Impact of thermo-physical properties on three-phase-interactions of molten salt, liquid metal and non-metallic inclusions
Final Report Abstract
Magnesium recycling processes typically are based on the use of fluxes, mostly containing MgCl2, KCl, NaCl and CaF2. They have the role of separation of surface oxides, gases or other contaminations from metal. It is state of the art that application of salt flux in contact with magnesium scrap leads inevitably to the formation of a sludge so called “black dross” containing the both metallic and oxidic magnesium as well as salt components. Up to now it has not been yet possible to recycle this sludge economically. The aim of this fundamental research work was to develop a characterisation tool for such complex solid-liquid structures, to systematically investigate the mechanisms, which allow/avoid this structure formation and to identify the basic parameters for a potential process implementation. In a lab scale experimental series a variety of industrial samples from typically inhomogeneous black dross were treated by vacuum distillation in order to separate metallic magnesium and salt from oxides. The investigations were carried out successfully through separation of approx. 62% condensate, as a mixture of evaporating metal and salt. Whether those two volatile pases can be detached selectively, was investigated too, showing that evaporation fromthe complex oxide/metal/salt structure (black dross) is indeed a stepwise process. However, a 100% separation of the substances was impossible. In order to distinguish between metal and salt in the condensed fraction, the distilled salt had to be reactivated, aschemically active components, especially fluorspar were proved to be absent there. The exploration to assess the chemical composition of the most active flux, able to coagulate the metallic droplets with the highest efficiency, was consequently conducted. A value of 5 wt % of fluoride components appeared to have the best effectivity on coalescence and CaF2 is the best technical choice against LiF and MgF2 as of its higher coagulation effectiveness in comparison with KF and NaF. Parallel to that, the thermo-physical properties of the selected salts - based on the most common industrial recycling salt - were investigated with focusing on density and surface tension, as viscosity supposed not to have a critical role in coalescence behaviour of the salt. It showed that the more different the surface tension as well as the less different the density of salt and magnesium, the bigger the coalescence efficiency and hence the lower loss of metallic magnesium during melting and that means the less production of unwanted black dross. This know-how can be taken as a base for future selection of molten salt mixtures in magnesium processes, helping to increase resource efficiency and to reduce costs of black dross processing as well as to support environmental aspects.
Publications
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“Recovery of Magnesium and Salt from Black Dross through Vacuum Distillation”, Conference Proceeding, 14th International Metallurgy and Materials Congress IMMC, Istanbul, 2008
S. Akbari, B. Friedrich
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“Potential of Minimizing Magnesium Losses in Black Dross through Optimization of Salt Fluxes”, Conference Proceeding, European Metallurgical Conference EMC 2009, Volume 4, pp. 1213 – 1234, Innsbruck, Austria, 2009
S. Akbari, M. Gökelma, B. Friedrich
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“Metal/Salt Interaction in Magnesium Recycling”, Slags and Fluxes in Modern Metallurgy, Proceedings of the International Workshop on Metal-Slag Interaction, Yalta, Crimea-Ukraine, 2011, ISBN 978-3-8440-0481-6
S. Akbari, B. Friedrich
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“Closing the Mg-Cycle by Metal and Salt Distillation from Black Dross“, Erzmetall, Vol. 66, No. 2, pp. 106-114, 2013
S. Akbari, B. Friedrich
