Can Fluorspar be replaced in steelmaking?

By

Eugene PretoriusBaker Refractories

2

I) IntroductionThe use of fluorspar in steelmaking is a controversial issue. A number of studies have shown thatthere are considerable environmental concerns regarding the use of fluorspar, and some plants hasopted not to use fluorspar for this very reason. While fluorspar has been banned as a deliberateadditive to the slags in these plants, the presence of fluorspar in mold fluxes has not beeneliminated. This technical note will not address any of the environmental concerns regarding theuse of fluorspar but will only focus on the technical aspects of this component in steelmaking. Anattempt is made to provide a better understanding on the behavior of fluorspar in slags and thendiscuss possible alternative to fluorspar in steelmaking slags.

II) The role of fluorspar in steelmaking slagsFluorspar 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 stablephase, Ca2SiO4. The following figures of the CaO-SiO2 system shows that once the saturationpoint of CaO has been reached at a specific temperature, the addition of more CaO to the slag willrapidly decrease the fluidity of the slag. It is important to note that it is dissolved lime in the liquidportion 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-SiO2 system1

3

This diagram has the following important features:

1. The composition of the CaO-saturated liquid at 1600°C is:% CaO – 56% SiO2 – 44

2. The equilibrium CaO-saturation phase in contact with this liquid is Ca2SiO4, which has amelting 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:

1600°C2912°F

% CaO - 64% SiO2 - 36C/S = 1.8

% CaO - 56% SiO2 - 44C/S = 1.3

C2S+L

L

1460°C2660°F CaO - Saturation

Refractory compatible (“Creamy”)

Liquidus Boundary

Figure 2. Enlarged area of the CaO-SiO2 phase diagram

This diagram shows the very small area of “workable” slags in this system. Slags with a basicityratio (C/S) > 1.8 will be completely solid at steelmaking temperatures. The lever rule can be usedto calculate the respective amounts of liquid and solid as shown in Figure 3.

0102030405060708090

100

1.3 1.4 1.5 1.6 1.7 1.8

C/S Ratio

% L

iqui

d

% Liq

0102030405060708090

100

1.3 1.4 1.5 1.6 1.7 1.8

C/S Ratio

% L

iqui

d

% Liq

Figure 3. % Liquid as a function of basicity (CaO/SiO2) in theCaO-SiO2 system at 1600°C

4

From the above discussion it is clear that the Ca2SiO4 phase is limiting the solubility of CaO in theslag. The addition of any component to the slag that will dissolve (destabilize) Ca2SiO4, willincrease the solubility of CaO in the slag. In the following figure the effect of the componentsB2O3, Al2O3, CaF2 and FeO on the Ca2SiO4 stability field, is demonstrated at 1600°C.

CaO

B2O3

Al2O3

CaF2

FeO

SiO2

B2O3

CaF2Al2O3FeO

1600°C

Ca2SiO4

Figure 4. The effect of different oxides on the liquidus phase relations of theCaO-SiO2 system at 1600°C

This figure clearly shows that B2O3 is the most potent flux to bring Ca2SiO4 into solution,followed by CaF2, then Al2O3 and finally FeO (in most steelmaking slags iron oxide ispredominately present as Fe2+)

III) Considering B2O3 as a flux.Figure 4 shows that the addition of B2O3 to a CaO-SiO2 slag will result in a rapid increase in thesolubility of CaO. The increase in CaO content as the B2O3 level increase is almost a linearrelationship and can be approximated by the following equation at 1600°C:

% CaO1600°C = 1.1 * %B2O3 + 56 (Applicable for B2O3 levels up to 15%)

5

The effect of B2O3 on the solubility of CaO and desulfurization is shown in the next table

Table 1. The effect of B2O3 on the solubility of CaO and desulfurization at 1600°C

% CaO 56 61.5 67% SiO2 44 33.5 23% B2O3 5 10Optical Basicity 0.691 0.711 0.733Sulfide Capacity -3.119 -2.822 -2.516Sulfur Distribution Coeff. 37.95 75.19 152.08Final Sulfur (%) 0.0284 0.0200 0.0124

Optical basicity, sulfide capacity correlations, and thermodynamic data were used to calculate thefinal sulfur in the steel. The following parameters were considered in the calculation:

Temperature (°C) 1600Slag Amount (kg) 2000Metal Amount (kg) 100000Initial Sulfur (%) 0.05Oxygen Level in Steel (ppm) 15

B2O3 and steelmaking concerns

The stability of B is compared to a number of typical steel components in Table 2.

Table 2. Thermodynamic stability of SiO2, B2O3, MnO, and Cr2O3 at 1600°C

Reaction ∆∆∆∆G° reaction at 1600°C (kJ/mole) Keq at 1600°CSi + O2 = SiO2 -576.102 1.16 x 1016

4/3 B + O2 = 2/3 B2O3(l) -554.177 2.84 x 1015

2 Mn + O2 = 2 MnO -485.184 3.39 x 1013

4/3 Cr + O2 = 2/3 Cr2O3 -428.705 9.01 x 1011

Table 2 shows that the stability of B is similar to that of Si and that a considerable amount of Bcould be dissolved in the steel under typical steelmaking conditions. While boron is a desirableelement in some grades of steel, in other grades of steel it could be detrimental to the physicalproperties of the steel. The use of B2O3 as a flux to increase the solubility of CaO would thereforebe limited to boron-containing grades. Furthermore, the amount of B2O3 that could be added tothe slag will be limited by the amount of B that can be tolerated in the steel, and still care would berequired, as control will not be easy.

B2O3 and Refractory concernsOne of the biggest drawbacks of utilizing B2O3 as flux in a slag, the potential for significantrefractory erosion. B2O3 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 B2O3 liquid at1600°C, which are 75% and 79%, respectively.

6

When B2O3 is utilized as a fluxing component in slags in contact with doloma refractories, CaO-saturation is an important slag requirement, since fired doloma refractories are lime-bonded. It isalso important to add the B2O3 in a pre-mixed form because the addition of concentrated amountsof B2O3 in one area of the slag could lead to significant localized refractory wear. CaO-saturationin 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 sourcesuch as doloma or magnesia is not always readily available as an additive to the slag. Whendoloma (~58 %CaO, 38% MgO) is used it is difficult to determine when the slag is MgO-saturatedbecause the doloma addition can result in a slag with a "creamy" consistency that could be CaO-saturated but not MgO-saturated. Furthermore, for some grades of steel, slags with high MgOcontent is not desirable because of the potential of spinel (MgAl2O4) inclusions in the steel. Anymagnesia-based refractory could therefore be vulnerable to significant refractory wear if in contactwith B2O3-containing slags. B2O3 is used in mould fluxes as a fluxing agent for the CaO-SiO2basic system, but this is at much lower temperatures and the B2O3 also has an effect on the overallcrystallization tendency, which is important.

IV) Considering CaF2 as a fluxAfter B2O3, CaF2 is the next strongest component to destabilize Ca2SiO4 and increase thesolubility of CaO in the slag. The phase diagram of the CaO-CaF2-SiO2 system is shown in Figure5.

Figure 5. Phase diagram of the CaO-CaF2-SiO2 system1

7

The most striking feature of this diagram is the tremendous increase in the solubility of CaO, whenCaF2 is added to CaO-SiO2 slags, or when SiO2 is added to CaO-CaF2 slags. The combined effectof SiO2 and CaF2 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 % SiO2 – 17 % CaF2 – 11

The saturation solubility of CaO at 1600°C in CaO-CaF2-SiO2 system, is plotted as a function ofSiO2 content in Figure 6.

50

55

60

65

70

75

10 15 20 25 30 35 40

% SiO2

% C

aO in

Sol

utio

n

Slag (a) in Fig. 2

Figure 6. Solubility of CaO as a function of SiO2 content inCaO-CaF2-SiO2 slags at 1600°C

The maximum in CaO solubility is at about 12% CaF2 in the slag. The addition of more CaF2 tothe slag, results in a decrease in CaO solubility along the CaO-saturation boundary. This isbecause the SiO2 content of the slag is diluted to below 17%. Again this shows why the maximumamount of fluorspar that would ever be required in a slag is 12%. The addition of more CaF2would also result in an increase in fluidity that could lead to increased refractory erosion.

Utilizing fluorspar as a fluxing precursorFluorspar is often used as fluxing precursor in stainless steel reduction slags. If fluorspar is addedjust before the reduction mix it will melt immediately and create some liquid in the slag so thatwhen the reductant is added it will be immersed in a partially liquid slag. The generation of asmall amount of early liquid slag could greatly enhance the reduction efficiencies and kinetics (dueto increased mass transfer/diffusion rates). The typical aim CaF2 levels in the final slag should beabout 3%, provided the slag contains considerable amounts of MgO (approximately 10%). If theMgO content of the final slag is less than 10% then higher CaF2 levels might be required to obtainadequate dissolution rates.

8

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 B2O3 itwas shown that any addition of B2O3 will increase the solubility of CaO. The same is not true forfluorspar in the CaO-SiO2-CaF2 system. The liquidus boundaries in Figures 4 and 5 show that asignificant increase in lime solubility will only occur when the CaF2 content of the slag exceedsabout 12% at 1600°C. Furthermore, the increase in CaO solubility at "constant" CaF2 content isstrongly dependent on the SiO2 content of the slag. The CaO solubility only increases as the SiO2content of the slag is decreased. Figure 7 shows the solubility of CaO as a function of theCaF2/(SiO2 + CaF2) ratio in the slag.

25

30

35

40

45

50

55

60

65

70

75

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

%CaF2/(%SiO2+%CaF2)

% C

aO a

t Sat

urat

ion

0% CaF2

0% SiO2

% CaO - 72% SiO2 -17% CaF2 -11

% CaO - 54% SiO2 -31% CaF2 -15

15

1313

12

25

60

Figure 7. The solubility of CaO as a function of the CaF2 and SiO2 content of the slag.The CaF2 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 CaF2 required to result in an increase in CaO solubility will probably be muchless. The increase in CaO solubility with increasing CaF2 content might even be linear, similar toB2O3. CaF2 is also a very good flux for MgO and any increase in the solubility of MgO, becauseof CaF2 addition, will also increase CaO solubility. This is because an increase in MgO solubilitywill 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 Ca2SiO4 by the formation ofCaMg-silicate phases and therefore acts as a flux for Ca2SiO4.

9

The phase relations for the CaO-MgO-SiO2-CaF2 system at 1600°C were inferred from availableCaF2-containing binary and ternary diagrams, and are shown in the next figure.

9080705040302010 60

10

20

30

40

90

80

70

������������

6 0

5 0

70

50

6 0

40

30

20

80

90

10

������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������

������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������

S + L

Ca2SiO4

Mg2SiO4

Ca3SiO5

SiO2 + CaF2

MgOCaO

5% CaF2

8% CaF2

12% CaF2

Figure 8. The system CaO-MgO-SiO2-CaF2 at 1600°C

This diagram shows the following important features:

1. An increase in CaO solubility as the CaF2 content of the slag increases2. 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 CaF2 content of the slag increases.

One of the most important features of this diagram is the increase in MgO solubility (at dualsaturation) as the CaF2 content of the slag increases. This has significant implications formagnesia-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 isCaO-saturated but MgO-unsaturated (“creamy” consistency), then the extent of refractory wearcould 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 severerefractory wear can occur in just one heat. The above is true for any slag, CaF2-containing or not,but the presence of CaF2 accelerates the wear because of its depression of the solidus temperatureof the slag, which causes deeper penetration into the refractory matrix.For some stainless steel grades with very low sulfur specifications, a second reduction slag mightbe required in the converter. Typically a mixture of lime and fluorspar is utilized. From Figures

10

5-7 it is clear that the amount of residual slag in the vessel, in combination with the lime andfluorspar 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 importantrequirement is that the slag stays reasonably liquid down to casting temperatures to facilitate alloyand wire additions. When all the fluorspar is added in a single step during reduction for fluiditycontrol, then extensive slagline refractory wear will occur. The preferred method is to add thefluorspar in steps after reduction as the slag cools, and only as needed. This could result insignificant 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 veryeffective in terms of increasing CaO solubility in aluminate slags (discussed later). The onlybenefit fluorspar could have for Al-killed grades is that it could act as the fluxing precursor beforethe Al is added. In these grades fluorspar is not normally necessary because the reaction of CaOand Al2O3 will form a liquid slag without high-melting intermediate phases such as Ca2SiO4. Theonly intermediate phase that can form, Ca3Al2O6, melts at 1535°C

Fluorspar and steel quality concerns

The elemental constituents of CaF2, Ca and F, has a very low solubility in steel so that there arenegligible interactions between CaF2 in the slag and the steel. This is in contrast to B2O3 andAl2O3 where significant slag-metal interactions are possible. The lack of slag/metal interaction ofthe F- with the steel is probably one of the main reasons why fluorspar is so popular as a fluidizingagent.

Fluorspar and Refractory concerns

In the previous discussion it was clearly shown that fluorspar in combination with SiO2 is a verypotent flux to bring CaO into solution. If lime is added to the slag until the slag is CaO-saturatedthere will be minimal refractory wear on lime-bonded (dolomite) refractories. However, ifadditional lime for saturation was not added, the presence of fluorspar in the slag could lead toaccelerated refractory wear. This slag will have a lower viscosity, a lower solidus temperature anda high capacity to bring lime into solution and will lead to a deeper slag penetration into therefractory and increased refractory wear.

It is important to note that it is not the presence of CaF2 that causes refractory wear in CaO-bondedrefractories but the lack of lime saturation. A very liquid silicate or aluminate slag that is CaOunsaturated, and contains no CaF2, will also be very aggressive to the refractories.

The addition of fluorspar to silicate and aluminate slags also results in an increase in the solubilityof MgO in the slag (Figure 8)2. This increase in MgO solubility could lead to significantrefractory wear if additional MgO is not added to the slag or if CaO-saturation is not maintained atall times. Most steelmaking refining slags are not MgO-saturated, because only lime is typically

11

available as an additive. Furthermore, the very high levels of MgO required for saturation mightbe undesirable from a steel quality perspective. High MgO slags in contact with steel with lowoxygen content could result in Mg pickup in the steel and lead to spinel inclusion formation in thesteel. Based on the discussion above, it is clear that dolomite refractories might be morecompatible in contact with fluorspar containing slags than magnesia-based refractories. Thesimple reason is that lime saturation (a dolomite refractory requirement) is much easier to achievein practical steelmaking than MgO saturation, or dual saturation.

V) Consider Al2O3 as a flux

From Figure 3 it can be seen that Al2O3 is the third "best" component to destabilize Ca2SiO4 andincrease the solubility of CaO. This figure also shows that a significant minimum amount ofAl2O3 would be required to result in an increase in CaO solubility at 1600°C. The increase in CaOsolubility above the Al2O3 threshold value is also linked to the SiO2 content of the slag, similar tothe case with CaF2 (Figure 9).

50

52

54

56

58

60

62

64

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Al2O3/(SiO2+Al2O3)

% C

aO a

t sat

urat

ion

0% Al2O3

0% SiO2

% CaO - 50.3% SiO2 - 23.2% Al2O3 - 26.5

% CaO - 62.2% SiO2 - 9.6% Al2O3 - 28.3

Figure 9. The solubility of CaO as Al2O3 is replacing SiO2 at 1600°C (2912°F)

This figure shows that in the CaO-Al2O3-SiO2 (CAS) system, the replacement of SiO2 with Al2O3will initially result in a decrease in CaO solubility. A large increase in CaO solubility only occurswhen the Al2O3 content of the slag exceeds about 25% Al2O3 and the SiO2 content of the slagdecreases from about 23% to 10% SiO2.

12

Figure 9 clearly shows the interdependence of the CaO solubility on the SiO2 and Al2O3 levels ofthe slag. The impact of these relationships is very significant on stainless steel production. Instainless steelmaking the Al2O3 in slag is generated by the partial replacement of FeSi by Al as areductant, or FeSi containing high levels of Al. It is therefore very important to do an accuratemass-balance calculation to ensure proper Al/Si reductant ratios in order to achieve the targetfluidity and desired CaO solubility.

From the evaluation of the CaO-SiO2-CaF2 and CaO-Al2O3-SiO2 systems, it is clear that Al2O3 isnot as potent as CaF2 to bring lime into solution, and considerably higher levels of Al2O3 would berequired 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-Al2O3-SiO2 (CMAS) system is very important. Fortunately, this system is well studied andthe "ternary isoplethal sections" at constant MgO and Al2O3 content are available.

Evaluation of the CaO-MgO-Al2O3-SiO2 system at constant MgO levelsFigure 10 shows the saturation levels of CaO at 1600°C as a function of the Al2O3/(SiO2 + Al2O3)ratio for slags containing MgO levels of 0%, 5% and 10%, respectively.

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

% Al2O3/(% SiO2 + % Al2O3)

% C

aO a

t Sat

urat

ion

0% MgO5% MgO10% MgO

5

% Al2O3 =

10

1520

22.5

25

23

27

39

10 15

19

2530

37

(A)

(B)

26.5

28.3

Figure 10. The solubility of CaO as a function of Al2O3 content at 1600°C(The actual Al2O3 levels of the slags are indicated in the figure)

13

This figure has the following important features:• The solubility of CaO initially decreases as Al2O3 replaces SiO2 as flux.• For slags with a (%Al2O3/(%SiO2 + %Al2O3) 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 insolution is the same for MgO levels from 0-10%, indicating that MgO is acting as flux in thiscomposition region, because total base in solution (MgO + CaO) increased. In practical terms,the %CaO/(%SiO2 + %Al2O3) ratio in the slag increases significantly as the MgO content ofthe slag increases from 0 – 10% at a (%Al2O3/(%SiO2 + %Al2O3) ratio of about 0.5.

• The minimum threshold Al2O3 level required in the slag before an increase in CaO solubility isrealized, decreases with increasing MgO content in the slag. At 0% MgO the minimum Al2O3threshold value is about 27%, for 5% MgO the Al2O3 level required is about 22%, and for 10%MgO the Al2O3 level required is about 15%. In all these cases the solubility of CaO onlyincreases when the SiO2 content of the slag is diluted. For example consider slags (A) and (B)in Figure 10 that contain 22.5 and 23% Al2O3, respectively (5% MgO). Slag (A) contains22.5% SiO2 and only has 50% CaO in solution, whereas slag (b), which contains only 12%SiO2, has 59% CaO in solution. This has a significant implication in terms of desulfurization.Not only are high levels of Al2O3 required in the slag (>22%), but the SiO2 level should bebelow 15%. These lower levels of SiO2, together with the higher levels of [Al], will result in adecreased oxygen potential in the steel.

• For slags containing 10% MgO, the solubility of CaO increases when the Al2O3 content of theslag exceeds about 15%. However, at an Al2O3 level of approximately 20%, the solubility ofCaO decreases, as the slags are now MgO saturated. From the quaternary system it can bedetermined that the maximum MgO level of the slag should be below 7.5 % to obtain the highCaO solubilities shown in Figure 10.

Evaluation of the CaO-MgO-Al2O3-SiO2 system at constant Al2O3 levelsThe 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 somesteelmaking process routes operate at much higher temperatures. For example, in stainlesssteelmaking operations the reduction temperatures are typically around 1700°C (3092°F), whilethe typical tapping temperatures from the stainless steel vessel are much lower (60-100°). Thislarge difference in the end of reduction temperature and the tapping temperature causes significantproblems in engineering refractory compatible slags that are also "workable" in the stainless steelvessel. A slag that is designed to be liquid and compatible with the refractories at 1700°C mightbecome too stiff at tapping temperatures. Alternatively, a slag designed to stay liquid down totapping temperatures could cause significant refractory wear during reduction step whentemperatures and turbulence in the vessel are high. Some slag regions in the CaO-MgO-SiO2-Al2O3 system further intensify this problem, as will be demonstrated by the following discussion.

In the CaO-MgO-SiO2 system, and the CaO-MgO-SiO2-Al2O3 system, MgO is acting as fluxingcomponent until the MgO saturation boundary is reached. For example in the CaO-SiO2 system,the solubility of basic oxide (CaO) is about 56% CaO at 1600°C, whereas in the CaO-MgO-SiO2,the solubility of basic oxides (CaO + MgO) is about 61% at dual saturation (saturated with bothCaO & MgO). Figure 11 shows the effect of MgO on the total base solubility (CaO + MgO) in theCMAS system. A significant shrinkage of the Ca2SiO4 stability field can be observed as the MgOcontent 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 Ca3SiO5 and lime (CaO).

14

The slag compositions that are of particular interest in this system are those that are dual saturatedwith respect to both MgO and CaO. The CaO-saturation phase could be Ca2SiO4, Ca3SiO5, orCaO depending on the temperature and Al2O3 content.

Figure 11. Saturation lines of CaO, Ca2SiO4, and Ca3SiO5 in the systemCaO-MgO-SiO2-Al2O3 at 1600°C and for MgO contents up to 16%.1

Phase relations at the 10% Al2O3 planeFor slags containing less than 10% Al2O3, the phase relations are similar at 1600°C and 1700°C,i.e., the solubility of CaO decreases with increasing Al2O3 content (Figure 10). However, forAl2O3 levels at, and greater than about 10%, an small area of high CaO solubility opens up at1700°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-SiO2-Al2O3 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% SiO2 20 27 31% Al2O3 10 10 10Equilibrium phases onthe liquidus boundary

Ca3SiO5 + MgO Ca2SiO4 + MgO Ca2SiO4 + MgO

15

10% 20%

60%

50%

CaO MgO

SiO2

10% Al2O3

1600°C

1700°Ca

b

c

Figure 12. Isothermal sections of the CaO-MgO-SiO2-Al2O3 systemthrough the 10% Al2O3 plane and temperatures of 1600° and 1700°C

Slags (a) and (b) have similar Al2O3 and MgO levels, but show a significant increase in CaOsolubility as the SiO2 content decreases from 27% (slag b) to 20% (slag a). This further highlightsthe importance of careful mass-balance calculations of reductant and alloy additives to obtain thedesired slag compositions. If slag (a) was targeted for desulfurization reasons at hightemperatures, it will become very stiff and unworkable at the 1600°C because the CaO solubilitydecreased from 59% to 46% over a 100°C interval. Furthermore, this small window of high CaOsolubility at 1700°C is only present in a very small MgO range (11-12%). It is very difficult tocontrol the MgO content of the slag that accurately under real steelmaking conditions.

Phase relations at the 15% Al2O3 plane

The phase relations in the CMAS system in the 15% Al2O3 plane are shown in Figure 13. At thisAl2O3 level the window of high CaO solubility at 1700°C has opened up considerably, but a smallwindow of high CaO solubility is now evident at 1600°C. On the 1700°C isotherm, the CaO-saturated phase in equilibrium with the liquid changes from Ca2SiO4, to Ca3SiO5 (g to h) andfinally to CaO (h to d). At point (d) the slag is dual saturated with respect to CaO and MgO.

16

f

de

SiO2

CaO MgO

50%

60%

10% 20%

15% Al2O3

1600°C1700°C

gh

Figure 13. Isothermal sections of the CaO-MgO-SiO2-Al2O3 systemthrough the 10% Al2O3 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 byopening a "window" of slags with high CaO solubility. However, from a more detailedperspective, the solubility of CaO actually decreases with increasing MgO content on the 1700°Cliquidus isotherm where lime is the equilibrium solid phase (points (h) to (d)). The following tableis comparison of CaO-saturated slags in the CAS system and the CMAS system (15% Al2O3) at1600° and 1700°C. This table shows that at 1700°C and 15% Al2O3, the right combination ofMgO and SiO2 can result in slags with a high dissolved CaO content at Al2O3 levels much lowerthan in the CAS system (27.7% Al2O3).

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°CSystem CAS CMAS CMAS CAS CMAS% CaO 62.5 58 46 64.4 62% MgO 10 13 7.5% SiO2 9.9 17 26 12.5 16% Al2O3 27.7 15 15 23.2 15

17

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 (highdissolved CaO content) can be generated with slags containing low levels of Al2O3 (12-15%) at1700°C, and to some extent at 1600°C. However, the fluidity of these slags are very sensitive tochanges in MgO contents and could became very stiff at lower temperatures (1600°C) if thecomposition of the slag is not carefully controlled. These diagrams further show that if the slagsare designed to be liquid at 1600°C (point f in Figure 13), then significant refractory wear ofdoloma based refractories can occur if these slags are in contact with the brick at 1700°C orhigher. The bonding phase in fired doloma refractories is lime (CaO) and Figure 13 and Table 4show that the solubility of CaO increases from 46% to 62% for a temperature increase from1600°C to 1700°C.

Phase relations at the 20% and 25 Al2O3 planes

The small "windows" of slags areas that had high CaO-solubilities at 10% and 15% Al2O3 have"opened" significantly at 20 Al2O3 and opened completely at 25% Al2O3 (Figure 14 and 15).

SiO2

CaO MgO

50%

60%

10% 20%

20% Al2O3

1600°C

1700°C

Figure 14. Isothermal sections of the CaO-MgO-SiO2-Al2O3 systemthrough the 20% Al2O3 plane and temperatures of 1600° and 1700°C

18

SiO2

CaO MgO

50%

60%

10% 20%

25% Al2O3

1600°C

1700°C

ij

Figure 15. Isothermal sections of the CaO-MgO-SiO2-Al2O3 systemthrough the 25% Al2O3 plane and temperatures of 1600° and 1700°C

These figures clearly show that the combination of MgO and Al2O3 results in a "shrinkage" of theCa2SiO4 stability field which opens up an area of slags with high CaO solubility. However, on thelime-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 forthe various Al2O3 levels. The solubility of MgO (at dual saturation) initially decreased withincreasing Al2O3 content up to about 20% Al2O3, but then increases again at higher Al2O3 levels.

19

Utilizing Al2O3 to increase the solubility of CaO in the slag

The discussion of the phase relations in the CAS and CMAS systems have clearly shown thatAl2O3 could be utilized to increase the solubility of CaO in the slag. While it is theoreticallypossible to generate slags with a high dissolved CaO content at Al2O3 levels as low as 15% at1700°C it would be difficult to consistently generate these slags under real steelmaking conditions.More realistic Al2O3 levels in the slag should be 20 - 25% Al2O3 (preferably 25%) in order togenerate slags with good fluidity at lower temperatures. The diagrams of the CMAS system alsoshowed the importance of MgO as a fluxing component to generate slags with high dissolved limecontents. The MgO content of the slag is very important in this system and should be controlled ina very tight range (8-11% MgO). Too low or too high MgO levels could result in very stiff slagswith poor desulfurizing properties.

The discussion of the diagrams also demonstrated the importance of the SiO2 content of the slagand its relationship with Al2O3 on the solubility of CaO. The solubility of CaO increases rapidlyas the SiO2 content of the slags is diluted at constant Al2O3 (Figures 10, 14 and 15).

Al2O3 and steel quality concerns

In practical steelmaking, the levels of Al2O3 required to generate slags with high CaO solubilitiesis >20% Al2O3. This means that the bulk of the steel deoxidant or reductant should be Al. Partialreplacement of FeSi by Al as a reductant in stainless steelmaking will be ineffective to increase thesolubility of CaO. Another important factor is the SiO2 content of the slag. The SiO2 that istransferred from the EAF slag, together with the transfer Si and Si in alloy additions, must beconsidered to determine the Al required in the reduction mix to ensure adequate SiO2 dilution(<15%).

The use of Al as a deoxidizer and reductant, and the resulting high Al2O3 slags, will have a largeeffect on the internal quality of the steel. The residual Al levels in the steel and the resultant lowerdissolved oxygen level will have a significant impact on the inclusion chemistry and the timing ofAl2O3 precipitation. The previous discussion on the CMAS system clearly demonstrated theimportance of MgO as flux in this system and that between 6 and 9% MgO would be required toenhance lime dissolution. Unfortunately, the maximum solubility of MgO in these high Al2O3slags is low (<11% MgO) so that slags with high MgO activities will be exposed to Al under fairlyreducing conditions. The potential for Mg reduction and spinel formation is a real possibility.

Al2O3 and Refractory concerns

The replacement of FeSi by Al in stainless steelmaking will result in higher reductiontemperatures, which will require more coolant additions. If these coolants are not added and hightemperatures (>1700°C) are prevalent during the reduction step, then significant refractory wear ofdoloma refractories can occur with slags with intermediate Al2O3 levels (10 - 20% Al2O3). Theaddition of sufficient lime to the slag to protect the refractories will result in liquid compatibleslags at high temperatures but very stiff "unworkable" slags at lower temperatures. It is mucheasier to design refractory compatible and "workable" slags if the Al2O3 content of the slag isbetween 25 and 30% Al2O3. These slags are less sensitive to variations in MgO levels and the

20

1600° and 1700°C liquidus isotherms are closer together resulting in reasonable slag fluidity as theslag cools down.

The following table summarizes the recommended target slag compositions in the CaO-MgO-Al2O3-SiO2 system.

Table 5. Target slag compositions and ranges in the CMAS system

Al2O3 range Comments0 – 10% Al2O3 No benefits in terms of CaO solubility. CaO solubility actually decreases as

Al is replacing FeSi10 - 20% Al2O3 A very large increase in CaO solubility occurs in very specific slag areas.

These slags are difficult to obtain and control under real steelmakingconditions. These slags could lead to significant refractory wear oralternatively very stiff unworkable slags at lower temperatures. It is best toavoid this slag composition range!

20 – 30% Al2O3 The ideal target range is 25 to 30% Al2O3. These slags can be designed to berefractory 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 fluxingcomponents CaF2, B2O3, and Al2O3 on the solubility of CaO in steelmaking slags.

Fluorspar is by far the most convenient component to use as a fluxing precursor and additive toincrease the solubility of CaO in the slag. The maximum levels of CaF2 that would be required toobtain the maximum CaO solubility is about 12% CaF2. Most operations could operate with slagswith much lower CaF2 levels, provided that some MgO is present in the slag (> 6% MgO). Animportant consideration is that the effect of CaF2 on steel quality is negligible and it is practicallyinert to the steel. Refractory compatible slags can be designed for fluorspar-containing slags athigh temperatures that will still maintain reasonable fluidity at lower temperatures.

B2O3 is the most potent of all three fluxes considered, not only in terms CaO solubility, but also interms of refractory wear. The addition of B2O3 as a flux is only an option in B-containing steelgrades and great care should be exercised to ensure refractory compatibility.

Alumina is a major slag component (> 25% Al2O3) in Al-killed grades and is very effective tobring lime into solution and to generate good desulfurizing slags. Magnesia-carbon refractoriesare typically used with these slags and with good results because the solubility of MgO in theseslags is fairly low (< 11% MgO). The use of Al2O3 as a flux (prefused Ca-Aluminate) in low-CSi-killed steel grades is common and very effective. However, in high-C grades the level of Al2O3that can be tolerated in the slag is low (<10 % Al2O3) because of castability issues (clogging). Atthese low levels, Al2O3 is actually worse than SiO2 to dissolve CaO and the only benefit is that itcan act as a fluxing precursor if added as prefused Ca-Aluminate. Unfortunately, these high-Csteel grades also have low sulfur requirements. In most cases a combination of SiO2 and CaF2 isused as fluxing additives to these grades to create slags with good desulfurization properties

21

without negative castability effects. The only alternative to using CaF2 in the slags of these gradesis to use scrap with very low sulfur levels.

Fluorspar is commonly used in combination with FeSi in stainless steel operations as a fluxingprecursor during the reduction step, and to increase the solubility of lime in the final reductionslag. The elimination of CaF2 and the partial replacement of FeSi by Al will not be effective inimproving reduction kinetics and increasing CaO solubility. For Al2O3 to be effective in theseslags, Al should be the bulk reductant addition and the FeSi addition and SiO2 content of the slagshould be carefully controlled. For these aluminate slags to be equivalent to a 8 - 10% CaF2-containing silicate slags in terms of lime dissolution and solubility, the Al2O3 content of the slagshould be around 25% Al2O3.

VII) References

1. Slag Atlas, edited by Verein Deutscher Eisenhüttenleute (VDEh). Verlag Stahleisen GmbH2. Pretorius, E.B. "The effect fluorspar in steelmaking slags". Unpublished technical document.