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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.5 ELECTROWINNING
3.5.1 Overview
Electrowinning is one of the two most widely used methods of metal recovery in the plating industry, the other being atmospheric evaporation (Section 3.2). Of the 318 plating shops responding to the Users Survey, 61 (or 19%) have employed this technology. Some shops have purchased or fabricated two or more units for different applications, resulting in a total number of 80 electrowinning units used by the 318 survey respondents.
Electrowinning is most frequently used to: (1) reduce the mass of inexpensive regulated metals (e.g., zinc, copper, lead) and cyanide being discharged to treatment and thereby reduce the quantity of treatment reagents used and the quantity of sludge generated and/or (2) recover expensive common metals (e.g., nickel and cadmium) or precious metals (e.g., silver and gold) for recovery/recycle and thereby reduce overall operating costs. In either case, electrowinning is most often applied for gross metal recovery from concentrated solutions such as drag-out rinses or ion exchange regenerant. Used in this manner, it is not sufficient as a stand-alone technology to meet discharge standards. Reticulate cathode, high surface area (HSA) or high mass transfer (HMT) cathode designs also make this technology applicable to some dilute metal bearing solutions (e.g., overflow rinses). The reticulate cathode units have been proven to be effective in maintaining the metal concentration of recirculated rinses to less than 1 mg/l. The HSA units have been advertised as a method of compliance (ref. 98) and in the late 1970's and early 1980's attempts were made to use the electrowinning technology in this manner. However, none of Users Survey respondents are currently discharging an effluent from an HSA or HMT unit without further treatment for metals removal. Some non-continuous discharges of batch-treated solutions are found. However, for these cases, the volume of the discharge is insignificant compared to total wastewater flow.
The basic unit of the electrowinning technology is the electrolytic cell: two electrodes (anode and cathode) are placed in a solution containing ions, where there occurs a movement of ions toward the charged electrodes. Dissolved metals in the electrolyte are reduced and deposited on the cathode. The deposited metal is removed by mechanical (e.g. scraping) or chemical means and either reused as anode material or sent off-site for refining/reuse or disposal.
The types of cathodes used in electrowinning can be grouped into three categories. These include, in order of increasing surface area: (1) flat plate, (2) expanded metal, wire mesh or reticulate plate, and (3) porous or woven carbon and graphite types. The flat plate cathodes are used for applications of gross metal recovery from concentrated solutions (e.g., >1 g/l of metal). The expanded metal, wire mesh, or reticulate plate and the porous or woven types are used for recovering metals from solutions with lower metal concentrations, with the latter group effective in some cases in the low mg/l range. Reticulate cathodes, which permit flow-through of the electrolyte, have an effective surface area of approximately 10 times the face or apparent area of the cathode. Porous or woven cathodes have internal pores that also permit flow-through of the electrolyte and have a surface area up to 13,000 times greater than the apparent area.
There are several common terms used in describing the equipment and processes relative to electrowinning. The basic electrolytic cell is composed of two electrodes, one anode (positive charge) and one cathode (negative charge). The chemical reactions that take place at the anode are oxidations and the reactions at the cathode are reductions. The solution is referred to as an electrolyte. When a direct current (D.C.) is applied to the cell, the anions present in the electrolyte migrate toward the anode and the cations migrate toward the cathode. An important controlling factor in the process is the amount of current flowing through the cell. The level of current is measured in amperes per unit area of electrode (typically, amperes per square foot) and is referred to as the current density. Current density affects the nature of the electroplated deposit, the distribution of the deposit, the current efficiency, and to some extent whether a deposit forms at all. In electrowinning, the theoretical quantity of metal that is deposited onto the cathode is described by Faraday's Law. This law states that the amount of chemical change produced by an electric current is proportional to the quantity of electricity used (ref. 350). Some of the electric current is used for reactions other than metal deposit. Electroplaters refer to the ratio of desired chemical change (deposit) to the total chemical change as the current efficiency, usually expressed as a percentage of current applied.
As indicated previously, the current density has a substantial impact on the rate of metal deposit. It is desirable to operate electrowinning processes at the maximum current density where good deposition still takes place. The current density should, however, not exceed that which deposits metal faster than ions can diffuse through the electrolyte. When the thin film of electrolyte surrounding the cathode is depleted of metal ions, a condition referred to as concentration polarization occurs. This results in an adverse effect on the current efficiency as well as the quality of the deposit due to excessive hydrogen evolution at the cathode and oxygen evolution at the anode. The allowable or critical current density is determined by the concentration of metal ions in the electrolyte and the thickness of the film surrounding the cathode. Innovations in the design of electrowinning devices have generally focused on extending the operating range of the process by: (1) increasing the surface of the cathode (i.e., high surface area), or (2) reducing the thickness of the film using agitation or heating (ref. 349, 351).
For most applications, the primary use of electrowinning is the recovery of metal. However, when performed with an electrolyte containing cyanide, the process also oxidizes some of the cyanide at the anode (alternatively CN can be oxidized with hypochlorite ions which result from the electrochemical oxidation of chloride ion in a basic medium). Although the anodic reactions are given less consideration in most applications, they can play an important role in the economic viability of the process by reducing the treatment reagent requirements for end-of-pipe treatment. Anodic reactions including cyanide destruction and organic complexing agent destruction (e.g., treatment of an electroless copper bath) were examined in detail by Waiux and Nguyen (ref. 123).