Barrier materials tested for cryolitic resistance methodology and expertise

Abstract

A method for testing barrier materials for cryolitic resistance to corrosive attack by molten fluoride and aluminum has been described. The method allows good reproducibility of measurements; an error analysis is given, factors affecting the accuracy of experiment are discussed, and ways of its improvement are suggested.As soon as an aluminum electrolysis cell is put in operation, its cathode blocks are fed with sodium and electrolyte. The electrolyte transfer is greatly assisted by sodium, which is formed by the reactions Al(liq) + 3NaF = 3Na (in C) + AlF 3 ,Na + + e -® Na (in Al).Sodium penetrates into the bottom block to render it wettable by molten fluoride; in this process, the front of molten fluoride is followed by a sodium front [1].The rate at which sodium penetrates into the bottom block is controlled by the blocks's material. Increasing the degree of graphitization of bottom blocks causes a significant decrease in sodium penetration rate [1]. A laboratory study has shown that the rate at which sodium penetrates into graphite is high enough -from 10 to 30 mm/h at 900°C [2]. For an amorphous graphite it was still higher, reaching 40 to 60 mm/h [2]. It is seen therefore that the life of a electrolysis cell is mainly controlled by the resistance to aggressive attack of barrier materials of the lining underlying the bottom blocks. Consequently, the choice of an appropriate barrier material is a major concern for technologists. Many aluminum manufacturers rely on the in-house methods of testing refractory materials for resistance to molten fluorides. Various testing methods have been described in some detail in [4].In this paper we are concerned with a method that has been in use over more than two years at the RUSAL Co. and Legkie Metally Research and Technology Center to test refractory materials for cryolitic resistance. The method is based on the direct attack of molten fluorides on test specimen that has formerly been suggested by Allaire and coworkers [5]. EXPERIMENTAL CELL AND TESTING UNITA schematic diagram of the cell for testing refractory materials is shown in Fig. 1. A specimen of refractory material 80 AE 54 AE 68 15 4 3 2 1 Fig. 1. Schematic diagram of a cell for testing refractory material for corrosive attack by electrolyte and aluminum: 1 ) graphite beaker; 2 ) specimen; 3 ) aluminum; 4 ) electrolyte.

Mechanism of aluminum carbide formation in aluminum electrolysis cells

“…The generated reaction of Al4C3 can also proceed in the presence of NaF and aluminum because NaF easily reacts with aluminum to form metal sodium (Eqs. 7 and 8) when temperatures exceed 900°C [29][30][31], as given by Al+3NaF=AlF3+3Na…”