Treatment of waste water containing chelated metals

Dr. Donald W. Kirk, Chemical Engineering and Applied Chemistry, University of Toronto, Toronto , Ontario , Canada M5S3E5

In the United States , over 450 000 tonnes of hazardous sludge, containing 20% heavy metals, is produced by electroplating companies annually. Chelating agents are often added to electroplating operations to keep metals in solution. However, these chelated metals cannot be recovered and reused in their original processes. Metal cations such as Co 2+, Co 3+, Ni 2+ and Cu 2+ that are complexed by chelating agents do not respond well to standard removal methods such as alkaline precipitation. EDTA is one of the strongest chelating agents that is used in industrial processes such as electroplating. This complexant was used to test the effectiveness of a variety of oxidants and precipitants. Of these, potassium peroxodisulphate, K 2S 2O 8, was found to be an extremely effective method to break the ‘EDTA’-metal complex and recover the metal. Experiments were conducted for the transition metals nickel, cobalt, and copper. Temperature increase from 25˚C to 100˚C showed decrease in precipitation time from the order of hours to the order of minutes. At 60˚C and a 20:1 stoichiometric ratio of K 2S 2O 8:EDTA, nickel precipitated the fastest at a time of 22 minutes with a metal recovery of 99.8%. Cobalt then precipitated at 25 minutes with a metal recovery of 86%. The slowest metal was copper that precipitated at 1 hour and 18 minutes with a recovery of 99.8%. Increasing the K 2S 2O 8 concentration decreases the precipitation time of all the metals and vice versa. Implementing peroxodisulphate oxidation as a potential method to recover and recycle metals is economically feasible. For a typical electroplating company, the chemical cost estimation is $80 US/m 3 of wastewater for a 0.02wt% metal solution.