Electro-refining process production of pure Zirconium

3 Electro-refining process production of pure Zirconium 

In the electro-refining step, the liquid metal (alloy) anode consists of zirconium and low melting more noble metals such as copper and tin, with a very low content of hafnium. The cathode is a pure zirconium sheet or block. The molten salt electrolyte is a mixture of alkali and alkali-earth chlorides, typically also containing some zirconium chloride. Upon applying a potential difference between the two electrodes, the following half-reactions take place. 

For metals at elevated temperatures, especially liquid metals, the exchange current density as defined by the Butler-Volmer equation is extremely large. Therefore, it is believed that no or very little Zr or Zr + ion forms at the anode. When the potential difference is not too high, the other metals in the alloy are not oxidised, because they are more noble than zirconium. Similarly, reduction of the alkali or alkali-earth chlorides in the salt does not occur either, because they are less noble than zirconium. 

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