Fundamental Research On Phosphorus Distribution Between 2CaO·SiO₂-3CaO·P₂O₅ Solid Solution And Liquid Slag Phases In CaO-SiO₂-Fe_tO-P₂O₅ Slags

In order to take advantage of domestic iron ore resources and solve the problem of the resources requirements because of very low average grade of iron ores, iron ores containing high phosphorus iron ores as a representive of low grade iron ores have been putting into ironmaking for alleviating crisis of ores resources shortage. Therefore, the demand of hot metal dephosphorization capacity will be harsher. At present, dephosphorization in BOF(Basic Oxygen Furnace) is also widely used. However, study on dephosphorization in BOF mainly concentrates on process and reaction between slag and hot metal, and studying on how distribution in slag is far from enough.This paper focuses on the CaO-SiO2-FetO-P2O5 slags which is major component of hot metal dephosphorization slag in BOF. Phosphorus existing forms, the law of slag compositions effect on phosphorus distribution, enrichment mechanism of phosphorus and mass transfer behavior of phosphorus in the slags were studied. The research could offer some helps for understanding mechanism of hot metal dephosphorization and theoretical foundation for industrial production.Existing forms of phosphorus in Ca O-Si O2-FetO-P2O5 slags was discussed by observing BOF slags obtained both in industry and laboratory with SEM, EDS and XRD, and theoretical calculation in thermodynamic. Results indicated that microscopic phases of the slags contained enrichment phosphorus phase with black irregular granular or strip shape, enrichment ferrum phase(matrix phase) with grey or dark grey, and RO phases with light gray or white tiny area. Phosphorus mainly existed as the form of n·2Ca O·Si O2-3Ca O·P2O5(denoted by n C2S-C3P) solid solution in the slags. Increasing of binary basicity(%Ca O)/(%Si O2) or phosphorus content were in favor of precipitating and growing up of C2S-C3 P solid solution. Phosphorus was independent existence as the form of C3 P when the phosphorus content in the slags reached to a certain value.Composition in various phases of Ca O-Si O2-FetO-P2O5 slags were observed and analyzed by SEM/EDS. The law of the slags compositions effect on phosphorus distribution between n C2S-C3 P solid solution and liquid slag phases were discussed. It implied that P2O5 activity coefficient in liquid slag phase(2 5P OL)g() increased with increasing total ferrum(T.Fe) content in liquid slag, and decreased with increasing Ca O content in liquid slag. P2O5 activity coefficient() in solid solution phase went 2 5P OSS)g(down with increasing binary basicity in total slag. An increased relationship was found to exist between P2O5 distribution ratio between the solid solution and liquid slag phases(LP) and T.Fe content in liquid slag. Enhancing binary basiciy in total slag hindered phosphorus enrichment from liquid slag phase to solid solution phase, however, increasing Fe O and P2O5 content in total slag were favor of the enrichment.The contents of Ca O and Si O2 both in the solid solution and liquid slag phases were observed and analyzed by SEM/EDS. The variations of Ca O and Si O2 content, both in n C2S-C3 P solid solution phase and liquid slag phase with increasing P2O5 content in total slag were discussed by both experimental and theoretical study. Formation mechanism of n C2S-C3 P solid solution in the slags was derived based on the ionic structure theory of molten slag. The Si O44- in the n C2S-C3 P solid solution can be replaced by PO43- in the liquid phase resulting in the increasing of free Ca2+ and Si O44-in liquid phase. When there was enough phosphorus in the liquid slag phase, all of the Si O44- in the solid solution phase would be substituted with the PO43-. The formula of describing formation process of n C2S-C3 P solid solution was deduced.The behavior of phosphorus transfer from Ca O-Si O2-FetO-P2O5 molten slags saturated with C2 S to C2 S particles was investigated. Phosphorus distribution between C2 S particles and the molten slag were observed and analyzed by SEM/EDS. The result shows that the C2 S particles reacted with the particles surrounded molten slag in the solid-liquid two phase zone to form the n C2S-C3 P solid solution. With the reaction time progressing, composition of the reaction layer was closer to n C2S-C3 P. Diffusion of phosphorus in molten slag and solid solution layer are both the restrictive step. Mass transfer coefficient of P2O5 in molten slag and in solid solution layer are 8 1=3.73 10Lbm s- -′ × and 13 2 15.00 10 SD m s- -= ′ × respectively, and the relationship between equivalent radius of C2 S particles and reaction time was obtained.