IE40797L - Aluminium by electrolysis, control of cell ledges - Google Patents

Abstract

1462332 Fused electrolyte production of aluminium SWISS ALUMINIUM Ltd 28 Jan 1975 [30 Jan 1974] 3598/75 Heading C7B In producing Al by electrolysis of Al 2 O 3 dissolved in a fluoride melt e.g. cryolite in a cell, the thickness of the lateral ledges (24), Fig. (not shown), of frozen electrolyte in the cell are controlled by (a) calculating at regular intervals e.g. between 2 secs and 5 mins, while the cell has no anode effects and no working operations influencing the cell resistance are being carried out, the instantaneous (ohmic) cell resistance, smoothing these values over a period e.g. between 1 min and 1 hour, and calculating the difference #R between this smoothed cell resistance and the base resistance established for the cell; (b) raising or lowering the anode beam (21) whenever #R exceeds a limiting value e.g. 0À5 microhms given for each cell to match the existing resistance with the base resistance; (c) reading the level of the anode beam by a level indicator e.g. potentiometer and storing; (d) at successive intervals not exceeding one day repeating steps (a) and (b) and reading the level of the anode beam as in (c); (e) calculating a difference #B from each two levels of the anode beam determined, corrected for changes of level due to tapping or metal addition; and (f) after each step (e) carrying out one or more known procedures, if necessary, which tend to bring back the thickness of the lateral ledges (24) to the preferred value such that areas of electrode surfaces are equal, depending on the sign and magnitude of the corrected #B value: e.g. changing the interpolar distance (d) or current, to charge the electric power of the cell, height of metal on cell bottom or of Al 2 O 3 layer (23) on the crust of frozen electrolyte above the melt or on the anodes, to charge the heat losses of the cell, the number of crust breakings per day, and/or the composition of the melt e.g. the concentration of electrolyte additives AlF 3 , LiF, CaF 2 , MgF 2 , NaCl. The calculations, and storing, may be effected by computer. The cell insulation (13) may be of chamotte. The carbon anodes (18) may be selfbaking, renewed by pouring green electrode paste into a steel jacket, or pre-baked; iron cathode bars (17) may be used. [GB1462332A]