Electrorefining, as the word suggests, is a way of purifying metals by using electrolysis. Copper can be purified by making the impure copper the anode in an electrolyte of copper sulphate. The cathode is made of pure copper. When an electric current is passed through the copper sulphate, positively charged copper ions from the anode are attracted to the cathode of pure metal. The impurities, which may be tiny amounts of other metals, such as mercury, gold and silver, fall to the bottom of the electrolyte.
Electro refining is one of a collection of electrochemical processes which are primarily concerned with the extraction of metals from their ores and or the subsequent refining of the metals to high purity.
The main advantages of electro refining processes are they are designed to handle a wide variation in the quality of the base scrap and conversely can provide a particularly high purity of end product material.
Electrochemical processing is used both in the primary extraction of metals from their ores and in the subsequent refining of metals to high purity. Both operations are accomplished in an electrolytic cell, a device that permits electrical energy to perform chemical work. This occurs by the transfer of electrical charge between two electrodes immersed in an ionically conducting liquid (electrolyte) containing metal dissolved as positive ions.
At the negatively charged cathode the metal cations acquire electrons (are reduced), and deposit as neutral metal atoms. At the positively charged anode there are two possible reactions, depending upon the type of cell. In an electrowinning cell the dissolution of the anode metal itself occurs. The more noble metals such as copper and zinc are electrolyzed from aqueous electrolytes, whereas reactive metals such as aluminum and magnesium are electrolyzed from electrolytes of their fused salts.
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: M ? Mn+ + ne- and at the cathode: Mn+ + ne- ? M.
Usually they are part of a larger operation to separate and recover pure metals from both scrap and primary ores. Therefore, the process must be designed to handle a variable-quality metal feed and lead to a concentration of all the metals present in a form which can be treated further. Electrorefining often provides a particularly high purity of metal.
Electrorefining processes using a molten salt or non-aqueous electrolyte are used and, indeed, are the subject of further development. This is due to the possibilities they offer for increasing current densities and refining via lower oxidation states not stable in water (e.g. refining of copper via Cu+ would almost halve the energy requirement). However, aqueous processes presently predominate due to their ease of handling, more developed chemistry and familiarity with aqueous process liquors and electrolytes.
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