Distribution Ratio of Sulfur between CaO-SiO₂-Al₂O₃-Na₂O-TiO₂ Slag and Carbon-Saturated Iron

Abstract

To explore the feasibility of hot metal desulfurization using red mud, the sulfur distribution ratio (L_S) between CaO-SiO₂-Al₂O₃-Na₂O-TiO₂ slag and carbon-saturated iron is evaluated in this paper. First, the theoretical liquid areas of the CaO-SiO₂-Al₂O₃ (-Na₂O-TiO₂) slag are discussed and the fluxing effects of Al₂O₃, Na₂O, and TiO₂ are confirmed. Then, L_S is measured via slag-metal equilibrium experiments. The experimental results show that L_S significantly increases with the increase of temperature, basicity, and Na₂O content, whereas it decreases with the increase of Al₂O₃ and TiO₂ content. Na₂O in the slag will volatilize with high temperatures and reducing conditions. Furthermore, based on experimental data for the sulfur distribution ratio between CaO-SiO₂-Al₂O₃-Na₂O-TiO₂ slag and the carbon-saturated iron, the following fitting formula is obtained: $\log L_S=45.584\Lambda +\frac{10568.406-17184.041\Lambda }{T}-8.529$

1. Introduction

Sulfur often deteriorates metal properties [1,2], especially metals’ toughness. To realize the deep desulfurization of steel [3,4], the process of hot metal desulfurization has become an economical and efficient method [2]. During this process, traditional lime-based slag has a high melting point. Therefore, CaF₂ is widely added as an additive to decrease the melting point and improve the solubility of lime [5]. However, because CaF₂ is toxic to the environment and human health, its use has been strictly restricted [6]. Hence, a desulfurizing slag with a much lower melting point, especially under conditions of high basicity, is required. Previous research [5,6,7,8,9,10,11,12] showed that other additives such as Al₂O₃, Na₂O, and TiO₂, could improve the desulfurization efficiency of lime-based slag. Niekerk and Dippenaar [5] determined that the Na₂O equivalent of CaO was 0.30, and adding Na₂O could significantly increase the sulfide capacity of silicate and lime-based slag. Pak and Fruehan [6] reported that the addition of Na₂O lowered the melting point and improved the fluidity of lime-based slag. Zhang’s experiments [7] obtained good slag fluidity and a much better separation between the slag and the melt phases, attributed to the fact that Al₂O₃ and Na₂O could act as a flux and decrease the melting point of the slag. Yajima et al. [8] found that, with the addition of Al₂O₃ to the CaO-SiO₂-FeO_x slag system at an oxygen partial pressure of 1.8 × 10⁻³ Pa, the liquid areas were enlarged. Park et al. and Sohn et al. [10,11] confirmed that TiO₂ decreased the viscosity of blast furnace slag by depolymerizing the slag structure.

Red mud is the residue discharged by the aluminum industry after the extraction of alumina. In addition to CaO, there is an abundance of Al₂O₃, Na₂O, and TiO₂ in red mud. However, it is piled up, pollutes the environment, and increases the burden on enterprises. The sulfur distribution ratio between CaO-SiO₂-Al₂O₃-Na₂O-TiO₂ slag and carbon-saturated iron is examined in this study to explore the feasibility of hot metal desulfurization using red mud. First, the liquid areas of the CaO-SiO₂-Al₂O₃ slag system, along with the effects of Al₂O₃, Na₂O, and TiO₂, are investigated thermodynamically using FactSage7.0 software. Then, the sulfur distribution ratio between these slag systems and carbon-saturated iron is examined using an equilibrium experiment in the laboratory.

2. Liquid Areas of the CaO-SiO₂-Al₂O₃ (-Na₂O-TiO₂) Slag System

The liquid areas of CaO-SiO₂-Al₂O₃ slag, simulated by the FactSage7.0 software (developed by CRCT, Montreal, QC, Canada and GTT-Technologies, Herzogenrath, Germany), are shown in Figure 1. Figure 1a shows that the liquid areas at 1400 °C, 1500 °C, and 1600 °C account for about 1/11, 1/4, and 1/2 of the whole diagram area, respectively. This indicates that high temperature is beneficial to the melting of slag. In Figure 1a, there are three lines that correspond to the CaO/SiO₂ of 0.25, 1, and 4, respectively. When CaO/SiO₂ is constant, the melting point of the slag decreases at first and then increases with an increase of Al₂O₃ content, indicating that adding Al₂O₃ can promote the melting of slag at low concentrations, whereas melting deteriorates above a certain content of Al₂O₃. When CaO/SiO₂ is 0.25, 1, and 4, the critical value of Al₂O₃ content is 13.7–19.3%, 15.3–19.1%, and 36.1–43.4%, respectively. Therefore, Al₂O₃ has the effect of a flux, but its concentration should be controlled. Figure 1b,c show the effects of Na₂O and TiO₂ on the liquid areas of CaO-SiO₂-Al₂O₃ slag at 1500 °C, respectively. Figure 1d shows the effect of a Na₂O and TiO₂ mixture (denoted as “NT”) at a mass ratio of Na₂O/TiO₂ = 2:1 on the liquid areas of CaO-SiO₂-Al₂O₃ slag at 1500 °C. The liquid areas become enlarged with the increase of Na₂O, TiO₂, and NT content, showing that these additives could all promote the melting of the slag. However, Na₂O and NT could promote the melting of the slag more effectively than TiO₂.

Therefore, when the temperature is relatively low, it is possible to obtain a good melting effect on slag with the addition of Al₂O₃, Na₂O, and TiO₂. For the actual desulfurization capacity of slag systems and the feasibility of hot metal desulfurization using red mud, slag–metal equilibrium experiments were carried out.

3. Experimental

3.1. Experimental Materials

Iron was prepared by melting electrolytic iron (Fe > 99.6%), high-purity FeS, and graphite particles in an induction furnace. The final iron contained 4.0% carbon and 0.3% sulfur. The base slag was obtained by mixing analytical-grade reagents (i.e., CaO, SiO₂, Al₂O₃, Na₂SiO₃, and TiO₂), and its composition varied around the composition of red mud. Na₂SiO₃ was added as a source of Na₂O [6]. With the help of a muffle furnace, CaO, SiO₂, Al₂O₃, and TiO₂ were calcined at 1000 °C for 2 h, and Na₂SiO₃ was roasted at 300 °C for 2 h to remove carbonates and hydroxides prior to use. The base slag was uniformly mixed in an agate mortar and then formed into cylinders with a diameter of 18 mm, at a pressure of 30 MPa for 2 min. The iron samples were shaped into cylinders with a diameter of 18 mm by wire cutting. The weight of the base slag was about 12 g, and that of the iron sample was about 15 g. The mass ratio of slag to metal was 0.8:1. A graphite crucible (OD = 25 mm, ID = 20 mm, H = 30 mm) was employed in the experiment.

3.2. Experimental Scheme

The experiment mainly consisted of two parts. For experiments T1–Ti3, the effect of temperature, basicity (i.e., CaO/SiO₂), and Al₂O₃, Na₂O, and TiO₂ content on the sulfur distribution ratio were measured by changing a single factor. For experiments 1–21, we referred to the method of uniform design [12]. Multiple factors were changed simultaneously, including temperature, basicity, Al₂O₃, Na₂O, and TiO₂ content. The test points were distributed as evenly as possible within the test range, so that each test point could be representative. The slag composition after equilibration is shown in

Table 1. Experimental data after equilibration.

No.	Temp./°C	Slag and Metal-Phase Composition (mass%)							Basicity	$\mathit{\Lambda }$	LS	logLS
		CaO	SiO2	Al2O3	Na2O	TiO2	[%S]	(%S)
T1	1400	48.29	27.87	17.84	1.80	4.20	0.01345	0.3788	1.73	0.7033	28.16	1.4496
T2	1450	48.99	27.99	17.92	0.89	4.21	0.01049	0.3784	1.75	0.7017	36.07	1.5571
T3	1500	49.47	27.70	18.13	0.40	4.30	0.00914	0.3817	1.79	0.7018	41.76	1.6208
R1	1450	39.14	37.17	17.47	2.13	4.09	0.01376	0.3769	1.05	0.6599	27.39	1.4376
R2		48.99	27.99	17.92	0.89	4.21	0.01049	0.3784	1.75	0.7017	36.07	1.5571
R3		54.04	22.29	18.78	0.55	4.34	0.00475	0.3815	2.42	0.7285	80.32	1.9048
A1		51.51	30.22	13.11	0.96	4.20	0.00783	0.3778	1.70	0.7063	48.25	1.6835
A2		48.99	27.99	17.92	0.89	4.21	0.01049	0.3784	1.75	0.7017	36.07	1.5571
A3		46.20	25.97	22.11	1.34	4.38	0.01300	0.3820	1.78	0.6980	29.38	1.4681
N1		49.74	28.38	17.38	0.33	4.17	0.01126	0.3727	1.75	0.7011	33.10	1.5198
N2		48.99	27.99	17.92	0.89	4.21	0.01049	0.3784	1.75	0.7017	36.07	1.5571
N3		48.01	27.44	18.79	1.46	4.30	0.00947	0.3695	1.75	0.7018	39.02	1.5913
Ti1		50.06	28.57	18.11	1.19	2.07	0.00973	0.3718	1.75	0.7049	38.21	1.5822
Ti2		48.99	27.99	17.92	0.89	4.21	0.01049	0.3784	1.75	0.7017	36.07	1.5571
Ti3		48.19	27.39	17.27	0.62	6.53	0.01062	0.3763	1.76	0.6995	35.43	1.5494
1	1400	45.36	27.11	18.66	2.85	6.02	0.01205	0.3762	1.67	0.6997	31.22	1.4946
2		53.86	23.43	13.88	3.41	5.42	0.00671	0.3854	2.30	0.7386	57.44	1.7591
3		42.81	31.01	19.46	2.03	4.69	0.01461	0.3797	1.38	0.6809	25.99	1.4147
4		53.22	26.69	15.78	0.74	3.57	0.01066	0.3733	1.99	0.7177	35.02	1.5444
5		36.00	34.27	22.51	3.50	3.72	0.01864	0.3801	1.05	0.6598	20.39	1.3095
6		47.94	27.35	19.13	2.51	3.07	0.01180	0.3831	1.75	0.7058	32.47	1.5114
7		56.74	24.49	14.77	1.78	2.22	0.00704	0.3778	2.32	0.7388	53.66	1.7294
8	1450	43.07	30.69	19.17	1.14	5.93	0.01328	0.3785	1.40	0.6790	28.50	1.4549
9		50.80	25.66	16.76	0.92	5.86	0.01083	0.3774	1.98	0.7127	34.85	1.5420
10		36.43	33.92	22.06	2.32	5.27	0.02115	0.3793	1.07	0.6576	17.93	1.2536
11		48.99	27.99	17.92	0.89	4.21	0.01049	0.3784	1.75	0.7015	36.07	1.5571
12		58.63	23.87	13.15	0.74	3.61	0.00671	0.3873	2.46	0.7427	57.72	1.7613
13		46.35	31.45	18.91	0.68	2.61	0.01175	0.3827	1.47	0.6856	32.57	1.5128
14		54.27	25.09	15.83	2.29	2.52	0.00746	0.3865	2.16	0.7310	51.81	1.7142
15	1500	39.55	32.00	21.95	0.74	5.76	0.01783	0.3789	1.24	0.6644	21.25	1.3274
16		49.71	27.19	16.98	0.68	5.44	0.00814	0.3731	1.83	0.7050	45.84	1.6611
17		57.58	24.03	13.39	0.26	4.74	0.00672	0.3696	2.40	0.7367	55.00	1.7401
18		44.20	30.36	19.98	0.46	5.00	0.01606	0.3719	1.46	0.6803	23.16	1.3645
19		54.97	25.67	15.06	0.21	4.09	0.00827	0.3744	2.14	0.7237	45.27	1.6560
20		44.44	31.18	21.14	0.39	2.85	0.01289	0.3811	1.43	0.6790	29.57	1.4708
21		54.60	27.13	15.77	0.12	2.38	0.00708	0.3662	2.01	0.7187	51.72	1.7138

.

3.3. Experimental Equipment

The experimental equipment included a horizontal resistance furnace, a gas-purification system, and a water-cooling device, whose schematic diagram is shown in Figure 2. A proportional-integral-derivative (PID) controller controlled the furnace with MoSi₂ heating elements. After being calibrated, a Pt-30%Rh/Pt-6%Rh thermocouple was used to measure the temperature. The temperature control range of the furnace was 25–1700 °C, and the temperature accuracy of the heating zone was ±2 °C. The water-cooling device was circulated with cooling water to control the temperature at the end of the furnace tube. The gas-purification system consisted of allochroic silica gel, a molecular sieve for dehydration, and copper and magnesium pieces (heated to 500 °C) for deoxidation. Through the gas-purification system, high-purity argon (Ar > 99.99%) was introduced into the horizontal furnace tube to protect the samples and graphite crucibles from being oxidized until the experiment’s conclusion.

3.4. Experimental Procedure

Pre-prepared slag and iron were placed in the graphite crucible. At the same temperature, an Al₂O₃ boat can hold five graphite crucibles simultaneously. They were positioned in the heating zone of the furnace with the help of a molybdenum bar. Ar gas was introduced into the furnace tube at the flow rate of 500 mL/min. The furnace was then switched on. Before this, an experiment had been carried out to determine the equilibrium time. Figure 3 shows the change of sulfur content with time. (i.e., in experiment T1, the mass ratio of slag to metal was 0.3:1.) It was found that the sulfur content was almost unchanged after 4 h. Therefore, an equilibrium time of 4.5 h was chosen to ensure a complete reaction [13,14]. When equilibrium was reached, the Al₂O₃ boat was pulled out of the furnace tube immediately for quenching. After being cooled, the slag and iron were separated. The slag was dried and then crushed into 200 mesh particles. The iron was washed with a steel brush and ultrasonically cleaned to remove surface residues. The sulfur content in slag and iron was detected by a carbon-sulfur analyzer (EMIA-920V2, HORIBA, Kyoto, Japan), and the composition of the slag was determined by an X-Ray fluorescence spectrometer (XRF-1800, Shimadzu, Kyoto, Japan). All experimental results are listed in Table 1.

4. Results and Discussion

4.1. Effect of Temperature

The temperature dependence of logL_S is shown in Figure 4. The data in Figure 4a are the results of experiments T1–T3, shown in Table 1. For these experiments, the slag compositions were similar. Figure 4b shows the dependence of logL_S on the temperature for all five components of the slag system (CaO-SiO₂-Al₂O₃-Na₂O-TiO₂), whose compositions varied significantly (i.e., experiments 1–21 in Table 1). As it can be seen from Figure 4a, logL_S increased from 1.45 to 1.62 when the temperature increased from 1400 °C to 1500 °C, for similar slag compositions. The desulfurization reaction is endothermic. Thus, the high temperature promotes the migration of sulfur from the metal into the slag phase. Simeonov et al. [13] investigated the sulfur distribution ratio between CaO-SiO₂-Al₂O₃-CaF₂ slag and carbon-saturated iron from 1450 °C to 1600 °C and learned that logL_S increased with the increase in temperature. The value of logL_S in Simeonov’s experiment was much larger than that in this paper, mainly because of the higher basicity in their study. (The basicity in their study was 4.4, whereas, here, it was only about 1.75). Lin et al. [15] measured the sulfur distribution ratio between CaO-SiO₂-Al₂O₃-MgO-TiO₂ slag and carbon-saturated iron from 1450 °C to 1550 °C. The value of logL_S in Lin’s experiment was much larger than that in this paper, because of the low content of weak acid oxides (Al₂O₃ = 10%, TiO₂ = 0–8%).

However, this trend did not hold for the data shown in Figure 4b, which suggests that the temperature has no significant effect on logL_S when the slag compositions vary significantly. Despite the different requirements of temperature, a similar tendency was observed in the research [16] on the phosphorus distribution ratio between the CaO-FeO-SiO₂-Al₂O₃-Na₂O-TiO₂ slag and the carbon-saturated iron, which suggests that the phosphorus distribution ratio showed much greater dependency on the slag composition, such as on basicity rather than on temperature. Together, these results indicate that, for hot metal pretreatment, slag compositions tend to have significant effects on the desulfurization/dephosphorization efficiency.

4.2. Effect of Basicity

Figure 5a shows the effect of basicity (i.e., CaO/SiO₂) on logL_S. The data are the results of experiments R1–R3, for which the other factors of the slag are similar, but the basicity is different. The value of logL_S increased from 1.44 to 1.90 with an increase in basicity from 1.05 to 2.42. This trend is in accordance with previous research [17,18,19,20]. The value of logL_S in Huang’s experiments [17] (shown in Figure 5a) was relatively low, mainly because of the high content of weak acid oxides (Al₂O₃ = 15%, TiO₂ = 25%). The increase of basicity implies the disintegration of the silicate network structure [18] and the increase of the free O²⁻ concentration in the slag. A high concentration of free O²⁻ thermodynamically promotes the desulfurization reaction [11].

The data in Figure 5b are the results of experiments 1–21. The temperature had three gradients, and the composition of slag was changed simultaneously at each temperature. Irrespective of Al₂O₃, Na₂O, and TiO₂ content and temperature, logL_S showed a strong dependence on the basicity of the slag, which linearly increased with the increase in basicity. This suggests that basicity has a much stronger effect on logL_S than other influencing factors, such as temperature, under this experimental condition.

4.3. Effect of Al₂O₃

Figure 6a shows the effect of Al₂O₃ content on logL_S. The data are the results of experiments A1–A3, for which all slag factors were similar, with the exception of Al₂O₃ content. As can be seen, the value of logL_S decreased from 1.68 to 1.47 with an increase in Al₂O₃ content from 13.11% to 22.11%. This was due to the fact that Al₂O₃ acted as an acidic oxide in the basic slag. Al₂O₃ consumed the free O²⁻ to form ${[\mathrm{AlO}]}_4^{5-}$-tetrahedron, which decreased free O²⁻ concentration and weakened the desulfurization capacity of the slag [21]. The same experimental trend was observed by Zhang et al. [19]. They obtained a much lower sulfur distribution ratio, even at a higher temperature (1500 °C), than the present work (1450 °C), mainly because of the lower basicity (1.17) of the slag in their study.

The data in Figure 6b are the results of experiments 1–21. The temperature had three gradients, and the composition of slag was changed simultaneously at each temperature. Irrespective of basicity, Na₂O and TiO₂ content, and temperature, logL_S also showed a strong dependence on Al₂O₃ content. This suggests that Al₂O₃ is also an important factor affecting the sulfur distribution ratio under this experimental condition.

4.4. Effect of Na₂O

Figure 7a shows the effect of Na₂O content on logL_S. The data are the results of experiments N1–N3, for which the other factors of the slag are similar. As can be seen, logL_S linearly increased from 1.52 to 1.59, with an increase in Na₂O content from 0.33% to 1.46%. This was because Na₂O is a strong basic oxide, and Na⁺ tends to have a strong affinity for S²⁻. Niekerk et al. [5] pointed out that the addition of Na₂O significantly increased the sulfide capacity of silicate and lime-based slag. Additionally, the Na₂O equivalent of CaO was determined to be 0.30. Pak and Fruehan [6,22] suggested that Na₂O could lower the melting point of a lime-based slag, reduce the consumption of acid oxides to CaO, and significantly increase the activity of CaO. Subsequently, the desulfurization capacity of the slag was enhanced.

The data in Figure 7b are the results of experiments 1–21. The temperature had three gradients, and the composition of the slag was changed simultaneously at each temperature. When other factors (e.g., temperature) changed, Na₂O content in the slag had no significant effect on logL_S. This could be due to the experimental conditions. Because of the strong reduction potential of hot metal, Na₂O can be reduced to metal Na, which can easily evaporate into the gas phase, especially at a high temperature. Therefore, the higher the temperature, the more Na vaporizes into the gas phase, and less residual Na₂O content remains in the slag. As shown in Figure 7b, the Na₂O content was 0.74–3.50% (square symbols in Figure 7b) at the lower temperature (1400 °C), while at a high temperature (1500 °C), the Na₂O content was only 0.12–0.74% (triangular symbols in Figure 7b). The promoting effect on desulfurization of higher Na₂O content was eventually counteracted by lower temperatures, as in the case where the negative influence of lower Na₂O content tended to be neutralized by higher temperatures. Comprehensively, Na₂O content showed no significant effect on logL_S under this experimental condition.

However, a different tendency was observed in the case of hot metal dephosphorization. The results of hot metal dephosphorization by Li [16] showed that, when changing various factors, such as temperature and slag compositions, the distribution ratio of phosphorus between the slag and iron-saturated iron (L_P) increased significantly with the increase of Na₂O content. This was mainly because hot metal dephosphorization was carried out in an oxidizing atmosphere, and Na₂O in the slag was hard to be reduced to metal Na. The volatilization of Na₂O is much weaker than that of metal Na. Finally, more Na₂O is maintained, and it plays the role of a strong basic oxide in the slag phase. Consequently, hot metal dephosphorization is promoted by high Na₂O content.

4.5. Effect of TiO₂

Figure 8a shows the effect of TiO₂ content on logL_S. The data are the results of experiments Ti1–Ti3, for which all factors of the slag were similar except for TiO₂ content. As can be seen, the value of logL_S decreased slightly from 1.58 to 1.55 with an increase in TiO₂ content from 2.07% to 6.53%. This is in accord with the results of Lin et al. and Tang et al. [15,18], who investigated the sulfur distribution ratio between CaO-SiO₂-Al₂O₃-MgO-TiO₂ slag and carbon-saturated iron at 1500 °C. The electrostatic potential of Ti⁴⁺ (5.88) is lower than that of Si⁴⁺ (9.76). Therefore, the Ti-O bond is weaker than the Si-O bond, which causes TiO₂ to exist as ${[\mathrm{TiO}]}_6^{8-}$-octahedron in a basic slag. Sommerville et al. [23] also suggested that TiO₂ acted as an acidic oxide in a basic slag and existed in the form of ${[\mathrm{TiO}]}_6^{8-}$-octahedron, which decreased the amount of free O²⁻ and weakened the desulfurization capacity of the slag.

The data in Figure 8b are the results of experiments 1–21. The temperature had three gradients, and the composition of the slag was changed simultaneously at each temperature. Although a relatively large fluctuation was observed, the total logL_S decreased with an increase in the TiO₂ content. This indicates that TiO₂ in the slag has also a thermodynamically negative effect on the desulfurization capacity of the slag.

4.6. Regression Analysis of logL_S

To further study the relationship between logL_S, $\Lambda$, and T, based on the data of experiments 1–21 in Table 1, the following regression equation was obtained by using the SPSS (Statistical Product and Service Solutions) software (SPSS10.91, SPSS Inc., Chicago, IL, USA) [24]. A comparison between the experimental values and the calculated values of logL_S is shown in Figure 9, which shows good agreement.

$$\log L_S=15.584\Lambda +\frac{10568.406-17184.041\Lambda }{T}-8.529,(\mathrm{r}=0.963)$$

(1)

where L_S is the sulfur distribution ratio; T is the temperature, °C; $\Lambda$ is the optical basicity of the slag [25]; r denotes the correlation coefficient.

When the temperature was 1400–1500 °C, basicity was 1.0–2.5, Al₂O₃ content was 12–22%, Na₂O content was 0–3%, and TiO₂ content was 2–6%. The equation could be helpful in predicting the sulfur distribution ratio between CaO-SiO₂-Al₂O₃-Na₂O-TiO₂ slag and carbon-saturated iron.

5. Conclusion

The sulfur distribution ratio between CaO-SiO₂-Al₂O₃-Na₂O-TiO₂ slag and carbon-saturated iron was re-evaluated in this study. Based on the theoretical calculation and equilibrium experiments, the conclusions are summarized as follows.

(1)

The thermodynamic calculation shows that high temperature helps the melting of the slag. Al₂O₃ is beneficial as a flux. However, the content should be controlled. Na₂O can promote the melting effect on the slag more effectively than TiO₂.

(2)

The experimental data suggest that the distribution ratio of sulfur between the slag and the carbon-saturated iron strongly increases with the increase of temperature, basicity, and Na₂O content, whereas it decreases with the increase of Al₂O₃ and TiO₂ content. Na₂O in the slag will volatilize because of the high temperature and the reducing conditions.

(3)

Based on the experimental data, for the distribution ratio of sulfur between CaO-SiO₂-Al₂O₃-Na₂O-TiO₂ slag and carbon-saturated iron, the following fitting formula is obtained:

$$\log L_S=15.584\Lambda +\frac{10568.406-17184.041\Lambda }{T}-8.529$$