Hot Metal Dephosphorization Process Research Articles

Phosphorus diffusion at the interface between 2CaO·SiO2 and CaO–SiO2–FeO–P2O5 slag

ABSTRACT The reaction behaviour of solid 2CaO·SiO2 (C2S) in steel slag is of great interest during the hot-metal dephosphorization process. In the present work, C2S samples were prepared and interfaced with steel slag to investigate the phosphorus diffusion behaviour at the interface of C2S at 1723K at the onset of the slag melting. The results showed that the phosphorus diffusion from the slag to C2S was very rapid with a significant diffusion coefficient of 4.44 × 10−9 m2·s−1. Phosphorus-enriched phases were formed continuously with increasing reaction time. As the reaction time increases, the composition of the diffusion layer becomes closer to 3CaO·P2O5 (C3P) and farther away from the C2S. The results indicate that a hot-metal dephosphorization process with much higher efficiency can be achieved by the careful selection of the interface characteristics between the C2S and steel slag.

Study on Mass Transfer of Phosphorus Based on New Hot-Metal Dephosphorization Kinetic Model

To investigate mass transfer of phosphorus in hot-metal dephosphorization process, it is necessary to reduce the deviation of traditional dephosphorization models in fitting accuracy. According to the change in hot-metal phosphorus content during dephosphorization, a new dephosphorization model was established by introducing the concept of “dynamic relative area”. With the help of literature data and comparative analysis of the new and old models, the rationality of the new model was verified. On this basis, in order to reveal the effects of CaO and its derivatives on mass transfer of phosphorus, based on the dephosphorization effect of FetO–CaO–SiO2–CaCl2 slags on high-phosphorus hot metal at 1673 K (1400 °C), the mass transfer process of phosphorus was investigated by calculating, analyzing, and discussing the overall mass transfer coefficients of phosphorus using CaO, Ca(OH)2, and CaCO3 with equal molar amount as fixatives (slag formers) in dephosphorizer respectively. The results show that the overall mass transfer coefficients of phosphorus under the action of Ca(OH)2 and CaCO3 are 0.0216 cm/s and 0.0363 cm/s, respectively, which are significantly lower than 0.0840 cm/s under the action of CaO.

Measures to Decrease and Utilize Steelmaking Slag

Steelmaking slag is artificially produced to remove, among others, the phosphorus from molten steel; approximately 120 kg of slag is formed per ton of steel. At present, steelmaking slag is mostly used for road construction and civil engineering materials. However, the slag cannot be sold at a high price and, in advanced countries, its market for civil work is shrinking and regulations to protect the environment are becoming more severe. There are two solutions to the problem of steelmaking slag: one is to decrease the generation of slag, and the other is the development of reuse methods. In this paper, the measures to decrease slag formation are first discussed based on the mass-balance calculation, and then the present and future states of slag valorization in Japan are reviewed. In the first part, the importance of the hot-metal dephosphorization process and the recycling of decarburized slag is clarified. In the second part, as a new field of slag valorization, the use of slag in marine environments is explained. Several studies to extract phosphorus and manganese from slag were also introduced.

Distribution Ratios of Phosphorus Between CaO-FeO-SiO2-Al2O3/Na2O/TiO2 Slags and Carbon-Saturated Iron

In order to effectively enhance the efficiency of dephosphorization, the distribution ratios of phosphorus between CaO-FeO-SiO2-Al2O3/Na2O/TiO2 slags and carbon-saturated iron ( $$ L_{\text{P}}^{\text{Fe-C}} $$ ) were examined through laboratory experiments in this study, along with the effects of different influencing factors such as the temperature and concentrations of the various slag components. Thermodynamic simulations showed that, with the addition of Na2O and Al2O3, the liquid areas of the CaO-FeO-SiO2 slag are enlarged significantly, with Al2O3 and Na2O acting as fluxes when added to the slag in the appropriate concentrations. The experimental data suggested that $$ L_{\text{P}}^{\text{Fe-C}} $$ increases with an increase in the binary basicity of the slag, with the basicity having a greater effect than the temperature and FeO content; $$ L_{\text{P}}^{\text{Fe-C}} $$ increases with an increase in the Na2O content and decrease in the Al2O3 content. In contrast to the case for the dephosphorization of molten steel, for the hot-metal dephosphorization process investigated in this study, the FeO content of the slag had a smaller effect on $$ L_{\text{P}}^{\text{Fe-C}} $$ than did the other factors such as the temperature and slag basicity. Based on the experimental data, by using regression analysis, $$ \log L_{\text{P}}^{\text{Fe-C}} $$ could be expressed as a function of the temperature and the slag component concentrations as follows: $$ \begin{aligned} \log L_{\text{P}}^{\text{Fe-C}} & = 0.059({\text{pct}}\;{\text{CaO}}) + 1.583\log ({\text{TFe}}) - 0.052\left( {{\text{pct}}\;{\text{SiO}}_{2} } \right) - 0.014\left( {{\text{pct}}\;{\text{Al}}_{2} {\text{O}}_{3} } \right) \\ \, & \quad + 0.142\left( {{\text{pct}}\;{\text{Na}}_{2} {\text{O}}} \right) - 0.003\left( {{\text{pct}}\;{\text{TiO}}_{2} } \right) + 0.049\left( {{\text{pct}}\;{\text{P}}_{2} {\text{O}}_{5} } \right) + \frac{13{,}527}{T} - 9.87. \\ \end{aligned} $$

A Thermodynamic Model for Predicting Phosphorus Partition between CaO-based Slags and Hot Metal during Hot Metal Dephosphorization Pretreatment Process Based on the Ion and Molecule Coexistence Theory

A thermodynamic model for predicting phosphorus partition L P between a CaO-based slags and hot metal during hot metal dephosphorization pretreatment process has been developed based on the ion and molecule coexistence theory (IMCT), i.e., the IMCT–L P model. The reaction abilities of structural units or ion couples in the CaO-based slags have been represented by the calculated mass action concentrations N i through the developed IMCT–N i model based on the IMCT. The developed IMCT–L P model has been verified to be valid through comparing with the measured L P as well as the predicted L P by two reported L P models from the literature. Besides the total phosphorus partition L P between the CaO-based slag and hot metal, the respective phosphorus partitions L P,i of nine dephosphorization products as P2O5, 3FeO·P2O5, 4FeO·P2O5, 2CaO·P2O5, 3CaO·P2O5, 4CaO·P2O5, 2MgO·P2O5, 3MgO·P2O5, and 3MnO·P2O5 can also be accurately predicted by the developed IMCT–L P model. The formed 3CaO·P2O5 accounts for 99.20 pct of dephosphorization products comparing with the generated 4CaO·P2O5 for 0.08 pct. The comprehensive effect of CaO+Fe t O, which can be described by the mass percentage ratio (pct Fe t O)/(pct CaO) or the mass action concentration ratio $$ {{N_{{{\text{Fe}}_{t} {\text{O}}}}} \mathord{\left/{\vphantom {{N_{{{\text{Fe}}_{t} {\text{O}}}}} {N_{\text{CaO}}}}} \right. \kern-0pt} {N_{\text{CaO}}}} $$ as well as the mass percentage product (pct Fe t O) × (pct CaO) or the mass action concentration product $$ N_{{{\text{Fe}}_{t} {\text{O}}}}^{5} \times N_{\text{CaO}}^{3} $$ , controls dephosphorization ability of the CaO-based slags. A linear relationship of L P against (pct Fe t O)/(pct CaO) can be correlated compared with a parabolic relationship of L P against $$ {{N_{{{\text{Fe}}_{t} {\text{O}}}}} \mathord{\left/{\vphantom {{N_{{{\text{Fe}}_{t} {\text{O}}}}} {N_{\text{CaO}}}}} \right. \kern-0pt} {N_{\text{CaO}}}} $$ , while the linear relationship of L P against (pct Fe t O) × (pct CaO) or $$ N_{{{\text{Fe}}_{t} {\text{O}}}}^{5} \times N_{\text{CaO}}^{3} $$ can be established. Thus, the mass percentage product (pct Fe t O) × (pct CaO) and the mass action concentration product $$ N_{{{\text{Fe}}_{t} {\text{O}}}}^{5} \times N_{\text{CaO}}^{3} $$ are recommended to represent the comprehensive effect of CaO+Fe t O on dephosphorization ability of the CaO-based slags. Furthermore, a parabolic relationship of L P against binary basicity or complex basicity CB2 and CB3 can be established at binary basicity in 1.8 or at complex basicity CB2 and CB3 in 2.0 corresponding to the maximum of dephosphorization ability of the CaO-based slags. However, the linear relationship between L P and optical basicity can only be correlated with the mathematically regressed $$ \varLambda_{\text{FeO}} $$ = 1.0 and $$ \varLambda_{{{\text{Fe}}_{ 2} {\text{O}}_{ 3}}} $$ =0.75. A great gradient of oxygen potential or oxygen activity a %,O between the dynamically formed metal film beneath slag–metal interface and hot metal bath is the main driving forces of hot metal dephosphorization by the CaO-based slags. The formed metal film with high oxygen content and low carbon content can dynamically be exchanged or replaced by hot metal elements from bath during hot metal dephosphorization process until dephosphorization products mainly as 3CaO·P2O5 are saturated in the CaO-based slags.

溶銑脱燐プロセスシミュレーションモデルの開発

溶銑脱燐プロセスシミュレーションモデルの開発

Thermodynamic Database of P2O5-containing Oxide Systems for the Dephosphorization Process in Steelmaking

AbstractThe Na2O-MgO-CaO-FeO-Fe2O3-Al2O3-SiO2-P2O5 system is a basic oxide system for the Basic Oxygen Furnace (BOF) process as well as the hot metal dephosphorization process. Numerous experimental investigations on this oxide system are being carried out to find out an advanced process route for P removal from molten iron. In spite of their industrial importance, however, phase equilibria in oxide systems containing P2O5 have not been well investigated due to the complexity of their chemistry. No systematic thermodynamic modeling of these systems has been conducted to date, either. In order to meet the strong demands of steelmaking and other industries, new systematic thermodynamic modeling of the P2O5-containing oxide systems (Na2O-MgO-CaO-FeO-Fe2O3-Al2O3-SiO2-P2O5) and key phase diagram experiments have been carried out over the past years. In the present study, the results of the thermodynamic modeling of unary, binary and ternary P2O5-containing systems and the applications of the thermodynamic database to the dephosphorization by multi-component slag in BOF process are presented in comparison with experimental data. All thermodynamic calculations were performed using FactSage thermodynamic software.

Phosphorus Distribution Equilibrium between CaO-FeO*-SiO<sub>2</sub>-P<sub>2</sub>O<sub>5</sub>(15%)-CaF<sub>2</sub>(B<sub>2</sub>O<sub>3</sub>) Slag System and Carbon-Saturated Hot Metal at 1573K

In order to understand the equilibrium condition of phosphorus between slag and hot metal saturated with carbon in the high phosphorus hot metal dephosphorization process, the dephosphorization capacity of CaO-FeO*-SiO2-P2O5(15%) -CaF2(B2O3) slag system at 1573 K was studied, by applying the method that phosphorus distribution ratio between slag and solid iron was firstly measured and the one between slag and hot metal was then calculated. The results showed that phosphorus distribution ratio decreased with increase in basicity of CaO/(SiO2 +P2O5), a proper increase in SiO2 content is helpful to the absorption of phosphorus into slag, an increase in FeO* content from 10.52% to 13.40% resulted in a dramatic reduction of phosphorus distribution ratio from 126.43 to 33.25, by completely substituting CaF2 with B2O3 at 2.5 of basicity and 0.07 of B2O3/CaO ratio not only was acquired a high phosphorus distribution ratio of 171, but also was decreased pollution to the environment.

Behavior of Vanadium and Niobium during Hot Metal Dephosphorization by CaO–SiO<sub>2</sub>–Fe<sub>t</sub>O Slag

V and Nb are key elements for the production of high-grade steel. These elements are produced by few countries, and V has been nominated as a national stockpile element in Japan. Despite the importance of these elements, there are no strategies to obtain stable resources for them. Some types of iron ore that contain V and Nb can be used as sources of these elements. This study clarified the possibility of extracting V and Nb by means of a conventional hot metal dephosphorization process without using CaF2 as a flux. First, the influences of V and Nb oxides on the distribution ratio of P2O5 between the solid solution and liquid phases in slag were measured. Then, the behaviors of V and Nb in hot metal dephosphorization by CaO–SiO2–FetO slag were investigated. Finally, a previously described hot metal simulation model was modified and the behaviors of various elements, including V and Nb, during hot metal dephosphorization were simulated, and the results were compared with experimental ones. The results are summarized as follows: (1) The addition of V and Nb oxides slightly increases the distribution ratio of P2O5 between the solid solution and liquid phases in slag. (2) The decreasing rate of the elements by the flux addition can be expressed as Nb > Mn > P > V, and these decreasing rates increase as the basicity increases. (3) The simulation results for the behavior of P and V or Nb are in good agreement with the experimental results; that is, the decreasing rate for Nb is greater than that for P, and the separation of Nb leaving P in the hot metal is easier than the separation of V.

Recycling Effects of Residual Slag after Magnetic Separation for Phosphorus Recovery from Hot Metal Dephosphorization Slag

In a previous study, the authors found that phosphorus exhibits remarkable segregations in industrial hot metal pretreatment slag where it exists as a Ca3P2O8–Ca2SiO4 solid solution together with a FeO–CaO–SiO2–MnO–MgO matrix. Since the magnetic property of each phase is significantly different, it is possible to separate the phases with the aid of a superconducting strong magnetic field. By applying a strong magnetic field of 0.5 to 2.5 T to the crushed slag, more than 60% of the concentrated phosphorus phase in the slag was recovered. If most of the phosphorus can be removed from the slag, the residual slag will comprise FeO–CaO–SiO2–MnO–MgO with less P2O5, and thus may be recycled to iron- and steel-making processes, such as sintering, hot-metal desiliconization, and hot-metal dephosphorization processes. In the present work, the recycling effect of subjecting the residual slag to the dephosphorization process was simulated based on a mass balance calculation. A significant reduction in total slag generation and CaO input was demonstrated by the mathematical model considering phosphorus recovery and recycling of residual slag as a dephosphorization agent. Using the waste input–output model, it was shown that phosphorus recovery from dephosphorization slag and the recycling of residual slag in a hot-metal dephosphorization process have potentially great environmental and economic benefits.

固体ＣａＯ‐溶融スラグ界面におけるＰ２Ｏ５含有相の微視的生成機構

It is quite important to reveal the microscopic reaction mechanisms and the role of the solid and liquid phases in the solid CaO coexisting flux in the hot metal dephosphorization process. In the present study, solid CaO piece and FeOx–CaO–SiO2–P2O5 slag with various FeOx and P2O5 contents, and CaO/SiO2 ratios of the slag were reacted at 1573 and 1673K. The interface between solid CaO and molten slag was observed and analyzed by SEM/EDS. Microscopic reaction mechanisms between solid CaO and molten slag was discussed with changing reaction times, slag compositions and temperatures. The CaO–FeOx phase adjacent to solid CaO, and the CaO–SiO2 or CaO–SiO2–P2O5 solid phase coexisting with the FeOx–CaO–SiO2 liquid slag next to the CaO–FeOx phase were observed for all slag compositions, temperatures and reaction times. Phosphorus was condensed as 2CaO·SiO2–3CaO·P2O5 phase more easily in the case of higher CaO/SiO2 ratio and higher FeOx content in slag. There was a linear relationship between P2O5 content in 2CaO·SiO2–3CaO·P2O5 phase and the distance from CaO–FeOx phase to 2CaO·SiO2–3CaO·P2O5 phase. The P2O5 content increased from CaO–FeOx boundary toward bulk slag, and P2O5 content in the condensed phase near the CaO–FeOx phase increased with increasing reaction time. This P2O5 concentration gradient tended to diminish. These results suggest that the condensation of phosphorus as 2CaO·SiO2–3CaO·P2O5 phase was controlled by P2O5 diffusion from bulk slag to reaction interface, not by absorption of P2O5 into 2CaO·SiO2 particle.

磁気分離法による溶銑脱リンスラグからのリン回収法で生成する残渣スラグのリサイクル効果

The authors have found in their previous work that phosphorus exhibits remarkable segregations in the industrial hot metal pretreatment slag and it exists as Ca3P2O8–Ca2SiO4 solid solution together with FeO–CaO–SiO2–MnO–MgO matrix. Since their magnetic property of each phase is significantly different, it is possible to separate each phase with the aid of superconducting strong magnetic field. By applying strong magnetic field of 0.5 to 2.5 T to the crushed slag, more than 60% of phosphorus concentrated phase in the slag has been recovered. If the most of phosphorus can be removed from the slag, the residual slag is basically FeO–CaO–SiO2–MnO–MgO with less P2O5 and thus it may be recycled to iron and steelmaking processes such as the sintering, hot-metal desiliconization, and hot-metal dephosphorization processes. In the present work, the recycling effect of the residual slag to the dephosphorization process is simulated based on the mass balance calculation. The significant reduction of total slag generation and CaO input has been demonstrated by the mathematical model considering phosphorus recovery and recycling of residual slag as a dephosphorization agent. It has also been indicated by the Waste Input–Output model that the phosphorus recovery from dephosphorization slag and the recycling of residual slag to hot metal dephosphorization process have great environmental and economical benefits.

The Dissolution Rate of Solid Lime into Molten Slag Used for Hot-metal Dephosphorization

Highly efficient dissolution of CaO into melt has an important role in reducing the amount of steelmaking slag produced in the hot-metal dephosphorization process because the existence of free-CaO in the melt prevents reuse of the slag. There are numerous studies related to the rate of CaO dissolution, however, in many cases the experimental conditions have been limited. The present work measured the dissolution rate of CaO into several kinds of slags: Fe t O-CaO-SiO 2 -X (X; CaF 2 , CaCl 2 , Al 2 O 3 and B 2 O 3 ) systems at hot-metal temperatures. To apply the results to practical operations, the efficiency of additives in dissolving CaO in the melts was compared using the enhancement factor based on the results.

Development of Analysis and Control Method for Hot Metal Dephosphorization Process by Computer Simulation.

In order to apply a coupled reaction model to the simulation of industrial scale hot metal pretreatment process, a new computer program of MACSIM (Mathematical Analysis Codes for Slag-metal Reaction and Injection Metallurgy) has been developed. In this program, the reactions by injected flux, slag on bath surface and top blown oxygen gas can be calculated and most of the parameters were determined based on the experimental results of the previous paper. The following phenomena were clarified by this program and the control of the hot metal dephosphorization process became possible.(1) In the case of hot metal pretreatment by torpedo car, about 50% of top blown oxygen gas had the same effectiveness as Fe oxide for dephosphorization reaction. The remaining oxygen contributed to the heat supply through decarburization and post combustion reactions.(2) When [P] concentration before treatment was high, the efficiency of oxygen and that of lime for dephosphorization were high but a clear dependence on temperature was not found.(3) The efficiency of oxygen and that of lime for dephosphorization mainly depended on CaO/O and hot metal composition ([P], [Si]) although dephosphorization treatment time and flux supply rate differed widely.

Effect of Stirring Energy, Temperature and Flux Composition on Hot Metal Dephosphorization Kinetics.

In order to extend the applicability of a coupled reaction model to the hot metal dephosphorization process, evaluation method for unknown parameters was investigated. The following points were clarified.(1) Mass transfer coefficient in metal phase was increased in proportion to ε1/2 and its activation energy was about 125 kJ/mol.(2) Ratio of mass transfer coefficient in metal phase to slag phase did not clearly depend on stirring energy, temperature, or flux composition.(3) Phenomenological rate parameter for CO evolution also did not clearly depend on stirring energy or flux composition but decreased with an increase in temperature.(4) Activity coefficients of FeO and P2O5 in CaF2 and CaCl2 containing oxide slag were able to be estimated by a regular solution model in which the interaction energy was expressed as a function of the CaF2, CaCl2 content.By the application of these results to the coupled reaction model, the change of concentration during the hot metal dephosphorization experiments was calculated without using a parameter fitting method.

溶銑脱りんおよび脱炭時のマンガン鉱石溶融還元による[Mn]上昇法

The smelting reduction process of Mn ore both in a dephosphorization and a decarburization converter has been examined in order to increase [Mn] at blow end of BOF. The new hot metal dephosphorization process in a top and bottom blowing converter (SRP : Simple Refining Process), in which BOF slag is used as a dephosphorization agent, is applied for the present study. The following results are obtained with the experiments in a 2 kg crucible and a 2 t laboratory scale converter : 1. 1.5%[Mn] at blow end of BOF can be achieved without [S] increase by the following features of the present process : (1) Simultaneous treatment of Mn ore reduction, dephosphorization and desulphurization of hot metal in a dephosphorization converter. (Coke is charged and combusted in some minutes to supply energy for the reduction of Mn ore.) (2) MnO-rich slag from a decarburization converter is used as a dephosphorization agent in a dephosphorization converter. (3) Less slag operation in a decarburization converter with large amount of Mn ore charge. 2. The yield of Mn is increased by about 10% by the recycle of BOF slag ; such high yield of Mn as 63% can be achieved in the present process.