Re-evaluation of standard free energy of formation of CaO
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[1]
also arisen in our previous studies. [6,7] The thermodynamic behavior of tramp elements in CaO-CaF2 melts at 1773 K was investigated under very low oxygen partial pressures, and the distribution ratios between these flux melts and molten iron were estimated as a function of oxygen partial pressure or the activity of calcium. On the other hand, K6hler and Engell tS] directly measured the distribution ratios of P and Sn between Ca-CaO based fluxes and molten iron and illustrated their relationship with the calcium activity. Comparing these two results, the distribution ratios in the former are one or two orders of magnitude larger than those in the latter. This discrepancy may again result from the uncertainty c f Eq. [2], which is used to calculate the activity of calcium from the Po~ value in the former case. With the above-mentioned questions as background, we have attempted to measure the AG ~ for Eq. [1] by using a chemical equilibration technique, following Eq. [10].
AG ~ = - 6 4 2 , 0 0 0 + 111 T [ J / m o l l
[2]
[Ca]in Ag + CO(g) = CaO(s) + C(s)
Communications Re-Evaluation of Standard Free Energy of Formation of CaO T. W A K A S U G I and N. SANO Calcium oxide is one of the most common refining fluxes in metallurgical processes, and its thermodynamic property is important in estimating an optimum operation condition. The standard free energy of formation of CaO which is generally accepted is available in J A N A F and C O D A T A tables tu2] and is expressed as Eq. [2] in the temperature range of 1400 to 1700 K, based on the second law method.
However, Kay and Subramanian [3j have recently raised a question as t 9 the validity of this value. They showed a large discrepancy for the activity solubility product, hca " ho, in liquid iron between the thermodynamically predicted value and the experimental one, although this may be explained by an unrealistically negative interaction parameter between calcium and oxygen. According to them, it is rather reasonable to consider that some errors in the seemingly established standard free energy of formation of CaO account for this difference. For this reason, Kay and Subramanian derived the standard free energy change for Eq. [ 1] from the data of Edmunds and Taylor, laj who measured the CO partial pressure equilibrated with CaO, CaC2, and C. According to Edmunds and Taylor, AG ~ for Eq. [3] is given as Eq. [4]. CaO(s) + 3C(s) = CaC2(s) + CO(g)
[3]
AG ~ = 390,000 -- 220 T [ J / m o l l
[4]
Combining Eq. [4] with the AG ~ of formation of CaC2 (Eq. [6]) and that of CO (Eq. [8]), Ca(l) + 2C(s) = CaC:(s)
[5]
AG ~ = - 6 0 , 2 0 0 - 26.3 T [ J / m o l l [5]
[6]
1 C(s) + ~ 02(g) = CO(g)
[7]
According to Eq. [10], 2 g of silver and 4 g of CaOsaturated slag (the CaO-CaF2 or CaO-CaCI2 system) in a graphite crucible were equilibrated under argon containing 10 pct CO at temperatures ranging from 1570 to 1831 K. A piece of CaO single crystal was used to make sure of the CaO saturation. Under these experimental conditions, the AG ~ for Eq. [10] is expressed as E
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