A Multi-step Thermodynamic Model for Alumina Formation during Aluminum Deoxidation in Fe-O-Al Melt

doi:10.1007/s40195-014-0203-7

Abstract

Abstract:

Based on the two-step nucleation mechanism, a multi-step thermodynamic model for alumina inclusion formation during aluminum deoxidation process was proposed in Fe-O-Al melt. Thermodynamic properties of metastable intermediates including (Al₂O₃)_nclusters for prenucleation and α-Al₂O₃ nanoparticle for growth process were calculated using density functional theory. Furthermore, Gibbs free energy change of forming the intermediate by reaction between the dissolved aluminum (Al) and oxygen (O) in the melt was calculated. The results indicated that the thermodynamics of (Al₂O₃)_nat steelmaking temperature are dependent on their structures, while that of α-Al₂O₃ nanoparticle are dependent on their size. The nuclei of α-Al₂O₃ which was originated from (Al₂O₃)_naggregated under a high supersaturation ratio of Al and O(R _s) in the melt. There existing excess oxygen because of the low R _s, but the secondary inclusions will be formed during the cooling process due to the excess oxygen. The nuclei lager than 20 nm can grow up spontaneously and instantaneously into primary inclusions because of thermodynamic drive. It is difficult to control the size of α-Al₂O₃ to be less than 20 nm, in the aluminum deoxidation process of the current conditions of steelmaking.

Key words: Multi-step thermodynamics, Fe-O-Al melt, Aluminum deoxidation, DFT, Nano-α-Al2O3

Guo-Cheng Wang, Qi Wang, Sheng-Li Li, Xin-Gang Ai, Da-Peng Li. A Multi-step Thermodynamic Model for Alumina Formation during Aluminum Deoxidation in Fe-O-Al Melt[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(2): 272-280.

Figures/Tables 9

Table 1 Fraction of the surficial atom (x) and expression of Δ r G m(nano) in the nanoparticles with different sizes

Nanoparticle size	x (%)	Δ _r G _m (kJ mol^-1)
2	16.631	-861.361 + 37.780 × 10^-2 T
5	6.653	-1,065.744 + 38.272 × 10^-2 T
10	3.326	-1,133.872 + 38.436 × 10^-2 T
20	1.663	-1,167.936 + 38.518 × 10^-2 T
30	1.109	-1,179.291 + 38.545 × 10^-2 T
40	0.832	-1,184.968 + 38.559 × 10^-2 T
50	0.665	-1,188.374 + 38.567 × 10^-2 T
75	0.444	-1,192.916 + 38.580 × 10^-2 T
100	0.333	-1,195.187 + 38.584 × 10^-2 T

Fig. 1 Structural model for DFT calculation: a (0 1 0) plane; b (1 0 0) plane; c (0 0 1) plane; d crystal

Fig. 2 Density of phonon states of α-Al2O3 crystal

Fig. 3 Thermodynamic properties of α-Al2O3 crystal: a S and C V; b F V, H and G

Fig. 4 Thermodynamic properties of α-Al2O3 nanoparticle: a \( C_{\text{V}}^{\text{nano}} \) versus temperature; b H nano versus temperature; c S nano versus temperature; d \( F_{\text{V}}^{\text{nano}} \) versus temperature; e \( U_{0}^{\text{nano}} \) versus nanoparticle size; f G nano versus temperature

Fig. 5 Gibbs free energy changes of various metastable phases' formation by Al reacting with O in Fe-O-Al melt

Fig. 6 Relationship between logc O versus log c Al at 1,873 K corresponding to various deoxidation products in Fe-O-Al melt

Fig. 7 Schematic illustration of nucleation and crystallization of deoxidation product from Fe-O-Al melt

Fig. 8 Gibbs free energy change of various deoxidation product formation by the reaction between Al and O at 1,873 K in Fe-O-Al melt

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