GB1132466A - - Google Patents

Abstract

1,132,466. Control system for electrolytic production of aluminium, REYNOLDS METALS CO. Sept.8, 1965 [Sept.29, 1964], No.38442/65. Heading C7B. In an electrolytic cell for the production of aluminium wherein alumina dissolved in a molten salt bath is reduced to aluminium metal by passing electric current through said bath, the method of operation comprises producing a function indicative of the rate of change of cell resistance and controlling the feeding of alumina into the bath in dependance on said function in such a manner as substantially to counteract fluctuations in the cell resistance due to variations in alumina concentration in the bath. Since for any value of anode-cathode spacing the cell resistance has a minimum value when a specific alumina content is present in the cell (although the actual value of the cell resistance at this alumina content is dependant upon the anodecathode spacing), by considering the rate of change of the cell resistance at any given time the alumina content may be maintained at a preferred value at or near said specific alumina content without reference to the actual value of the anode-cathode spacing, similarly by adjusting the alumina content to the preferred value with reference to the rate of change of cell resistance, the anode-cathode spacing may then be altered to a predetermined value by adjusting said spacing until a corresponding predetermined value of cell resistance is reached. Whilst direct voltage is applied to the cell an alternating component appears superimposed on the direct voltage and by detecting variations in this alternating component, upset conditions within the cell e.g. caused by maladjustment of the anode blocks or by sludge accumulations, may be detected. The feedrate of alumina to the cell may be varied by varying the periods between feeds or by alternating between a high rate and a low rate of feed, one higher and the other lower than the rate of consumption thereof. Reduction Cell. The reduction cell shown comprises a steel shell 11 lined with an insulating layer 12 and carbonaceous lining 13 in which are embedded iron rods 18 which are connected with a cathode bus-bar 19, the cell containing in operation a pool of molten aluminium 14 and molten bath of alumina-containing electrolyte 15, covered by a crust 17 consisting of frozen electrolyte containing additional alumina. Suspended above the cell are a mechanical alumina feeder 24 supplied from a bin 23, and individually adjustable pre-baked carbon anodes 16 removably attached to bridge bus-bar 22.