Activity of Iron Oxide in Steelmaking Slag
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FORMATION, AND ACTIVITY, OF FeO
SEVERAL
workers have attempted to measure the free energy of formation of ferrous oxide and its activity in molten slag.[1–24] Equilibration of liquid iron and molten slag with H2/H2O or CO/CO2 gas mixtures was the most common technique employed by these workers.[1–6,10,12–15] A summary of the investigations reported by the different authors is listed in Table I. Most of the earlier workers[1,3–9,13–17] followed the assumption that oxygen in liquid steel remained dissolved in the form of ferrous oxide (FeO), and hence, under equilibrium, the activity of FeO was identical in slag and steel. Further, oxygen was assumed to behave as an ideal Henrian solute in liquid iron. Thus, the activity of FeO could be defined as aFeO ¼ ½mass pct O=½mass pct Osatd: where [mass pct O] = oxygen content of liquid steel in equilibrium with the slag, and [mass pct O]satd. = oxygen content of liquid steel in equilibrium with pure liquid FeO. These assumptions were probably necessitated by the nonavailability of published values of the interaction parameters between oxygen and other solutes in liquid iron. However, some workers[18,19] persisted with these assumptions even after Sigworth and Elliott,[20] in 1974, published a compilation of the interaction parameters between most solutes of importance in steelmaking.
SOMNATH BASU, Researcher, is with Research & Development, Tata Steel, Jamshedpur 831 001, India. Contact e-mail: somnathbasu@ tatasteel.com ASHOK KUMAR LAHIRI, formerly Professor, is with the Indian Institute of Science, Bangalore 560012, India, is Visiting Scientist, Tata Steel. SESHADRI SEETHARAMAN, Professor, is with the Royal Institute of Technology, Stockholm 100 44, Sweden. Manuscript submitted November 21, 2007. Article published online May 16, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
STOICHIOMETRY OF FERROUS OXIDE
It has been reported that solid FeO is not stoichiometric and is more appropriately described as FexO, where x = 0.95 to 0.98.[10] Fetters and Chimpan observed that pure ferrous oxide melt, in equilibrium with oxygen saturated iron, contained approximately 94 mass pct FeO and 6 mass pct Fe2O3, corresponding to FeO1.03.[5] They adopted the notation FeOx, where x depended upon temperature and composition. However, Deo and Boom observed that this nonstoichiometry is of negligible significance in steelmaking operations.[25] They suggested that the free energy of formation of ‘‘FeO’’ and that of FexO may be considered as the same. This convention has been followed up on in the present work.
III.
EXPERIMENTATION
Simultaneous distribution of phosphorus and oxygen between liquid steel and slag was recently measured by the present authors.[26,27] The experimental conditions differed from those of earlier investigators,[1,2,5,6,13,15,17,23] because the liquid steel contained both oxygen and phosphorus while P2O5 and MgO were simultaneously present along with CaO, SiO2, and FeOx in the molten slag. Liquid steel and synthetic slag of previously determined quantity and com
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