Coalescence and sedimentation of liquid iron droplets during smelting reduction of converter slag with mechanical stirring

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To improve the sedimentation efficiency of iron droplets during iron extraction process from converter slag by smelting reduction, a mechanical stirring is proposed and the coalescence and sedimentation behavior of iron droplets under experimental crucible scale are studied based on the established coalescence model. Coalescence mechanism of droplets is elucidated and the effects of fluid flow, rotating speed and structure of the impeller are examined. The model is validated by the high-temperature experiment results. The results show that with rotating flow in radial direction and “double roll” pattern in axial direction, the settling mode of droplets is no longer a monotonous vertical uniform sedimentation, but a spiral sedimentation with acceleration, deceleration and eventual uniform velocity stages. The mechanical stirring improves the sedimentation efficiency by 3.4% and reduces the total settling time by 6.8%. The coalescence mode with mechanical stirring is mainly due to the oblique collision, instead of the chase collision of the large droplet chasing the small one without stirring. Although the coalescence of the colliding droplet pairs becomes difficult due to the increased relative velocities of droplets, the mechanical stirring greatly improves the coalescence number of iron droplets by a factor of 5.0 because of the increased collision probability. With the impeller rotating speed increased from 50 to 80 rpm, the coalescence of colliding iron droplet pairs is more difficult but the cumulative coalescence number and sedimentation efficiency increase. The same trend is also obtained for the three different impeller structures of ‘Y’ shape, straight shape and cross shape. Therefore, higher rotating speed (80 rpm) and cross-shape impeller are more favored during smelting reduction process.