Viscosity Property and Structure Analysis of FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ Slags

Abstract

In order to clarify the viscosity of FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ system as the main components of vanadium slag with varied Cr₂O₃ and TiO₂ contents, the viscosity and structure characteristics of the slag were investigated by the rotating cylinder method and Raman spectroscopy, respectively. The results showed that the viscosity was decreased to 3.5 Pa·s at temperature above 1534 K for the FeO–SiO₂–V₂O₃ system, and the polymerization degree of this system was low due to the main structure of silicate as monomer. Meanwhile, V³⁺ mainly existed in the form of V–O–V as a chain structure in FeO–SiO₂–V₂O₃ system, which slightly enhanced the polymerization degree of the slag. With the introduction of 6 mass% Cr₂O₃ into FeO–SiO₂–V₂O₃ system, the viscosity increased rapidly and was decreased to 3.5 Pa·s until temperature higher than 1767 K, and the polymerization degree of the slag was enhanced drastically due to the formation of Q² and Cr–O–Cr band in a chain structure as well as the formation of Q³ species in a sheet structure. Furthermore, part of the chromium existed in the form of the high melting point of spinel (FeCr₂O₄). With the introduction of 13 mass% TiO₂ into the FeO–SiO₂–V₂O₃–Cr₂O₃ system, the viscosity decreased and was of 3.5 Pa·s at 1624 K, and the polymerization degree of the slag became weak due to the formation of discrete Si–O–Ti and Ti–O–Ti inhibiting the formation of sheet structure and hampering the crystallization of FeCr₂O₄ in molten slag, which was advantageous to decrease the viscosity of FeO–SiO₂–V₂O₃–Cr₂O₃–TiO₂ system.

1. Introduction

It is well known that vanadium-titanium magnetite (VTM) is a characteristic resource in China, and the main route for the extraction of vanadium is by oxidizing the vanadium-bearing hot metal in converter to form vanadium-bearing slag and semi-steel.^1,2,3) Since the blowing smelting time is merely 3 to 5 minutes, the thermodynamics and the dynamics conditions for oxidizing vanadium in hot metal are very important during the blowing smelting process.^4,5) Thus, the properties of the slag, such as melting temperature and viscosity, are very crucial to improve the process conditions.

With the gradual consumption of high quality VTM, the low-grade bearing chromium VTM (namely the Hongge type V-bearing titanomagnetite, with an approximate composition of 45.5 mass% T.Fe, 10.6 mass% TiO₂, 0.4 mass% V₂O₅, and 0.2 mass% Cr₂O₃)^6,7,8) has caused much attention and is beginning to be utilized in recent years. It is reported that the proven reserves of Hongge type V-bearing titanomagnetite is about 2.0 billion tons in Panxi and Chengde regions of China.^9,10) However, during the vanadium-extraction process, the viscosity of the slag increases after the chromium in the hot metal is oxidized to Cr₂O₃ and dissolves into the slag. Thus, the dynamics condition for vanadium-extraction becomes worse, and the yield ratio of vanadium is decreased.^11,12,13,14) Though the viscosity of Cr₂O₃-bearing slag would decease with increasing of the temperature, the oxidation of vanadium would be suppressed according to the selective oxidation theory, and thus the practical blowing smelting temperature is usually controlled below 1653 K.^15,16,17) Therefore, study on the influence of Cr₂O₃ on the properties of FeO–SiO₂–V₂O₃(–TiO₂) system slag is very important to improve the blowing smelting technology for the vanadium-extraction from the chromium-bearing hot metal.

Many studies on the properties of CaO–SiO₂–TiO₂–FeO system slag have been carried out to promote the use of VTM in blast furnaces. Besides, stainless steel slag containing Cr₂O₃ of the Al₂O₃–CaO–Cr₂O₃ system has been studied to obtain a high yield ratio of chromium.^18,19,20) However, few studies on the properties of vanadium slag, especially the Cr₂O₃-bearing slag system of FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ have been reported. Meanwhile, the properties of vanadium slag are high oxidizability and low basicity ( $R=\frac{\sum w(\text{Basic oxides})}{\sum w(\text{Acidic oxides})},$ with the basic oxides of CaO, MgO, MnO, FeO, and the acidic oxides of SiO₂, TiO₂, P₂O₅),²¹⁾ which is different from that of converter slag and blast furnace slag (as listed in Table 1). Thus, it is necessary to study the properties of chromium-bearing vanadium slag. Considering that the physicochemical properties of melts strongly depend on the structure characteristics which are closely related to the polymerization degree of molten slag. Therefore, the fundamental information on the structure of Cr₂O₃-bearing vanadium slag is significantly crucial to the vanadium-extraction process.

Table 1. Typical chemical compositions of different slag, mass%.

Species of slag	FeO	SiO2	V2O3	Cr2O3	MnO	TiO2	MgO	Al2O3	CaO	P2O5
Cr2O3-bearing vanadium slag	35.47	14.41	14.44	11.18	9.13	8.28	2.32	2.35	2.42	0.00
Ordinary vanadium slag	37.23	16.68	16.36	2.74	10.56	9.94	2.13	2.13	2.23	0.00
Converter slag	17.73	17.96	0.00	0.00	1.68	1.23	8.99	3.73	46.92	1.76
Blast furnace slag	2.86	36.23	0.00	0.00	2.14	0.00	12.73	4.92	39.89	1.23

In this work, the viscosity and structure of the FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ system with different content of Cr₂O₃ and TiO₂ were investigated using the rotating cylinder method and Raman spectra, respectively. The purpose of this study is to provide the structure information of vanadium slag and its relationship with the viscosity of Cr₂O₃-bearing vanadium slag, and to provide theoretic basis for optimizing the vanadium-extraction process.^16,22,23)

2. Experimental Procedures

Reagent grade powders of Cr₂O₃ (>99.50 mass%), V₂O₃ (>99.50 mass%), FeC₂O₄ (>99.50 mass%), TiO₂ (>99.50 mass%), and high purity SiO₂ (>99.99 mass%) were used as raw materials. These five kind powders were dried at 473 K for 4 hours in a drying oven to remove moisture, and then were well mixed in ball mill in the required proportion according to the actual components of Cr₂O₃-bearing vanadium slag as shown in Table 2 (with external addition of TiO₂ and Cr₂O₃). Then the mixed powders were pressed into tablet samples and heated at 1823 K for 2 h in a platinum crucible to prepare pre-melted slag under H₂/CO₂ gas flowing atmosphere. After heating, the sample was rapidly taken out from the furnace and quenched by water to avoid the oxidation of elements during the cooling process. According to oxygen-potential diagram of the elements as shown in Fig. 1,^4,5,7) the rank of the stability of various oxides in the slag was TiO₂>SiO₂>V₂O₃>Cr₂O₃>FeO, which indicated that FeO was the most unstable oxide in the FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ system slag. In order to protect FeO from oxidation, the oxygen partial pressure was ensured 10⁻³ Pa by changing the composition of H₂/CO₂ according to Reaction (1)–(3).²¹⁾ In addition, during the heat process, the sample was held at 873 K for 2 h to decarburize FeC₂O₄.   

$$\begin{array}{l}{\mathrm{H}}_2\left(\mathrm{g}\right)+\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)={\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)\\ \mathrm{\Delta }{G}^0=77\hspace{0.17em}\hspace{0.17em}205-64.12T\begin{array}{cc}& \text{J/mol}\end{array}\end{array}$$(1)

  

$$\begin{array}{l}\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)+1/2{\mathrm{O}}_2\left(\mathrm{g}\right)=\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)\\ \mathrm{\Delta }{G}^0=-281\hspace{0.17em}\hspace{0.17em}000+85.23T\begin{array}{cc}& \text{J}/\text{mol}\end{array}\end{array}$$(2)

  

$$\begin{array}{l}{\mathrm{H}}_2\left(\mathrm{g}\right)+1/2{\mathrm{O}}_2\left(\mathrm{g}\right)={\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\\ \mathrm{\Delta }{G}^0=-203\hspace{0.17em}\hspace{0.17em}795+21.11T\begin{array}{cc}& \text{J}/\text{mol}\end{array}\end{array}$$(3)

Table 2. Typical chemical composition (with external addition of TiO₂ and Cr₂O₃) of the studied slag systems, mass%.

Sample No.	FeO	SiO2	V2O3	TiO2	Cr2O3
1	52.12	28.59	19.29	0.00	0.00
2	52.12	28.59	19.29	0.00	6.00
3	52.12	28.59	19.29	13.00	6.00
4	52.12	28.59	19.29	13.00	10.00

Fig. 1.

Oxygen-potential diagram of the main elements in hot metal containing vanadium.

After completion of the pre-melting process, 150 g of the pre-melted sample was put into a 5 mass% MgO-stabilized zirconia crucible (with purity (MgO+ZrO₂)>99.9 mass% and operating temperature >2273 K) and heated in a rotatory viscometer under the oxygen partial pressure of 10⁻³ Pa.²⁰⁾ When the temperature reached the target temperature, it was maintained for more than 30 min to ensure that the molten slag was homogenized. Subsequently, the molybdenum bob was immersed in liquid slag for 10 mm and rotated at a fixed speed of 180 r/min, and the viscosity was measured at different temperatures (with temperature step of 5 K) during temperature dropping process. The schematic illustration of a rotatory viscometer and the dimension of crucible and a bob were shown in Fig. 2. By measuring the relative torque exerted on the inner cylinder (bob) using this apparatus, the viscosity of the sample was calculated based on the reference relationship between the viscosity and the relative torque value, which was obtained using a standard oil of known viscosity at calibration stage. Meanwhile, calibration measurements of the apparatus were carried out at room temperature using a standard oil of known viscosity before measuring the viscosity.^24,25)

Fig. 2.

Schematic illustration of the apparatus for viscosity measurements.

In order to verify the effects of Cr₂O₃ and TiO₂ on the structure characteristics of the FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ system slag, 8 g of the pre-melted slag was placed in a zirconia crucible (with inner diameter of 15 mm and height of 30 mm) for the molten state in a resistance furnace at approximately 1823 K for 4 hours under the oxygen partial pressure of 10⁻³ Pa. High temperature melts were also quenched in water to form the glasses. To be reminded, the samples were rapidly taken out from the furnace and quenched by water (with the cooling rate about 500 K/s). The whole process of taking out sample and quenching cost less than 8 s and water temperature was lower than 298 K to avoid any precipitation of crystalline phase during the cooling process.²⁶⁾

The quenched slags were characterized by phase compositions, microstructures, and structural properties. The phase compositions were examined by X-ray powder diffraction (XRD; X’pert PRO, PANalytical, Netherlands) using Cu Kα1 radiation (λ=1.5406 Å) with a step of 0.02° (2θ) and a scanning rate of 2°/min from range of 10° to 90°. The microstructures as well as the element distribution of the slag were determined by scanning electron microscopy (SEM; SSX-550, Shimadzu, Japan) attached with energy dispersive X-ray analyzer (EDX). The structural properties were analyzed by Raman spectroscopy (Horiba Jobinyvon HR800) using an excitation wavelength of 633 nm with the laser power of 2 mw at room temperature in the frequency range of 50–2000 cm⁻¹. And the spectra of Raman were fitted by assuming Gaussian line shapes for the peaks of different structural units.

3. Results and Discussion

3.1. Viscosity Property of FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ System Slags

Figure 3 shows the viscosity changes of the samples at varying temperatures with different contents of Cr₂O₃ and TiO₂. It is observed that the viscosity of all the samples first decreased rapidly and then decreased gradually with increasing of temperature, and it finally was closed to a constant value. According to Arrhenius equation (η=A·e^E/RT), the viscosity was decreased with increasing of temperature due to the depolymerization of the complex polymers in the molten slag. But when the depolymerization of the complex polymers was finished and the simple structures with stability in the molten slag were regenerated at a certain temperature range, the viscosity was closed to a constant value.²¹⁾ However, the constant value was different for different samples. It was about 0.7 Pa·s for sample 1 at temperature above 1650 K, but it was about 3.0 Pa·s for sample 2 at temperature above 1800 K, about 1.4 Pa·s for sample 3 at temperature above 1700 K, and about 3.4 Pa·s for sample 4 at temperature above 1810 K, respectively. During the final stage of the blowing in extraction-vanadium process, amount of iron is dragged into the slag to form dispersion of molten iron (namely the entrapped iron) while the viscosity of vanadium slag is above 5 Pa·s.²⁷⁾ According to practical process, if the viscosity of the vanadium slag is controlled below 3.5 Pa·s, the entrapped iron could be avoided during blowing process, and it is benefit to improve the yield rate of iron and the grade of vanadium slag.²⁷⁾ The critical temperatures for the viscosity decreasing to 3.5 Pa·s were 1534 K, 1767 K, 1624 K and 1803 K for sample 1, sample 2, sample 3, and sample 4 respectively, which indicates that the viscosity of the samples increased significantly with increasing the Cr₂O₃ content, but decreased with introduction of TiO₂.

Fig. 3.

The viscosity of the vanadium slag with various content of Cr₂O₃ and TiO₂ at given temperatures.

3.2. Raman Spectroscopy

All original spectra for glassy samples with different contents of Cr₂O₃ and TiO₂ are shown in Fig. 4. It is observed that the dominant peaks of the Raman spectra for the FeO–SiO₂–V₂O₃ system slag were at about 600–1000 cm⁻¹. With the introduction of 6 mass% Cr₂O₃ into this system, new strong Raman bands appeared at about 252, 550–800, and 1300 cm⁻¹, while the bands at about 70–113, 379, and 750–1050 cm⁻¹ of the Raman spectra became weaker, especially for the relative intensity of the main bands at 750–1050 cm⁻¹ which was down to about 50%. With the introduction of 6 mass% Cr₂O₃ and 13 mass% TiO₂ into the FeO–SiO₂–V₂O₃ system, the relative intensity of the bands at about 180–320, 800–1050, and 1300 cm^–1 of the Raman spectra became weaker. In addition, with further increase of Cr₂O₃, the Raman bands near 970 and 1300 cm⁻¹ became stronger.

Fig. 4.

Original Raman spectra for quenched samples with different contents of Cr₂O₃ and TiO₂ at room temperature.

3.3. Phase Compositions and Microstructure

Figure 5 shows the XRD patterns of the quenched samples with different Cr₂O₃ and TiO₂ contents. For the sample without Cr₂O₃ and TiO₂ addition, the crystalline phases was detected (sample 1), which indicated that the FeO–SiO₂–V₂O₃ system slag melted completely and did not form any spinel. For the sample with addition of Cr₂O₃, new stronger peaks of the spinel (FeCr₂O₄) appeared (sample 2), while the intensity of the peaks became weaker with the introduction of 13 mass% TiO₂ (sample 3). Furthermore, the peak intensity of the spinel became stronger with further increase of the Cr₂O₃ addition (sample 4).

Fig. 5.

XRD pattern of the quenched sample.

To further investigate the effect of Cr₂O₃ on the microstructure of the molten slag, SEM/EDX was employed for the microstructure and compositional analysis of the phases on the polished surfaces of the quenched sample 4, as shown in Fig. 6. It is observed that the coexisting phases were composed of liquid, spinel (FeCr₂O₄), and a spot of ZrO₂, which verified the results of X-ray diffraction analysis. With addition of Cr₂O₃ and TiO₂ in this sample, FeO, SiO₂, V₂O₃ TiO₂ and a part of Cr₂O₃ formed the homogenous liquid as listed in Fig. 6(c). In addition, some of these Cr₂O₃ reacted with FeO to form the spinel (FeCr₂O₄) as listed in Fig. 6(b), which was unfavorable for the viscosity of FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ system slag. Owing to a slight dissolution of the zirconia crucible into the slag, Zr element was detected in the liquid and spinel, as shown in Figs. 6(a) and 6(d). Meanwhile, it is also could be known from the Figs. 5 and 6 that the low melting point of solids (spinel and olivine) were not found in the quenched samples, thus it was considered that the precipitation of crystalline phase was avoided during the quenching process.

Fig. 6.

SEM/EDS pattern of the quenched Sample 4. (a) SEM; (b) Spinel; (c) Liquid; (d) ZrO₂.

3.4. Discussion

The different Raman bands are related to the different structure units, as summarized in Table 3. According to previous research results, the main band at about 830–1060 cm⁻¹ corresponds to the silicate sites for the Q⁰, Q¹, Q², and Q³ structure units (superscripts 0, 1, 2, and 3 are the numbers of bridging oxygen per ${\text{SiO}}_4^{4-}$ tetrahedra).^16,22) The bands at about 600–750 cm⁻¹ are attributed to the structural units of titanium in the melts, but 830–850 cm⁻¹ are attributed to Ti⁴⁺ which is substituted for Si⁴⁺ in tetrahedron to form Ti–O–Si structure units.^16,22) The bands at about 690–710 and 730–750 cm⁻¹ are assigned to the structural units of vanadium and chromium in melts, respectively.^17,23,28,29) Considering that the structural behavior of irons is different in the different slag system, the present work investigated the structural behavior of various ions in FeO–SiO₂–V₂O₃–Cr₂O₃–TiO₂ system.

Table 3. Assignments of Raman bands in spectra of FeO–SiO₂–V₂O₃–Cr₂O₃–TiO₂ glass system.

Raman centered shift/cm−1	Raman assignment	References
420–440	Antisymmetric stretching vibrations of Cr–O–Cr	[13, 23]
600–750	O–Si–O and O–Ti–O deformation vibration	[16]
730–750	Symmetric stretching vibrations of Cr–O–Cr	[17, 23]
690–710	Vibrations of V–O–V structure group	[28, 29]
830–850	Vibrations of Ti–O–Si or Ti–O–Ti structure groups, or both	[16, 22]
850–880	${\text{SiO}}_{\text{4}}^{\text{4}-}$ with zero bridging oxygen in a monomer structure (Q0)	[16, 22]
900–930	${\text{Si}}_{\text{2}}{\text{O}}_{\text{7}}^{\text{6}-}$ with one bridging oxygen in dimmer structure unit (Q1)	[16, 22]
950–980	${\text{Si}}_{\text{2}}{\text{O}}_{\text{6}}^{\text{4}-}$ with two bridging oxygen in chain structure unit (Q2)	[16, 22]
1040–1060	${\text{Si}}_{\text{2}}{\text{O}}_{\text{5}}^{\text{2}-}$ with one bridging oxygen in sheet structure unit (Q3)	[16, 22]

Table 4. Coefficient of scattering of Qⁿ.

Qn	Q0	Q1	Q2	Q3
θn	1	0.514	0.242	0.09

The chemical bonds between oxygen and vanadium in V₂O₃-bearing slag are divided into three types:^28,29) (1) threefold coordinated oxygen O₃ belonging to the chains at about 698 cm⁻¹; (2) twofold coordinated oxygen O₂ constituting bridges between two chains at about 524 cm⁻¹; (3) apex oxygen O₁ belonging to the short V–O bond at about 992 cm⁻¹. The band at about 698 cm⁻¹ was assigned to the stretching of the V–O₂–V bond; the second vibrational mode at 524 cm⁻¹ was assigned to the stretching vibration of the V–O₃ bond in the medium region.^28,29) In addition, the third type at 992 cm⁻¹ was assigned to the stretching vibration mode of the short V–O₁ bond.^28,29) In the present work, the first and second types were employed to interpret Fig. 4, and it is worth to notice that the first type is the main existing form of V³⁺ in the melt. The peak located at about 698 cm⁻¹ was considered to be attributed to the mode involving the bond stretching vibration localized within the V–O–V bridges, and forming ‘rails’ of ladders to increase the polymerization degree of the melt.^30,31) However, the combined capacity of Si⁴⁺ and O²⁻ is stronger than V³⁺ and O²⁻,^30,31) which meant a larger number of V₂O₃ entered the silicate network to enhance slightly the polymerization degree of melts. Meanwhile, little V₂O₃ as monomer in the third type decreased the silicate network. Accordingly, the first and second types could consistently predict the polymerization degree of sample 1 with the present results as listed in Table 5.

Table 5. Deconvolved results of Raman spectra for FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ slag.

Sample number	Q0/%	Q1/%	Q2/%	Q3/%	The average bridging oxygen	n(NOB/T)
1	96.21	0.00	3.79	0.00	0.08	3.87
2	7.99	52.72	38.34	0.95	1.32	2.70
3	0.00	84.00	16.00	0.00	1.16	2.84
4	0.00	0.00	88.43	11.57	2.12	1.88

With the introduction of Cr₂O₃ into the slag, Raman spectra was changed greatly. In the first case, the presence of a shoulder at about 745 cm⁻¹ was considered due to the symmetric stretching vibration of Cr–O–Cr.^32,33) In the lower wavenumber region, a band at about 439 cm¹ was observed and assigned to be the antisymmetric stretching modes of Cr–O–Cr.^32,33) The plenty of the Cr–O–Cr bonds were consequence of the constraints imposed by their membership of a ring structure, which included Cr–O–Cr, V–O–V, and Si–O–Si bonds.³⁴⁾ Meanwhile, Dines’s revealed that SiO₂ could stabilize the supported Cr³⁺ in tetrahedral coordination to enhance the ring structure.^32,35) The band at 258 cm⁻¹ observed for this slag could probably be attributed to the bending/deformation modes of the [Cr³⁺O₆] units in the structure. In the second series, the spectrum of pure FeCr₂O₄ shows well-defined peaks at about 1030, 737, 677, 516, 595, 495, and 250 cm⁻¹,^33,36) so the results are consistent with the XRD and SEM/EDS analysis.

Three possible models for the structural role of titanium, suggested in previous reports on the structure of Ti-bearing glasses, were considered: (1) Ti⁴⁺ substitutes for Si⁴⁺ in tetrahedral coordination in the structural units in melts; (2) Ti⁴⁺ forms TiO₂-like clusters with Ti⁴⁺ in tetrahedral coordination; (3) Ti⁴⁺ as a network modifier possibly occurs in five-fold or six-fold coordination.^16,21) As can be seen from Fig. 4 (sample 3-4), the second and third models could be employed to interpret the results. It means that Ti⁴⁺ as a network modifier could possibly occur in five-fold or six-fold coordination, which would significantly decrease the polymerization degree of silicates. Meanwhile, a small number of Ti⁴⁺ formed TiO₂-like clusters with Ti⁴⁺ in tetrahedral coordination. As the first model stated, Ti⁴⁺ is substituted for Si⁴⁺ in tetrahedral coordination in structural units of the glassy slag, so the number of average bridging oxygen would be significantly larger and increase to 2.23 for samples with 13 mass% TiO₂ addition, and the polymerization degree of the silicates should be significantly enhanced according to Li’s study.¹⁶⁾ Accordingly, the first model could not consistently predict the number of average bridging oxygen of silicates with the present results, as listed in Table 5. The second and the third models are most appropriate to describe the role of titanium in structure. The relative intensity of the Raman bands of spinel (FeCr₂O₄) became weaker, while the band at about 670 cm⁻¹ of the Raman spectra for Cr–O–Cr, Si–O–Si, and Ti–O–Ti became stronger with the introduction of TiO₂ in the slag. According to Dines’s study, the crystallization ability of the SiO₂–TiO₂ system decreases with an increase of TiO₂.²⁹⁾ Another reason for this could be increased numbers of discrete Si–O–Ti and Ti–O–Ti structural units, which would hamper the crystallization of FeCr₂O₄ as main crystallization product of slag.³²⁾ Furthermore, Healy and Schottmiller suggested that SiO₂ and Cr₂O₃ could form liquid SiO₂–Cr₂O₃ slag above 1573 K.³⁷⁾ Similarly, TiO₂ could form liquid TiO₂–Cr₂O₃ slag at 1773 K to increase the solubility of Cr₂O₃ in FeO–SiO₂–V₂O₃–Cr₂O₃–TiO₂ system slag, according to the present study.

The bands at 830–1000 cm⁻¹ could correspond to the silicate sites for the Q⁰, Q¹, Q², and Q³ structure units,^16,22) as shown in Table 3. Considering that the molar fractions of different structure units are related to the band areas, all samples were deconvolved using the Gauss-Deconvolution method by assuming contribution from the structural units of Qⁿ with the minimum correlation coefficient r²≥0.99 to study the effect of different components. Different Qⁿ species could be described as follows:   

$${\mathrm{Q}}^{\mathrm{n}}={\mathrm{Q}}^{\mathrm{n}+1}+{\mathrm{Q}}^{\mathrm{n}-1}$$(4)

As the scattering coefficient of Qⁿ is different in the Raman spectra, the mole fraction of the silicate structure units can be calculated according to the following equation:   

$$x_{\text{n}}={\theta }_n\cdot A_{\text{n}}$$(5)

where x_n is the mole fraction of the silicate structure units, A_n is the area fraction of each structural unit. θ_n listed in the Table 4 is the scattering coefficient of Qⁿ.²²⁾ The number of non-bridging oxygen in the silicate slag could be calculated by the following equation:   

$$n\left(\frac{\text{NOB}}{\text{T}}\right)=\sum x_{\text{n}}\cdot (4-n)$$(6)

where n(NOB/T) is the number of non-bridging oxygen in the silicate slags. In addition, the average number of bridging oxygen in each sample is also used to explain the change of the silicate structures in the melts, which could be estimated by the area ratio of each structural unit (Qⁿ) multiplied by the number of its bridging oxygen. The best-fit simulations are shown in Figs. 7(a)–7(d) by the Gauss-Deconvolution method. The summary of the deconvolution and calculation results is listed in Table 5.

Fig. 7.

Deconvolved results of Raman spectra for samples with different Cr₂O₃ and TiO₂ contents.

As can be seen from Table 5, the change of the number of non-bridging oxygen was contrary to the average number of bridging oxygen, which verified the accuracy of the calculation results. Table 5 also shows that the number of non-bridging oxygen rapidly decreased while the content of Cr₂O₃ increased from 0 to 10% in the slags, which could be explained by the Cr₂O₃ playing the role of network formation to increase the polymerization degree of the silicate slag. This result meant that the majority of Cr³⁺ formed the bands of Cr–O–Cr with Cr³⁺ in tetrahedral coordination and changed the polymerization degree of the silicates. In addition, Table 5 also shows that the number of non-bridging oxygen slightly increased as the content of TiO₂ increased from 0 to 13% in mass, indicating that the polymerization degree of silicates became weaker due to the decrease of Q³ in the sheet structure unit and Q² in the chain structure as well as the increase of Q¹, O–Ti–O, and Si–O–Si in the monomer structure. As the field strength (change/radius) of the cations is Si⁴⁺>Cr³⁺>V³⁺>Ti⁴⁺, the bond lengths of Si–O, Cr–O, V–O, and Ti–O are 1.60, 1.62, 1.88, and 1.90 Å, respectively.^16,30,32) The combined capacity between the cations and O²⁻ is also presented by comparing it with the bond length; the rank of the stability is Si–O>Cr–O>V–O>Ti–O, which is opposite to their bank length, as proposed by Zhang, Hino, and Baddour-Hadjean’s work.^22,30,32) Thus, it could be conclude that the addition of TiO₂ broke up the 3-dimensional networks formed by Si and O, which was consistent with the conclusion proposed by Park’s work.¹⁶⁾

Correlation between the structural information and physiochemical properties of vanadium slag could naturally be expected. Many researchers have reported that the viscosity and melting temperature of vanadium slag increased with an increase of w(Cr₂O₃) and decreased with an increase of w(TiO₂).^13,16) According to the present structural study, the polymerization degree increased drastically with Cr₂O₃ introduction because of the formation of Cr–O–Cr in chain structures. The plenty of high melting point of solid spinel (FeCr₂O₄, with melting temperature about 2273 K) formed in molten slag would increase the viscosity and melting temperature of vanadium slag. According to the literature,²¹⁾ if the high melting point solid exists in the liquid, it not only affects the homogenized liquid but also forms the plenty of phase interface in liquid to generate inner friction, which could dramatically increase the liquid viscosity. Thus it is considered that the effect of viscosity was for the bond of Cr–O–Cr and the precipitation of FeCr₂O₄ phase, but the effect of FeCr₂O₄ was greater than that of the Cr–O–Cr bond. On the contrary, the polymerization degree became weaker with an increase of TiO₂ due to a decrease of ${\text{Si}}_{\text{2}}{\text{O}}_{\text{5}}^{\text{2}-}$ as a sheet structure and an increased number of discrete Si–O–Ti as well as Ti–O–Ti structural units, which would hamper the crystallization of FeCr₂O₄ as the main crystallization product of the melts, and thus the melting temperature and viscosity of the vanadium slag could be decreased.

4. Conclusions

The viscosity and the structure of FeO–SiO₂–V₂O₃–TiO₂–Cr₂O₃ system slag with varied Cr₂O₃ and TiO₂ contents were investigated by the rotating cylinder method and Raman spectroscopy, respectively. Based on the above results, the following conclusions have been drawn:

(1) The viscosity of the FeO–SiO₂–V₂O₃ system slag was decreased to 3.5 Pa·s at temperatures above 1450 K. With the introduction of 6 mass% Cr₂O₃ into the FeO–SiO₂–V₂O₃ system, the viscosity increased rapidly and was decreased to 3.5 Pa·s until temperature higher than 1770 K. With the introduction of 13 mass% TiO₂ into the FeO–SiO₂–V₂O₃–Cr₂O₃ system, the viscosity decreased and reached to 3.5 Pa·s at a low temperatures of 1630 K. But the viscosity increases again after increase of the Cr₂O₃ to 10 mass% in the FeO–SiO₂–V₂O₃–Cr₂O₃–TiO₂, and the viscosity was decreased to 3.5 Pa·s until temperature higher than 1803 K.

(2) V³⁺ mainly existed in the form of V–O–V as a chain structure in the FeO–SiO₂–V₂O₃ system, which slightly enhanced the polymerization degree of molten slag. A small number of V³⁺ formed V–O as a monomer structure in the FeO–SiO₂–V₂O₃ system, but it showed little effect on the polymerization degree of molten slag.

(3) Part of the chromium existed in the form of solid spinel (FeCr₂O₄), and most of the others existed in the bond of Cr–O–Cr to form network structures, which enhanced the polymerization degree of molten slag. Besides, a small number of chromium formed the [Cr³⁺O₆] units in the FeO–SiO₂–V₂O₃–Cr₂O₃ system, which showed little effect on the polymerization degree of the molten slag.

(4) Ti⁴⁺ mainly existed in the form of discrete Si–O–Ti and Ti–O–Ti and it decreased Q³ in sheet structures which hampered the crystallization of FeCr₂O₄ in molten slag and decreased the polymerization degree of the FeO–SiO₂–V₂O₃–Cr₂O₃–TiO₂ system. A small number of titanium existed in the form of [TiO₄] as clusters in molten slag, which showed little effect on the polymerization degree of the molten slag.

(5) The polymerization degree of silicate structures was lower in the FeO–SiO₂–V₂O₃ system, which existed mainly as a monomer structure (Q⁰) in Raman spectroscopy. With the introduction of 6 mass% Cr₂O₃ into the FeO–SiO₂–V₂O₃ system, the polymerization degree of the silicate structure was enhanced significantly due to the formation of Q² in a chain structure and Q³ in a sheet structure. With the introduction of 13 mass% TiO₂ into the FeO–SiO₂–V₂O₃–Cr₂O₃ system, Q² and Q³ decreased in the silicate structure, and the polymerization degree of the silicate structure became weaker, which could decrease viscosity of the FeO–SiO₂–V₂O₃–Cr₂O₃–TiO₂ system slag.

Acknowledgment

The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51174049, 51174052, 51374057, 51374062) which has made this research possible.

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