THE INFLUENCE OF SLAG EVOLUTION ON BOF DEPHOSPHORISATION

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1 THE INFLUENCE OF SLAG EVOLUTION ON BOF DEPHOSPHORISATION A thesis submitted in fulfillment of the requirements for the award of the degree MASTER OF ENGINEERING From UNIVERSITY OF WOLLONGONG By MARK SWINNERTON, BE(HONS) Materials Engineering 25

2 Acknowledgments Many thanks must be given to the people who have supported me throughout the writing of this thesis. Firstly, to Professor Dippenaar and Doctor Monaghan, your patience with the many drafts and lengthy discussion is greatly appreciated. During the time of experimentation, many thanks must go to Blue Scope Steel, Central Laboratories, and particularly Peter Salter. During the writing of this document, I must acknowledge the support of my family, and particularly Marnie Elizabeth, in helping me draw this work to a conclusion. ii

3 Abstract A study has been conducted to examine the influence of slag evolution on BOF dephosphorisation. An experimental technique was developed where slag / metal emulsion samples were obtained from the BOF during processing. The observed evolution of slag composition was consistent with many previous studies, where in the first half of the blow slags high in (%SiO 2 ) and low in (%FeO) are generated. During the second half of the oxygen blow, (FeO) generation and lime dissolution improve the slag basicity and contribute to the transfer of phosphorus from metal to slag. Poor levels of phosphorus removal during the centre region of the blow were found to coincide with a minimum in the (%FeO) composition. By the end of blow, 7% of the mass of phosphorus in the furnace is present in the slag, and the extent of phosphorus removal is proportional to the slag basicity and the (%FeO) concentration in the slag. By utilising mass balance calculations to estimate the mass of slag in the furnace, it was shown that at tap, approximately 3% of lime and 5% of magnesia remain undissolved in a liquid slag saturated in both components. It was concluded that the mass of lime added to the furnace could be reduced without influencing the dissolved concentration of (%CaO), and therefore without deteriorating the extent of dephosphorisation. iii

4 Table of Contents Certification Acknowledgments Abstract Table of Contents List of Figures List of Tables.i.ii.iii.iv.vii.xi CHAPTER 1 INTRODUCTION.1 CHAPTER 2 LITERATURE REVIEW The Basic Oxygen Furnace The Removal of Phosphorus from the Steel Melt Thermodynamics of Phosphorus Removal Slag Basicity The Role of Basic Slags in Dephosphorisation The Supply of Oxygen for Reaction Kinetics of Dephosphorisation Characteristics During the Oxygen Blow The Evolution of Slag and Metal Composition The Removal of Phosphorus During the Blow Fluxing Practices Examining the Phosphorus Equilibria The Activity of Phosphorus in Slag Literature Summary...4 iv

5 CHAPTER 3 EXPERIMENTAL TECHNIQUES Determination of Composition Physical Sampling Analysis of Samples.47 CHAPTER 4 EXPERIMENTAL RESULTS Slag Analyses The (%P) Concentration in the Slag The (%FeO) Concentration in the Slag The (%SiO 2 ) Concentration in the Slag The (%CaO) Concentration in the Slag The (%MgO) Concentration in the Slag The (%MnO) Concentration in the Slag The (%CaF 2 ) Concentration in the Slag The (%Al 2 O 3 ) Concentration in the Slag Metal Analyses The [%C] Concentration in the Metal The [%Si] Concentration in the Metal The [%Mn] Concentration in the Metal The [%P] Concentration in the Metal The [%O] Concentration in the Metal Temperature Results The Effect of Assumptions and Errors Relevant Calculated Values.65 v

6 CHAPTER 5 DISCUSSION The Evolution of Slag Composition The Distribution of Fluxes in the Furnace The Estimated Mass of Slag in the Furnace The Dissolution of Lime The Dissolution of Magnesia Phosphorus Removal Emulsion [P] Concentrations The Extent of Phosphorus Removal.88 CHAPTER 6 CONCLUSIONS.94 CHAPTER 7 RECOMMENDATIONS.95 CHAPTER 8 BIBLIOGRAPHY.96 Appendix 1 Calculation of MgO Saturation.1 Appendix 2 Calculation of Log L P According to Selin.13 Appendix 3 Summary of Experimental Results.15 Appendix 4 Full Chemical Analysis.111 Appendix 5 Experimental Conditions.117 Appendix 6 Statistical Analysis of Slag Composition Results.136 Appendix 7 The Mass of Slag in the Furnace.154 Appendix 8 The Influence of (%Fe t O) on Log L P.158 vi

7 List of Figures Figure Title Page 1 An overview of the major steps in the BOF operation. 3 2 Representation of the BOF during the oxygen blow. 4 3 The affect of MgO on the solubility isotherms at 16 C in the system 1 (CaO+MgO)-SiO 2 -FeO in equilibrium with liquid iron. 4 The solubility limit of (MgO) in MgO-SiO 2 -CaO-Fe t O slag plotted as a 11 function of the slag V-Ratio at 16 C 5 The (%P 2 O 5 )/[%P] (mass%) ratio plotted as a function of (%FeO) content of the slag. Reproduced from Balajiva et a 12 6 Influence of po 2 on the CaO-FeO-SiO2 phase diagram The bulk slag composition plotted as a function of processing time 2 (reproduced from van Hoorn). 8 The bulk metal composition plotted as a function of blowing time in a 21 3 ton converter at Corus (formally Hoogovens) 9 Slag evolution path with different initial hot metal silicon contents. 22 (Source: Obst, K., Schűrmann, E. Műnchberg, W. Mahn, G. and Nolle, D. Arch. Eisenhűttenwes., 51 (198) 47.) 1 Changes in composition of slag during oxygen blowing for various 22 BOF practices: I, van Hoorn et al. and II, Bardenheurer et al. 11 Changes in composition of slag during oxygen blowing for various 23 BOF practices: AA, Nilles et al., and BB, Baker (Source: Baker, R., Corus (formally British Steel) Corporation Report, code CAPL/SM/A/31/74). 12 Comparison of (%FeO) and [%P] concentrations as a function of 24 blowing time (data obtained from Cicutti et al.). 13 The percentage of phosphorus input into the converter absorbed in the 25 slag phase (%P S ) as a function of the mass of oxygen blown into the furnace. Curves reproduced from Kreijger et al. Curve I shows findings when lime was added in one batch, and curve II shows the findings when lime was added lime over the first 6 minutes of the blow. 14 The relationship between the dissolved phosphorus concentration in the 27 metal at the end of blowing and the further reduction in phosphorus concentration as a consequence of argon stirring in the BOF for 2-3 min. (Reproduced from Pistorius et al.) 15 Dissolution of CaO in liquid slag at Corus (formally Hoogovens) 29 (reproduced from Kreijger et al.). 16 The influence of coated CaO slag additions on the V-Ratio (of liquid 3 slag) and slag melting point as a function of blowing time. The curves are reproduced from Choi et al.). 17 The influence of coated CaO additions to the slag on the dissolved phosphorus concentration in steel as a function of blowing time. Curves are reproduced from Choi et al. 3 vii

8 Figure Title Page 18 Equilibrium phosphorus distribution ratios at 162 C. The four 33 diagrams represent cross sections through the (CaO) -(FeO) -(SiO 2 ) system at fixed (FeO) levels where phases are at equilibrium with the liquid slag. (After Bardenheuer and Oberhauser). 19 The variation of logγ P 2 O with optical basicity, Λ for various slag 5 39 compositions and 16 C. 2 Typical Sampling Regime for the BOF Schematic diagram of the Port Kembla BOF (side view on the left and 43 top view on the right). The vertical axes of the oxygen and sub-lances are shown, along with the metal and slag sampling positions. 22 The main components of a sub-lance probe. The entrance to the 44 sampling chamber is uncovered for representation only. 23 A representation of the two areas inside the sample head. The entrance 45 to the sample chambers opens into Area A, which is used to collect slag from the emulsion and to provide a path for metal to transfer into Area B when sampling form the metal bath. Area B is a lolli-pop metal sampling chamber. 24 The (%P) concentration in the slag for the emulsion, in-blow, afterblow 51 and tap samples plotted as a function of processing time 25 The (%FeO) concentration in the slag for the emulsion, in-blow, afterblow 52 and tap samples plotted as a function of processing time 26 The (%SiO 2 ) concentration in the slag for the emulsion, in-blow, afterblow 53 and tap samples plotted as a function of processing time. 27 The (%CaO) concentration in the slag for the emulsion, in-blow, afterblow 54 and tap samples plotted as a function of processing time. 28 The (%MgO) concentration in the slag for the emulsion, in-blow, afterblow 55 and tap samples plotted as a function of processing time. 29 The (%MnO) concentration in the slag for the emulsion, in-blow, afterblow 56 and tap samples plotted as a function of processing time. 3 The bulk metal [%C] concentration plotted as a function of processing 59 time. The estimated values are determined by assuming a linear relationship of carbon concentration with time between the hot-metal and in-blow samples. 31 The [Mn] concentration in the metal plotted as a function of processing 6 time. The sample 7669 at 1min has been removed as an outlier. 32 The measured and estimated [%P] concentration in the metal plotted as 61 a function of processing time. 33 The emulsion, in-blow, after-blow and tap temperature measurements 63 plotted as a function of processing time. The estimated values are determined by assuming a linear increase in temperature with time between the hot-metal and in-blow measured temperatures. 34 Plot of measured slag compositions. The slag analysis is adjusted so that SiO 2 +Fe t O+CaO+MgO=1. The solid lines represent liquidus isotherms plotted for 16 C and 1% MgO. The block arrow indicates the evolution of composition. 66 viii

9 Figure Title Page 35 Plot of measured emulsion slag compositions. The slag analysis is adjusted so that SiO 2 +Fe t O+CaO+MgO=1. The solid lines represent liquidus isotherms plotted for 16 C and 1% MgO. 36 Plot of measured tap slag compositions. The slag analysis is adjusted so that SiO 2 +Fe t O+CaO+MgO=1. The solid lines represent liquidus isotherms plotted for 16 C and 1% MgO. 37 The measured (%Fe t O) concentration plotted as a function of processing time. The solid line shows the trend reported by Van Hoorn. 38 Calculated values for Mass(Slag). Three data points are plotted for each sample, representing the output for mass balances of phosphorus, manganese, and silicon. 39 A plot showing the percentage difference between Mass(Slag) calculations using [P] and [Mn] and the value obtained using [Si]. Each sample is plotted as a pair of results. Positive values indicate that the relevant calculated value for Mass(Slag) is greater than that calculated using [Si]. 4 The calculated mass of CaO in the slag (calculated by multiplying (%CaO) by Mass(Slag)). The value of Mass(Slag) was calculated using Equation [43]. The solid lines represent the average values for emulsion and tap sample sets. 41 The term (%CaO Sampled ) plotted as a function of processing time. The term (%CaO Sampled ) is calculated using Equation [47] (the estimated mass of slag in the furnace is obtained using Equation [43]). The solid line represents the line of best fit and is calculated using the least squares method. The dashed line represents results reported by Thornton et al. 42 The calculated mass of MgO in the slag (calculated by multiplying (%MgO) by Mass(Slag). Three values are presented for each sample, which correspond to the three methods of calculating Mass(Slag) in Section The term (%MgO Sampled ) plotted as a function of processing time. The term (%MgO Sampled ) is calculated using Equation [49] (the estimated mass of slag in the furnace is obtained using Equation [43]). The solid line represents the line of best fit and is calculated using the least squares method. 44 The measured (%MgO) total plotted as a function of slag V-Ratio. The V-Ratio is defined in Equation [15]. The solid line represents the saturation limit of MgO in slag at 16 C in a Fe t O-SiO 2 -CaO-MgO slag as predicted by Bock et al. 45 Plot of the term (%MgO) oversat as a function of processing time. The term (%MgO) oversat is calculated using Equation [5]. 46 The experimental L P plotted as a function of processing time. The solid line represents the line of best fit and is calculated using the least squares method. 47 The term %P S plotted as a function of processing time. The solid line represents the trend observed by Kreijger et al ix

10 Figure Title Page 48 The tap experimental log L P plotted against (%FeO). The term B 91 represents the V-Ratio. The three lines are calculated using Equation [25] with a constant V-Ratio and a temperature of 1635 C. 49 The emulsion experimental log L P plotted against (%FeO). The term B represents the V-Ratio. The three lines are calculated using Equation [25] with a constant V-Ratio and a temperature of 155 C Calculation of the term ( MgO) ref.( ) is data by Selin held for 1h; 11 ( ) is data by Selin held for 6h; (x) is data by Suito et al. All figures refer to wt% MgO. The dash-dotted lines show double saturation boundaries. 51 Estimation of, the equilibrium phosphorus distribution between 14 L P, ref CaO-FetO-MgO sat -SiO 2 -.4wt%P slags and liquid iron at 16 C (full lines). ( ) is data by Selin held for 1h; ( ) is data by Selin held for 6h; (x) is data by Suito et al. All figures refer to L P ratio. The dashdotted lines show double saturation boundaries. 52 Interpretation of the elements in a box plot A box plot of the emulsion and tap (%P) concentration in the slag A boxplot of the emulsion and tap (%FeO) concentration in the slag A boxplot of the emulsion and tap (%SiO 2 ) concentration in the slag A boxplot of the emulsion and tap (%CaO) concentration in the slag A boxplot of the emulsion and tap (%MgO) concentration in the slag A boxplot of the emulsion and tap (%MnO) concentration in the slag. 152 x

11 List of Tables Figure Title Page 1 Composition of metal droplets (.6<d<1.2mm) obtained from a BOF 18 emulsion. 2 Analysis of fluxing agents at BHP Steel Port Kembla The analytical uncertainty (95% confidence interval) for metal 49 chemical analysis. 4 Overview of experimental slag results. Slag analysis presented in wt%. 5 5 The dissolved oxygen concentration in the metal measured by an 62 oxygen sensor. 6 Slag and metal analysis for measured samples (bold indicates estimated 15 values). 7 Henrian and Raoultian activity data for slag and metal samples in 17 addition to predicted saturated phases. 8 Additional values calculated directly from experimental results Slag and metal analysis for all samples The estimated mass of slag in the furnace at the time of sampling 156 calculated using equations [43, 52, and 53]. 11 The predicted values for Log L P calculated using Equation [25]. Calculations were performed using the conditions listed above for temperature and slag composition. The values for Log L P are calculated for various slag V-Ratios. 159 xi

12 1 CHAPTER 1 INTRODUCTION The removal of phosphorus in steelmaking operations has been a subject of extensive research since the inception of early steelmaking technologies. Phosphorus removal remains a key area of research because of its detrimental effect on the mechanical properties of steel. 1, 2, 3, 4 Phosphorus is primarily introduced to integrated steelmaking through blast furnace additions of iron ore, coke, alloys, and re-circulated converter slag. 5 The conditions in ironmaking are unsuitable for the removal of phosphorus, 6, 7, 8, 9 which has subsequently led to the development of steelmaking practices to manage the removal of this impurity. Even since the days of the early pioneers such as Huntsman, Bessemer, and Siemens, phosphorus removal has fundamentally dictated process selection in steelmaking. 2 The importance of slag in the Bessemer process was not fully realised until Sidney Thomas linked phosphorus removal to the slag properties in It was at this time that the use of basic fluxes was introduced. It was found that the addition of lime to the furnace could aid in the removal of phosphorus from the molten metal. It is often considered that the introduction of basic processes revolutionised steel production. Although the basic process was initially developed in the Bessemer converter, it was only a few years later that the open-hearth process adopted similar practices. The use of basic slags to produce low phosphorus steel is a practice that is still used in modern steelmaking. In recent years, some evidence 9,1 has suggested that the supply of high quality, low phosphorus, iron ore products may become depleted. This may lead to higher phosphorus contents in molten iron, which must correspond to an increasing requirement for oxygen steelmaking practices to better manage high phosphorus loads. Hot metal from the blast furnace can be dephosphorised 5 in combination with desiliconisation. Additions of Ca, Mg, or rare earth metals are used to remove phosphorus from the hot metal prior to the steelmaking process.

13 2 The management of high phosphorus loads in BOF vessels has been made considerably more difficult by a number of factors including: 1) The requirement for low phosphorus steels in the continuous casting process. It is known that phosphorus can lead to the formation of low melting point films, which can in turn lead to operational difficulties including uncontrolled metal escapes. 2) The need for high furnace tapping temperatures in order to accommodate the continuous casting process. High temperature in the furnace detracts from the thermodynamics of phosphorus removal. 5 3) More stringent environmental laws, and strong financial incentives to conserve refractory linings, have put pressure on the use of Fluorspar 11 as a slag agent. Without this fluidising agent in the slag, achieving efficient transfer of phosphorus from the metal to the slag becomes more difficult. This study has been conducted in conjunction with Blue Scope Steel Port Kembla to study the extent of dephosphorisation during the blow of the BOF. It will be shown that a slag saturated in CaO and MgO is generated by the end of blow, and that a reduction in the addition of these fluxes should be possible without any impact to the extent of dephosphorisation.

14 3 CHAPTER 2 LITERATURE REVIEW Prior to experimentation and sampling, a background investigation has been carried out in which existing literature concerning dephosphorisation in steelmaking and the evolution of slag in the BOF has been reviewed. 2.1 The Basic Oxygen Furnace Basic Oxygen Steelmaking (BOS) was first introduced in the early 195 s at Linz and Donawitz (Austria). Rapid uptake of this process by steel manufactures has lead to the process now accounting for more than 5% of world steel production. 12 The BOS process uses inputs of blast furnace hot metal, scrap steel, fluxes, and oxygen to produce a molten steel product. The operation of a Basic Oxygen Furnace (BOF) can be divided into a number of distinct steps as shown in Figure 1. Scrap Bin Charging Blowing Tapping Slag Off Slag Oxygen Lance Refractory Lining Slag Slag Stopper Hot Metal Tap Hole Scrap Bath Level Ar / N 2 Tuyeres Bulk Alloying Steel Slag Slag Rake Steel Ladle Slag Pot Slag Pot Slag Rake Slag Off Slag Wash Figure 1 An overview of the major steps in the BOF operation.

15 4 After charging molten iron from the blast furnace, scrap metal, and some fluxes to the furnace, a lance is lowered from which oxygen is delivered at a supersonic velocity. Within the first few minutes, lime and other fluxes are gradually added to the furnace. This is done to flux the oxides formed by the oxidation of iron, silicon, and manganese. 1 A slag is formed primarily consisting of CaO, SiO 2, MnO, FeO, P 2 O 5, and MgO. MgO is added to the slag to reduce refractory wear. In the Port Kembla BOF, oxygen is blown into the furnace for a period of approximately 16 minutes. During that time, a slag-metal emulsion forms in which the majority of refining occurs. A representation of the slag-metal emulsion is shown in Figure 2. Oxygen Lance Refractory Lining Slag / Metal Emulsion [C] + 1/2O 2 CO [Si] + O 2 SiO 2 [C] + (FeO) CO + Fe 1/2O 2 [O] Metal Figure 2 Representation of the BOF during the oxygen blow.

16 5 2.2 The Removal of Phosphorus from the Steel Melt In more than 6 years of experimental research into dephosphorisation, a range of chemical reactions have been proposed to describe the transfer of phosphorus between metal and slag. Early researches such as Balajiva 13, Winkler 14, and Turkdogan 15 used a range of molecular representations to describe dephosphorisation. 2[ P ] + 5[ O] ( P 2 O5)..[1] 2[ P ] + 5[ O] + 4( CaO) 4CaO. P 2 O )..[2] ( 5 2[ P ] + 5( FeO) ( P 2O5 ) + 5[ Fe]..[3] where [X] represents species dissolved in the metal phase and (Y) represents species dissolved in the slag phase. Even at the time they were proposed, researchers expressed doubts about the validity of such chemical expressions. Consequently, by the 197 s it was common to assume phosphorus existed in molten slags as an ionic species. This resulted in different expressions for the phosphorus reaction: [ P ] + [ O] + ( O ) ( PO )..[4] [ P ] + ( FeO) + ( O ) 2 2 ( PO + Fe 3 4 ) 5 2..[5] 17 [ P ] + ( FeO) + ( CaO) Ca ( PO ) ) + [ Fe]..[6] 18 ( Despite it being generally accepted that dephosphorisation occurs through an ionic dissociation into the slag, for convenience, many researchers still use Reaction [1] to perform analysis. Regardless of the equation used, there are three common understandings implied by the above equations: 1) A highly basic slag is required to accept phosphorus from the metal into the slag. 2) A high oxygen potential must exist to force phosphorus from the metal to the slag. 3) Low temperature promotes the thermodynamics of phosphorus removal.

17 Thermodynamics of Phosphorus Removal In steelmaking, products and reactants in chemical reactions are generally present in the form of a solution. 19 The extent to which the free energy of a species i in solution differs from the free energy of that species under standard conditions, is quantified by the activity of the species. Slags can be treated as concentrated solutions 4 so that the activities of the components of the slag are referred to in the Raoultian standard state. The Raoultian activity a i can be calculated using Equation [7]: a = γ N..[7] i i i where N i is the mole fraction of the species i in solution, and γ i is the Raoultian activity coefficient of the species. The activity coefficient γ i is usually experimentally determined. When the solute concentration is very low, as is the case for elements dissolved in the metal phase, the activities of the components in the metallic solution use infinite dilution as the standard state. The Henrian activity h i is used in the calculation of free energy and can be calculated from Equation [8]: h = f ( wt%) i i i..[8] 2 where h i is the Henrian activity, f i the Henrian activity coefficient, and (wt%) i the mass percentage of the species i in solution. This activity has the property that, for ideal behaviour in dilute solutions, the activity coefficient f i is equal to 1. The amount by which the Henrian activity coefficient deviates from ideal Henrian behaviour is quantified by interaction parameters 21 according to Equation [9]: j ei ( wt% j) + j 2 log fi = ri ( wt% j)..[9] e i r i j j where and are (respectively) the first and second order interaction parameters of species j with species i.

18 7 It is common in literature to describe the transfer of phosphorus from the metal to the slag using Reaction [1]: [ ] [ ] %) 2 P ( 1%) + 5 O (1 ( P 2O5 ) liquid (Reaction 1) The equilibrium constant K 1 for Reaction [1] is given in Equation [1]: ( a P ) 2O5 2 P ] [ ao K 1 =..[1] 5 [ a ] Equation [1] can be re-written using Equations [7] and [8] to give: K 1 = ( f [ P] γ P O 2 [% P]) ( f 2 5 N P O 2 5 [ O] [% O]) 5..[11] 22 The value of K 1 is a function of temperature and is reported in Equation [12]: 3716 log K 1 = [12] 23,24 T In many cases, researchers have developed an equilibrium quotient k x, which assumes ideal behaviour for slag components when Raoultian actually applies. Ideal behaviour is often assumed as a first approximation when no activity data are available. The quotient for Reaction [1] is shown in Equation [13]: (% P2 O5 ) k 1 =..[13] 2 5 [ P] [ O] The distribution of phosphorus between the slag and metal is expressed in this document as the phosphorus distribution ratio L P. In all cases L P refers to Equation [14]: ( mass% P) L P =..[14] [ mass% P]

19 Slag Basicity The concept of slag basicity appears to have been first introduced by Herty for steelmaking slags 25 in the mid 192 s; he used molecular theory to propose a simple concentration ratio, the so-called V-Ratio: % CaO V Ratio =..[15] % SiO 2 The slag basicity defined by the V-Ratio gives a general indication of the extent of depolymerisation of the melt, 25, 26, 27, 28 and is often used as a simple indicator of the trend in changes of oxide activities with composition. 29 A different method of describing the slag basicity is through the use of optical basicity. 3,31 With this method, basicities are regarded in terms of electron donor power rather than just a simple concentration ratio. Measuring shifts in the UV spectrum following the inclusion of trace metal ions allows the experimental measurement of optical basicity values. Duffy and Ingram 32,33 were the first to correlate these measurements with the Pauling electronegativity. They produced an optical basicity scale, where by definition, the optical basicity Λ for CaO is equal to 1. One of the most useful aspects of the treatment developed by Duffy and Ingram is the ability to calculate the optical basicity for a slag directly from its constituent components. The optical basicity can be determined using Equation [16]: Λ = X iλi and Ni ( x) X i =..[16] Ni ( x) where Λ is the known optical basicity of oxide i, N is the mole fraction, and (x) i represents the number of oxygen atoms in the oxide i. i Later work by Nakamura 34 used electron densities to calculate optical basicity yielding slightly different values to Duffy et al. Has limited applicability to slags containing transition metals. 3,35

20 The Role of Basic Slags in Dephosphorisation The importance of slag basicity is clearly demonstrated when Reaction [4] is used to represent dephosphorisation [ P] + [ O] + ( O ) ( PO ) (Reaction 4) Basic slags (indicating high concentrations of O 2- ) are essential to the achievement of efficient dephosphorisation because they allow phosphate ions to be readily accepted into the slag. 26 In the past, a number of authors 5,15, 34, 35 have expressed concern at assuming all phosphorus exists in the slag as monomeric ions been based on the following reaction: PO 4 3. This concern has ( PO ) ( P O ) + ( O )..[17] Selin 37,38 has reported that for phosphorus levels in slag of less than 1.5%, it is reasonable to assume that all phosphorus is present in the slag as monomeric phosphate anions. For slags with higher phosphorus contents, it may be necessary to include the dimer-ion equilibrium in calculations. The basicity of the slag in the BOF can be increased through the addition of basic fluxes to the furnace. When very basic compounds such as Na 2 O or BaO are used, extensive dephosphorisation of steel has been reported. 39, 4, 41, 42, 43 However, these compounds are generally considered too expensive and hazardous for industrial use. The most common method of controlling the basicity of slag in the furnace is through the addition of lime (consisting primarily of CaO) as a fluxing agent. Additions of the compound CaF 2 (Fluorspar) are often made to the BOF to lower the slag melting point and viscosity. Studies 8,11,44 into large (~4%) and small (~3%) CaF 2 additions to slag have shown no direct effect on the slag basicity. However, it has been found 11,44,45 that in slags with CaF 2 additions between 1-4wt%, the phosphate capacity can be increased by up to a factor of three. 45 Such increases are explained 46 by the

21 1 addition of CaF 2 decreasing the activity coefficient of (P 2 O 5 ), and increasing the activity coefficient of (FeO) in the slag. It has also been claimed that CaF 2 additions increase the fluidity of the slag, which in turn allows more CaO to be dissolved (Choi et al. 47 suggested this is facilitated by suppressing the formation of dicalcium silicate). An opposing view has been published by Turkdogan, 25 who found that CaF 2 does not affect the phosphorus distribution ratio. He showed that the impact of CaF 2 in moderate proportions is equivalent to that of CaO for slags saturated with both dicalcium silicate and magnesio-wűstite. The major components of steelmaking slags are CaO, MgO, SiO 2, and FeO. The phase diagram of these components is shown in Figure 3. Figure 3 The affect of MgO on the solubility isotherms at 16 C in the system (CaO+MgO)-SiO 2 -FeO in equilibrium with liquid iron. The solubility of (MgO) in steelmaking slags is of critical importance 48 because the refractory linings of most oxygen furnaces are constructed from MgO. The solubility limit of (MgO) in the slag typically decreases during the course of the oxygen blow, which corresponds to increasing dissolved lime concentrations in the slag. 49 The saturation limit of (MgO) in the slag is plotted as a function of the slag V-Ratio in Figure 4.

22 11 (MgO) Saturation %C ao / %Si O 2 Figure 4 The solubility limit of (MgO) in MgO-SiO 2 -CaO-Fe t O slag plotted as a function of the slag V-Ratio at 16 C. It follows from Figure 4 that for a slag V-Ratio of greater than 3, that (MgO) solubility limit is in the range of 4-6 wt%. This represents a relatively low solubility limit, which should contribute to low rates of refractory wear. For a slag V-Ratio of less than 2, the (MgO) solubility limit is greater than 8 wt% meaning that a large mass of MgO must be added to the furnace as flux to avoid excessive refractory wear. In a study into the solubility of (MgO) in steelmaking slags, Selin 38 determined that various minor oxides are able to substitute for the effect of (SiO 2 ) on the saturation limit of (MgO). The equation derived to quantify the saturation limit of (MgO) is summarised in Equation [18]. Details on the application of this equation are shown in Appendix 1. [ α W + β W ] [( MgO) 6] δ W ( MgO) + ε ( 1873) ( MgO) = ( MgO) T sat ref Al Ti ref V ref and ( ) = ϕ(% CaO',% FeO) where % CaO ' = % CaO +.75 % MnO.[18] MgO ref The term (MgO) ref is empirical and the function ϕ can be interpreted graphically in Appendix 1. W x represents the mass of substituting oxides. The terms α, β, δ, and ε are constants.

23 12 In a study by Morales et al., 5 it was shown that increased additions of MnO do not adversely affect the dephosphorising ability of the slags studied. In the case of Al 2 O 3, several authors 1, 51, 52 have shown that concentrations of Al 2 O 3 greater than 3.5wt% have a negative effect on dephosphorisation. This is primarily caused by dilution and reduced slag basicity. Balajiva et al. 13 studied the effect of iron oxide on the equilibrium phosphorus distribution between slag and metal. The phosphorus distribution ratio between a CaO- SiO 2 -Fe t O slag and liquid iron was found to increase with increasing (%FeO) concentration in the slag to a maximum, after which it decreases. The observed trends from that study are shown in Figure 5. Figure 5 The (%P 2 O 5 )/[%P] (mass%) ratio plotted as a function of (%FeO) content of the slag. Reproduced from Balajiva et al. The trends in Figure 5 that show a maximum in the (%P 2 O 5 )/[%P] ratio which has been explained 5 by the combined effect of:

24 13 1) The oxygen potential increasing with increasing (%FeO) concentration in the slag. This promotes the oxidation of phosphorus from the metal leading to an increase in the (%P 2 O 5 )/[%P] ratio. 2) As the (%FeO) concentration in the slag continues to increase, the mass percentage of (%CaO) in the slag decreases correspondingly. At the points of maxim observed in Figure 5, the negative effect on the (%P 2 O 5 )/[%P] ratio of reducing (%CaO) concentration in the slag outweighs the positive effect of increasing oxygen potential. Selin 38 used the same technique as Balajiva, but involved (%Fe t O) meaning both Fe 2+ and Fe 3+ ions were counted. This had the effect of shifting the maximum in the ratio to (%Fe t O) = 2-22%.

25 The Supply of Oxygen for Reaction At low carbon concentrations in the BOF, the (FeO) activity in the slag determines the concentration of dissolved oxygen in the steel. 15,44,5 It is necessary to calculate the Henrian activity coefficient of oxygen in the liquid metal such that: h = ho K and [% O ] =..[19] 18 f O O a FeO where f O can be calculated from interaction parameters and the metal composition using Equation [9]. The equilibrium relationship between oxygen and iron can be described using Reaction [2]. The equilibrium constant for Reaction [2] is shown in Equation [21]. FeO Fe Liquid + [O]..[2] Liquid [ ho ][ afe] 6372 K 19 = and log K 19 = [21] ( a ) T FeO The physical state of iron oxide in the slag depends on the temperature, oxygen potential, slag composition, and is very difficult to accurately predict. 53, 54, 55, 56 In the formulation of the equilibrium constants, the total iron dissolved in the slag as oxides (T.Fe) is usually converted to the stoichiometric formula FeO and denoted by Fe t O using the following relationships: %Fe t O = %FeO (analysed) +.9 %Fe 2 O 3 (analysed) %Fe t O = %T.Fe (total as oxides) Embedded metal droplets from the slag emulsion are sometimes surrounded by a wűstite layer. 57 This layer could inhibit dephosphorisation by limiting the access of basic slag to the reaction site.

26 15 The actual distribution of oxygen between slag and metal is not easily determined, since it is a function of a number of variables including lance height and oxygen flow-rate. 16 Bergman 35 used optical basicity to propose the following: [%O] =. 6867Λ where Λ = X Λ and i i Ni( x) X i =..[22] N ( x) i In Equation [22], X i is the equivalent cation fraction of the oxide i in the slag, Λi is the optical basicity of oxide i, N i is the mole fraction of oxide i, and (x) represents the number of oxygen atoms in the oxide i. Trentini 58 showed that the oxygen potential of the slag shifts the liquidus line for the ternary (FeO)-(CaO)-SiO 2 ) phase diagram. The liquidus line at various oxygen potentials is shown in Figure 6. Figure 6 Influence of po 2 on the CaO-FeO-SiO2 phase diagram. The relationship observed by Trentini is caused by a shift in the Fe 2+ / Fe 3+ ratio. The change in the position of the liquidus line represents a change in the FeO and Fe 2 O 3 concentrations.

27 Kinetics of Dephosphorisation To this point, the description of dephosphorisation in the BOF has been limited to thermodynamic aspects. In any process however, some understanding of the reaction kinetics is desirable to aid in making an informed analysis. The thermodynamic aspects of a reaction can describe to what extent that reaction can occur, but it is the reaction kinetics that determines the processing time necessary to obtain the required extent of refining. The rate of refining in the BOF depends on: 1) The rate at which reactants and products move to and from the reaction sites. 2) The rate at which the reagents react at the reaction sites. 3) The interfacial area of reaction. 4) The rate of heat transfer to or from the reaction site. For any chemical reaction occurring in the BOF, a number of reaction steps must take place for that reaction to proceed. Those reactions steps involve the three mechanisms above, and may typically involve a sequence of steps such as: Mass Transfer of Reagent 1 to the Reaction Interface Mass Transfer of Reagent 2 to the Reaction Interface Reaction at the Interface Mass Transfer of Product into the Bulk Slag Many reaction interfaces exist within the BOF. Some of these include: 12 Within or directly under the oxygen jet. In the cavity emulsion or cavity four-phase (liquid slag, metal, gas, undissolved lime or solid slag) dispersion; In the slag-metal emulsion.

28 17 At the interface between the metal bath and liquid slag. In the slag foam between droplets and the oxidising slag. Due to the many phases, and many reaction interfaces inside the BOF, it is possible for combinations of metal-slag, slag-gas, and metal-gas reactions to occur. A consequence of a reaction occurring through a series of reaction steps is that one or more of those steps must be rate limiting and control the overall rate of the reaction. It is known that for most reactions in the BOF, the high temperature results in rapid chemical reaction, meaning that mass transfer in either the metal or the slag phase is generally found to the rate-limiting step. 1 Studies into the reaction kinetics of BOF dephosphorisation have concluded that the rate-limiting step of reaction is mass transfer. Evidence has suggested that mass transfer control can occur in either the metal or the slag phase depending on the process or experimental specifics. Although several major models exist to describe inter-phase mass transfer kinetics, the dynamics of the BOF emulsion require some additional considerations. The dephosphorisation reaction proceeds at all metal-slag interfaces. The most significant is when metal droplets are ejected into the slag, transfer part of their phosphorus content to the slag, and fall back to the metal phase. Oeters 18 has published a detailed description of the kinetics of this process where it was found that: 1) For an individual metal droplet in the emulsion, the surfactant effect of oxygen in the droplet causes the mass transport of phosphorus in the droplet to be performed exclusively by diffusion. 2) The rate-limiting step for an individual droplet in the emulsion is found almost exclusively in the metal phase. 3) The rate of dephosphorisation in the emulsion is dependent on the size of the metal droplets. 59 Small droplets are more extensively dephosphorised than large ones. The common models used to describe mass transfer kinetics are The Two Film (Lewis/Whitman), the Boundary Layer, and the Surface Renewal models.

29 18 4) The bulk dephosphorisation kinetics in the BOF are primarily controlled by the residence time of droplets in the emulsion, and the emulsification rate which refers to the quantity of metal that is distributed as droplets in the emulsion. To further examine the reaction rates in the slag-metal emulsion, the relationship between reaction rate and surface area is shown in Equation [23]. k( C J = S ).[23] where J is the concentration flux, S is the surface area available for reaction, k is the rate constant, and C is the change in composition. In the early 197 s, Meyer 6 sampled (by collecting material ejected through the tap hole) the BOF emulsion to examine claims that decarburisation of the metal occurred within the slag-metal emulsion. Samples were obtained between 7 and 17 minutes into the blow. It was concluded that after 7 or 8 minutes into the blow, 3% or more of the total metallic charge is present in the slag-metal emulsion. The emulsified metal has a substantially lower carbon concentration than the average metal bath carbon concentration, indicating that significant reaction was occurring inside the emulsion. Kozakevitch 57 summarised earlier work by Hergat, 61 publishing a number of substantial findings relating to the slag-metal emulsion and to dephosphorisation. The data in Table 1 shows the composition from three randomly collected groups of metallic droplets in the emulsion. The composition of the drops is compared to the metal bath composition at the time of sampling. Table 1 Composition of metal droplets (.6<d<1.2mm) obtained from a BOF emulsion. C % Mn % P % S % Si % Metal Bath st Group of Drops nd Group of Drops rd Group of Drops

30 19 When comparing the results from the emulsion and from the metal bath in Table 1, a number of facts are evident: 1) The [%Si] concentration in the bath and emulsion is extremely low. This result is expected due to the strong driving force for the oxidation of silicon. 2) The [%C] concentration varies between droplet groups, but is consistently lower than the bath concentration. The variation in [%C] between droplet groups is explained by Kozakevitch 57 as resulting from a specific droplet groups residence time in the emulsion. 3) The [%P] concentration of the emulsion droplet groups is considerably less than the bath [%P] concentration. It is reported that in 95% of drops, the phosphorus concentration is at least 5 times lower than in the bath. The results from Kozakevitch in Table 1 support the theory that significant refining occurs within the emulsion. The contrast in the dissolved phosphorus concentration in the metal between the bath and droplets in the emulsion suggests 22,57,58 that dephosphorisation occurs largely in the emulsion. To conclude, the majority of dephosphorisation in the BOF occurs through droplet residence in the slag-metal emulsion. The transfer of phosphorus from the droplets to the slag is limited by mass transport in the metal phase. Other investigators 6 have suggested a process of temporary oxygen over-saturation resulting from gas nucleation difficulties. Little evidence has been presented to support either view.

31 2 2.3 Characteristics During the Oxygen Blow The extent of refining in the BOF is dynamic and the dephosphorisation equilibria changes throughout the period of the oxygen blow. Previous research into slag evolution is discussed in this section The Evolution of Slag and Metal Composition Numerous studies 16,51, 62, 63, 64, 65 have examined the evolution of slag and metal composition during the blow. The evolution of slag and metal composition as a function of processing time reported by Van Hoorn 62 is shown in Figure 7 and in Figure FeO CaO Wt % in Slag MnO SiO Fe 2 O 3 % 1% 2% 3% 4% 5% 6% 7% 8% 9% 1% Processing Time Figure 7 The bulk slag composition plotted as a function of processing time (reproduced from van Hoorn).

32 21 Figure 8 The bulk metal composition plotted as a function of blowing time in a 3 ton converter at Corus (formally Hoogovens). During the first few minutes of the blow, silicon, manganese and iron are oxidised. 62,64,65 All three oxides contribute to the initial rapid fluxing of lime. After several minutes the silicon oxidation is complete and manganese removal temporarily halted, while the (%FeO) concentration is influenced by the decarburisation reaction. 64 Throughout the blow the (%CaO) concentration increases as dissolution slowly occurs. The decreasing concentrations of (%MnO) and (%SiO 2 ) are explained by the effects of dilution as the slag mass increases with the dissolution of fluxes. The increases in [%Mn] and [%P] concentrations during the middle of the blow is termed reversion, and is primarily driven by the reduction of (%FeO) in the furnace during that period. In many plants lime is added to the furnace gradually for the first 1 to 12 minutes of the blow. Some plant studies 82 have shown that under certain conditions, high basicity slags can be reached in as little as 5 minutes. The influence of hot metal silicon on slag evolution can be seen in Figure 9.

33 22 (%CaO) 6 5 [Si] HM =.8 wt% [Si] HM =.6 wt% 3 (%SiO 2 ) 8 [Si] HM =.4 wt% (%FeO) Figure 9 Slag evolution path with different initial hot metal silicon contents. (Source: Obst, K., Schűrmann, E. Műnchberg, W. Mahn, G. and Nolle, D. Arch. Eisenhűttenwes., 51 (198) 47.) The early generation of liquid slag is important because liquid foaming slags act as a buffer for metal droplets ejected from the jet cavity. If these are not caught, skull formation around the mouth of the BOF is often reported as a problem. 73 The evolution of slag composition varies between different steel plants. The evolution of slag composition at four different steel plants are presented in Figure 1 and in Figure 11. Figure 1 Changes in composition of slag during oxygen blowing for various BOF practices: I, van Hoorn et al. and II, Bardenheurer et al. 66

34 23 The slag compositions shown in Figure 1 are recalculated to give CaO + MgO + FeO + SiO 2 = 1%. Curve I shows the slag evolution reported by van Hoorn et al. (also shown in Figure 7). This slag evolution is claimed to lead to low metal dispersion in the slag, minimum slopping, and good phosphorus removal. Such a practice has also been reported to reduce refractory lives due to the generally acidic evolution of the slag. Slag practice II, from the Mannesmann works in Germany, 66 generates a very basic slag within a short blowing period. Low slag MgO saturation levels and high rates of slopping have been reported with this slag practice. Numerous authors 22,57,67 have reported slag paths similar to Bardenheuer. Nilles 68 reported poor phosphorus removal when using slag practice II and classified the slag as over-oxidised. Two additional slag evolution paths are presented in Figure 11. Figure 11 - Changes in composition of slag during oxygen blowing for various BOF practices: AA, Nilles et al., and BB, Baker (Source: Baker, R., Corus (formally British Steel) Corporation Report, code CAPL/SM/A/31/74). 1 Path AA is reported 62,68 to give the best refining conditions, particularly for carbon oxidation. Nutt et al. 48 reported a slag evolution in the range of AA which resulted in problems with dry (silicate saturation) slag. They claimed that path AA was underoxidised and that path BB was more appropriate for higher basicity, and therefore better phosphorus removal. An internal Corus (formally British Steel) report by Baker 69 reported that slag paths similar to BB result in an over-oxidised slag and slopping.

35 The Removal of Phosphorus During the Blow The rapid generation of (FeO) early in the blow in addition to early dissolution of lime leads to oxidation of phosphorus from the metal. During the middle stages of the blow, the dissolved phosphorus concentration in the bulk metal increases. 16,47,51,64,68 This behaviour is called reversion, and the typical extent to which the phosphorus reverts from the slag into the metal is shown in Figure 12 in the region between 4% and 7% of the blowing time. wt% (FeO) (FeO).9 [P] Blowing Time (%) wt% [P] Figure 12 Comparison of (%FeO) and [%P] concentrations as a function of blowing time (data obtained from Cicutti et al.). A significant decrease in the (%FeO) concentration in the slag causes the change in phosphorus distribution. The decreased (%FeO) concentration is caused by: 1) A high rate of reduction through the reaction ( FeO ) + [ C] Fe + ( CO) 2) The high dissolution rate of lime, which dilutes the effective concentration of (%FeO) in the slag. The (%FeO) concentration in the slag is linked with the dissolved phosphorus concentration in the metal through the availability of [O] for dephosphorisation. According to Reaction [2], the concentrations of (%FeO) and [%O] are related:

36 25 FeO Fe Liquid + [O] (Reaction 2) Liquid If the dephosphorisation reaction is represented by Reaction [1], the impact of decreasing the available dissolved oxygen in the metal phase can be interpreted by applying Le-Chátelier s principle. 2[ P ] + 5[ O] P 2 O 5 (Reaction 1) Kreijger et al. 65 used the term %P S to study the extent of phosphorus removal from the metal phase during the blow. The term %P S is defined as the percentage of phosphorus input into the converter that is absorbed in the slag phase. The extent of phosphorus removal was studied for two lime addition methods: I II CaO was added in a single batch at the beginning of the blow. CaO was gradually added over the first 6 minutes of the blow. The evolution of %P S is plotted against the volume of oxygen blown in Figure %PS I II Oxygen Blown 1 3 Nm 3 Figure 13 The percentage of phosphorus input into the converter absorbed in the slag phase (%P S ) as a function of the mass of oxygen blown into the furnace. Curves reproduced from Kreijger et al. Curve I shows findings when lime was added in one batch, and curve II shows the findings when lime was added lime over the first 6 minutes of the blow.

37 26 The data in Figure 13 highlights the importance of basic slags in dephosphorisation (see Sections and for more detail on slag basicity). In Curve I, Kreijger et al. proposed that the slag contained a higher concentration of dissolved (CaO) during the middle of the blow than in Curve II, and that the more basic slag resulted in better dephosphorisation and less evidence of reversion. Conditions during the final stages of the blow are highly favourable for dephosphorisation. 51 These conditions include: 1) A low rate of decarburisation (leading to high dissolved oxygen concentrations) 2) A highly basic slag which lowers γ P 2 O and favors the oxidation of phosphorus 5 from the metal to the slag. 3) Increased liquid slag mass. 4) High FeO concentrations in the slag. After the oxygen blow has concluded, the rate-limiting kinetic step in the transfer of phosphorus from the metal to the slag is mass transport in the metal phase. The use of argon stirring in the BOF after the blow allows more time for refining to occur and results in improved dephosphorisation. 4,1,16,7 The relationship between the dissolved phosphorus concentration in the metal and the further reduction in the phosphorus concentration as a consequence of 2-3 minutes of argon stirring is shown in Figure 14.