Recycling of aluminum – effect of fluoride additions on the salt viscosity and on the alumina dissolution

The objectives of this work are to study the influence of fluoride additions to molten salt flux used in aluminum recycling on the viscosity and on the dissolution of alumina. These parameters were studied to clarify the phenomena that occur inside the rotary furnace, because there is still very little fundamental research about this process. First, an apparatus was set to measure the viscosity of molten salt fluxes. Second, equimolar mixtures of NaCl and KCl with NaF, KF and CaF₂ additions up to 5 wt% were prepared and melted in a laboratory furnace. Alumina rods were immersed in the molten flux, and the measurements of the reduction of the external diameter were taken at periodic times. The additions of fluorides diminish the viscosity of the equimolar mixture of NaCl and KCl, and fluoride additions also improve the dissolution velocity and corrosive attack on alumina.