Chemical Composition and Morphology of Oxidic Ceramics at Filtration of Steel Deoxidised by Aluminium

On the boundary of any two phases a transition region is formed, designated as interphase, the composition of which is not identical with composition of the internal part of any of two phases. This transition region – interphase – is formed by effect of surface forces, given by the character and composition of both contacting phases, and its thickness is usually of the order of molecular dimension. If this interphase is being enriched as a result of influence of surface forces by certain substance due to its concentration inside the phase (for example solid phase), such accumulation of the given substance at the contact of two phases is called adsorption. Among the physical forces that cause adsorption in solutions particularly the capillary forces take their effect, whereas the filtered molten steel can be regarded as multi-component liquid solution. Ceramic filter is characterised always by certain porosity. If we take into account the fact that relative surface of solid phase increases with its porosity, then the higher is porosity of ceramic filter, the more distinct is manifestation of phenomena related to adsorption at flow of molten steel through ceramic filter. In this case we distinguish physical adsorption, which takes place predominantly at lower temperatures, and chemical adsorption, which manifests itself intensively at high temperatures. When the melt enters the ceramic filter, the temperature of which is in the first phase close to the temperature of surrounding atmosphere, it is possible to expect effect of physical adsorption, and gradually with increasing temperature of ceramic filter up to the value of the steel flowing through the filter, i.e. 1600 °C, it is possible to expect effect of chemical adsorption. Adsorption at these conditions is already connected with physical-chemical reactions between the molten steel flowing through the filter, non-metallic particles, i.e. inclusions in steel and oxidic ceramics of the relevant filter. Adsorption effects can take place at the boundary of solid porous surface of ceramics, which is in contact with molten steel flowing through the filter capillary thanks to characteristic distribution of flow rate of molten steel through the capillary hole 1-4 . The reason is that at laminar and turbulent flow through the filter capillary the flow rate w of molten steel on the interphase boundary of the type (solid phase / molten metal) is approaching the zero value, which means that molten steel can here penetrate the porous surface of ceramics, which gradually melts and mixes with melt of filtered steel.