Dynamics of formation and properties of cryolite-alumina crusts

Abstract

This paper describes the results of an experimental study that looked at the cryolite-alumina crust formation rate. The density, strength and thickness of the obtained crusts were measured. Alumina from five different suppliers was used for crust formation. The paper describes physical and chemical properties of the alumina products used. Two types of alumina products are conventionally distinguished: A — the ones made in Russia and neighbouring countries; B — imported sandy alumina. The paper describes the experimental unit, as well as the experimental methods applied. Graphs are included showing the temperature-time relationship at different alumina fill levels. The obtained data were used to calculate the crust formation rate (V), the heat flow (Q) through the alumina crust and the heat conductivity of the crust (λк). The calculation technique is also described in the paper. Graphs are included that show the relationship between the molten electrolyte rise level in alumina fill and time for different types of alumina. There was observed an almost linear dependence of the electrolyte rise level on time, and the crust was growing continuously as long as it stayed in contact with electrolyte. It is shown that, as compared with type A aluminas, sandy aluminas (type B) form a cryolite-alumina crust 2–3 times quicker due to a quicker penetration of electrolyte. Because of the high concentration of α -Al2O3 and –45 μm particles in type A aluminas, the crusts formed with them are inferior to the ones formed with type B aluminas in terms of heat conductivity, thickness and strength. The obtained results can be used to calculate the heat losses occurring in cells through the cryolite-alumina crust. Process personnel can use the given data to interpret the reaction of the cell to a different type of alumina used.