Optimization of the efficiency and sustainability of salt fluxes for melt cleaning and refining of aluminium alloys

This study aims to deepen the understanding of the fluxing process in the aluminium industry and is designed based on two main research streams: the first regards fluxes for refining during recycling, while the second concerns fluxes for melt cleaning during foundry operations. Two fluxes were formulated for refining purposes during recycling, while two other fluxes were sourced externally and for melt-cleaning operations in foundries. The relevant chemical and physical properties of the fluxes were assessed using different characterization methods. The influence of cryolite additions to a 95 wt.% NaCl - 5 wt.% KCl flux was investigated to evaluate its ability in dissolving aluminium oxide, its solubility in water, critical for the treatment and recovery of salt cakes. A flux containing 70 wt.% NaCl and 30 wt.% KCl and enriched with 25 wt.% borax was formulated to assess its effectiveness in reducing the Fe concentration from a secondary AlSi9Cu3(Fe) alloy, by considering different flux introduction methods as well as various melt holding times after fluxing. This study developed a methodology to assess the performance of melt cleaning fluxes, focusing on their impact on melt cleanliness and tensile properties of the AlSi9Cu3(Fe) alloy. Two different commercial fluxes were evaluated, as well as an untreated melt which served as a reference. Scrap and ingot of the selected alloy were melted and treated with the fluxes. Melt cleanliness was evaluated by means of the reduced pressure test (RPT) and hydrogen monitoring. Tensile samples were cast and tested, followed by Weibull analysis on the elongation to fracture. Dross samples were weighed to assess the melting yield and analysed by means of XRD. Lastly, the developed procedure was employed to analyse the effect of the quantity of added flux on the melt cleanliness and tensile properties of the same alloy. Cryolite additions to the refining flux in concentrations below 5 wt.% do not yield a significant effect on its melting temperature and they lead to an increase in viscosity. Further cryolite additions reduce its melting temperature and its viscosity, while also improving the flux’s ability to dissolve aluminium oxide, which is beneficial for the improvement of the metal recovery. Nonetheless, cryolite additions hinder the salt’s dissolution in water, which is relevant for the recovery of salt cakes. Flux additions should be limited to the minimum necessary to ensure proper metal recovery because of the environmental concerns related to cryolite usage. The refining flux enriched with borax shows potential for reducing the Fe concentration from Al alloys, achieving a 22% decrease when the interface between the melt and the flux remains undisturbed. However, stirring reverses this effect by disrupting the interfacial stability, highlighting the need for improved processing conditions to stabilize the reaction products. The developed methodology provides a practical tool for preliminary flux assessment, allowing for process optimization and control. The procedure allowed to assess the chemical composition of the two fluxes, which exhibited differences, as well as the evaluation of their melting temperature, which was comparable. The experimental procedure also allowed to highlight differences in sensitivity to melt cleanliness between RPT and tensile testing. Weibull statistics highlighted the effects of flux treatment and processing temperature control on the mechanical properties of the alloy. Fluxing exhibited a beneficial effect on the melting yield. When compared to the recommended dosage by flux producers of 0.5 wt.%, using smaller flux amounts can lead to reduced mechanical performance as a result of residual oxides and porosity in the castings. Increasing the flux beyond this level provides slight improvements in mechanical properties, however flux usage should be kept to the lowest effective amount to limit environmental impact and costs.