Study On The Slag/Metal Separation From H₂-reduced High Phosphorus Oolitic Hematite For Direct Reduction Ironmaking

High phosphorus oolitic hematite ore reserves largely in China,but it cannot be achieved economic development and utilization by traditional physic mineral processing owing to its complex mineral phases,unique concentric oolitic texture and high phosphorus content.In this paper,a process of H₂ reduction followed by the slag/metal separation has been proposed for the utilization of high phosphorus oolitic hematite from western Hubei Province,which can not only obtain effective separation of iron grains and P-bearing slag,but also other silica and alumina gangue minerals can be removed completely.The high phosphorus oolitic hematite of 120-160 mesh used in this experiment was with iron grade of 56.27 wt.%in the form of hematite and with phosphorus content of 0.65wt.%in the form of apatite.Moreover,the hematite was tightly embedded with quartz,chlorite,apatite and other gangue minerals into circular oolitic particles.Based on the above mineral characteristics of this ore,thermodynamic of possible chemical reactions between apatite and hydrogen in the H₂ reduction under different gangue systems were calculated by using HSC chemistry^?5.1 software,which showed that phosphorus in the fluorine or hydroxyapatite are more likely to be reduced into P₂ vapors.In the Ca₅?PO₄?₃F-SiO₂-Al₂O₃ and Ca₅?PO₄?₃OH-SiO₂-Al₂O₃ systems,the starting temperature of P₂ formation reaction could be decreased to 1350 K and 1248 K at P_P2=100Pa,respectively.Considering the selective reduction between the apatite and iron oxide,about 200 g ore fines were isothermally reduced in the rotary horizontal resistance furnace by a flow rate of 1 L/min H₂ at 1073 K and the metallization of reduced ore fines was87.12%after 5 h reduction.Besides,the surface of reduced fines became porous,the distribution of gangue elements in the oolitic structure were not significantly changed,and the phosphorus still existed in the form of apatite after reduction,which created good conditions for the removal of gangue minerals in the following slag/metal separation experiment.In the process of slag/metal separation,batches of 11-12 g reduced ore fines with a proper amount of additives CaO and CaF₂ into the high-alumina crucibles were put in an induction furnace at the predetermined temperature range from 1400-1600°C for melting.By gradually optimizing the melting factors,the obtained optimum conditions were melt separation temperature of 1873 K,basicity of 2,melt separation time of 10 min,CaF₂mixing ratio of 4%and metallization of reduced fines of 85.91%.Under such conditions,the dephosphorization,dealumination and desiliconization were up to 97.54%,99.94%and 99.99%respectively.Correspondingly,the P,Al and Si content in the metal product could be decreased to 0.027 wt.%,0.004 wt.%and 0.0013 wt.%with 86.3%of iron recovery,which can achieve phosphorus reduction,impurity removal and iron recovery simultaneously.Additionally,the effect of basicity on dephosphorization was greater than that of additives CaF₂ and melting time,and Al and Si in metal product were easier to remove than that of P owing to the formation of low melting temperature melts by reactions among CaO,FeO and gangue mineral phases of Al₂O₃ and SiO₂.Moreover,the increase of metallization of reduced fines lead to the decline of content of unreduced iron oxide,which decreased oxidizability and fluidity of slag and was harmful to dephosphorization.After melting,the obtained iron and slag blocks were subjected to mounting,grinding and polishing for the SEM-EDS examination of P-containing slag inclusion in slag and metal phases,which indicated that at 1873 K,when the basicity was 1 and the melting time was 8 min,P-containing slag inclusion mainly existed in the form of iron phosphate and some dissolved phosphorus was also observed in the iron matrix.When the basicity and melt separation time increased to 2 and 10 min respectively,P-containing slag inclusion presented in the form of compound slag inclusion,which distinctly exhibited two different areas.The dark-colored area was iron phosphate phase and the light-colored area was iron oxide phase.With the addition of 4%CaF₂ additive into the reduced fines under above melt separation conditions,the dark-colored area of compound P-containing slag inclusion became smaller and gradually transformed into the single FeO slag inclusion.Thus the evolution of P-containing slag inclusion in the metal product was Fe₂P+Fe₃?PO₄?₂?Fe₃?PO₄?₂?FeO.Moreover,the slag obtained after the slag/metal separation was mainly composed of phosphorus rich phase,iron rich phase and general phase.The phosphorus rich phase existed in the form of black irregular polygonal or oval shape in the range of5-10?m.Under the high melting temperature of 1873 K with relatively longer melting time more than 8 min,phosphorus in slag continuously diffused to the 2CaO·SiO₂particles,and eventually formed the 2Ca O·SiO₂-3CaO·P₂O₅ solid solution as the rich phosphorus phase.In this paper,based on the mineral characteristics of high phosphorus oolitic hematite,the reaction behavior of P-contaning minerals in the H₂ reduction was explored,the optimum conditions for the slag/metal separation was obtained and the influence of melting conditions on the distribution of phosphorus in slag or metal phase was also revealed,which provide foundational support for the further research on H₂ reduction followed slag/metal separation process and comprehensive utilization of high phosphorus oolitic hematite.