Fundamentals For Phosphorus Migration Control In Coal-based Reduction Of High Phosphorus-containing Oolitic Hematite Ore

High phosphorus-containing oolitic hematite ore is characterized by complex structure,micro-fine grained minerals,and high phosphorus content.It is difficult to obtain high iron grade concentrates with low phosphorus content from oolitic hematite ore using conventional separation methods.Coal-based reduction followed by magnetic separation has been proved to be a feasible method to recover iron from oolitic hematite ore.However,a considerable amount of phosphorus obtruded into the metallic phase from gangue minerals during the reduction process.High quantities of dephosphorization agent are needed to obtain low phosphorus reduced iron,which results in high cost and environmental pollution.With this background,a novel approach is proposed to enhance phosphorus enrichment in the metallic phase through controlling reduction conditions.The high phosphorus-containing metallic phase was then refined to low phosphorus molten steel and high phosphorus-containing slag to be used as a phosphate fertilizer.The fundamental research of phosphorus migration contral in reduction process and its resource utilization was carried out systematical,which enriched the coal-based reduction followed by magnetic separation technology.The main work and achievements of the thesis are as follows:The mineralogical properties of the high phosphorus-containing oolitic hematite ore was analyzed by chemical analysis,X-ray diffraction,optical microscope,and mineral liberation analyzer.The results demonstrated that the ore contains 42.21%total iron and consists of hematite(hematite-limonite)and gangue minerals,such as quartz,chamosite.The phosphorus content of ore reaches 1.31%,which exists in the form of aphanitic apatite.The micro-fine grain size of hematite(hematite-limonite)disseminates in gangue minerals and forms special layered oolitic texture.The reduction behavior of apatite and the enrichment of phosphorus in metallic phase were investigated using an orbital box-type furnace at different reduction conditions based on the thermodynamic analysis and analog computation.The kinetics of apatite reduction and phosphorus migration were studied through kinetic model fitting method,respectively.The occurrence states of phosphorus in gangue phase and metallic phase were analyzed by scanning electron microscopy and energy dispersive spectroscopy.The results indicated that the apatite could be reduced to phosphorus at temperatures ranging from 1473 K to 1548 K.A higher reduction temperature,C/O molar ratio and a longer reduction time were favorable for the apatite reduction.The most suitable mechanism for the apatite reduction was first-order reaction.Both the apparent activation energy and pre-exponential factor of apatite reduction decreased slightly with increasing C/O molar ratio.The phosphorus in gangue phase existed in the form apatite at initial reduction stage but in the form of CaO-SiO2-Al2O3-FexO-P2O5 with the extension of reduction time.Moreover,the replacement between phosphorus and iron can result in the formation of Fe-P solid solution whilst the FexP compound will form when the phosphorus content of metal phase exceeds its solubility.The migration of phosphorus in the metallic phase improved with increasing temperature,C/O molar ratio,and reduction time.The migration process of phosphorus could be divided into two stages,early and late.The mechanisms of the early and later stages were interfacial reaction and diffusion equations,respectively.Both the apparent activation energy and pre-exponential factor exhibited a decreasing trend with increasing C/O molar ratio at the early or later stages.Some phosphorus distribution in metallic phase as Fe-P solid solution but others coexisted in the form of both FexP compound and Fe-P solid solution.On the basics of the thermodynamic analysis and analog computation,the dephosphorization of high phosphorus-containing metallic phase was performed at different slag series compositions using a vertical tube-type furnace.The dephosphorization kinetics was discussed through two-film theory.The experimental results were agreed with thermodynamic analysis and analog computation and determined the suitable compositions of slag with a basity of 3.5,FeO content of 55%,and Al2O3 content of 6%.The phosphorus content of metallic phase was decreased to 0.2%and dephosphorization ratio reached 88.51%at 1873 K with a slag ratio of 0.2 after 20 min dephosphorization.Meanwhile,the P2O5 content of final slag was as high as 14.41%.The second dephosphorization results indicated that the mother liquor of different steels could be obtained through controlling the dosage of second slag and the final phosphorus content of metallic phase.The apparent dephosphorization rate constant of the high phosphorus-containing metallic phase were in the range of 1.141?2.363×10-3 g/(cm2·s)whilst the total mass transfer coefficient were ranging from 2.47×10-3 cm/s to 3.38×10-3 cm/s.The restriction factor of dephosphorization process was the mass transfer between slag and metallic phase.The effects of slag series compositions on the phase compositions and the phosphorus distribution behavior of high phosphorus-containing slag were analyzed by X-ray diffraction and scanning electron microscopy and energy dispersive spectroscopy.The solubility of slag was also studied by chemical analysis.The results demonstrated that the slag mainly comprised Ca2Al2SiO7,Ca2SiO4,Ca5(PO4)2SiO4,FeO,and Ca2Fe2O5.A higher initial basicity or a lower FeO and Al2O3 content of led to off-CaO existed in the slag.The FeO and Ca2Fe2O5 content of slag increased with increasing initial basicity and FeO content.The high phosphorus-containing slag could be divided into solid solution,liquid slag,and small amounts of RO phases.A higher initial basicity or FeO content were favorable for the precipitation of solid solution from liquid slag phase.The migration of phosphorus from liquid slag to solid solution phase was enhanced with increasing initial FeO content but decreased with increasing Al2O3 content.The solubility of slag increased with increasing initial basicity within a certain range.However,a higher FeO and Al2O3 content resulted in a lower solubility.The solubility of slag also increased with improving grinding fineness,and it reached 93.81%?94.54%when the high phosphorus-containing slag was crushed down to 90%passing 23.8-20.7?m.The results of this study provided a new alternative method for the utilization of high phosphorus-containing oolitic hematite ore and served as a reference for the exploitation of other refractory iron ores.