GB1242280A - Improvements in method and apparatus for controlling the production of aluminium - Google Patents

Abstract

1,242,280. Producing aluminium by the fused electrolyte process. GENERAL ELECTRIC CO. Nov.28, 1968 [Dec. 7, 1967], No.56469/68. Heading C7B. [Also in Divisions G2 and G3] In a control arrangement for preventing the occurrence of the "anode effect" in a series of aluminium-producing electrolytic reduction cells by maintaining the alumina concentration in the electrolyte within predetermined limits, the resistance of each cell is periodically computed from cell voltage and current measurements to obtain respective "long" and "short" period resistance values corresponding to the average cell resistance during these periods, the values so obtained are compared after each computation and, when the difference between them exceeds a predetermined value (indicative of the onset of the "anode effect"), a predetermined quantity of alumina is automatically added to the electrolyte of the cell, after which the anode position is automatically adjusted (if necessary) to ensure that the resistance of the cell is maintained at a predetermined value. Also, to ensure that the "long" period average resistance value used for comparison with the "short" period resistance value is the lowest so far obtained, the initially computed "long" period value is stored as a base value which is compared with each successive "long" period value and is replaced by lower value (if such occurs) which then becomes the new base value for further comparisons. In one embodiment the cells are controlled by a digital computer with A/D and D/A converters interposed between the measuring and control devices for each cell, these devices comprising the current and voltage sensors, 30, 31, an actuator 36 controlling crust-breaking knives 35, an actuator 39 controlling the supply of alumina to the electrolyte 14 and an actuator 21 controlling the position of the anode 12. Each cell in the line is periodically scanned and controlled (if necessary), the signals from sensors 30, 31 being supplied to a sampling unit 50 which is controlled by a timing unit 51 and calculates the average resistance during a "short" period of thirty seconds from six samples obtained during this period. The output of unit 50 is supplied to an averaging unit 53 which calculates an average resistance during a "long" period of five minutes from ten consecutive outputs of the unit 50. A storage unit 57 and comparator 58 enable the lowest output of the unit 53 to be obtained, stored and applied to a comparator 60 where it is compared with the output of unit 50 after passage of this output through a filter 54. The output of the filter 54 is dependent on the last and previous values of filtered resistance and on the last and previous values of average resistance calculated by the unit 53. When the difference between the signals applied to comparator 60 exceeds a predetermined value (dependent on the characteristics and operating conditions of the cell), an output signal is applied to a sequence controller 65 which actuates in this order a crust break controller 61, an alumina feed controller 67 and an anode-position controller 55. The controller 61 controls and counts the number of times the knives 35 perform their crust-breaking action, the controller 67 controls and registers the (predetermined) amount of alumina fed to the cell and the controller 55 automatically adjusts the anode position (if necessary) in accordance with a resistance signal from the unit 50 and a set-point determined by the operator and dependent on such factors as anode deterioration temperature of the electrolyte and quantity of aluminium in the cell. Anode adjustment maintains the cell resistance within a predetermined "dead band" around the set-point and so maintains the cell temperature within a desired range. Predetermined times are allotted for the operations of controllers 55, 61, 67 and for stabilization of the cell after these operations. The controller 65 then generates a signal to initiate operation of the units 53, 54 and so resume the resistance-monitoring operation previously described. A typical cycle of operations is described Fig. 3 (not shown) and the mathematical functions of the units 50, 53, 54 are discussed. In another embodiment, Fig.4 (not shown) analogue components are used and the operation is generally similar to that previously described, except that after the crust break and alumina adding operation a check is made on the resistance of the cell (using the "short" period value) and further operations and checks are performed as necessary. The analogue components include a magnetic amplifier, millivolt/current converters, RC filter units, a PI controller and relays, the latter serving among other purposes to provide standard resistance values and to ensure equilibrium of the PI controller at the beginning of a resistance monitoring operation.