Thermodynamics of carbon in liquid manganese and ferromanganese alloys

Abstract

The thermodynamics of carbon in manganese and ferromanganese melts were studied to predict the refining limit of carbon during the decarburization of molten ferromanganese. The equilibrium carbon content in a Mn-C melt was determined by the C-CO equilibrium in the presence of pure solid MnO at 1673 to 1773 K. The activities of manganese and carbon in the Mn-C melt were then calculated from the experimental results, the equilibrium constant for the reaction, and the Gibbs-Duhem equation integrated by the Belton-Fruehan treatment. The standard free-energy change of carbon dissolution in the manganese melt was determined to be 41,700 — 59.6 T J/g · atom, with the standard state taken as 1 wt pct carbon in solution. The effect of iron on the activity coefficient of carbon in ferromanganese was determined by measuring the carbon solubility in Mn-Fe melts. The first- and second-order interaction parameters between carbon and iron in ferromanganese melts were determined. The activity coefficient of carbon in the ferromanganese alloy melt can be expressed as $$\log fc = 0.0308 (pct C) + 0.0136 (pct C)^2 + 0.00867 (pct Fe)$$ where the interaction parameters are independent of temperature in the temperature range of 1673 to 1773 K. The thermodynamic parameters determined in the present study could predict the equilibrium carbon content in the ferromanganese melt accurately for various melt compositions and CO partial pressures.