Distribution of Al and Ca between Si and CaO-Al 2 O 3 -SiO 2 Slags 38

Knowledge about the distribution of Ca and Al between the slag and the silicon melt is essential, and several researchers have studied the equilibrium between silicon and CaO-Al2O3-SiO2slags. However, the data in the low SiO2region is inadequate. In the follow-ing sections, the most relevant litterateur will be presented. Details from the findfollow-ings of relevant studies are given in table 2.1.

Morita et al. [61] studied the distribution of slag forming elements between the CaO-Al2O3-SiO2 slag system at 1550 °C and 1600 °C. They performed two experiments with CaO-Al₂O₃-SiO₂slags with high CaO and Al₂O₃contents, with some of the most relevant slag compositions being XSiO2 = 0.094, XCaO = 0.432, XAl2O3 = 0.474 and XSiO2 = 0.071, XCaO= 0.367, XAl₂O₃ = 0.562. They found that when the alloys are equilibrated with slags in the lower SiO2region, the Ca and Al contents of the alloys will be high, with the highest obtained Ca and Al concentrations being 10.4 wt% Ca and 6.46 wt% Al, and for the second experiment, 14.9 wt% Ca and 4.46 wt% Al were obtained. Considering the Al content, this study is in good agreement with the study obtained from Weiss and Schw-erdtfeger [62] even though the temperature difference of 50 K in regards to the aluminum content of the SiO2saturated slags. This is a strong suggestion that the aluminum content in SiO2saturated slags is relatively independent of temperature.

Ahn et al. [41] measured the distribution of Ca and Al between molten silicon and SiO₂ -rich CaO-Al2O3-SiO2at 1550 °C (1823 K) they found that the concentrations of Al in Si follow an exponential trend along the liquidus line when the slag is saturated with SiO2. The results from their extrapolated work are also in good agreement with the results ob-tained from Morita et al. [61] concerning the Al content. However, the Ca concentration was significantly higher for both studies than the ones obtained from Weiss and Scherdt-feger [62] at 1500 °C, which is an indication of that the calcium content increases with increasing temperatures. When comparing with the work of Fujiwara et al. [63], the ob-tained values for Al is lower, but Fujiwara et al. [63] had higher Al2O3content initially, and the experiments were run at 50 K higher.

The study conducted by Morita et al. (2000) [61] is also in good agreement with the study by Fujiwara et al. [63] which equilibrated silicon with Al2O3 and CaAl12O19and CaAl12O19saturated CaO-Al2O3-SiO2slags. The results from the studies obtained from Morita et al. (2000) [61] and Fujiwara et al. [63] are plotted together in fig. 2.15 and fig. 2.16 showing the results for the slag composition alongside with the liquidus saturated with the aluminate compounds.

Figure 2.15: The relation between XCaO/XSiO2+XCaO and log [mass% Ca] in Si based al-loys in equilibrium with the CaO-AlO1.5SiO2 slags saturated with AlO1.5, CaO·12 AlO1.5 or CaO·4 AlO1.5, taken from Morita et al. [61]

Figure 2.16: The relation between XCaO/XSiO₂+XCaO and log [mass% Ca] in Si based al-loys in equilibrium with the CaO-AlO1.5SiO2 slags saturated with AlO1.5, CaO·12 AlO1.5 or CaO·4 AlO1.5, taken from Morita et al. [61]

Table 2.1:Experimental details and results from different researchers, with important parameters as equilibrium time, the activity coefficients of Ca And Al, initial and final Ca and Al concentration, etc.

Reference T(°C) Equilibrium

time (h) Atmosphere Slag lnγCa lnγAl

Morita et al. [61] 1550 and 1600 18 Ar CaO-Al₂O₃-SiO₂

(CaO-Al2O3-rich) -¹⁴³⁰⁰_T + 1.55 -³⁶¹⁰_T + 0.425

Ahn et al. [41] 1550 6 Ar CaO-Al₂O₃-SiO₂

(SiO₂-rich) -7.25 -0.77 and -0.68 Dumay et al. [42]

(Experiment 20) 1450 and 1550 12-48 Ar-10%He CaO-Al₂O₃-SiO₂ -5.81 -0.43 Weiss et al. [62] 1500 and 1600 1/2-48 Ar CaO-Al₂O₃-SiO₂

(SiO₂-rich) NA -0.60

Fujiwara et al. [63] 1600 3-5 Ar CaO-Al2O3-SiO2

(CaO-Al₂O₃-rich) NA NA

Reference Si mass Slag mass Initial Ca Final Ca Initial Al Final Al

Morita et al. [61] 8 g 16 g NA 0.3-15.9 wt% NA 0.22-5.7 wt%

Ahn et al. [41] 5 g 10 g NA 0.025-0.073 wt% NA 0.015-0.032 wt%

Dumay et al. [42]

(Experiment 20) 60 g (FeSi65) 60 g 0.001-0.19 wt% 0.019-0.074 wt% 0.015-0.26 wt% 0.23-0.52 wt%

Weiss et al. [62] 2 g 5 g NA

0.0003-0.023 NA 0.0016-0.18 wt%

Fujiwara et al. [63] 2.0-3.5 g 4-6 g 0.02 wt% 8 wt% 0.4 wt% 10 wt%

2.5 Activities in the CaO-Al

O

-SiO

Slag System

Rein and Chipman [64] determined the activity of AlO1.5 and CaO, SiO2 in the ternary system CaO-Al2O3-SiO2 at 1500-1600 °C (1773-1873 K) by measuring the distribution of silicon between the slag and Fe-Si-C alloys saturated with graphite or silicon carbide under a CO atmosphere. Their plotted isoactivity diagram for CaO, AlO1.5and SiO2 are shown in figure 2.17 and 2.18, respectively.

Morita et al. (2000) [61] determined the activity of SiO₂by equilibrating the CaO-Al₂O₃ -SiO2 slag with Si at 1550 and 1600 °C and used the equilibrium distribution of the Ca and Al between the ternary slag and silicon a Gibbs-Duhem integration. In a later study, Morita et al. (2002) [65] determined the activities of CaO and Al₂O₃, and also here, their work fits well with the values obtained from Rein and Chipman [64], however, at activities above 30 mol% Al2O3, their activities are lower.

Kang et al. [66] determined the activity of SiO₂ in CaO – Al₂O₃– SiO₂ melts with low SiO2content (<10 wt%) at 1600 °C, and compared the results with the study by Morita et al. (2000) [61] By equilibrating the melts with copper as the reference metal at controlled partial pressures, the isoactivity of SiO2 were drawn from the measured values. They found that SiO2 shows a positive deviation from their previous data, as well as a relation between the CaO and SiO2content where the SiO2activity decreases with increasing CaO content. Their results also showed a higher activity of SiO2than the ones obtained from Rein and Chipman [64], as well as a more drastic decrease in the activity of SiO₂ with increasing CaO-content. Their obtained isoactivity values are plotted together with the ones obtained from Rein and Chipman [64] in figure 2.19

Mao et al. [26] did a thermodynamic assessment of the CaO-Al₂O₃-SiO₂system, and the activities they obtained fit relatively well with the ones obtained from Rein and Chipman.

As a summary, the work by Morita et al. (2000) [61] at 1550 °C is in close agreement with the work by Rein and Chipman [64]. Therefore, Rein and Chipman’s work is considered the most representative for the CaO – Al2O3– SiO2system at 1600 °C.

Figure 2.17:The isoactivity diagram for CaO and AlO1.5at 1600 °C, by Rein and Chipman. [64]

Figure 2.18:The isoactivity diagram for SiO2at 1600 °C, by Rein and Chipman. [64]

Figure 2.19:The isoactivity diagram for SiO2at 1600 °C, by Kang et al. compared with the results obtained from Rein and Chipman (dotted lines) in the isothermal region. [66]