Effects of valence control of iron ions on radiative heat transfer in mould flux

The present study aims to investigate the possibility of reduction of radiative heat flux in the continuous casting process by valence control of iron ions in mould flux. The compositions of samples were designed on the basis of a practical mould flux, where the basicity defined as CaO/SiO₂ was also varied from 0.6 to 1.4 to change the valence of iron ions. Mixtures of oxide and fluoride powders were melted in platinum crucibles in air and then the melts were quenched into brass moulds to obtain glassy samples. In addition, graphite crucibles were also used to melt samples having the basicity of 1 at lower partial pressure of oxygen. Some glassy samples were heat-treated for crystallisation. Glassy and crystallised samples were subjected to chemical analyses and optical measurements of apparent reflectivity and transmissivity. The concentration ratio of Fe³⁺/Fe²⁺ increased with increasing basicity but decreased by melting in graphite crucibles. Increasing concentration ratio of Fe³⁺/Fe²⁺ leads to an increase of radiative heat flux for the glassy samples but to a decrease for the crystallised samples: the effect of valences of iron ions is more prominent in the glassy samples. In the crystallised samples, on the contrary, the degree of crystallinity affects radiative heat flux more strongly than the valence of iron ions.