Proper MgO/Al 2 O 3 Ratio in Blast-Furnace Slag: Analysis of Proper MgO/Al 2 O 3 Ratio Based on Observed Data

: With the gradual depletion of high-quality iron-ore resources, ironmaking blast furnaces have gradually shifted to using high-content Al 2 O 3 iron ore. Under this condition, the addition of MgO in the slag is adopted to alleviate operation problems caused by the high content of Al 2 O 3 . However, the proper value of the MgO/Al 2 O 3 ratio for blast-furnace slag systems (Al 2 O 3 = 8–25%) is not systematically studied. In this paper, we discuss the proper MgO/Al 2 O 3 ratio on the basis of blast-furnace slag systems under different Al 2 O 3 contents. On the basis of thermodynamics and phase-diagram analysis, it could be concluded that: (1) the MgO/Al 2 O 3 ratio is not limited when Al 2 O 3 in slag is less than 14%, (2) the MgO/Al 2 O 3 ratio is required to be in the range of 0.40–0.50 when Al 2 O 3 in the slag is 15–17%, and (3) the MgO/Al 2 O 3 ratio should be 0.45–0.55 when Al 2 O 3 in the slag is larger than 18%. The proper MgO/Al 2 O 3 ratio value has been established in various industries, and it has achieved significant economic and social benefits. Author Contributions: conceptualization, F.S.; methodology, F.S. and H.Z.; software, X.H.; validation, H.Z.; formal analysis, F.S. and H.Z.; investigation, H.Z. and X.J; resources, Q.G.; data curation, H.H. and F.L; writing— original draft preparation, X.H.; writing—review and editing, H.Z.; visualization, Q.G.; supervision, F.S.; project administration, F.S. and H.Z.; funding acquisition, Q.G.


Introduction
The massive use of Al2O3-enriched iron ore inevitably leads to the increase in Al2O3 content in blast-furnace (BF) slag; thus, there exist the two following problems in BF production: First, the increase in slag viscosity. Figure 1 shows the isoviscosity diagram of the CaO-SiO2-Al2O3 ternary slag system at 1500 °C, where the straight line is the isobasicity line, and the curve is the isoviscosity line [1]. Industry practices request that slag viscosity be below 0.4 Pas [2]; thus, slag basicity (CaO/SiO2, wt %) should be about 1.1, and Al2O3 content in slag should be 9-13%. Meanwhile, slag viscosity exceeds 0.4 Pas as Al2O3 content in the slag increases at constant slag basicity, leading to difficult BF operation. Therefore, on the basis of phase-diagram analysis, we see that, while maintaining BF temperature and slag basicity, an increase in slag viscosity caused by the high content of Al2O3 in the slag is one of the reasons that make BF operations difficult [3][4][5][6].
The second problem is the relative decrease in the desulfurization ability of slag. Keeping slag basicity constant, CaO content in the slag decreases as the content of Al2O3 in the slag increases, worsening the slag's desulfurization ability [7,8]. In addition, the poor kinetic conditions of desulfurization caused by increased slag viscosity are another reason for the weakened desulfurization. Therefore, in view of thermodynamics and kinetics, high Al2O3 content has a negative impact on slag desulfurization. In fact, overall, the operational problem of high-Al2O3 BF slag is mainly related to the increase in slag viscosity. BF operation problems of high-Al2O3 slag stem from the increase in slag viscosity; thus, it is necessary to reduce slag viscosity while maintaining the proper melting point of slag. Although increasing slag basicity helps to improve slag viscosity, it significantly increases the melting point of the slag [9]. However, lowering slag basicity helps to reduce its melting point, further deteriorating its desulfurization ability of slag [10]. The addition of proper MgO can increase the liquid-phase region, reduce the melting point of slag, and improve its fluidity. T. Paananen [11] found that MgO significantly accelerates the reduction of magnetite to iron. M. Matsumura et al. [12], J. Chang [13], and X. Qin et al. [14] found that proper MgO can decrease the melting temperature and increase the desulfurization ability of slag. Therefore, the current popular method for adjusting the metallurgical properties of high-Al2O3 BF slag is to add MgO into the slag.
Regarding the influence of MgO on slag viscosity, Figure 2 shows the isoviscosity diagram of the CaO-SiO2-Al2O3-MgO quaternary slag system with a SiO2 content of 35% [1]. With the addition of MgO, slag viscosity is reduced. For BF slag with a basicity of about 1.15, adding a certain amount of MgO gives a proper slag viscosity within the range of 0.3 to 0.4 Pas. In addition, MgO is an alkaline substance. Under the condition of constant binary basicity, CaO content is reduced by about 0.4% for each added 1% of MgO. Since the desulfurization ability of MgO is about 0.7 times that of CaO [15], the addition of MgO can maintain the desulfurization ability of slag, keeping it unchanged from a thermodynamic point of view. If the MgO were added with an appropriate simultaneous increase in basicity, the desulfurization ability of the slag would be improved. In addition, as mentioned above, the addition of MgO can improve slag fluidity and the kinetic conditions of slag desulfurization. Therefore, the addition of MgO is an effective measure for high-Al2O3 BF slag operations.
According to the above analysis, in order to ensure a proper viscosity for the high Al2O3 BF slag, it is necessary to add MgO. This topic, apart from the studies mentioned above, has also been studied by many other researchers [16][17][18]. H. He et al. [16], and Y. Liu [17] obtained that slag viscosity decreases with the increase in MgO content in a definite range. T. Talapaneni [3] and Tingle. L [18] found that slag viscosity increases with an increase in the Al2O3 content, and decreases with an increase in MgO content, by using FTIR and Raman spectroscopy. However, most studies are only qualitative analyses, and rarely mention the addition method of MgO under different Al2O3 contents in production. In fact, MgO is also proper for BF slag with a lower content of Al2O3. However, what is the proper amount of MgO addition, or what is the proper MgO/Al2O3 ratio? So far, there has been no clear answer. In this paper, phase-diagram analyses with laboratory experiments and production practices were conducted to determine the proper MgO/Al2O3 ratio and formulate operational guidelines.

Analysis of Proper MgO/Al2O3 Ratio
The proper MgO/Al2O3 ratio should be considered in two aspects, that is, fluidity (viscosity) and the melting point of BF slag.

BF Slag Viscosity
An experiment was carried out to study the effect of MgO/Al2O3 on viscosity. Slag was prepared from a mixture of reagent-grade CaO, SiO2, Al2O3, and MgO. Slag compositions are listed in Table 1. A vertical-resistance furnace was used. Slag of 140 g was placed in a graphite crucible into the hot zone of the furnace. The sample was heated to 1500 °C and maintained at 1500 °C for 30 min to sufficiently ensure complete melting and homogenization. A rotating Mo spindle connected to a viscometer was employed in the high-temperature viscosity measurement. Figure 3 shows the variation of slag viscosity with basicity (R) of 1.1 and 1.2, and Al2O3 content of 12%, 15%, and 18% at 1500 °C under laboratory conditions. The present results showed the same trend as that of T.Iida et al [19]. When the Al2O3 content in the slag was less than 15%, slag viscosity increased slightly with the decrease in the MgO/Al2O3 ratio. However, when Al2O3 content in the slag reached 18%, for the slag with lower basicity (R = 1.1), slag viscosity increased rapidly with the decrease in the MgO/Al2O3 ratio, due to the melting point rapidly rising (see Section 3.3 for specific discussion). When the MgO/Al2O3 ratio was 0.25, slag viscosity reached 0.5 Pas. Therefore, according to Figure 3, for high-Al2O3 BF slag, the lower limit of the MgO/Al2O3 ratio should not be less than 0.25.   Figure 4 is a phase diagram of a CaO-SiO2-Al2O3-MgO quaternary slag system when Al2O3 is 20% [1]. If the content of MgO in BF slag exceeds 16% (in this case, MgO/Al2O3 = 0.80), the melting point of BF slag is as high as 1500 °C (dashed line in Figure 4), reaching or even exceeding the slag temperature during BF production (1500-1550 °C), which leads to the incomplete melting of slag in the BF hearth. To ensure the safety of BF production, the melting point of slag is usually controlled to below 1400 °C [20]. Therefore, the upper limit of the MgO/Al2O3 ratio of BF slag should not be larger than 0.80 according to the melting point of BF slag. In addition, the content of Al2O3 in BF slag is not higher than 20% in most cases. Because the liquid phase decreases with the increase in Al2O3 at a fixed temperature, it is feasible to take 20% Al2O3 as the upper limit and separately discuss it.

Melting Point of BF Slag
As mentioned above, considering various constraints, such as the viscosity and melting point of slag, the MgO/Al2O3 ratio of BF slag should be controlled within the range of 0.25-0.80.

Discussion (Three-Segment Control of Proper MgO/Al2O3 Ratio)
According to the above analysis, the MgO/Al2O3 ratio of BF slag and of high-Al2O3 slag should be controlled between 0.25 and 0.80. However, the above range is too wide and thus needs to be refined. In addition, in order to comprehensively investigate the mechanism of the MgO/Al2O3 ratio, the proper MgO/Al2O3 ratio for low-Al2O3 slag should also be demonstrated. The following is the respective discussion.

Less Than 14% Al2O3 in Slag
According to the phase diagram of the CaO-SiO2-MgO-Al2O3 quaternary system when SiO2 is 35% ( Figure 5) [1], if the Al2O3 content in the slag is 12-14% and even less, the melting point of BF slag is below 1400 °C with MgO less than 20% (shadow in Figure 5). Then, there is no limitation to the amount of added MgO, and it can be added depending on resource and production-cost conditions. Table 2 shows the composition of the BF slag of a European steel company. Although the MgO content in the BF slag is as high as 17.5%, its melting point is around 1390 °C (elliptical area in the figure), that is, this BF production can still normally proceed because the Al2O3 content in the slag is only 13%. Therefore, the MgO/Al2O3 ratio of BF slag is not limited when Al2O3 in the slag is less than 14%.   For slag containing 15-17% Al2O3, attention should be paid to slag sensitivity on temperature during operation. According to the study of T. Koshida et al. [21], for slag containing 15-17% Al2O3, viscosity is acceptable when slag temperature is high, but viscosity rises rapidly when the slag temperature is lower than around 1425 °C. That is to say, slag containing 15-17% Al2O3 is very sensitive to temperatures of around 1425 °C; thus, attention should be paid to changes in slag temperature. It is necessary to ensure that slag temperature is sufficient to prevent poor BF performance caused by a sudden increase in slag viscosity.

Higher Than 18% Al2O3 in Slag
Similarly, the MgO/Al2O3 ratio should be 0.45-0.55 to keep viscosity in the range of 0.3-0.4 Pas for slag containing 18-20% Al2O3. Therefore, a proper MgO/Al2O3 ratio is 0.45-0.55 for slag containing more than 18%Al2O3. In addition, from the perspective of the melting point of slag, the relationship between the MgO/Al2O3 ratio and Al2O3 content of slag can also be obtained as shown below. Figure  7 shows the effect of the MgO/Al2O3 ratio on the melting point for slag containing different Al2O3 contents [1]. For slag containing 10% Al2O3, an MgO/Al2O3 ratio of less than 2.0 can maintain the melting point below 1425 °C. That is, there is no limitation on the MgO/Al2O3 ratio under this condition, and MgO can be added according to raw-material conditions and production costs. For slag containing 20% Al2O3, the MgO/Al2O3 ratio should be controlled to not exceed 0.6 to keep the melting point of slag lower than 1425 °C. The proper MgO/Al2O3 ratio obtained on the basis of the melting point of BF slag was highly consistent with that obtained on the basis of viscosity analysis. That is, both viscosity and melting-point analysis gave the same scientific theoretical basis for controlling a proper MgO/Al2O3 ratio.
In summary, the three-segment control of proper MgO/Al2O3 ratio for BF slag obtained by thermodynamics and phase-diagram analysis is listed in Table 3.  The emphasis here is that the MgO/Al2O3 ratio is "proper". Above the proper value, it is not impossible to carry out BF operations, but it would lead to a substantial increase in production costs.
A higher MgO/Al2O3 ratio not only increases the consumption of slag, energy, and magnesium resources, but also affects sintering production and quality. Conversely, if the MgO/Al2O3 ratio were too low, the furnace condition would be unstable. In addition, the above-mentioned proper MgO/Al2O3 ratio was optimized on the basis of the CaO-SiO2-Al2O3-MgO slag system. If other components, such as TiO2, MnO, and FeO, are added to the system, this proper MgO/Al2O3 ratio may still work, but slightly differently. Table 4 shows the production data for a steel company to implement the proper MgO/Al2O3 ratio. In 2013, the company's BF slag contained 15.5% Al2O3 and an MgO/Al2O3 ratio of 0.51. To reduce production costs, and energy and MgO consumption, on the basis of the three-segment control theory of the proper MgO/Al2O3 ratio, the company gradually reduced MgO content in the sinter from 1.8% in 2013 to 1.5% in 2017. As a result, the MgO content in BF slag was decreased from 7.96% in 2013 to 7.02% in 2017. The fuel ratio fell to an average of 492.5 kg/tHM in 2017, a decrease of 1.5 kg/tHM from the fuel ratio in the first half of 2014. At the same time, BF production was stable and smooth, and the quality of pig iron was excellent, with an (S) decrease from 0.4 to less than 0.025 in the hot metal. Reducing the MgO/Al2O3 ratio not only reduces the fuel ratio, but also saves resources, reduces costs, and decreases CO2 emissions. The carbon content of fuel is calculated to be 85%, and CO2 emissions can be reduced by 4.7 kg/tHM per ton of iron. The annual output of iron in 2017 was 7.2 million tons, and CO2 emissions could be reduced by 34,000 tons per year. A proper MgO/Al2O3 ratio has significant economic and social benefits.

Conclusions
On the basis of thermodynamics and phase-diagram theory, analysis was conducted on production practices regarding the proper MgO/Al2O3 ratio of BF slag, and the following conclusions were obtained: (1) A quantitative three-segment control theory of the proper MgO/Al2O3 ratio was obtained regarding BF slag containing Al2O3 of 10-25%.
(A) The MgO/ Al2O3 ratio is not limited when the Al2O3 in the slag is less than 14%, and MgO can be added according to resource and production-cost conditions. (B) The MgO/Al2O3 ratio is required to be between 0.40-0.50 when the Al2O3 in the slag is 15-17%, but the temperature should be closely monitored due to slag sensitivity to temperature.
(C) The proper MgO/Al2O3 ratio should be 0.45-0.55 when the Al2O3 in the slag is larger than 18%.
(2) Industrial practices showed that a proper MgO/Al2O3 ratio can help to achieve great economic and social benefits when used.

Conflicts of Interest:
The authors declare no conflict of interest.