Effect of Cr₂O₃ Addition on Viscosity and Structure of Ti-bearing Blast Furnace Slag

Abstract

The viscous flow of CaO–SiO₂–MgO–TiO₂–Al₂O₃–Cr₂O₃ slag (CaO/SiO₂=0.9–1.3, Cr₂O₃=0–4 mass%) were investigated to promote understanding of the effect of Cr₂O₃ addition and basicity (CaO/SiO₂) on the viscous behavior of slag containing TiO₂. The viscosity of slag was found to significantly increase with increasing Cr₂O₃ content at a fixed basicity. The increase in basicity could alleviate the viscosity of the slag to a particular extent. And this study involved the use of theoretical calculations and experiments to prove that Cr₂O₃ in the BF slag could easily react with MgO and Al₂O₃ to generate spinel phase (MgCr₂O₄, MgCrAlO₄) with high melting points, there increasing the viscosity of the slag. The perovskite phase with high melting points is easily crystallized out at high temperatures with increasing Cr₂O₃ content and basicity which makes the viscosity of the slag increase sharply with increasing the CaO/SiO₂ ratio. In addition, this study analyzed the silicate structure of the slag via infrared spectroscopy. The addition of Cr₂O₃ has little effect on the silicate structure of the slag. The increasing basicity enabled the slag to become simple in structure. However, the viscosity of the slag increased rapidly because of the precipitation of the high melting phase perovskite.

1. Introduction

Hundreds of millions of tons of vanadium titanium-magnetite resources can be found in China’s Panxi region.¹⁾ With nearly 40 years of development, a technological process of steel and vanadium products centered on vanadium-titanium magnetite beneficiation, blast furnace process, converter vanadium extraction, converter steelmaking, continuous casting production has already established. However, for the Hongge mining area with the largest reserve, one of the four main mining areas in the Panxi region, resource utilization is still underdeveloped. The main problem is that Hongge ores contain Cr₂O₃ of 0.49%–0.82% in addition to iron, vanadium, and titanium.²⁾ Cr₂O₃, a refractory oxide, affects the blast furnace reduction and slagging process, furthermore, chromium in molten iron significantly affects the follow-up converter vanadium extraction process. Therefore, the systematic and comprehensive investigation of these effects has significance to the large-scale development and utilization of Hongge mineral resources.

Research on the viscous flow characteristics of Ti-bearing BF slag are extensive. Zhang et al.³⁾ showed TiO₂ additions can decrease the slag viscosity under Ar at a fixed basicity, and TiO₂ may function as a network modifier. Satio et al.⁴⁾ studied the effect of TiO₂ in the CaO–SiO₂–MgO–Al₂O₃ slag system at 10 and 20 mass% TiO₂ content. TiO₂ lowered the viscosity of the slag and the corresponding activation energy of viscous flow decreased with TiO₂ content. Shankar et al.,⁵⁾ Park et al.,⁶⁾ Sohn et al.^7,8) Liao et al.⁹⁾ studied quinary slag systems (CaO–SiO₂–Al₂O₃–MgO–TiO₂) and found that, TiO₂ additions can decrease the slag viscosity under Ar at a fixed basicity, and with increasing the CaO/SiO₂ ratio can also decrease the slag viscosity at a fixed TiO₂ content. Park et al.⁶⁾ investigated the silicate structure of quinary slag systems via infrared spectroscopy and Raman spectroscopy analysis, suggested that TiO₂ depolymerizes the slag by modifying the silicate structure of the slag and TiO₂ functions as a network modifier. However, few scholars have studied the effect of Cr₂O₃ content on the viscosity of Ti-bearing blast furnace slag. Thus, research comprehensively studies a chromium-containing high-titanium blast furnace slag via thermodynamic calculation, viscosity measurement, infrared spectroscopy analysis.

2. Experimental

2.1. Materials

The experimental samples used in this study were based on the chemical composition of on-site slag from Panzhihua steel (Table 1). A pure chemical reagent was used to prepare the samples. Before preparation, the reagent was dried for 6 h at the 120°C to remove moisture. In addition, CaO was added in the form of CaCO₃ with the same molar mass. The chemical components of the prepared slag sample are shown in Table 2.

Table 1. Chemical composition of BF slag from Panzhihua Iron and Steel Corporation, wt%.

CaO	SiO2	MgO	Al2O3	TiO2
27.0	24.3	8.3	14.4	22.3

Table 2. Experimental composition and measured values from the present experiment.

No.	Composition (wt%)						C/S	Viscosity (Pa·S)
	CaO	SiO2	Al2O3	MgO	TiO2	Cr2O3		1400°C	1425°C	1450°C	1475°C	1500°C
1#	29.3	26.7	14	8	22	0	1.1	0.26	0.22	0.19	0.16	0.14
2#	28.8	26.2	14	8	22	1	1.1	0.27	0.23	0.21	0.18	0.15
3#	28.3	25.7	14	8	22	2	1.1	0.30	0.24	0.22	0.18	0.15
4#	27.8	25.2	14	8	22	3	1.1	0.32	0.27	0.23	0.19	0.16
5#	27.2	24.8	14	8	22	4	1.1	0.40	0.30	0.25	0.20	0.17
6#	25.6	28.4	14	8	22	2	0.9	0.47	0.37	0.30	0.24	0.19
7#	27.0	27.0	14	8	22	2	1.0	0.38	0.30	0.24	0.20	0.16
8#	28.3	25.7	14	8	22	2	1.1	0.30	0.24	0.22	0.18	0.15
9#	29.5	24.5	14	8	22	2	1.2	0.77	0.29	0.23	0.17	0.13
10#	30.5	23.5	14	8	22	2	1.3	−	−	0.92	0.21	0.13

2.2. Apparatus and Procedure

In the present work, viscosity measurements were made by the rotating cylinder method using a high-temperature rotary viscosimeter. The experimental setup for the viscosity measurement is shown in Fig. 1. MoSi₂ was used as the heating element; the furnace tube is made of Aluminum oxide; the furnace bottom was sealed so that the crucible and test structure were not oxidized after the inert gas flew in. The measuring pole and measuring head in the rotary test were made of molybdenum. The reducing conditions were not considered in the experiment, thus, a molybdenum crucible (45×120 mm) was used. In the experimental process, the furnace was protected under Ar of 99.9% (0.1 L·min⁻¹). The experimental temperature was 1500°C. The prepared slag sample of 200 g was placed in the molybdenum crucible for melting and was kept at 1500°C for 5 h. A molybdenum bar was used to stir the slag every 15 min so that the components of the slag are uniformly mixed. The viscosity was measured after the end of thermal insulation.

Fig. 1.

Experimental apparatus for slag viscosity measurements and crucible.

2.3. Calibration

The prepared slag sample of 200 g was placed in the molybdenum crucible for melting and was kept at 1500°C for 5 h. In addition the experimental temperatures, such as 1500, 1475, 1450°C were calibrated by the standard thermocouple. The furnace temperatures were controlled by the PID program and the temperature fluctuation was limited to 1 degree. A molybdenum bar was used to stir the slag every 15 min so that the components of the slag were uniformly mixed. At first, when the temperature reduced to the experimental temperature point, it took 0.5 h to make sure the temperature remains constant. 60 viscosity data were obtained at the same time. Then can get the average of the viscosity data.

It was calibrated by using silicone oil with 1, 5, 10 poise at room temperature. The temperature was 25°C. It took 0.5 h every kind of silicone oil during the calibration to obtain the viscosity in Table 3. A curve-fitting was finished with the standard viscosity data. Comparing with obtained viscosity, the degree of fitting is 0.999988.

Table 3. Testing data at room temperature.

silicone oil	Standard Viscosity/Pa.s	Obtained Viscosity/Pa.s
1 poise	0.95	0.940
5 poise	4.92	4.938
10 poise	9.80	9.792

At elevated temperature, the temperature was set to 1400°C and 1475°C. The sample was Hi Boron Glass. The prepared sample of 200 g was placed in the molybdenum crucible for melting and was kept at 1500°C for 5 h. The furnace temperatures were controlled by the PID program and the temperature fluctuation was limited to 1 degree. A molybdenum bar was used to stir the slag every 15 min so that the components of the slag were uniformly mixed. At first, when the temperature reduced to the experimental temperature point, it took 0.5 h to make sure the temperature remains constant. 60 viscosity data were obtained at the same time. Then can get the average of the viscosity data in Table 4. Finally, compared the average viscosity with NIST Certificate Data.

Table 4. Testing data at elevated temperature.

Temperature/°C	Viscosity/Pa.s	NIST Certificate Data/Pa.s
1475	1.276	1.22±0.06
1400	1.544	1.46±0.07

It has a small range of viscosity in Tables 3 and 4. So the author thinks the data is reliable.

3. Thermodynamic Calculation

Research on the influence of Cr₂O₃ on the viscous flow characteristics of quinary slag systems (CaO–SiO₂–Al₂O₃–MgO–TiO₂) remain scarce. Therefore, this study uses the FactSage software to calculate theoretically the dissolved amount of Cr₂O₃ added to the quinary slag systems and the phase change after the slag crystallizes.

3.1. Dissolved Percentage of Cr₂O₃ in Ti-bearing BF Slag

In this study, the influence of different binary basicities on the dissolved amount of Cr₂O₃ in the quinary slag systems (CaO–SiO₂–Al₂O₃–MgO–TiO₂) at 1200°C to 1600°C is calculated by the FactSage software.

In the quinary slag systems (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass% TiO₂) of 100 g, the relationship between the binary basicity and the solubility percentage of Cr₂O₃ in quinary slag systems is shown in Fig. 2. The solubility of Cr₂O₃ in the quinary slag systems increase gradually with increasing temperature. And the solubility of Cr₂O₃ in the slag systems gradually decreases with increasing the CaO/SiO₂ ratio at a given temperature. However, the maximum solubility percentage of Cr₂O₃ in slag system is less than 1%. Therefore, the Cr₂O₃ in the Ti-bearing blast furnace slag is soluble only in small amounts.

Fig. 2.

Effect of basicity on the solubility of Cr₂O₃.

3.2. Phase Transformation in Crystallization of Chromium Containing High-Titanium Slag

In this study, the researchers theoretically calculated the phase crystallization and transformation of the quinary slag systems (CaO–SiO₂–Al₂O₃–MgO–TiO₂) and the senary slag systems (CaO–SiO₂–Al₂O₃–MgO–TiO₂–Cr₂O₃) at constant CaO/SiO₂ of 1.1 slowly cooled to 1000°C from 1600°C by the FactSage software.

3.3. Effect of Cr₂O₃ Content on Phase Transformation in Crystallization of Ti-bearing BF Slag

Figure 3 shows the change after the phase crystallization of the quinary slag systems (29.3 mass%CaO-26.7 mass%SiO₂-14 mass%Al₂O₃-8 mass%MgO-22 mass% TiO₂) cooled to 1000°C from 1600°C at constant CaO/SiO₂ of 1.1. The slag is present in the liquid phase at 1600°C and begins to crystallize out the perovskite phase (CaTiO₃) at 1436°C. With decreasing temperature, the amount of the liquid phase gradually reduces and that of the perovskite gradually increases. When the temperature is reduced to 1324°C, it begins to crystallize out the titanium spinel phases (TiFe₂O₄). The anorthite (CaAl₂Si₂O₈) is crystallized out when the temperature is reduced to 1234°C. The clinopyroxene (CaMgSi₂O₆, CaAlSi₂O₆) and melilite phases (Ca₂MgSi₂O₇, Ca₂AlSi₂O₇) begin to be crystallized out at 1224°C. The liquid phase disappears at 1220°C, the clinopyroxene, perovskite, anorthite, titanium spinel and melilite phases gradually increase. The clinopyroxene phase increases the most remarkably, which accounted for 40%.

Fig. 3.

Phase crystallization change of the quinary slag system cooled to 1000°C from 1600°C at constant CaO/SiO₂ of 1.1.

Figure 4 shows the change after the phase crystallization of the senary slag systems (CaO-SiO₂-8 mass%MgO-14 mass% Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃) cooled to 1000°C from 1600°C at constant CaO/SiO₂ ratio of 1.1. The spinel phases (MgCr₂O₄, MgCrAlO₄, MgAl₂O₄) are crystallized out at 1600°C with Cr₂O₃ additions. With decreasing temperature, the amount of the liquid phase gradually reduces. The slag begins to crystallize out the perovskite phase (CaTiO₃) at 1438°C. When the temperature is reduced to 1235°C, it begins to crystallize out the anorthite (CaAl₂Si₂O₈). The clinopyroxene (CaMgSi₂O₆, CaAlSi₂O₆) begins to be crystallized out at 1225°C. The liquid phase disappears at 1220°C. With Cr₂O₃ adding, the temperature that the slag begins to crystallize out the spinel phases (MgCr₂O₄, MgCrAlO₄, MgAl₂O₄) increase obviously. In case of Cr₂O₃=2 mass%, the melilite phases disappear.

Fig. 4.

Phase crystallization change of the senary slag system cooled to 1000°C from 1600°C at constant CaO/SiO₂ of 1.1.

It is well known that crystallinity has a big effect on viscosity. With lowering temperature, the solid phase increase inevitably. Compared Fig. 3 with Fig. 4, The spinel phases (MgCr₂O₄, MgCrAlO₄, MgAl₂O₄) are crystallized out at 1600°C with Cr₂O₃ additions. So the viscosity increase from 0.14 to 0.15 Pa s. With decreasing temperature, the amount of the liquid phase gradually reduces and the viscosity increase with crystallinity.

Figure 5 shows the impact of Cr₂O₃ content on the spinel phases generated in the crystallization of Ti-bearing blast furnace slag. The spinel phases (MgAl₂O₄) begin to be crystallized out at 1324°C without Cr₂O₃ additions. The spinel phases (MgCr₂O₄, MgCrAlO₄, MgAl₂O₄) begin to be crystallized out at 1600°C with Cr₂O₃ additions, and the amount of spinel phases crystallized out gradually increase with gradual increasing Cr₂O₃ content.

Fig. 5.

Effect of Cr₂O₃ content on spinel phases in the crystallization of the Ti-bearing BF slag.

Figure 6 shows the effect of Cr₂O₃ content on the perovskite phases generated in the crystallization of the Ti-bearing blast furnace slag. The temperature at which the perovskite phases start to be crystallized out with Cr₂O₃ additions increases. The activity of MgO decreases with Cr₂O₃ additions in the Ti-bearing BF slag which can easily react with MgO and Al₂O₃ to generate spinel phase (MgCr₂O₄, MgCrAlO₄), and the perovskite phase increases with the activity of CaO increasing. Moreover, when Cr₂O₃ content is increased by 2 mass% each time, the temperature at which the perovskite phases start to be crystallized out increases more obviously. The temperature at which the perovskite phases start to be separated out through the crystallization increases from 1436°C to 1441°C with Cr₂O₃ additions from 0 to 4 mass%.

Fig. 6.

Effect of Cr₂O₃ content on perovskite phases in the crystallization of the Ti-bearing BF slag.

3.4. Effect of Basicity on Phase Transformation in the Crystallization of Chromium Containing High-Titanium Slag

Figure 7 shows the impact of basicity (CaO/SiO₂=0.9–1.3) on phase transformation in the senary slags (CaO-SiO₂-8 mass%MgO-4 mass%Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃) cooled to 1000°C from 1600°C. When the basicity is increased to 1.0 from 0.9, the sphene phases (CaSiTiO₅) disappear, the amount of olivine phases gradually decreases, and the amount of perovskite phases and clinopyroxene phases gradually increase. When the binary is increased to 1.1 from 1.0, the olivine phases disappear, the anorthite phases gradually decreases, and the clinopyroxene phases significantly increases. When the binary basicity is increased to 1.2 from 1.1, the clinopyroxene phases gradually decrease, and the melilite phases are crystallized out. The clinopyroxene phases continue to decrease and the amount of melilite phases gradually increase with the CaO/SiO₂ ratio increases to 1.3 from 1.2.

Fig. 7.

Impact of basicity on phase transformation in the crystallization of the senary slag systems (CaO-SiO₂-8 mass%MgO-4 mass%Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃).

Figure 8 shows the impact of basicity on the spinel phase generated in the crystallization of the senary slags (CaO-SiO₂-8 mass%MgO-4 mass%Al₂O₃-22 mass% TiO₂-2 mass%Cr₂O₃). The amount of spinel phases crystallized out increase gradually with increasing the CaO/SiO₂ ratio and decreasing temperature. The spinel phases separated out via crystallization increase by about 0.5% with the CaO/SiO₂ ratio increases to 1.3 from 0.9 at 1600°C.

Fig. 8.

Impact of basicity on spinel phase transformation in the crystallization of the senary slag systems (CaO-SiO₂-8 mass%MgO-4 mass%Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃).

The MgCr₂O₄ is easily formed with Cr₂O₃ and MgO, which can be expressed by the follow equation   

$$\text{MgO}+{\text{Cr}}_{\text{2}}{\text{O}}_{\text{3}}\leftrightarrow {\text{ MgCr}}_{\text{2}}{\text{O}}_{\text{4}}$$

The amount of spinel phase (MgCr₂O₄) crystallized out increase gradually with increasing of the CaO/SiO₂ ratio, because of that higher CaO/SiO₂ ratio would provide additional free oxygen ions (O²⁻) which made the reaction.

Figure 9 shows the impact of basicity on the perovskite phase generated in the crystallization of the senary slags (CaO-SiO₂-8 mass%MgO-4 mass%Al₂O₃-22 mass% TiO₂-2 mass% Cr₂O₃). The temperature at which the perovskite phases are crystallized out and gradually increases with gradual increase in basicity. And the amount of perovskite phases increases since the CaTiO₃ is more easily formed with TiO₂ and CaO with the activity of CaO increasing by increasing the CaO/SiO₂ ratio. When the CaO/SiO₂ ratio increases to 1.3 from 0.9, the temperature at which the perovskite phases are crystallized out gradually increases to 1473°C from 1381°C; that is, the perovskite phases is easily crystallized out at high temperatures with increasing of basicity.

Fig. 9.

Impact of basicity on perovskite phase transformation in the crystallization of the senary slag systems (CaO-SiO₂-8 mass%MgO-4 mass%Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃).

4. Results and Discussion

4.1. Effect of Cr₂O₃ Content on Viscosity of the Ti-bearing BF Slag

Figure 10 shows the effect of Cr₂O₃ content on the viscous behaviour of the quinary slag (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂) with the constant CaO/SiO₂ ratio of 1.1, where the Cr₂O₃ content was varied between 0 and 4 mass%. The viscosity of the slag increases obviously with Cr₂O₃ additions, and the viscosity of slag increases gradually with increasing Cr₂O₃ content. The increase in Cr₂O₃ content considerably influences on the viscosity of the slag at lower temperature.

Fig. 10.

Effect of Cr₂O₃ content on the viscosity of the the quinary slag (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂).

Figure 11 shows the effect of Cr₂O₃ content on the viscosity–temperature curve of the quinary slag systems (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂) in the continuous reduction in temperature with the constant CaO/SiO₂ ratio of 1.1. The critical temperature of the slag rises with Cr₂O₃ additions. Moreover, the trend toward increase is greater with Cr₂O₃ increasing by 2% each time. Based on the measured viscosity–temperature curve, the calculation can be made by using the temperature corresponding to the tangent point between the 45° line and the viscosity–temperature curve as the critical temperature (Table 5).

Fig. 11.

Effect of Cr₂O₃ content on the viscosity–temperature curve of the quinary slag (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂) in the continuous reduction in temperature with a basicity=1.1.

Table 5. Effect of Cr₂O₃ content on the critical temperature of the slag.

CaO/SiO2	Cr2O3, wt%
	0	1	2	3	4
1.1	1302°C	1312°C	1329°C	1329°C	1348°C

Figure 12 shows the impact of the undissolved Cr₂O₃ on the viscosity of the slag with the binary basicity of CaO/SiO₂=1.1. With increasing the undissolved content of Cr₂O₃, the viscosity of the slag increases gradually under a certain temperature; the higher temperature can decrease the undissolved Cr₂O₃ and then subsequently resulting in a low viscosity.

Fig. 12.

Effect of the undissolved Cr₂O₃ Addition on Viscosity–Temperature Curve of Ti-Bearing BF Slag in Temperature.

4.2. Effect of Cr₂O₃ Content on Viscous Flow Activation Energy of the Ti-bearing BF Slag

Temperature is one main factor that influences slag viscosity.¹⁰⁾ In General, the viscosity of the slag decreases with increasing temperature. The temperature dependence of the viscosity is usually expressed by Weymann–Frenkel’s equation:¹¹⁾   

$$\eta ={\text{ATe}}^{\text{E}\eta /\left(\text{RT}\right)}$$

where A is a proportionality constant, E_η is the apparent activation energy for viscous flow, R is the gas constant and T is the absolute temperature. The apparent activation energy for slag E_η represents the frictional resistance for viscous flow, and the variations in the apparent activation energy can suggest a change in the structure of the molten slag or more directly a change in the cohesive flow units comprising the slag structure.

Based on the experimental data, the apparent activation energies for viscous flow of the senary slag were evaluated according to Weymann–Frenkel’s equation. Figure 13 shows the curve of the slag representing the relationship ln(η/T) –1/T. For these two, they take on a linear relationship, which indicates that the relationship between viscosity and temperature conforms to the Wayman–Frankel’s equation. The results are presented in Table 6. It can be noted that the apparent activation energy increases with increasing Cr₂O₃ content.

Fig. 13.

Temperature dependence of the viscosity of the senary slag with various Cr₂O₃ contents.

Table 6. Effect of Cr₂O₃ content on the apparent activation energy of the viscous flow of the slag.

NO.	Wayman–Frankel Equation	Activation Energy Eη/kJ·mol−1
1#	y=20121 x−20.80, R2=0.999	167.29
2#	y=18717 x−19.91, R2=0.997	155.61
3#	y=21253 x−21.35, R2=0.992	176.70
4#	y=21839 x−21.61, R2=0.999	181.57
5#	y=26329 x−24.10, R2=0.992	218.90

4.3. Effect of Basicity (CaO/SiO₂) on the Viscosity of the Chromium Containing High-Titanium Slag

Figure 14 shows the effect of basicity on the viscosity behavior of the senary slag systems (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃). The viscosity of the slag decreases gradually with increasing basicity when the CaO/SiO₂≤ 1.1. Because higher CaO/SiO₂ ratio is likely to have depolymerized the slag silicate network structure and lowered the viscosity by providing additional free oxygen ions (O²⁻). The viscosity of the slag decreases with the increase in basicity with the CaO/SiO₂ ratio of 1.2 at above 1450°C, however, the viscosity of the slag increases sharply with the temperature drops to 1400°C from 1425°C. The viscosity of the slag increases rapidly when the CaO/SiO₂ ratio is 1.3 with the temperature drops to 1450°C from 1475°C. This result is the same with the CaO/SiO₂ ratio of 1.2. What’s more, the slag has the nature of a basic slag, and the slag has a strong crystallization ability and crystallized out continuously, which increases the viscosity of the slag.

Fig. 14.

Effect of the CaO/SiO₂ ratio on the viscosity of the senary slags (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃).

Figure 15 shows the effect of basicity on the viscosity–temperature curve of the senary slag systems (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂-2 mass% Cr₂O₃) in the continuous reduction in temperature. The critical temperatures of the slag corresponding to the basicity are shown in Table 7.

Fig. 15.

Effect of CaO/SiO₂ ratio on the viscosity–temperature curve of the senary slags (CaO-SiO₂-8 mass%MgO-14 mass%Al₂O₃-22 mass%TiO₂-2 mass%Cr₂O₃) with continuous reduction in temperature.

Table 7. Effect of CaO/SiO₂ ratio on the critical temperature of the senary slag.

Cr2O3	CaO/SiO2
	0.9	1.0	1.1	1.2	1.3
2%	1310°C	1319°C	1329°C	1392°C	1446°C

The critical temperature of the slag gradually rises with the CaO/SiO₂ ratio increasing. When the basicity of the slag increases to 1.1 from 0.9 and the critical temperature of the slag increases to 1329°C from 1310°C respectively, the viscosity curve changes slightly and has no obvious turning point, which indicates that the slag has the nature of a acidity slag. However, the critical temperature of the slag rises significantly when the basicity of the slag increases to 1.3 from 1.1. And the viscosity curve has an obvious turning point, which shows that the slag has the nature of a basic slag. The critical temperature increases to about 80°C to 120°C compared with that of the slag with a basicity of 0.9–1.1.

4.4. X-ray Diffraction Analysis

The direct study of the structure of slag at high temperatures has faced several difficulties. In this study adopts the water quenched method for the slag which kept the silicate structure. Spectroscopy study was conducted to obtain the structural information of the quenched slag at high temperatures. Figure 16 shows the XRD patterns of the slag samples quenched from 1500°C. The slag samples are basically amorphous, which keep the structure of the samples intact at high temperatures.

Fig. 16.

The XRD patterns of the as-quenched samples from 1500°C with varying Cr₂O₃ content in slags at CaO/SiO₂ of 0.9–1.3.

As can be seen, the diffraction peak of the XRD patterns of the slag samples with varying Cr₂O₃ content changes slightly since the content of Cr₂O₃ in the slag sample changes within a relatively smaller scope; the crystallization peak gradually increases with increasing Cr₂O₃ content, it’s consistent with the theoretical calculation. when the binary increase, it is easy to precipitate perovskite phase at high temperature. Cr₂O₃ easily react with MgO and Al₂O₃ to generate spinel phase (MgCr₂O₄, MgCrAlO₄). The crystallization peak also gradually increases with increasing basicity. To analyze accurately the phases generated via crystallization at high temperatures, the XRD patterns of slag sample No.5^# (Cr₂O₃=4 mass%) and No.10^# (CaO/SiO₂=1.3) are taken as an example for analysis (Fig. 17).

Fig. 17.

The XRD patterns of the as-quenched NO. 5 and NO. 10 samples from 1500°C.

Cr₂O₃ and MgO easily combine to generate MgCr₂O₄ with additions Cr₂O₃, and the phenomenon of isomorphous substitution between Cr³⁺ and Al³⁺ occurs, which generates of magnesia-chrome-aluminum spinel (MgCrAlO₄),¹²⁾ and is consistent with the calculation of the FactSage. The MgCr₂O₄ and MgCrAlO₄ are spinel phases with high melting points (about 1800°C) and have better crystallization properties.¹²⁾ Therefore, the Ti-bearing BF slag contains MgCr₂O₄ and MgCrAlO₄ with Cr₂O₃ additions at 1500°C, which enables the viscosity of the slag to be higher than that of an ordinary Ti-bearing BF slag. And the amount of MgCr₂O₄ and MgCrAlO₄ increase with increasing of Cr₂O₃ content, which makes the viscosity of the slag increase. The molybdenum in the phase may be derived from the molybdenum crucible or molybdenum probe dissolved in the slag in small amounts because of erosion.

Based on previous studies,^7,13) the viscosity of the Ti-bearing BF slag would decrease with MgO additions. However, Cr₂O₃ reacts with MgO to generate the magnesia-chrome spinel with Cr₂O₃ additions at 1500°C, which reduces the activity of MgO. Thus the viscosity of the slag increases, and the melting temperature rises.

4.5. FTIR Analysis

The FTIR results of the CaO–SiO₂–MgO–Al₂O₃–TiO₂–Cr₂O₃ (CaO/SiO₂=1.1) slags is shown in Fig. 18 as a function of the wavenumbers at different Cr₂O₃ contents. FTIR spectra of silicate slags are typically focused within the wavenumber of region between 1200 cm⁻¹ and 400 cm⁻¹.^14,15,16) This region represents the symmetric stretching vibration bands of the [SiO₄] tetrahedra, the asymmetric stretching vibrations of the [AlO₄] tetrahedra, and the symmetric Si–O bending vibration bands, etc.

Fig. 18.

FTIR result of as-quenched samples from 1500°C with varying Cr₂O₃ content in the slag at CaO/SiO₂ of 1.1.

The vibration band between the wavenumbers of 1200 cm⁻¹ and 800 cm⁻¹ is the [SiO₄] tetrahedra symmetric stretching vibration band, which known to be the convoluted band of the NBO/Si transmission troughs from NBO/Si of 1–4. NBO/Si is the non-bridged oxygen per unit of silicon atom, where lower NBO/Si pertains to the more polymerized slag structure.⁶⁾ According to Mysen et al.,¹⁷⁾ NBO/Si assigned values from 1 to 4 are structurally defined as sheets (1100 cm⁻¹ to 1050 cm⁻¹), chains (980 cm⁻¹ to 950 cm⁻¹), dimmers (920 cm⁻¹ to 900 cm⁻¹), and monomers (880 cm⁻¹ to 850 cm⁻¹), respectively. This decrease in the depth of the transmission trough indicates a depolymerization of the slag structure as suggested in previously published literature.^16,18) Little significant differences could be ascertained from the symmetric Si–O bending vibration band with Cr₂O₃ additions. Given that the added Cr₂O₃ accounts for less than 4% in amount, there is slightly or no effect on the silicate structure of the slag for the present slag compositions.

The FTIR results of the CaO–SiO₂–MgO–Al₂O₃–TiO₂–Cr₂O₃ (Cr₂O₃=2 mass%) slag is shown in Fig. 19 as a function of the wavenumbers with varying CaO/SiO₂ ratio. There is no obvious change for the depth of the transmission trough with the CaO/SiO₂ ratio of 0.9–1.1. The depth of the symmetric Si–O bending vibration band from 1100 cm⁻¹ to 960 cm⁻¹ decreased with CaO/SiO₂ ratio of 1.2 and 1.3. With increasing basicity, the concentration of O²⁻ increases, which depolymerized complex silicate in the slag. By this time, the viscosity of the slag shall have been decreased, but the perovskite phase (CaTiO₃) with a high melting point sharply increases the slag viscosity.

Fig. 19.

FTIR result of as-quenched samples from 1500°C with varying CaO/SiO₂ ratio in the slag at Cr₂O₃ of 2 mass%.

5. Conclusion

The influences of Cr₂O₃ and basicity on phase transformation in crystallization of the CaO–SiO₂–MgO–Al₂O₃–TiO₂–Cr₂O₃ slag systems were investigated using the FactSage software, and the viscosity of the senary slag were investigated using the rotating method. Furthermore, the XRD and FTIR were analyzed the quenched slag. The following conclusions were obtained.

(1) Cr₂O₃ additions increased the viscosity by reacting with MgO and Al₂O₃ to generate MgCr₂O₄ and MgCrAlO₄ with high melting points, and the amount of spinel phases (MgCr₂O₄, MgCrAlO₄) and the perovskite phases (CaTiO₃) crystallized out gradually increase with increasing Cr₂O₃ content, the temperature at which the perovskite phases start to be crystallized out increases more obviously, which provided by FactSage and XRD. The apparent activation energy increases with increasing Cr₂O₃ content.

(2) The viscosity of the slag decreases gradually with increasing basicity with the CaO/SiO₂≤ 1.1 at a constant Cr₂O₃ content. The perovskite phases with high melting points is easily crystallized out at high temperatures with increasing of basicity which makes the viscosity of the slag increase sharply below 1475°C with the CaO/SiO₂ ratio of 1.3. The amount of spinel phases (MgCr₂O₄, MgCrAlO₄) crystallized out increase gradually with increasing the CaO/SiO₂ ratio which also increase the viscosity of the slag. The viscosity of the slag below 1425°C with the CaO/SiO₂ ratio of 1.2 has the same tendency. The melting temperature is 70°C to 120°C above the slag with the CaO/SiO₂ ratio of 0.9–1.1.

(3) The FTIR analysis indicates the polymerization degree of the slag slightly changes with Cr₂O₃ additions. However, the polymerization degree of slag decreases with increasing basicity. The viscosity of the slag increases sharply since the perovskite is crystallized out at high temperatures.

Acknowledgements

The authors are especially grateful to the Key Project of Chinese National Programs for Fundamental Research and Development (Grant No. 2013CB632603).

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