Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 557 of 707

powder also entrains mother liquor, which is virtually impossible to rinse out completely. This results in an acidic, wet powder, often contaminated with halite ions, which in turn render the recovered metal powder difficult or impossible to reuse or sell. 2. High-surface-area recovery cells deposit the metal on some type of fibrous or filamentous substrate. In some cases, the plated metal is dis- carded or sold as a low- volume residue, while in others, the deposited metal is stripped chemically or electrochemically so that the end result is a concentrated solution of the metal that was recovered. 3. True EMR or electrowinning approaches recover a solid slab or sheet of relatively high-purity metal, that can be easily handled, weighed, assayed, or transported and sold for the best available price in the secondary met- al markets. In certain recovery applications or circumstances, the elec- trodeposited metal is pure enough to be reused as anode material in the originating plating process. This type of cell usually applies some type of moving or rotating cathode, or alternatively, a high solution velocity over fixed cathodes. To reduce the effect of electrode polarization common to low metal ion con- centrations and to increase ion diffusion rates at the electrodes, it is recom- mended the solution be heated. Otherwise, plate-out of metal from these low con- centration solutions will be hindered. Strong air agitation is another method for providing adequate mechanical mixing, but it removes heat from the system, thus reducing operating rates. Air agitation may also add to the load on air pollution control equipment. In addition to the use discussed earlier under concentrate recovery methods, ion exchange can be used for several other applications, which include recuperation of noble metals, recovery of metals from rinsewater in combination with electrolyt- ic metal recovery, and the purification of some process solutions such as chromate baths. Ion Exchange In gold recovery, ion exchange is effective in collecting essentially at traces from a dilute rinse stream. Historically, such gold-laden ion exchange resins were burned by a gold refiner who recovered the ash. Currently some companies are offering a tolling service to regenerate the ion exchange resin chemically and return it to the user. In either case the primary disadvantages are the difficulty in assaying a het- erogeneous mass of metal-laden ion exchange beads and the high tolling charges from the refiner or processor. Both of these factors preclude recovery of maximum gold value. A second emerging application involves linking two recovery techniques; ion exchange and EMR. In this scheme, as shown in Figure 11, the ion exchange bed is used to collect metal ions from dilute rinsewater and the acid formed in the elec- trowinning operation serves to regenerate the ion exchange resin. SLUDGES AS BYPRODUCTS There has been a steady increase in the number of companies interested in using metal- bearing waste treatment sludge as a feedstock in their manufacturing processes; nevertheless, most mixed sludge has no value. In fact, the generator often 556

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