GB1440694A - Method and apparatus for producing metal - Google Patents

Abstract

1440694 Process control in electrolytic AC production NATIONAL-SOUTHWIRE ALUMINUM CO 21 Feb 1974 [21 Feb 1973] 7915/74 Heading C7B [Also in Division G3] Aluminium is produced from its oxide using an electrolytic reduction cell in which the process is controlled in accordance with measurements of low frequency variations in the resistance of the cell. In the described embodiment, during the electrolysis of alumina dissolved in cryolite, the voltage and current across the bath are periodically measured and divided to form a digital signal representing the bath resistance. The signal is separated by digital filters into components representing the D.C. resistance which varies slowly with time, and a noise component which oscillates at a low frequency. Each component is compared with pre-set threshold values, and control signals are produced if the measured values fall outside the threshold limits. The apparatus includes four separate control circuits, the outputs of which are connected through logic elements and memory circuits to mechanical devices for breaking the crust on the bath, feeding in alumina, and raising or lowering the anode of the cell. A first circuit instigates normal crust breaking and feeding at predetermined times. A second circuit responds to a measured D.C. resistance outside limits by causing incremental anode movement until the error condition is removed. A third circuit corrects the anode effect due to cell starvation by responding to a cell voltage exceeding a Zener breakdown voltage and producing crust breaking, anode lowering and alumina feeding. The fourth circuit is actuated if the noise component of bath resistance exceeds a given level for more than a predetermined time, causing the anode to be raised and then subsequently lowered back to its original position. This circuit also resets the threshold values for the D.C. bath resistance measurements.