RECOVERY OF ZINC AND CADMIUM FROM CYANIDE PLATING WASTES USING A WEAKLY ACIDIC CATION EXCHANGE RESIN

RALPH EDWARD FROST, Purdue University

Abstract

Although the quantity of wastes produced by the electroplating industry is small in comparison to that of other industrial activities, the waste is toxic to aquatic and other life forms. Current pollution control regulations require effective removal of the toxic constituents from plating effluents. Rising costs for process and treatment chemicals, sludge disposal, energy, and water have resulted in a shift from conventional end-of-pipe treatment methods to the development and use of closed-loop recycle techniques. While recovery systems exist for several of the various types of electroplating wastes, efficient methods of recovering metal ions, cyanide, and water from alkaline cyanide plating wastes are needed. Ion exchange resins have been shown to be technically feasible for this purpose. The widespread use of these methods has been limited, however, due to the high costs for regenerant chemicals and problems associated with disposal of the regenerant solutions which typically contain 200 to 300% of the quantity of salts removed from the wastewater. Prior research in this area has centered on the use of strongly acidic cation exchange resins and both strong and weak base anion resins. In this study, the feasibility of accomplishing the cation exchange step using a weakly dissociated cation resin (Amberlite IRC-84) was evaluated. Preliminary ion exchange tests showed that IRC-84 resin operated in the hydrogen cycle could achieve high operating capacities when treating both zinc and cadmium cyanide plating rinsewaters. The method could not remove copper ions from a simulated cuprous cyanide plating waste. Additional exhaustion studies with zinc and cadmium were conducted to assess the performance of the process in terms of column capacity and effluent quality for a range of influent feed compositions, concentrations, and operating flowrates. The results of these studies showed that capacities in excess of 74 Kilograins as CaCO(,3) per cubic foot of resin were achievable for zinc cyanide wastes containing as much as 510 mg/l Zn, 1330 mg/l Na, and 684 mg/l CN at a flowrate of 2 gpm/ft('3). At one-half this influent concentration and a flowrate of 1.0 gpm/ft('3) the theoretical capacity of the resin was achieved. For cadmium plating waste feeds containing up to 316 mg/l Cd, 900 mg/l Na, and 640 mg/l CN and a flowrate of 2 gpm/ft('3) a capacity of 70 Kgr/ft('3) was achieved. The presence of sodium carbonate in the influent feeds was shown to result in increasing sodium leakage. This occurrence was due to the inability of the weak acid cation resin to completely remove sodium associated with bicarbonate. Metal ion leakage was consistently below 1 to 3 mg/l. Regeneration of the exhausted resin with concentrated sodium cyanide solutions was found to be effective for recovering both zinc and cadmium in a form suitable for reuse. The concentrations of the metal ions in the recovered solutions were a factor of 2 to 3 times below that of typical plating baths. Fractionation of the regenerant volume into appropriate segments led to recovery of 87% of both metals at sufficiently high concentrations to permit direct return to the plating bath.

Degree

Ph.D.

Subject Area

Civil engineering|Energy

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