Renovation of Electroplating Rinse Waters with Coupled-transport Membranes

A new membrane process is described that can be used to recoverplating chemicals from electroplating rinse waters. The process, called coupled transport, is based on liquid membranes that are comprised of a thin, polymeric microporous support containing a metal-ion complexing agent within the pores. Metal ions present in the rinse solutions are transported across these membranes and can be concentrated into a relatively small volume. The results of a small-scale field test are presented in which hollow-fiber coupled-transport membranes were used to recover chromium from chrome-plating rinse waters. The results of a 125-day test are used as the basis of an economic analysis that predicts a I . 7-year payback period for a full-sized coupledtransport unit. INTRODUCTION: THE USE OF MEMBRANES IN THE TREATMENT OF ELECTROPLATING RINSE WATERS Electroplating rinse waters are dilute solutions of plating chemicals that result frow dragout of the chemicals from the plating bath into the rinse baths. The discharge of these rinse waters not only creates pollution problems, but also results in a loss of valuable chemicals. Current processes for treating the rinse waters involve precipitation o f the toxic components (mostly metal ions) as a sludge. These sludges must then be disposed of in hazardous waste sites. where they remain as a threat to the environment. There is little doubt that in the future the favored processes for rinse water treatment will be those that permit recovery of the plating chemicals in a reusable or valuable form. Several membrane processes fall into this category. The ma.jor composition of electroplating rinse waters are metal ions such as Ni". Cu", and Cr207'. The ions can be recovered from the rinse solutions and concentrated using basically the two types of membrane separation processes shown in Figure I . In the process in Figure I a. a membrane is used that is permeable to water but impermeable to ions. Thus. when water flows through the membrane the rinse water becomes more concentrated in ions and reusable rinse is produced on the opposite side of the membrane. The only such process that has been used for treating electroplating rinse waters is reverse osmosis. In reverse osmosis. water is forced through the membrane by pressure on the upstream 'W C. Babcock E. D. LaChapelle Bend Research, Inc Bend, Oregon R. W. Baker Mt. View. California side-typically about 400 psi. In the process shown in Figure Ib, the membrane is permeable to ions but impermeable to water. In this case, as ions permeate the membrane the rinse water becomes more dilute and a concentrated solution of ions is produced on the opposite side of the membrane. Several suct, processes are currently being considered for treating electroplating rinse waters. These include electrodialysis. for which the driving force to concentrate metal ions is derived from an electric field imposed across the membrane, and Donnan Dialysis and coupled transport, in which the energy for concentrating the metal ions is derived from the flow of other types of ions across the membrane. Of the membrane processes with potential for treating electroplating rinse water, reverse osmosis is the most highly developed due to extensive research and development over the past 20 years to apply the process to water desalting. And, although reverse osmosis has been used to a 1imi:ed extent in the electroplating industry, it has inherent drawbacks that may prevent its widespread use. Foremost Fig. 1-Two Methods of Treating Electroplating Rinse Water with Membranes