The Strengthening Mechanism Of P-gasification From Converter Slag And The Circular Utilization Of Molten Slag

Converter slag is a by-product of the steelmaking process with large output and low utilization rate.The converter slag cannot be used on a large scale,due to the the harmful elements of sulfur,phosphorus and the self-decomposing phases of free CaO,MgO in them.So far,there are many ways to treatment the slag outside the furnaces,but all of them have not been applied on industrial level.“Management from the origin”,such as slag-remaining operation,is a new concept on the management of converter slag,which can significantly reduce the amount of converter slag,however,sulfur and phosphorus in the slag also are circulated at the same time,which have a bad effect on the quality of the molten steel.Existing studies have confirmed that most of sulfur can be removed by gasification during the slag-splashing process.In order to remove phosphorus from the slag,a reductant,such as carbon,was put into the furnace in advance,and phosphorus can be separated from the slag in the manner of gasification.Firstly,the basic research on P-gasification of converter slag was carried out in the ways of thermodynamic analysis and laboratory experiments.The reaction is mainly occurred at the interface between slag and nitrogen,and there are fewer dephosphorization reactions at the surface of CO bubbles in the slag.The gaseous Pcontaining reduction products from steel slag are P₂,PO,P4 and P,while mainly dominated by P₂.FeO and MnO in the slag were reduced simultaneously,and FeO has a greater effect on the reaction of P₂O₅.Secondly,the actual thermodynamics and kinetics of P₂O₅ and FeO reduction in slag were studied.The results showed that the actual reduction Gibbs free energy of P₂O₅ is lower than that of FeO,and the driving force of P₂O₅ reduction is stronger during the experiment.The reduction of P₂O₅ in the slag is controlled by mass transfer,while the reduction of FeO is controlled by the interface chemical reaction.An optimization scheme for P-gasification is proposed and the reaction rate equation for the reduction of P₂O₅ and FeO is determined.Thirdly,the mineral phases of the slag were study.The results show that the average melting point of converter slag is 1404℃ and the transition temperature of viscosity is in the range of 1410-1440 ℃,so the suitable temperature for P-gasification reaction is needto larger than 1440 ℃.SEM-EDS and XRD were used to characterize the samples,and it is found that in the process of P-gasification in converter slag,most of the large particles,slab-like phases of 2CaO·SiO₂,3CaO·SiO₂ were broken to the phases of particles or long band.While,CaO·Fe₂O₃ and RO phases disappeared,which were reduced to form a multi-composite phase mainly composed of Ca,Mg,and Fe.Afer reduction,the free phases of CaO and MgO in the slag disappeared,and the amount of 3CaO·SiO₂ and3 CaO·MgO·2SiO₂ increased.phosphorus was mainly accumulated in the phases of 2CaO·SiO₂ and 3CaO·SiO₂,where the P-gasification reaction was mainly occurred.Lastly,laboratory experiments and industrial tests were performed on P-gasification reaction of converter slag,and the results showed that temperature had the greatest impact on the rate of P-gasification,followed by FeO,while basicity and carbon equivalents have the similar effect,and the last was N₂.In the orthogonal experiment,when the basicity is 3.6,the content of FeO is 30%,the flowing rate of nitrogen is 0.4Nm³/ h,the temperature is 1720 ℃ and the carbon equivalent is 1.5 times,the rate of Pgasification is the highest and the value is 75.2%.The best combination is AⅣBⅣCⅡDⅤEⅢ.The optimized process of P-gasification operation is divided into two stages.In When 1.5times carbon equivalent,12000 Nm³/h of top flowing N₂ and 650Nm³/h bottom blowing N₂ were used,the rate of P-gasification was largest at the level of 40%.After application of steelmaking with the circulating slag after P-gasification process,the technologies of "nitrogen-oxygen mixed blowing" at ignition process,the strengthen technology of dephosphorization in slag-remaining operation and the safety process of steelmaking in slag-remaining operation were developed.In this paper,the process of steelmaking with the circulating slag after P-gasification was proposed.After the application of the technology,the output of converter’s new slag was effectively reduced and the adverse effect of phosphorus on the molten steel was alleviated.Also,the process of steelmaking in slag-retaining operation was proceed smoothly,which further improved the quality of molten steel.Meanwhile,a new stable smelting technology for low-silicon and low-heat molten iron was provided.Figure 93;Table 43;Reference 136...