Recycling of copper from metal finishing wastewaters using electrodialysis ion exchange

A new design, called electrodialysis ion exchange (or EDIX), has been development at the University of Connecticut that is a pollution prevention technique for metal finishers and electroplaters. By integrating the concepts of electrodialysis and ion exchange, the EDIX system separates and concentrates dilute concentrations of copper or nickel ions from wastewaters for recycle to the plating bath while the purified water is reused as process rinse waters. The EDIX cell is a five compartment electrodialysis cell, separated by ion exchange membranes, with ion exchange resins in each compartment. The resins mainly aid in improving the conductivity of the cell. Resins are continuously regenerated on-line by an electric field, so there is no need for additional chemicals.^ The EDIX system has been shown to be operational on a pilot-plant scale for a feed flowrate of 1 liter per minute. For a feed concentration of 5 ppm Cu$\sp{2+}$ ions, a Cu$\sp{2+}$ concentrate concentration of 60 ppm and purified water concentration of 0.4 ppm Cu$\sp{2+}$ ions were obtained using a current density of 11 mA/cm$\sp2$ with a membrane area of 480 cm$\sp2.$ Most of the EDIX experiments were operated under current-limiting conditions (rather than molar flow-limiting conditions) such that problems of Cu(OH)$\sb2$ precipitation and copper reduction inside the cell were not an issue. Low counter flowrates allow higher concentrate concentrations due to the higher residence time of the ions in the concentrate compartment. Higher current densities allow higher exit concentrations by shifting the equilibrium of the ions in solution with the ions adsorbed on the resins. Differences in using strong and weak cation resins play an important role in the location of these resins inside the EDIX cell. Heterogeneous membranes, as opposed to the homogeneous type, perform better for Cu$\sp{2+}$ ion transport across the cell. A new technique and experimental apparatus was developed for characterizing membranes in terms of their effective diffusion coefficients.