STABILITY OF FLUORINE-FREEMOULD FLUXES SiO2-CaO-Al2O3- B2O3-Na2O FOR STEEL CONTINUOUS CASTING
Lin Wang1, Jianqiang Zhang1, Yasushi Sasaki1, Oleg Ostrovski1, Chen Zhang2, Dexiang Cai2
1School of Materials Science and Engineering, UNSW Australia, Sydney, 2052, Australia
2Steelmaking Research Department, Baosteel Group Corporation Research Institute, Shanghai, 201900, China
Keywords: mould flux, B2O3, Na2O, thermogravimetric analysis, evaporation
Abstract
B2O3 and Na2O are key components of fluorine-free mould fluxes for continuous casting, but both are highly volatile which affects the flux stability. This paper investigates evaporation of the SiO2-CaO-Al2O3-B2O3-Na2O fluxes (Na2O: 6-10 wt%, CaO/SiO2 ratio: 0.8-1.5) in the temperature range 1573 to 1673K using a thermogravimetric analysis method. The weight loss as a result of the flux evaporation increased with increasing temperature. The rate of evaporation increased significantly with the increase in the Na2O content. The effect of the CaO/SiO2 ratio in the range 0.8-1.3 on the evaporation rate was marginal. However, the increase of this ratio to 1.5 slowed down evaporation after approximately 1000 s. When 6.2 wt% Na2O was added to the SiO2-CaO-Al2O3-B2O3 flux, the apparent activation energy Ea for the evaporation reaction decreased from 365 to 193 kJ/mol, while a further increase in the Na2O content to 9.1 wt% had a minor effect on Ea. The high evaporation rate of boracic fluxes in the presence of B2O3 and Na2O was attributed to the formation of highly volatile NaBO2.
1. Introduction
Conventional commercial mould fluxes usually contain 4-10 wt% of fluoride to maintain heat transfer, melting temperature, and viscosity appropriate for the steel continuous casting[1].
However, the emission of fluorine causes water pollution, equipment corrosion, and health hazards [2]. Therefore, development of the fluorine-free mould fluxes is important to decrease environmental impact of metallurgical industry and equipment corrosion [2, 3, 4]. Na2O is added to the fluorine-free mould fluxes to accelerate the crystallisation of mould fluxes and to decrease their melting temperature [5]. It was also suggested that the combination of B2O3 and Na2O leads to the formation of boracic phase which might replace cuspidine, which is a key crystal phase in the fluorine-containing fluxes [2, 6].
However, high volatility of sodium containing compounds is a limitation in the industrial use of the Na2O-containing boracic fluxes [7]. Besides, B2O3 itself might also evaporate at high temperatures. High volatility of Na2O and B2O3 can change the chemical compositions of mould fluxes, and therefore their physicochemical properties. It was also reported that CaO/SiO2 ratio could affect the flux evaporation rate because of the alternation of flux physicochemical properties, e.g. viscosity [8]. Thus, knowledge of kinetics and mechanism of evaporation of mould fluxes containing both B2O3 and Na2O is important for the development of the F-free mould fluxes. Research in this area has been quite limited [8, 9, 10]. The purpose of this work was to investigate the evaporation of CaO-SiO2-Al2O3-B2O3-Na2O fluxes, focussing on effects of the Na2O content and CaO/SiO2 ratio.
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
2. Experimental procedure 2.1 Sample preparation
In this study, slag samples were prepared using chemical reagents of CaCO3, Na2CO3, SiO2, Al2O3, and B2O3 powders, with several carbonates being substituted for oxides due to their stability in air. The mixture of components was ground in an agate mortar for 20 min, and placed to the furnace in a high-purity graphite crucible. After heating and melting at 1400°C, and holding at this temperature for 20 min, the slag was poured onto a steel plate and cooled to the room temperature. Subsequently, the slag sample was crushed and ground, forming homogenized fine powder. Compositions of fluxes analysed by X-ray fluorescence are listed in Table I. Fluxes No. 1, 4, 5 and 6 have varied Na2O contents from 0 to 9.1 wt% at a fixed CaO/SiO2 ratio of 1.3, while Fluxes No. 2, 3, 5 and 7 have varied CaO/SiO2 ratios from 0.8 to 1.5 at Na2O contents in the narrow range 7.8-8.4 wt%.
Table I. Chemical compositions (wt%) of test fluxes
Flux No. CaO/SiO2 ratio CaO SiO2 Al2O3 B2O3 Na2O
1 1.3 51.1 39.2 3.2 6.4 0
2 0.8 36.8 44.9 3.5 6.6 8.2
3 1.0 40.5 40.8 3.6 6.7 8.4
4 1.3 47.2 36.4 3.6 6.6 6.2
5 1.3 46.2 35.7 3.5 6.7 7.9
6 1.3 45.3 35.3 3.8 6.5 9.1
7 1.5 49.5 32.8 3.4 6.5 7.8
2.2 Thermogravimetric measurement
The evaporation experiments were conducted using STA449-F1 calorimeter (NETZSCH Instruments, Germany) in the isothermal mode at 1300, 1350 and 1400C in Ar atmosphere.
Pt/Ph crucibles with 6 mm inside diameter and a volume of 84 μL were employed as the containers for the slag samples. The crucibles were cleaned after each use, using the solution of 25 wt% HCl.
Before the furnace was heated, the chamber was evacuated and purged with argon gas for 300 s to ensure a steady gas flow. The sample was then heated to a pre-determined temperature with a heating rate of 50 K/min, the maximum heating rate for the apparatus. The gas flow rate was kept constant at 70 ml/min for the duration of the experiment, which was reported to be above the starvation rate[11]. Therefore, evaporated species at the interface are expected to be immediately swept away from the liquid/gas interface. The sample was kept at the
experimental temperature for 60 min. During the Figure 1. Weight loss of Flux No.7 in parallel experiments at 1350oC.
experiment, the weight change and the temperature of the sample were recorded every 0.1 s.
Parallel experiments for some selected fluxes demonstrated a good reproducibility of the experimental results (Figure 1).
3. Results and discussion
3.1 Weight loss of fluxes
The measured weight loss of fluxes (represented as m/m0, where m is the weight change and m0 is the original sample weight) over time during isothermal conditions (1300-1400°C) is shown in Figure 2. Figures 2a, c and e show the weight loss of fluxes with a fixed CaO/SiO2
ratio at 1.3 but varied Na2O contents at different temperatures. In general, the weight loss increased with the increasing temperature. For Flux No.1 containing 6.4 wt% B2O3 and no Na2O, the weight loss was less than 0.002% at all temperatures for 1 h. The addition of 6.2 wt%
Na2O produced a significant weight loss (Figures 2a and c). With further increase of the Na2O content, the increase in the evaporation slowed down. No change in the evaporation rate was observed at 1300°C when Na2O increased from 7.9 to 9.1 wt% (Figure 2a).
Figures 2b and d show the evaporation rates of fluxes with a fixed Na2O content at approximately 8 wt% but varied CaO/SiO2 ratios at 1300 and 1350C, respectively. Increase in the CaO/SiO2 ratio from 0.8 to 1.3 had no significant influence on the weight loss at 1300°C (Figure 2b). However, when the CaO/SiO2 ratio increased further to 1.5, the sample weight changed slowly after around 1000s, although there was no difference in the evaporation rate before this time (Figures 2b and d).
Figure 2. Weight loss as a function of time at (a,b)1300°C, (c,d)1350°C, and (e)1400°C with (a,c,e) a fixed CaO/SiO2 but varied Na2O content, and (b,d) a fixed Na2O but varied CaO/SiO2.
3.2 Evaporation rate and activation energy
The evaporation rates were analysed during first 10 min of the weight measurements. To take this short time only is by considering the flux chemistry change because of evaporation which would alter the original flux evaporation rate. Evaporation rate k, s-1, was expressed as:
𝑘 =∆𝑚 𝑚𝑡⁄ 0
𝑖 (1)
∆𝑚 = 𝑚𝑖− 𝑚0 (2) where ∆𝑚 𝑚⁄ 0 is the flux mass change, and 𝑚𝑖, g, is the flux mass at time 𝑡𝑖, s. Table II shows the calculated evaporation rates of different fluxes. The evaporation rate increased with the increase of the Na2O content at the fixed CaO/SiO2 ratio of 1.3, while the CaO/SiO2 ratio at the constant Na2O content had almost no effect on k.
Table II. Evaporation rate k (s-1) of fluxes
Flux No. 1300°C 1350°C 1400°C
1 -3.08×10-7 -8.21×10-7 -1.90×10-6
2 -1.93×10-5 -- --
3 -1.93×10-5 -- --
4 -1.54×10-5 -2.43×10-5 --
5 -1.81×10-5 -2.89×10-5 --
6 -1.87×10-5 -3.34×10-5 -4.91×10-5
7 -2.03×10-5 -2.89×10-5 --
*-- Evaporation rates were not measured under these conditions
The apparent activation energies for the evaporation process were estimated from the Arrhenius plots using data on k obtained at different temperatures:
ln𝑘 = ln𝐴 +𝑅𝑇𝐸𝑎 (3) where A is the pre-exponential factor, Ea is the apparent activation energy, kJ/mol, R is the gas constant, 8.314 J/(mol.K), and T is the absolute temperature, K. Values of Ea were obtained from the slope of lnk vs 1/T (shown in Figure 3) and listed in Table III. Addition of 6.2 wt%
Na2O to the CaO-SiO2-Al2O3-B2O3 system significantly decreased Ea. A further increase in the Na2O content from 6.2 to 9.1 wt% had a minor effect on Ea. Experimental data for fluxes containing 7.9 and 9.1 wt% Na2O were available only at 1300 and 1350°C, what is not sufficient for the accurate estimation of the apparent activation energy.
Figure 3. Arrhenius plots for fluxes with varying Na2O contents.
Table III. Calculated Ea as a function of Na2O content
Flux No. Na2O, wt% Ea, kJ/mol R2
1 0 365 0.99
4 6.2 193 --
5 7.9 203 --
6 9.1 211 0.98
3.3 Estimation of vapour pressures of Na, O2, B2O3 and NaBO2
The evaporation of B2O3 at high temperatures from the B2O3 containing fluxes was observed in works [10, 12]. However, the vaporisation of fluxes containing Na2O, which is a common component in the mould fluxes, has significant differences as the vaporization species include Na2O and also other compounds. Generally, vaporisation of Na2O and B2O3 can be described by the following reactions [8, 9]:
(B2O3) = B2O3(g) (4) (Na2O) = 2Na(g) +1
2O2(g) (5) (B2O3) + (Na2O) = 2NaBO2(g) (6) In these reactions (Na2O) and (B2O3) are oxides dissolved in the flux. The weight loss of Flux No.1 (Figures 2a, c and e) was very low, indicating evaporation of B2O3 can be neglected.
Therefore, Na, O2 and NaBO2 are considered as the major evaporative species. Once the evaporation starts, gaseous species evaporate from the interface and leave the system with the carrier Ar gas. After a certain time, a steady-state is established in which the concentrations of gaseous species in argon gas become constant [13].
The flux of each gaseous component JA, mol/s, is defined as:
𝐽𝐴=∆𝑚𝑡𝑀𝐴
𝐴 (7) where MA is molecular weight, g/mol, and ∆mA is the mass loss of component A (A: B2O3, Na, O2, NaBO2).
In order to simplify the problem, it is assumed that there is no convection occurring inside the crucible[9]:
𝐽𝐴
𝑆 =(𝑃𝐴𝑖−𝑃𝐿𝑅𝑇𝐴𝑏)𝐷𝐴−Ar (8) where i and b represent the interface and bulk, respectively. S is the area of the interface, and L is boundary layer thickness which is assumed to be the distance between the surface of liquid slag and the crucible top, cm. 𝑃𝐴𝑖 and 𝑃𝐴𝑏 are the partial pressures at the interface and in the bulk gas, respectively. 𝑃𝐴𝑏 is assumed to be zero since the gas flow rate is not starved under the experimental conditions. 𝐷𝐴−Ar is the diffusion coefficient of gaseous species through Ar gas, cm2/s, which can be calculated by Chapman-Enskog equation[14]:
𝐷𝐴−Ar= 0.0018583 × 𝑇3/2
𝑃∙𝜎𝐴−Ar2 ∙Ω𝐴−Ar× (1
𝑀𝐴+ 1
𝑀Ar)1 2⁄ (9) where P is the pressure, atm, 𝑀𝐴 and 𝑀Ar are molecular weights of gaseous species and Ar, and Ω𝐴−Ar is the collision integral for diffusion, which is a function of the dimensionless temperature kT/εA-Ar[14].
Parameters 𝜎𝐴−Ar, Å and 𝜀𝐴−Ar, J, are parameters of the Lennard-Jones potential which can be estimated from the following equations [14]:
𝜎𝐴−Ar=12(𝜎𝐴+ 𝜎Ar) (10) 𝜀𝐴−Ar= (𝜀𝐴𝜀Ar)1/2 (11)
In the present study, the values 𝜀Ar𝑘 and σAr taken from [15] are 122.4K and 3.432 Å, respectively.
The values of εA and σA can be estimated by the following empirical equations [16]:
𝜀𝐴
𝑘 = 1.92𝑇𝑚,𝐴 (12) 𝜎𝐴= 1.222𝑉𝑚,𝐴
1
3 (13) where k is Boltzmann constant, 1.38×10-23 J/K, Tm is the melting temperature, K, and Vm,A is the molar volume at melting temperature, cm3/mol. Table IV shows the calculated 𝜀𝑘𝐴 and σA using Equations 12 and 13.
Table IV. Parameters 𝜀𝑘𝐴 and σA for different gaseous species Gaseous
species MA, g/mol ρliq, g/cm3 Vm, cm3/mol Tm, K 𝜀𝑘𝐴, K σA, Å
B2O3(g) 69.62 2.46 28.30 723 1388.16 3.724
Na(g) 22.99 0.93 24.80 371 712.32 3.560
O2(g) 32.00 -- -- -- 113[15] 3.433[15]
NaBO2(g) 65.80 2.46 26.75 1239 2378.88 3.655
Calculated κT/εA-Ar are shown in Table V. Using collision integrals together with the Lennard- Jones potential for the prediction of transport properties of gases at low densities [15], values of Ω𝐴−Ar for different species are presented in Table V. Estimated 𝐷𝐴−Ar values using Equation 9 are also shown in Table V.
Table V. Estimated diffusion coefficients of gaseous species in Ar gas (10-4 m2/s)
Parameter 1300°C 1350°C 1400°C
𝑘𝑇/𝜀B2O3−Ar 3.79 3.91 4.03
ΩB2O3−Ar 0.8952 0.8897 0.8845
𝐷B2O3−Ar 2.02 2.13 2.24
𝑘𝑇/𝜀Na−Ar 5.29 5.46 5.63
ΩNa−Ar 0.8428 0.8428 0.8129
𝐷Na−Ar 2.96 3.1 3.36
𝑘𝑇/𝜀O2−Ar 13.29 13.71 14.13
ΩO2−Ar 0.7025 0.7025 0.7025
𝐷O2−Ar 2.29 3.28 3.43
𝑘𝑇/𝜀Na2BO2−Ar 2.90 2.99 3.08
ΩNa2BO2−Ar 0.9588 0.9500 0.9418
𝐷Na2BO2−Ar 1.94 2.05 2.17
The rate of total weight loss of the molten flux can be calculated using the following equation:
∆𝑚
𝑡 = ∑ 𝑀𝐴𝐽𝐴 (14) Table VI shows the estimated vapour pressures of major gaseous species for Flux No.5 using thermochemical software Factsage 7.0. Clearly, the vapour pressure of NaBO2 is the highest, indicating it is a dominant volatile species.
Table VI. Estimated vapour pressure 𝑃𝐴𝑖 (Pa) for major gaseous species for Flux No.5
PA 1300°C 1350°C 1400°C
B2O3(g) 4.72×10-4 1.31×10-3 3.35×10-3
Na(g) 1.38 2.84 5.60
O2(g) 0.34 0.71 1.39
NaBO2(g) 58.37 104.16 177.96
By combining Equations 8 and 14, total weight loss can be estimated from the following equation:
∆𝑚𝑅𝑇𝐿
𝑡𝑆 = ∑ 𝑀𝐴𝐷𝐴−Ar𝑃𝐴𝑖 (15) The calculated weight loss for Flux No.5, compared with the experimental measurement, is shown in Table VII. In general, the measured values are slightly lower than the calculated ones, but both are in reasonable agreement.
Table VII. A comparison of measured and estimated weight loss ∆𝑚𝑡 (10-9 g/s) for Flux No.5
Temperature 1300°C 1350°C
Measured 513.83 900.56
Calculated 611.05 1141.82
4. Conclusions
In the present study, stability of the fluorine-free mould flux SiO2-CaO-Al2O3-B2O3-Na2O for the steel continuous casting was investigated, and the following results were obtained:
(1) The weight loss as a result of the flux evaporation increased with the increasing temperature.
The rate of evaporation increased with the increasing Na2O content, while the effect of the CaO/SiO2 ratio on the flux evaporation rate was marginal.
(2) The apparent activation energy Ea for the evaporation reaction decreased from 365 to 193 kJ/mol when 6.2 wt% Na2O was added to the SiO2-CaO-Al2O3-B2O3 flux. A further increase in the Na2O content to 9.1 wt% had a minor effect on Ea.
(3) The high rate of evaporation of the SiO2-CaO-Al2O3-B2O3-Na2O system was attributed to the formation of highly volatile NaBO2.
Acknowledgements
This project was financially supported by Baosteel through BAJC, Abel Metal, and Australian Research Council (ARC LP130100773).
References
1. E. Brandaleze et al., Mould Fluxes in the Steel Continuous Casting Process, in Science and Technology of Casting Processes. 2012.205-233.
2. A.B. Fox et al., Development of Fluoride-free Fluxes for Billet Casting. ISIJ International, 2005. 45(7), 1051-1058.
3. G.H. Wen et al., Development of Fluoride-free Mold Powders for Peritectic Steel Slab Casting.
ISIJ International, 2007. 47(8), 1117-1125.
4. J.L. Klug et al., Fluorine-Free Mould Powders for Slab Casting: Crystallization Control in the CaO-SiO2-TiO2-Na2O-Al2O3 System. Steel Research International, 2012. 83(12), 1186-1193.
5. C.-B. Shi et al., Crystallization Characteristics of CaO-Al2O3-Based Mold Flux and Their Effects on In-Mold Performance during High-Aluminum TRIP Steels Continuous Casting.
Metallurgical and Materials Transactions B, 2014. 45(3), 1081-1097.
6. S.Y. Choi et al., Properties of F-free Glass System as a Mold Flux: Viscosity, Thermal Conductivity and Crystallization Behavior. Journal of Non-Crystalline Solids, 2004. 345, 157- 160.
7. R.G.C. Beerkens, Modeling the kinetics of volatilization from glass melts. Journal of the American Ceramic Society, 2001. 84(9), 1952-1960.
8. Z.T. Zhang, S. Sridhar, and J.W. Cho, An Investigation of the Evaporation of B2O3 and Na2O in F-Free Mold Slags. Isij International, 2011. 51(1), 80-87.
9. M. Li, T. Utigard, and M. Barati, Kinetics of Na2O and B2O3 Loss From CaO-SiO2-Al2O3
Slags. Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science, 2015. 46(1), 74-82.
10. V.L. Stolyarova, G.G. Ivanov, and S.V. Stolyar, Vaporization and thermodynamic properties of melts in the Na2O-B2O3-SiO2 system. Glass Physics and Chemistry, 2002. 28(2), 112-116.
11. M. Persson, S. Seetharaman, and S. Seetharaman, Kinetic Studies of Fluoride evaporation from slags. ISIJ international, 2007. 47(12), 1711-1717.
12. V.L. Stolyarova et al., Thermodynamic properties of silicate glasses and melts: VII. System MgO-B2O3-SiO2. Russian Journal of General Chemistry, 2010. 80(12), 2405-2413.
13. J. Szekely and N.J. Themelis, Rate phenomena in process metallurgy (New York, NY:
Wiley-Interscience, 1971), 439.
14. R.B. Bird, W.E. Stewart, and E.N. Lightfoot, Transport phenomena (New York, NY: John Wiley & Sons, 2007), 525-542.
15. R.B. Bird, W.E. Stewart, and E.N. Lightfoot, Transport phenomena (New York, NY: John Wiley & Sons, 2007), 864-866.
16. R.B. Bird, W.E. Stewart, and E.N. Lightfoot, Transport phenomena (New York, NY: John Wiley & Sons, 2007), 22.
