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CaO-AI203Melts

MASAKI KUWATA and HIDEAKI SUITO Carbon distribution ratios between CaO-AI:O3 slags and Fe-0.0003 to 0.8 mass pct AI-0.2 to 5.6 mass pct C alloys were measured at 1873 K in an A1203, CaO, or graphite crucible. The carbon distribution ratios were dependent on the oxygen potential, determined by the Al___/(AIzO3)equilibrium, not by the C/CO (Pco = 1 atm) equilibrium. The (mass pct C)/ac_ ratios were proportional to the activity of A1 in logarithmic form with a slope of 2/3, indicating that carbon in slag is dissolved as C-'- ion. Solubilities of carbon in CaO-A1203 slags were also measured at 1873 K under the COCO2-Ar gas mixtures in an A1203 or graphite crucible. It was found that C :- ion is present in the range of log Po~(atm) < - 15 and CO~- ion in the range of log Po~(atm) > - 7.

I.

INTRODUCTION

I N previous works, the solubilities of carbon in the CaOCaO-A1203, [1,3-61 CaO-SiOz-Al203, {I,2,3.51 CaOA1203-CaF2,t71 and CaO-SiO2-CaFzt21 slags were measured in a gas-slag experiment, where a slag sample was equilibrated with a graphite crucible under CO, Ar, and/or N 2 gas mixtures. Although the possibility of the existence of carbide ions such a s C 2 - , [3.4'6,7] and C 2-[I] has been discussed, no experimental verification concerning the form of carbide ion was made in previous solubility measurements. A gas-slag-solid graphite equilibrium can be written in a general form as SiO2,]L2]

(n+m/2) C (gr) + m/2 02- = C, ~- + m/2 CO

[I]

where m is the charge on the carbide ion and n is the number of atomic carbon units in the ion. Since a graphite crucible was used in previous works, It-T1 the activity of carbon is unity. Thus, the following relation is deduced by using the equilibrium constant of Reaction [1], Kl: log (mass pct C) = (--m/2) log Pco + log (K, 9 ao..- ~'2/fc,~-)

II.

[2]

where ao2- and f c , - - are the activity of 02- ion and the activity coefficient of C~'- ion, respectively. Pco is the partial pressure of CO gas. The solubility of carbon in CaO-A1203 slags at a given slag composition and temperature was found to be inversely proportional to the partial pressure of CO at carbon saturation in previous studies.V.