Development and utilization of fly ash

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(1) Definition of fly ash: In a narrow sense, fly ash is the powdery residue carried out of the boiler by the flue gas after coal powder is burned. It is a siliceous or silico-aluminous material and also constitutes a high-quality, environmentally friendly building material. It possesses a slight cementing property; in China it is called fly ash, while abroad it is referred to as slag or flying ash； Broadly speaking, it also includes the slag discharged from the bottom of the boiler, commonly referred to as slag. (2) Composition and properties of fly ash Table 1 Main chemical composition of fly ash Component SiO2 Al2O3 Fe2O3 CaO MgO SO3 Content (%): 38–54 23–38 4–6 3–10 0.5–4 0.1–1.2 The main oxide components of fly ash in China include SiO2, Al2O3, Fe2O3, CaO, TiO2, MgO, K2O, Na2O, and SO3; the order of their contents generally follows that listed above. The composition of fly ash varies depending on factors such as the origin of the coal, the method and degree of coal combustion, etc. The main chemical composition values are shown in Table 1. The composition of fly ash varies widely, which determines the differences in its properties. Statistical data show that the basic physical properties of fly ash in our country are shown in Table 2. Table 2 Physical properties of fly ash Property Density (g·cm-3) Bulk density (g·cm-3) Specific surface area (cm2·g-1) Melting point (℃) Results 1.9-2.9 531-1261 1180-6530 1250-1450 Fly ash can be classified into two types based on particle size: ordinary type (0.105 mm) and ultra-fine type (0.042 mm) ； In terms of phase composition, there are both crystalline minerals such as mullite, magnetite, and quartz, as well as amorphous glass phases, amorphous carbon, and secondary limonite; the glass phase accounts for over 50% of the total ； Based on their shape, they can be divided into hollow (i.e., tubular) and solid microspheres. (3) Activity of fly ash: The activity of fly ash includes two aspects: physical activity and chemical activity. Physical activity is the sum of effects such as fly ash particle effects and micro-aggregate effects; it refers to all physical effects that are unrelated to the properties of the chemical elements themselves, yet can enhance the cementing activity of the products and improve their performance (such as strength, impermeability, and wear resistance). It is the most practically valuable reactivity that enables fly ash to be fully utilized directly, and it is the main source of early reactivity. Chemical reactivity refers to the property in which components such as soluble silica and aluminum oxide slowly react with water and lime at room temperature to form insoluble, stable calcium aluminosilicate salts; it is also known as pozzolanic reactivity. The activity of fly ash is a comprehensive reflection of the particle size, morphology, degree of vitrification, and composition of fly ash particles, and it is also an important parameter determining its application value. The activity level of fly ash is not constant; it can be activated through artificial means. The three commonly used methods are as follows: mechanical grinding, hydrothermal synthesis, and alkaline activation. Among them, the alkali solution activation method has the strongest effect on fly ash. (4) Current development and utilization of fly ash: China is a country with abundant coal resources, with approximately 70% of the coal being used for thermal power generation. In 2000, the amount of ash generated was 160 million tons. From the establishment of the country in 1949 to the end of 2000, the total amount of ash stored in thermal power plants across the country reached 2.2 billion tons, with ash storage sites occupying 440,000 mu of land and fertile farmland. Research on the comprehensive utilization of fly ash dates back to the 1920s, but it was not until the 1950s that it received attention. Our country established specialized agencies in the 1960s to carry out work in this area. The initial direction of development was the production of building products and construction materials, but progress was slow. Later on, the technologies for utilizing fly ash became increasingly mature. Besides being used in construction materials, it is also widely applied in agriculture, transportation, industry, water management, and other fields. Its specific applications are as follows: (1) Use in construction projects. Currently, this is the field where the most amount of fly ash is utilized in China and around the world. Fly ash is primarily composed of silica and aluminum oxide; therefore, it can replace clay components in mixtures used for the production of cement and concrete. At the same time, fly ash can also be used to produce ceramsite, mortar, etc. (2) Applications of fly ash in agriculture, forestry, and animal husbandry. The use of fly ash in these sectors involves improving soil quality and using it as a covering material for land preparation, thereby promoting crop cultivation with the aim of increasing yields, improving the ecological environment, and cultivating high-quality forage. (3) Applications of fly ash in the chemical industry: Fly ash can be used to produce molecular sieves and water-repellent powders; it can also serve as a filler for polymer materials, as a filling agent in the production of asphalt shingles, and as a raw material for recovering valuable elements. (4) The fine utilization of fly ash: Fly ash is a mixture containing a variety of substances such as iron, floating particles, and sinking particles. Its effective utilization involves separating these components one by one and making full use of those with high added value, based on their respective properties, so as to make the best use of everything available and further enhance the economic value of the comprehensive utilization of fly ash. (5) Application of fly ash in wastewater treatment. Fly ash can be used to treat domestic wastewater, urban wastewater, printing and dyeing wastewater, wastewater from the chemical fiber and pulp and paper industries, as well as other industrial wastewaters; for example, it can be used to treat fluoride-containing wastewater, chromium- and phosphorus-containing wastewater, and to purify oil extraction wastewater and recycled waste lubricants. Xu Sen et al. used fly ash to treat domestic wastewater, achieving a significant removal of organic matter. The pH value and temperature of the wastewater had little impact on the treatment efficiency; the greater the amount of fly ash used, the better the treatment effect. However, when the amount of fly ash added exceeded 1% of the volume of wastewater to be treated, the improvement in treatment efficiency slowed down. Zhu Hongtao et al. used a combination of fly ash and iron filings to treat printing and dyeing wastewater; this method relies primarily on electrochemical processes, along with mechanisms such as reduction, physical adsorption, and coagulation. Experimental studies were conducted to determine the optimal conditions for practical application, and the results showed that the removal rates of COD and color in the treated printing and dyeing wastewater reached 77% and 95%, respectively. Liu Han used modified fly ash to treat oily wastewater, enabling it to meet discharge standards. It mainly discusses the wastewater treatment mechanism of fly ash and its modification methods. Using modified fly ash to treat oil-containing wastewater at both high and low concentrations yielded satisfactory results. Li Yafeng et al. conducted experimental studies on the treatment of high-concentration fluoride- and phosphorus-containing chemical wastewater using lime and fly ash, examining the effects of factors such as the amount of lime used, stirring time, and contact time, while also determining the adsorption capacity of fly ash. The experimental results show that the lime coagulation-precipitation followed by fly ash filtration process is effective for treating high-concentration fluorine- and phosphorus-containing chemical wastewater; after treatment, the levels of fluoride, phosphorus, and all other parameters meet the **discharge standards. As research progresses and technology advances, the ways and fields of application will gradually expand. Research and development on fly ash in our country began early, which enabled the application technologies related to fly ash to be widely adopted on a relatively reliable basis. However, much work remains to be done to further improve the value and utilization level of fly ash. This post was last edited by ldhappyxun2006 on 2007-12-29 20:52.]