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The microstructure of inorganic polymers (IP) formed from fayalite slag was investigated as a function of the composition of different activating solutions. The starting slag was 80 wt% amorphous, and after activation using sodium silicate solutions with varying SiO2/Na2O molar ratios, the amorphous phase dissolved and a binder phase was formed. The morphology of this binder, including the population and size of remnant particles and pores, was dependent on the particular activating solution used, and became denser as the level of silicate rose. 57Fe Mössbauer spectroscopy revealed that the IP synthesis reaction is combined with the oxidation of Fe2+ from the fayalite slag to Fe3+ in the inorganic polymer binder. The reaction extent varied and could be quantified using the absorption areas of these ions. Data corroborate that the Fe2+ ions in the amorphous part of the fayalite slag and the Fe3+ ions in the new binder phase had an average oxygen‐coordination number of 5.

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Abstract:The purpose of present work was to provide mineralogical insight into the rare earth element (REE) phases in bauxite residue to improve REE recovering technologies. Experimental work was performed by electron probe microanalysis with energy dispersive as well as wavelength dispersive spectroscopy and transmission electron microscopy. REEs are found as discrete mineral particles in bauxite residue. Their sizes range from <1 µm to about 40 µm. In bauxite residue, the most abundant REE bearing phases are light REE (LREE) ferrotitanates that form a solid solution between the phases with major compositions (REE,Ca,Na)(Ti,Fe)O 3 and (Ca,Na)(Ti,Fe)O 3 . These are secondary phases formed during the Bayer process by an in-situ transformation of the precursor bauxite LREE phases. Compared to natural systems, the indicated solid solution resembles loparite-perovskite series. LREE particles often have a calcium ferrotitanate shell surrounding them that probably hinders their solubility. Minor amount of LREE carbonate and phosphate minerals as well as manganese-associated LREE phases are also present in bauxite residue. Heavy REEs occur in the same form as in bauxites, namely as yttrium phosphates. These results show that the Bayer process has an impact on the initial REE mineralogy contained in bauxite. Bauxite residue as well as selected bauxites are potentially good sources of REEs.

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Clustering of alumina inclusions during liquid processing of steel significantly influences its cleanliness and mechanical properties. We have therefore studied the effect of alumina inclusion morphology on their clustering behavior in molten iron. Alumina inclusions were extracted from iron samples taken at 1 min after Al addition. Dendritic, spherical, plate-like, faceted and clustered alumina inclusions were identified and their clustering degrees were measured. The clustering degree increases in the order of spherical, dendritic, plate-like and faceted inclusions. To explain this, attractive force between two alumina particles with different shape combinations, i.e., sphere-sphere (S-S), sphere-plane (S-P), plane-plane (P-P), was calculated based on the theory of spontaneous cavitation. The attractive force is influenced significantly by particle shape. S-S type has the smallest attractive force and the shortest acting length. P-P type has the largest attractive force and the longest acting length. This explains that spherical inclusions have the lowest clustering degree. The lower clustering degree of plate-like inclusions, compared with faceted inclusions, is due to that molten iron wets plate-like inclusions better.

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