Kinetic studies of calcium carbide hot metal desulfurization by powder injection

Calcium carbide was injected into 2.5 ton heats of carbon-rich iron to investigate the kinetics of desulfurization. Four different top slag initial conditions were studied: no slag, a dry slag which was a mechanical mixture of lime and silica, a liquid slag, and the dry slag modified by plunging aluminum into the iron before calcium carbide injection. The first two slag conditions produced incubation periods of slow initial desulfurization due to the slags' inability to absorb the plume reaction products. The latter two slag conditions did not exhibit incubation. After the powder injection was stopped, the rate of desulfurization was proportional to the sulfur content of the iron. The apparent first order rate constants were dependent on the gas flow rate and the slag volume. The reactions occurring in the plume were separated into the following steps: (1) entrainment of sulfur from the bulk of the vessel into the plume, (2) sulfur diffusion through boundary layers around the particles and bubbles, (3) calcium vapor diffusion through the product layers, and (4) calcium vapor formation. Each of these possible controlling steps was analyzed. A mathematical model coupled the reactions occurring at the particle interfaces with the gas and particle velocities and liquid velocities and entrainment rates obtained from plume dynamics. The dependencies of the apparent rate constants on the gas and solids flow rates suggest that 20 to 40 percent of the particles enter the melt. The remainder are positioned on the carrier gas bubble interfaces.