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1、MODELLING OF THERMAL AND SODIUM EXPANSION IN PREBAKED ALUMINIUM REDUCTION CELLSYang Sun1, Qianpu Wang2, Ketil . Rye3, Morten Srlie4, Harald A. ye11Department of Chemistry, Norwegian University of Science and Technology; Trondheim, N-7491, Norway2Innovation Centre, National Research Council Canada; 3
2、250 East Mall; Vancouver V6T 1W5, Canada3Elkem Aluminium Mosjen; Mosjen, N-8655, Norway4Elkem Aluminium ANS Research; P.O. Box 8040 Vaagsbygd; Kristiansand S, N-4675, NorwayAbstractA two dimensional transient mathematical model in prebaked aluminium reduction cells was developed to study cathode cha
3、nges during heat up and early operation using the ANSYS program. In this nonlinear finite element model, material non- linearity and influence of temperature on the mechanical properties were taken into account. Then sodium concentration distribution with time in the cathode carbon was calculated, a
4、nd the stress distribution and deformation were calculated and analyzed.IntroductionOne of the main reasons for early failure of Hall-Heroult cells is the crack of the cathode carbon blocks when the stress exceed their mechanical strength. These stresses are induced by physical and chemical factors1
5、. Usually the physical factors include thermal expansion and/or thermal shock. The chemical factors may include chemical reactions between bath, aluminium, sodium and cathode carbon block, which result in the cathode carbon blocks expansion. Among the chemical forces, sodium expansion of the cathode
6、 carbon block is the most important.After the aluminium reduction cells startup, sodium can be absorbed in carbon by the following reactions2:Al(l) + 3NaF(in cryolite) = 3Na(in carbon) + AlF3(in cryolite) (1)or directlyNa+ + e- = Na(in carbon) (2)Direct discharge of Na+ will not normally take place
7、with an aluminium pad as the electrochemically active cathode. The deposition potential of pure sodium or sodium saturated alloy is 1.44V compared to 1.20V for aluminium. However, the energy requirement for the electrochemical co-reduction of sodium is decreased if the activity of deposited sodium i
8、s reduced3. During cell startup on bath the carbon cathode block surface is the electrochemically active cathode. Especially if the blocks contain amorphous carbon material (anthracite), the sodium uptake/absorption into the carbon may be fast and the sodium activity at the carbon surface is low, i.
9、e. aNa(C)1. Hence the sodium deposition potential may be shifted enough for direct sodium reduction (Eqn.(2) to take place. Herstad et al.4 observed in an inverted laboratory cell (cathode on top, anode on bottom) that aluminium droplets did not form at the amorphous cathodesurface during initial el
10、ectrolysis but formed readily on a graphitized cathode surface. While there is a low sodium uptake in the graphitized cathode, the sodium uptake in the amorphous cathode was high and the diffusion rate much larger5.The practice of adding soda ash to the electrolyte during startup on bath is done in
11、part to compensate for the loss of sodium into the cathode carbon. For a mid-sized cell the quantity of soda ash addition can typically be about 750kg during the first day or two. Once the soaking time is over and metal is added to the cell, Eqn.(1) is believed to be the primary cause for sodium upt
12、ake in the metal and then (mainly) through the metal to the carbon bottom. Since the sodium concentration in aluminium in industrial cells is in the range of only about 50-120ppm3, the sodium uptake into carbon once a metal pad is formed slows down considerably. This is experienced in cells preheate
13、d on metal. When bath is added after such a preheat it is normally not necessary to add large amounts of soda ash to compensate for loss of sodium. The electrolyte acidity remains quite stable with only a gentle increase over time as sodium slowly is absorbed into the lining.Sodium that penetrates i
14、nto the carbon lattice of the cathode block results in the bottom block expansion and creates mechanical stresses. The stresses caused by sodium expansion and thermal expansion are the two most significant forces acting on the cathode.Two basic methods can be used to study the stress and strain of t
15、he aluminium reduction cell6. One is on-line measurement on an operating cell, the other is mathematical model calculation.In the first method, a pressure sensor is inserted between the cathode carbon block and the steel sidewall for on-line measurement of the pressure. The displacement of the steel
16、 shell with time is also measured. But excessive heat may damage the pressure sensor, and the electromagnetic field can have an effect on it too.In the second method, the stress and strain in the whole cell can be calculated, the results can then be compared with data from the first method.A large study of the sodium expansion for all types of cathode carbon, from 100% amorphous to fully graphitized blocks has been performed2. Dewing7 pro
