Intelligent control of the feeding of aluminum electrolytic cells using neural networks
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I.
THE PHYSICAL PROCESS
A. The Electrolytic Cell Aluminum is obtained by electrolysis of alumina, Al2O3, inside electrolytic cells in which the alumina, dissolved in an electrolyte made mainly of cryolite (Na3AlF6), is reduced according to the following reaction: 2Al2O3 1 3C → 4Al 1 3CO2
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˚) A constant electric current at high intensity (50 to 300 kA and low voltage (about 4 V) passes through the cell from the carbon anode to the electrolyte, also called bath, and on toward the carbon cathode. The latter is built in a box shape to collect the liquid metal (Figure 1). The oxygen coming from alumina decomposition goes to the anode and combines with the anode carbon to form the CO2 gas. The anode is thus gradually consumed in the process. Alumina is fed into the cell to make up for the alumina consumed. Feeding is carried out by batch or by ‘‘point feeding,’’ as shown in Figure 1. The hard crust of alumina on top of the cell is broken by a crust breaker, and a predetermined amount of alumina is dumped through the opening in the crust (2 to 3 kg per dump). The feeding rate is determined by changing the frequency of the point feeding (usually 1- to 2-minute intervals) and changing the total length of the feeding period. A good control of the cell is required in order to maintain its operation close to the target values of the main
A. MEGHLAOUI, formerly Postdoctoral Fellow, University of Quebec at Chicoutimi, is Development Engineer, Dubai Aluminium, POB 3627 Dubai, United Arab Emirates (UAE). R.T. BUI and L. TIKASZ, Professors, are with the University of Quebec at Chicoutimi, Department of Applied Science, Chicoutimi, PQ, Canada G6H 2B1. J. THIBAULT, Professor (currently on sabbatical), is with Laval University, Department of Chemical Engineering, G1K 7P4, Quebec, PQ, Canada. R. SANTERRE, Consultant, is with Alcan International Ltd., Jonquiere, PQ, Canada G7S 4K8. Address correspondence to R.T. Bui. Manuscript submitted April 1, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
process variables. The most important of these variables are the cell resistance (about 20 mV) and the alumina concentration in the bath (2 to 3 wt pct). The two are related through a characteristic curve giving cell resistance as a function of concentration. Depending on the state of the cell, the shape of the characteristic curve may vary. Figure 2 illustrates this relationship with three curves chosen to represent three possible cell states referred to as states 1, 2, and 3. The state of a cell is determined by a number of elements describing the operation of the cell, such as the thermal condition, the initial concentration, the stability of the cell, and the alumina feedstock. The relative importance of each of those elements taken separately cannot be readily quantifiable. As a consequence, it is reasonable to refer to the overall condition of the cell as ‘‘cell state’’ in the general sense of the term. In a given situation, the number of typical states required to describe the evolution of a cell may be three or four and possibly
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