Influence of Basicity and MgO on Fluidity and Desulfurization Ability of High Aluminum Slag

Introduction

Viscosity is an important parameter that indicates the liquidity of blast furnace (BF) slag. It influences the BF operation in many ways, such as desulfurization, coke consumption, and gas permeability in the furnace. In addition, melting temperature of the slag has a great influence on the BF slag fluidity [1–2]. When the slag temperature is below the melting point, the so-called short slag appears, which means the sedimentation of solid particles and a rapid increase of slag viscosity. Thus, to maintain BF slag in the liquid region is important to improve the fluidity of BF slag.

Many Chinese steel companies use iron ores from Australian and Indian, where the Al₂O₃ content is high: about 2.00% in Australian ore and 1.88% in Indian ore. By contrast, domestic ore contains generally less than 1% Al₂O₃. With the improvement of burden preparation technique and the increase of injection ratio of coal, the slag amount is increasingly reduced. These factors lead to a continuous increase of Al₂O₃ content in slag. In some companies, the Al₂O₃ content in slag reaches or exceeds 16%.

The fluidity and desulfurization capacity of high-aluminum slag have been investigated by numerous metallurgical scientists [3–6]. There are common opinions and controversies. The Kungliga Tekniska högskolan (KTH) model, which is developed by Swedish Royal Institute of Technology, predicts that the increase of Al₂O₃ improves the viscosity of BF slag. The study of Shen Fengman in China shows that viscosity of slag is in a low range and has a good stability [7], when the binary basicity(hereafter referred to as R₂) of slag is within the normal range between 1.05 and 1.20 and Al₂O₃ content in the range of 7–15%.

Sulfur partition ratio is a common parameter to characterize the desulfurization capacity in BF iron-making process [8]. The study of Hu Xiayu in China shows that raising the content of MgO in high aluminum slag can increase the desulfurization capacity [9]. However, the thermodynamic model proposed by Shi Chengbin shows that the desulfurization capacity of free CaO is 97% at 1,500°C, while it is only 3% for free MgO [10].

In this paper, the viscosity–temperature curve was measured to investigate the influence of w(Al₂O₃), w(MgO), R₂ and slag temperature on the fluidity of high aluminum slag. The relationship between sulfur distribution coefficient and slag composition as well as furnace temperature was analyzed through the stepwise regression method by means of SPSS program, in order to obtain key factors on the desulfurization capacity of high aluminum slag.

Investigation methods

Experimental and detection

Slag sample preparation

Practical slag from a 4,000 m³ BF in M steel was used as an experimental basis which has a composition shown in Table 1. With this composition, the influence of the contents of S, TiO₂, FeO, and K₂O on the slag fluidity can also be studied.

Table 1:

The composition of 4,000 m³ blast furnace slag in M steel.

Composition	SiO2	Al2O3	CaO	MgO	FeO	TiO 2	MnO	K2O	Na2O	S
%	32.44	15.83	38.21	8.94	0.385	1.18	0.16	0.44	0.50	1.04

The BF slag was used as the main material to make up experimental slag, while the contents of CaO/SiO₂, MgO, and Al₂O₃ were manipulated by adding chemical-grade reagents. The total weight percentage of SiO₂, CaO, Al₂O₃, and MgO was controlled at 100%. Due to the high absorbent of CaO in air, Ca(OH)₂ was used in the preparation of slag samples. The slag sample was heated to 1,530°C in a high-temperature furnace and hold for 30 min. Afterward it was put on an iron plate and rapidly cooled down and crushed.

Experimental equipment

Slag viscosity was measured with an RTW-08-type physical characteristics identifier, which is illustrated in Figure 1. The slag was heated to 1,500°C and hold for 30 min, then cooled down at a rate of 3°C/min.

Figure 1:

Schematic diagram of experimental apparatus: 1, torque sensor; 2, rotating rods; 3, high temperature furnace; 4, graphite crucible; 5, probe; 6, thermocouple; 7, control cabinet; 8, computer system; 9, N₂.

The temperature of molten slag cannot be measured simultaneously. The calibrating thermocouple was placed in the graphite crucible which was filled with high aluminum powders. Then the temperature was raised, stabilized, and decreased. The temperature difference was calculated from the two thermocouples that were located at the bottom and inside of the crucible. This value will be used for drawing the temperature–viscosity curve.

Experiments and field data investigation

Three principles of selecting components of the slag samples were followed: (1) The melting temperature of tapped slag should be below 1,400°C. Considering the influence of other components in the slag, this temperature limit can be raised to 1,420–1,430°C for a quaternary slag system. (2) When the slag temperature is below the normal operating range in BF, the viscosity of slag is preferably below 0.5 Pa s. In the worst case, it cannot exceed 2.0 Pa s. (3) The slag should have a good chemical stability (isothermal section of the diagram) and a good thermal stability (viscosity–temperature curve).

Single factor influence

The influence of w(Al₂O₃) on slag fluidity

Figure 2 illustrated the relationship between slag viscosity and temperature when R₂ is 1.18, w(MgO) is 9.0% and w(Al₂O₃) is between 15.0% and 20.0%.

Figure 2:

Effect of w(Al₂O₃) on viscosity of slag.

Figure 3 shows the relationship between slag viscosity and w(Al₂O₃) at temperatures of 1,440°C, 1,470°C, and 1,500°C. The slag viscosity increases with the increase of w(Al₂O₃). Below 1,470°C, the viscosity is higher than 0.5 Pa s. Above 1,500°C, the viscosity is continuously decreased to 0.4 Pa s.

Figure 3:

Effect of w(Al₂O₃) on viscosity of slag.

The relationship between melting temperature and Al₂O₃ is shown in Figure 4. As seen in Figure 4, the melting temperature of slag increases with the increase of w(Al₂O₃). And when w(Al₂O₃) is greater than 18.0%, the melting temperature of slag increases sharply.

Figure 4:

Effect of w(Al₂O₃) on T_m of slag.

The influence of w(MgO) on the fluidity of slag

Figure 5 shows the influence of MgO content and temperature on the slag viscosity with R₂ of 1.18 and the content Al₂O₃ of 16%.

Figure 5:

Effect of w(MgO) on viscosity of slag.

Figure 6 shows the relationship between slag viscosity and w(MgO) at temperatures of 1,440°C, 1,470°C, and 1,500°C. The relationship between the melting temperature and w(MgO) is shown in Figure 7. From these two figures, it can be obtained that when the w(MgO) is in the range of 8–12%, the slag viscosity and the melting temperature increase with the content of w(MgO). However, when the w(MgO) is greater than 12%, the slag viscosity decreases. Therefore, w(MgO) should be controlled below 12%. As shown in Figure 8, the reason can be explained that increasing the MgO content of slag could improve the transform from silica composite anions (such as (Si₆O₁₈)^12–, (Si₄O₁₂)^8–, (Si₃O₉)^6–) to melilite (such as (Si₂O₇)^4–), which reduces the slag viscosity. However, when MgO content is higher than 12%, high melting point materials such as spinel (2,135°C) and periclase (2,800°C) are formed, increasing the slag viscosity.

Figure 6:

Effect of w(MgO) on viscosity of slag.

Figure 7:

Effect of w(MgO) on T_m of slag.

Figure 8:

CaO–SiO₂–MgO–Al₂O₃ quadruple phase diagram (Al₂O₃% = 15%).

The influence of R₂ on slag fluidity

When R₂ = 1.12–1.25, w(MgO) = 8.5%, and w(Al₂O₃) = 16.5%, the relationship between slag viscosity and temperature is shown in Figure 9.

Figure 9:

Effect of temperature on viscosity of slag.

The relationship between slag viscosity and R₂ is shown in Figure 10. The slag has a minimum viscosity at an R₂ of 1.15 in the range of 1.12–1.25. When R₂ is greater than 1.20 and temperature above 1,470°C, the slag viscosity can also be controlled below 0.5 Pa s. The relationship between the melting temperature and R₂ is shown in Figure 11. The melting temperature of slag increases with the increase of R₂.

Figure 10:

Effect of R₂ on viscosity of slag.

Figure 11:

Effect of R₂ on T_m of slag.

The fluidity of high aluminum slag

When w(Al₂O₃) is 20%, the relationship between slag viscosity and temperature is shown in Figure 12. The melting temperature and slag viscosity are shown in Table 2.

Figure 12:

Effect of temperature on the viscosity of slag (w(Al₂O₃) = 20%).

Table 2:

Effect of MgO on the viscosity of slag.

Sample no.	R2	w(MgO)/%	w(Al2O3)/%	Viscosity (η/Pa s)			Tm/°C
				1,440/°C	1,470/°C	1,500/°C
16	1.30	4.0	20.0	0.74	0.50	0.40	1,385
17	1.25	6.0	20.0	0.62	0.48	0.39	1,381
18	1.25	8.0	20.0	0.70	0.51	0.42	1,400
19	1.20	8.0	20.0	0.63	0.50	0.45	1,375
20	1.20	12.0	20.0	2.44	0.80	0.64	1,435
21	1.30	12.0	20.0	1.84	1.21	0.98	1,450
22	1.00	8.0	20.0	1.47	1.14	0.95

The slags of sample nos 16, 17, 18, and 19 have a low viscosity. At 1,500°C, the viscosity is below 0.5 Pa s. At 1,440°C, the viscosity is below 0.8 Pa s. Melting temperature of no. 17 slag is 1,381°C. Melting temperature of no. 19 slag is 1,375°C.

The viscosity of nos 20 and 21 is greater than 0.60 Pa s, when the temperature is 1,500°C. The melting temperature of no. 20 is 1,435°C. And the melting temperature of no. 21 is 1,450°C; therefore, their chemical stability is poor. No. 22 slag (R₂ equals 1.0) was acidic, whose viscosity–temperature curve is smooth and has no obvious turning point. But its viscosity is high, close to 1.0 Pa s at 1,500°C. In order to ensure a good flowability of this slag, a high-temperature operation should be adopted.

As a summary, w(MgO) in slag should be less than 12% for high aluminum slag, independent of R₂. The w(MgO) should be controlled at less than 8% when Al₂O₃ is 20%.

Discussion of an optimal high aluminum slag system

Three kinds of slag systems are used worldwide. One is a high basicity (R₂ = 1.20–1.25) with a low MgO (4–7.0%) slag system; the second is a medium basicity (R₂ = 1.12–1.18) with a medium to high MgO content (8–10%); the last one is a low basicity (R₂ = 1.00–1.10) with high MgO (11–13%) slag system.

The high basicity (R₂ = 1.20–1.25) and low MgO (4–7%) slag system is short and has a poor stability. Chinese Baosteel and Japanese giant BF (both the BF effective volumes are larger than 4,000 m³) adopted this slag system. This slag system requires a low slag ratio and stable raw material and fuel. The temperature of hot metal can be raised to above 1,490°C with a smooth furnace operation. As we concluded from our experiment, the slag temperature is the most important factor on the slag fluidity. When the slag temperature is high than 1,470°C, the slag viscosity can be controlled below 0.5 Pa s, as demonstrated by no. 16 and 17 slag. When the slag ratio is below 300 kg/tHM, the high basicity and low MgO slag system should be adopted besides a high slag temperature. R₂ should be controlled between 1.20 and 1.30 and w(MgO) below 8%.

In China, the compositions of raw materials and fuel fluctuate in a wide range. Meanwhile, the amount of slag is also high (the slag ratio is 320–400 kg/tHM), and the w(Al₂O₃) is in the range of 14–17%. Therefore, the medium basicity (1.10–1.17) with high to medium MgO (12%) slag system, such as nos 11 and 12 slag, is widely used. This slag system can ensure a hot metal with a low silicon and a low sulfur, when the hot metal temperature is higher than 1,460°C. This slag system should also meet the principle of R₂ above 1.15, where the viscosity is minimized, and w(MgO) does not exceed 12%. At the same time, when the content of MgO is low, the binary basicity should be increased properly, vice versa.

In the former Soviet Union, the raw materials of BF are all from domestic, the amount of BF slag is generally large and w(Al₂O₃) is between 7% and 10%. In order to improve the slag fluidity, the slag system of higher w(MgO) (11–13%) and the lower basicity (R₂ ≤ 1.0) was hence adopted. In North America, some BFs, which use the high proportion of pellets, also adopt this slag system. This slag system has a wider liquid-phase region, and hence it is stable and has low viscosity. But its desulfurization performance is poorer than the others.

Conclusions

1. The increase of w(Al₂O₃) has an obvious effect on the viscosity of high aluminum slag and the melting temperature. The increase of w(MgO) plays a role in reducing the viscosity of the high aluminum slag and the melting temperature. When the R₂ is 1.15, the slag viscosity is minimum. The rising of R₂ increases the melting temperature of the slag. When the slag temperature is greater than 1,500°C, the viscosity of most slag systems can be controlled below 0.5 Pa s.

2. When the w(Al₂O₃) is 20% in the BF slag, the w(MgO) should not be higher than 12%, independent of R₂. The basicity should be adjusted to between 1.20 and 1.30, and the w(MgO) should be reduced to below 8%. The slag system with high basicity and high MgO is not suitable for a high aluminum slag. When w(MgO) is low, the basicity should be increased, vice versa.

3. The temperature of hot metal, w[Si], and w(CaO) are the main factors determining the slag desulfurization capacity. MgO has a small influence. Meanwhile, maintaining a higher and stable temperature of hot metal is an important guarantee for a smooth operation for BF and also for a high slag desulfurization capacity.