The Effect of Fluorspar in Steelmaking slags. Eugene Pretorius Baker Refractories

Transcription

1 The Effect of Fluorspar in Steelmaking slags By Eugene Pretorius Baker Refractories 1

2 Introduction Fluorspar (CaF 2 ) is probably the "black sheep" of the steelmaking industry and has been blamed for more refractory slagline failures than anything else. For this reason CaF 2 is considered evil and the use of CaF 2 has been banned in many shops. In this section the effect of fluorspar on steelmaking slags and refractories will be evaluated in a practical and scientific manner, and hopefully it will dispel some of the myths regarding this component. It will show the considerable advantages of using fluorspar in steelmaking slags, but it will also show the possible devastating effect on refractories if fluorspar is used inappropriately. This paper will not address any of the environmental concerns regarding the use of fluorspar but will only focus on the technical aspects of this component in steelmaking. Fluorspar as desulfurizing agent One of the biggest misconceptions in the steelmaking industry is that fluorspar will desulfurize the steel. It is not fluorspar but dissolved lime in the slag that is responsible for desulfurization. The amount of CaO that can be dissolved in a slag is a function of slag composition and temperature. Once the CaO-saturation point of a slag is reached, any further addition of lime to the slag will only increase the viscosity of the slag (further precipitation of CaO or CaO-rich phases), and will inhibit desulfurization. The addition of fluorspar to some slags will increase the solubility of CaO and thus give the slags greater desulfurization potential. It is this increased CaO solubility (assuming that the lime is added) that increases the sulfide capacity of the slag, which results in improved desulfurization. The addition of CaF 2 to a fully liquid slag, without adding lime to maintain CaO-saturation, will do nothing for desulfurization but drastically increase refractory wear. Unfortunately, it is common to find operators adding fluorspar to a liquid slag, without additional lime, in an attempt to lower the sulfur content of the steel. The component fluorspar by itself is not a good desulfurizing component. This is demonstrated in the following table that shows the optical basicity values for the most common steelmaking components (higher optical basicity values are better for desulfurization). Table 1. Optical basicity values for typical slag components Component Optical Basicity Value () CaO 1.00 MgO 0.78 Al 2 O SiO CaF

3 From this table it is clear that CaO in solution has the highest desulfurizing ability and CaF 2 is a distant third behind MgO. Even the combination of CaO and SiO 2 (lowest basicity) to form a CaO-saturated slag, will still desulfurize better than a pure CaF 2 slag. This is shown in the table below, where optical basicity, sulfide capacity correlations and thermodynamic data were used to calculate the final sulfur in the steel. The following parameters were considered in the calculation: Temperature ( C) 1600 Slag Amount (kg) 2000 Metal Amount (kg) Initial Sulfur (%) 0.05 Oxygen Level in Steel (ppm) 15 Table 2. Calculated slag and metal parameters for a CaO-saturated CaO-SiO 2 slag and a pure CaF 2 slag at 1600 C. CaO-SiO 2 Slag CaF 2 Slag % CaO 56 % SiO 2 44 % CaF Optical Basicity Sulfide Capacity Sulfur Distribution Coeff Final Sulfur (%) Why is CaF 2 added to a slag? 1) To increase the solubility of basic components (CaO & MgO) in the slag 2) To act as a fluxing precursor 3) To maintain fluidity in the slag as the slag temperatures decreases CaF 2 will increase the solubility of CaO in silicate slags and therefore increase the effective sulfide capacity of the slag. In aluminate slags (> 25% Al 2 O 3 ) the addition of CaF 2 does not increase the solubility of CaO in the slag. The effect of CaF 2 in aluminate slags will be discussed in detail later. When should CaF 2 be added to a slag. The addition of CaF 2 to a stiff silicate slag (CaO over-saturated) will increase the fluidity of the slag as CaF 2 will melt immediately at steelmaking temperatures. More importantly, it will bring more CaO into solution. In contrast, the addition of CaF 2 to a stiff high-alumina slag will only slightly increase the fluidity of the slag (liquid CaF 2 ), since CaF 2 has a limited effect on the solubility of CaO in these slags. 3

4 Fluorspar should never be added in the stir-eye of a ladle, but instead to the bulk of the slag (middle of the ladle). If the CaF 2 is added in the stir-eye of a ladle, it will not only dissolve the solid lime in the slag, but also dissolve the basic components of the slagline refractories in the vicinity of the stir area. Unfortunately, CaF 2 added in this fashion does not discriminate between undissolved lime in the slag or CaO and MgO in the slagline refractories. The addition of CaF 2 in the stir-eye obviously results in a very liquid slag with a very high CaF 2 concentration locally. The chemical potency of this slag to dissolve basic oxides and the turbulence of the stir-eye can lead to significant localized refractory wear. If CaF 2 is used as a ladle slag additive, it is generally recommended to add CaF 2 in a premixed form together with lime and silica sand or Ca-Aluminate. The phase relations of CaF 2 with various oxides. In order to understand the behavior of CaF 2 in slags it is important to first evaluate the binary phase relations of CaF 2 with other components and then consider more complicated higher order systems. The system CaO-CaF 2 The following figure shows the phase diagram of the CaO-CaF 2 system. Figure 1. Phase diagram of the CaO-CaF 2 system 4

5 This diagram shows a few very important features: 1. Pure CaF 2 is liquid at steelmaking temperatures 2. An eutectic exists between CaO and CaF 2 at 1360 C ( F), and the composition of the slag at the eutectic is: 83% CaF 2 and 17% CaO - Point (a) on the diagram 3. The solubility of CaO in CaF 2 at 1600 (2912 F) is not very high and the composition of the slag at CaO saturation is: 26.6% CaO and 73.4% CaF 2 Point (b) on the diagram. The solubility of CaO in pure CaF 2 is much lower than SiO 2, Al 2 O 3, or FeO, as shown in the next table: Table 3. Solubility limit of CaO in the following systems at 1600 C (2912 F) System % CaO at saturation CaO-SiO 2 56 CaO-Al 2 O 3 61 CaO-FeO 48 CaO-CaF 2 26 Fluorspar is rarely added by itself to a slag but normally added in combination with lime (usually premixed). Consider the amounts of liquid that will be present if the following CaO-CaF 2 mixtures are heated to 1600 C: Table 4. Amounts of liquid and solid for different CaO-CaF 2 mixtures at 1600 C Mixture Composition % Liquid % Solid 90% CaO 10% CaF % CaO 20% CaF % CaO 30% CaF % CaO 40% CaF It is important to note that the composition of the liquid phase for all the mixtures does not change (Point (b) in Figure 1). From the evaluation of the CaO-CaF 2 system the following question then arises: Why is fluorspar so popular as a flux since the ability of CaF 2 dissolve lime by itself is very limited? What other components are needed in the slag together with CaF 2 to affect an increase in CaO solubility? The evaluation of the CaO-CaF 2 -SiO 2 system (Figure 2) provides significant insight on the combined effect of CaF 2 and SiO 2 to drastically increase the solubility of CaO in the slag. 5

6 The CaO-CaF 2 -SiO 2 system Figure 2. Phase diagram of the CaO-CaF 2 -SiO 2 system The most striking feature of this diagram is the tremendous increase in the solubility of CaO when CaF 2 is added CaO-SiO 2 slags or when SiO 2 is added to CaO-CaF 2 slags. The combined effect of SiO 2 and CaF 2 results in a very high CaO solubility and the maximum solubility at 1600 C is shown by point (a) on the diagram. The composition of the slag at this point is approximately the following: % CaO 72 % SiO 2 16 % CaF 2 12 The CaO content (saturation) on the 1600 C isotherm in Figure 2 was traced from the CaO-SiO 2 binary to the CaO-CaF 2 binary system. This saturation solubility of CaO in CaO-CaF 2 -SiO 2 system is plotted as a function of the SiO 2 content of the slag in Figure 3. 6

7 75 Slag (a) in Fig % CaO in Solution % SiO2 Figure 3. Solubility of CaO as a function of SiO 2 content in CaO-CaF 2 -SiO 2 slags at 1600 C The maximum in CaO solubility is at about 12% CaF 2 in the slag. The addition of more CaF 2 to the slag, while still maintaining CaO saturation, results in a decrease in CaO solubility. This is because the SiO 2 content of the slag is diluted to below 18%. From this it is clear that the maximum amount of fluorspar that would ever be required in a slag is 12%. The addition of more CaF 2 would either result in a decrease in CaO solubility or an increase in fluidity that could lead to an increase in refractory erosion. The diagrams shown above are especially useful in designing flux recipes for rapid liquid formation. Consider the following mixtures in Table 5: Table 5. The combined effect of SiO 2 and CaF 2 in the solubility of CaO at 1600 C Mixture 1 Mixture 2 Mixture 3 % CaO % SiO % CaF % Liquid 0 40* 100 % Solid *The composition of the liquid for mixture 2 only contain about 27% CaO in solution. 7

8 The combination of CaF 2 and SiO 2 in the right proportions can create slags in the CaO- CaF 2 -SiO 2 system with excellent desulfurization abilities as shown in the Table 6 below. (Using the same parameters listed earlier) Table 6. Calculated slag and metal parameters for CaO-saturated slags at 1600 C. CaO-SiO 2 Slag CaO-CaF 2 Slag CaO-CaF 2 -SiO 2 slag (slag (a) Fig. 2) % CaO % SiO % CaF Optical Basicity Sulfide Capacity Sulfur Distribution Coeff Final Sulfur (%) While the theoretical slags discussed above show tremendous potential in terms of desulfurization, they are difficult to attain under real steelmaking conditions. The principal reason is that most real steelmaking slags also contain the components MgO and Al 2 O 3, which will also influence the solubility of CaO. Most slagline refractories are MgO based so that MgO saturation becomes an important requirement in steelmaking slags. The effect of MgO on the solubility of CaO will be discussed later in more detail. Although the theoretical slags of the CaO-CaF 2 -SiO 2 system has limited applicability as target slags for ladle applications, they are very important in terms of designing flux and additions recipes. This system shows that CaF 2 -CaO-SiO 2 mixtures have the potential to go into solution much faster than CaF 2 -CaO mixtures. The phase diagram in Figure 2 also shows that for slags containing SiO 2 and CaF 2 in approximately a 1:1 ratio is almost parallel to the slopes of the liquidus lines in the diagram. This is a very important observation and should be considered for flux recipes. Consider for example, mixtures 2 and 3 in Table 5. Mixture 3 could potentially melt completely if exposed to steelmaking temperatures, whereas Mixture 2 would require SiO 2 from another source (steel deoxidation) to become fully liquid. This also shows that combining the CaF 2 with silica sand might minimize the amount of CaF 2 added to a slag. The addition of CaF 2 to high-alumina slags In the previous section on the CaO-SiO 2 -CaF 2 system, a tremendous increase in the solubility of CaO was demonstrated for slags containing a combination of SiO 2 and CaF 2. The question arises whether the same fluxing effect will be present if CaF 2 is combined with Al 2 O 3. The following figure shows the isothermal section of the CaO-Al 2 O 3 -CaF 2 system at 1600 C. 8

9 Figure 4. Isothermal section through the CaO-Al 2 O 3 -CaF 2 system at 1600 C From this figure, it is clear that CaF 2 in combination with Al 2 O 3 does not show that same behavior as it does in combination with SiO 2. The addition of Al 2 O 3 to the CaF 2 -CaO system does result in a significant increase in the solubility of CaO. However, the addition of CaF 2 to the CaO-Al 2 O 3 system results in a decrease in the solubility of CaO. It is therefore clear that CaF 2 is not a good flux to increase the solubility of CaO in CaO- Al 2 O 3 slags. The replacement of Al 2 O 3 with CaF 2 results in a decrease in the solubility of CaO as indicated by the two CaO-saturated slags in the following table: Table 7. CaO-saturated slags in the CaO-Al 2 O 3 -CaF 2 system at 1600 C Slag (a) Slag (b) % CaO % Al 2 O % CaF 2 20 Comparison to the CaO-Al 2 O 3 -SiO 2 system This section is mainly concerned with the effect of CaF 2 on slags, but it is useful to consider phase relations in the CaO-Al 2 O 3 -SiO 2 system for comparison. Figure 5 shows the isothermal section of the CaO-Al 2 O 3 -SiO 2 system at 1600 C. 9

10 Figure 5. Isothermal section through the CaO-Al 2 O 3 -SiO 2 system at 1600 C. This diagram shows a few very important features: 1. The phase field of Ca 2 SiO 4 is much larger in this system than the CaO-CaF 2 -SiO 2 system indicating that CaF 2 is more potent than Al 2 O 3 to bring Ca 2 SiO 4 into solution. 2. The addition of Al 2 O 3 to CaO-SiO 2 slags initially results in a decrease in CaO solubility, but when the SiO 2 content of the slag drops below about 20%, a significant increase in CaO solubility is observed. 3. The addition of SiO 2 to CaO-Al 2 O 3 slags results in a very small increase in CaO solubility, indicating SiO 2 is actually a better flux than CaF 2 in CaO-Al 2 O 3 slags. 4. A maximum in the solubility of CaO on the CaO-saturated phase boundary is also observed, similar to that in the CaO-CaF 2 -SiO 2 system. However, this maximum CaO solubility value in the Al 2 O 3 system is significantly lower than the CaF 2 system, as indicated by the following table. 10

11 Table 8. Maximum CaO solubility limits of slags in the CaO-CaF 2 -SiO 2 and CaO-Al 2 O 3 -SiO 2 systems. CaO solubility maximum in CaO-CaF 2 -SiO 2 system CaO solubility maximum in CaO-Al 2 O 3 -SiO 2 system % CaO % SiO % CaF 2 12 % Al 2 O 3 28 Based on the phase relations in the systems discussed so far, it is expected that slags in the CaO-Al 2 O 3 -SiO 2 -CaF 2 system will probably show similar phase relations as slags of the system CaO-SiO 2 -CaF 2. The ability of CaF 2 to increase the solubility of CaO will be probably be limited until the SiO 2 content of the slag approaches about 15% SiO 2. The implication of the above to steelmaking practice is the following: In high-al 2 O 3 slags with low SiO 2 content (< 6% SiO 2 - Al-killed grades where the Si content of the steel is restricted), the addition of CaF 2 to slag will probably not result in an increase in the solubility of CaO in the slag (improve desulfurization). However, the addition of CaF 2 to these slags will probably result in a depression of the solidus temperature of the slag (complete solidification at lower temperatures). For the other grades of Al-killed steel or Al/Si-killed steel, the addition of CaF 2 to high- Al 2 O 3 slags that contain appreciable amounts SiO 2 (10-15%), might increase the solubility of CaO significantly and hence improve desulfurization, or increase refractory wear if not careful (catch 22). The use of CaF 2 as a fluxing precursor in high-al 2 O 3 slags. For most Al-killed steel grades, the amount of slag carried over from an EAF or BOF to the ladle is minimized. In some shops the slag is even raked off before further processing at the ladle refining station. In these cases a new synthetic slag has to be made from scratch. The volume of the new synthetic slag should be adequate to cover the steel and stabilize the arcs at the ladle furnace. The following could be a typical target ladle slag composition: % CaO 55 % MgO 7 % Al 2 O 3 25 % SiO 2-13 There are a number of ways this composition can be attained in a ladle. The choice of raw materials used will have a dramatic impact on the time it will take for the slag to be fully liquid and homogeneous in the ladle. The advantages and disadvantages of the different raw materials are summarized in the next table. 11

12 Table 9. Advantages and disadvantages of different raw material additions Raw Material Advantages Disadvantages Residual Very good fluxing precursor: Could cause Si and P reversions. Carryover Slag Liquid and hot. It s free. Require slag deoxidants (CaC 2, Al) Prefused Ca- Aluminate Lime & Dolimite Good fluxing precursor: Melts immediately when heated to steelmaking temperatures Inexpensive. Required for steel cleanliness and quality, and refractory compatibility Relatively inexpensive compared to prefused Ca-Aluminate to reduce the FeO and MnO Very expensive. (Check that the composition is approximately 50% CaO and 50% Al 2 O 3 ) Very refractory at steelmaking temperatures. Need a fluxing precursor to get it into solution Bauxite (or other high- Al 2 O 3 material) Very refractory at steelmaking temperatures. Need a fluxing precursor to get it into solution Silica Sand Inexpensive Can not be used in steel with low Fluorspar Good fluxing precursor: Melts immediately when heated to steelmaking temperatures. Relatively inexpensive compared to prefused Ca-Aluminate. Si specifications Could cause extensive refractory damage if the amounts and method of addition is not controlled In most shops it is important to create a liquid and desulfurizing slag as soon as possible in the ladle. In order to achieve this goal a certain amount of fluxing precursor is needed in the ladle. The fluxing precursors are added to bring the bulk addition of lime and high- Al 2 O 3 materials into solution. The fluxing precursor will melt immediately when heated to steelmaking temperatures and then provide the liquid medium into which the other refractory components can be dissolved. The possible fluxing precursors are: Residual Carryover slag Prefused Ca-Aluminate & Fluorspar It now becomes a choice between economics and steelmaking requirements. The addition of large amounts of Ca-Aluminate could drastically decrease the time to make a liquid slag and process the heat. However, the addition of more prefused Ca-aluminate also greatly increases the cost of producing the steel. Residual carryover slag is also an excellent fluxing precursor. However, the amount of carryover slag that can be tolerated will depend on the grade of steel and the composition of the carryover slag. If P and Si reversions are a concern, then the amount of carryover slag must be carefully controlled. An obvious goal therefore would be to balance the amount of carryover slag with the amount of Ca-aluminate added. It is certainly possible to only use the refractory end-member components such as lime, dolomite, bauxite (or another high Al 2 O 3 material) and silica. All the components individually are solid at steelmaking temperatures and if added together will have to react 12

13 first to create intermediate liquid phases and eventually become a liquid slag. Slags formed this way normally take a long time to become fully liquid and homogeneous (at least 30 minutes of vigorous stirring and arcing). During this time little or no desulfurization occurs due to the inhomogeneity of the slag and the limited amount of CaO in solution. In the discussion above the advantages and disadvantages of carryover slag and prefused Ca-aluminate was illuminated. The question now arises whether flourspar could be utilized as a fluxing precursor in order to decrease the dependence on prefused Caaluminate and/or carryover slag as fluxing precursors. In the discussion of the CaO- Al 2 O 3 -CaF 2 system it was shown that CaF 2 does not increase the solubility of CaO. However, fluorspar might be used as a fluxing precursor to enhance the kinetics of Al 2 O 3 and CaO dissolution. The added fluorspar will melt immediately and provide the liquid medium to enhance reaction between CaO and Al 2 O 3. The next figure shows significant fluxing of Al 2 O 3 by CaF 2 in the Al 2 O 3 -CaF 2 system. Figure 6. Phase diagram of the Al 2 O 3 -CaF 2 system The solubility of Al 2 O 3 in CaF 2 at 1600 C is about 50% Al 2 O 3 as shown by the small circle in the figure. This high Al 2 O 3 slag would then be very effective to bring the added lime into solution. The partial replacement of some of the prefused Ca-aluminate with fluorspar could drastically decrease flux cost and hence the cost per ton of steel produced. However, when fluorspar is utilized considerable care should be taken to ensure CaO and/or MgO saturation in the slag to minimize refractory wear. To section above can be summarized as follows: The addition of CaF 2 to high-alumina slags will probably not increase the solubility of CaO and hence the sulfide capacity of the slag. However, the addition of CaF 2 as a fluxing precursor when the pure 13

14 component end-members lime and bauxite are added to create a synthetic slag, might improve the kinetics of slag formation, which in turn might improve the overall kinetics of desulfurization. One major drawback on the use of CaF 2 in these slags is that the addition of CaF 2 significantly increases the solubility of MgO in the slag (discussed in a later section). Great care should be taken to ensure that the slag is either CaO or MgO saturated, or both, in order to limit the extent of refractory wear. The effect of CaF 2 on Refractories. The biggest reservation on the use of CaF 2 in the steelmaking industry (besides environmental concerns) has been the effect of fluorspar on refractory wear. In some shops fluorspar has been used for many years without any negative effect in refractory life, whereas in other shops, the introduction of fluorspar resulted in significant increased refractory wear. The principal reason for increased refractory wear when using fluorspar is the lack of CaO and/or MgO saturation in the slag. Fluorspar in combination with silica is a very potent flux to bring basic oxides into solution. If fluorspar is added to a slag without the matching basic oxide additions to maintain saturation, then dissolution of the basic refractories will occur. It is not the presence of CaF 2 that is causing refractory wear but rather the lack of CaO (or MgO) saturation in the slag. In some cases the misuses of spar; either added at the wrong time or for the wrong reason will also contribute to increased refractory wear. The effect of CaF 2 on CaO-bonded refractories (dolomite) In the previous discussion it was clearly shown that fluorspar in combination with SiO 2 is a very potent flux to bring CaO into solution. If lime is added to the slag until the slag is CaO-saturated there will be minimal refractory wear on lime-bonded (dolomite) refractories. However, if additional lime for saturation was not added, the presence of fluorspar in the slag could lead to accelerated refractory wear. This slag will have a lower viscosity, a lower solidus temperature and a high capacity to bring lime into solution and will lead to a deeper slag penetration into the refractory and increased refractory wear. It is not the presence of CaF 2 that is causing refractory wear in CaO-bonded refractories but the lack of lime saturation. A very liquid silicate or aluminate slag that is CaO unsaturated, and contains no CaF 2, will also be very aggressive to the refractories. The effect of CaF 2 on magnesia refractories Most slagline refractories are MgO based, so it important to evaluate the effect of CaF 2 on the solubility of MgO. The inferred phase diagram of the MgO-CaF 2 system is shown in Figure 7. There is some discrepancy in the literature on the exact location of the MgOsaturation boundary, so the diagram was redrawn utilizing the data from a number of ternary diagram which were in agreement on the position of the MgO saturation curves. The solubility of MgO in CaF 2 at 1600 C is about 48 wt%. This is significantly higher than CaO in the equivalent CaO-CaF 2 system (26.6% CaO). The composition of the 14

15 eutectic (12% MgO) is similar to that of the CaO-CaF 2 system (17% CaO). The phase diagram for the MgO-CaF 2 -SiO 2 system is shown in Figure Temperature ( C) L 1300 MgO CaF2 Wt% CaF2 Figure 7. Redrawn Phase diagram of the MgO-CaF 2 system Figure 8. Phase diagram of the MgO-CaF 2 -SiO 2 system 15

16 From this figure it is clear that there is some discrepancy in the solubility of MgO in pure CaF 2 slags. This figure indicates about 62% MgO at 1600 C whereas the binary system MgO-CaF 2 indicates about 48% MgO. Most other ternary diagrams indicate a MgO solubility on the binary join of about 48% MgO. However, the important feature of this diagram is the significant increase in the solubility of MgO when CaF 2 is added to the MgO-SiO 2 system. If it is assumed that the MgO solubility is about 48% at 1600 C on the MgO-CaF 2 join, and then the 1600 C isotherm can be redrawn as follows: 40% MgO SiO2 CaF2 MgO 60% CaF2 Figure 9. Isothermal section at 1600 C in the MgO-CaF 2 -SiO 2 system in the proximity of the MgO apex. The composition of the slag at the maximum MgO saturation point on the 1600 C isotherm is approximately: % MgO 58 % SiO 2 22 % CaF 2-20 The inferred phase relations as shown by the preceding figures are similar to that observed in the CaO-SiO 2 -CaF 2 system. From these figures it is clear that CaF 2 is also a very potent flux for MgO. This means that considerable care should be used when using CaF 2 -containing in slags in contact with MgO-based refractories. The saturation levels of MgO in these slags are significantly higher when the slag contains CaF 2 than when the slag just contains SiO 2 and/or Al 2 O 3 as fluxing components. Most of the discussions so far have centered on the phase relations of fluorspar with only one or two components. The reason for this, is the phase diagrams for these systems are 16

17 available. The phase relations of fluorspar with multi-component slags, and typical steelmaking slags, are not available. The relations observed in the simple systems have to be extrapolated and approximated for the more complex systems. Most basic steelmaking slags contain appreciable amounts of MgO and are mostly in contact with magnesia-based refractories. It is therefore very important to approximate the effect of fluorspar on the CaO and MgO phase boundaries in silicate and aluminate slags. The effect of MgO on CaO solubility Before the effect of CaF 2 on complex slags will be discussed it is important to evaluate the effect of MgO on the solubility of CaO. A key slag requirement for compatibility with magnesia-based slaglines is MgO saturation. A typical practical slag aim is dual saturation with respect to CaO and MgO. These dual saturation slags always have the lowest MgO solubility as shown in the next two figures. Slags compatible with Magnesia-C refractories: MgO-Saturated SiO 2 S+L C (2912 F) Isothermal Section 80 Sea area (all liquid) 70 Land area (all solid) Swamp area (liquid + solid) CaO O 60 C 2 S + L Ca 2 SiO 4 70 C 2 S + L + M Ca 3 SiO 5 M+L Slags compatible with both dolomite and Magnesia-C refractories: MgO and CaO-Saturated P 50 M 2 S + L Mg 2 SiO 4 40 M 2 S + L + M MgO Figure 10. The 1600 C isothermal section for the CaO-MgO-SiO 2 system 17

18 % MgO dissolved in the Slag Slag O (Dual Saturated) Slag P % CaO/% SiO C 1600 C Figure 11. The solubility of MgO as a function of slag basicity in the CaO-MgO-SiO 2 system. The next table shows the decrease in CaO solubility when MgO is added to the CaO-SiO 2 and CaO-Al 2 O 3 binary systems until dual saturation is achieved: Table 10. The effect of MgO on the solubility of CaO at 1600 C CaO-SiO 2 CaO-MgO-SiO 2 CaO-Al 2 O 3 CaO-MgO-Al 2 O 3 Saturation CaO Saturation Dual Saturation CaO Saturation Dual Saturation % CaO % MgO % SiO % Al 2 O While dual saturation (CaO and MgO) is recommended for magnesia-based slaglines, it is not a requirement for dolomite slaglines where only CaO saturation is required. It is therefore possible to create low-mgo, but CaO-saturated slags, that could have considerable better desulfurization properties than the dual saturated slags. 18

19 The evaluation of the effect of CaF 2 on dual saturated steelmaking slags is difficult since very limited phase diagrams are available for these complex systems. The few diagrams that are available will be utilized but in most cases the phase relations will be inferred from the lower order systems. Consider the CaO-MgO-SiO2-CaF 2 system The inferred phase relations for this system at 1600 C are shown in the next figure. SiO 2 + CaF S+L Ca 2 SiO 4 70 Ca 3 SiO % CaF 2 8% CaF 2 12% CaF 2 Mg 2 SiO CaO Figure 12. The system CaO-MgO-SiO2-CaF 2 at 1600 C MgO This diagram shows the following important features: 1. An increase in CaO solubility as the CaF 2 content of the slag increases 2. A decrease in CaO solubility on the CaO-saturation curve as the MgO content of the slag increases towards dual saturation. 3. An increase in MgO solubility at dual saturation as the CaF 2 content of the slag increases. One of the most important features of this diagram is the increase in MgO solubility (at dual saturation) as the CaF 2 content of the slag increases. This has significant implications for magnesia-based slaglines. If fluorspar-containing slags are in contact with magnesia refractories, then significant refractory wear can occur if the slag is not 19

20 MgO or CaO saturated. If the slag is CaO-saturated but MgO-unsaturated ( creamy consistency), then the extent of refractory wear could be minimized even though the slag is not fully chemically compatible with the refractories. However, if the slag is also CaO unsaturated (very liquid or watery in consistency) then severe refractory wear can occur in just one heat. The above is true for any slag, CaF 2 -containing or not, but the presence of CaF 2 accelerates the wear because of its depression of the solidus temperature of the slag, which causes deeper penetration into the refractory matrix. Consider the CaO-MgO-Al 2 O 3 -CaF 2 system It was shown earlier in the CaO-Al 2 O 3 -CaF 2 system that the addition of CaF 2 did not increase the solubility of CaO. Similar phase relations are observed in the CaO-MgO- Al 2 O 3 -CaF 2 system, which is shown in the next figure. Figure 13. Phase diagram of the CaO-MgO-CaO-CaF 2 system at 10% Al 2 O 3 The compositions of the slags at dual saturation in the systems CaO-MgO-Al 2 O 3 and CaO-MgO-Al 2 O 3 -CaF 2 are shown in the next table. The addition of CaF 2 to the CaO- MgO-Al 2 O 3 system results in only a small increase in the solubility of CaO but a very large increase in the solubility of MgO in the slag. This diagram and table once again demonstrate the vulnerability of magnesia-based slagline refractories to CaF 2 -containing slags. The use of CaF 2 in high-al 2 O 3 slags therefore should be avoided if possible. The benefits of using CaF 2 as a cheap fluxing precursor might be offset by the potential of increased refractory wear. 20

21 Table 12. Slag compositions at dual saturation in the systems CaO-MgO-Al 2 O 3 and CaO-MgO-Al 2 O 3 -SiO 2 at 1600 C. CaO-MgO-Al 2 O 3 system CaO-MgO-Al 2 O 3 -CaF 2 system % CaO % MgO % Al 2 O % CaF 2 12 The extent of MgO-dissolution and refractory wear can be minimized if CaO-saturation is maintained at all times, but if the slag is both CaO and MgO unsaturated (very watery) then extensive refractory wear could occur. Summary and Conclusions The use of fluorspar in steelmaking is a controversial issue. A number of studies have shown that there are considerable environmental concerns regarding the use of fluorspar, and many plants has opted not to use fluorspar for this very reason. While fluorspar has been banned as a deliberate additive to the slags in these plants, the presence of fluorspar in mold fluxes has not been eliminated. This paper is not an attempt to justify the use of fluorspar in steelmaking but is rather geared to provide a better understanding on the behavior of CaF 2 for the steel plants that still use this component. Fluorspar can be very effective to increase the solubility of CaO in silicate slags but is not very effective to increase CaO solubility in aluminate slags. The method of fluorspar addition could have a big impact on the effectiveness of fluorspar to bring lime into solution and the amounts required to do so. Fluorspar should never be added in its pure form to a slag but rather in combination with lime and silica. Lime, silica, and CaF 2 mixtures are much more effective to go into solution than lime and CaF 2 mixtures. The addition of fluorspar to silicate and aluminate slags results in an increase in the solubility of MgO in the slag. This increase in MgO solubility could lead to significant refractory wear if additional MgO is not added to the slag or if CaO-saturation is not maintained at all times. Most steelmaking refining slags are not MgO-saturated, because only lime is typically available as an additive. Furthermore, the very high levels of MgO required for saturation might be undesirable from a steel quality perspective. High MgO slags in contact with steel with low oxygen content could result in Mg pickup in the steel and lead to spinel inclusion formation in the steel. Based on the discussion above, it is clear that dolomite refractories might be more compatible in contact with fluorspar containing slags than magnesia-based refractories. The simple reason is that lime saturation (a dolomite refractory requirement) is much easier to achieve in practical steelmaking than MgO, or dual saturation. 21