Treatment of Plating Wastewaters Containing Metal Cyanide Complexes by a Gas Membrane-Ion Exchange Process

Abstract

A novel gas rnembrane-ion exchange process (GM-IX) is described to treat metal plating wastewaters containing zinc cyanide and recover usable forms of zinc and cyanide. The process uses a strongly basic anion exchange resin to selectively concentrate the metal cyanides from dilute plating wastes. Following exhaustion, the resin is regenerated under acidic conditions. The metal cyanide complexes are broken down as a result of HCN production under these acidic conditions, and the acidic regenerant containing the HCN is recirculated through a gas rnenbrane module that is supplied with a caustic stripping solution. Volatile HCN migrates across the gas perneable membranes to be absorbed and neutralized in the NaOH solution. The products of the process include concentrated sodium cyanide solution containing some excess sodium hydroxide, and concentrated acidic zinc sulfate solution that can be treated by an electrowinning process to recover zinc metal and a reusable acid for regeneration. Results of mass transfer studies indicate that the rate of HCN transfer across the gas permeable membrane is rapid and complete. The transfer is largely limited by diffusion within the lumen of the hollow fibers but also is influenced by the reaction rate of HCN with base outside the fibers. The gas permeable membrane provides the basis for effective separation and recovery of cyanide from zinc and cadmium plating wastes. Initial ion exchange is necessary to concentrate dilute metal cyanide solutions such as rinsewaters, but if the wastewater is already concentrated, only acidification is required. Th e zinc cyanide precipitate formed in the resin during regeneration was found to be Zn(CN)2(s), Recovery of cyanide from acidified zinc and cadmium cyanide solution is fast and complete when the solution pH is sufficiently acidic to convert the cyanide to the HCN form; above pH 2.0 the rate of cyanide recovery was slowed by the 1ow percentage of HCN present at equilibrium. In concentrated zinc cyanide solutions such as may be encountered during regeneration a lower pH is required to convert all the complexed cyanide to HCN. To achieve good regeneration, a strong acid solution and sufficient contact time must be provided to ensure complete cyanide removal. A kinetic model interfaced with a chemical equilibrium model successfully predicted the rate of cyanide recovery as a function of operating pH. Agreement between the predicted and the observed cyanide recovery rates indicated that the dissolution rates of Zn(CN)2(s), does not appear to limit the cyanide recovery rate. Rather, the rate is controlled by the cyanide speciation equilibria. Solutions containing copper and silver cyanides are Iess readily treated. The use of organic acids to catalyze the breakdown of these metal cyanides may aid these less tractable solutions. Cyanide cannot be completely recovered simply by acidification of concentrated copper cyanide solutions because of the formation of a cuprous cyanide precipitate. With addition of FeC13,, cuprous cyanide precipitate can be oxidized and cyanide can be completely recovered in the NaCN form.