Figure 4. Schematic diagram for electrolytic recovery of metals.
Far dilute solutions, electrowinnlng can be difficult because of the low mass-transfer rates; however, mass transfer rates can be enhanced both by agitation and by increasing the effective surface area of the cathode.
Among the modified electrolytic reactors that have been designed with electrodes that either enhance mixing or have large surface areas are the concentric cylinder; parallel, porous plates; the rotating cylinder; packed-bed reactor; fluidized-bed reactor; and the carbon fiber reactor.
Although the electrodes used in these reactors are more effective in the removal of metals from solution, their design makes it difficult to remove the metal once it has been plated onto the cathode. For example, the use of a reactor with parallel stainless steel cathodes generally allows for the production of a compact layer of metal that can be mechanically removed and sold as scrap. Conversely, the use of a reactor with a high electrode area results In the deposition of metal within pores of a cathode, which generally makes mechanical removal of the metal impossible. In thfs case, recovery of the metal must be accomplished by leaching the deposited metal out of the cell by anodic dissolution.
Prgtreatment reaulrements--In many cases, the wastewater must be filtered before it is fed through the electrolytic reactor. Grease and oil increase the reaction time if present (n the wastewater. This is particularly true with reactors that use porous or packed-bed electrodes because particulates can potentially clog the reactor. Adjustment of pH is a necessary pretreatment measure because the waste pH affects metal speciation.
Posttreatment requirement;--Posttreatment may be required to recover metals from the cathodic regenerating solution if the solution cannot be reused directly as bath makeup. Also, recovered wastewater mist eventually be disposed of or treated because of the buildup of organtcs and other impurities 1n the bath. Stripped metals and metal-laden cathodes can be shipped off site to smelters or reclamation facilities.
Performance data-Table 26 summarizes the performance of several electrolytic reactors on specific metal/cyanide wastes. These data reflect the performance of a particular electrolytic reactor on a specific waste stream; therefore, the data should not be taken as a general indicator of performance.
Performance can be assessed In terms of rate of metal removal from solution or current efficiency. The rate of attal removal can be determined either by performing a metal mass balance on inlet arid outlet streams or by weighing the amount of metal that is deposited on the cathode. The latter technique may not be very accurate when metal concentrations are very low. Current efficiency compares the actual amount of metal (or other contaminants) removed with the amount that could theoretically be removed at a given current. In practice, a high current efficiency is not necessarily equivalent to a high rate of removal because the removal rate increases with the current.
TABLE 26. SUMMARY OF METAL RECOVERY RATES WITH ELECTROLYTIC TECHNOLOGY*
Estimated Metal metal recovery rate, Recovery Reuse value, value, kg/week technique" application $/kg i/yr
Acid zinc concentrate from ion-exchange (2.5 g/L Zn 5
Nickel plating, drag-out 23
Copper plating, off-1ine-drag-
Cadmium plating, drag-out 0.7
Acid copper plating, drag-out 23
Acid zinc plating, drag-out 16
Bright nickel plating, drag-out 30
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