Elaboration and Characterization of Mullite Refractory Products from Moroccan Andalusite
Enrichment of schist mined in Morocco resulted in an extract essentially made of andalusite, quartz and muscovite, which was investigated with the aim to synthesize mullite refractory materials. X-ray diffraction analysis was used to assess phase transformations upon heating at different temperatures. Mullite formation starts as early as 1200℃ and seems almost total at 1450℃. This process correlates with a weak shrinkage (3.5% at 1450℃) and the formation of an important glass phase. Green compacts obtained by unidirectional pressing the extract powder were heated under air in the temperature range 1200℃ - 1600℃. Chemical and mechanical properties of these materials were investigated and correlated with their microstructure.
Valorization of Finnish mining tailings for use in the ceramics industry
The present study valorized Finnish mining tailings waste to identify opportunities for the use of ceramics technologies. On the basis of their mineralogical and chemical contents, the five selected tailings wastes represented felsic mining tailings (FMT) rich in quartz and alkali feldspars, mining tailings dominated by Mgand Fe-bearing minerals (MgFeMT), and mining tailings rich in carbonate minerals (CMT). Preliminary pilot studies indicated that the FMT materials are potential secondary raw materials for mullite-type ceramics. An Al additive was needed, since the Al2O3 content of the studied tailings was too low for mullitization. In addition, carbonate-bearing tailings with Ca silicates can be applicable for chemically bonded phosphate ceramic (CBPC) synthesis. Based on a literature review, FMT are viable source materials for the production of geopolymers, but a high initial Si:Al ratio (in quartz-rich FMT) may lead to partial geopolymerization. Preliminary results from the geopolymerization of pre-heated phlogopite mica mixed with metakaolin gave promising findings, with the formation of a geopolymer having good compressive strength. The findings support the viability of MgFeMT materials rich in phlogopite mica for the production of alkali-activated ceramics.
Behavior of paramagnetic iron during the thermal transformations of kaolinite
Kaolinites with various degrees of structural order and iron content were heated and subsequently analyzed via electron paramagnetic resonance. Iron was present in two different states in the heated materials, either as dilute structural Fe 3+ ions or in concentrated Fe 3+ phases. During metakaolinization, the environment of dilute Fe 3+ ions changed, following modifications of the Al 3+ coordination, and the Fe 3+ concentration increased. With the breakdown of metakaolinite, the diffusion of Fe 3+ ions induced their exsolution in superparamagnetic iron-rich domains (Fe 3+ clusters in γ-Al 2 O 3 and/or Fe 3+ oxide nanophases), which produced a decrease in the dilute Fe 3+ concentration. The subsequent breakdown of γ-Al 2 O 3 and the formation of mullite made the dilute Fe 3+ concentration increase again, because of the incorporation of Fe 3+ ions in the mullite structure.