Wonbaek Kim

Previously, mineral carbonation of blast-furnace slag was carried out to sequestrate CO₂ and attain pure CaCO₃ crystals. In this process, amorphous silica-alumina nanoparticles were obtained as a byproduct. In this study, the crystallization of these nanoparticles on calcination at various temperatures in air was examined using TGA-DTA, XRD, MAS-NMR spectroscopy, and FT-IR spectroscopy. The precursor nanoparticles (Si:Al = 78:22 mol %) were prepared using the solution extracted from blast-furnace slag (BFS) with acetic acid at room temperature. The XRD analysis showed that the initial amorphous state was retained up to 800 °C, and decomposition to amorphous silica and mullite started after calcination at 950 °C. At temperatures between 1150 °C and 1250 °C, amorphous silica crystalized to cristobalite, which eventually melted to glassy silica at 1500 °C. The mullite crystals initially adopted a metastable tetragonal phase and transformed to a stable, needle-like orthorhombic phase at higher temperatures. ²⁷Al MAS-NMR spectroscopy revealed that octahedrally coordinated Al was favored up to a temperature of 800 °C as a result of the dehydration process and transformed into tetrahedrally coordinated Al at higher temperatures. A microstructural examination revealed that the initially randomly-oriented mullite developed into stable, needle-like grains owing to anisotropic grain growth in the presence of a glass phase at high temperatures. This study suggests that the recycling of BFS can be exploited for the procurement of a mullite-type ceramic material.

Blast-furnace slag (BFS) has been used as a feedstock for CO₂ sequestration by indirect mineral carbonation to produce calcium carbonate precipitates and solid residues. The most-abundant elements in these residues, Si and Al, are usually considered to be impurities that need to be removed in acid-dissolution processes involving BFS. The co-production of value-added materials from these residues is an attractive option for strengthening the economic competitiveness of mineral carbonation methods. In view of this, we separated the Si and Al, as their hydrated forms, during the dissolution of BFS in acetic acid prior to carbonation. During the sol-gel processing of Si-Al nanoparticles, a catalyst is usually required during the hydrolysis and subsequent condensation processes. In this study, only condensation occurs because the low-concentrations of acetic acid used facilitate in-situ hydrolysis during the dissolution process. Aging was carried out not only to structurally arrange the Si and Al but also to oxidize the marginal Fe(II) to reddish Fe(III). Silica-alumina nanoparticles (78% Si and 22% Al) were prepared by a simple sol-gel route at ambient pressure. These nanoparticles were amorphous and below 20 nm in size. Fourier-transform infrared (FT-IR) studies reveal that the nanoparticles consist of Si–O–Si and Si–O–Al bonds. ²⁷Al nuclear magnetic resonance (NMR) spectroscopy reveals a significant resonance corresponding to tetra-coordinated Al inside the particle framework.

Blast furnace slag (BFS), a calcium-rich industrial byproduct, has been utilized since 2005 as a mineral carbonation feedstock for CO₂ sequestration, producing calcium carbonate precipitates. In this study, the conditions for the dissolution of Ca and Si in acetic acid, and subsequent carbonation, were elaborated. For this purpose, the retardation of the polymerization of silicon was attempted by varying the concentration of acetic acid, temperature, and leaching time. An inductively coupled plasma (ICP) analysis revealed that both the Ca and Si dissolved completely within 30 min in 5% acetic acid at room temperature. This high dissolution value can be attributed to the fact that Ca was bound to O rather than to Si, as determined by X-ray photoelectron spectroscopy (XPS). The use of CO₂-absorbed monoethanolamine enabled the complete carbonation of BFS at ambient conditions without the need for a pH swing. The presence of dissolved silica was found to affect the polymorphs of the precipitated CaCO₃. We believe that this process offers a simple method for manipulating the composites of products obtained by mineral carbonation diminishing the leaching residues.

The effects of CO₂ flow rate and calcium source on the particle size (PS) and specific surface area (SSA) of CaCO₃ particles were evaluated using microbubble generator (MBG) and air diffuser (AD) systems. The carbonate mineralization (CM) and precipitated calcium carbonate (PCC) methods were employed to produce CaCO₃ using gypsum and Ca(OH)₂ as the calcium sources, respectively. The CaCO₃ particles prepared using the MBG were smaller (with larger specific surface area) than those obtained using the conventional AD, regardless of the calcium source. The average PSs were 2–3 and 7–9 μm for the MBG and AD systems, respectively. Moreover, the PS and SSA of the particles prepared using the MBG were not greatly affected by the CO₂ injection rate. This study clearly demonstrates that the use of an MBG ensures the stable production of fine CaCO₃ particles using various calcium sources and a wider range of CO₂ flow rates.