Recycling of aluminium is constantly increasing as the demand of aluminium is growing and it only necessitates 5% of the energy needed for the primary production. Contaminated scraps, Used Beverage Cans for example, are often recycled in Tilting Rotary Furnaces with a salt flux, which is generally composed of NaCl, KCl and a low addition of fluoride (Na3AlF6 or CaF2). The functions of the salt flux are to collect the impurities such as oxides, protect the metal against oxidation and promote the coalescence. This thesis focuses on the understanding of slag formation in TRF and its influence on the metal losses. Indeed, the collection of impurities transforms the initial salt flux into a salt slag, which contains up to 50% of non-metallic particles at the end of the recycling process. This modification has a large impact on salt functions and therefore on the aluminium entrapment in salt slag. In this work, industrial salt slags are characterized to define their final state. Their formation is studied in demo- and lab-scale trials to understand the origin of non-metallic particles. The two main steps of UBC's recycling process in TRF, decoating and melting, are separated in order to better analyse the phenomena taking place in the furnace. The coagulation of metallic aluminium in evolving salt slag is finally investigated by conducting trials with synthetized salt slags to understand the mechanisms in liquid and granular salt slags.