Synthesis and Characterization of 12CaO·7Al2O3 Slags: The Effects of Impurities and Atmospheres on the Phase Relations

Abstract

Synthesis of crystalline slags of 12CaO·7Al2O3 phase from the corresponding melt compositions in different atmospheric conditions and different purities is investigated. Observations using a thermogravimetry coupled with differential thermal analysis showed that the dehydration of a zeolitic 12CaO·7Al2O3 phase occur at 770 °C to 1390 °C before it congruently melts at 1450 °C. The X-ray diffraction pattern of the slag showed that a single 12CaO·7Al2O3 phase is produced from a mixture, which has small SiO2 impurity with a 49:51 mass ratio of CaO to Al2O3. A scanning electron microscope and electron probe micro-analyzer showed that a minor Ca-Al-Si-O-containing phase is in equilibrium with a grain-less 12CaO·7Al2O3 phase. Moreover, 12CaO·7Al2O3 is unstable at room temperature when the high-purity molten slag is solidified under oxidizing conditions contained in an alumina crucible. On the other hand, a high-temperature in-situ Raman spectroscopy of a slag that was made of a higher purity CaO-Al2O3 mixture showed that 5CaO·3Al2O3 phase is an unstable/intermediate phase in the the CaO-Al2O3 system, which is decomposed to 12CaO·7Al2O3 above 1100 °C upon heating in oxidizing conditions. It was found that 5CaO·3Al2O3 is present at room temperature when the 12CaO·7Al2O3 dissociates to a mixture of 5CaO·3Al2O3, 3CaO·Al2O3, and CaO·Al2O3 phases during the cooling of the slag at 1180 °C ± 20 °C in reducing atmosphere. It is proposed that low concentrations of Si stabilize 12CaO·7Al2O3 (mayenite), in which Si is a solid solution in its lattice, which is named Si-mayenite. Regarding the calculated CaO-Al2O3-SiO2 diagram in this study, this phase may contain a maximum of 4.7 wt pct SiO2, which depends on the total SiO2 in the system and the Ca/Al ratio.

High Temperature Treatment of Selected Iron Rich Bauxite Ores to Produce Calcium Aluminate Slags

“…The presence of this phase was not expected, since the experiments were performed in an open induction furnace, where the presence of humidity from the atmosphere can be assumed. Other stabilizing agents for the C 12 A 7 phase are reported to be, among others, C, S, and SiO 2 [11][12][13]. However, none of these agents seem to stabilize the C 12 A 7 phase in the Turkish slag, and it would be of future interest to justify the exact reason for this deviation.…”

Aluminothermic Reduction of Manganese Oxide from Selected MnO-Containing Slags

“…After the mechanical treatment, the milled dross 2 was partitioned by sieving and a particle size of 1.25–2 mm was further used. The microstructures and characteristics of these dross samples were studied by the Zeiss Ultra 55 Scanning Electron Microscope (SEM, Carl Zeiss Microscopy GmbH, Jena, Germany), coupled with energy-dispersive X-ray spectroscopy (EDS) and X-Ray Diffraction (XRD) (Bruker D8 A25 DaVinci X-ray Diffractometer with CuKα radiation with LynxEye™ SuperSpeed Detector [ 25 , 26 ] (Bruker Corporation, Billerica, MA, USA). In this study, a high-purity Al metal (99.9%) was used for a trial.…”

Valorization of Aluminum Dross with Copper via High Temperature Melting to Produce Al-Cu Alloys

“…The produced metal and slag were separated after breaking the crucibles and subjected to a structural characterization by SEM/EDX. Moreover, the produced final slags from experiments 2 and 3 were studied by X-Ray Diffraction (XRD) analysis using a Bruker D8 A25 DaVinci X-ray Diffractometer with CuKα radiation with LynxEye™ SuperSpeed Detector (Bruker Corporation, Billerica, MA, USA) [ 26 , 27 ].…”