Optimum Dewatering and Metal Recovery of Metal Plating Waste Sludges.

摘要

Bench scale experimentation was initiated to evaluate the solubilities and sludge characteristics associated with selected heavy metals and their hydroxides for both homogeneous and heterogeneous solutions. Data suggests that in a mixed-metal solution, mixed-metal co-precipitants form and the point of zero charge (PZC) for these precipitants depends upon the specific metal content of the colloidal precipitants. In general the presence of chromium (III) depresses the overall PZC while nickel and cadmium, when major waste components, tend to raise the PZC. Aging was found to impact chromium hydroxide dewatering but it is not clear that the theoretical aspects of aging chemistry can be used to predict changes. High molecular weight anionic polymers were effective in conditioning metal sludges with respect to cake formation/filtrate clarity but help dewatering rates very little. Previously developed relationships for inorganic sludges which allowed predictions about sand bed, centrifugation, and vacuum filtration dewatering processes based on gravity thickening data were found to hold for metal sludges. Based on these relationships and the laboratory data gathered here it is concluded that vacuum filtration is the only reasonable process to insure a handleable sludge. If direct reclamation of the metals from the mixed hydroxide slurry is to be attempted centrifugation may be employed. Based on a literature review and a basic economic evaluation, liquid/liquid extraction was recommended for further study as the metal recovery process of choice. (Author)