Metal Hydroxides from Electroplating: Sludge Characterization and Metal Recovery.

摘要

The purpose of this project was to investigate the precipitation of metals from plating facilities, to determine the dewatering characteristics of the resultant metal hydroxide slurries and to evaluate the solvent extraction process for metal recovery and reuse. Individual metals were found to precipitate according to the theoretical solubility calculations, however, mixed metals solutions containing Cr (III) often would form 'co-precipitants.' The co-precipitation would result in certain metals precipitating at pH levels below the range of predicted insolubility. Colloidal metal hydroxides remaining after settling could be removed by filtration using either a gravity filter or diatomaceous precoat pressure filter. Metal removal was effective (>95%) but the filtrate metal concentrations were not sufficiently low to meet effluent requirements. Sludges produced by hydroxide precipitation varied depending upon the pH of precipitation and the specific metals in the sludge. Sludge dewatering characteristics were found to be determined by the mean particle size of the flocs. Polymers improved dewatering rates by increasing the mean floc size. Freshly precipitated chronium hydroxide sludges were found to pass through filtering media because of floc break-up. Polymer conditioning was necessary to prevent sludge penetration through the filter media. Solvent extraction could be used to selectively extract individual metals from a mixed metal sludge but the process was not economical. (Author)