Determination of Area Fraction of Free Lime in Steelmaking Slag Using Cathodoluminescence and X-ray Excited Optical Lumi
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INTRODUCTION
IN the steelmaking process, huge amounts of steelmaking slag, which is a byproduct of the conversion of carbon-rich molten iron to steel in basic oxygen furnaces (BOFs) and the conversion of scrap to steel electric arc furnaces (EAFs), are produced globally. Recently, the reuse of steelmaking slag has been aggressively tackled in the iron and steelmaking industry to realize an environmentally friendly society. The reuse rate of steelmaking slag is close to 100 pct in developed countries such as the United States, Japan, Germany, and France.[1,2] In these countries, almost half of the reused steelmaking slag is used in road construction, e.g., road bases, embankments, and asphaltic concrete.[1–6] Steelmaking slag contains free lime (f-CaO), which can cause volumetric expansion because it doubles in volume upon reacting with the water and carbon dioxide in the air.[7,8] Therefore, prior to its use in road construction, steelmaking slag is aged, i.e., exposed to
SUSUMU IMASHUKU and KAZUAKI WAGATSUMA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Contact e-mail: [email protected] Manuscript submitted March 31, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
air, sprayed with hot water, or steamed,[9] to accelerate the hydration and carbonation reactions of f-CaO. Various aging periods have been proposed, e.g., 1 to 18 months for exposure periods in air because the required aging period depends on the amount of f-CaO in the steelmaking slag.[6] If the aging is insufficient, the residual f-CaO is likely to create serious problems, such as road expansion and cracking.[6] As such, determining the f-CaO content in steelmaking slag before and after aging is crucial to efficiently and safely use steelmaking slag for road construction. Several methods to determine the f-CaO content in steelmaking slag have been proposed, such as ethylene glycol extraction followed by the determination of calcium in the ethylene glycol via titration, atomic absorption spectrophotometry, or inductively coupled plasma-atomic emission spectrometry (ICP-AES),[9–11] X-ray diffraction (XRD),[12–15] and infrared spectrophotometry.[16] Among these methods, the ethylene glycol extraction method is the most commonly used. However, the f-CaO content determined applying this extraction method also includes the Ca(OH)2 content in the steelmaking slag because both are extracted in the ethylene glycol.[15,17] To obtain an accurate f-CaO measure, a combined method of the ethylene glycol extraction and thermogravimetry, which can also determine the Ca(OH)2 content in steelmaking slag, has been proposed.[18] Extraction via ethylene glycol is a complicated and time-consuming procedure requiring skillful
techniques, e.g., controlling the particle size of the steelmaking slag and monitoring the temperature and extraction time.[18] Therefore, a simple method to rapidly determine the f-CaO content in steelmaking slag would contribute to the reduction of the time needed to
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