GB976031A - Improvements in or relating to electrolytic cells for the production of aluminium - Google Patents

Abstract

<PICT:0976031/C6-C7/1> An apparatus for making aluminium by the three layer process in which a hard refractory metal cathode extends through the top crust, and the intermediate fused electrolyte to the liquid metal layer, and an anode extends into the intermediate zone, is provided with a sacrificial cathode located between the current-carrying cathode and anode and extending only into the top crust, whereby current flowing from the anode through the crust is prevented from reaching the current-carrying cathode. Corrosion of the latter and deposition of sodium products on it are avoided. The latter is also of a "hard metal", i.e. nitrides or carbides of Ti, Ta, Zn, or Nb. An electrolytic cell comprising a refractory brick wall 12, and a bottom cell lining 16 disposed on an insulating layer 14, has a layer of molten aluminium 22, electrolyte 24, and crust 26. The electrodes are the current-carrying cathode 20 of hard metal, and a carbon anode 28. The sacrificial cathode 30 is disposed between the latter and is also of hard metal, SnO2 or other corrosion-resistant material, and is connected electrically to the cathode. A number of forms of the sacrificial cathode are illustrated, e.g. as of spike, fork, or spade shape. In another modification it takes the form of a frame connected directly to the negative bus-bar and extending as a sheet screen between a series of main cathodes and anodes. The screen is preferably made of a high chromium cast iron. In a further modification, the relatively small main cathodes are mounted in a cluster centrally in the cell, the anodes on the outer edge, and the sacrificial cathodes of larger cross-section than the main cathodes are disposed between them.