The use of sodium β-alumina for controlling the rate of sodium addition to aluminium alloys
Abstract Sodium, when added at concentrations of 30–200 ppm, can be used to modify the microstructure and properties of a number of aluminium alloys. This has traditionally been achieved by physically adding a sodium rich alloy or flux to the melt. However, this method suffers from considerable inefficiency since much of the sodium burns or evaporates on the surface of the metal generating fumes and dross. A much cleaner and more efficient process is under development using a sodium ion conducting solid electrolyte to separate a source of sodium from the molten aluminium alloy. Under the influence of an applied potential, sodium is transported through the solid electrolyte with the rate of sodium addition to the melt controlled by the current flow. An additional advantage of using an electrochemical sodium addition method is that it lends itself to control via a solid state sodium sensor. This can be used to continuously measure changes in sodium activity in the alloy leading to improved process control. In practice, this system gives a much greater level of control over the rate of sodium addition and because the additions are made continuously within the molten metal, dross and fumes are virtually eliminated from the process. Experiments have been performed in laboratory, intermediate and industrial environments and this has been shown to be a viable method of controlling the sodium content of aluminium alloys at industrial scales. Small portions of solid electrolyte have been shown capable of withstanding a current density in excess of 1.3 A cm−2. A standard 200 × 30 mm electrolyte tube has been operated at a current density of approximately 0.6 A cm−2, supplying sodium at a rate of 1.8 g min−1 to a launder of aluminium. This rate of addition from a single tube is able to continuously supply the desired quantity of sodium to two tonnes per hour of aluminium alloy. Alternatively, smaller static pots of aluminium may be rapidly brought up to the desired sodium concentration and maintained at that level by adding more sodium to compensate for any evaporation.