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The equilibrium diagram for bloomery operation is here.
Raw data is from Richardson, Jeffes, & Withers (1948–52); for the methods used see Rosenqvist (1974); Wagner (1990). Energies in the following are in calories, temperatures in °C, partial pressures in atmospheres, carbon contents in wt%.
In a reaction X + CO = Y + CO2, with standard free energy of formation ΔG°, the equilibrium partial pressures are related by
Where R = 1.987 cal/°K·mol) is the gas constant. We are interested in plotting this ratio on a logarithmic scale, so we use:
In this reaction the partial pressures of CO and CO2 are related by
For the carburisation of iron this becomes:
where aC is the activity of carbon in iron, for which a rough approximation is:
where W is the weight percentage of carbon in iron.
In order to obtain a simple ratio between partial pressures we need another relation between pCO and pCO2. This is:
where A = 0.21 atm is the partial pressure pO2 of oxygen in atmospheric air. Combining gives
Richardson, F. D., J. H. E. Jeffes, and G. Withers. 1948–52. ‘The thermodynamics of substances of interest in iron and steel making from 0°C to 2400°C. I. Oxides. II. Compounds between oxides. III. Sulphides’. Journal of the Iron and Steel Institute: I: 1948, 160: 261–70; II: 1950, 166: 213–34 + 245; III: 1952, 171: 165–75.
Rosenqvist, Terkel. 1974. Principles of extractive metallurgy. (McGraw–Hill series in materials science and engineering New York: McGraw–Hill.
Wagner, Donald B. 1990. ‘Ancient carburization of iron to steel: A comment’. Archeomaterials 4.1: 111–117. donwagner.dk/REHD/REHD.html
Last edited by DBW 25 February 2023