Iyer and Scott (200 1) made a comprehensive summary of the high value-added utilization of fly ash, including the preparation of zeolite, the synthesis of mullite, the production of glassy materials, the preparation of composite materials, the adsorbent used for waste treatment, the solidification of waste, the recovery of useful metals and mineral materials, and the application of fly ash in agriculture. However, as they pointed out in their conclusion, most of the research results in these important development fields are still in the laboratory stage, and a lot of work needs to be done to realize industrial production. Querol et al. (2002) made a special comment on the research progress of preparing zeolite from fly ash, pointing out that although the proportion of zeolite synthesized from fly ash is small, this research has attracted much attention because of the environmental benefits of zeolite.
China has also made outstanding contributions to the utilization of fly ash, and a large number of patented technologies have appeared (Wei Rongsen, 2004). In the field of high value-added utilization of fly ash, there are mainly synthesizing mullite (Shao et al., 1997), preparing zeolite (Wang Deju et al., 2002), manufacturing composite materials (et al., 2005), extracting alumina (Zhao et al., 2003) and producing compound fertilizer (et al., 2002). Querol et al., 2002; Chandra et al., 2005; Rohatgi et al., 2006).
The following focuses on the synthesis of mullite and cordierite from sintered fly ash.
(1) Synthesis of Mullite
Mullite is a mineral, named after it is produced in Mur Island in northern Scotland. Mullite has high melting point (about 1890℃), high shear modulus, good creep resistance, thermal shock resistance and corrosion resistance, and is widely used in refractory and ceramic industries. Schneider et al. discussed in detail the chemical composition, crystal structure, physical and chemical characteristics, industrial synthesis and utilization of mullite and mullite products in mullite ceramic in 1994. In 2008, combined with recent research results, the structure and characteristics of mullite were summarized (Schneider et al., 2008). Table 1. The thermo-mechanical properties of mullite and other advanced oxide ceramics are listed.
Table 1. Thermomechanical properties of mullite and other advanced oxide ceramics
(According to Schneider et al., 2008)
1cal = 4。 184J .
At present, the consumption of fused mullite in the world is (1 ~ 2) × 104t/a, and that of sintered mullite is (50 ~ 1 ~2) × 104t/a (Zhang Xiuqin et al., 2002). Because mullite is a product formed under special conditions of high temperature and low pressure, it is extremely rare in nature, and no deposits with industrial value have been found so far. Mullite for industrial use comes from artificial synthesis, including sintered mullite, fused mullite and chemical mullite. The main raw materials used are silica, kaolinite, bauxite or industrial alumina. Synthesized according to the theoretical ratio of mullite. Among them, mullite synthesized by sintering natural raw materials accounts for the vast majority of industrial mullite production.
There are not many literatures about the synthesis of mullite from sintered fly ash at home and abroad, and the earliest research can be found in the literature of Ohtake et al. (199 1). The method comprises the following steps: mixing the treated fly ash and γ-Al2O3 according to the ratio of 1: 1 and heating to1400 DEG C, with the content of 80%. Huang et al. (1994) and Huang et al. (1995) mixed F-treated fly ash with Al2O3 in the approximate ratio of 1: 1 in the range of 65400 ~/600℃, and more than 85% mullite was synthesized. Jung et al. (200 1) also used the term containing 8. 27% Fe2O3 and 3. 57% stoichiometric ratio of CaO and al2o 3(7 1. 8% alumina, 28%. 2% SiO2, Al2O3/ SiO2 mass ratio of 2. 55).
Domestic literature on mullite synthesis from fly ash appeared in 1994, mainly studying the relationship between mullite content and firing temperature and chemical composition (Chen et al., 1994). M50, M60 and M70 series mullite products can also be synthesized from the treated fly ash and industrial alumina, and some physical and chemical properties can even reach the national first-class mullite standard (Sun Junmin et al., 65438+). The production cost of mullite synthesized from fly ash and alumina is 20% ~ 30% lower than that prepared from kaolin and alumina (Zhou Zhonghua, 2003).
It is feasible to synthesize mullite from fly ash in theory and practice, and there are three main problems:
The first problem is that the content of impurities in fly ash is high, especially Fe2O3, CaO, MgO and TiO2, and the content of K2O and Na2O is often higher than that of conventional raw materials. Therefore, the synthesis of mullite from fly ash basically needs impurity removal, and the degree of impurity removal depends on the quality requirements and economic feasibility of the synthesized mullite. According to the national industry standard "All Natural Materials Sintered Mullite" (YB/T5267— 1999), some requirements are difficult to meet, such as the content of Fe2O3 is less than 1. 0%, the content of TiO2 is less than 2. 0%, K2O+Na2O content is less than 0. 3%. The quality index of mullite is set for bauxite sintering, while fly ash is an industrial waste discharged from coal burning in power plants, and there is no national standard for synthesizing mullite. Fortunately, the new national industry standard "Sintered Mullite" (YB/T5267—2005) implemented on February 1 2005 has been promulgated, and the maximum allowable amounts of Fe2O3, TiO2 and K2O+Na2O in the new standard are relaxed to 1. 5%, 3.5% and 2. 5% respectively. According to the experimental results, it contains 8. 27% Fe2O3( Jung et al., 200 1), or 2. 22% titanium dioxide (Jiangfeng Chen et al., 2007), or 2. 8% (Huang et al., 1995), or 0 .64% Na2O. However, materials with mullite content over 60% have good high-temperature thermal stability (Chen et al., 1994). Therefore, mullite synthesized from fly ash can also be widely used in refractory or ceramic industry.
The second problem is the addition of industrial alumina, because the Al2O3 content in fly ash is generally 15.2% ~ 36. 1%, with an average of 26. 1% (Jiangfeng Chen et al., 2005). A large amount of industrial alumina must be added to synthesize M50, M60 and M70 mullite, while in 2005 it was 1%. Therefore, it is necessary to use fly ash with high Al2O3 content or fly ash with improved Al-Si ratio after treatment, so as to obtain mullite products with high added value under economically feasible conditions.
The third problem is the synthesis conditions. Different researchers use different molding pressure, synthesis temperature and constant temperature time when synthesizing mullite from fly ash. This is because the chemical composition of fly ash in different power plants is different, and even the chemical composition of fly ash in the same power plant will change due to different coal sources, resulting in different amounts of industrial alumina added in the ingredients. So far, no economic and practical conclusion has been reached. The change of chemical composition of fly ash directly affects the proportion of raw materials for synthesizing mullite.
(2) synthesizing cordierite
Cordierite is also a raw material widely used in ceramics and refractory minerals. It is a low melting point (about 1470℃) phase in MgO-Al2O3-SiO2 system, and it has the advantages of extremely low thermal expansion coefficient and excellent thermal shock resistance in mullite and other advanced oxide ceramics (Chowdhury et al., 2007). Because cordierite also has high resistivity and low dielectric constant (table 1. 5), it is often used as a ceramic substrate instead of corundum substrate in microelectronics industry (Camerucci et al., 200 1, 2003).
There are two main synthetic raw materials for industrial cordierite: one is "clay+talc+(alumina or silica)" and the other is "clay +Mg (OH)2+ a small amount of additives" (Yalamac et al., 2006). Because the chemical composition of fly ash is similar to clay minerals, it can be used to synthesize cordierite instead of clay minerals.
Table 1. Comparison of properties between cordierite and corundum matrix
(According to Winnie et al., 1995)
Similar to the synthesis of mullite, although there are reports of synthesizing cordierite from fly ash, the quantity is very small. 1995, Sampathkumar et al. published the article "Synthesis of α-cordierite (Indian stone) from fly ash" for the first time in the Bulletin of Materials Research. The raw material they use is "fly ash+talc+alumina". According to the stoichiometric ratio of cordierite (MgO 13. 8%，Al2O3 334.8%，SiO25 1。 4%), an ideal cordierite mineral was synthesized at 1370℃, and XRD analysis showed that only cordierite phase existed. The material properties (including thermal expansion coefficient) of the synthesized samples are equivalent to those of cordierite synthesized from conventional raw materials.
Kumar et al. (2000) used "original fly ash+talc+alumina" and "treated fly ash+talc+alumina" to obtain relatively pure cordierite raw materials at 1350 ℃× 2 h, and used flotation and magnetic separation to remove carbon and iron from fly ash. The main crystalline phase of the obtained cordierite sample is α-cordierite, and the secondary crystalline phases are β-cordierite and mullite. In addition, Fe- cordierite phase with low diffraction peak intensity was also found in the samples synthesized from the original fly ash. The sample synthesized at 1350℃ has the highest density. The experiment also shows that mullite, spinel (Mg, Al) and α-Al2O3 are formed under the synthesis conditions of 9 15℃ ×2 h; β-cordierite, mullite, spinel (Mg, Al) and α-Al2O3 appear at 1200℃ × 2 h; 13 15℃ × 2 h, spinel phase disappears and a new α-cordierite phase appears, accompanied by β-cordierite and mullite. The density of cordierite synthesized by removing carbon and iron from fly ash is lower than that of cordierite synthesized from original fly ash, and its physical properties are equivalent to those of industrial cordierite, and its fracture modulus is better than that of industrial cordierite with the increase of temperature.
Goren et al. (2006) synthesized α-cordierite with only one crystal phase at 1350℃ × 3 h and 1375℃ × 1 h using "fly ash+talc+fused alumina and silica", and showed that sintering temperature and sintering time have the same important influence on the recrystallization of cordierite. When the sintering temperature is 1300℃ ×3 h, the obtained cordierite sample contains magnesia-alumina spinel and timely secondary crystals in addition to the main crystal phase α -cordierite
There is no report on the synthesis of cordierite from fly ash in China, and only a few documents are about the preparation of cordierite glass-ceramics from fly ash at the temperature of 1000℃ (Shao et al., 2004; How, 2005; Liu Hao et al., 2006), in addition to fly ash, the raw materials used are alumina, basic magnesium carbonate and quartz sand to make up for the deficiency of Al2O3, MgO and SiO2 in fly ash. The advantages of cordierite glass-ceramics prepared by this method are that cordierite has low dielectric constant, low thermal expansion and high strength, and excellent glass-ceramics can be obtained. The disadvantage is that the base glass or mother glass needs to be melted at 1500℃ × 2 h, then quenched, crushed and melted at least three times to ensure uniformity, then nucleated at 800 ℃× 2 h after molding and crystallized at1000 ℃× 2 h. The general technological process of preparing glass-ceramics by sintering method is: batching → melting → quenching → crushing → forming → sintering. The process is relatively complex and the energy consumption is high. Zhang Xuebin et al. (2006) used alumina with a content of 32. In the range of1100 ~1350℃, 40% pore-forming agent (starch) was added to trial-produce cordierite porous ceramics. The optimum sintering condition is1300 ℃× 4 h.
Compared with mullite (3Al2O3 2SiO2), cordierite (2mgo 2al2o35sio2) prepared from high-alumina fly ash has the greatest advantage that it uses abundant high-quality low-priced talc (or talcum powder) in China as raw material to replace the expensive industrial alumina (or bauxite) added in mullite preparation, which reduces the preparation cost. Because the content of Al2O3 in mullite minerals is as high as 7 1. The sulfur content in cordierite minerals is only 34. 8%. The production process of synthesizing cordierite from high alumina fly ash is similar to that of synthesizing mullite, but the problem is also the pretreatment of impurities in fly ash. But for cordierite synthesized from high alumina fly ash, MgO in fly ash is a favorable component, because the theoretical value of MgO in cordierite is 13. 8%.