GB1046705A - Improvements in or relating to the operation of electrolytic reduction cells for theproduction of aluminium - Google Patents

Abstract

In start-up of an electrolytic fused bath cell for producing aluminium and having at least two electrodes which function as anodes during normal cell operation, the temperature of the cell is raised by passing current from at least one of the electrodes to the other electrode or electrodes through an electrical resistance heating material in contact with the electrodes. Preferably the cell has more than two anodes and are connected to provide equal numbers of positive and negative electrodes during the heating step. If the cell anodes are carried on hangers connected to the anode bus bar and are arranged in one or more rows, alternate electrodes in a row may be insulated from the anode bus bar by interposing a layer of electrically insulating material and are connected to a cathode bus bar. Alternatively, if the anodes are arranged in two or more rows, one of the rows may be made cathodic. The anodes may be of the pre-baked or self-baking type, the cathode connections to the molten aluminium pool formed during normal operation may be of one or more of the carbides or borides of Ti, Zr, Ta, Nb, and the cell bottom although it may be formed of a non-conductor is typically of self-baking carbon paste or carbon blocks cemented by such paste. The cathode connections may be introduced either from the top of the cell or through the cell side walls or bottom. During the start-up heating stage the refractory hard metal cathode connections are protected against thermal shock by coating at least a part of their length with a refractory cement with or without asbestos wool or with a refractory powder such as alumina contained within a metal cap as of aluminium. The resistance heating material which is placed on the cell bottom and contacted by the heating anodes may be a carbon grist but due to difficulty in removal of such material it is preferred to employ graphite or carbon blocks. During the cell heating stage frozen flux and cryolite or powdered alumina are added around the anodes to form a retaining crust when molten flux or solid flux and solid aluminium is later added when the cell has reached the required temperature and the anodes are raised off the cell bottom for starting aluminium electrolysis.