Crystallization Behavior of the CaO-Al 2 O 3 -MgO System Studied with a Confocal Laser Scanning Microscope
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INTRODUCTION
THE reclamation of waste slag from ironmaking and steelmaking processes has allowed the steel industry to obtain additional revenue from this previously unvalued process waste. According to recent data, most of the blast furnace slags are recycled and used in either construction or roadside gravel. However, steelmaking process waste contain significant amounts of MgO, Al2O3, FeO, CaF2 and other oxides and halides that limit its widespread use. Furthermore, the size and density of the cooled slags vary and can require additional crushing and separating that substantially increases the costs of recycling steelmaking slags. However, the strength and density of the cooled slags typically depend on the formation of crystalline phases, the ratio of the crystalline volume to the amorphous volume of the slag, and the morphology of the crystalline phases. Agarwal et al.[1] observed a significant increase in the cutting time for crystalline slags compared to glassy slags. Because the fraction of crystalline and amorphous slags during solidification and the crystal morphology are dependent upon the cooling rate, it is essential to provide fundamental information related to the crystallization characteristics of various slag systems to enable control of their eventual structure and composition. Furthermore, the crystallization of oxide slag is also an important topic in the control of radiative and conductive heat transfer in continuous casting.[2–4] Ryu et al.[5] showed the effect of Al2O3 additions of up to 25 wt pct on crystallization in the CaO-SiO2-CaF2 SUNG SUK JUNG, Graduate Student, and IL SOHN, Associate Professor, are with the Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea Contact e-mail: [email protected] Manuscript submitted April 21, 2012. Article published online September 1, 2012. 1530—VOLUME 43B, DECEMBER 2012
system using a confocal laser scanning microscope (CLSM). The results indicated a change in the crystalline phase from 3CaO Æ 2SiO2 Æ CaF2 to 2CaO Æ SiO2 Æ Al2O3 as the SiO2 was replaced with Al2O3. Al2O3 addition also increased the crystallization temperature of the melt and shortened the incubation time of the crystallization, as indicated by the time temperature transformation (TTT) diagram. Zhang et al.[6] also observed the crystallization behavior in mold fluxes with varying Al2O3/SiO2 ratios using a CLSM and observed that the crystallization temperature increased with increasing Al2O3/SiO2. Thus, depending on the chemical composition of the melt and the cooling conditions, distinct differences in the primary crystal phases and the morphology were observed. In a more recent study by Heulens et al.[7], isothermal crystallization in the CaO-Al2O3-SiO2 system was accelerated by wo¨llastonite particles, which resulted in a faceted crystal being observed at 1643 K (1370 °C) and a dendritic nonfaceted crystal being observed at 1593 K (1320 °C). Engstrom et al.[8] analyzed slag samples from commercial basic oxygen furnace (BOF) and EAF (elect
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