Electrorefined aluminum

The electrorefining of many metals can be carried out using molten salt electrolytes, but these processes are usually expensive and have found Httie commercial use in spite of possible technical advantages. The only appHcation on an industrial scale is the electrorefining of aluminum by the three-layer process. The density of the molten salt electrolyte is adjusted so that a pure molten aluminum cathode floats on the electrolyte, which in turn floats on the impure anode consisting of a molten copper—aluminum alloy. The process is used to manufacture high purity aluminum. 

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... 

With the progress of electrolysis the concentration of aluminum (and of other base impurities) increases as a result of this the contamination of the cathode deposit also increases. A stage may be reached when the contamination exceeds acceptable limits, thereby calling for a premature termination of electrolysis. It is for this reason that it is desirable to purify and recycle the electrolyte wherever possible so that electrorefining could be conducted for extended periods, without having to contend with the problem of excessive contamination. 

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,... 

Electrolysis, the process of using an electric current to bring about chemical change, is employed to produce sodium, chlorine, sodium hydroxide, and aluminum (Hall-Heroult process) and is used in electrorefining and electroplating. 

Electrorefining in aqueous media is extensively applied for the production of copper, nickel, lead, tin, cobalt, silver, and gold, while in molten salt electrolytes it is practically limited to aluminum. 

Kamavaram V, Reddy RG (2003) Physical and thermal properties of ionic liquids used in aluminum electrorefining at low temperatures, in Das SK (ed), Aluminum, TMS, San Diego... 

Wang XR (2006) Electrorefining of aluminum with BMIC-Ald ionic liquids. Master Thesis, Kunming University of Science and Technology, Kunming (in Chinese)... 

Production of Aluminum Electrorefining of Metals Metal Plating Electrolysis of Sodium Chloride... 

Uses Reduction of iron ore in blast furnaces as source of synthesis gas refractory furnace linings in electrorefining of aluminum and other high-temp, service electrodes in electrolytic reduction of AI2O3 to aluminum filter medium fuel electrothermal prod, of phosphorus, silicon carbide, and calcium carbide plastics additive providing wear reduction in PTFE, creep resist., chemical inertia and stability, thermal conductivity and stability... 

Several principles are exemplified in the Hoopes aluminum electrorefining process. Accordingly, crude aluminum is comelted to form a dense, lower-layer anode beneath a molten chloride electrolyte. A pure aluminum cathode floats above the melt. Since copper is more noble than aluminum, during electrolysis, the crude aluminum is preferentially dissolved into the molten chloride phase prior to electrodeposition as pure metal in the upper metal layer. 

Chart 6 depicts the change in the structure of the electrowinning and electrorefining of nonferrous metals between 1958 and 1963. The most important changes in the structure of this component were a decline in the relative importance of tinplating, a moderate increase in the share of aluminum, and a large increase in the importance of copper. 

Copper, however, is used in applications where purity is important. Copper, when pure, is ductile and an excellent electrical conductor, so it needs to be refined to be used in electrical wiring. Copper anodes (blister copper) are suspended in a water solution containing sulfuric acid and copper sulfate with steel cathodes. Electrolysis results in dissolution of copper from the anode and migration of copper ions to the cathode, where purified metal is deposited. The result is copper of 99.9 percent purity. A similar procedure may be used in recycling copper. Other metals that are electrorefined include aluminum. 

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