Produção de membranas cerâmicas do tipo fibra oca utilizando pentóxido de nióbio

Abstract

The membrane separation process has gained emphasis and has been used in a wide variety of applications over the years, such as filtration and gas separation processes. Ceramic membranes are being developed and used in processes that require high temperatures and chemical stability, conditions in which the polymer membranes fail. Ceramic membranes with hollow fiber geometry present a larger area per volume of membrane, which guarantees a higher productivity when compared to flat membranes. Brazil is the world's largest producer of niobium pentoxide (Nb2O5), being the application of this compound little explored in the country. Therefore, in this study, Nb2O5 is proposed for the first time as the starting material for the production of ceramic hollow fiber membranes by the phase inversion and sintering process. The membranes were made using high purity Nb2O5 powder as received and after wet milling process. TG/DTA analyzes showed the thermal stability of the material with increasing the temperature and confirmed its high purity. XRD and Raman spectroscopy analyzes revealed the same monoclinic crystalline phase, H-Nb2O5, for the ceramic material at all the sintering temperatures evaluated. The as received material had a high average particle size (D50 of 42.9 μm), which caused the formation of porous and fragile membranes. The membranes produced with the ground material and no air-gap region formed filaments from the outer and inner surfaces of the precursor, and a sponge region remained at the center. Air-gap of 5 and 15 cm formed a filamentous region near the inner surface and a sponge region from the outer surface of the membranes. The increased in the sintering temperature promoted a process of grain densification and pore closure, increasing the mechanical resistance of the produced membranes and reducing their water permeability. The membrane produced without an air-gap region and sintered at 1200°C had a lower mechanical strength (7.91 MPa) and a higher water permeability (3.83 L h-1 m-2 kPa-1) when compared to the membrane produced with 5 cm of air-gap (24.2 MPa and 2.32 L h-1 m-2 kPa-1). The membrane made with ground powder, air-gap of 5 cm and sintered at 1100°C showed the highest water flow (7.38 L h-1m-2 kPa-1) while the membrane produced with ground powder, 5 cm of air-gap and sintered at 1300°C presented the highest mechanical strength, 93.72 MPa. Thus, Nb2O5 presents as a promising material for the production of ceramic hollow fiber membranes, being suggested the use of the powder with average particle diameter of less than 1 μm and sintering temperature between 1200 and 1300°C in order to guarantee mechanical resistance and permeability.