A paradigm shift in the electrowinning of metals

摘要

In the electrowinning of metals, a metal compound is electrolysed as a solution or as a molten salt with appropriate electrodes. The metal ion discharges at the negative electrode (cathode) to produce the metal. This traditional approach to metal electrowinning is given a paradigm shift by the discovery of the FFC (Fray, Farthing and Chen) Cambridge process by researchers led by Derek J. Fray of the Materials Chemistry Department of the Cambridge University, England. In this process, instead of being dissolved in the electrolyte, the metal compound remains in the solid state and is the cathode during electrolysis. Chemical changes are induced in the compound by combined thermal and electrochemical means to produce the pure metal at the cathode. The FFC Cambridge process is a hightemperature molten salt electrolysis process and is named after the inventors Fray, Farthing and Chen. The process was patentedt in December 1999 for the production of a host of metals/semimetals such as Ti, Si, Ge, Zr, Hr, Sm, U, Al, Nd, Mo, Cr, Nb or their alloys with their oxide, sulphide, carbide or nitride as the starting material. The extraction of titanium from titanium dioxide has been reported as a case study. Titanium dioxide is used as the solid cathode and graphite as the anode in the hightemperature (approx 950 deg C) FFC cell with molten calcium chloride, which possesses the required electrochemical and thermodynamic properties for the metal extraction, as the electrolyte in the process. During electrolysis, the oxygen present in the Solid cathode is reduced to oxide ions. Under the influence of the applied potential and the thermal conditions existing in the cell, the oxide ions leave the cathode, which finally gets converted to pure metal. The oxide ions are transported through the molten electrolyte to the graphite anode, where they are discharged as oxygen or carbon dioxide.