Low-grade bauxite ores are not favorable in the conventional Bayer process for alumina production, as they are producing more bauxite residue (red mud) and accompanying lower alumina yield than high-grade ores. In the current study, the thermodynamics and characterization of calcium-aluminate slags and pig iron produced from smelting reduction of high iron-and silica-containing bauxites are studied. Coke and limestone are used to reduce the iron oxide and adjust the basicity of slag during smelting. There is evidence that complete iron separation from bauxite is feasible through smelting-reduction process, and up to 99.9 pct of iron can be eliminated. Moreover, it is shown that the partial separation of silicon, titanium, and other elements from the Al 2 O 3 -containing slag occurs. The phase compositions and the distribution of elements between the metal and slag phases provide information about the high-temperature behavior of the bauxite components during smelting reduction. Employing electron microscopy analysis, it is indicated that the morphologies of CaOAEAl 2 O 3 , 12CaOAE7Al 2 O 3 , 2CaOAEAl 2 O 3 AESiO 2 , and CaOAEAl 2 O 3 AESiO 2 phases in the slag, as well as the complex oxides of Ca-Al-Si-Ti in the slag behave differently as the mass ratio of Al 2 O 3 / (Fe 2 O 3 + SiO 2 ) in the bauxite changes. It is also shown that the phases of slag produced from smelting-reduction below 5 K s À1 of cooling rate are proper for further leaching process.

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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.

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