In this work, the crystallization behavior of synthesized FetO-rich electric arc furnace (EAF) waste slags with a basicity range of 0.7 to 1.08 was investigated. Crystal growth in the melts was observed in situ using a confocal laser scanning microscope, and a delayed crystallization for higher-basicity samples was observed in the continuous cooling transformation and time temperature transformation diagrams. This result is likely due to the polymerization of the melt structure as a result of the increased number of network-forming FeO4 and AlO4 units, as suggested by Raman analysis. The complex incorporation of Al and Fe ions in the form of AlO4 and FeO4 tetrahedral units dominant in the melt structure at a higher basicity constrained the precipitation of a magnetic, nonstoichiometric, and Fe-rich MgAlFeO4 primary phase. The growth of this spinel phase caused a clear compositional separation from amorphous phase during isothermal cooling at 1473 K leading to a clear separation between the primary and amorphous phases, allowing an efficient magnetic separation of Fe compounds from the slag for effective remediation and recycling of synthesized EAF waste slags for use in higher value-added ordinary Portland cement.
The crystallization behavior of a calcium-aluminate system with various MgO content from 2.5 to 7.5 wt pct and CaO/Al 2 O 3 ratios between 0.8 and 1.2 has been examined using a confocal laser scanning microscope (CLSM). CCT (continuous cooling transformation) and time temperature transformation (TTT) diagrams were constructed to identify the primary crystal phase of slag at different compositions and at cooling rates between 25 and 800 K/minutes. In the slag at a CaO/Al 2 O 3 ratio of 1.0, crystallization temperature increased during isothermal and continuous cooling with higher MgO content, and the shortest incubation time was observed at 5 wt pct MgO. When MgO content was fixed to be 5 wt pct, crystallization temperature increased with lower CaO/Al 2 O 3 ratio. According to the slag composition, cooling rates and temperature, the primary phase could be CA, or C 5 A 3 , or C 3 A, or C 3 MA 2 , or MgO, and the crystal morphology changes from dendrites to faceted crystals to columnar crystals in this composition range.
The solidification of a multicomponent stainless steel slag and dust composite has been studied by thermodynamic calculations using Factsage and analyses of samples using EPMA and Rietveld refinement of synchrotron X-ray powder diffraction after various cooling rates. At an apparent cooling rate of 1 K/min, the content of spinel (10.6 wt%) was less than thermodynamically calculated (16.6 wt %), largely because of difficulties in the diffusion of depleted ions (including Cr, Mn, and Ni) in the liquid with very gradual compositional gradients. Melilite showed a uniform but distorted crystal structure of P2 1 2 1 2 and its content (48.9 wt%) was larger than the calculated result (32.3 wt%). At apparent cooling rates of 10 K/min and 50 K/min, a slight decrease in spinel and a significant decrease in melilite were observed, and the spinel was divided into two regions with an identical space group of Fd 3m but with distinguished composition and lattice parameters. However, the amorphous proportion consistently increased with the cooling rate from 29.4 wt% at 1 K/min to 69.6 and 92.9 wt% at 10 K/min and 50 K/min respectively.
K E Y W O R D Shazardous waste, phase transformations, spinels, X-ray methods
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