The removal of heavy metals from dredged sediments by mechanical Denver flotation: the contribution of true flotation and entrainment

M. Vanthuyne and A. Maes

Abstract
This study investigated the possibility of removing Cd, Cu, Pb and Zn from four Belgian anoxic sediments, using Denver flotation. The sediments were sampled in, respectively, the River Schijn, the Gent-Terneuzen Canal, the Zuid-Willemsvaart Canal, and the Brussels Docks, all of which have a different (in situ) particle-size distribution. Two froth fractions were collected after froth stabilization: froth 1 (during the first five minutes) and froth 2 (during the second five minutes). For all sediments, the heavy-metal-concentrating factors decreased in froth 2 to values close to 1, which means an absence of flotation selectivity. Except for Cd, higher heavy-metal flotation selectivities were observed in froth 1 for the River Schijn sediment (with concentrating factors of 2.87-3.15) compared with the other three sediments (with concentrating factors of 1.71-2.17) and at similar mass recoveries (20.4-25.5%). The heavy-metal recoveries by entrainment were predicted for froth 1 and froth 2, based on the experimental Fe-concentrating factors of froth 2. For all sediments, the total heavy-metal recoveries in froth 2 were shown to be entirely due to entrainment. The higher Cu, Pb and Zn flotation selectivity in froth 1 from the River Schijn sediment was ascribed to the higher heavy-metal recoveries by true flotation (65.7-68.9%), as compared with the other three sediments (23.9-49.2%), and was explained by the observation that a higher heavy-metal fraction was present in the >20-um particle-size fraction. For the sediments from the Gent-Terneuzen Canal, the Zuid-Willemsvaart Canal, and the Brussels Docks, the large contribution of entrainment (50.8-76.1%) strongly decreased the heavy-metal flotation selectivities in froth 1.

Key words: entrainment, flotation, heavy metals, remediation, sediment

Land Contamination & Reclamation, 15 (1), 15-30 (2007) 

DOI 10.2462/09670513.844 

Updated: 21-Dec-2009