Reduction of Alkaline-Earth Metals from Slag Melts: Thermodynamic Aspects

On the basis of phase diagrams of the binary silicate systems SrO–SiO2, BaO–SiO2, and CaO–SiO2, the activation of components at invariant (eutectic and monotectic) points of these systems is determined. Solidification at invariant eutectic points l1 and l2 is regarded as corresponding to the chemical reactions $${{{\text{L}}}_{{{{e}_{1}}}}}$$ (KSiO2 (L) + lMeO (L) → CSiO2 (S) + α(MeO⋅SiO2) (S), $${{{\text{L}}}_{{{{e}_{2}}}}}$$ (mSiO2 (L) + nMeO (L) → (MeO⋅SiO2) (S) + b(2MeO⋅2SiO2) (S). The values of Δ $$G_{T}^{^\circ }$$ and the equilibrium constants are found for these reactions. For specified temperatures and known activities of the components in metal melts that are in equilibrium with slag, the values of aMeO in the slags are determined. In homogeneous slag melts, the activity of the oxides of alkaline-earth metals is determined from the reaction constants of equilibrium reduction of these metals from slags by silicon in ferrosilicon metallic melts. In homogeneous slag melts, $${{a}_{{{\text{Si}}{{{\text{O}}}_{{\text{2}}}}}}}$$ is plotted against $$f\left( {{{x}_{{\left( {{\text{Si}}{{{\text{O}}}_{2}}} \right)}}}} \right)$$ at 1600 and 1700°C. Data on the activation of alkaline-earth metals (Sr, Ba, Ca) in high-silica metallic melts are used to determine the dependence of log a(SrO) on $$f\left( {{{x}_{{\left( {{\text{Si}}{{{\text{O}}}_{2}}} \right)}}},{{x}_{{\left( {{\text{Si}}} \right)}}}} \right)$$ at 1493°C and the dependence of log a(BaO) on $$f\left( {{{x}_{{\left( {{\text{Si}}{{{\text{O}}}_{2}}} \right)}}},{{x}_{{\left( {{\text{Si}}} \right)}}}} \right)$$ at 1450°C. From a three-parameter diagram in the coordinates a[Si], $${{a}_{{\left( {{\text{Si}}{{{\text{O}}}_{{\text{2}}}}} \right)}}}$$ , and a(MeO) (for alkaline-earth metals), the dependence of a(SrO) on f(a[Si] and $${{a}_{{\left( {{\text{Si}}{{{\text{O}}}_{{\text{2}}}}} \right)}}}$$ ) at 1493°C and the dependence of a(BaO) on a[Si] and $${{a}_{{\left( {{\text{Si}}{{{\text{O}}}_{{\text{2}}}}} \right)}}}$$ at 1450°C are plotted. It is shown that low equilibrium values of a(SrO) and a(BaO), log a(SrO) = f( $${{a}_{{\left( {{\text{Si}}{{{\text{O}}}_{{\text{2}}}}} \right)}}}$$ , a[Si]) ≤ –4, and log a(BaO) = f( $${{a}_{{\left( {{\text{Si}}{{{\text{O}}}_{{\text{2}}}}} \right)}}}$$ , a[Si] ≤ –3 may be attained with equilibrium activity of silicon in metallic melts a[Si] > 0.5 in the reduction of strontium and a[Si] > 0.7 in the reduction of barium.