ALUMINIUM ELECTROLYSIS CELL OPERATION, ALUMINA FEED RATE ALTERNATED ABOVE AND BELOW AN AVERAGE RATE DETERMINED FROM THE CELL RESISTANCE AND ITS FIRST AND SECOND TIME DERIVATIVES

摘要

Regulating the alumina content in the cryolite-based bath of an electrolytic aluminium production cell comprises varying the alumina supply rate, as a function of the value and change of cell resistance (R) calculated from the cell terminal potential difference, in alternate phases of under-supply with slow rates (CL) of alumina introduction (phase 1) and phases of over-supply with rapid (CR) or very rapid (CUR) rates of alumina introduction (phase 2) w.r.t. a reference or theoretical rate (CT) corresponding to the mean theoretical alumina consumption of the cell. Each regulation cycle of duration (T) comprises: (a), at the end of each regulation cycle 'i', calculating the average resistance 'R(i)', the resistance change rate or slope 'P(i)', the rate of change of the resistance slope or curve 'C(i)' and a forecasted value of the resistance slope at the instant 't(i+1)' or the extrapolated slope 'PX(i)' (= P(i) + C(i) x T) which is an estimate of the future resistance slope 'P(i+1)' at the end of regulation cycle 'i+1'; (b) comparing the value 'R(i)' with a target value 'Ro' to ascertain whether the anodes require displacement for reducing or for increasing the anode-to-metal distance; and (c) regulating the alumina supply as a function of the values of the slope 'P(i)', the curve 'C(i)' and the extrapolated slope 'PX(i)' to compensate anticipated alumina content changes.