Pyrometallurgical and Hydrometallurgical Treatment of Calcium Aluminate-containing Slags for Alumina Recovery
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The pyrometallurgical and hydrometallurgical part of the Pedersen process has been studied. The process can be seen as one of the most potent alternative processes to recover alumina from bauxite more sustainably than the well-known Bayer process as it does not produce bauxite residue (red mud), which has numerous issues concerning its disposal and environmental risk. Pyrometallurgical part The thermodynamics and characterization of calcium aluminate slags and pig iron produced from the smelting-reduction of low-grade bauxites have been studied. Coke and limestone are used to reduce the iron oxide and adjust the basicity of slag during smelting. It is observed that complete iron separation from bauxite is feasible through the smelting-reduction process, and up to 99.9 wt.%Fe can be eliminated. Moreover, it is shown that silicon, titanium and other elements partial separation from the Al2O3-containing slag occurs. The phase compositions and the distribution of elements between the metal and slag phases also provide information about the high-temperature behavior of the bauxite components during smelting-reduction. The phase composition of the obtained slag shows that the slag has leachable phases, i.e., 12CaO·7Al2O3 and CaO·Al2O3, that are necessitated for the next leaching process in the Pedersen process. Moreover, to investigate the stability of the 12CaO·7Al2O3 phase in a CaO–Al2O3 slag at room temperature, a synthesis of crystalline slags of the phase from a corresponding melt composition in different atmospheric conditions and different purities are studied. Observations using a thermogravimetry coupled with differential thermal analysis showed that the dehydration of a zeolitic 12CaO·7Al2O3 phase occurs at 770 – 1390 °C before it congruently melts at 1450 °C. X-ray diffraction (XRD) pattern of the slag showed that a single 12CaO·7Al2O3 phase is produced from a mixture, which has small SiO2 impurity with 49:51 mass ratio of CaO to Al2O3. Scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA) results show that a minor Ca-Al-Si-O-containing phase is in equilibrium with a grain-less 12CaO·7Al2O3 phase. The 12CaO·7Al2O3 phase 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 higher purity CaO–Al2O3 mixture showed that 5CaO·3Al2O3 phase is an unstable/intermediate phase in the CaO-Al2O3 system, which is decomposed to 12CaO·7Al2O3 above 1100 °C upon heating in oxidizing conditions. It is proposed that low concentrations of silicon stabilize 12CaO·7Al2O3 (or also known as mayenite mineral), in which silicon is a solid solution in its lattice, which is named Si-mayenite. Regarding the calculated CaO-Al2O3-SiO2 diagram in the study, this phase may contain a maximum of 4.7 wt.%SiO2, which is depending on the total SiO2 in the system and the Ca/Al ratio. Hydrometallurgical part The leaching characteristics, kinetics and mechanism of both synthetic CaO-Al2O3 and real slags in alkaline solution at atmospheric pressure have been studied. The purpose of the study is to have a better understanding of the leaching part of the Pedersen process. The crystalline slags containing CaO·Al2O3, 3CaO·Al2O3, CaO·2Al2O3, and 12CaO·7Al2O3 phases, and leaching residues (predominantly CaCO3) are characterized by XRD and semi-quantitative analysis. Of the leaching characteristics in a solution containing 120 g/L Na2CO3, the slag with the highest amount of 12CaO·7Al2O3 phase is the most leachable one in the CaO-Al2O3 system with about 95% of alumina extraction. The leaching extent is confirmed by employing Inductively Coupled Plasma-High Resolution-Mass Spectrometer (ICP-HR-MS) analysis, and it decreases by 0.4% for every percent of the bayerite (Al(OH)3) formation during the leaching. The less stable form of CaCO3, i.e., vaterite, is formed over the leached slag particles that consist 33 – 49 wt% CaO, while 3CaO·Al2O3·6H2O, a hydrogarnet phase, precipitated at relatively low concentrations (< 6 wt%) in all residue. The non-bridging oxygen (NBO) over tetrahedral structure (T) index shows that the atomic structure may affect the leaching extent of the slags, the lower NBO/T index of the phase is the more difficult for the phase to leach or depolymerize. However, the 12CaO·7Al2O3 phase is an exceptional case where it has “free” O-ions at the center of the cage structure, which makes it easily depolymerize, therefore, the NBO/T index for the 12CaO·7Al2O3 phase becomes irrelevant. Furthermore, the morphology and size evolution of the obtained residue measured with a laser particle analyzer indicates the agglomeration behavior of the residue particles during the leaching process. In an investigation on leaching kinetics and mechanism, the highest alumina recovery up to 90.5 % is obtained after the slag is leached by 10 wt.% Na2CO3 solution, at low temperatures (30 – 45 °C) within 90 min. It is shown that the rate of alumina recovery is high at the beginning of leaching and is then slow down due to the calcite layer product nucleation and growth at the surface of slag. The wet-grinding leaching and vigorous stirring increase the possibility of the collision between both particles and the stirrer that breaks the calcite layer, yielding less residue agglomeration and better recovery compared to the slow and mild agitations. A surface observation of the slag using electron microscopy shows that the calcite starts to nucleate at the unleachable phase as the best deposition site which has the least mass transfer barrier in the system. The apparent activation energy of the leaching reaction is calculated as 10.8 – 19.9 kJ/mol which indicates the reaction is diffusion rate-limited as revealed by the applied kinetic models.