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Electrorefining is an age-old process. It was first demonstrated experimentally by von Leuchtenberg in 1847. Elkington, however, patented the process in 1865 and developed the first successful plant in Pembrey, Wales in 1869. The first commercial electrorefining plant in the United States was constructed by Balbach Smelting & Refining Co. in 1883. The Newark, New Jersey plant produced 2-3 tons of electrolytic copper per day during the first year.
Electrorefining utilizes the electrochemical dissolution of an impure copper anode in an electrolytic cell containing a copper sulfate-sulfuric acid solution. Copper ions are transported to the cathode where they are deposited with suitable purity. The basic electrochemical reactions involved are:
Anodic Reaction: Cu (impure) ® Cu2+ + 2e(Eq. 1)
Cathodic Reaction: Cu2+ + 2e ® Cu (pure)(Eq. 2)
Net Reaction:Cu (impure)® Cu (pure)(Eq. 3)
As dissolution occurs, impurities are released either as solid phases or as aqueous species. Purification results from differences in the oxidation/reduction potentials of the individual ions. First, more electrochemically noble impurities, such as gold and silver, will remain at or near the anode. The elements that are less noble than copper will dissolve along with copper into the bulk electrolyte. At the cathode, copper being the most noble element in solution will deposit preferentially. Careful control of the system's voltage, current density, temperature, electrolyte contamination, and deposit morphology ensures an extremely pure copper deposit.
In an industrial tankhouse, a constant current is utilized to control the production rate. Typical cathodic current densities in commercial electrorefining are between 200-300 A m-2. Higher current densities can cause problems such as anode passivation and/or cathode contamination. Theoretically, the net reaction (Eq. 3) for copper electrorefining is 0.0 V. In practice, overvoltages at the anode and cathode and the resistance in the electrolyte and electrical system result in the need for an applied voltage of approximately 0.3 V. Tseidler has indicated that 70-80% of the voltage is consumed by electrolyte resistance, approximately 5% by polarization, and the remainder in the resistance of the electrical system including the electrodes and electrical contacts . If the voltage becomes too high, contamination can occur by the deposition of impurities such as the Group VB elements on the cathode. Contamination can also occur because of dendritic growth which traps impurity laden electrolyte within the deposit.
Electrolyte temperature is another important system parameter. Increased temperature reduces the resistance of the electrolyte, which lowers the cost of production. The temperature also controls the solubility of impurities in the solution. A decrease in temperature can cause precipitation of impurity phases. However, heating the electrolyte adds cost to the operation. Thus, most refineries compromise and have electrolyte temperatures between 60 and 65 oC.
Electrolyte impurity concentrations can lead to cathode contamination. Elements that typically accumulate in the electrolyte are nickel, arsenic, antimony, bismuth, and iron. The Group VB elements can be particularly problematic because their standard reduction potentials are very close to that of copper. Another concern that may arise from these elements is the formation of floating slimes. Floating slimes are composed of antimony arsenate, SbAsO4, and bismuth arsenate, BiAsO4. These phases form by precipitation away from the anode and can float to the cathode causing contamination. Impurities are typically maintained at acceptable levels in the electrolyte by removal in a bleed stream by electrolysis and crystallization. Recently, ion exchange and solvent exchange have been implemented in the removal of impurities, particularly the Group VB elements (As, Sb, and Bi).
Morphology is also extremely important in maintaining cathode purity. Nodules and dendrites are the most common surface defects that lead to impurities in the cathode. A nodule forms when a conductive particle remains on the cathode surface. The deposit will then grow around the particle encasing it. This results in a bump on the surface. The bump will then act as a ledge which other particles can come to rest on. Thus the nodule becomes larger with the growth of the copper case and more contaminated with slime particles.
Uninhibited growth of the copper electrodeposit will typically lead to the formation of dendrites. These surface irregularities can lead to entrapment of electrolyte or colloidal particles into the deposit. Additives, such as, thiourea, glue, and chloride, are added to the electrolyte to inhibit the formation of dendrites. While the complete mechanism in which these additives interact with the cathodic surface is not fully understood, the results are reproducible. Thiourea and glue, which are usually added to maintain concentration of 1-3 mg l-1 (ppm), act as leveling agents. It is believed that these organic molecules adsorb onto high spots on the deposit. This blocks these sites, causing deposition to occur at the low areas, which result in a smooth deposit. Chloride is contained in the makeup water or added as NaCl with typical concentrations being 40-60 mg l-1 (ppm). Cl- acts as a grain refiner for the deposit. Proper concentrations of these additives result in a smooth and dense cathode with suitable purity.
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Anode passivation can be a significant problem in the commercial electrorefining process.
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