Structural Properties of Molten CaO–SiO₂–P₂O₅–FeO System

Introduction

The physicochemical properties and transport behaviors of molten slag have important influence on most metallurgical smelting processes. Essentially, the physicochemical properties of molten slag are determined by the structure of the slag. Therefore, the structural information of molten slag is of great interest for understanding their physicochemical properties and transport behaviors in metallurgical fields. The structural properties of some simple oxide systems have been investigated through various sorts of experimental techniques, e. g. nuclear magnetic resonance (NMR), X-ray diffraction, X-ray absorption, neutron diffraction, Raman, infrared spectroscopy and so on [1, 2]. However, taking into account most metallurgical slag systems are extremely complex and the difficulties of experiments at high temperatures, molecular dynamic (MD) simulation has been widely employed to calculate the structural and transport properties of molten slag at high temperatures [3, 4, 5].

It is well known that the CaO–SiO₂–P₂O₅–FeO system is one of the most important component parts in molten converter slag. In the early stage of converter steelmaking, the viscosity of molten slag directly affects the dissolution rate of lime, slag–metal interface reaction and mass transfer of phosphorus from hot metal to the slag system [6, 7]. As the viscosity of molten slag is determined by its structure, it is necessary to investigate the structural properties of CaO–SiO₂–P₂O₅–FeO system. So far as is known to the author, most of the previous studies mainly concentrated on the structural information of simple binary or ternary silicate systems and aluminate systems [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. However, only a few studies have reported on the steelmaking slag system. Wang et al. [20] investigated the effect of P₂O₅ and Fe_tO on the viscosity and slag structure in steelmaking slags through viscosity measurement, Raman, magic angle spinning-nuclear magnetic resonance (MAS-NMR) and FTIP spectra measurements. The results show that the viscosity of molten steelmaking slag increases slightly with increasing P₂O₅ content, while decreases with increasing Fe_tO content. The degree of the polymerization of quenched steelmaking slag is found to increase with increasing P₂O₅ content and decrease with increasing Fe_tO content. Our previous work [21] studied the structural properties of the ternary molten CaO–SiO₂–P₂O₅ slag system by MD simulations. The results show that the concentration of free oxygen decreases remarkably with the increase of P₂O₅ content. The P ion has a tendency to promote the polymerization of phosphosilicate melts. Mercier et al. [22] also observed that the glass network structure became more and more polymerized while increasing P₂O₅ content in SiO₂–Na₂O–CaO–P₂O₅ bioglasses using Si and P MAS-NMR.

Because of the importance of the molten CaO–SiO₂–P₂O₅–FeO system, it is necessary to investigate and discuss it thoroughly. Almost all P₂O₅ concentrates in Ca₂SiO₄–Ca₃P₂O₈ solid solution in steelmaking slag; therefore, in this paper, we performed a MD simulation and emphatic studied the effect of basicity (mass ratio of CaO/SiO₂) on the structural properties of molten CaO–SiO₂–P₂O₅–FeO slag system.

Computational details and experiments

In this work, MD simulations are performed with the parallel package Materials Explorer 5.0 using the born mayer huggins (BMH) function [23, 24]:

where U(r_ij) is the interatomic potential; q_i and q_j are the charges of ion i and ion j; r_ij represents the distance between ion i and ion j; A_ij, B_ij and C_ij are parameters for BMH function. The first term is Coulombic interaction, the second term is inter-core short-range repulsion and the last term represents van der Waals attraction.

The charge of Ca, Si, P, Fe and O are 2, 4, 5, 2 and −2, respectively. The Coulombic potential was calculated by using the Ewald summation. All the noncoulombic parts of the pair potentials were subjected to a short-range cutoff of 10 Å. The interatomic potential parameters of the CaO–SiO₂–P₂O₅–FeO slag system in this study were determined by using empirical parameters model. Optimization of the empirical parameters based on Hirao and Kawamura [25] was carried out by Lammps program. The principle of the optimization is energy minimization. The optimized parameters A_ij, B_ij and C_ij are listed in Table 1.

The slag composition, total number of atoms and other detailed information of the CaO–SiO₂–P₂O₅–FeO system are presented in Table 2. To eliminate the edge and surface effects, three-dimensional periodic boundary condition was employed for each simulation process. The Newton motion equations were numerically integrated with a step of 1 fs. The initial temperature was set at 4,000 K for 15,000 steps to eliminate the effect of the initial distribution. Then, the temperature was decreased slowly to 2,000 K, 1,873 K and 1,673 K for 10,000 steps, respectively. The calculation was equilibrated at 1,673 K for another 25,000 steps. All simulations were performed in NVT (constant number of particles, volume and temperature) ensemble. In order to ensure that the CaO–SiO₂–P₂O₅–FeO system is a homogeneous molten slag system, the slag compositions are designed based on the CaO–SiO₂–P₂O₅–FeO phase diagram [26]. It can be seen in Figure 1 that all the slags are located in the liquid phase region at 1,673 K.

To check the validity of the numerical calculations, Raman spectroscopic analysis of the quenched slag samples have been conducted. The slag samples listed in Table 2 were prepared using analytical reagents. The mixture of slag sample was charged into a corundum crucible and melted at 1,673 K. The molten samples were held at this temperature for 1 h. Then, the sample was quenched quickly with liquid nitrogen. Raman spectroscopic analysis was performed on a Horiba LabRAM HR Evolution system.

Figure 1:

Slag compositions in CaO–SiO₂–P₂O₅–FeO system.

Results and discussion

Bond lengths, angles and coordination numbers

The calculated bond lengths of Si–O, P–O, Fe–O and Ca–O for No. 2 slag are 1.61 Å, 1.53 Å, 2.05 Å and 2.30 Å, respectively. The results are in a good agreement with the previous data in molten CaO–SiO₂–P₂O₅ system [27]. The coordination numbers of Si–O, P–O, Fe–O and Ca–O are equal to 3.86, 3.97, 5.86 and 5.10, respectively. Figure 2 shows the bond angle distributions of O–Si–O and O–P–O of No. 2 slag. The angles of O–Si–O in [SiO₄] tetrahedron and O–P–O in [PO₄] tetrahedron are 109.5° and 109.0°, respectively. The results are in a good agreement with the theoretical value of 109.5°.

Figure 2:

The distributions of O–Si–O and O–P–O bond angles for No. 2 slag.

Table 3 shows the coordination statistics of Si and P with varying slag basicity. It can be seen that the proportions of 4-coordinated P (P^IV) and Si (Si^IV) ions are beyond 98 % and 95 %, respectively, which means [SiO₄] and [PO₄] tetrahedrons are prominent in the molten CaO–SiO₂–P₂O₅–FeO slag system. The variation of slag basicity has little effect on the coordination of P–O and Si–O. It can be concluded that (SiO₄) and (PO₄) tetrahedrons are extremely stable in this slag system.

Table 3:

Coordination statistics of Si and P, %.

R	SiIV	SiV	SiVI	PIV	PV
0.6	96.75	3.25		100
0.9	96.76	2.31	0.93	98.11	1.89
1.2	95.60	4.40		100
1.5	96.23	3.14	0.63	100

Distributions of Onb, Ob and Of

It is well known that the form of oxygen can be divided into bridge oxygen (Ob), non-bridge oxygen (Onb), free oxygen (Of) and tricluster oxygen. However, the proportion of tricluster oxygen is very low (<4 %) according to the calculation of Fan et al. [28]. In the present calculation, the tricluster oxygen was not found in the CaO–SiO₂–P₂O₅–FeO system. The distributions of the aforementioned three type of oxygen are presented in Figure 3. It can be seen that when the basicity increases from 0.6 to 1.5, the proportion of Ob decreases from 30.1 % to 10.2 %, on the contrary, the proportion of Onb increases from 66.3 % to 77.3 %, and the proportion of Of increases from 3.6 % to 12.6 %. This implies that the polymerization degree of slag decreased by increasing CaO content.

Ob can be further subdivided into three groups, i. e. P–O–P, Si–O–P and Si–O–Si. Onb can be further subdivided into two groups, i. e. P–O–Ca, Si–O–Ca. AsFigure 3(b) shows, the proportion of Si–O–Si decreases sharply with increasing slag basicity, which means CaO plays the role of network former in the CaO–SiO₂–P₂O₅–FeO system.

Figure 3:

Proportions of different oxygen types with varying slag basicity.

Distributions of Qⁿ

The polymerization of the slag structure can be estimated by the parameter Qⁿ, where n denotes the number of Ob in a tetrahedron, i. e. Qⁿ(Si) represents the proportion of Ob in [SiO₄] tetrahedron, Qⁿ(P) represents the proportion of Ob in [PO₄] tetrahedron. The calculated distributions of Qⁿ for P and Si are shown in Figure 4. It can be found that the proportions of Q⁰(Si) and Q⁰(P) show a remarkable uptrend with the increase of slag basicity. With the basicity increasing from 0.6 to 1.5, the proportion of Q⁰(Si) increases from 6.2 % to 44.5 %, and the proportion of Q⁰(P) increases from 12.3 % to 67.6 %. The proportions of Q²(Si), Q³(Si), Q⁴(Si), Q¹(P), Q²(P) and Q³(P) also decrease obviously with the increase of slag basicity. When basicity increased to 1.5, Q⁴(Si), Q³(P) and Q⁴(P) disappeared completely. It can be concluded that CaO can make the complex structure of slag become simple.

Figure 4:

Effect of slag basicity on (a) Qⁿ(Si) and (b) Qⁿ(P).

Raman spectra

Figure 5 displays the Raman spectra for the CaO–SiO₂–P₂O₅–FeO system. It can be seen that the No. 1 slag has a noticeable 600 cm⁻¹ band. However, it shifts to a higher frequency band with increasing slag basicity. The 600 cm⁻¹ band is assigned to the vibration of a 3-member ring [29]. It is proposed that the strain on the ring would be attacked by the free oxygen added by CaO [30]. The disappearance of this band and subsequent appearance of a higher frequency band with increasing slag basicity have been also observed by Kline et al. [31]. The peak at around 440 cm⁻¹ is assigned to the P–O–P bending [32]; however, it fade away gradually with increasing slag basicity, which means the structure of CaO–SiO₂–P₂O₅–FeO system becomes simple with increasing slag basicity.

In the region of 800–1,200 cm⁻¹, a peak at around 950 cm⁻¹ is most evident, which can be attributed to the Q² unit. With increasing slag basicity, the peak fades away gradually. The peak for Q² and Q³ turn to lower frequency band, which also implies that the structure of the slag became simple. To quantitatively investigate thestructure of CaO–SiO₂–P₂O₅–FeO system, deconvolution of the Raman curves was performed in the range of 800–1,150 cm⁻¹. As the [SiO₄] and [PO₄] tetrahedrons may be copolymerized, and the phosphorus-related peaks are very weak, it is not easy to obtain the phosphorus-related bands by deconvoluted results [20]. Therefore, the Qⁿ(P) are ignored in the present experimental study. Figure 6 displays the deconvoluted results. Based on the curves, the mole fractions of different structural units can be calculated. It can be seen that with the slag basicity increasing from 0.6 to 1.5, the proportion of Q⁰(Si) increases from 6.2 % to 43.9 %, the proportion of Q²(Si) and Q³(Si) decreases from 64.3 % to 39.9 % and 12.2 % to 5.4%, respectively. Both the calculated Q¹(Si) and experimental Q¹(Si) show a trend of first increasing and then decreasing. Therefore, the proportions of structural units showed in Table 4 are consistent with the numerical modeling results showed in Figure 4. It also can be concluded that the degree of polymerization of CaO–SiO₂–P₂O₅–FeO system decreases with increasing slag basicity.

Figure 5:

Raman spectra of the slag samples.

Figure 6:

Deconvoluted results of the Raman spectral curves at different slag basicities.

Table 4:

The proportions of structural units.

	Q0	Q1	Q2	Q3
No. 1	6.2	17.1	64.3	12.2
No. 2	9.5	43.6	40.3	6.5
No. 3	10.8	50.1	32.9	6.2
No. 4	43.9	10.6	39.9	5.4

Conclusion

The molten CaO–SiO₂–P₂O₅–FeO system plays an important role in metallurgical process. As this slag system has not been studied adequately, a MD simulation was carried out to calculate the structural information of this quaternary slag system. The simulated results demonstrate that the [SiO₄] and [PO₄] tetrahedrons are prominent in this molten system. With slag basicity increasing from 0.6 to 1.5, the proportion of Ob decreases, while the proportion of Onb increases. The polymerization degree of slag decreases by increasing slag basicity. The proportions of structural units observed by Raman spectroscopic analysis are in agreement with the data calculated by MD simulation. The results would help us to better understand the fundamental structural properties and physicochemical properties of molten metallurgical slags. Future work will characterize the structural information through experiments.