Desulfurization of iron alloys in vacuo-free energy of formation of SiS (v)

Abstract

The rate of desulfurization of Fe-C-Si-S meltsin vacua (10 ώm pressure) has been investigated using 15 kg alloys melted by induction heating. For the sulfur contents from 0.008 to 0.1 pct investigated, the rate is a first-order type with respect to sulfur. Over this sulfur concentration range and in the presence of carbon up to saturation and silicon from 0 to 5 pct, the sulfur activity is sufficiently high that the fractional surface coverage by adsorbed sulfur is within 0.75 < θS < 1. The rate equation derived to satisfy the experimental findings for the limiting case of θS → 1 indicates that sulfur is evolved primarily via two activated reactions involving (SiS2)° and (S2)°. These complexes then produce SiS and S vapor species. At 1600°C the rates of formation of these volatile species are about one fourth of those for free vaporization of SiS and sulfur, respectively. The apparent heats of activation are ∼47 kcal for SiS and ∼37 kcal for sulfur vapor. Using an apparatus involving a Knudsen cell and the Bendix mass spectrometer, the enthalpy of reaction SiS2(s) + Si(s) = 2SiS(v) has been measured, giving 102.2 kcal. Combining this with other thermodynamic data, the free energy of formation of SiS vapor is evaluated as Si(s) + 1/2S2(v) = SiS(v) δF° = 15,500-19.5T from 1000° to 1686°K Si(l)+ 1/2S2(v)= SiS(v) δF°= 3500 -12.4T above 1686°K