Porous ceramics are-widely used in a variety of environmental fields such as solid-liquid separation and hot gas filtration due to their high mechanical strength, excellent chemical resistance, and high thermal stability. Conventional porous ceramics are usually made of particle-packing of inorganic powders, which are usually characterized by a low porosity, a high self-weight, a low permeability and a high filtration resistance. It is thus necessary to develop novel porous ceramics with a high porosity, a high mechanical strength and a low packing density. Mullite whisker-based porous ceramics have attracted recent attention in the treatment of various wastewaters. Compared with porous ceramics prepared by conventional particle-packing, mullite whisker-based porous ceramics are constructed by whisker interlocking, resulting in fibrous structures with a high porosity, a high pore connectivity and a low packing density. However, these mullite porous ceramics are seldom used in industry due to the high cost of the high-purity starting materials. Fly ash is a major by-product of coal combustion. The elemental analysis shows that fly ash is mainly composed of unburned carbon, metal oxides (i.e., Si, Al, and Fe) and other inorganic materials. Fly ash can be used to prepare mullite porous ceramics due to the presence the high content of aluminum silicates. However, the effect of holding time on the growth of mullite whiskers and the permeance-selectivity of the fibrous structures is still unclear. In order to elucidate the growth mechanism of mullite whiskers based on gas-solid reaction, the effects of sintering temperature and holding time on the microstructure, phase composition and filtration properties of the porous ceramics were investigated.

Fly ash powder provided by Huaneng Thermal Power Plant in Zibo, China, and Al(OH)₃ purchased from Sinopharm Chemical Reagent Co. Ltd., China, were used as major materials. AlF₃·3H₂O purchased from Sinopharm Chemical Reagent Co. Ltd., China, was employed as a catalyst for mullite whisker growth. In order to prepare porous ceramics with a high fracture strength, sodium carboxymethyl cellulose (CMC) from Sinopharm Chemical Reagent Co., China was chosen as a binder. Fly ash, Al(OH)₃, AlF₃·3H₂O and CMC were mixed and ground in a ball mill, and then the slurry was dried with a rotary evaporator and sieved through a stainless steel sieve to obtain a homogeneous mixture. After dry pressing, the green body was sintered in a muffle furnace. The effect of holding time on the formation and growth of mullite whiskers was elucidated at different holding time (i.e., 2, 4 h and 6 h). The microstructure of the porous ceramics was determined by scanning electron microscopy (SEM). The phase composition of the porous ceramics was analyzed by X-ray diffraction (XRD). The selectivity of the porous ceramics was investigated by oil-in-water emulsion separation experiments.

To clarify the growing mechanism of mullite whiskers based on solid-vapor reaction, the influences of sintering temperature and holding time on the microstructure, phase composition and filtration performance of the porous ceramics are investigated. The gas AlF₃·3H₂O acts as a catalyst for the formation and growth of mullite whiskers. The thermogravimetric analysis curves indicate that the mullite whisker growth follows a gas-solid reaction mechanism, where AlF₃·3H₂O acts as a catalyst for the formation and growth of mullite whisker. The mass loss of the samples sintered at different temperatures is similar while the shrinkage increases from 2.0% to 4.2% as the holding time increases. A liquid phase formed during the sintering process enhances the particles rearrangement, the mass diffusion and migration, and the densification of the specimens. The XRD patterns indicate that a high sintering temperature and a long holding time were favorable for the formation of the mullite phase. The whisker aspect ratio is related to the sintering temperature and the concentration of the atmosphere provided by the additives. The analysis of the length-to-diameter ratio of the mullite whiskers reveals that the increase in holding time favors the growth of mullite whiskers. Based on the analysis of the pore structure of the porous ceramics, the sintering temperature is a key factor affecting the porosity, water absorption and pore size of the porous ceramics, while the holding time has a slight effect. According to the results of flux tests on the specimens sintered at 1 200 ℃ for 6 h, the permeation flux decreases from 210.61 L/(m²·h) to 192.35 L/(m²·h). When the trans-membrane pressure increases from 0.1 bar to 0.4 bar, the oil rejection of the porous ceramic reaches 90%.

The sintering temperature affected shrinkage of the porous ceramics. For the specimen sintered at 1 100 ℃, only a slight shrinkage of approximately 0.2% occurred during sintering. For the specimen sintered at 1 200 ℃, the shrinkage increased from 2.0% to 4.2% due to the formation of silica-rich liquid phase. Also, the high sintering temperature favored the formation of mullite phase. The growth of mullite whiskers increased both in porosity and pore size of the whisker-constructed porous ceramics. With the growth of mullite whiskers, the porosity of the porous ceramics increased from 56.4% to 57.6% for the specimen sintered at 1 200 ℃, and the average pore size increased from 0.75 to 1.24 μm. The emulsion test indicated that the permeate flux decreased from 210.61 to 192.35 L/(m²·h) for the specimen sintered at 1 200 ℃. Oil rejection of the porous ceramics decreased from appropriately 90% to 87% as TPM increased from 0.1 to 0.4 bar.