Can Fluorspar be replaced in steelmaking? By Eugene Pretorius Baker Refractories
I) Introduction 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 some 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 technical note 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. An attempt is made to provide a better understanding on the behavior of fluorspar in slags and then discuss possible alternative to fluorspar in steelmaking slags. II) The role of fluorspar in steelmaking slags Fluorspar is utilized for the following reasons: 1. To increase the solubility of CaO in the slag and hence improve desulfurization of the steel. 2. To act as a fluxing precursor in ladle and stainless steel reduction slags. 3. To maintain fluidity in the slag as the slag temperature decreases (VOD and ladle slags). In simple silicate slags, the solubility of CaO is limited by the precipitation of the very stable phase, Ca 2 SiO 4. The following figures of the CaO-SiO 2 system shows that once the saturation point of CaO has been reached at a specific temperature, the addition of more CaO to the slag will rapidly decrease the fluidity of the slag. It is important to note that it is dissolved lime in the liquid portion of the slag that desulfurizes the steel. The addition of more lime to a CaO-saturated slag, results in a rapid decrease in slag fluidity, which will negatively effect desulfurization. Figure 1. Phase diagram of the CaO-SiO 2 system 1 2
This diagram has the following important features: 1. The composition of the CaO-saturated liquid at 1600 C is: % CaO 56 % SiO 2 44 2. The equilibrium CaO-saturation phase in contact with this liquid is Ca 2 SiO 4, which has a melting point of 2130 C. 3. The solidus temperature of this liquid is about 1464 C. 4. The area of interest in this diagram has been circled and is shown in the next figure: Liquidus Boundary % CaO - 64 % SiO 2-36 C/S = 1.8 1600 C 2912 F C 2 S + L % CaO - 56 % SiO 2-44 C/S = 1.3 L 1460 C 2660 F CaO - Saturation Refractory compatible ( Creamy ) Figure 2. Enlarged area of the CaO-SiO 2 phase diagram This diagram shows the very small area of workable slags in this system. Slags with a basicity ratio (C/S) > 1.8 will be completely solid at steelmaking temperatures. The lever rule can be used to calculate the respective amounts of liquid and solid as shown in Figure 3. % Liquid 100 90 80 70 60 50 40 30 20 10 0 1.3 1.4 1.5 1.6 1.7 1.8 C/S Ratio %Liq Figure 3. % Liquid as a function of basicity (CaO/SiO 2 ) in the CaO-SiO 2 system at 1600 C 3
From the above discussion it is clear that the Ca 2 SiO 4 phase is limiting the solubility of CaO in the slag. The addition of any component to the slag that will dissolve (destabilize) Ca 2 SiO 4, will increase the solubility of CaO in the slag. In the following figure the effect of the components B 2 O 3, Al 2 O 3, CaF 2 and FeO on the Ca 2 SiO 4 stability field, is demonstrated at 1600 C. SiO 2 1600 C B 2 O 3 CaF 2 Al 2 O 3 FeO Ca 2 SiO 4 B 2 O 3 CaO Al 2 O 3 CaF 2 FeO Figure 4. The effect of different oxides on the liquidus phase relations of the CaO-SiO 2 system at 1600 C This figure clearly shows that B 2 O 3 is the most potent flux to bring Ca 2 SiO 4 into solution, followed by CaF 2, then Al 2 O 3 and finally FeO (in most steelmaking slags iron oxide is predominately present as Fe 2+ ) III) Considering B 2 O 3 as a flux. Figure 4 shows that the addition of B 2 O 3 to a CaO-SiO 2 slag will result in a rapid increase in the solubility of CaO. The increase in CaO content as the B 2 O 3 level increase is almost a linear relationship and can be approximated by the following equation at 1600 C: % CaO 1600 C = 1.1 * %B 2 O 3 + 56 (Applicable for B 2 O 3 levels up to 15%) 4
The effect of B 2 O 3 on the solubility of CaO and desulfurization is shown in the next table Table 1. The effect of B 2 O 3 on the solubility of CaO and desulfurization at 1600 C % CaO 56 61.5 67 % SiO 2 44 33.5 23 % B 2 O 3 5 10 Optical Basicity 0.691 0.711 0.733 Sulfide Capacity -3.119-2.822-2.516 Sulfur Distribution Coeff. 37.95 75.19 152.08 Final Sulfur (%) 0.0284 0.0200 0.0124 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) 100000 Initial Sulfur (%) 0.05 Oxygen Level in Steel (ppm) 15 B 2 O 3 and steelmaking concerns The stability of B is compared to a number of typical steel components in Table 2. Table 2. Thermodynamic stability of SiO 2, B 2 O 3, MnO, and Cr 2 O 3 at 1600 C Reaction G reaction at 1600 C (kj/mole) K eq at 1600 C Si + O 2 = SiO 2-576.102 1.16 x 10 16 4/3 B + O 2 = 2/3 B 2 O 3(l) -554.177 2.84 x 10 15 2 Mn + O 2 = 2 MnO -485.184 3.39 x 10 13 4/3 Cr + O 2 = 2/3 Cr 2 O 3-428.705 9.01 x 10 11 Table 2 shows that the stability of B is similar to that of Si and that a considerable amount of B could be dissolved in the steel under typical steelmaking conditions. While boron is a desirable element in some grades of steel, in other grades of steel it could be detrimental to the physical properties of the steel. The use of B 2 O 3 as a flux to increase the solubility of CaO would therefore be limited to boron-containing grades. Furthermore, the amount of B 2 O 3 that could be added to the slag will be limited by the amount of B that can be tolerated in the steel, and still care would be required, as control will not be easy. B 2 O 3 and Refractory concerns One of the biggest drawbacks of utilizing B 2 O 3 as flux in a slag, the potential for significant refractory erosion. B 2 O 3 is a more powerful flux than fluorspar to dissolve basic oxides (CaO & MgO) as evidenced by the very high solubility of the CaO and MgO in a pure B 2 O 3 liquid at 1600 C, which are 75% and 79%, respectively. 5
When B 2 O 3 is utilized as a fluxing component in slags in contact with doloma refractories, CaOsaturation is an important slag requirement, since fired doloma refractories are lime-bonded. It is also important to add the B 2 O 3 in a pre-mixed form because the addition of concentrated amounts of B 2 O 3 in one area of the slag could lead to significant localized refractory wear. CaO-saturation in the slag is achieved by adding lime to maintain a slag with a "creamy" consistency at all times. Maintaining MgO saturation in a slag is more difficult for a number of reasons. A MgO source such as doloma or magnesia is not always readily available as an additive to the slag. When doloma (~58 %CaO, 38% MgO) is used it is difficult to determine when the slag is MgO-saturated because the doloma addition can result in a slag with a "creamy" consistency that could be CaOsaturated but not MgO-saturated. Furthermore, for some grades of steel, slags with high MgO content is not desirable because of the potential of spinel (MgAl 2 O 4 ) inclusions in the steel. Any magnesia-based refractory could therefore be vulnerable to significant refractory wear if in contact with B 2 O 3 -containing slags. B 2 O 3 is used in mould fluxes as a fluxing agent for the CaO-SiO 2 basic system, but this is at much lower temperatures and the B 2 O 3 also has an effect on the overall crystallization tendency, which is important. IV) Considering CaF 2 as a flux After B 2 O 3, CaF 2 is the next strongest component to destabilize Ca 2 SiO 4 and increase the solubility of CaO in the slag. The phase diagram of the CaO-CaF 2 -SiO 2 system is shown in Figure 5. Figure 5. Phase diagram of the CaO-CaF 2 -SiO 2 system 1 6
The most striking feature of this diagram is the tremendous increase in the solubility of CaO, when CaF 2 is added to 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 high CaO solubility, as shown by point (a) on the diagram (1600 C). The composition of the slag at this point is approximately the following: % CaO 72 % SiO 2 17 % CaF 2 11 The saturation solubility of CaO at 1600 C in CaO-CaF 2 -SiO 2 system, is plotted as a function of SiO 2 content in Figure 6. 75 Slag (a) in Fig. 2 70 % CaO in Solution 65 60 55 50 10 15 20 25 30 35 40 % SiO2 Figure 6. 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, results in a decrease in CaO solubility along the CaO-saturation boundary. This is because the SiO 2 content of the slag is diluted to below 17%. Again this shows why the maximum amount of fluorspar that would ever be required in a slag is 12%. The addition of more CaF 2 would also result in an increase in fluidity that could lead to increased refractory erosion. Utilizing fluorspar as a fluxing precursor Fluorspar is often used as fluxing precursor in stainless steel reduction slags. If fluorspar is added just before the reduction mix it will melt immediately and create some liquid in the slag so that when the reductant is added it will be immersed in a partially liquid slag. The generation of a small amount of early liquid slag could greatly enhance the reduction efficiencies and kinetics (due to increased mass transfer/diffusion rates). The typical aim CaF 2 levels in the final slag should be about 3%, provided the slag contains considerable amounts of MgO (approximately 10%). If the MgO content of the final slag is less than 10% then higher CaF 2 levels might be required to obtain adequate dissolution rates. 7
Utilizing Fluorspar to increase the solubility of CaO in the slag Fluorspar is most commonly added to slag in order to increase the solubility of CaO in the slag, and hence improve the desulfurization capacity of the slag. In the previous discussion on B 2 O 3 it was shown that any addition of B 2 O 3 will increase the solubility of CaO. The same is not true for fluorspar in the CaO-SiO 2 -CaF 2 system. The liquidus boundaries in Figures 4 and 5 show that a significant increase in lime solubility will only occur when the CaF 2 content of the slag exceeds about 12% at 1600 C. Furthermore, the increase in CaO solubility at "constant" CaF 2 content is strongly dependent on the SiO 2 content of the slag. The CaO solubility only increases as the SiO 2 content of the slag is decreased. Figure 7 shows the solubility of CaO as a function of the CaF 2 /(SiO 2 + CaF 2 ) ratio in the slag. 75 70 12 % CaO - 72 % SiO2-17 % CaF2-11 % CaO at Saturation 65 60 55 50 45 40 0% CaF2 15 13 % CaO - 54 % SiO2-31 % CaF2-15 13 25 35 60 30 25 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 %CaF2/(%SiO2+%CaF2) 0% SiO2 Figure 7. The solubility of CaO as a function of the CaF 2 and SiO 2 content of the slag. The CaF 2 contents of each of the slags are shown on the figure. In more complex steelmaking slags that also contain considerable amounts of MgO, the "minimum" level of CaF 2 required to result in an increase in CaO solubility will probably be much less. The increase in CaO solubility with increasing CaF 2 content might even be linear, similar to B 2 O 3. CaF 2 is also a very good flux for MgO and any increase in the solubility of MgO, because of CaF 2 addition, will also increase CaO solubility. This is because an increase in MgO solubility will shift the dual saturation point downward towards the CaO-MgO boundary of the diagram. Furthermore, the presence of MgO in the slag limits the stability of Ca 2 SiO 4 by the formation of CaMg-silicate phases and therefore acts as a flux for Ca 2 SiO 4. 8
The phase relations for the CaO-MgO-SiO 2 -CaF 2 system at 1600 C were inferred from available CaF 2 -containing binary and ternary diagrams, and are shown in the next figure. SiO 2 + CaF 2 10 90 30 20 S+L 80 70 40 60 50 50 60 Ca 2 SiO 4 70 Ca 3 SiO 5 80 5% CaF 2 8% CaF 2 12% CaF 2 Mg 2 SiO 4 40 30 20 90 10 CaO 10 20 30 40 50 60 70 80 90 Figure 8. 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 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. For some stainless steel grades with very low sulfur specifications, a second reduction slag might be required in the converter. Typically a mixture of lime and fluorspar is utilized. From Figures 9
5-7 it is clear that the amount of residual slag in the vessel, in combination with the lime and fluorspar additions, can yield slags with high desulfurization capabilities. Utilizing fluorspar to maintain slag fluidity In VOD operations all the reduction slags stays in the ladle until the steel is cast. An important requirement is that the slag stays reasonably liquid down to casting temperatures to facilitate alloy and wire additions. When all the fluorspar is added in a single step during reduction for fluidity control, then extensive slagline refractory wear will occur. The preferred method is to add the fluorspar in steps after reduction as the slag cools, and only as needed. This could result in significant refractory performance improvements and also decreased fluorspar consumption. The effect of fluorspar in aluminate slags Fluorspar can be very effective in increasing the solubility of CaO in silicate slags but it is not very effective in terms of increasing CaO solubility in aluminate slags (discussed later). The only benefit fluorspar could have for Al-killed grades is that it could act as the fluxing precursor before the Al is added. In these grades fluorspar is not normally necessary because the reaction of CaO and Al 2 O 3 will form a liquid slag without high-melting intermediate phases such as Ca 2 SiO 4. The only intermediate phase that can form, Ca 3 Al 2 O 6, melts at 1535 C Fluorspar and steel quality concerns The elemental constituents of CaF 2, Ca and F, has a very low solubility in steel so that there are negligible interactions between CaF 2 in the slag and the steel. This is in contrast to B 2 O 3 and Al 2 O 3 where significant slag-metal interactions are possible. The lack of slag/metal interaction of the F - with the steel is probably one of the main reasons why fluorspar is so popular as a fluidizing agent. Fluorspar and Refractory concerns 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 important to note that it is not the presence of CaF 2 that causes 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 addition of fluorspar to silicate and aluminate slags also results in an increase in the solubility of MgO in the slag (Figure 8) 2. 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 10
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 saturation, or dual saturation. V) Consider Al 2 O 3 as a flux From Figure 3 it can be seen that Al 2 O 3 is the third "best" component to destabilize Ca 2 SiO 4 and increase the solubility of CaO. This figure also shows that a significant minimum amount of Al 2 O 3 would be required to result in an increase in CaO solubility at 1600 C. The increase in CaO solubility above the Al 2 O 3 threshold value is also linked to the SiO 2 content of the slag, similar to the case with CaF 2 (Figure 9). 64 62 % CaO - 62.2 % SiO2-9.6 % Al2O3-28.3 % CaO at saturation 60 58 56 54 0% Al2O3 % CaO - 50.3 % SiO2-23.2 % Al2O3-26.5 0% SiO2 52 50 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Al2O3/(SiO2+Al2O3) Figure 9. The solubility of CaO as Al 2 O 3 is replacing SiO 2 at 1600 C (2912 F) This figure shows that in the CaO-Al 2 O 3 -SiO 2 (CAS) system, the replacement of SiO 2 with Al 2 O 3 will initially result in a decrease in CaO solubility. A large increase in CaO solubility only occurs when the Al 2 O 3 content of the slag exceeds about 25% Al 2 O 3 and the SiO 2 content of the slag decreases from about 23% to 10% SiO 2. 11
Figure 9 clearly shows the interdependence of the CaO solubility on the SiO 2 and Al 2 O 3 levels of the slag. The impact of these relationships is very significant on stainless steel production. In stainless steelmaking the Al 2 O 3 in slag is generated by the partial replacement of FeSi by Al as a reductant, or FeSi containing high levels of Al. It is therefore very important to do an accurate mass-balance calculation to ensure proper Al/Si reductant ratios in order to achieve the target fluidity and desired CaO solubility. From the evaluation of the CaO-SiO 2 -CaF 2 and CaO-Al 2 O 3 -SiO 2 systems, it is clear that Al 2 O 3 is not as potent as CaF 2 to bring lime into solution, and considerably higher levels of Al 2 O 3 would be required in the slag to get the same amount of CaO into solution. Most steelmaking slags also contain MgO so that consideration of the phase relations in the CaO- MgO-Al 2 O 3 -SiO 2 (CMAS) system is very important. Fortunately, this system is well studied and the "ternary isoplethal sections" at constant MgO and Al 2 O 3 content are available. Evaluation of the CaO-MgO-Al 2 O 3 -SiO 2 system at constant MgO levels Figure 10 shows the saturation levels of CaO at 1600 C as a function of the Al 2 O 3 /(SiO 2 + Al 2 O 3 ) ratio for slags containing MgO levels of 0%, 5% and 10%, respectively. % CaO at Saturation 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 0% MgO 5% MgO 10% MgO 5 10 10 % Al 2 O 3 = 15 15 20 19 22.5 (A) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 25 25 23 (B) % Al 2 O 3 /(% SiO 2 + % Al 2 O 3 ) 26.5 30 27 28.3 39 37 Figure 10. The solubility of CaO as a function of Al 2 O 3 content at 1600 C (The actual Al 2 O 3 levels of the slags are indicated in the figure) 12
This figure has the following important features: The solubility of CaO initially decreases as Al 2 O 3 replaces SiO 2 as flux. For slags with a (%Al 2 O 3 /(%SiO 2 + %Al 2 O 3 ) ratio < 0.5 the solubility of CaO decreases as the MgO content of the slag increases. However, at ratio of about 0.5, the amount of CaO in solution is the same for MgO levels from 0-10%, indicating that MgO is acting as flux in this composition region, because total base in solution (MgO + CaO) increased. In practical terms, the %CaO/(%SiO 2 + %Al 2 O 3 ) ratio in the slag increases significantly as the MgO content of the slag increases from 0 10% at a (%Al 2 O 3 /(%SiO 2 + %Al 2 O 3 ) ratio of about 0.5. The minimum threshold Al 2 O 3 level required in the slag before an increase in CaO solubility is realized, decreases with increasing MgO content in the slag. At 0% MgO the minimum Al 2 O 3 threshold value is about 27%, for 5% MgO the Al 2 O 3 level required is about 22%, and for 10% MgO the Al 2 O 3 level required is about 15%. In all these cases the solubility of CaO only increases when the SiO 2 content of the slag is diluted. For example consider slags (A) and (B) in Figure 10 that contain 22.5 and 23% Al 2 O 3, respectively (5% MgO). Slag (A) contains 22.5% SiO 2 and only has 50% CaO in solution, whereas slag (b), which contains only 12% SiO 2, has 59% CaO in solution. This has a significant implication in terms of desulfurization. Not only are high levels of Al 2 O 3 required in the slag (>22%), but the SiO 2 level should be below 15%. These lower levels of SiO 2, together with the higher levels of [Al], will result in a decreased oxygen potential in the steel. For slags containing 10% MgO, the solubility of CaO increases when the Al 2 O 3 content of the slag exceeds about 15%. However, at an Al 2 O 3 level of approximately 20%, the solubility of CaO decreases, as the slags are now MgO saturated. From the quaternary system it can be determined that the maximum MgO level of the slag should be below 7.5 % to obtain the high CaO solubilities shown in Figure 10. Evaluation of the CaO-MgO-Al 2 O 3 -SiO 2 system at constant Al 2 O 3 levels The slags and phase relations discussed so far were all at a temperature of 1600 C (2912 F). However, it is also important to consider the phase relations at higher temperature since some steelmaking process routes operate at much higher temperatures. For example, in stainless steelmaking operations the reduction temperatures are typically around 1700 C (3092 F), while the typical tapping temperatures from the stainless steel vessel are much lower (60-100 ). This large difference in the end of reduction temperature and the tapping temperature causes significant problems in engineering refractory compatible slags that are also "workable" in the stainless steel vessel. A slag that is designed to be liquid and compatible with the refractories at 1700 C might become too stiff at tapping temperatures. Alternatively, a slag designed to stay liquid down to tapping temperatures could cause significant refractory wear during reduction step when temperatures and turbulence in the vessel are high. Some slag regions in the CaO-MgO-SiO 2 - Al 2 O 3 system further intensify this problem, as will be demonstrated by the following discussion. In the CaO-MgO-SiO 2 system, and the CaO-MgO-SiO 2 -Al 2 O 3 system, MgO is acting as fluxing component until the MgO saturation boundary is reached. For example in the CaO-SiO 2 system, the solubility of basic oxide (CaO) is about 56% CaO at 1600 C, whereas in the CaO-MgO-SiO 2, the solubility of basic oxides (CaO + MgO) is about 61% at dual saturation (saturated with both CaO & MgO). Figure 11 shows the effect of MgO on the total base solubility (CaO + MgO) in the CMAS system. A significant shrinkage of the Ca 2 SiO 4 stability field can be observed as the MgO content of the slag increases. Also important to note is that the solubility of the basic oxides (primarily CaO) is much higher in the stability areas of Ca 3 SiO 5 and lime (CaO). 13
The slag compositions that are of particular interest in this system are those that are dual saturated with respect to both MgO and CaO. The CaO-saturation phase could be Ca 2 SiO 4, Ca 3 SiO 5, or CaO depending on the temperature and Al 2 O 3 content. Figure 11. Saturation lines of CaO, Ca 2 SiO 4, and Ca 3 SiO 5 in the system CaO-MgO-SiO 2 -Al 2 O 3 at 1600 C and for MgO contents up to 16%. 1 Phase relations at the 10% Al 2 O 3 plane For slags containing less than 10% Al 2 O 3, the phase relations are similar at 1600 C and 1700 C, i.e., the solubility of CaO decreases with increasing Al 2 O 3 content (Figure 10). However, for Al 2 O 3 levels at, and greater than about 10%, an small area of high CaO solubility opens up at 1700 C as shown in Figure 12. Three slags of particular interest on this diagram are labeled a, b, and c and their compositions are listed in the table below. Table 3. Slag compositions in the CaO-MgO-SiO 2 -Al 2 O 3 system at 1600 and 1700 C Slag (a) Slag (b) Slag (c) Temperature 1700 C 1700 C 1600 C % CaO 59 50.5 46 % MgO 11 12.5 13 % SiO 2 20 27 31 % Al 2 O 3 10 10 10 Equilibrium phases on the liquidus boundary Ca 3 SiO 5 + MgO Ca 2 SiO 4 + MgO Ca 2 SiO 4 + MgO 14
SiO 2 10% Al 2 O 3 50% 1600 C 60% c a b 1700 C CaO 10% 20% MgO Figure 12. Isothermal sections of the CaO-MgO-SiO 2 -Al 2 O 3 system through the 10% Al 2 O 3 plane and temperatures of 1600 and 1700 C Slags (a) and (b) have similar Al 2 O 3 and MgO levels, but show a significant increase in CaO solubility as the SiO 2 content decreases from 27% (slag b) to 20% (slag a). This further highlights the importance of careful mass-balance calculations of reductant and alloy additives to obtain the desired slag compositions. If slag (a) was targeted for desulfurization reasons at high temperatures, it will become very stiff and unworkable at the 1600 C because the CaO solubility decreased from 59% to 46% over a 100 C interval. Furthermore, this small window of high CaO solubility at 1700 C is only present in a very small MgO range (11-12%). It is very difficult to control the MgO content of the slag that accurately under real steelmaking conditions. Phase relations at the 15% Al 2 O 3 plane The phase relations in the CMAS system in the 15% Al 2 O 3 plane are shown in Figure 13. At this Al 2 O 3 level the window of high CaO solubility at 1700 C has opened up considerably, but a small window of high CaO solubility is now evident at 1600 C. On the 1700 C isotherm, the CaOsaturated phase in equilibrium with the liquid changes from Ca 2 SiO 4, to Ca 3 SiO 5 (g to h) and finally to CaO (h to d). At point (d) the slag is dual saturated with respect to CaO and MgO. 15
SiO 2 15% Al 2 O 3 1600 C 1700 C 50% 60% f g h e d CaO 10% 20% MgO Figure 13. Isothermal sections of the CaO-MgO-SiO 2 -Al 2 O 3 system through the 10% Al 2 O 3 plane and temperatures of 1600 and 1700 C Figure 12 and 13 show that MgO has a very important overall fluxing effect in the system by opening a "window" of slags with high CaO solubility. However, from a more detailed perspective, the solubility of CaO actually decreases with increasing MgO content on the 1700 C liquidus isotherm where lime is the equilibrium solid phase (points (h) to (d)). The following table is comparison of CaO-saturated slags in the CAS system and the CMAS system (15% Al 2 O 3 ) at 1600 and 1700 C. This table shows that at 1700 C and 15% Al 2 O 3, the right combination of MgO and SiO 2 can result in slags with a high dissolved CaO content at Al 2 O 3 levels much lower than in the CAS system (27.7% Al 2 O 3 ). Table 4. The maximum solubility of CaO in the CAS and CMAS systems Temperature 1600 C 1600 C (e) 1600 C (f) 1700 C 1700 C System CAS CMAS CMAS CAS CMAS % CaO 62.5 58 46 64.4 62 % MgO 10 13 7.5 % SiO 2 9.9 17 26 12.5 16 % Al 2 O 3 27.7 15 15 23.2 15 16
While the "slag window" for high CaO slags at 1700 C is not that sensitive to MgO levels (5-12%), it is much more sensitive at 1600 C (9-11% MgO). The following conclusion can be drawn from the above diagrams: Good desulfurizing slags (high dissolved CaO content) can be generated with slags containing low levels of Al 2 O 3 (12-15%) at 1700 C, and to some extent at 1600 C. However, the fluidity of these slags are very sensitive to changes in MgO contents and could became very stiff at lower temperatures (1600 C) if the composition of the slag is not carefully controlled. These diagrams further show that if the slags are designed to be liquid at 1600 C (point f in Figure 13), then significant refractory wear of doloma based refractories can occur if these slags are in contact with the brick at 1700 C or higher. The bonding phase in fired doloma refractories is lime (CaO) and Figure 13 and Table 4 show that the solubility of CaO increases from 46% to 62% for a temperature increase from 1600 C to 1700 C. Phase relations at the 20% and 25 Al 2 O 3 planes The small "windows" of slags areas that had high CaO-solubilities at 10% and 15% Al 2 O 3 have "opened" significantly at 20 Al 2 O 3 and opened completely at 25% Al 2 O 3 (Figure 14 and 15). SiO 2 20% Al 2 O 3 1600 C 50% 1700 C 60% CaO 10% 20% Figure 14. Isothermal sections of the CaO-MgO-SiO 2 -Al 2 O 3 system through the 20% Al 2 O 3 plane and temperatures of 1600 and 1700 C MgO 17
SiO 2 25% Al 2 O 3 1600 C 1700 C 50% 60% i j CaO 10% 20% Figure 15. Isothermal sections of the CaO-MgO-SiO 2 -Al 2 O 3 system through the 25% Al 2 O 3 plane and temperatures of 1600 and 1700 C MgO These figures clearly show that the combination of MgO and Al 2 O 3 results in a "shrinkage" of the Ca 2 SiO 4 stability field which opens up an area of slags with high CaO solubility. However, on the lime-saturated liquidus boundaries, the solubility of CaO decreases with increasing MgO content. For example, on the 1700 C liquidus boundary the solubility of CaO decreases from about 64% (0% MgO at point (i)) to about 56% where the slag is dual saturated (11% MgO at point (j)). Another interesting feature of these diagrams is the MgO content of the slag at dual saturation for the various Al 2 O 3 levels. The solubility of MgO (at dual saturation) initially decreased with increasing Al 2 O 3 content up to about 20% Al 2 O 3, but then increases again at higher Al 2 O 3 levels. 18
Utilizing Al 2 O 3 to increase the solubility of CaO in the slag The discussion of the phase relations in the CAS and CMAS systems have clearly shown that Al 2 O 3 could be utilized to increase the solubility of CaO in the slag. While it is theoretically possible to generate slags with a high dissolved CaO content at Al 2 O 3 levels as low as 15% at 1700 C it would be difficult to consistently generate these slags under real steelmaking conditions. More realistic Al 2 O 3 levels in the slag should be 20-25% Al 2 O 3 (preferably 25%) in order to generate slags with good fluidity at lower temperatures. The diagrams of the CMAS system also showed the importance of MgO as a fluxing component to generate slags with high dissolved lime contents. The MgO content of the slag is very important in this system and should be controlled in a very tight range (8-11% MgO). Too low or too high MgO levels could result in very stiff slags with poor desulfurizing properties. The discussion of the diagrams also demonstrated the importance of the SiO 2 content of the slag and its relationship with Al 2 O 3 on the solubility of CaO. The solubility of CaO increases rapidly as the SiO 2 content of the slags is diluted at constant Al 2 O 3 (Figures 10, 14 and 15). Al 2 O 3 and steel quality concerns In practical steelmaking, the levels of Al 2 O 3 required to generate slags with high CaO solubilities is >20% Al 2 O 3. This means that the bulk of the steel deoxidant or reductant should be Al. Partial replacement of FeSi by Al as a reductant in stainless steelmaking will be ineffective to increase the solubility of CaO. Another important factor is the SiO 2 content of the slag. The SiO 2 that is transferred from the EAF slag, together with the transfer Si and Si in alloy additions, must be considered to determine the Al required in the reduction mix to ensure adequate SiO 2 dilution (<15%). The use of Al as a deoxidizer and reductant, and the resulting high Al 2 O 3 slags, will have a large effect on the internal quality of the steel. The residual Al levels in the steel and the resultant lower dissolved oxygen level will have a significant impact on the inclusion chemistry and the timing of Al 2 O 3 precipitation. The previous discussion on the CMAS system clearly demonstrated the importance of MgO as flux in this system and that between 6 and 9% MgO would be required to enhance lime dissolution. Unfortunately, the maximum solubility of MgO in these high Al 2 O 3 slags is low (<11% MgO) so that slags with high MgO activities will be exposed to Al under fairly reducing conditions. The potential for Mg reduction and spinel formation is a real possibility. Al 2 O 3 and Refractory concerns The replacement of FeSi by Al in stainless steelmaking will result in higher reduction temperatures, which will require more coolant additions. If these coolants are not added and high temperatures (>1700 C) are prevalent during the reduction step, then significant refractory wear of doloma refractories can occur with slags with intermediate Al 2 O 3 levels (10-20% Al 2 O 3 ). The addition of sufficient lime to the slag to protect the refractories will result in liquid compatible slags at high temperatures but very stiff "unworkable" slags at lower temperatures. It is much easier to design refractory compatible and "workable" slags if the Al 2 O 3 content of the slag is between 25 and 30% Al 2 O 3. These slags are less sensitive to variations in MgO levels and the 19
1600 and 1700 C liquidus isotherms are closer together resulting in reasonable slag fluidity as the slag cools down. The following table summarizes the recommended target slag compositions in the CaO-MgO- Al 2 O 3 -SiO 2 system. Table 5. Target slag compositions and ranges in the CMAS system Al 2 O 3 range Comments 0 10% Al 2 O 3 No benefits in terms of CaO solubility. CaO solubility actually decreases as Al is replacing FeSi 10-20% Al 2 O 3 A very large increase in CaO solubility occurs in very specific slag areas. These slags are difficult to obtain and control under real steelmaking conditions. These slags could lead to significant refractory wear or alternatively very stiff unworkable slags at lower temperatures. It is best to avoid this slag composition range! 20 30% Al 2 O 3 The ideal target range is 25 to 30% Al 2 O 3. These slags can be designed to be refractory compatible with reasonable fluidity at lower temperatures VI) Summary and Conclusions This technical note has attempted to provide a better understanding on the effect of the fluxing components CaF 2, B 2 O 3, and Al 2 O 3 on the solubility of CaO in steelmaking slags. Fluorspar is by far the most convenient component to use as a fluxing precursor and additive to increase the solubility of CaO in the slag. The maximum levels of CaF 2 that would be required to obtain the maximum CaO solubility is about 12% CaF 2. Most operations could operate with slags with much lower CaF 2 levels, provided that some MgO is present in the slag (> 6% MgO). An important consideration is that the effect of CaF 2 on steel quality is negligible and it is practically inert to the steel. Refractory compatible slags can be designed for fluorspar-containing slags at high temperatures that will still maintain reasonable fluidity at lower temperatures. B 2 O 3 is the most potent of all three fluxes considered, not only in terms CaO solubility, but also in terms of refractory wear. The addition of B 2 O 3 as a flux is only an option in B-containing steel grades and great care should be exercised to ensure refractory compatibility. Alumina is a major slag component (> 25% Al 2 O 3 ) in Al-killed grades and is very effective to bring lime into solution and to generate good desulfurizing slags. Magnesia-carbon refractories are typically used with these slags and with good results because the solubility of MgO in these slags is fairly low (< 11% MgO). The use of Al 2 O 3 as a flux (prefused Ca-Aluminate) in low-c Si-killed steel grades is common and very effective. However, in high-c grades the level of Al 2 O 3 that can be tolerated in the slag is low (<10 % Al 2 O 3 ) because of castability issues (clogging). At these low levels, Al 2 O 3 is actually worse than SiO 2 to dissolve CaO and the only benefit is that it can act as a fluxing precursor if added as prefused Ca-Aluminate. Unfortunately, these high-c steel grades also have low sulfur requirements. In most cases a combination of SiO 2 and CaF 2 is used as fluxing additives to these grades to create slags with good desulfurization properties 20
without negative castability effects. The only alternative to using CaF 2 in the slags of these grades is to use scrap with very low sulfur levels. Fluorspar is commonly used in combination with FeSi in stainless steel operations as a fluxing precursor during the reduction step, and to increase the solubility of lime in the final reduction slag. The elimination of CaF 2 and the partial replacement of FeSi by Al will not be effective in improving reduction kinetics and increasing CaO solubility. For Al 2 O 3 to be effective in these slags, Al should be the bulk reductant addition and the FeSi addition and SiO 2 content of the slag should be carefully controlled. For these aluminate slags to be equivalent to a 8-10% CaF 2 - containing silicate slags in terms of lime dissolution and solubility, the Al 2 O 3 content of the slag should be around 25% Al 2 O 3. VII) References 1. Slag Atlas, edited by Verein Deutscher Eisenhüttenleute (VDEh). Verlag Stahleisen GmbH 2. Pretorius, E.B. "The effect fluorspar in steelmaking slags". Unpublished technical document. 21