Structural Design And Property Research Of Silicon Carbide Porous Ceramics Used For High-temperature Filtration

A high-temperature filtration system is a key subsystem of cleaning coal power stations, which affects the stable operation and generation efficiency of these power stations. The core component of the high-temperature filtration system is an asymmetrical porous ceramic candle filter, composed of support and filtration membrane. The working life, filtering efficiency, and filtering precision of a high-temperature filtration system are determined by the strength and pore structures of porous ceramic candle filters. The main disadvantages in the porous ceramic candle filters are their low strength and permeability at present.The aim of this thesis is to fabricate silicon carbide (SiC) porous ceramic supports with high strength and permeability and filtering membranes with low filter pressure drop. The influences of ceramic binder contents, green body molding pressures, SiC particle diameters, sintering temperatures, and pore former types on the microstructure of SiC porous ceramic supports have been systematically studied. The relationship of flexural strength and permeability to the microstructure parameters, like porosity, pore diameter, pore shape, and tortuosity, has been also deeply analyzed, and the preparation technology, which can simultaneously improve the flexural strength and permeability of SiC porous ceramic supports, has been found out. The results showed that the SiC porous ceramic support with the flexural strength28.16MPa, porosity38.7%, darcy permeability1.99×10-11m2, and non-darcy permeability1.16×10-7m can be obtained using the preparation technology parameters of SiC particle diameter239μm, ceramic binder content20wt%, pore former made up of graphite and active carbon10wt%, green body molding pressure6MPa, and sintering temperature1300℃Three measures have been used to improve the structural homogeneity and pore connectivity of SiC porous ceramic supports. First, the SiC particles used for fabricating porous ceramic supports were reshaped by ball milling to increase the flowability of SiC particles in the process of preparing support green body. Second, a design method of ceramic binder according to ternary phase diagrams is given. The ceramic binder of K2O-SiO2-Al2O3system with a controllable melting point and forming phases was designed to match the physical properties of SiC. Finally, a preparation technology was designed to homogeneously distribute the ceramic binder fine powder on the surface of SiC particles and particle necks. SiC particles were first coated by the ceramic binder fine powder, and then coated by flake graphite. The SiC porous ceramics fabricated by such a SiC particle composite have high pore connectivity and permeability.It is necessary for filtration membranes to have homogeneous structure and intact surface for effective filtration. A filtration membrane including a fiber transition layer composed of mullite fibers and aluminosilicate fibers was designed to lie between the support and the filtration membrane, which was able to decrease the thickness of the filtration membrane in the membrane fabricating process on the condition of keeping the filtration membrane intact and homogeneous. Moreover, the thickness of the filtration membrane became more controllable and the filter pressure drop also decreased, which is helpful to increase the filter life.