Effect of basicity on physical properties of ladle slags of CaO ‒ SiO₂ ‒ Ce₂O₃ ‒ Al₂O₃ ‒ MgO system

Introduction

Viscosity stands out as one of the most crucial physical properties of slag, as metallurgical processes hinge on phenomena influenced by heat and mass transfer within both slag and metal [1; 2]. Exploring the utilization of rare earth element (REE) oxides represents a promising avenue for decreasing the viscosity of refining slags. Investigations into the impact of cerium oxide additives on slag’s physical properties have revealed that cerium oxide reduces both viscosity and crystallization temperature [3 ‒ 5]. Recent studies have demonstrated that incorporating rare earth oxides into slag can diminish the activity of Al₂O₃ oxide in the slag while enhancing the slag’s adsorption capacity for Al₂O₃ inclusions in the metal [6 ‒ 8]. Additionally, the equilibrium between refining slag containing Ce₂O₃ and molten steel deoxidized with aluminum hints at the potential for introducing a small amount of cerium, which may transfer into the steel [9 – 11], thereby facilitating micro-alloying and modification [12]. However, to date, the influence of basicity on the physical properties of cerium-containing ladle slags remains unexplored in both domestic and foreign literature.

This study aimed to investigate the physical properties of slags within the СаО ‒ SiO₂ ‒ Ce₂O₃ ‒ Al₂O₃ ‒ MgO system. By consolidating research findings, we sought to generate new insights into the impact of basicity on the viscosity, temperature of crystallization onset, and structure of cerium-containing slags within the studied oxide system.

Research methods

The slags of the СаО ‒ SiO₂ ‒ Ce₂O₃ ‒ Al₂O₃ ‒ MgO oxide system were melted in a resistance furnace using graphite crucibles under an argon atmosphere. Analytical grade oxides were calcined for 2 – 3 h at a temperature of 800 °C prior to melting. Viscosity measurements of the slags were conducted in graphite crucibles using an electric vibrating viscometer, with continuous cooling of the melt from a homogeneous-liquid to a solid state [13]. A molybdenum rod with a diameter of 1.5 mm served as the measuring spindle. The temperature of the slag was monitored using a VR 5/20 tungsten-rhenium thermocouple. The crystallization temperature of the slags was determined according to Frenkel’s theory of viscous flow. For this purpose, graphs were plotted in coordinates lnη – 1/T, with breaks indicating the onset temperature of slag crystallization [14]. The results of viscosity and crystallization temperature measurements are presented in Table 1 and Fig. 1.

The structure of test slag samples was investigated using a Raman microscope spectrometer U 1000 with a laser having an excitation wavelength of 532 nm. The obtained spectra, within the wave number range of 450 – 1250 cm\(^‒\)¹, are depicted in Fig. 2. The observed lines in such spectra can be clearly attributed to vibrations of the molecules of the material under study. Depending on the frequency, intensity, and shape of these lines, conclusions about the structure of the slags can be drawn [15]. This transformation can be attributed to the characteristics of the slag structure. Fig. 2 displays the Raman spectra of slag samples with varying basicity, featuring peaks: in the low-frequency region with wave numbers around 600 cm\(^‒\)¹, representing Al ‒ O stretching vibrations in [AlO₆] octahedra within the range of slag basicity from 2.0 to 2.5; as basicity increases to 3.5 – 5.0 units, peaks emerge in the region of about 550 cm\(^‒\)¹, attributed to the transverse motion of bridging oxygen inside the Al ‒ O ‒ Al bond; peaks in the range of 650 ‒ 800 cm\(^‒\)¹, reflecting Al – O stretching vibrations in [AlO₄] tetrahedra. Peaks in the higher wave numbers region (800 – 950 cm\(^‒\)¹) are associated with the silicate structure ([SiO₄]-tetrahedra). The intensity and shape of these peaks allow for the evaluation of the influence of basicity on the structure of the formed slags and their viscosity.

For further quantitative determination of changes in structural units with different basicity of the slag, the Raman spectra (Fig. 2) were deconvoluted by the Gaussian method using the PeakFit software with the correlation coefficient of minimum 0.99. The results of deconvolution of the silicate region of Raman spectra are shown in Fig. 3 and Table 2.

Results and discussion

The viscosity dependence of the studied slags on temperature, within a basicity range of 2.0 to 5.0, is illustrated in Fig. 1. As basicity increases, slags transition gradually from fluid states with low crystallization temperatures to those with higher viscosity and crystallization onset temperatures (Table 1).

The slag structure formed at a basicity of 2.0 is low-polymerized. As noted earlier, it is characterized by the presence of [AlO₆]-octahedra, which function as grid modifier (Fig. 2), along with two depolymerized structural units of silicon: [SiO₄]⁴\(^–\) with an increased proportion of non-bridging oxygen to 0.48 (\(Q_{{\rm{Si}}}^0\)) and [Si₂O₇]⁶\(^–\) with a proportion of one bridging oxygen increased to 0.52 (\(Q_{{\rm{Si}}}^1\)) Fig. 3, Table 2). This structure arises due to the presence of structure modifiers – calcium and cerium oxides – in the slag. Their dissociation in melts releases additional O²\(^–\) ions, which interact with [AlO₄]- and [SiO₄]-tetrahedra, disrupting the aluminate and silicate melt structures [16; 17]. Consequently, these slags exhibit a low crystallization temperature (1397 °C) and low viscosity (0.20 and 0.16 Pa·s) at temperatures of 1500 and 1550 °C (Table 1).

As the basicity increases to 2.5, the silicate structure becomes more complex. Its polymerization degree rises from 0.52 to 0.59, while the proportion of non-bridging oxygen decreases from 0.48 to 0.41, and the proportion with one bridging oxygen increases from 0.52 to 0.59 (Table 2). In high-basicity slags containing Al₂O₃, Al³⁺ ions are absorbed by the silicate structure, acting as grid-forming agents, thus enhancing the complexity of the silicate structure [18]. The increased complexity of the slag structure elevates the onset temperature of crystallization to 1419 °C and viscosity to 0.22 and 0.17 Pa·s at temperatures of 1500 and 1550 °C (Table 1).

As the basicity increases to 3.5 and 5.0, a peak emerges in the region of about 550 cm\(^–\)¹, attributed to the transverse motion of bridging oxygen inside the Al – O – Al bond. The relative Al – О – Al intensity gradually increases with increasing basicity, while the opposite trend is observed for [AlO₆] octahedra. This indicates an enhanced Al – O – Al bond and a decreased proportion of [AlO₆] octahedra, leading to polymerization of the aluminate grid. Additionally, an increase in the slag basicity from 3.5 to 5.0 results in higher peak intensity in the region of wave numbers 650 – 800 cm\(^–\)¹. This is attributed to symmetrical stretching vibrations [AlO₃]³\(^–\) (\(Q_{{\rm{Al}}}^2\)) and [Al₂O₅]⁴\(^–\) (\(Q_{{\rm{Al}}}^3\)) [16; 17], indicating that the aluminate structure in the molten slag (Fig. 2) has become more complex, resulting in an increase in the crystallization onset temperature to 1463 and 1497 °C. Viscosity increases to 0.26 and 0.18 Pa·s at temperatures of 1500 and 1550 °C and a basicity of 3.50 and to 0.41 and 0.23 Pa·s at temperatures of 1500 and 1550 °C and a basicity of 5.0.

Alongside the polymerization of the aluminate structure at basicities of 3.5 and 5.0, the silicate structure becomes simpler, and the polymerization degree decreases from 0.47 to 0.12 (Table 2). CaO oxide can act not only as a grid modifier but also as a charge compensator due to the excess of Ca²⁺ ions formed with increasing basicity. Са²⁺ cations will compensate for the polymerized structural units of [AlO₄]-tetrahedra, leading to the formation of a more stable tetrahedral structure and resulting in increased slag viscosity [19; 20]. Cerium ions can act as charge compensators and stabilize the aluminate structure [3; 21; 22].

Conclusions

Experimental studies of the physical properties of the slags within the СаО ‒ SiO₂ ‒ Ce₂O₃ ‒ Al₂O₃ ‒ MgO oxide system have indicated that increasing the basicity from 2.0 to 5.0 leads to higher viscosity and temperature of crystallization onset. This phenomenon is attributed to the structure of the resulting slags. With the augmentation of basicity, the aluminate structure becomes more complex, while the silicate structure becomes simpler due to an excess of Ca²⁺ ions, which serve as charge compensators for the polymerized structural units of [AlO₄] tetrahedra. Overall, the slags within the studied oxide system containing 15 % Ce₂О₃ exhibit sufficiently high fluidity within the considered basicity range.