Electrorefining /electrowinning

Electrolytic Processes. The electrolytic procedures for both electrowinning and electrorefining beryUium have primarily involved electrolysis of the beryUium chloride [7787-47-5], BeCl2, in a variety of fused-salt baths. The chloride readUy hydrolyzes making the use of dry methods mandatory for its preparation (see Beryllium compounds). For both ecological and economic reasons there is no electrolyticaUy derived beryUium avaUable in the market-place. 

In the field of electrowinning and electrorefining of metals, titanium has an advantage as a cathode, upon which copper particularly can be deposited with finely balanced adhesion that allows the electrodeposited metal to strip easily when required. Titanium anodes are also being employed as a replacement for lead or graphite in the production of electrolytic manganese dioxide. 

Besides copper passing into the solution as ions, the total concentration of the solution in respect of S04 ions (not discharged) and Cu2+ ions (copper is depositing on the cathode) remains constant. The electrolysis merely transfers copper from the anode to the cathode. With a platinum or a carbon anode, it is found that the color of the solution fades as copper is deposited, whereas with copper electrodes, the color does not change. The two situations, in fact, represent respectively the electrowinning and the electrorefining processes introduced later in the section on process classification. 

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 ... 

In comparison to electrorefining, the electrowinning process of copper, when using a copper sulfate-sulfuric acid electrolyte, presents an interesting situation. In the electro-... 

Electrochemical Reduction of C02 in Non-Aqueous Solvents 321 Use of Acetonitrile in Electrowinning and Electrorefining of Copper 323... 

Beryllium chloride, [CAS 7787-47-5], BeCF, with a melting point of 440 C, is used as a component of molten salt baths for electrowinning or electrorefining of the metal. The compound hydrolyzes readily with atmospheric moisture, evolving HO, so protective atmospheres are required during processing. 

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. 

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)... 

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. 

An electrowinning or electrorefining electrolyte is not a simple aqueous solution of a metal salt. The electrolyte contains many compounds that all have specific functions. Some compounds have a positive effect, whereas others are detrimental. The electrolyte component types and their roles are listed in Table 3. 

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