Observation of the crystallization behavior of a slag containing 46 wt pct CaO, 46 wt pct SiO 2 , 6 wt pct Al 2 O 3 , an
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Due to its important role in continuous casting process, crystallization of mold slags has been investigated by many authors using different techniques such as differential thermal analysis, direct casting experimentation, the single hot thermocouple technique and more recently the double hot thermocouple technique (DHTT).[1–8] Time-temperaturetransformation (TTT) and continuous-cooling-transformation (CCT) diagrams have been constructed for several types of slags. The crystallization of a mold slag is influenced by the cooling rate,[1,2] the holding temperature,[3,7] the chemistry[6,7] and the presence of humidity in the gas phase.[5] This communication documents the crystallization behavior of a limesilica-alumina slag with a low Na2O content slag and compares the results with previous studies. Experiments were carried out by means of a DHTT. Details of the experimental setup have been given elsewhere.[1–5,7,8] The synthetic slag used in this study was prepared by mixing and melting pure oxides in a graphite crucible. After melting, the slag was ground to a fine powder and decarburized in a muffle furnace at 900 °C for 15 hours. The purity of oxides used and the slag chemical composition are listed in Table I.
After the crystallization was completed, the sample was quenched, taken out of the thermocouples, put in a plastic container, and kept in a dessicator for scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD) analyses. Figure 1 shows a TTT diagram for a lime-silica-alumina slag containing 2 wt pct of sodium oxide. The crystallization start point at different temperatures was determined by the direct observation method using the DHTT. The obtained curve shown in Figure 1 indicates a double nose rather than the classical C-shape curve, indicating that at least two different types of crystals precipitated depending upon the details of the experiment. Figure 1 can be viewed as a juxtaposition of two different C curves with two distinct noses at 1200 °C and 1125 °C, respectively. The XRD analyses were conducted on quenched samples for crystal identification. Three selected samples, one inside of each of the two C curves (1100 °C and 1200 °C) and one on the intersection of the curves (1150 °C), were analyzed. A Rigaku diffractometer with a fixed sample holder was used for this purpose. The results are summarized in Table II. These results show an existence of two different phases in all three samples: an amorphous (glassy) phase and a crystalline phase. The crystalline phases in sample CASN 1 and CASN 3 were identified as di-calcium silicate (Ca2SiO4) and tri-calcium silicate (Ca3SiO5), respectively. The crystals in sample CASN 2 were a mixture of the two different crystal types. Ca2SiO4 crystals have rhombohedral structure and Ca3SiO5 crystals are monoclinic. A clear difference in crystal morphology was also observed. At low temperatures, 950 °C, the crystals were small and had a rod shape. At higher temperatures, the crystals are dendritic. These results confirm those
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