Fundamental Research On The Dephosphorization Of Converter Slag Under Low Basicity

In the converter dephosphorization process, it is difficult for solid CaO to dissolve into the liquid slag in short time because of the high melting point of molten slag. To pursue the dephosphorization rate of molten slag, the solid CaO will be added into slag continuously, but a mass of solid CaO are not utilized and remain in slag after the operation. Thus the low efficiency of CaO utilization leads to a high production cost and the wasting of resource, and the free CaO of slag also hinders the recycling of waste slag. Therefore, the effective utilization of solid CaO is the urgent problem to be solved under the premise of high dephosphorization rate.At present, only small quantity of slag are used in advanced foreign converter dephosphorization technology. In another word, the high dephosphorization rate of molten slag could be obtained under low basicity. This technology not only save the production cost but also decrese the amount of waste slag, so it is the future development direction of converter dephosphorization technology. Because the phenomena that the high dephosphorization rate of molten slag could be obtained under low basicity can not be explained by technological study, so it is necessary to study the properties of molten slag and the reaction mechanism of dephosphorization. But people only know the kinds of phases after dephosphorization reaction, the formation process of P2O5 enrichment phase and its influence factors are also rarely systematically studied. In order to make full use of solid CaO for removing phosphorus and investigate the reaction mechanism of dephosphorization under low basicity. In the present study, the effects of CaO particles, holding time and temperature on the formation of P2O5 enrichment phase under low basicity were investigated, Calculation model of activity for the components of CaO-SiO2-FeO-P2O5 slag was established, the influences of FeO, temperature and basicity on the activity of the components were discussed. The CaO-SiO2-FeO-P2O5 slag and solid CaO were selected to study the formation process and reaction mechanism of 2CaO·SiO2-3CaO·P2O5 solid solution. The dephosphorization kinetics of CaO-SiO2-FeO-P2O5 slag was studied, the restrictive step of dephosphorization was confirmed by experiments, then the dephosphorization kinetic equation of CaO-SiO2-FeO-P2O5 slag was established. The liquidus equilibrium phase relations of CaO-SiO2-FeOx-P2O5 slag system were calculated by thermodynamic software, then the influence of temperature and oxygen partial pressure on the equilibrium phase relations and liquidus were also investigated. The following conclusions were obtained.(1) Increasing the particle size of CaO properly was beneficial to the formation of 2CaO·SiO2-3CaO·P2O5 solid solution. When the granular CaO is low in slag, increasing the proportion of granular CaO could promote the formation of 2CaOSiO2-3CaO·P2O5 solid solution. But the 2CaO·SiO2-3CaO·P2O5 solid solution decreased when the granular CaO is high in slag. Increasing the temperature properly was beneficial to the dephosphorization reaction. Prolonging the reaction time could enlarge the size of 2CaO-SiO2-3CaO·P2O5 solid solution and improve the content of phosphorus in 2CaO-SiO2-3CaO·P2Os solid solution(2) The components with higher activity at 1300℃ were FeO, CaO-SiO2,2CaO·SiO2 and 3FeO·P2O5. When w(FeO) was less than 15%, with increasing w(FeO), the activity of 2CaO·SiO2 and 3FeO·P2O5 in the slag increased, while activity of 3CaO-P2O5 decreased, and the 2CaO-SiO2-3CaO-P2O5 solid solution layer thickened gradually. When w(FeO) was more than 15%, with increasing w(FeO), the activity of 2CaO-SiO2 and 3FeO-P2O5 in the slag decreased, the 2CaO·SiO2-3CaO·P2O5 solid solution layer thinned. When w(CaO)/w(SiO2) was less than 2.0, with increasing w(CaO)/w(SiO2), the activity of 2CaO-SiO2 increased, however the activity of 3FeO·P2O5 was non-change, and the 2CaO·SiO2-3CaO·P2O5 solid solution layer thickened gradually. When the w(CaO)/w(SiO2) was more than 2.0, with increasing w(CaO)/w(SiO2), the activity of 2CaO·SiO2 and 3FeO·P2O5 decreased, besides the activity of 3CaO-SiO2 increased, because the reaction between 2CaO·SiO2 and 3CaO·P2O5 has reached equilibrium, so the 2CaO·SiO2-3CaO·P2O5 solid solution layer changed little. With increasing temperature, the activity of 2Ca·OSiO2 decreased, but the activity of 3FeO·P2O5 changed little, the 2CaO·SiO2-3CaO·P2O5 solid solution layer has a thickening trend, but the change was not obvious.(3) The components of CaO, SiO2, FetO and P2O5 existed mainly in forms of 2CaO·SiO2 and 3FeO·P2O5 in molten slag, the 2CaO·SiO2 layer formed on the surface of the CaO solid was mainly due to the precipitation of 2CaO·SiO2 compound, and then with the diffusion of CaO and 3FeO·P2O5 to the interface between CaO and 2CaO·SiO2 layer, the 2CaO·SiO2-3CaO·P2O5 and CaO-FeO solid solution were formed.(4) The restrictive step of dephosphorization in CaO-SiO2-FeO-P2O5 slag were the mass transfer of phosphorus in the slag and the mass diffusion of phosphorus in the 2CaOSiO2-3CaO·P205 solid solution layer. By measuring the mass transfer coefficient k of phosphorus in the slag boundary layer and the mass diffusion coefficient DC of phosphorus in the solid solution layer, the kinetic equations of various basicity at 1300℃ could be obtained.The parameters of different conditions were substituted in kinetic equations, then the conversion rates of phosphorus in the slag were calculated, the results agreed well with the experimental results. Comparing the different slag systems, the mass transfer coefficient k and the mass diffusion coefficient Dc could be adjusted properly to solve kinetic equation. Therefore, the kinetic model in this paper could be predict the dephosphorization rate of CaO-SiO2-FeO-P2O5 slag systems, the results could provide theoretical basis for practical production.(5) The liquid regions in CaO-SiO2-FeOx and CaO-SiO2-FeOx-P2O5(10%) slag systems enlarged with the increase of temperature. In the CaO-SiO2-FeOx slag system, with decreasing oxygen partial pressure, the liquid regions extended rapidly to the direction of high FeO content, but the spinel solid solution disappeared. In the CaO-SiO2-FeOx-P2O5(10%) slag system, with increasing w(P2O5), the liquid regions extended to the direction of CaO-FeOx boundary and the stable region of SiO2 and CaSiO3 enlarged when oxygen partial pressure were 100Pa and 1Pa, the primary crystals area of spinel solid solution disappeared and the stable region of SiO2, CaSiO3, a’-Ca2SiO4, CaO decreased when oxygen partial pressure was 0.001Pa.