Fundamental Study On Application Of Limestone In BOF Steelmaking

Using limestone replacing a portion of lime as a slagging flux is a new technology in converter steelmaking process.It is a key way for saving energy,reducing emissions and lowering operation costs.The process can utilize the heat of hot metal more efficiently,reduce the comsuption of scrap,improve the economic efficiency,and also mitigate the overall CO₂ emissions.Still,the process is questioned because of the poor slagging and the low ratio of dephosphorization.It is therefore necessary to investigate the related fundamental issues deeply and thus laying fundations for the widespread application of the technology.The work pertaining to slagging with limestone in BOF steelmaking was carried out in four aspects including calculations of material and heat balance,high temperature experiments,kinetic analysis of limestone decomposition and water model experiments.A partition model for carbon dioxide that can react with elements more easily oxidized in the bath was established based on the change of reaction Gibbs free energies.The material and heat balance of slagging with limestone in BOF steelmaking was analyzed to verify the feasibility of the technology,followed by the determination of the largest limestone substitution under the laboratory conditions.For solving the possible problem using lumpy limestone in BOF steelmaking,the method of granular limestone injected into the hot metal was proposed and studied-Employing a high temperature tube furnace,limestone samples were calcined at converter temperatures and the activity of calcination product of limestone was examined.The effect of temperature and calcination time on lime activity was thus clarified.The calcination behavior of limestone samples in hot metal and molten slag was studied by utilizing the method of rotating cylinder.The effect of temperature and rotation speed on the thickness of lime layer was thus been illuminated.Finally,a kinetic model of decomposition of limestone was built to determine the controlling step of the process.Adopting relavant experimental data,the model was used to obtain the corresponding macro-kinetic parameters,which were employed to predict conversion evolution during calcination.On the basis of similarity law,a 1:6 water model was built to estimate mixing time of bath,penetration ratio of particles and particles distribution under conditions of top and bottom injection,followed by operating parameter optimization.The main conclusions of the present study are as follows.?1?Temperature and heat in converters can meet the need of limestone calcination.CO₂ from limestone calcination can replace a portion of oxygen.Under the condition of this study,the largest limestone substitution is about 70%.Using limestone in BOF steelmaking can reduce the operation cost by 37 RMB/t approximately.?2?Particles size and calcination temperature are the main factors influencing the activity of limestone after calcination.Under the condition of natural atmosphere,the activity of limestone samples of different sizes varies with different temperatures and calcination periods.Calcined at converter temperature,the calcination time can be significantly reduced.The limestone of 0.03 m in radius can be completely calcined within 14 min at 1300 ?.?3?Calcined in molten slag or hot metal,the thickness of lime layer increases with calcination time.Temperature is the main factor affecting limestone calcination,which is faster with a higher temperature.However,rotation speed can hardly impact the process.There exist three layers on the cross section of calcined samples in hot metal,including a reacted layer with iron penetration,a lime layer and an unreacted limestone layer.Limestone can be completely calcined in hot metal or molten slag.Considering the converter smelting time,the biggest particle size of limestone is about 0.02-0.024m.?4?The decomposition mechanism of limestone follows the unreacted shrinking core model.Calcined in molten slag or hot metal,the temperature at the reaction interface is higher than the critical decomposition temperature,i.e.,902 ?.The heat conduction and CO₂ migration through lime layer simultaneously determine the rate of limestone decomposition.Calcined in molten slag,a dense dicalcium silicate shell that inhibits CO₂ transport is expected to form at the sample surface.As a result,the produced CO₂ accumulates at the reaction interface,leading to a locally high temperature.However,the effect of dicalcium silicate shell is of little importance when calcined in hot metal,resulting in a faster decomposition reaction and a lower interface temperature.?5?For both top and bottom injection experiments,particle penetration ratio increases as solid-gas ratio increases and with the same solid-gas ratio,the penetration ratio increases as particle size enlarges.The penetration ratio with top lance injection is greater than the one with bottom injection.However,the particles can quickly distribute in the molten pool with bottom lance injection.Under the present laboratory conditions,the best combination of operating parameters is as follows,bottom gas flow of 1.96 Nm³·h^-1,top lance height of 258 mm,solid-gas ratio of 30?40 and powder size of 2.12×10^-4?3.8×10^-4m.Correspondingly,the best combination of operating parameters for the prototype converter is as follows,bottom gas flow of 450 Nm³·h^-1,top lance height of 1550 mm,solid-gas ratio of 30?40.The suitable particle size is?1?5?× 10^-3m when limestone is injected by top lance or bottom lance.