Applied Fundamental Research On Medium And High Phosphorus Hot Metal Refining By Duplex Process In Converter

Iron ore resources are precious non-renewable natural resources, which are the indispensable material foundation of the existence and development of human society. With the rapid development of iron and steel industry, the supply of high quality, low phosphorus iron ores becomes depleted gradually. In recent years, some evidence has suggested that the development and utilization of low grade complex iron ores represented by high phosphorus iron ores have attracted more and more attention of metallurgists. In the paper, according to synthetic analysis for the vast majority of literatures, the advantages and disadvantages of different dephosphorization treatment like mineral separation, chemical method, refining method and microorganism method were analyzed. In the author's view, the refining method, especially the duplex process, is an effective way to utilize the high phosphorus iron ores in our country. Taking duplex process refining as subject, the paper concentrates on the enrichment of fundamental research for medium and high phosphorus hot metal refining by studying the thermodynamics and kinetics of dephosphorization, the dissolution behavior of lime and the resource utilization of duplex refining slags. The feasibility of refining medium and high phosphorus hot metal by duplex process was also discussed. The comprehensive evaluation result could provide a theoretic basis and direction for industrial application of duplex process.Dephosphorization thermodynamics of high phosphorus hot metal (0.42%) was investigated by using CaO-FetO-SiO2 slags, which contained small amounts of Na2O and Al2O3. The results show that the dephosphorization ratio, log LP , logγP2O5 and log C POwere 74~91%, 1.75~1.92, -15~-17 and 19~21, respectively. It was found that the classical and experiential formulae about dephosphorization analysis are not applicable to the case of medium and high phosphorus hot metal refining. Based on the analysis for the effect of optical basicity and slag components on log LP , logγP2O5 and log C PO, the correlations between optical basicity, slag components and these thermodynamic parameters were developed by regression analysis method. The kinetics of dephosphorization was studied by using CaO-FetO-SiO2-(Al2O3, Na2O) slags. The results indicate that both 2~6% Al2O3 and Na2O could increase the initial dephosphorization velocity. Na2O could also inhibit the rephosphorization phenomena. Low Si hot metal had better dephosphorization effect. Based on the double film theory, a dephosphorization dynamic model was set up. The present paper made a systematic and multi-angle analysis of the model from the following three aspects: the apparent dephosphorization rate constant kP, the overall mass transfer coefficient kO and the mass transfer parameter A·kO. The experimental results show that kP were in the range of 0.868~8.602×10-3g/(cm2·s). kO were in the range of 0.005~0.024cm/s. Al2O3 and Na2O could obviously increase kO. Si content in hot metal was lower, kO was higher. In all the 11 heats of experiments, the average A·kO in the first two minutes were in the range of 0.16~0.36cm3/s. As the reaction process continues, the value of A·kO decreased gradually. To the heats without rephosphorization phenomena, the value of A·kO decreased gradually and finally approach zero. To the heats with rephosphorization phenomena, the value of A·kO decreased gradually and finally become negative. Based on the Robertson model, a coupled reaction model to the medium and high phosphorus hot metal dephosphorization process was set up. The calculated results agreed well with the measured data. The relationship between the oxygen activity at slag-metal interface aO* and hot metal and slag components were discussed. The higher the FeO content in slag was, the lower the C, Si and Mn in hot metal were and the higher the aO* at slag-metal interface was. The effect of GCO and slag amount on calculation were also discussed. The higher the slag amount was, the lower the GCO was and the higher the aO* was. Higher aO* facilitates dephosphorization.The dissolution behavior of lime into CaO-FetO-SiO2-Al2O3-Na2O-P2O5 slag was investigated with the help of quadratic regression orthogonal composite design method. The influence of different factors and interactions between major factors were discussed. The dissolution rate of CaO in CaO-SiO2-FetO-P2O5-X was studied by rotating cylinder method. From the analysis of experiment results, it was concluded that the dissolution rate was controlled by the mass transport of CaO in liquid phase. The effect of additives to CaO-SiO2-FetO-P2O5 slag on raising the dissolution rate of CaO was CaF2 > Na2O > Al2O3. The convective mass transfer equation and relational expression between linear velocity and CaO dissolution rate in the high P2O5 slag were obtained by regression analysis. The calculated mass transfer coefficient were in the range of 7.63×10-4~1.70×10-3. Based on the Arrhenius equation, the apparent activation energy for the dissolution rate was calculated to be 43 kcal/mol. For the medium and high phosphorus hot metal refining, in order to accelerate the slagging process under low fluorine or fluorine-free condition, a series of technology parameters such as adequate hot metal temperature,ΣFeO/SiO2 in the slag, stirring and so on should be guaranteed. In the aspect of resource utilization of duplex refining slags, mineralogy of De-P and De-C slags were investigated by SEM, EDS and XRD. Viscosity characters of De-P slags and melting characters of De-C slags were also discussed. In view of the high P2O5 content in De-P slags, the way of developing steel slag phosphate fertilizer was put forward. Due to the formation of fluorine containing apatite Ca5(PO4)3F, solubility of slag samples in citric acid decreased rapidly with CaF2 addition. To get a higher solubility, production of steel slag phosphate fertilizer should follow the fluorine-free route. The recovery of phosphorus from De-P slag was studied in present work through slag modification and magnetic separation. It was found that most of the phosphorus could be separated by magnetic separation after slag modification. Consequently, the recovery of phosphorus from De-P slag is made possible by this method. The activities of typical slag components were calculated by regular ion solution model and applied to a previous mathematical model to calculate the substitution of dephosphorizing agent by return slags. According to the calculated data, the effect of De-C slag substitution on dephosphorization was checked with experiments in laboratory. The results show that 20% De-C slag in dephosphorizing agent is suitable in the present experiments. Furthermore, the structure of CaO-SiO2-P2O5-(CaF2) molten slags were investigated by molecular dynamics simulation. The results achieved here are very important for the enrichment of molten slag structure theory, especially for further realize the structure and properties of high P2O5 content slags.In order to evaluate the feasibility of refining medium and high phosphorus hot metal by duplex process, 80t scale plant trails have been conducted. For the [P] 0.25~0.30% hot metal, the average dephosphorization ratio in De-P furnace was 73%, the average P2O5 content in dephosphorized slag was 5.01%. For the [P] 0.37~0.44% hot metal, the average dephosphorization ratio in De-P furnace was 67.6%, the average P2O5 content in dephosphorized slag was 7.25%. The P2O5 content in dephosphorized slag was 7.25%. Both the P2O5 content in two slags were much higher than that in conventional BOF slags. The phosphorus enriched De-P slag could be used as raw material in fertilizer production. After less slag semi-steel refining in De-C furnace, the end point phosphorus content in steel were less than 0.015%, which satisfy the requirement of low phosphorus steel refining. In accordance with such a particular process for medium and high phosphorus hot metal refining, the primary dephosphorization technology were assessed comprehensively. The authors concluded that it is feasible to refining the medium and high phosphorus hot metal by duplex process. The duplex process for steel making has a bright future in the development and utilization of high phosphorus iron ores in China.