Ions to Enhance Water Recycle J. P. H. Sukamto, S. D. Rassat,(a,b) W. E. Lawrence,(b) G. B. Josephson,(b) V. V. Viswanathan,(a,b) D. P. Mendoza,(a,b) A. S. Lea, J. S. Young, R. T. Hallen,(a,b) R. J. Orth,(a,b) W. A. Wilcox,(a,b) M. A. Lilga,(a,b) S. A. Bryan,(a,b) B. J. Malmberg,(a,c) Y. Gu,(a,c) L. L. Edwards,(a,c) and P. Thorn(d) Supported by the Forest Products, Industry of the Future Program, Office of Industrial Technologies, Department of Energy. (a) EMSL User. (b) Environmental Technology Division. (c) University of Idaho. (d) Weyerhaeuser. Reducing wastewater discharges from pulping and bleaching operations through the recovery of bleach plant filtrates is currently limited by the build-up of non-process cations (e.g., potassium, K+), soluble anions (e.g., chloride, Cl-), and organic compounds. Without the potassium and chloride, the sodium and sulfur in the discharges could be available for recycling and reuse. The cost of effectively removing these soluble ions, then, is essential for maximizing filtrate recovery and minimizing corrosion and recovery boiler downtime (Ricketts 1994; Mannisto et al. 1995; Sharp 1996; Jordan and Bryant 1996). PNNL, in collaboration with the University of Idaho and Weyerhaeuser, evaluated the technical feasibility and economic viability of using electrochemically active membrane materials for the selective separation of K+ from a mixture of mainly K+ and sodium (Na+) cations. Separation of Cl- is more straightforward because the main competing anions are sulfates and bicarbonates (i.e., monovalent/divalent separation). In FY1999, we evaluated the feasibility of synthesizing electroactive membranes to increase process efficiency, and we established electrochemical preparation procedures for making nickel hexacyanoferrate (NiHCF) films with optimum properties for separating K+ from Na+. An assessment of the impact of this process on overall mill operation was made using the University of Idaho’s mill simulator, the General Energy and Material Balance System (WinGEMS). WinGEMS was used to evaluate recycle strategies and how new technologies impact K and Cl levels throughout the mill. New simulation tools have been integrated into WinGEMS to accurately model the impact of K and Cl buildup due to increased recycling of mill effluent streams. A new pulp sorption model has been developed that includes Na+ and K+ competition for pulp binding sites, as well as pH effects. The WinGEMS Kraft recovery furnace model has been enhanced to handle K and Cl compounds. Results from a full mill simulation showed that use of PNNL’s system to treat the electrostatic precipitator (ESP) catch and ash from the recovery boiler can lower the K levels in the "as fired" black liquor by 35%, decreasing ash and dust stickiness (Sukamto et al. submitted). The ESP catch is an attractive stream to treat since it contains the highest fraction of KCl. Both simulants and actual ESP catch samples were used in this study. The functional requirements for electroactive membranes for our application are electronic and ionic conductivity, and membranes that are free from pinholes. There are two pinhole issues that were of interest: 1) physical pinholes that would allow mixing between process streams and 2) chemical pinholes in the NiHCF coating that would lead to low separation factors. The first is easily assessed by the use of a scanning electron microscope, while the second can only be verified by flow tests. Shown in Figures 5.6a and b are typical scanning electron micrographs (SEM) of a bare 635 x 635 mesh stainless steel electrode and after it has been coated with polypyrrole (PPY), respectively; Figure 5.6b shows no visible pinholes. (The SEMs were obtained using a LEO 982 field emission scanning electron microscope equipped with an Oxford ISIS energy dispersive x-ray microanalysis system.) This was further verified by using membranes, prepared in the similar fashion as that shown in Figures 5.6a and b, in a flow cell where one of the streams was colored with a fluorescein dye. After more than 17.5 hours of testing at 10 mL/min., there was no evident of dye transferring across the ESIX membrane. Figure 5.6. (a) Scanning electron micrograph of the base 635 x 635 mesh stainless steel electrode. (b) After coating with polypyrrole. To impart selectivity for K+ over Na+, NiHCF materials were incorporated within the PPY membranes. After NiHCF was deposited within the membranes, the nickel and iron distributions were determined. Typical results are shown in Figures 5.7a, b, and c. Figure 5.7a shows the polypyrrole-coated 635 x 635 stainless steel mesh electrode. The nickel and iron distributions are shown in Figures 5.7b and c, respectively (the light areas indicate the presence of nickel or iron). The results show that both nickel and iron are distributed uniformly, which suggest that NiHCF was successfully incorporated within the membranes. Figure 5.7. (a) Scanning electron micrograph of a polypyrrole-coated 635 x 635 stainless steel mesh. (b) Nickel map of (a). (c) Iron map of (a). In addition, the presence of electroactive NiHCF was confirmed by infrared data (not shown). However, flow test results (not shown) suggest that there are areas in the membranes that do not have NiHCF. Consequently, the separation factors of the membranes were not as high as that obtained from batch-contact studies of NiHCF. We also showed that slow growth mode electrochemical deposition of NiHCF materials resulted in a more stoichiometric nickel to iron ratio compared to other electrochemical methods reported in the literature. The more stoichiometric ratio was found to correlate with faster transport of cations through the NiHCF films (Sukamto et al. 1999). References Ricketts, J.D. "Considerations for the Closed-Cycle Mill," TAPPI Journal. 77, 43 (1994). Mannisto, H., E. Mannisto, and P. Winter, "Technical and Economic Implications of Converting Bleached-Kraft Mills to Low-Effluent Operation," TAPPI Journal. 78, 65 (1995). Sharp, W. B. A., "Anticipated Effects of System Closure on Corrosion in Chemical Recovery Equipment," TAPPI Journal. 79, 161 (1996). Jordan, J. M. and P. S. Bryant, "Cluster Rule Impact on Recovery Boiler Operations: Chloride and Potassium Concentrations in the Kraft Liquor Cycle," TAPPI Journal. 79, 108 (1996). Sukamto, J. H., S. D. Rassat, G. J. Josephson, W. E. Lawrence, D. P. Mendoza, V. V. Viswanathan, R. T. Hallen, R. J. Orth, M. A. Lilga, B. J. Malmberg, Y. Gu, and L. L. Edwards, "Electrically Switched Ion Exchange (ESIX) for the Removal of Potassium to Enhance Water Recycle Opportunities," TAPPI Journal, submitted. Sukamto, J. H., M. F. Buehler, S. D. Rassat, R. J. Orth, M. A. Lilga, and R. T. Hallen, "An Electroactive Film on a Substrate and Methods of Making," Patent Application filed August 1999. William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147