Development of Process Slags for Cu-Cr- Recycling Processes

In standard remelting processes of metallic copper scrap it is not necessary to add slag components. Mostly graphite is used to protect the melt against oxidation. If alloying elements have high affinity for oxygen like Mg, Zr or Cr the remelting takes place under vacuum or cover gas. Unfortunately, re-melting under such conditions is expensive and takes place typically in small scale batches. Therefore, depending on the mass flow of the alloy these processes are mostly not cost effective. In a current research programme IME investigates exemplary the recycling process of Cu-Cr-alloys, materials mostly used for welding electrodes, high temperature magnetic coils or recuperators. The Cr content of this material varies between 0.4 and 1.2 mass-%. Due to its ignoble character a huge amount of Cr becomes slagged while remelting in large open induction furnaces, despite of the use of common graphite protection layers. Besides that, the mixture of partially molten oxidic phases and graphite produces a highly viscous slag which has to be taken out of the furnace regularly by hand. The paper will present the results of thermochemical modelling performed for different slag systems appropriate for chromium containing copper melts. Subsequent equilibrium tests under inert gas atmosphere as well as kinetic experiments under defined atmospheres allow assessment of the slag’s oxygen permeability .The main objectives are to minimize the losses of the valuable metals and to create a fluid low viscous slag which can easily be tapped after melting from the metal surface without strong interaction to the furnace lining.