PREDICTING THE ENVIRONMENTAL STABILITY OF TREATED COPPER SMELTER FLUE DUST

Column tests were conducted to determine the leachability of As, Cd, Cu, Fe and Pb from copper smelter flue dust that had been treated by the Cashman Process in an effort to recover metals while rendering the residue inert. Between 100 and 300 mug/l As [predominantly As(V)] leached from the residue from a batch-reactor test, while between 1000 and 1400 mug/l As leached from residue from a continuous-reactor test. Electron microprobe analyses of the two materials identified scorodite (FeAsO4.2H2O) as the principal As-bearing phase. The pHs of leachate from the batch- and continuous-reactor residues were approximately 5.0 and 4.0, respectively. Cadmium and Cu leachate concentrations decreased through the tests, while Pb equilibrated at approximately 200 and 1300 mug/l for the batch and continuous residues, respectively. The measured Eh did not agree with the Eh calculated from either the As(III)/As(V) or Fe(II)/Fe(III) couple. Modeling of system chemistry using MINTEQA2 indicated that scorodite controlled the As concentration of the leachate. The variability of leachate As and metal concentrations between the batch- and continuous-reactor residues indicates that the process conditions failed to produce residues of satisfactory stability.