Study on the Effect of Binders on the Properties of Mullite Porous Ceramics for Flue Gas Filtration

. Fused mullite particles are used to prepare mullite porous ceramics by gel-casting method, using polysilicon waste and ρ-Al 2 O 3 powder as the binder, and starch as the pore forming agent. The content of the pore forming agent was 20%, and the ratio of the mass of fused mullite particles to the mass of the binder was 9:1, 8:2, 7:3, and 6:4, respectively. The effects of binder content on the properties of mullite ceramics in terms of phase composition, microstructure, apparent porosity, compressive strength, bulk density, pore size distribution, and filtration pressure drop were investigated. It was found that prepared porous mullite possessed relatively high apparent porosity (56.64-58.19%), and low bulk density (1.28-1.31g/cm 3 ). With increasing binder content, the sintering shrinkage rate increased from 2.07 to 5.92%, the compressive strength increased from 1.01MPa to 6.08MPa. Therefore, the preparation of mullite ceramics was a near net size preparation, and the prepared porous ceramics meet the requirements of flue gas filtration.


Introduction
The high-temperature flue gas emitted by power plants, steel plants, cement plants, and other factories contains many dust particles, which cause serious pollution to the human living environment. Therefore, flue gas filtration has become a key concern in modern society. Various filter membranes have gradually emerged, but ceramic membrane has become the preferred material for harsh environments due to their excellent chemical stability and high temperature resistance [1][2][3]. At present, many porous ceramic membranes are used as filter materials [4][5][6][7]. Therefore, a porous ceramic membrane is a promising candidate material for gas filtration applications.
The toxic and expensive chemical reagents such as tert-butanol (TBA), acrylamide (AM), and N,N'methylene-bisacrylamide (MBA) are often used as solvents, monomers, and cross-linking agents in the preparation of porous ceramics by gel-casting [8][9][10][11]. The use of these reagents will cause some harm to the human body. Therefore, it is an urgent problem to use a gelcasting method with environmentally friendly and lowcost solvents and binders to prepare porous ceramics.
This study selected deionized water as the solvent, which is easy to obtain, low-cost, and environmentally friendly. The selection of polysilicon waste and ρ-Al 2 O 3 is used as a binder, the selected adhesive does not cause any pollution to the environment and meets the social requirements of green development. At the same time, realizing the recycling and utilization of waste, reducing resource waste, and reducing economic pressure for waste treatment.

Raw materials
The ρ-Al 2 O 3 powers (Purity ≥90%, average particle size ≤1 μm, Chinalco Zhengzhou Research Institute) and polysilicon waste (Dv(50) = 6.71 μm, Jinzhou Dongsheng Silicon Technology Co., Ltd.) were used as the binders to synthesize doped sintered phase mullite. Commercial fused mullite particles (Purity ≥95%, Yixing Pengyang Refractories Co., Ltd., Wuxi, China) with a particle size distribution in the range of 200-300 μm were used as the main raw materials. Soluble starch (Tianjin Hengxing Chemical Reagent Manufacturing Co., Ltd.) was used as the pore forming agent and deionized water was used as the solvent.

Preparation process
The composition of polysilicon waste obtained by ICP analysis is shown in Table 1. In order to synthesize the mullite phase, the mass ratio of polysilicon waste to ρ-Al 2 O 3 powder was determined to be 4:17 based on stoichiometric calculations. The content of the pore forming agent is 20%, and the ratio of the mass of fused mullite particles to the mass of the binder is 9:1, 8:2, 7:3, and 6:4, respectively. According to the content of the binder, the samples are represented as S1, S2, S3, and S4, respectively. The details of the weight of various raw materials are shown in Table 2. Weigh the raw materials according to Table 2 and mix at a speed of 300 r/min for 30 minutes. Add deionized water, mechanical stirring, and ultrasonic oscillation for 20 minutes to obtain a uniform ceramic slurry. The slurry is poured into the mold, dried, demolded, and sintered at 1600 ℃.

Characterization
The composition of the sintered ceramic body was analyzed using X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany). The microstructure and morphology of porous ceramics were observed using field emission scanning electron microscopy (FE-SEM, Model Ultra Plus, ZEISS, Germany) together with X-ray energy dispersive spectroscopy (EDS, Oxford, UK). The apparent porosity and bulk density were measured by Archimedes method with water as the medium. The pore size distribution of the sample was detected using the mercury pore method (Autopore IV9500, Micromerics Instrument Corp., Norcross, GA, USA). The room temperature compressive strength was measured using a hydraulic press (Type 5015, Jinan Shijin Company, China) according to GB/T 5072-2008. The sintering shrinkage of the sample was measured using the vernier caliper. The sintering shrinkage was obtained by dividing the diameter difference before and after sintering by the diameter before sintering. A selfmade pressure drop tester was used to measure the pressure drop, and the inner pipe diameter of the device was 20 mm. The ceramic sample used for testing has a diameter of 20mm and a thickness of 10mm. The tested gas flow rate was 25-250L/h, and the flow velocity was 1.33-13.26m/min. Fig.1. XRD patterns of the samples of S1-S4 with different binder addition: (a) the green bodies of the samples of S1-S4;

phase composition
(b) mullite samples of S1-S4. Fig.1 (a) shows the XRD pattern of the green body with different binder addition. The main crystal phase is mullite. With the increases in binder content, the diffraction peak intensity of boehmite and bayerite phases gradually increased, which can be explained by the hydration reaction of ρ-Al 2 O 3 . The continuous formation of boehmite and bayerite phases reduces the fluidity of the slurry and makes the slurry gradually form gel. Fig.1 (b) shows the XRD pattern of the sintered sample. It can be seen that the phase composition of sintered sample materials is all mullite phase. Boehmite and bayerite form stable alumina after high-temperature treatment, and then react with silicon waste to form mullite. Meanwhile, the results show that the slurry formed after the hydration of ρ-Al 2 O 3 powder disperses uniformly. Fig.2 shows the microscopic morphology of mullite porous ceramics prepared with different binder additions. The surface of fused mullite particles is flat, evenly distributed as a matrix in the sample, and stacked to form a skeleton. The irregular mullite phase formed by polysilicon waste and ρ-Al 2 O 3 powder is uniformly distributed at the edges of fused mullite particles. The irregular mullite phase serves as a binder to tightly connect the skeleton formed by the fused mullite particles. As the binder content increases, the amount of fine particles covered between and on the surface of fused mullite particles gradually increases. This represents a more stable connection between the contact points and the fused mullite particles. Therefore, it helps to improve the strength of mullite ceramics.  Fig.3 (a) shows the apparent porosity and bulk density of the sintered sample. The results show that the apparent porosity ranges from 56.64% to 58.19%, and the bulk density ranges from 1.28 to 1.31g/cm3. High temperature sintering causes the reaction between binders to generate a mullite phase. The sintering process produces a certain degree of shrinkage, and the contact points of fused mullite are more tightly connected. As the binder content increases, the large pores between the fused mullite are occupied by the binder, resulting in a decrease in porosity. The large pores are filled with binder, resulting in an increase in bulk density. When the binder content is high, the porosity is 56.64%, which is still at a high level. This provides a possibility for the preparation of porous mullite with high permeability. Fig.3. Apparent porosity, bulk density, pore size distribution and average diameter of pore of S1-S4 with different binder addition: (a) apparent porosity and bulk density; (b) pore size distribution; (c) average diameter of pore. Fig.3 (b) shows the pore size distribution of porous mullite ceramics prepared with different binder contents. The pore sizes of the samples are mainly distributed in the range of 35μm to 120μm, and all of them show a unimodal pattern in the curve. Fig.3 (c) shows the average pore size decreased from 45.105μm to 16.291μm with the increase of binder content from 8% to 32%. The results show that porous mullite ceramics with large pore sizes can be prepared by gel casting.

Sintering behavior and pressure drop
A three-dimensional spatial structure formed by the mutual accumulation of large particles of fused mullite, mainly mixed with ρ-Al 2 O 3 powder and silicon waste as binders. The increase of binder enhances the formation of hydration products in the slurry and promotes the shrinkage of pore size after sintering. The content of the binder directly affects the porosity and pore size, which both affect gas filtration. Therefore, the content of the binder affects gas filtration by regulating porosity and pore size.  Fig.4 (a) shows the sintering linear shrinkage and compressive strength of porous mullite ceramics prepared with different binder contents. The linear shrinkage and compressive strength of the samples increased from 2.07% to 5.92% and from 1.01 to 6.08MPa, respectively, with the increase of the addition of binder. Although the shrinkage rate gradually increases, it is generally at a relatively low level, meeting the requirements for near-net size preparation. During the sintering process, the decomposition of boehmite and bayerite phases, as well as the formation of the mullite phase by combining with polysilicon waste at high temperatures, will result in shrinkage. The increase in binder will cause greater pore shrinkage, which also leads to an increase in sample shrinkage. The shrinkage of the sample after sintering can effectively enhance the binding degree between particles and improve the overall mechanical strength of the sample. Fig.4 (b) shows the pressure drop curves of mullite porous ceramics prepared with different binder contents in the flow rate range of 1.33-13.26m/min. It can be seen from the pressure drop curve that the pressure drop value of all samples increases with the increase of gas velocity. When the gas flow rate is 1.33m/min, the flow rate is at a lower level, and the pressure drop between samples does not exceed 1hPa. However, when the average gas velocity increased, the difference between samples increased significantly, from 1.62hPa to 12.53hPa. Therefore, porous ceramics for gas filtration can be prepared by adjusting the pressure drop value by selecting an appropriate binder content.

Conclusion
Based on the above results and discussion, the conclusions are obtained as follows: (1) Porous mullite ceramics were successfully prepared by gel-casting using fused mullite particles as the skeleton, ρ-Al 2 O 3 powder, and polysilicon waste as the binder, and starch as pore forming agent.
(3) By adjusting the content of the binder, porous ceramics with different pore sizes were prepared for flue gas filtration. This study provides a good approach and method for preparing environmentally friendly and pollution-free filtration ceramics, and further exploration of the filtration efficiency of different porous ceramics will be conducted in the future.