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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.5 ELECTROWINNING
3.5.5 Costs
3.5.5.1 Capital Costs
The capacity requirement for conventional electrowinning depends most heavily on the amount of metal to be recovered and the rate of metal deposition. Factors that influence the rate of metal deposition are (ref. 39 and 128):
- Electrode type and area
- Agitation rate (or in general, mass transfer)
- Solution chemistry
- Electrical variables
- Temperature
The cathode surface area depends on the size and number of the cathodes employed in the unit. The agitation rate, average metal concentration in the rinse solution, solution conductivity and temperature all influence the current density that can be maintained and still result in an even, homogeneous metal deposit on the cathode. The higher the current density allowed, the higher the rate of metal deposition per unit area of cathode (ref. 39). Exhibits 3-42 and 3-43 present useful data for cathode sizing exercises.
Exhibit 3-43. Maximum Current Density for Copper Electrowinning A nearly linear relationship between cost and capacity is displayed in Exhibit 3-44. Capital costs, therefore, can be estimated once capacity requirements are determined. Most vendors refer to capacity in terms of amperage; more precisely, the maximum amperage setting on a unit's rectifier. Less commonly, capacity is expressed in terms of total cathode area. The rectifier and electrodes comprise the majority of the cost for most units; other contributing components are the fluid containment tank, pumps, filters and optional metering devices.
The strategy for determining the appropriate capacity of an electrowinning unit for a specific application is straightforward in theory: match the expected plate-out rate of the unit with the application's waste metal generation rate. For drag-out tank applications, such as those diagrammed in Exhibit 3-32, the rate at which metal is introduced into the tank is determined either by direct analysis, or by a method such as that proposed in Section 2.5.3.1. The plating rate of the metallic species in question is obtained from the Faraday Table presented in Exhibit 3-42. Capacity requirement in amperes, therefore, is the introduction rate in g/hr divided by the plating rate in g/amp-hr. This quotient requires an adjustment for expected current efficiency (that portion of current available to the cell that actually is employed reducing the target metal on the cathode) before serving as a reliable guide for required capacity. Current efficiency for electrolytes with high metallic concentrations will approach theoretical levels, but it may range down below < 10% of the theoretical rate for electrolytes concentrations of metal below 100 mg/l.
Other application configurations lend themselves to similar analysis. For units employed to electrowin metal from ion-exchange regenerant, the volume and concentration of the regenerant is required for capacity sizing. These quantities will be known from the analyses required for ion exchange sizing. The time available for electrowinning is limited by the time between regenerations. Spent regenerant may be contaminated by several species of metallic ions; this will make the calculation of appropriate cell amperage less accurate. For batch dumps, concentration of metal in the spent bath and the dump period are usually available data. The following formula applies to regenerant or process batches alike.
amps = [g/batch]/[g/amp-hr]/[hours/batch or cycle]
Cell amperage is not likely to approach the maximum rectifier output. It is limited by the maximum practical cathode current density for the metallic species being electrowinned which, in turn, is a function of metallic concentration. Thus, in practice, it is the total cathode area and the concentration of metal in the electrolyte that best determines the practical amperage capacity of a unit. Exhibit 3-43 demonstrates the considerable effect of concentration (and temperature) on maximum current density for copper electrowinning; similar curves are to be expected for other metals.
Low concentration electrolytes present significant sizing implications. Plating proceeds at a fraction of high concentration current densities and efficiency is lost as current is wasted reducing hydrogen at the cathodes. Units lacking design features aimed at reducing the thickness of the polarization depletion layer and not equipped with high surface area cathodes will be least efficient in these environments. At concentrations of <100 mg/l, a reasonable current efficiency estimate may be 10% or less. These factors must be offset with larger capacity units. Success at low concentrations will also depend on the metallic species being electrowinned, the presence of multi-valent cations in the electrolyte (such as tin and iron, which further lower efficiency by oxidizing to higher valence at the anode and reducing to lower valence at the cathode, thereby wasting current and yet staying in solution) and the time available for electrowinning (eventually, units so designed can reduce concentrations of certain metals to below compliance levels).
Anode and cathode construction will significantly impact the cost of an electrowinning unit. A list of cathode and anode materials is displayed in Exhibit 3-36. Materials options for a specific application are limited by the peculiarities of the electrolyte being electrowinned and by the manufacturer of the unit. Cost differences can be significant; e.g., ELTECH Systems Corporation offers itís units with either graphite or DSAÆ (proprietary rare earth coating) anodes. The graphite anodes were quoted in 1993 at $80, while the DSA were $335. For a Retec 25 (26 anodes) this represents a cost difference of $6,630.
3.5.5.2 Operating Costs
Typical operating costs for this technology are shown in Exhibit 3-45. Respondents employing this technology reported their annual operating costs to be only $4,100/yr on average, roughly split between the labor and non-labor categories. This technology is not labor-intensive nor expensive to run. Operating costs components are labor, electrode replacement, maintenance and energy.
Labor costs are largely installation- and application-specific. Units used in batch configurations may require considerable solution transport, pre-adjustment of the electrolyte, cleaning of salt deposits and adjustment of batch releases from the unit. On the other hand, dedicated drag-out rinse units treating fluids with lower total dissolved solids may require only occasional cleaning and rare maintenance of any kind. In all, little scheduled maintenance beyond cleaning and replacement of corroded connectors is to be expected.
Energy costs will comprise only a small percentage of total operating costs for most applications. For large units, however, energy costs may more significant in relation to total operating costs as other expenses benefit from economies of scale. Predicting energy costs for a given application is complicated by the fact that several variables affecting total power consumption are difficult or impossible to know prior to actually running the unit. Conductivity, required voltage, rectifier efficiency, and current efficiency are either unknown or will vary with time or batch to batch. Once the unit is operating, energy costs will become easy to assess. In the example given in Exhibit 3-46, electrical costs were $0.4198/lb of Cu; these costs reflect metal removal to <5 mg/l and should be typical of similar applications.
Electrode replacement costs depend on their construction and life expectancy. Stainless steel cathodes are usually peeled or scraped free of plating deposits and reused. Reticulated cathodes must be replaced after they accumulate sufficient metal to lower their effectiveness, roughly 5-10 pounds/sq. ft. of cathode area. Anodes may, as in the case of DSAÆ and other proprietary materials, be semi-permanent and last in excess of 5 years. Graphite anodes are well-known to ìmeltî or gradually exfoliate carbon particles into the electrolyte; this obviously shortens their operating life.
The example offered in Exhibit 3-46 has total operating costs at $6.81/lb of Cu. Unlike a drag-out rinse application, considerable labor was involved transporting and adjusting spent process baths prior to electrowinning. Labor, at $3.64/lb, was by far the largest operating cost component.