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Al2O3 ratio on the structure and properties of blast furnace slags: A molecular dynamics simulation
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
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Effect of MgO/Al2O3 ratio on the structure and properties of blast furnace slags: A molecular dynamics simulation ⁎
Chunhe Jianga, Kejiang Lia, , Jianliang Zhanga,b, Qinghua Qina, Zhengjian Liua, Minmin Suna, Ziming Wanga, Wang Lianga a b
School of Metallurgical and Ecological Engineering, University of Science and Technology, Beijing 100083, PR China School of Chemical Engineering, The University of Queensland, St Lucia,QLD, 4072, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Slags Viscosity Molecular dynamics Aluminosilicates MgO/Al2O3 ratio
SiO2-Al2O3-CaO-MgO is the most significant slag system in the blast furnace ironmaking process and it is very important to investigate the microstructure and viscosity of the system. In this paper, molecular dynamics simulations were carried out to explore the effects of MgO/Al2O3 ratio on structure and properties of the system. Based on the self-diffusion coefficients, the viscosities were calculated by Einstein-Stokes equation and compared with the experimental value and the Factsage value. The results showed that with the increase of MgO/Al2O3 ratio, the stability of [SiO4]4− and [AlO4]5− tetrahedron became weaken and the relative proportions of bridge and non-bridge oxygen showed a decrease. And due to the increase of MgO, more Mg2+ ions are used as network modifiers to reduce the degree of polymerization of the system, resulting in a decrease in the viscosity, which is consistent with experimental results. Finally, based on the present study, in the case of increasing Al2O3 content of blast furnace slag, the fluidity of slag could be adjusted by controlling MgO/Al2O3 ratio, thereby providing a basis for stable operation of blast furnace ironmaking.
1. Introduction Slag viscosity plays an important role on the stability and productivity in the blast furnace (BF) operation [1–4]. In the field of ironmaking, the slag has poor fluidity in the case of higher viscosity, which can lead to bad effects on blast furnace operation. For instance, with higher viscosity, it will be very difficult for primary and bosh slag to flow down inside the furnace, while it will be hard for final slag to be tapped outside the blast furnace. By adjusting the compositions of the slag, the melting point and viscosity of the BF slags could be optimized to a proper level, benefiting the smooth operation of BF ironmaking [5–7]. For example, an appropriate increase of basicity could reduce the slag viscosity, but it will lead to higher melting temperature. So, it is necessary to adjust the slag composition to achieve a proper smelting state with a proper smelting point and viscosity. With the degradation of iron ore resources, the charged content of Al2O3 in BF raw materials gradually increases and the content of Al2O3 in BF slag has risen sharply due to the utilization of iron ore resources with high alumina content. At present, the highest content of Al2O3 in BF slags can reach about 22%, while the lowest is only 8%. Generally, the Al2O3 content of BF slag is 15–18 wt% [5–7]. When the content of Al2O3 in the BF slag is too high, the degree of polymerization of the slag will increase, leading to
⁎
the increase of the viscosity of the slag. Due to the higher Al2O3 content in the slag, not only the viscosity of the slag becomes larger, but also the melting point of the slag will further increase (exceeding 1400 °C and even reaching 1500 °C), resulting in difficulty to melt the slag. Reducing basicity can relieve this bad effect, but it will weaken the desulfurization capacity of the slag. However, adding appropriate amount of MgO can make up for the disadvantage of high Al2O3 content [8,9]. The structure properties of aluminosilicate melts with different contents of MgO have already been studied extensively by diverse modeling and experimental methods [10–16]. For example, Kim et al. [17] revealed the influence of MgO on the viscosity of the CaO-SiO220 wt%-Al2O3-MgO slag system with the basicity from 1.0 to 1.2 and the MgO content in the range of 5–13 wt% at 1500 °C. Zhang et al. [18] revealed the influence of MgO/Al2O3 ratio on viscosity of BF slag with high Al2O3 content and concluded that the suitable value of the MgO/ Al2O3 ratio of the slag decrease with the Al2O3 concentration increasing. Li et al. [19] investigated the effects of MgO/Al2O3 ratio and basicity on the viscosities of CaO-MgO-SiO2-Al2O3 slag by experiments and modeling and concluded that the addition of MgO decreased the viscosity of slag due to the network-modifying characteristic of this component. Therefore, the effect of MgO and Al2O3 on the BF slags has been discussed systematically. However, few researches regarding the
Corresponding author at: 30 Xueyuan Road, Haidian District, Beijing 100083, PR China. E-mail address: [email protected] (K. Li).
https://doi.org/10.1016/j.jnoncrysol.2018.06.043 Received 2 April 2018; Received in revised form 29 June 2018; Accepted 30 June 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Jiang, C., Journal of Non-Crystalline Solids (2018), https://doi.org/10.1016/j.jnoncrysol.2018.06.043
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loops were carried out to ensure equilibrium till no further variation of potential energy. The mean square displacement of atoms was monitored regularly to ensure the molten state of the simulated system. In the final stage, the system was equilibrated at 1773 K for 1000 ps and the simulation data was collected. The ISSACS program [25] was used to analyze the simulation trajectories. PDF, CN, MSD, diffusion coefficient and viscosity were calculated with the methods introduced in the Supporting information (S1).
mechanism of BF slags with different ratios of MgO/Al2O3 content have been carried out. Based on the high content of Al2O3 in BF slags, an indepth understanding about the structure of BF slags with different ratios of MgO/Al2O3 is essentially important of further optimize the slag composition and properties. Due to the limitations of high-temperature experiments, it is difficult to explain the mechanism of macroscopic properties changes from the perspective of microstructure. Therefore, MD simulations have been widely applied to the metallurgical field in recent years [20–22]. This article uses MD simulations to study the effect of MgO/Al2O3 ratio on the structure and properties of blast furnace slag. By comparing the changes in the atomic structure, it was found that the increase of MgO/Al2O3 ratio could improve the degree of depolymerization of the system by increasing more network modifiers, resulting in a decrease in the viscosity. The present study provides a theoretical basis for the industrial operation of blast furnace slag.
3. Results and discussion 3.1. Local structural order and bond lengths The network structure and stacking structure of various atoms in the melt are shown in Fig.1 and the effective ionic radii of various atoms are as following: Si4+(0.4 Å), Al3+(0.535 Å), Ca2+(1.0 Å), Mg2+(0.72 Å) and O2−(1.40 Å) [26]. By observing the change of the atom position and the structure of the atomic network in Fig.1, it could be clearly found that the network structure of the system is mainly composed of [SiO4]4− tetrahedron and [AlO4]5− tetrahedron. Meanwhile, it can also be found that O2– ions are distributed around Si4+ and Al3+ ions from the stacking structure. In addition, Ca2+ ions and Mg2+ ions appeared to be in a free state in the system, with no bonding with O2– ions. And it could be found that Ca2+ and Mg2+ ions principally existed in the interstitial sites or the hole of network structure. The most of Ca2+ and Mg2+ ions existed around the O2– ions. Based on the characteristics of Ca2+ and Mg2+ ions as alkaline cations, it can be concluded that this part of the ions were providing charge compensation. The influence of MgO/Al2O3 ratio on the Pair distribution functions (PDF) of various ion-oxygen pairs in the SiO2-CaO-MgO-Al2O3 system are shown in Fig.S2–S4. The simulation results for different Al2O3 contents have been shown in separate columns for comparison. The first peaks of every iron-oxygen PDFs, which correspond to the most probable bond distances, are adopted to estimate the bond length for different pairs. These distances have been summarized in Table 2. For SieO bonds, there was no influence of enhancing MgO/Al2O3 ratio on the nearest neighbor bond length (1.619 Å) and the second neighbor distances changed slightly. In the case of AleO bonds, the nearest neighbor bond length (1.731 Å) was not influenced by the different MgO/Al2O3 ratios and the second neighbor distances showed the same change with the SieO bonds. However, the intensities of the first peak in SieO and AleO pairs increased with the enhancing of MgO/Al2O3 ratios, which implied that the stability of the structure decreased. And the bond length of CaeO and MgeO showed the same trend which hasn't changed. Thus, the variation of MgO/Al2O3 ratios do not influence on the local structures and bond lengths obviously. The coordination numbers (CNs) for atom i (Si. Ca. Mg. Al) around O are determined by numerical integration of the PDFs within the cutoff radius corresponding to the first valley of the PDFs. The calculation results for the coordination numbers have been shown in Fig.S5–S7 and Table 2. The coordination number of SieO is 4.02, which indicates that the majority of Si atoms were tetrahedrally coordinated with oxygen. For the AleO bonds, the coordination numbers is about 4.2, which is greater than the expected coordination “4” in a tetrahedra structure. With the increase of MgO/Al2O3 ratios, the amplitude of variation in SieO CNs was significantly smaller than that of AleO, indicating that the [AlO4]5− tetrahedrons was more easily destroyed, and further showing that the stability of [SiO4]4− tetrahedrons is better than [AlO4]5− tetrahedrons. As compensation cations and network modifiers, the Ca2+ and Mg2+ ions did not form any stable structural units. And the CN curves of CaeO and MgeO pairs didn't show any platform and the coordination numbers seem to increase as the proportion of MgO/Al2O3 increasing.
2. Simulation methods Fifteen CaO-MgO-Al2O3-SiO2 (CMAS) slag systems covering different ratios of MgO/Al2O3 were selected for molecular dynamics simulations. These quaternary slags were also selected to cover different Al2O3 contents and Table S1 summarizes the composition data and free funning temperature. The concept of free running temperature is defined in supporting information S1. And as these system's free running temperature are below 1773 K, these different systems are always free to flow during the simulation. The notations A10, A16 and A22 indicate the Al2O3 content of 10%, 16% and 22%. And the following numbers represent the group number. With the increase of group number, the MgO/Al2O3 ratio increased. A total number of almost 10,000 atoms are used in every simulation model. All atoms were placed in a cubic model box and the size of the simulation box was determined by their molar masses and densities. The simulation temperature was chosen as 1773 K, which was similar to the BF slag temperature. A Nose-Hoover thermostat was used for the NVT simulations, with periodic boundary conditions. All molecular dynamics simulations were carried out using CMAS94 model (Matsui 1994) [23]. This potential can be written as:
U (rij ) =
qi qj rij
+ f (Bi + Bj ) exp
(Ai + Aj − rij ) (Bi + Bj )
−
Ci Cj r6
(1)
where rij is the interatomic distance between atoms i and j, f is a standard force of 4.184 kJ·mol−1, and qij, Aij, Bij and Ci are energy parameters peculiar of the kind of atom i. The first term of equation represents the long-range Coulomb potential. The second and third term represent van der Waals and repulsions, respectively. Various parameters used have been listed in Table 1. LAMMPS package [version 16 Feb 2016] [24] was used to perform these simulations with an integration time-step of 1 fs. Long range Coulomb interactions were evaluated by the Ewald summation method with a cut-off distance of 10 Å and the corresponding cut-off distances for the short-range Born &Van der Waals was chosen to be 8.0 Å. The simulation was started at T = 5000 K from a random configuration of different atoms; the system was thermalized for 100 ps and then cooling down to 1773 K in 322.7 ps with a cooling rate of 1013 K/s. And then the system was kept 500 ps to achieve equilibrium. Several iteration Table 1 CMAS94 model parameter s used in this simulation. Ion
q(e)
A(Å)
B(Å)
C(Å3(kJ/mol)1/2)
Si Ca Mg Al O
1.890 0.945 0.945 1.4175 −0.945
0.7204 1.1720 0.8940 0.7852 1.8215
0.023 0.040 0.040 0.034 0.138
49.30 45.00 29.05 36.82 90.61
2
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Fig. 1. The network structure and stacking structure of melt: atomic distribution of Ca2+ and Mg2+ ions in the A16-3 sample.
system. Further study is required about the similarities and differences between the effects of Mg2+ and Ca2+ ions on the structure.
3.2. Oxygen bonding networks The structure of the aluminosilicate system is mainly a network structure connected by oxygen and the types of oxygen could be divided in to bridge oxygen, non-bridge oxygen, oxygen triclusters and free oxygen. Among them, bridge oxygen is coordinated by two tetrahedral structures, which can be divided in to Si-O-Si, Si-O-Al and Al-O-Al. Nonbridge oxygen is coordinated by one tetrahedral structures, including OeSi and OeAl. And there is a positive correlation between the ratio of bridge oxygen to non-bridge oxygen and degree of polymerization. The evolution of different types of oxygen bonding networks is shown in the Fig.2. Ca2+ ions and Mg2+ ions could produce non-bridge oxygen by breaking the Si-O-Si, Si-O-Al and Al-O-Al bonds, thus weakening the tetrahedral structure. In this article, simulation experiments were conducted by controlling the basicity and changing the proportion of MgO/Al2O3. With the increase of MgO/Al2O3 ratio, the concentration of bridging oxygen showed a decreasing trend, while the concentration of non-bridging oxygen showed an increasing trend, which indicates that the polymerization degree increased. Similarly, the increase of free oxygens also has a degrading effect on the polymerization degree. The evolution of different types of bridge oxygen is shown in Fig.3. It could be found that the bridge oxygen Si-O-Si and SiO-Al shown a decrease trend with the increase of MgO/Al2O3 ratio. And the bridge oxygen Al-O-Al showed an opposite trend. While keeping the MgO/Al2O3 ratio constant, the bridge oxygen Si-O-Si showed a decrease trend and the other types of bridge oxygen showed an increase trend. In Fig.4, the BO-Ca means the bridge oxygen was charge compensated by Ca2+ ions and the BO-Mg means the bridge oxygen was charge compensated by Mg2+ ions. As shown in the Fig.4, it is found that a part of Mg2+ ions supply additional charge compensation and the concentration of BO-Ca showed a fluctuating trend. Because the relative content of Ca2+ ions showed a decrease trend with the increase of MgO/Al2O3 ratio, it is hard to analyze the interaction of Mg2+ and Ca2+ ions in the
3.3. Transport properties The function of the mean square displacement (MSD) about time could be generated through the statistical analysis of the atomic trajectory. The MSD vs time cure for each atom is linear, indicating that each type of diffusion unit is free to flow in the system. And the selfdiffusion coefficient would be calculated on the basis of the MSD curves of particles. The computed results for various ions are shown in Fig.5. The diffusion ability of various atoms could be summarized as follows: Mg2+ > Ca2+ > Al3+ > O2– > Si4+. Due to the [SiO4]4− tetrahedron was the most stable structure unit in the melt, the movement of Si4+ ions were imprisoned, leading to its self-diffusion coefficient is the lowest in any trials. As amphoteric oxide, a part of Al3+ ions formed network modifier, which had stronger diffusion capability than the network former. Thus, the self-diffusion coefficient of Al3+ ions is greater than Si4+ and O2– ions. For Mg2+ and Ca2+ ions, the diffusion ability of both ions is greater than that of other three ions due to the basis properties of alkaline metals. And smaller radius of Mg2+ ions as compared to Ca2+ resulted in a higher diffusivity of Mg2+ in the system. Although the evolution of the self-diffusion coefficients of different atoms are not exactly analogous, the total self-diffusion coefficient of all atoms showed an obvious increase trend, suggesting that the liquidity of the system has become better. It could be found that when the MgO/Al2O3 ratio is the same, the total diffusion coefficient of the system shows an increasing trend with the Al2O3 content increases, except for the ratio of MgO/Al2O3 is 0.4. It is because that when the Al2O3 content is 10%, the proportion of the increased MgO content in the system is smaller than others, resulting in that insufficient network modifiers were provided and the degree of polymerization decreased slowly, which is consistent with the trend of bridge oxygen changes.
Table 2 Average first (r1) and second (r2) peak positions and coordination numbers in different samples. Si-O
A10-1 A10-2 A10-3 A10-4 A10-5 A16-1 A16-2 A16-3 A16-4 A16-5 A22-1 A22-2 A22-3 A22-4 A22-5
r1(Å) 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619
Al-O r2(Å) 4.169 4.169 4.181 4.169 4.181 4.181 4.181 4.181 4.181 4.194 4.181 4.194 4.181 4.181 4.194
CN 4.017 4.017 4.019 4.019 4.020 4.018 4.019 4.021 4.024 4.024 4.019 4.022 4.024 4.027 4.031
r1(Å) 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731
Ca-O r2(Å) 4.256 4.256 4.256 4.256 4.256 4.256 4.256 4.269 4.256 4.269 4.269 4.269 4.269 4.269 4.269
CN 4.178 4.178 4.188 4.202 4.218 4.182 4.204 4.214 4.231 4.260 4.207 4.225 4.263 4.275 4.297
3
r1(Å) 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369
Mg-O r2(Å) 4.756 4.744 4.756 4.756 4.756 4.744 4.744 4.744 4.769 4.756 4.744 4.744 4.744 4.756 4.756
CN 7.639 7.510 7.584 7.632 7.704 7.603 7.649 7.695 7.790 7.804 7.700 7.791 7.857 7.890 7.926
r1(Å) 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994
r2(Å) 4.444 4.431 4.431 4.456 4.444 4.431 4.431 4.444 4.456 4.444 4.444 4.444 4.444 4.444 4.456
CN 5.183 5.232 5.342 5.346 5.364 5.282 5.285 5.322 5.418 5.402 5.366 5.402 5.420 5.519 5.550
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Fig. 2. The concentration of bridging, nonbridging and free oxygen as a function of MgO/Al2O3 ratio.
And when the MgO/Al2O3 ratio is 0.4, the total diffusion coefficient of the system is between the other two, which shows that the fluidity of the system is somewhere in between. 3.4. Variation of viscosity The viscosities of aluminosilicates are shown in Fig.6(a). Since there is no experiment results of viscosity in the same composition system, some similar composition experiments are shown in Fig.6(b) for comparison. Since the viscosity mode in Factsage is optimized to fit the experimental data for pure oxides and selected binary and ternary systems, it is accurate to estimate viscosity of slag system, especially the most common SiO2-CaO-MgO-Al2O3 sytem [27–31]. By comparison, it could be found that the calculated value of MD is close to that of Factsage, which indicates that the calculated results are in good agreement. And due to the low alkalinity of the experimental components, it may cause the experimental value to be slightly larger than the simulated value. It was found that the viscosity showed a decrease trend with the increase of MgO/Al2O3, which is consistent with the experimental results. It is further explained that in the case of constant Al2O3 content, the addition of MgO leaded to more network modifier to depolymerize the network structure, resulting in the reduction of the polymerization degree, which is consistent with the literature [32]. This mechanism could be explained by Fig.7. Mg2+ and Ca2+ ions act in a similar manner, with one part acting as compensation cations and the other as network modifiers. When the compensation cations are sufficient, the Mg2+ ions will depolymerize the network structure of the system as network modifiers, reducing the ratio of bridge oxygen and non-bridge oxygen, which result in a decrease in the degree of
Fig. 4. Variation of bridging oxygen charge compensated by Ca2+ or Mg2+ ions.
polymerization and viscosity of the system. However, keeping MgO/Al2O3 ratio invariant, the viscosity also showed a decreasing trend with the increase of Al2O3 content, except for the first group (MgO/Al2O3 = 0.4), which is consistent with changes in total diffusion coefficient. However, it is interesting that the
Fig. 3. The evolution of different types of bridge oxygen: (a) Si-O-Si; (b) Si-O-Al; (c) Al-O-Al. 4
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Fig. 5. Self-diffusion coefficients of various with different MgO/Al2O3 ratios: (a)Al2O3 content is 10%; (b)Al2O3 content is 16%; (c)Al2O3 content is 22%; (d)total selfdiffusion coefficient of all atoms in different system.
And during the process of Al2O3 content increased, the proportion of SiO2 presented a decreasing trend and [SiO4]4− tetrahedron is more stable, which may lead to inconsistent changes in viscosity and BO concentrations.
concentration of BO showed an increasing trend. Through the previous analysis, it could be found that the [SiO4]4− tetrahedron is more stable than the [AlO4]5− tetrahedron. In Fig.3, it could be found that keep the MgO/Al2O3 ratio constant, the bridge oxygen Si-O-Si showed a decrease trend and the bridge oxygen Al-O-Al showed an opposite trend, which could make the structure instability, with the increase of Al2O3 content.
Fig. 6. Viscosity of MD simulation results and experimental data [33–36] 5
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Fig. 7. The depolymerization of network structure with MgO and CaO.
of Prof. Mansoor Barati of the department of materials science at the University of Toronto, Canana. The authors acknowledge the financial support of the National Science Foundation of China (51774032), the Chinese Fundamental Research Funds for the Central Universities (FRFTP-17-086A1), and the National Key Research and Development Program of China (2017YFB0304300 & 2017YFB0304303).
3.5. Guidance for slag structure optimization The operation of the blast furnace is closely related to the fluidity of the slag. And it has always been the concern of people to understand the impact of changes in the composition of blast furnace slag. Based on the research in this paper, it was found that the effect of MgO on the aluminosilicate system is similar to that of CaO. Both of them could reduce the degree of polymerization as network modifiers, resulting in better fluidity. Nowadays, with the decrease of low-alumina ore, the content of Al2O3 in blast furnace slag is increasing day by day, resulting in increased viscosity. Based on the characteristics of MgO, the viscosity of the slag could be reduced by increasing MgO/Al2O3 ratio without change of the basicity, so as to ensure the better smelting state of blast furnace.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jnoncrysol.2018.06.043. References [1] I. Sohn, D.J. Min, A review of the relationship between viscosity and the structure of calcium-silicate-based slags in ironmaking, Steel Res. Int. 83 (7) (2012) 611–630. [2] J.H. Park, D.S. Kim, Effect of CaO-Al2O3-MgO slags on the formation of MgO-Al2O3 inclusions in ferritic stainless steel, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 36 (4) (2005) 495–502. [3] N. Saito, N. Hori, K. Nakashima, K. Mori, Viscosity of blast furnace type slags, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 34 (5) (2003) 509–516. [4] A. Shankar, M. Gornerup, A.K. Lahiri, S. Seetharaman, Estimation of viscosity for blast furnace type slags, Ironmak. Steelmak. 34 (6) (2007) 477–481. [5] K. Sunahara, K. Nakano, M. Hoshi, T. Inada, S. Komatsu, T. Yamamoto, Effect of high Al2O3 slag on the blast furnace operations, Tetsu Hagane 92 (12) (2006) 875–884. [6] F. Shen HZ, X. Jiang, G. Wei, Q. Wen, Influence of Al2O3 in blast furnace smelting and discussions on proper w(MgO)/w(Al2O3), Ironmak. Steelmak. 49 (2014) 1. [7] D. Papanastassiou, P. Nicolaou, A. Send, The effect of Al2O3 and MgO contents on the properties of the blast furnace slag, Stahl Eisen 120 (7) (2000) 59–64. [8] C. Wang, J.L. Zhang, K.X. Jiao, Z.J. Liu, Influence of basicity and MgO/Al2O3 ratio on the viscosity of blast furnace slags containing chloride, Metall. Res. Technol. 114 (2) (2017). [9] X.F. Zhang, T. Jiang, X.X. Xue, B.S. Hu, Influence of MgO/Al2O3 ratio on viscosity of blast furnace slag with high Al2O3 content, Steel Res. Int. 87 (1) (2016) 87–94. [10] M. Bouhadja, N. Jakse, A. Pasturel, Structural and dynamic properties of calcium aluminosilicate melts: a molecular dynamics study, J. Chem. Phys. 138 (22) (2013) 224510. [11] Gao Y-m, Wang S-b, C. Hong, X.-j. Ma, F. Yang, Effects of basicity and MgO content on the viscosity of the SiO2-CaO-MgO-9wt%Al2O3 slag system, Int. J. Miner. Metall. Mater. 21 (4) (2014) 353–362. [12] L. Yao, S. Ren, X. Wang, et al., Effect of Al2O3, MgO, and CaO/SiO2on viscosity of high alumina blast furnace slag, Steel Res. Int. 87 (2) (2016) 241–249. [13] D. Liang, Z.M. Yan, X.W. Lv, J. Zhang, C.G. Bai, Transition of blast furnace slag from silicate-based to aluminate-based: structure evolution by molecular dynamics simulation and Raman spectroscopy, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 48 (1) (2017) 573–581. [14] L. Mongalo, A.S. Lopis, G.A. Venter, Molecular dynamics simulations of the structural properties and electrical conductivities of CaO–MgO–Al 2 O 3 –SiO 2 melts, J. Non-Cryst. Solids 452 (2016) 194–202. [15] S. Ren, Q.C. Liu, J.L. Zhang, M. Chen, X.D. Ma, B.J. Zhao, Laboratory study of phase transitions and mechanism of reduction of FeO from high Ti-bearing blast furnace primary slag by graphite, Ironmak. Steelmak. 42 (2) (2015) 117–125. [16] Z.M. Yan, X.W. Lv, Z.D. Pang, W.C. He, D. Liang, C.G. Bai, Transition of blast furnace slag from silicate based to aluminate based: sulfide capacity, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 48 (5) (2017) 2607–2614. [17] H. Kim, W.H. Kim, I. Sohn, D.J. Min, The effect of MgO on the viscosity of the CaOSiO2-20 wt%Al2O3-MgO slag system, Steel Res. Int. 81 (4) (2010) 261–264.
4. Conclusions Molecular dynamics simulation was carried out on SiO-CaO-MgOAl2O3 systems with different MgO/Al2O3 ratio to study the structure and properties of these systems. These simulations were able to reproduce the microstructure difference and experimentally observed trend in slag viscosity with different MgO/Al2O3 ratio. These simulations have highlighted the effect of Mg2+ ions as network modifiers to depolymerize the structure of aluminosilicate. The influence of MgO/ Al2O3 ratio on local structure was negligible. And it was found that the nearest neighbor bond length of SieO and AleO were not influenced and the [SiO4]4− tetrahedron is more stable than [AlO4]5− tetrahedron with the increase of MgO/Al2O3 ratio. While increasing the MgO/Al2O3 ratio, the proportion of bridge oxygen to non-bridge oxygen showed a decreasing trend, as a result of more network modifiers (Mg2+). As excepted, the diffusion ability of various atoms is as follows: Mg2+ > Ca2+ > Al3+ > O2– > Si4+. And The total self-diffusion coefficient of all atoms showed an increasing trend, which reflects that the viscosity became lower. With the increase of MgO/Al2O3 ratio, the degree of polymerization of system decreased, which led to a decrease in the viscosity and is consistent with the experimental results. Based on the results of this paper, the properties of blast furnace slags with different Al2O3 content could be adjusted by addition of appropriate MgO content. Acknowledgement All these computations were performed on the GPC supercomputer at the SciNet HPC Consortium in the Compute/Calcul Canada National Computing Platform. SciNet is funded by the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund - Research Excellence; and the University of Toronto. The authors acknowledge the technical support 6
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Effect of MgO/Al2O3 ratio on the structure and properties of blast furnace slags: A molecular dynamics simulation ⁎
Chunhe Jianga, Kejiang Lia, , Jianliang Zhanga,b, Qinghua Qina, Zhengjian Liua, Minmin Suna, Ziming Wanga, Wang Lianga a b
School of Metallurgical and Ecological Engineering, University of Science and Technology, Beijing 100083, PR China School of Chemical Engineering, The University of Queensland, St Lucia,QLD, 4072, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Slags Viscosity Molecular dynamics Aluminosilicates MgO/Al2O3 ratio
SiO2-Al2O3-CaO-MgO is the most significant slag system in the blast furnace ironmaking process and it is very important to investigate the microstructure and viscosity of the system. In this paper, molecular dynamics simulations were carried out to explore the effects of MgO/Al2O3 ratio on structure and properties of the system. Based on the self-diffusion coefficients, the viscosities were calculated by Einstein-Stokes equation and compared with the experimental value and the Factsage value. The results showed that with the increase of MgO/Al2O3 ratio, the stability of [SiO4]4− and [AlO4]5− tetrahedron became weaken and the relative proportions of bridge and non-bridge oxygen showed a decrease. And due to the increase of MgO, more Mg2+ ions are used as network modifiers to reduce the degree of polymerization of the system, resulting in a decrease in the viscosity, which is consistent with experimental results. Finally, based on the present study, in the case of increasing Al2O3 content of blast furnace slag, the fluidity of slag could be adjusted by controlling MgO/Al2O3 ratio, thereby providing a basis for stable operation of blast furnace ironmaking.
1. Introduction Slag viscosity plays an important role on the stability and productivity in the blast furnace (BF) operation [1–4]. In the field of ironmaking, the slag has poor fluidity in the case of higher viscosity, which can lead to bad effects on blast furnace operation. For instance, with higher viscosity, it will be very difficult for primary and bosh slag to flow down inside the furnace, while it will be hard for final slag to be tapped outside the blast furnace. By adjusting the compositions of the slag, the melting point and viscosity of the BF slags could be optimized to a proper level, benefiting the smooth operation of BF ironmaking [5–7]. For example, an appropriate increase of basicity could reduce the slag viscosity, but it will lead to higher melting temperature. So, it is necessary to adjust the slag composition to achieve a proper smelting state with a proper smelting point and viscosity. With the degradation of iron ore resources, the charged content of Al2O3 in BF raw materials gradually increases and the content of Al2O3 in BF slag has risen sharply due to the utilization of iron ore resources with high alumina content. At present, the highest content of Al2O3 in BF slags can reach about 22%, while the lowest is only 8%. Generally, the Al2O3 content of BF slag is 15–18 wt% [5–7]. When the content of Al2O3 in the BF slag is too high, the degree of polymerization of the slag will increase, leading to
⁎
the increase of the viscosity of the slag. Due to the higher Al2O3 content in the slag, not only the viscosity of the slag becomes larger, but also the melting point of the slag will further increase (exceeding 1400 °C and even reaching 1500 °C), resulting in difficulty to melt the slag. Reducing basicity can relieve this bad effect, but it will weaken the desulfurization capacity of the slag. However, adding appropriate amount of MgO can make up for the disadvantage of high Al2O3 content [8,9]. The structure properties of aluminosilicate melts with different contents of MgO have already been studied extensively by diverse modeling and experimental methods [10–16]. For example, Kim et al. [17] revealed the influence of MgO on the viscosity of the CaO-SiO220 wt%-Al2O3-MgO slag system with the basicity from 1.0 to 1.2 and the MgO content in the range of 5–13 wt% at 1500 °C. Zhang et al. [18] revealed the influence of MgO/Al2O3 ratio on viscosity of BF slag with high Al2O3 content and concluded that the suitable value of the MgO/ Al2O3 ratio of the slag decrease with the Al2O3 concentration increasing. Li et al. [19] investigated the effects of MgO/Al2O3 ratio and basicity on the viscosities of CaO-MgO-SiO2-Al2O3 slag by experiments and modeling and concluded that the addition of MgO decreased the viscosity of slag due to the network-modifying characteristic of this component. Therefore, the effect of MgO and Al2O3 on the BF slags has been discussed systematically. However, few researches regarding the
Corresponding author at: 30 Xueyuan Road, Haidian District, Beijing 100083, PR China. E-mail address: [email protected] (K. Li).
https://doi.org/10.1016/j.jnoncrysol.2018.06.043 Received 2 April 2018; Received in revised form 29 June 2018; Accepted 30 June 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Jiang, C., Journal of Non-Crystalline Solids (2018), https://doi.org/10.1016/j.jnoncrysol.2018.06.043
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loops were carried out to ensure equilibrium till no further variation of potential energy. The mean square displacement of atoms was monitored regularly to ensure the molten state of the simulated system. In the final stage, the system was equilibrated at 1773 K for 1000 ps and the simulation data was collected. The ISSACS program [25] was used to analyze the simulation trajectories. PDF, CN, MSD, diffusion coefficient and viscosity were calculated with the methods introduced in the Supporting information (S1).
mechanism of BF slags with different ratios of MgO/Al2O3 content have been carried out. Based on the high content of Al2O3 in BF slags, an indepth understanding about the structure of BF slags with different ratios of MgO/Al2O3 is essentially important of further optimize the slag composition and properties. Due to the limitations of high-temperature experiments, it is difficult to explain the mechanism of macroscopic properties changes from the perspective of microstructure. Therefore, MD simulations have been widely applied to the metallurgical field in recent years [20–22]. This article uses MD simulations to study the effect of MgO/Al2O3 ratio on the structure and properties of blast furnace slag. By comparing the changes in the atomic structure, it was found that the increase of MgO/Al2O3 ratio could improve the degree of depolymerization of the system by increasing more network modifiers, resulting in a decrease in the viscosity. The present study provides a theoretical basis for the industrial operation of blast furnace slag.
3. Results and discussion 3.1. Local structural order and bond lengths The network structure and stacking structure of various atoms in the melt are shown in Fig.1 and the effective ionic radii of various atoms are as following: Si4+(0.4 Å), Al3+(0.535 Å), Ca2+(1.0 Å), Mg2+(0.72 Å) and O2−(1.40 Å) [26]. By observing the change of the atom position and the structure of the atomic network in Fig.1, it could be clearly found that the network structure of the system is mainly composed of [SiO4]4− tetrahedron and [AlO4]5− tetrahedron. Meanwhile, it can also be found that O2– ions are distributed around Si4+ and Al3+ ions from the stacking structure. In addition, Ca2+ ions and Mg2+ ions appeared to be in a free state in the system, with no bonding with O2– ions. And it could be found that Ca2+ and Mg2+ ions principally existed in the interstitial sites or the hole of network structure. The most of Ca2+ and Mg2+ ions existed around the O2– ions. Based on the characteristics of Ca2+ and Mg2+ ions as alkaline cations, it can be concluded that this part of the ions were providing charge compensation. The influence of MgO/Al2O3 ratio on the Pair distribution functions (PDF) of various ion-oxygen pairs in the SiO2-CaO-MgO-Al2O3 system are shown in Fig.S2–S4. The simulation results for different Al2O3 contents have been shown in separate columns for comparison. The first peaks of every iron-oxygen PDFs, which correspond to the most probable bond distances, are adopted to estimate the bond length for different pairs. These distances have been summarized in Table 2. For SieO bonds, there was no influence of enhancing MgO/Al2O3 ratio on the nearest neighbor bond length (1.619 Å) and the second neighbor distances changed slightly. In the case of AleO bonds, the nearest neighbor bond length (1.731 Å) was not influenced by the different MgO/Al2O3 ratios and the second neighbor distances showed the same change with the SieO bonds. However, the intensities of the first peak in SieO and AleO pairs increased with the enhancing of MgO/Al2O3 ratios, which implied that the stability of the structure decreased. And the bond length of CaeO and MgeO showed the same trend which hasn't changed. Thus, the variation of MgO/Al2O3 ratios do not influence on the local structures and bond lengths obviously. The coordination numbers (CNs) for atom i (Si. Ca. Mg. Al) around O are determined by numerical integration of the PDFs within the cutoff radius corresponding to the first valley of the PDFs. The calculation results for the coordination numbers have been shown in Fig.S5–S7 and Table 2. The coordination number of SieO is 4.02, which indicates that the majority of Si atoms were tetrahedrally coordinated with oxygen. For the AleO bonds, the coordination numbers is about 4.2, which is greater than the expected coordination “4” in a tetrahedra structure. With the increase of MgO/Al2O3 ratios, the amplitude of variation in SieO CNs was significantly smaller than that of AleO, indicating that the [AlO4]5− tetrahedrons was more easily destroyed, and further showing that the stability of [SiO4]4− tetrahedrons is better than [AlO4]5− tetrahedrons. As compensation cations and network modifiers, the Ca2+ and Mg2+ ions did not form any stable structural units. And the CN curves of CaeO and MgeO pairs didn't show any platform and the coordination numbers seem to increase as the proportion of MgO/Al2O3 increasing.
2. Simulation methods Fifteen CaO-MgO-Al2O3-SiO2 (CMAS) slag systems covering different ratios of MgO/Al2O3 were selected for molecular dynamics simulations. These quaternary slags were also selected to cover different Al2O3 contents and Table S1 summarizes the composition data and free funning temperature. The concept of free running temperature is defined in supporting information S1. And as these system's free running temperature are below 1773 K, these different systems are always free to flow during the simulation. The notations A10, A16 and A22 indicate the Al2O3 content of 10%, 16% and 22%. And the following numbers represent the group number. With the increase of group number, the MgO/Al2O3 ratio increased. A total number of almost 10,000 atoms are used in every simulation model. All atoms were placed in a cubic model box and the size of the simulation box was determined by their molar masses and densities. The simulation temperature was chosen as 1773 K, which was similar to the BF slag temperature. A Nose-Hoover thermostat was used for the NVT simulations, with periodic boundary conditions. All molecular dynamics simulations were carried out using CMAS94 model (Matsui 1994) [23]. This potential can be written as:
U (rij ) =
qi qj rij
+ f (Bi + Bj ) exp
(Ai + Aj − rij ) (Bi + Bj )
−
Ci Cj r6
(1)
where rij is the interatomic distance between atoms i and j, f is a standard force of 4.184 kJ·mol−1, and qij, Aij, Bij and Ci are energy parameters peculiar of the kind of atom i. The first term of equation represents the long-range Coulomb potential. The second and third term represent van der Waals and repulsions, respectively. Various parameters used have been listed in Table 1. LAMMPS package [version 16 Feb 2016] [24] was used to perform these simulations with an integration time-step of 1 fs. Long range Coulomb interactions were evaluated by the Ewald summation method with a cut-off distance of 10 Å and the corresponding cut-off distances for the short-range Born &Van der Waals was chosen to be 8.0 Å. The simulation was started at T = 5000 K from a random configuration of different atoms; the system was thermalized for 100 ps and then cooling down to 1773 K in 322.7 ps with a cooling rate of 1013 K/s. And then the system was kept 500 ps to achieve equilibrium. Several iteration Table 1 CMAS94 model parameter s used in this simulation. Ion
q(e)
A(Å)
B(Å)
C(Å3(kJ/mol)1/2)
Si Ca Mg Al O
1.890 0.945 0.945 1.4175 −0.945
0.7204 1.1720 0.8940 0.7852 1.8215
0.023 0.040 0.040 0.034 0.138
49.30 45.00 29.05 36.82 90.61
2
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Fig. 1. The network structure and stacking structure of melt: atomic distribution of Ca2+ and Mg2+ ions in the A16-3 sample.
system. Further study is required about the similarities and differences between the effects of Mg2+ and Ca2+ ions on the structure.
3.2. Oxygen bonding networks The structure of the aluminosilicate system is mainly a network structure connected by oxygen and the types of oxygen could be divided in to bridge oxygen, non-bridge oxygen, oxygen triclusters and free oxygen. Among them, bridge oxygen is coordinated by two tetrahedral structures, which can be divided in to Si-O-Si, Si-O-Al and Al-O-Al. Nonbridge oxygen is coordinated by one tetrahedral structures, including OeSi and OeAl. And there is a positive correlation between the ratio of bridge oxygen to non-bridge oxygen and degree of polymerization. The evolution of different types of oxygen bonding networks is shown in the Fig.2. Ca2+ ions and Mg2+ ions could produce non-bridge oxygen by breaking the Si-O-Si, Si-O-Al and Al-O-Al bonds, thus weakening the tetrahedral structure. In this article, simulation experiments were conducted by controlling the basicity and changing the proportion of MgO/Al2O3. With the increase of MgO/Al2O3 ratio, the concentration of bridging oxygen showed a decreasing trend, while the concentration of non-bridging oxygen showed an increasing trend, which indicates that the polymerization degree increased. Similarly, the increase of free oxygens also has a degrading effect on the polymerization degree. The evolution of different types of bridge oxygen is shown in Fig.3. It could be found that the bridge oxygen Si-O-Si and SiO-Al shown a decrease trend with the increase of MgO/Al2O3 ratio. And the bridge oxygen Al-O-Al showed an opposite trend. While keeping the MgO/Al2O3 ratio constant, the bridge oxygen Si-O-Si showed a decrease trend and the other types of bridge oxygen showed an increase trend. In Fig.4, the BO-Ca means the bridge oxygen was charge compensated by Ca2+ ions and the BO-Mg means the bridge oxygen was charge compensated by Mg2+ ions. As shown in the Fig.4, it is found that a part of Mg2+ ions supply additional charge compensation and the concentration of BO-Ca showed a fluctuating trend. Because the relative content of Ca2+ ions showed a decrease trend with the increase of MgO/Al2O3 ratio, it is hard to analyze the interaction of Mg2+ and Ca2+ ions in the
3.3. Transport properties The function of the mean square displacement (MSD) about time could be generated through the statistical analysis of the atomic trajectory. The MSD vs time cure for each atom is linear, indicating that each type of diffusion unit is free to flow in the system. And the selfdiffusion coefficient would be calculated on the basis of the MSD curves of particles. The computed results for various ions are shown in Fig.5. The diffusion ability of various atoms could be summarized as follows: Mg2+ > Ca2+ > Al3+ > O2– > Si4+. Due to the [SiO4]4− tetrahedron was the most stable structure unit in the melt, the movement of Si4+ ions were imprisoned, leading to its self-diffusion coefficient is the lowest in any trials. As amphoteric oxide, a part of Al3+ ions formed network modifier, which had stronger diffusion capability than the network former. Thus, the self-diffusion coefficient of Al3+ ions is greater than Si4+ and O2– ions. For Mg2+ and Ca2+ ions, the diffusion ability of both ions is greater than that of other three ions due to the basis properties of alkaline metals. And smaller radius of Mg2+ ions as compared to Ca2+ resulted in a higher diffusivity of Mg2+ in the system. Although the evolution of the self-diffusion coefficients of different atoms are not exactly analogous, the total self-diffusion coefficient of all atoms showed an obvious increase trend, suggesting that the liquidity of the system has become better. It could be found that when the MgO/Al2O3 ratio is the same, the total diffusion coefficient of the system shows an increasing trend with the Al2O3 content increases, except for the ratio of MgO/Al2O3 is 0.4. It is because that when the Al2O3 content is 10%, the proportion of the increased MgO content in the system is smaller than others, resulting in that insufficient network modifiers were provided and the degree of polymerization decreased slowly, which is consistent with the trend of bridge oxygen changes.
Table 2 Average first (r1) and second (r2) peak positions and coordination numbers in different samples. Si-O
A10-1 A10-2 A10-3 A10-4 A10-5 A16-1 A16-2 A16-3 A16-4 A16-5 A22-1 A22-2 A22-3 A22-4 A22-5
r1(Å) 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619 1.619
Al-O r2(Å) 4.169 4.169 4.181 4.169 4.181 4.181 4.181 4.181 4.181 4.194 4.181 4.194 4.181 4.181 4.194
CN 4.017 4.017 4.019 4.019 4.020 4.018 4.019 4.021 4.024 4.024 4.019 4.022 4.024 4.027 4.031
r1(Å) 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731 1.731
Ca-O r2(Å) 4.256 4.256 4.256 4.256 4.256 4.256 4.256 4.269 4.256 4.269 4.269 4.269 4.269 4.269 4.269
CN 4.178 4.178 4.188 4.202 4.218 4.182 4.204 4.214 4.231 4.260 4.207 4.225 4.263 4.275 4.297
3
r1(Å) 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369 2.369
Mg-O r2(Å) 4.756 4.744 4.756 4.756 4.756 4.744 4.744 4.744 4.769 4.756 4.744 4.744 4.744 4.756 4.756
CN 7.639 7.510 7.584 7.632 7.704 7.603 7.649 7.695 7.790 7.804 7.700 7.791 7.857 7.890 7.926
r1(Å) 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994 1.994
r2(Å) 4.444 4.431 4.431 4.456 4.444 4.431 4.431 4.444 4.456 4.444 4.444 4.444 4.444 4.444 4.456
CN 5.183 5.232 5.342 5.346 5.364 5.282 5.285 5.322 5.418 5.402 5.366 5.402 5.420 5.519 5.550
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Fig. 2. The concentration of bridging, nonbridging and free oxygen as a function of MgO/Al2O3 ratio.
And when the MgO/Al2O3 ratio is 0.4, the total diffusion coefficient of the system is between the other two, which shows that the fluidity of the system is somewhere in between. 3.4. Variation of viscosity The viscosities of aluminosilicates are shown in Fig.6(a). Since there is no experiment results of viscosity in the same composition system, some similar composition experiments are shown in Fig.6(b) for comparison. Since the viscosity mode in Factsage is optimized to fit the experimental data for pure oxides and selected binary and ternary systems, it is accurate to estimate viscosity of slag system, especially the most common SiO2-CaO-MgO-Al2O3 sytem [27–31]. By comparison, it could be found that the calculated value of MD is close to that of Factsage, which indicates that the calculated results are in good agreement. And due to the low alkalinity of the experimental components, it may cause the experimental value to be slightly larger than the simulated value. It was found that the viscosity showed a decrease trend with the increase of MgO/Al2O3, which is consistent with the experimental results. It is further explained that in the case of constant Al2O3 content, the addition of MgO leaded to more network modifier to depolymerize the network structure, resulting in the reduction of the polymerization degree, which is consistent with the literature [32]. This mechanism could be explained by Fig.7. Mg2+ and Ca2+ ions act in a similar manner, with one part acting as compensation cations and the other as network modifiers. When the compensation cations are sufficient, the Mg2+ ions will depolymerize the network structure of the system as network modifiers, reducing the ratio of bridge oxygen and non-bridge oxygen, which result in a decrease in the degree of
Fig. 4. Variation of bridging oxygen charge compensated by Ca2+ or Mg2+ ions.
polymerization and viscosity of the system. However, keeping MgO/Al2O3 ratio invariant, the viscosity also showed a decreasing trend with the increase of Al2O3 content, except for the first group (MgO/Al2O3 = 0.4), which is consistent with changes in total diffusion coefficient. However, it is interesting that the
Fig. 3. The evolution of different types of bridge oxygen: (a) Si-O-Si; (b) Si-O-Al; (c) Al-O-Al. 4
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Fig. 5. Self-diffusion coefficients of various with different MgO/Al2O3 ratios: (a)Al2O3 content is 10%; (b)Al2O3 content is 16%; (c)Al2O3 content is 22%; (d)total selfdiffusion coefficient of all atoms in different system.
And during the process of Al2O3 content increased, the proportion of SiO2 presented a decreasing trend and [SiO4]4− tetrahedron is more stable, which may lead to inconsistent changes in viscosity and BO concentrations.
concentration of BO showed an increasing trend. Through the previous analysis, it could be found that the [SiO4]4− tetrahedron is more stable than the [AlO4]5− tetrahedron. In Fig.3, it could be found that keep the MgO/Al2O3 ratio constant, the bridge oxygen Si-O-Si showed a decrease trend and the bridge oxygen Al-O-Al showed an opposite trend, which could make the structure instability, with the increase of Al2O3 content.
Fig. 6. Viscosity of MD simulation results and experimental data [33–36] 5
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Fig. 7. The depolymerization of network structure with MgO and CaO.
of Prof. Mansoor Barati of the department of materials science at the University of Toronto, Canana. The authors acknowledge the financial support of the National Science Foundation of China (51774032), the Chinese Fundamental Research Funds for the Central Universities (FRFTP-17-086A1), and the National Key Research and Development Program of China (2017YFB0304300 & 2017YFB0304303).
3.5. Guidance for slag structure optimization The operation of the blast furnace is closely related to the fluidity of the slag. And it has always been the concern of people to understand the impact of changes in the composition of blast furnace slag. Based on the research in this paper, it was found that the effect of MgO on the aluminosilicate system is similar to that of CaO. Both of them could reduce the degree of polymerization as network modifiers, resulting in better fluidity. Nowadays, with the decrease of low-alumina ore, the content of Al2O3 in blast furnace slag is increasing day by day, resulting in increased viscosity. Based on the characteristics of MgO, the viscosity of the slag could be reduced by increasing MgO/Al2O3 ratio without change of the basicity, so as to ensure the better smelting state of blast furnace.
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4. Conclusions Molecular dynamics simulation was carried out on SiO-CaO-MgOAl2O3 systems with different MgO/Al2O3 ratio to study the structure and properties of these systems. These simulations were able to reproduce the microstructure difference and experimentally observed trend in slag viscosity with different MgO/Al2O3 ratio. These simulations have highlighted the effect of Mg2+ ions as network modifiers to depolymerize the structure of aluminosilicate. The influence of MgO/ Al2O3 ratio on local structure was negligible. And it was found that the nearest neighbor bond length of SieO and AleO were not influenced and the [SiO4]4− tetrahedron is more stable than [AlO4]5− tetrahedron with the increase of MgO/Al2O3 ratio. While increasing the MgO/Al2O3 ratio, the proportion of bridge oxygen to non-bridge oxygen showed a decreasing trend, as a result of more network modifiers (Mg2+). As excepted, the diffusion ability of various atoms is as follows: Mg2+ > Ca2+ > Al3+ > O2– > Si4+. And The total self-diffusion coefficient of all atoms showed an increasing trend, which reflects that the viscosity became lower. With the increase of MgO/Al2O3 ratio, the degree of polymerization of system decreased, which led to a decrease in the viscosity and is consistent with the experimental results. Based on the results of this paper, the properties of blast furnace slags with different Al2O3 content could be adjusted by addition of appropriate MgO content. Acknowledgement All these computations were performed on the GPC supercomputer at the SciNet HPC Consortium in the Compute/Calcul Canada National Computing Platform. SciNet is funded by the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund - Research Excellence; and the University of Toronto. The authors acknowledge the technical support 6
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