FUNDAMENTAL STUDY ON THE REMOVAL OF CALCIUM ALUMINATE INCLUSIONS BY METALLURGICAL SLAGS

Abstract

Solid calcium aluminate non-metallic inclusions (CaO∙2Al2O3 (CA2) and CaO∙6Al2O3 (CA6)) usually form due to inadequate calcium additions during calcium treatment, and/or interactions among refractories, slag and steel. These inclusions negatively affect steel cleanliness and production efficiency by causing submerged entry nozzle clogging. Most non-metallic inclusions (NMIs) are removed through flotation to the steel – slag interface and dissolved into the molten slag. Therefore, it is critical to better understand the dissolution behaviors of CA2 and CA6 inclusions in metallurgic slags. This study focuses the dissolution kinetics and mechanisms of CA2 and CA6 particles in metallurgical slags using high-temperature confocal laser scanning microscopy (HT-CLSM). The role of intermediate solid products in the dissolution process is also examined via HT-CLSM combined with post-analysis techniques. Dissolution kinetics of CA2 and CA6 particles in the synthetic CaO-Al2O3-SiO2-(MgO) steelmaking slags were examined under varying temperatures, CaO/Al2O3 (C/A) ratios, and SiO2 and MgO contents of slag, along with the time dependency of the projection area of the particle during the dissolution process. The results indicate that the dissolution rates of CA2 and CA6 particles increase with higher temperature, increased MgO content of slag, or lower SiO2 content of slag. At 1550°C, the dissolution rates of CA2 and CA6 particles increased with an increase in slag C/A ratio from 0.9 to 1.8 but declined at 3.8. However, at 1600°C, a higher C/A ratio led to an increased dissolution rate of CA2 particles. Additionally, the particles' motion, dissociation, and interaction with gas bubbles were observed during the experiments, with the first two accelerating dissolution while gas bubbles hindered it. The impact of porosity (φ) and velocity (ν) on the dissolution process of CA2 particles in CaO-SiO2-Al2O3 steelmaking slag was in-situ investigated at 1550°C. Increasing the porosity from 0.08 to 0.20 raised the average dissolution rate (total dissolution time/initial radius of the particle) from 0.35 to 0.59 μm/s. Moreover, particle motion was also observed, highlighting its importance in modeling. A novel mathematical model, incorporating both the motion and porosity of particles, was developed and validated against the current experimental and literature data. The model predictions demonstrated that the total dissolution kinetics of CA2 particles was enhanced by an increase in φ, ν and driving force (∆C), and a decrease in slag viscosity (μ). The parameter sensitivity analysis found that the order of parameter influence is for 0.25 ≤ Xi/Xr(i) ≤ 2, ∆C > φ > ν > μ, and for 2 < Xi/Xr(i) ≤ 4, φ> ∆C > μ > ν. Interrupted dissolution experiments were carried out at 1550℃ for CA2 particles in slags with C/A of 1.8 and 3.8, and CA6 particles in slag with C/A of 3.8 and slag with 8 wt.% MgO, respectively. Post analysis using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) combined with thermodynamic calculations revealed their dissolution paths and the impacts of the intermediate solid products on their dissolution kinetics and mechanisms. It was found that intermediate solid layer formed around the undissolved CA2 and CA6 particles in slag with a C/A ratio at 3.8 was melilite, reducing their dissolution rates. Conversely, the formation of randomly dispersed fine solid particles around and within the undissolved CA6 particle in slag with 8 wt.% MgO did not impede their dissolution. No intermediate solid products were observed for the CA2 particle in slag with a C/A ratio of 1.8, consistent with the thermodynamic predictions. Based on experimental findings and thermodynamic predictions, the dissolution behaviors of the CA2 and CA6 particles in molten steelmaking slags are categorized into two types: (1) direct dissolution: without formation of intermediate solid products or formation of randomly dispersed intermediate solid products, but they do not affect ∆C and do not retard the dissolution kinetics. (2) indirect dissolution: an intermediate solid layer forms around the particle, inhibiting the transport of species and reducing ∆C. The current work provides a comprehensive understanding of the removal of solid calcium aluminate inclusions, including the dissolution kinetics and mechanisms of CA2 and CA6 particles, by CaO-Al2O3-SiO2-(MgO) metallurgical slags at steelmaking temperature. The findings offer valuable insights for optimizing slag composition in clean steel production, particularly during calcium treatment in secondary steelmaking.