Potential electrorefining

Electrorefining. Electrolytic refining is a purification process in which an impure metal anode is dissolved electrochemicaHy in a solution of a salt of the metal to be refined, and then recovered as a pure cathodic deposit. Electrorefining is a more efficient purification process than other chemical methods because of its selectivity. In particular, for metals such as copper, silver, gold, and lead, which exhibit Htfle irreversibHity, the operating electrode potential is close to the reversible potential, and a sharp separation can be accompHshed, both at the anode where more noble metals do not dissolve and at the cathode where more active metals do not deposit. 

The quantities pure and impure denote the activities of the pure and the impure metals respectively. For a pure metal the activity pure is unity and thus ° = -(R T/n F) In impure. From this equation it can be noted that the potential requirement in electrorefining is quite small and that the process depends on the activity of the impure metal. 

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... 

Electrorefining has been used for the purification of many common as well as reactive metals. It has been seen that the emf or the potential required for such a process is usually small because the energy needed for the reduction of the ionic species at the cathode is almost equal to that released by the oxidation of the crude metal at the anode. Some metals, such as copper, nickel, lead, silver, gold, etc., are refined by using aqueous electrolytes whereas molten salt electrolytes are necessary for the refining of reactive metals such as aluminum,... 

Interesting potential applications of molten salts are electroplating and electrorefining of refractory metals and rare earth metals. Electrowinning of titanium has been tested on a pilot scale. Electrodeposition of refractory compounds like TiB2 has also been demonstrated. Due to space limitations these more exotic applications of molten salts will not be treated here. However, short chapters on molten salt batteries and fuel cells are included. 

Electroraffination — (see also electrorefining) Purification of metals by means of dissolution and subsequent electrodeposition. Common method in - electrometallurgy for the removal of impurities from raw metals. Upon anodic dissolution the metallic constituents of the anode are dissolved as cations, oxyanions, or complex ions. All impurities - whether metallic or not - are also dissolved or will fall to the bottom of the cell. At the cathode set to a suitable potential (in most cases only fractions of one volt are needed) the desired metal is deposited. Less noble metals stay in solution, they can be recovered by processing the electrode solution. Metals more noble than the metal under consideration are in most cases not dissolved anodically, instead they settle in the solid deposit at the cell bottom. From this residue they can be recovered. 

Electrorefining — Electrolytic process aimed at the purification of a metal (M). Impure metal anodes are elec-trochemically dissolved in a suitable electrolyte (solution of a M salt) to form ions of the desired element, which are reduced at the cathodes, effecting a selective deposition of M with high purity. Depending on its nature, the anode impurities are left as anodic slimes (collected from the bottom of the electrolytic cell) or as ions in the electrolyte (continuously bled to a purification circuit). This performance can be easily understood by noting that the elements with higher reduction potential than M will not undergo oxidation and thus are re-... 

Table 2 lists standard electrode potentials for some metals and some other reactions common in electrorefining and electrowinning. The metals with high equilibrium potential are noble metals. They are often difficult to dissolve but deposit easily. The metals with low equilibrium potentials are active metals that dissolve easily but are more difficult to reduce. 

The electrolytic production of materials is one of the oldest branches of electrochemical technology. Electrowinning and electrorefining of metals, electroplating, and electrolytic gas production are but a few examples. While still at an evolutionary stage, electroprocessing of materials presents enormous potential opportunities and could well have a significant commercial impact. A few examples are described below and are not intended to be all inclusive. 

The crude copper that is subjected to electrorefining contains tellurium as an impurity. The standard reduction potential between tellurium and its lowest common oxidation state, Te +,is... 

Alternatives to aqueous processing or dry or nonaqueous processing techniques have heen tried hut none, until recently, have been used on a true industrial scale for fuel reprocessing. Examples of these include separations based on (1) differences in volatility of the hahdes, especially fluorine compounds (2) molten salt (liquid-liquid) extraction where fuel dissolved in a molten salt is then contacted with a heat resistant low volatility second phase such as 100% TBP or a liquid metal and, (3) electrorefining, where controlling the cell potential results in removal of a metal from a molten salt by the selective deposition (reduction) on a cathode. 

A variation of the PUREX process is being proposed by the US Department of Energy as a possible alternative partitioning scheme for the transmutation of wastes. This aqueous process called UREX, only removes uranium from spent commercial LWR fuel and leaves plutonium in the HLW stream with the other minor actinides and fission products. Nonaqueous pyroprocessing, a variation of the ANL electrorefining process, is then proposed to be used to separate both the plutonium and minor actinides so that they can be transmuted in an ADS. For further information related to potential modifications of this process for other accelerator transmutation of waste applications, see ANL-99/15 (1999). 

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