Desulfurization of bath smelter metal

The purpose of the present work was to determine the mechanism and optimal conditions for desulfurizing bath smelter metal with a CaO-CaF2 flux. The minimum silicon (0.1 pct), or aluminum (0.3 pct), contents in the metal for optimal rates were determined. It was found that 8 to 10 pct CaF2 at 1450 °C is required and that the rate below the CaO-CaF2 eutectic temperature (1360 °C) is very slow. It is proposed that a liquid phase at the surface of the CaO particles is required, which is provided by the addition of CaF2. The Si or Al is required to reduce the number of phases for the reaction from three, when carbon is controlling the oxygen potential, to two when Si or Al is; two-phase reactions are inherently faster than those involving three phases. For the optimal conditions, the rate is controlled by mass transfer of sulfur in the metal to the CaO-CaF2 surface. A simple model for continuous desulfurization indicates 95 pct desulfurization can be achieved at high production rates for metal containing 0.10 to 0.15 pct Si using a CaO-10 pct CaF2 flux at 1450 °C.