Particular Aspects Concerning Instability at Interface LCAK Steel- Slag (CaO-Al2O3) due to Transfer of Sulphur, Evaluated Using the Model Reichenbach-Linde Adapted - NIȚĂ, Petre Stelian (The Annals of “Dunarea de Jos” University of Galati. Fascicle IX, Metallurgy and Materials Science, 2013)