Tantalum joined the list of 43 critical raw materials (CRM) within the latest report of the European Commission, published in 2017. Global Ta and Nb ore production, publicly known as Coltan, is dominated by D.R. Congo and Rwanda accounting for a combined share of at least 60%. Recycling constitutes an evolving chance for European countries to lower their complete reliance on importation. Consequently, tin slags as well as process residues like dusts or sludges are processed by pyrometallurgical treatment and hence offer great economic potential due to the content of valuable Ta ₂O ₅. Flowability tests with various atmospheres are performed to evaluate the similarity of industrial and synthetic slags. These experiments, thermodynamic FactSage simulations plus slag viscosity calculations provide a solid foundation for follow up high-temperature furnace rheometer trials which deepen fundamental knowledge about such melts and improve existing process steps.