Role of mixing in powder injection desulphurisation processes

Mixing times were measured in one-fifth scale water models of 150 and 240 t torpedo cars during simulations of the injection of calcium carbide and limestone. Contrary to intuition, it was found that the powder had a negligible effect on mixing times, even at high loadings and velocities, unless the particles changed the trajectory of the carrier gas. Measured mixing times were used in a kinetic model for desulphurisation in the ascending three phase plume. It was found that the mixing rates in the model torpedo cars were always fast enough not to be rate controlling, in marked contrast to steelmaking vessels. Control of commercial operations with calcium carbide, lime, and magnesium lies with diffusion through boundary layers around the particles or the bubbles on which the particles reside. The major benefit of gas releasing compounds is to increase the bubble interfacial area, not to increase mixing rates as has been commonly assumed.