Role of composition and oxygen partial pressure on microstructural and crystalline phase evolution in aluminosilicate derived aggregates

Spherical aggregates, approximately 1mm in diameter, derived from either kaolinite or bauxite are used in tonnage quantities to aid the extraction of oil and natural gas. Aggregates intended for this application are referred to as proppants and key characteristics include low density and high strength, which are influenced by processing temperature and variation in raw ore chemistry. Kaolinite and bauxite ores doped with varying concentrations of K2O and Fe2O3 were sintered at different temperatures to elucidate composition-processing-property relationships. The dopants are known to form low temperature ternary eutectics with Al2O3 and SiO2 and are anticipated to facilitate low temperature densification and enhance mullite formation. In addition, proppants doped with Fe2 O3 were studied under varying oxygen partial pressures to further enhance low temperature densification by manipulating the valence state of iron.;Microstructure and crystalline phase assemblage were evaluated by scanning electron microscopy and X-ray diffraction, respectively, and correlated with results of density and strength measurements obtained by gas pycnometry and diametral compression, respectively. Results indicate that dopant enhanced densification can simultaneously improve strength and processing economy of proppants by lowering sintering temperatures up to 100°C. Controlled atmosphere studies revealed that manipulating the valence of iron produces unique microstructures that may be useful in a number of different applications. These microstructures include aggregates with metallic coatings and aggregates with core-shell microstructures, which exhibit a porous core enclosed by a relatively dense outer shell.