Fluid dynamics simulation of chromium recovery from AOD slags during reduction with ferrosilicon additions

Abstract

The kinetic and thermodynamic efficiencies of chromium recovery from the slag in an AOD process during the slag deoxidation period with ferrosilicon is a complex process affected by slag basicity, temperature, the dissolution kinetics of the reducing agents, and intermixing of the slag/metal/alloy addition phases within the vessel. The present work focused on a study and simulation of the fluid dynamics and mixing behavior of ferrosilicon additions in the AOD process for the production of stainless steel. In order to visualise the mixing behaviour, a two dimensional slice model of an AOD vessel was first built and filled with ZnCl₂ solution and then covered with silicone oil in order to represent the bulk liquid steel and slag phases respectively. The injection of argon/oxygen process gases was simulated using air or helium. Particle trajectories and intermixing of the phases was monitored with a high speed camera. To simulate the 75FeSi additions, wooden spheres were used. A three-dimensional version of the AOD model was then constructed, in order to confirm the adequacy of the slice model. Similar particle mixing behaviour was observed. Both models demonstrated that the ascending gas/liquid plume created two asymmetric mixing zones; a smaller rapidly recirculating mixing zone adjacent to the side wall locating the tuyere(s), and a larger mixing zone in the main part of the vessel. The process efficiency during the reduction period is governed by the thermal behaviour of ferroalloy as well as the mixing efficiency in the main large mixing zone. To enhance the recovery efficiency for chromium, small Fe–Si particles are suggested, given their wide distribution within the slag phase, and their large interfacial areas and rapid melting times for Cr₂O₃ reduction from the slag.