The present work aims to understand the mechanism and rate of iron oxide formation in molten mould
flux initially having no iron oxide to discuss how to keep the mild cooling performance of the flux. The
concentration change of iron oxides in mould fluxes was measured at 1 891 K as a function of time in variations
of three physico-chemical conditions: (i) oxygen partial pressures in atmospheres (air and argon), (ii)
SiO2 activities in mould fluxes and (iii) oxygen concentrations in molten irons, the last being provided by
Al-killed and non-killed operations on electrolytic iron samples. The following findings have been obtained:
(i) iron oxide concentrations increase with holding time and reach ca 3.5 mass% within 1.8 ks, independently
of atmospheres, (ii) higher SiO2 activity leads to higher viscosity of mould flux and lower iron oxide
concentrations and (iii) iron oxide concentrations decrease to ca 0.5 mass% due to lower oxygen concentrations
in molten Al-killed iron. Thus, the following mechanism has been proposed: oxygen dissolved in
molten iron is primarily oxygen source and reacts with iron to form iron oxides at the metal/flux interface,
which oxides diffuse into molten flux phase. A kinetic discussion has given the total reaction rate constant
as k = 7.5 × 10?6 cm?s?1 and suggested that the rate be dominated by iron oxide transfer through the
boundary layer. To suppress iron oxide formation, additions of reducing agents would be more efficient
than controls of oxygen partial pressures.
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