Investigation On Physical And Chemical Properties Of P-bearing Steelmaking Slags During The Selective Enrichment Process Of Phosphorus

China has become the biggest crude steel producer in the world and the annual steelmaking slags production of China has surpassed 100 million tons in the resent years.The slags contain lots of valuable elements.However,due to the low grade of steelmaking slags,only 20%of the slags can be recycled,causing great waste of resources and pollution of the environment.With the accumulation of steelmaking slags in recent years,how to recycle the valuable elements and use the slags high value-added have drawn much attention during the past decades.Among the utilization ways,the most effective way of utilizing the steelmaking slags was recycled into the iron-and steelmaking process,which may inevitably result in the enrichment of phosphorus in the slags and further increase the burden in the following dephosphorization process.Thus,lowering the phosphorus content in steelmaking slags is the primary concern for further utilization of P-bearing steelmaking slags,which is beneficial to the sustainable development of metallurgical enterprises.In the present study,the selective crystallization and phase separation?SCPS?method has been proved to be the most efficient way to recycle the phosphorous in steelmaking slags.During the SCPS process,the physical and chemical conditions can be controlled to enrich the phosphorus into a certain phase and then the P-enriched phase can be separated and used as phosphorus fertilizer,and the residues may be recycled into iron-and steelmaking process.Based on the composition and characteristics of phosphorus in steelmaking slags,the present study carried out an investigation on physical and chemical properties of P-bearing steelmaking slags during the enrichment of phosphorus modified by different kinds of additives,such as:1)indispensable composition of steelmaking slags,Fe_tO;2)amphoteric modifiers,Al₂O₃ and TiO₂;3)modifier with strong flux,CaF₂.Some conclusions were drawn as follows:?1?The structural and dynamic properties of CaO-SiO₂-P₂O₅ ternary glasses with varying P₂O₅ content from 0 to 15 wt%was performed by molecular dynamics?MD?simulation,Raman spectra and X-ray photoelectron spectra?XPS?analysis.It can be seen that P₂O₅ acts as an acidic oxide and has a more affinity to Ca2-ions,resulting the removal of Ca²⁺ ions bonded with Si-NBO?non-bridging oxygen?,which leads to the formation of Si-O-Si and Ca-O-P linkages.In addition,almost all P5 inos displayed as Qp?Qpn,n is the number of bridging oxygen in one[PO₄]-tetrahedra units?and a small fraction ofP⁵⁺ ions behave as Q_p¹?P-O-P?and P-O-Si.The enhanced degree of polymerization?DOP?can be detected from the increase of Xxi3 and X_P¹/X_P⁰?mole fraction of QSii or Q_pⁱ?.?2?The roles of Fe_tO?0?25 wt%?in CaO-SiO₂-Al₂O₃-MgO-Fe_tO-P₂O₅ slag system were investigated through measurements of viscosity combined with the Raman spectra,FTIR and NMR analysis.The result shows that the viscosity of slag decreases with increasing Fe_tO content and the degree of polymerization?DOP?of present slags decreases with increasing Fe_tO content,which is beneficial to the mass transfer and then the crystallization of P-enriched phase.?3?In the current study,the selective crystallization behavior of CaO-SiO₂-Al₂O₃-MgO-Fe_tO-P₂O₅ steelmaking slags modified through Al₂O₃ was investigated by non-isothermal DSC,XRD,SEM/EDS and Raman spectra techniques.It was found that an increase of Al₂O₃ content accelerated the precipitation of dystectic MgFeAlO₄,copolymerized by[AlO₄]-tetrahedra and[FeO₄]-tetrahedra units.It was also found that the Al₂O₃ addition enhances the crystallization ability of the slags.In addition,the content of phosphorus in P-enriched phase can reach a high value as 28.71 wt%for the slags modified by 15.17 wt%AlO3,which meets the requirement of phosphate fertilizer?the steelmaking slags can be used as fertilizer when P₂O₅>10 wt%?.?4?The viscous flow and crystallization behavior of CaO-SiO₂-MgO-Al₂O₃-Fe_tO-P₂O₅-TiO₂ steelmaking slags have been investigated over the range of TiO₂ content from 0 to 10 wt%.The results indicated that the viscosity of the slags slightly decreased with increasing TiO₂ content.The constructed non-isothermal continuous cooling transformation?CCT?diagrams revealed that the addition of TiO₂ lowered the crystallization temperature.This can mainly be ascribed to that addition of TiO₂ promotes the formation of[TiO6]-octahedra units,and consequently the formation of MgFe₂O₄-Mg2TiO₄ solid solution.Moreover,the decreasing viscosity has a significant effect on enhancing the diffusion of ion units,such as Ca²⁺ and[TiO₄]-tetrahedra,from bulk melts to crystal-melt interface.The crystallization of CaTiO3 and CaSiTiO5 was consequently accelerated,which can improve the phosphorus content in P-enriched phase?n2CaO·SiO₂-3CaO·P₂O₅?.For slag with 10 wt%TiO₂,the solid solubility of phosphorus in P-enriched phase can reach a high value as 30.17 wt%which is close to the solid solubility of P₂O₅ in theory.?5?The role of fluorine in the CaO-SiO₂-MgO-Al₂O₃-Fe_tO-P₂O₅ steelmaking slags was investigated through analyzing the viscous flow and crystallization behaviors of the slags.It was found that the viscosity decreased with increasing CaF₂ content,which resulted from the decrease of the degree of polymerization?DOP?of the structures,as proved by O1s X-ray photoelectron spectroscopy?XPS?and magic angular spinning nuclear magnetic resonance?MAS-NMR?analysis.For slag without CaF₂,MgFe₂O₄ was formed first,then nC2S-C3P was formed.However,for the slags contained CaF₂,the primary crystal transformed to Ca₅?PO₄?₃F.For the slag contained 3 wt%CaF₂ the P₂O₅ content in Ca₅?PO₄?₃F phase can reach 27.35 wt%.In short,the addition of Al₂O₃ and TiO₂ contributes to the enrichment of phosphorus in steelmaking slag,and the content of P₂O₅ in P-enriched phase can meet the phosphorus fertilizer index.However,with the addition of CaF₂,the P-enriched phase transformed to Ca₅?PO₄?₃F.As the use of Ca₅?PO₄?₃F is more difficult,the steelmaking slag cannot be efficiently used.