Patent ID: 6033550
Filing Date: 2000-03-07
Classification: C25C

Abstract:
A process for control of the alumina content of the bath in a cell for production of aluminum by electrolysis of alumina dissolved in a molten cryolite-base salt, the said process employing alumina feed at a rate modulated as a function of the value and change of the resistance R of the cell as calculated from the difference of electric potential measured at the cell electrode terminals, phases of alumina underfeeding with introduction of alumina at a slow rate CL (phase 1) being alternated with phases of alumina overfeeding with introduction of alumina at a fast rate CR or ultrafast rate CUR (phase 2) compared with a reference rate or theoretical rate CT corresponding to the mean theoretical rate of alumina consumption of the cell, characterized by control cycles of duration T, comprising the following sequence of operations in each cycle:A/ At the end of each control cycle i, the mean resistance R(i), the rate of change of resistance or resistance slope P(i) and the rate of change of the resistance slope or curvature C(i) are calculated and a prediction is made of the value of the resistance slope at time t(i+1) or extrapolated slope PX(i)=P(i)+C(i).times.T, which is an estimate of the future resistance slope P(i+1) at the end of control cycle i+1;B/ The value R(i) is compared with a setpoint value Ro, and on this basis there are transmitted the following commands to move the anode frame position: shorten the anode-metal distance (pot squeeze), or lengthen the anode-metal distance (pot unsqueeze);C/ The alumina feed is controlled as a function of the values of the slope P(i), curvature C(i) and extrapolated slope PX(i) in order to compensate for variations in alumina content by anticipating them.