Effect of B₂O₃ on Structure of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ Glassy Systems

Abstract

Titanium-bearing blast furnace slag is an important metallurgical waste, but presently, it is difficult for it to be effectively utilized. B₂O₃ is an important modifier to greatly promote the mass diffusion of crystallization of Ti-enriched phases in molten slags. To furtherly understand the effect of B₂O₃ on the structure of Ti-bearing slag, CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses of various B₂O₃ were investigated by combining Raman, FT-IR, and X-ray photoelectron spectroscopy. The results showed that BO₃ was the dominant structure, which decreased slightly as the B₂O₃ content increased, while BO₄ increased. Three coordination forms (TiO₄, TiO₅, and TiO₆) of the Ti-structure were discovered in the prepared Ti-bearing glasses. The percentage of TiO₄ gradually increased and became the main structural unit as the increased B₂O₃. The increase of BO₄ and TiO₄ leaded to an increased amount of network connection units, such as SiO₄, to increase the degree of polymerization of the prepared Ti-bearing glasses.

1. Introduction

Vanadium–titanium magnetite is an important ore resource, of which there are abundant reserves in China, especially in the Panxi area, which account for more than 90% of China’s reserves.^1,2) During the ironmaking process, vanadium–titanium magnetite is commonly used as a raw material for extracting iron. After smelting in a blast furnace, most of the titanium is transferred to slag, referred to as Ti-bearing blast furnace slag. More than 3 million tons of Ti-bearing blast furnace slag is produced annually, with a total accumulation of more than 7 million tons in China.^3,4,5) The accumulation of titanium slag not only wastes valuable titanium resources but also occupies a large amount of land and seriously pollutes the environment. Since the titanium components in blast furnace slag are mainly dispersed in the mineral phase, such as perovskite, carbon (nitrogen) titanium, and a small amount of spinel and diopside,^6,7,8) this makes it considerably difficult to extract Ti from blast furnace slag. Presently, the utilization rate of titanium in Ti-bearing blast furnace slag is low. Therefore, new economical and efficient methods of treating Ti-bearing blast furnace slag are of great significance for the comprehensive utilization of titanium-bearing slag.

Modifiers are normally added to blast furnace slag, which can affect the precipitation, growth, and morphology of crystals in molten blast furnace slag. Much research has been done on Ti-bearing blast furnace slag modifiers to improve the Ti-enrichment process.^9,10,11) B₂O₃ is a typical acidic oxide¹²⁾ that can change the melting behavior and decrease the viscosity of slags to greatly promote the mass diffusion of crystallization of Ti-enriched phases in molten slags. Previous reports have shown that B₂O₃ can affect the composition of crystals in solution to some extent.¹³⁾ Sun et al.¹¹⁾ investigated the effect of the addition of SiO₂ and B₂O₃ on the crystallization behavior of Ti-bearing blast furnace slag using a single hot thermocouple technique. They found that the addition of 3 and 7 wt% B₂O₃ can promote the transformation of the primary crystalline phase from perovskite to rutile, while the precipitation of perovskite is suppressed. It can be concluded that adding a slight amount of B₂O₃ can have a great effect on the crystallization process of Ti-bearing blast furnace slag.

The physicochemical properties of slag are closely related to its structure. A better understanding of the influence of B₂O₃ on the structure of Ti-bearing blast furnace slag is of great significance for further improving Ti-bearing slag utilization. Sun et al.¹⁴⁾ reported that B₂O₃ acts as a typical network-forming oxide and exists dominantly as triangular BO₃. Liang et al.¹⁵⁾ studied the coordination structure of a CaO–SiO₂–TiO₂–B₂O₃ system by molecular dynamics simulation, reporting that boron changes from a frame structure to a layered structure in the slag, which promotes its fluxing performance. Li et al.¹⁶⁾ analyzed the structure of CaO–SiO₂–TiO₂–B₂O₃ glasses and found that the content of bridging oxygen (BO) and the degree of polymerization increase with the increase of B₂O₃, and an equilibrium reaction between non-bridging oxygen (NBO) atoms, tetrahedral BO₄, and trigonal BO₃ is established. Yang et al.¹⁷⁾ also found that the introduction of B₂O₃ increased the degree of polymerization of a silicate network, forming a 3D borate structure.

Many researchers have investigated the effect of B₂O₃ on the structure of silicate slags. However, there have been few studies on glass or melts bearing both TiO₂ and B₂O₃. The influence of the addition of B₂O₃ on the Ti structure in silicate is also unclear. Therefore, the effect of B₂O₃ on the structure of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glassy systems was investigated in this study. Raman spectroscopy, FT-IR, and X-ray photoelectron spectroscopy (XPS) detection techniques were employed, and the effect of B₂O₃ on the Ti structure of silicate was analyzed.

2. Experimental Methods

2.1. Sample Preparation

Glasses were synthesized using analytical-grade powders of CaO, Al₂O₃, SiO₂, TiO₂, and H₃BO₃, with H₃BO₃ being the source of B₂O₃. The chemical composition of the investigated CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses is listed in Table 1. In order to obtain CaO, CaCO₃ was calcined in a muffle furnace at 1273 K for 6 h. After carefully weighing the ingredients, the prepared samples were grinded in an agate mortar for 40 min. Then, the mixed powder sample was placed into a platinum crucible and heated to 1773 K, which was held for 2 h in a high-temperature tube furnace. The heating and melting process were kept in air atmosphere. Afterwards, the molten slags were quenched with water to obtain glassy samples. To identify the glassy state, the prepared samples were analyzed by X-ray diffraction (XRD), as shown in Fig. 1.

Table 1. The chemical compositions of slag samples with different B₂O₃.

Sample number	Composition (mol%)
	CaO	Al2O3	SiO2	TiO2	B2O3
1	41.4	7.2	41.4	10	0
2	40.158	6.984	40.158	9.7	3
3	39.33	6.84	39.33	9.5	5
4	38.502	6.696	38.502	9.3	7

Fig. 1.

XRD pattern of the quenched samples. (Online version in color.)

2.2. Spectroscopy Analysis

This study combined XPS, FT-IR, and Raman spectroscopy techniques to detect and analyze the structures. XPS was performed to detect glass powder samples in 100–200 mesh, including the spectra of B1 s and O1s, which using ESCALAB250Xi XPS designed by Thermo Fisher under the pressure of 5 × 10⁻⁹ Pa. The FT-IR spectra, with a spectral resolution of 4 cm⁻¹, were recorded in the wavenumber range of 4000–400 cm⁻¹ using an iS50 spectrometer (Thermo Fisher, USA). The LabRAM HR Evolution type micro-confocal Raman spectrometer (HORIBA Jobin Yvon S.A.S, France) with an excitation wavelength of 532 nm was used. The power of the light source was 1 mW. Raman spectra were recorded in the wavenumber range of 120–3000 cm⁻¹.

3. Results and Discussion

3.1. X-ray Photoelectron Spectroscopy Analysis

3.1.1. B1s XPS Spectra

The B1s spectra were smoothened by averaging the raw data of the CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses, as shown in Fig. 2(a). There was a main peak in the binding energy range of 190–194 eV. With the increased amount of B₂O₃, the intensity of the main peak was enhanced, and the whole peak moved slightly toward the higher binding energy direction. In order to explore the coordination information of boron in CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses, the B1s XPS smoothed spectra were further analyzed. As reported in a previous work,¹⁸⁾ B³⁺ ions can form two coordination bonds (BO₃ and BO₄) in a silicate network, which correspond to the bending energy at about ~191 and ~192 eV in the B1s XPS spectra, respectively. For the present B1s spectra, the whole peak slightly moved toward the higher binding energy direction, indicating that the tetrahedral BO₄ had slightly increased. These results were supported by the further fitting of B1s spectra results using Gaussian functions, taking the example of 3 mol% B₂O₃, as shown in Fig. 2(b).

Fig. 2.

B1s XPS spectral of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses. (a) the spectra smoothened by averaging raw data, (b) the fitted spectral of 3 mol% B₂O₃ sample by Gaussian functions attributed to two different coordination bonds (BO₃, BO₄) of B. (Online version in color.)

The changes of BO₃ and BO₄ with the increased B₂O₃ calculated from the deconvolved results of the B1s XPS spectra are shown in Fig. 3. With the increasing content of B₂O₃, the proportion of BO₃ decreased, while BO₄ increased. The triangular BO₃ usually formed a 2D structure, which was the dominant structure compared with the 3D structure, tetrahedral BO₄.¹⁴⁾ This kind of 2D structure was simpler and less structurally complex compared with the 3D structures, such as tetrahedral SiO₄ and BO₄. Additionally, for the investigated system of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃, Ca²⁺ is the charge-compensator to make BO₄ structure, which consumed the total cation in the system leading to the increase of polymerization degree. Therefore, it was concluded that the increasing percentage of tetrahedral BO₄ may make the structure of the glass more polymerized.

Fig. 3.

The change of the percentage of BO₃ and BO₄ of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses. (Online version in color.)

3.1.2. O1s XPS Spectra

As shown in Fig. 4(a), there was one main peak located at the 527–535 eV binding energy of the O1s spectra in all glassy samples. According to previous works,^{19,20,21,22,23)} the main peak of the O1s spectrum can be distinguished by two main oxygen contributions in the silicate glass network due to the bridging oxygen (BO) and NBO atoms. The NBO atoms were located in the low binding energy peak at 530–531 eV, and the peak of BO atoms corresponded to 531–533 eV binding energy. From Fig. 4(a), the O1s XPS spectra slightly moved toward the high binding energy direction with the increased B₂O₃ content, showing that the bridging oxygen increased. All O1s XPS spectra were fitted using two Gaussian functions (BO and NBO), taking the example of 0 mol% B₂O₃ shown in Fig. 4(b). The fitting results are presented in Fig. 5. It was observed that the fraction of bridging oxygen increased and that of non-bridging oxygen decreased with the increasing content of B₂O₃, which demonstrated that the addition of B₂O₃ enhanced the degree of polymerization of the Ti-bearing silicate structure. This maybe because the tetrahedral BO₄ had a 3D structure,¹⁴⁾ which participated in the SiO₄ network to increase the network connectivity.

Fig. 4.

O1s XPS spectra for the samples. (a) the original spectral, (b) the fitted spectral of 0 mol% B₂O₃ sample. (Online version in color.)

Fig. 5.

Change of the percentage of BO and NBO of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ system. (Online version in color.)

3.2. FT-IR Analysis

The FT-IR transmission spectra between 500 and 1600 cm⁻¹ are shown in Fig. 6. According to previous studies,^{14,16,24,25,26)} the structural assignments of B-bearing silicate glasses are summarized in Table 2. The FT-IR spectra curves were divided into three domains: the 600–800, 800–1200, and 1200–1600 cm⁻¹ bands.^14,16,26) The highest intensity bands in 800–1200 cm⁻¹ range corresponded to the combined stretching vibrations of the SiO₄ and BO₄ tetrahedral network. It was observed that the intensity of the band centered at about 845 cm^–1 gradually weakened with the increase of B₂O₃, which was assigned to Q⁰ (Si).¹⁶⁾ This may have increased the degree of polymerization of the silicate structure. The weaker band at 600–800 cm⁻¹ was attributed to the bending vibrations of [AlO₄]⁵⁻ and B–O–B.^24,25,26) Especially, the band at about 710 cm⁻¹ was assigned to the bending vibrations of bridging oxygen formed by two trigonal BO₃ units, and the signal of the absorption region became more pronounced as the B₂O₃ content increased, which indicated that BO₃ may be the main structural unit in the B structure. This had been proved in the results of the B1s spectra. According to Li et al.,¹⁶⁾ Kamitsos et al.,²⁷⁾ and Hidi et al.,²⁸⁾ two bands centered at about 1210 and 1350 cm^–1 were assigned to the stretching vibrations of tetrahedral BO₄ and BO₃ antisymmetric stretching vibration in the domain of 1200–1600 cm⁻¹. Similarly, the signal of trigonal BO₃ gradually became stronger with the increase of B₂O₃, while the signal of tetrahedral BO₄ was not obvious. Combined with the previous analysis, it can be concluded that trigonal BO₃ was the main type of boron-related structural group in the investigated Ti-bearing glassy slags.

Fig. 6.

FTIR spectra of B₂O₃ doped CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses between 500–1600 cm⁻¹ wave number. (Online version in color.)

Table 2. Assignments of FT-IR bands in spectra of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses.

Wavenumber (cm–1) FT-IR	FT-IR assignment	Reference
600–800	[AlO4]5− or B–O–B bending vibration	14, 16, 24, 25, 26
~710	the bridging oxygen between two BO3 trigonal	14, 16, 23, 27, 28
800–1200	the stretching vibrations of SiO4 or BO4 tetrahedral	14, 16, 23, 24, 25, 39
~845	SiO44−with zero bridging oxygen in a monomer structure (Q0)	16
~1210	the stretching vibrations of BO4 tetrahedral	14, 16, 27, 28
~1350	BO3 Anti-symmetric stretching vibration	14, 16, 27, 28

3.3. Raman Spectroscopy Analysis

The original Raman spectra for CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses at room temperature are shown in Fig. 7. The spectra can be subdivided into two main regions, referring to the [SiO₄] tetrahedron (850–1100 cm⁻¹) and the Ti structure (600–850 cm⁻¹),^29,30,31) respectively. Ti can form three coordination forms of fourfold (~850 cm⁻¹), fivefold (~780 cm⁻¹), and sixfold (~700 cm⁻¹).^31,32) According to previous studies,^33,34,35) the [SiO₄] tetrahedron structural units have been separated into five types—Qⁱ (i = 0, 1, 2, 3, 4, where i represents the number of bridge oxygen)—which is based on the number of bridging oxygen atoms in each SiO₄ structural unit. In the present work, all original Raman spectra were fitted by the Gaussian deconvolution method (the minimum correlation coefficient r² ≥ 0.99),^34,36) as exhibited in Figs. 8(a)–8(d). It can be seen in Fig. 8(a) that there were two stronger bands at about 926 and 997 cm⁻¹, and two relatively weaker bands near 892 and 1042 cm⁻¹. Based on related research,^16,33,34,35) the Raman band centered at about 926 cm^–1 was defined as Q¹, due to Si₂O₇⁶⁻ having one bridging oxygen in the dimer structure unit. Another band at about 997 cm⁻¹ was referred to as Q², due to Si₂O₆⁴⁻ having two bridging oxygen atoms in the chain structure unit. Similarly, the Raman bands centered at about 892 and 1042 cm⁻¹ were assigned to Q⁰ in the monomer structure and Q³ in the sheet structure. There were three bands at about 698, 791, and 856 cm⁻¹ that corresponded to TiO₆, TiO₅, and TiO₄ structural units, respectively. The Raman band with the highest peak intensity was due to the TiO₅ structural unit in the B₂O₃-free glass. However, with the introduction of B₂O₃, the TiO₄ structural unit gradually increased and became the main structural unit.

Fig. 7.

Original Raman spectra for CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses with different B₂O₃ contents at room temperature. (Online version in color.)

Fig. 8.

Deconvolved results of Raman spectra for CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses with different B₂O₃ contents. (Online version in color.)

The area percentages of each structural unit of the CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses are shown in Table 3. The ratio of the dimer (Q¹) and sheet (Q³) gradually increased, while the ratio of the monomer (Q⁰) and chain (Q²) decreased as the B₂O₃ content increased, which revealed that the addition of B₂O₃ is beneficial to enhancing the connectivity of the [SiO₄] tetrahedral network structure. These results are consistent with the work done by Li et al.¹⁶⁾

Table 3. Deconvolved results of Raman spectra for CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses.

Sample number	Percentage of each unit (%)
	TiO4	TiO5	TiO6	Q0	Q1	Q2	Q3
1	28.67	55.48	15.85	25.79	25.01	29.45	19.75
2	36.22	45.30	18.48	22.23	33.75	22.61	21.41
3	41.08	37.04	21.89	20.18	36.90	20.42	22.51
4	44.64	31.93	23.43	20.09	37.11	17.81	24.99

According to some studies,^30,34,37) the average number of bridging oxygen atoms of each sample (BO/Si) can be estimated by the area ratio of each structural unit in the Raman spectra multiplied by the number of its bridging oxygen atoms, as shown in Fig. 9. It was observed that the number of BO/Si slightly increased with the increasing B₂O₃ content, which was consistent with the results of the O1s XPS spectra results. Additionally, it can be seen that the intensity of the TiO₄ and TiO₆ structural units slightly increased, while the relative intensity of the TiO₅ unit decreased with the increasing content of TiO₂. This demonstrated that the Ti structure can be changed by the addition of B₂O₃ in silicates. According to Mysen et al.,³⁸⁾ Ti⁴⁺ can be substituted for Si⁴⁺ in the structural units of Ti-bearing silicate glasses when Ti⁴⁺ exists in tetrahedral coordination to enhance the network structure as a network former, while TiO₅ and TiO₆ work as network modifiers, which decreased the degree of polymerization of CaO–Al₂O₃–SiO₂–TiO₂ glasses. It can be concluded BO₄ and TiO₄ increased with the increasing content of B₂O₃, which could lead to the increase of network connection units, such as SiO₄, to increase the degree of polymerization of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses.

Fig. 9.

Effects of B₂O₃ content on the number of the average bridging oxygen.

The structural information of slags could be used to explain the change of properties. Ren et al.,⁸⁾ investigated the effect of B₂O₃ on precipitation behavior of perovskite and anosovite crystals, showed that the increasing B₂O₃ content in the present slags suppress the precipitation of perovskite and promote the formation of anosovite crystal. Zhang et al.,⁴⁰⁾ also found that the perovskite was suppressed by B₂O₃ addition in Ti-bearing blast furnace slag. From the present structure investigation, as the increase of B₂O₃, both the TiO₄ and BO₄ increased. The formation of BO₄ would consume Ca²⁺ as a charge compensator. These results probably suppressed the perovskite precipitation.

4. Conclusions

The structures of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glassy systems with various B₂O₃ contents were investigated by XPS, FT-IR, and Raman spectroscopy techniques. The following conclusions were obtained:

(1) B³⁺ ions could form trigonal BO₃ and tetrahedral BO₄ structural units in Ti-bearing silicate networks. BO₃, as the dominant structure of the boron-related structural group, decreased slightly with the increasing content of B₂O_3, while BO₄ slightly increased.

(2) Three coordination forms of TiO₄, TiO₅, and TiO₆ were discovered in Ti-bearing glasses. The percentage of the TiO₄ unit gradually increased and became the main structural unit in the Ti structure as the B₂O₃ increased_.

(3) There was an increase of BO₄ and TiO₄ units as the B₂O₃ content increased, which could lead to the increase of network connection units, such as SiO₄, to increase the degree of polymerization of CaO–Al₂O₃–SiO₂–TiO₂–B₂O₃ glasses.

Acknowledgements

This work was supported by Natural Science Foundation of China (51704050, 51774054); Fundamental and Frontier Research Project of Chongqing (cstc2018jcyjAX0791) and China Postdoctoral Science Foundation (2017M612905, 2018T110944) are gratefully acknowledged.

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