Effects of PO2 at Flux State on the Fluorine Dissolution from Synthetic Steelmaking Slag in Aqueous Solution

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This paper studies effect of oxygen partial pressure at molten flux state on the fluorine dissolution from synthetic steelmaking slag solidified from the flux. The PO2s were set as 6.6×10−8, 8.1×10−5, 2.9×10−3, 1.5×10−2, 1.1×10−1, or 6.6×103 Pa, by changing the CO/CO2 ratio of environmental flowing gas. The Ca : Al : Fe atomic ratio of major cations of the slag (oxide) is 23 : 9 : 13, and minor constituents are Mg (~3 at%) and F (~0.3 at%). The phases and their distribution is characterized using SEM, EDS (point and mapping), Rietveld refinement of XRD. Increase of PO2 reduces fluorine dissolution, and the amounts of dissolution are higher than 1 mg/L when processed at PO2<10−2 Pa, but are about or less than 0.5 mg/L when processed at PO2>10−2 Pa. These results indicate that the low PO2 processed slag has large fraction of low density Mayenite (Ca12Al14O33) with most of fluorine in its sparse crystal structure, and liberates large amount of fluorine. On the contrary, the high PO2 processed slag is mainly composed of high density Brownmillerite (Ca2AlxFe2−xO5), and the fluorine exists as CaF2 particles not to liberate fluorine easily. It is concluded that the fluorine dissolution is strongly governed by the amount of phases, the density and empty space of each crystal structure to accommodate fluorine, and their distribution.