Electrolytic refining cell voltage

The technical process of copper refining is an important example of an electrolytic concentration cell. Two copper electrodes operate in the same Cu++-containing electrolyte, one as anode, the other as cathode. Clearly, at equilibrium, E - 0. The current flow at the anode will dissolve Cu as Cu++, raising the concentration there. On the other hand, at the cathode, Cu++ deposits as Cu. Consequently, near the cathode, the Cu++ concentration decreases. Thus, a cell voltage will establish that opposes the applied voltage and leads to a loss of energy. Note that the aim of the process is the... 

The experimental work done to date in the reaction kinetics of SO2 depolarized electrolyzers, electrocatalyst evaluation, and tests on small experimental cells has led to projections of cell voltage for a mature technology. Table III presents these projections, as a function of electrolyte concentration and operating temperature. These projections will be refined, as the development work progresses, to expand the matrix of variables, increase the confidence level in the figures, and define an optimum set of operating parameters. 

Figure 6.44. Nyquist plot of AC impedance spectra of the segmented cell measured during different operating conditions. The plotted data of the total cell and segment Seg03 is uncorrected for the employed voltage and current gains [43], (Reprinted from Journal of Power Sources, 123(2), Bender G, Wilson MS, Zawodzinski TA. Further refinements in the segmented cell approach to diagnosing performance in polymer electrolyte fuel cells, 163— 71, 2003, with permission from Elsevier and the authors.)...

To interpret this inconsistency, a refined model is outlined below for the cell shown in Figure 4.1. The current density, j, is still expressed by the Faraday law (Eq. 4.2) and Ohm s law (Eq. 4.3). However, Eq. 4.3 cannot apparently apply if L approaches zero. In such a case the current is controlled by the intrinsic rate of the faradaic process, and not by the conductivity of the solid electrolyte, o. Hence, the cell voltage E consists of the ohmic, 011111, and activation, E, parts. The corresponding current density is then expressed as follows ... 

In the case of precious metals, optimization of the electrolytic cell is not usually critical Indeed, a number of radically different types and size of reactor compete satisfactorily for a given application, especially when the concentration of dissolved metal is relatively high. Cell voltage (and, hence, the electrolytic power requirement) and electrolyte agitation costs arc not usually critical rather, security of the deposited metal from theft and the need to produce a relatively pure metal in a form suited to refining (or reuse) is paramount... 

The copper obtained from this process is about 99% pure, yet this is not pure enough for most uses, especially those involving electrical conductivity. To refine the copper further, it is made the anode of an electrolytic cell containing copper sulfate solution. With careful control of the voltage to regulate the half-reactions that can occur, the copper is transferred from the anode (where it is about 99 % Cu) to the cathode where it can be deposited as 99.999% Cu. At the anode there is oxidation of copper,... 

In electrorefining, anodes of impure metals are immersed in an electrolyte that contains a salt of the metal to be refined—often a sulfate—and possibly sulfuric acid. When current is passed through, metal dissolves from the anode and is redeposited on the cathode in purer form. Control of the applied voltage is necessary to prevent dissolution of less easily oxidized metal impurities. Material that is not oxidized at the anode falls to the bottom of the electrolysis cell as anode slime. This slime can be further processed for any valuable materials it may contain. 

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