Polymeric Structure Evolution Behavior Analysis of Aluminosilicate-Based Smelting Slag

: The physical and chemical properties of the CaO–SiO 2 –Al 2 O 3 –MgO slag system depend on structure evolution caused by the synergistic mechanism among the components at high temperature, especially the structural complexity of the anion group. In this study, Si–O, Al–O, and Al– Si–O anion groups were found in the high-temperature slag systems at 1773 K. The [Si 2 O 5 ] 2 − (Q 3 ) structural unit of the silicate polymer changed to [Si 2 O 6 ] 4 − (Q 2 ), [Si 2 O 7 ] 6 − (Q 1 ), and [SiO 4 ] 4 − (Q 0 ) as the binary basicity ( R 2 ) and ω (MgO)/ ω (Al 2 O 3 ) increased, while there was a trend toward a simplified structure. That is, the stability of each structural unit weakened, and the relative content of non-bridging oxygen increased. As a result, the degree of polymerization of slag decreased, and the experimental results of the relative content of non-bridge oxygen can be fitted well with the calculations from the reference documents. At the same time, the number of Si–O–Al structural units gradually decreased. The 27 Al-MAS-NMR spectroscopy results showed that the structure of aluminate changed from [AlO 4 ] 5 − tetrahedral structure to [AlO 5 ] 7 − pentahedral and [AlO 6 ] 9 − octahedral structure, and the degree of polymerization of the slag decreased. Comprehensive analysis showed that basicity controlled within 1.2 with the ω (MgO)/ ω (Al 2 O 3 ) ratio; less than 0.55 allows for suitable fluidity and energy-saving ability, which is beneficial for the stable and smooth production of the blast furnace.


Introduction
Metallurgical smelting slags are multi-component substances whose microstructural information plays an important role in mastering the physical and chemical properties, optimized slag forming route, and slag-forming practices of blast furnace (BF) [1][2][3][4][5]. Moreover, the macro properties are affected by the anionic groups contained therein. Zhang et al. [6] studied the influence of the Al2O3/SiO2 ratio on the viscosity of the CaO-Al2O3-SiO2 melt and found that a maximum viscosity can result from the opposite variation tendencies of the mean bond strength and the degree of polymerization of the melt with the Al2O3/SiO2 ratio. The mean bond strength decreased when SiO2 was substituted with Al2O3 due to the lower bond strength of the Al-O bond compared to the Si-O bond, decreasing the viscosity, while the degree of polymerization (DOP) was enhanced by the charge compensation effect of the Al 3+ ion, which increased the viscosity. Through infrared and Raman spectroscopy, Liao et al. [7] found that the DOP of the [SiO4] 4− tetrahedral structure decreased with the increase of the ω(Al2O3)/ω(SiO2) ratio but increased sharply when the ratio was greater than 0.56. Park et al. [8] used NBO/T or Q to respectively represent the number of non-bridging oxygen or bridging oxygen carried by Si atoms and analyzed the structure information of the slag through infrared spectroscopy. They found that Al2O3 acted as a network former that could increase the DOP. Kim et al. [9] found that the infrared spectral intensity decreased when the [SiO4] 4− tetrahedron had the lower NBO/Si, while the infrared spectral band increased with the Al2O3 content. McMillan et al. [10] found that increasing CaO content, which may reflect a decrease polymerization of the aluminate units, and the coordination Al is detected by 27 Al-MAS-NMR, there are [AlO4] 5− and [AlO6] 9− structure in the glasses. Sunahara et al. [11] found that in the process of blast furnace (BF) smelting, with the increase of high quality Al2O3, problems such as dripping zone, poor fluidity of slag, excessive slag accumulation, and increased pressure can happen, but it can be solved by reducing R2 and increasing MgO content. Moreover, many studies combined different raw material conditions, then analyzed the changes in slag structure and metallurgical properties, and many optimization methods have bene put forward, which are the basis for the selection of BF raw materials, as well as ensuring the reasonable flow slag can be provided [12][13][14]. In addition, other scholars have studied these polymers at the corresponding high temperatures in glass and silicate, and used the spectral data for detection and analysis [15][16][17][18]. These results can be used as a reference for the study of non-iron elements in the field of metallurgy. However, with the demand for more reasonable slag-making systems in China, research on the microstructural evolution of anionic agglomeration in BF slag with different R2 and ω(MgO)/ω(Al2O3) ratios is currently not detailed enough.
In the paper, the samples of CaO-SiO2-Al2O3-MgO slag are quenched at 1773 K, then the X-ray diffraction (XRD), Fourier transform infrared (FTIR), and the 27 Al-SS-MAS-NMR spectroscopy are used for detection. Then, the existence form of the polymer and microstructure anionic group evolution behavior are explored with different R2 (R2 = ω(CaO)/ω(SiO2)), ω(MgO)/ω(Al2O3), and NBO/T is the ratio of the non-bridging oxygen and the Si atom number, which is used to express the degree of polymerization (DOP), and the Q is the structural unit of silicates. Moreover, contrastive analysis is carried out between experiment and reference calculation results. In this way, the mutual synergy mechanism of the component is analyzed, from which basic reference data for the study of microstructure of non-iron elements in the smelting process, especially BF slag, are provided.

Experiment Schemes
The slag samples used in the experimental research were all prepared with pure chemical analysis reagents, which were obtained from Tianjin Zhiyuan Chemical Reagent Co., Ltd. China. The purity was 99% and the chemical composition including R2 and ω(MgO)/ω(Al2O3) of the CaO-SiO2-Al2O3-MgO slag system is shown in Table 1. Before the experiment, the raw materials were fully dried for 10 h at 1773 K in an electrothermal constant-temperature dry oven, Tianjin Taisite intelligent instrument Co., Ltd. China, type 202-0AB. The water was removed, each group of slag samples was 50 g, and the samples were mixed evenly in the agate mortar, then placed in a φ45 × 30 mm graphite pot and heated to 1773 K to melt fully in the muff furnace (Luoyang Tairui intelligent electric furnace Co., Ltd. China, type TRL-18LB). The high-temperature samples were removed and poured into water for rapid cooling to ensure that the structure of the water-quenched slag was consistent with high-temperature melt slag. After drying for 10 h at 1773 K, the slag was ground in an agate mortar to a particle size of 74 μm. Finally, 300 mg of the sample was weighed for XRD, FTIR, and 27 Al-SS-MAS-NMR analyses, and the structure of the molten slag at high temperatures was determined.

Phase Analysis
According to XRD detection, it was found that when the temperature was increased to 1773 K, the curves of the slag with different R2 and ω(MgO)/ω(Al2O3) ratios showed large peaks ( Figure 1). It is clear that the prepared CaO-SiO2-Al2O3-MgO slag systems had completely melted at this temperature. Thus, the structure of the samples tested is consistent with that of the melt slag at high temperatures.

Anion Group Structure Evolution Analysis
Spectral analysis is an effective method used to detect the DOP of slag. The influence of R2 and the ω(MgO)/ω(Al2O3) ratio on the structural units in the slag was first analyzed by FTIR spectroscopy.
segments of different silicate and aluminate structural units by the references and analysis [12,19].  Figure 2 shows the FTIR detection results of water-quenching BF slag with different R2 when the temperature is 1773 K, (ω(MgO)/ω(Al2O3) = 0.50). It can be seen that with the increase of R2, the [AlO4] 5-antisymmetrical bending vibration intensity at 720-630 cm −1 decreased gradually; this indicates that the number of [AlO4] 5-tetrahedral structural units in the form of tetra-coordination aluminum is reduced. In addition, the symmetrical bending vibration strength of Si-O-Al at about 500 cm −1 decreased with increasing R2. At the same time, the number of Si-O-Al structural units decreased. The symmetrical stretching vibration of the silicate structural unit Qn (n = 0, 1, 2, 3) at 1200-800 cm −1 did not change significantly as R2 increased. Thus, it is necessary to fit the peak divisions at 1200-800 cm −1 into four peaks corresponding to the Q0, Q1, Q2, and Q3 structural units. The content of each unit can then be characterized by calculating the relative contents of each peak area.  Figure 3 shows the peak division results of the FTIR spectra of BF slag in the wavenumber range of 1200-800 cm −1 under different R2 conditions. It can be seen that with the increase of R2, the corresponding peak center positions of the Q0, Q1, Q2, and Q3 structures gradually moved to a low wavenumber region, respectively decreasing from 844.3 to 828.1 cm −1 , 904.7 to 892.3 cm −1 , 982.5 to 976.9 cm −1 , and 1056.6 to 1038.0 cm −1 . This shows that the stability of the tetrahedral elements in the slag structure decreased with the increase of R2 and tended to simplify in structure. 1200 1100 1000 900 800 700 600 500 400 300 200  The relative area of each peak corresponding to each structural unit Qn (n = 0, 1, 2, 3) was calculated according to the peak division results. Then, the curve of the relative content of each structural unit was obtained with increasing R2 (Figure 4). It can be seen that as R2 increased from 0.9 to 1.3, the relative content of Q0 and Q1 gradually increased but the range of change was not large, from 7.0% to 10.0% and from 19.2% to 21.4%, respectively, while the relative content of Q3 decreased significantly from 61.8% to 13.4% and the relative content of Q2 increased significantly from 12.0% to 55.1%. As R2 increased, Ca 2+ and free oxygen ions (O 2− ) were elevated, which interacted with bridge oxygen. As a result, the complex Q3 structural unit changed into more simple Q0, Q1, and Q2 structural units, thereby eventually reducing the viscosity of the slag. The DOP of the slag can be more intuitively represented by the relative content of non-bridging oxygen (NBO/T) in the slag structure. Based on previous research [12,20,21], the theoretical calculation of NBO/T can be written as: where X is the mole fraction of oxide, mol%.  Compared with the theoretical value, the relative content of Qn obtained based on spectral detection can reflect the actual slag polymerization more accurately. The relationship between NBO/T and the relative content of Qn is shown in Equation (2): According to Equations (1) and (2), the theoretical and detected values of the relative content of non-bridging oxygen were calculated for different R2. As shown in Figure 5, a good correlation between the theoretical and detected values was observed. Therefore, Equation (2) can be used for the calculation of NBO/T. At the same time, the relative content of non-bridging oxygen gradually increased with R2, indicating that Ca 2+ and free oxygen ions (O 2− ) are network modifiers in the slag structure, and the increase of R2 led to the decrease of the DOP. The symmetrical stretching vibration of the silicate structural unit Qn (n = 0, 1, 2, 3) at 1200-800 cm −1 did not change significantly. Thus, it is necessary to fit the peak divisions at 1200-800 cm −1 into four peaks corresponding to the Q0, Q1, Q2, and Q3 structural units, respectively. The relative content of each unit can then be characterized by calculating the relative contents of each peak area.       The relative area of each peak corresponding to each structural unit Qn (n = 0, 1, 2, 3) was calculated according to the peak division results. The change curve of the relative content of each structural unit with the increase of the ω(MgO)/ω(Al2O3) ratio was then obtained (Figure 8). It can be seen that the relative content of Q2, Q1, and Q0 gradually increased as the ω(MgO)/ω(Al2O3) ratio increased from 0.35 to 0.55. The number of Mg 2+ ions in the slag structure had increased and interacted with the bridge oxygen, transforming the more complex Q3 structural unit into simpler Q0, Q1, and Q2 structural units. Thus, the overall DOP decreased and the macroscopic viscosity was reduced [22][23][24]. Moreover, the Q2 and Q1 curves were smooth and steady when the ω(MgO)/ω(Al2O3) ratio is above 0.55. The percent content of Q2 and Q1 was higher than that of the others, while continuously increasing the MgO content was detrimental to the energy-saving ability of the BF. Therefore, the ω(MgO)/ω(Al2O3) ratio should be controlled at less than 0.55. To better analyze the evolution behavior of the silicon-oxygen anion groups, the schematic diagram of its structural evolution is shown in Figure 9.  To better understand the structure of Al in the slag and the change of its structural form in relation to R2 and the ω(MgO)/ω(Al2O3) ratio, 27 Al-SS-MAS-NMR analysis was employed. According to previous studies [25][26][27], images from the 27 Al-SS-MAS-NMR spectrum can be divided into three parts: −10-30 ppm, the [AlO6] 9− octahedral structure of six-coordinated Al; 30-60 ppm, the [AlO5] 7− pentahedral structure of five-coordinated Al; and 60-80 ppm, the [AlO4] 5− tetrahedral structure of four-coordinated Al. To better reflect the variation of the structural unit content of each coordinating Al with increasing R2 and ω(MgO)/ω(Al2O3) ratio, Gaussian peak partitioning was performed on the obtained NMR spectrum.
The 27 Al-SS-MAS-NMR peak results for different R2 are shown in Figure 10. The peak points do not clearly show changes in the content of Al structural units for each ligand, so calculations of the relative contents of each peak area are needed in Figure 11. The relative content of the [AlO4] 5− tetrahedral structural units of tetra-coordinated Al gradually decreased from 66.1% to 20.0% as R2 increased from 0.9 to 1.3. The relative content of the [AlO5] 7− pentahedral units and the [AlO6] 9− octahedral increased gradually from 26.7% to 52.8% and from 7.2% to 27.2%, respectively. The results show that Al in slag changed from [AlO4] 5− tetrahedral structure to [AlO5] 7− pentahedral structure and [AlO6] 9− octahedral structure with increasing of R2, the [AlO4] 5− structural unit of tetrahedral Al is a network former in the slag structure, and the complexity of the slag structure increases with the Al content. In addition, the [AlO5] 7− unit of five-coordinated Al and the [AlO6] 9− unit of sixcoordinated Al are network modifiers in the slag structure, simplifying the slag structure will increasing Al content. Therefore, the increase of R2 leads to the transition from [AlO4] 5− to [AlO5] 7− and [AlO6] 9− , reducing the DOP of the slag, while an R2 above 1.2 is not conducive to the transition of [AlO6] 9− .    .6% to 44.8% and from 6.4% to 28.7%, respectively. As the ω(MgO)/ω(Al2O3) ratio increased, the structure of Al in the slag changed from [AlO4] 5− tetrahedral structure to [AlO5] 7− pentahedral structure and [AlO6] 9− octahedral structure ( Figure 14). The content of [AlO5] 7− octahedral units was highest, but its relative content decreased when the ω(MgO)/ω(Al2O3) ratio was above 0.55. In regard to energy-saving ability in the production of BF, the ω(MgO)/ω(Al2O3) ratio should be controlled at less than 0.55.

Conclusions
The influence of R2 and ω(MgO)/ω(Al2O3) on the microstructural evolution of the CaO-SiO2-Al2O3-MgO quaternary slag system was investigated by XRD, FTIR, and 27 Al-SS-MAS-NMR analyses, and the following conclusions can be drawn: (1) The CaO-SiO2-Al2O3-MgO quaternary slag system entered the melting state when the temperature was increased to 1773 K, and the main structural units of the polymer were as follows: (2) With the increase of R2 and ω(MgO)/ω(Al2O3), the Q3 structural unit of the silicate anion group changed to Q0, Q1, and Q2, the DOP decreased, and the corresponding peak center position of each structural unit moved gradually to the lower wavenumber region. This indicates that the stability of each structural unit weakened and had a tendency to simplify in structure. The experimental data of the non-bridged oxygen content were compared with the formula in reference documents, and the fitting degree was found to be in agreement.
(3) The 27 Al-SS-MAS-NMR results showed that R2 can lead to the transformation of the Al in slag from [AlO4] 5− tetrahedral structure to [AlO5] 7− pentahedral structure and [AlO6] 9− octahedral structure. Increasing the ω(MgO)/ω(Al2O3) ratio is more likely to cause the slag to eventually transform into [AlO6] 9− octahedral structure, which functions as a network modifier in the slag structure. This would then reduce the DOP of the slag. In general, when the basicity is controlled within 1.2, especially when the ω(MgO)/ω(Al2O3) ratio is less than 0.55, the fluidity is suitable and energy can be saved in BF production.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

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