Anode reaction, electrorefining

If the fire-refined copper is to be cast into anodes for electrorefining, the oxygen content of the copper is lowered to 0.05—0.2%. If the copper is to be sold directly for fabrication, the oxygen level is adjusted to 0.03—0.05%, which is the range for tough-pitch copper. The principal reactions of fire refining are... 

The electrowinning processes essentially use anodes that do not dissolve anodically. In electrorefining, however, an impure metal is anodically dissolved as metal ions and subsequently these ions are reduced at the cathode to yield the pure metal the cell reactions are ... 

Electrowinning is done from purified solutions. The anodes are insoluble metal or oxide. In the electrowinning of most metals, oxygen is evolved from decomposition of water at the anode. Electrowinning of some metals is done from chloride solutions with evolution of chlorine gas as anode reaction. The metal to be recovered is deposited on the cathode as in the electrorefining process. 

In electrorefining, the anodic reaction is oxidation and dissolution of metal by the generic reaction (6). The cathodic reaction in both electrowinning and electrorefining is the reduction and deposition of metal ion by the generic reaction (7)... 

The anode reactions are, however, difierent. In electrorefining, a soluble anode is used, dissolution occuring at high current efiiciendes ... 

In electro winning, the cathode reaction is the same as for electrorefining (see eq. 31). However, because of the use of insoluble anodes, oxygen is released at the anode. 

Anodic oxidation liberates some amount of energy whereas the cathode reaction consumes some energy. Thus, from the point of view of energy consumption, electrorefining is quite an efficient process which involves the overall reaction... 

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

These chemical reactions possibly precede the electrochemical reactions. Thus the electrochemical reactions in the case of molybdenum oxides may be taken to be similar to those which occur in electrorefining, i.e., electrochemical dissolution of molybdenum from the impure metallic molybdenum anode and subsequent deposition at the cathode. The combination of the chemical and the electrochemical reactions occurring at the anode can be represented in the following way ... 

In the electrorefining of copper, copper(I) sulfate in the AN-H20-H2S04 solution is electrolyzed using a coarse copper electrode as anode and a pure copper electrode as cathode. The reaction at the anode is Cu° —> Cu+, while that at the cathode is Cu+ —> Cu°. Compared to the conventional process which uses aqueous acidic solutions of copper(II) sulfate, this method is advantageous in that the quantity of electricity is one-half and the electric power is also small. Moreover, the low quality AN used in this method is available at low price and in large quantity as a by-product of chemical industries. 

The reactions (20) to (22) form the copper equilibrium on the electrode surfaces. Concentration of Cu(I) on the cathode surface affects the deposition rate. The maximum net rate of Cu+ production is at about —50 mV versus Cu/CuSC>4 and at higher overpotentials it decreases. Disturbing the Cu(II)—Cu(I)—Cu equilibrium can cause the formation of copper powder, but this is more a problem on the anode. For the current densities commonly used in electrorefining, the cathode overpotential is between 50 and 100 mV. The system is mainly charge transfer controlled and the effect of mass-transfer polarization is small. If Cu(I) concentration on the cathode surface decreases, mass-transfer polarization will increase, causing more uneven deposit. 

Electrorefining Nickel Metal Anodes In the refining of nickel metal anodes, the principal reaction at the anode is the dissolution of nickel metal as nickel ions. The principal cathodic reaction is the reduction of nickel ions from solution. Nickel anodes are made by reducing nickel oxide with coke at temperatures up to 1550 °C and casting in molds. The practice is designed to obtain anodes with the desired strength and crystal size. 

Copper may also be recovered from leach solutions electrolytically. Electrowinning requires the use of an insoluble anode such as hard lead, comparable to the liberator cell used for liquor purification in copper electrorefining. Consequently, there are net electrochemical reactions involved in electrowinning (Eqs. 13.20 and 13.22), as opposed to the situation with electrorefining, so that about 1.7 V are required for this step. This results in a much higher electrical power consumption of about 2.8 kWh/kg copper for electrowinning, compared to about 0.2 kWh/kg for electrorefining. 

In an electrorefining process, the anode is the impure metal and the impurities must be lost during the passage of the metal from the anode to the cathode during electrolysis, i.e. the electrode reactions are, at the anode... 

In electrorefining the cathode reaction is the reverse of that at the anode and therefore, in the ideal case, the cell voltage is only required to drive the current... 

Electrorefining has its ftmdamentals based on the principles of electrochemical kinetics. The anodes are artificially oxidized by loosing electrons and freeing cations, which migrate towards the cathode surfaces through the solution under the influence of an applied current in opposite direction. As a result, the cations AT " combine with electrons to form reduction reactions. This is the source of electrolytic metal deposition, which in turn, becomes adhered as a layer on the cathode surfaces. This layer increases in thickness as electrolysis proceeds up to an extend determined by experimentation or industrial practice. The refining process can be represent by a reversible electrochemical reactions of the form... 

Consider the bipolar electrode for electrorefining metal M. Thus, the rate of formation (reduction) and dissolution (oxidation) are treated as steady-state quantities. Despite that the initial formation of a thin film at the cathode face and dissolution rates are controlled by reactions at the electrode-electrolyte interfaces, assume a steady-state diffusion mechanism. Derive expressions for the weight gain at the cathode side and the weight loss at the anode side. 

It can be seen from the data in Table 4.3 that in order to obtain pure metals at the cathode, the current density is always low. On the other hand, with the exception of tin, the current efficiencies are good and the celt voltages can be low. fn electrorefining the cathode reaction is the reverse of that at the anode and therefore, in the ideal case, the celt voltage is only required to drive the current through the electrolyte. In practice, there may also be overpotemials associated with the anode and cathode reactions, and in the cases of nickel and cobalt these are considerable because the couples are very irreversible. 

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