| dc.description.abstract | Climate change has clearly become an urgent global issue. There is substantial evidence showing that the rapid rise in greenhouse gas emissions, especially carbon dioxide, is primarily caused by industrial development and increased human activities, leading to negative environmental impacts. Therefore, reducing greenhouse gas emissions is essential to prevent the worsening of climate change. Recently, Taiwan officially released the ＂Taiwan 2050 Net-Zero Emissions Pathway Blueprint,＂ aiming to achieve the goal of net-zero emissions by 2050. Carbon capture, utilization, and storage (CCUS) technology is considered the most promising carbon reduction strategy. Capturing and storing carbon dioxide in minerals, geological formations, or oceans is seen as the most effective and feasible method.
In this study, electric arc furnace slag was used as the material for calcium ion extraction, as it contains a high amount of calcium, which can react with carbon dioxide to form carbonate precipitates. These carbonates can be reused in construction materials, promoting the concepts of circular economy and environmental sustainability. Ammonium chloride was employed as the extracting agent due to its high selectivity for calcium ions, enhancing the extraction efficiency. Moreover, microwave technology was used to accelerate the dissolution of calcium ions and speed up the reaction, significantly improving the overall experiment′s efficiency.
The extraction experiments examined the effects of four parameters—reaction time, particle size, ammonium chloride concentration, and liquid-to-solid ratio—on the extraction of calcium, magnesium, and silicon ions. The results showed that as the reaction time increased, the extraction concentrations of calcium and magnesium ions rose, while silicon ions formed silica gel due to prolonged heating, resulting in a decrease in their concentration. Smaller particle sizes with larger surface areas improved the extraction efficiency. Higher ammonium chloride concentrations promoted the dissolution of calcium and magnesium ions. Since ammonium chloride is more selective for calcium ions, the magnesium ion concentration was lower under the same conditions. However, silicon ions exhibited the opposite trend because high concentrations of ammonium chloride prioritized the binding with calcium ions in the slag, inhibiting the dissolution of silicon ions. A lower liquid-to-solid ratio also helped increase the extraction concentrations of calcium, magnesium, and silicon ions. The optimal extraction conditions were a reaction time of 10 minutes, particle size ? 74 μm, ammonium chloride concentration of 2 M, and a liquid-to-solid ratio of 5:1 (calcium ions: 52,000 ± 563 mg/L; magnesium ions: 1,100 ± 4.3 mg/L; silicon ions: 64.8 ± 8.4 mg/L).
In the carbonation reaction tests, the effects of reaction time, temperature, and pH on carbonation efficiency were examined. The results indicated that increasing reaction time and temperature only slightly improved carbonation efficiency, whereas pH was the critical factor. At higher pH levels, the amount of calcium carbonate precipitated increased significantly, successfully carbonating 84.1 ± 0.59 g CO2 per kg of slag. To confirm the properties of the precipitated calcium carbonate, XRD and SEM analyses were conducted, showing that the calcium carbonate precipitated at 25°C formed as vaterite crystals, while at 60°C, it formed as calcite crystals. Using microwave heating equipment results in carbon emissions of only 0.066 kg CO2e, demonstrating that microwave heating not only shortens reaction time and increases extraction concentration but also saves more energy. | en_US |