한양대학교

It has been reported that about 45 million metric tons of end-of- life electronic products such as printed circuit board (PCB) are discarded worldwide per annum. Furthermore, the amount of discarded lithium-ion battery (LIB) is expected to grow rapidly as the demand for electric vehicles increases. The industrial wastes such as PCBs, LIBs, and catalysts contain significant amounts of valuable metals, carbonaceous materials, and inorganic parts. Valuable metals can be recovered from PCBs (e.g., copper, gold, silver, palladium, etc.), waste catalyst (e.g., platinum, etc.), and LIBs (e.g., lithium, cobalt, nickel, manganese, etc.). It is expected that the demand for the recovery of valuable metals from the industrial wastes has grown to achieve the sustainable development in the metallurgy.

Little experimental work to understand the viscous flow behavior of iron-compound bearing calcium-aluminosilicate melts based on structural knowledge from the viewpoint of molecular dimensions has been carried out. Therefore, the viscosity of the CaO-SiO2-Al2O3-FetO-MgO slag was investigated in the present study with different Al2O3 and SiO2 contents in conjunction with structure analysis using micro-Raman spectroscopy and iron redox equilibria in the slags.

The oxygen pick-up in molten steel with the formation of iron-bearing calcium aluminate slags spontaneously occur during Ruhrstahl-Heraeus (RH) process for producing ultra-low carbon steel. Then, aluminum is added to deoxidize the steel melt, resulting in the formation of alumina-rich nonmetallic inclusions as a deoxidation product in the molten steel. Because the presence of the alumina-rich inclusions in the steel causes a decline in the mechanical properties and various other problems in steelmaking and continuous casting processes, it is necessary to remove the harmful alumina-rich inclusions.

Because a study looking into the viscous flow behavior and structure of low-silica and iron-containing calcium aluminate melts has yet to be fully developed, in the present study, the viscosity of the CaO–Al2O3–FetO–SiO2–MgO system, which is representing the RH refining slag, was investigated with different C/A ratios and various iron oxide contents; this was done in conjunction with structural analysis in the molecular dimensions using micro-Raman spectroscopic analysis.

The alumina-rich oxide inclusions are spontaneously formed by aluminum addition for the deoxidation of molten steel in the steelmaking processes. Experimental studies have demonstrated that the removal rate of alumina-rich oxide inclusions in molten steel are critically affected by the viscosity of the slag, i.e., the lower the viscosity, the higher the inclusion removal rate. The ladle refining slag for the Al-killed steel is generally the CaO-Al2O3 based system, whereas the CaO-Al2O3-FetO based melts are adopted in a specific operation such as Ruhrstahl-Heraeus (RH) refining vessel. Thus, even though the comprehensive understanding of the viscosity-structure relationship of the CaO-Al2O3-FetO based melts is key to optimize the RH refining slag for best steel quality, there are few fundamental studies on the structural understanding of the CaO-Al2O3-FetO based system.

Because the iron redox equilibrium reactions and the coordination of Fe3+ and Al3+ are critically affected by the chemical composition of the melts, it is difficult to understand the relationship between the thermophysical properties and structural changes in iron oxide-containing calcium aluminate melts. Therefore, in the present study, the effect of the compositional changes on the iron redox equilibria and structure of the CaO-Al2O3-FetO system was investigated with different activities of CaO at 1823 K using micro-Raman spectroscopic analysis.

It is necessary to remove the inclusions from molten steel to enhance the cleanliness of steel products in secondary refining processes. There are following consecutive processes for the removal of inclusions, viz. (i) flotation of inclusions to the steel melt-slag interface, (ii) separation into the slag phase and (iii) dissolution into the slag phase, among which the final step was reported to be a rate controlling process for the removal of inclusions.

Even though a few reports on the structural changes in the low-silica calcium aluminosilicate melts have been published by glass scientists and chemical geologists, the relationship between the viscosity and the structure of the melts, which potentially correspond to the secondary refining ladle slags, is not fully developed yet. Therefore, in the present study, the structure-viscosity relationship of the low-silica (SiO2 ≤ 10 wt%) calcium aluminosilicate melts was investigated by employing the rotating-cylinder viscosity measurement in conjunction with the Raman spectroscopy measurement for linking the macroscopic thermophysical property and molecular (ionic) structural information. Furthermore, the influence of CaF2 on the structure-property relationship was discussed

Knowledge of the structure of silicate melts is of great importance in metallurgical processes. The structure of silicate melts is the dominant factor affecting the transport properties, such as viscosity, conductivity, density, etc. Therefore, there have been numerous attempts to measure and/or predict these macroscopic thermophysical properties as a function of the composition of silicates based on the microscopic view of structures.

In the current study, the effects of CaO/SiO2 ratio, SiO2 content, and Ca-Mg substitution on the distribution of silicate anionic species in the CaOSiO2-MgO slag system were investigated by micro-Raman spectroscopic analysis of melt-quenched glass samples. Furthermore, the relationships among the structural and physicochemical properties of silicate melts were evaluated.

In the present study, calcium silicate glass samples containing MnO or MgO were prepared by batch melting and quenching method. Recently, Park unveiled that the sulfide capacity of the CaO–SiO2–MnO slag is strongly dependent on the effect of CaO and MnO on the depolymerization behavior of silicate networks.

From these previous works, the quantitative information on the structure of the CaO–SiO2–MO (M=Mn and Mg) slags and the interrelationship between structure and thermodynamic properties are still highly necessitated. Consequently, in the present study, not only a distribution of silicate anionic species but also a chemical speciation of free oxygen, non-bridging oxygen and bridging oxygen in the CaO–SiO2–MO (M=Mn and Mg) slags are investigated by employing micro–Raman spectroscopic analysis for melt–quenched samples. Furthermore, the thermochemical properties such as sulfide capacity and excess free energy of mixing of oxide components are evaluated using a concentration of free oxygen as well as a degree of polymerization of silicate melts.

Knowledge of the structure of silicate melts and glasses is of great importance for both earth sciences and materials sciences, such as metallurgical processes and glass sciences. The structure of silicate melts and glasses is the dominant factor affecting the transport properties, such as viscosity, conductivity, density, etc. Therefore, there have been numerous attempts in the fields of chemical geology, metallurgy, and glass science to measure and/or predict these macroscopic thermophysical properties as a function of the composition of silicates based on microscopic view of structures.

Viscosity is of particular importance because it controls the rate of transport of matter and thus, of energy. Generally, the largest changes in the viscosity of silicate melts are found when metal oxides, MO (where M=alkali, alkaline earth, transition metals, etc.) are added to SiO2. The associated variation in activation energy follows a similar trend. As the MO content increases, the microheterogeneous structure transforms gradually to discrete polyanions, and finally to isolated [SiO4]4– ions.

Over limited temperature intervals, electrical conductivity or resistivity follows an Arrhenius-type law, as does viscosity. The activation energy has been shown to decrease slightly with increasing MO content.

Vibrational spectroscopy, specifically Raman spectroscopy was originally used to probe the local anionic structure of silicate glasses. The bands assigned to antisymmetric stretching of Si–O– (non–bridging oxygen; NBO) and Si–O0 (bridging oxygen; BO) bonds occur in the 850–1200 cm–1 region, whereas Si–O–Si bending modes are found between 500 and 700 cm–1. The frequencies of the stretching modes decrease with decreasing degree of polymerization of the glass, viz. increasing NBO/Si. There are several types of units, such as SiO2 (fully polymerized), Si2O5 (sheet), SiO3 (chain), Si2O7 (dimer), and SiO4 (monomer). Based on a nuclear magnetic resonance (NMR) spectra of silicate glasses, the stoichiometric notations for each unit were replaced by the so–called Qn concept as follows: Q4 (NBO/Si=0), Q3 (NBO/Si=1), Q2 (NBO/Si=2), Q1 (NBO/Si=3), and Q0 (NBO/Si=4).

In the present study, calcium silicate glasses containing MgO or MnO were prepared by batch melting and quenching. Recently, Park reported that sulfur absorption ability, viz. the sulfide capacity of the CaO–MnO–SiO2 melts, is strongly dependent on the effect of CaO and MnO on the depolymerization of silicate networks. Consequently, in the present study, the effect of Mn2+ and Mg2+ ions on the distribution of silicate anionic species in calcium silicate systems was investigated by micro–Raman spectroscopic analysis of melt–quenched samples. Furthermore, the relationships among the structural and thermophysical properties of silicate melts, such as viscosity, density, and conductivity were evaluated.

Even though there have been several studies on the physicochemical properties of the CaO–SiO2–MnO slag due to its importance in smelting and refining processes for the production of manganese ferro–alloys as well as high manganese steel grades, the systematic and fundamental approach on the structure of this slag system is scarcely found.

Recently, Park unveiled that the sulfide capacity of the CaO–SiO2–MnO slag is strongly dependent on the effect of CaO and MnO on the depolymerization behavior of silicate networks. From these previous works, the quantitative information on the structure of the CaO–SiO2–MnO slag is still highly necessitated.

Consequently, in the present study, the effects of lime to silica ratio, silica content, and Ca–Mn substitution on the distribution of silicate anionic species in the CaO–SiO2–MnO slag are investigated by employing micro–Raman spectroscopic analysis for melt–quenched samples. Furthermore, the relationships among the structural information, thermophysical properties such as viscosity, density and electrical conductivity are developed.

The relationship between thermophysical properties and the structure of silicate melts is of importance to understand the microscopic origin of the thermodynamic properties not only in the metallurgical community but also in chemical geology. The viscosity, which determines the working temperature and process kinetics of the system, CaO-SiO2-Al2O3-MgO melt, has been measured by many researchers. The structure of the subsystems with mainly binary and ternary was investigated with the aid of various experimental techniques. Further, we can find some reports on the measurement or optimization of thermodynamic activities of each component in this quaternary system at high temperatures. Furthermore, there has been considerable interest in the amphoteric behavior of alumina, which acts as a network modifier or network former according to the composition of aluminosilicate melts.

Although there are several studies on viscosities, structures, and thermodynamic properties of CaO-SiO2-Al2O3-MgO slag, comprehensive explanations integrating all three factors together at a given system are still insufficient. Park et al. measured the viscosity and Fourier transform-infrared (FT-IR) transmitting spectra of CaO-SiO2-Al2O3(-MgO) melts and glasses and revealed the amphoteric behavior of alumina based on the changes of activity coefficient of alumina and silica.

In the present study, in line with previous works, the effect of alumina on the relationship between viscosity and structure of the CaO-SiO2-Al2O3-15mol%MgO ((mol%CaO)/(mol%SiO2), B=1.2) system is investigated by employing a viscometer using the rotating cylinder method and FT-IR spectra, respectively. In addition, the original Darken's excess stability function was introduced in order to understand the thermophysical phenomena and the role of alumina based on thermodynamics.

Because the viscosity of the CaO-SiO2-CaF2 system is very important in primary and secondary refining as well as continuous casting processes, several researchers measured the viscosity of this slag system. Recently, Park et al. had an attempt to interrelate the viscosity with the structural analysis by applying the rotating cylinder method for molten slags at temperatures from 1473 to 1823 K. The characterization of structural analysis was operated by the Fourier-transform infra-red (FT-IR) spectra for quenched samples. The authors observed that a decrease of the viscosity as well as the activation energy for viscous flow of the slags (B=1.0 and 1.3) in a Newtonian flow region is originated from a decrease in the degree of polymerization due to a network modifying role of F- ions in silicate melts. Although the role of F- ions in the (de)polymerization reaction of silicate melts is still in a room for further discussion based on various analytical methods such as FT-IR, Raman, X-ray photoelectron spectroscopy (XPS), and 19F nuclear magnetic resonance (NMR) spectroscopy, it is simply believed that the existence of F- ions decreases the viscosity of silicate melts, as noted by Sasaki et al.