Low Effluent Processing in the Pulp and Paper Industry: Electrodialysis for Continuous Selective Chloride Removal

Abstract

Pollution prevention is currently a major focus of the United States pulp and paper industry. Significant process changes are inevitable to implement low effluent processing. The kraft pulping process is prevalent for the production of wood pulp. About 50 million tons of wood pulp are produced annually in the United States alone using the kraft process. Water consumption is currently roughly between 30 and 200 m3 of water per ton of air dry bleached kraft pulp. In-process recycling of water is now being implemented by many mills to reduce the use of increasingly scarce water resources and to reduce the need for waste-water treatment. Mass balance considerations and industrial experience show that nonprocess elements, which are detrimental to the kraft process, such as chloride and potassium, will quickly build up once water use is significantly reduced. High concentrations of chloride and potassium can cause corrosion and lead to more frequent mill shutdowns due to fouling of heat exchanger surfaces in the kraft recovery furnace. Electrodialysis with monovalent selective anion and cation exchange membranes was explored here to selectively remove chlorine as sodium and potassium chloride from a feed stream with very high ionic strength. Experiments with model solutions and extended tests with the actual pulp mill materials were performed. Very good selectivities and current efficiencies were observed for chloride over sulfate. The outstanding performance of the process with actual mill materials containing organic and inorganic contamination shows great promise for rapid transfer to the pilot scale. This work is an example of the usefulness of membrane separations as a kidney in low effluent industrial processing.