Cryolite's effect on the physicochemical properties of a NaCl/KCl flux for aluminium refining

Salt fluxes with fluoride additions are necessary to recycle contaminated and oxidized aluminium scrap. Cryolite is a particularly effective fluoride in separating molten aluminium from the oxides, but its use needs to be minimized due to environmental concerns. This study aims to investigate the effect of various cryolite additions in the range of 2-15 wt.%, on the viscosity, recycling characteristics, and recoverability of a 95 wt.% NaCl/5 wt.% KCl salt flux mixture. Viscosity measurements of the molten salts were carried out using a rotational rheometer. The Al2O3dissolution capability of the salt was evaluated as the weight loss of alumina spheres in the molten salt. Finally, the dissolution time of the salt mixtures in distilled water was evaluated, which is relevant for the recovery of salt cakes. A maximum value of viscosity was found at 5 wt.% cryolite. Higher cryolite contents in the salt led to higher alumina dissolution rates and an increase in the time needed for the dissolution of salt in water. Cryolite additions are beneficial in terms of oxide dissolution but lead to greater viscosity of the salt flux. Moreover, cryolite also reduces the dissolution of the salt in water, which may hinder the recovery of the salt cakes.