Fly ash is the fine ash collected from the flue gas after the combustion of the power plant boiler. The main components are silicon dioxide and aluminum oxide and a small amount of iron oxide, calcium oxide, magnesium oxide, sodium oxide, potassium oxide and sulfur oxide. . The massive consumption of coal produces a large amount of fly ash. According to statistics, China produces about 500 million tons of fly ash every year. As electricity consumption increases, fly ash emissions also increase year by year. my country’s fly ash output was 686 million t in 2017, 715 million t in 2018, 748 million t in 2019, 781 million t in 2020, and 925 million t in 2024. Unlike common industrial by-products, fly ash is complex in composition and can cause serious environmental problems if not handled properly. If my country can rationally utilize the accumulated fly ash as a resource, it can not only effectively solve the problem of environmental pollution, but also create huge economic value and realize the high-value utilization of resources.

Mineralogical properties of fly ash

In terms of mineralogy, fly ash is mainly composed of vitreous and crystalline phases, and also contains a certain amount of unburned carbon. Among them, the glass body components are mostly aluminosilicate glass bodies, which usually account for more than 60% of the mass fraction of fly ash; the crystal phase components of fly ash are mainly mullite, quartzite and iron ore. The proportion is the largest, accounting for about 6% to 15% of the total; unburned carbon generally does not exceed 5% in fly ash.

In general, the smaller the fineness of the fly ash, the smaller the loss on ignition, the larger the specific surface area, the more active sites and the higher the activity, and the fly ash with high activity is easy to react with other substances to form new substances. for resource utilization.

Extract valuable elements from fly ash

Extract aluminum. The main processes for extracting aluminum from fly ash are alkali sintering and leaching. Alkali sintering method refers to the method of adding alkali sintering agent to activate fly ash at high temperature, and then extracting aluminum. Limestone and soda lime are commonly used as sintering agents to extract aluminum. The lime sintering method is mature and easy to operate, but this method has a large amount of lime, high energy consumption, and a large amount of silica slag by-products; the soda lime method effectively reduces the consumption of limestone, reduces the reaction temperature, and improves the extraction rate. But the problem of silicon slag has not been completely solved.

The leaching methods mainly include inorganic acid leaching and alkali leaching. The research shows that the main factors affecting aluminum leaching rate in acid leaching process are acid type, acid concentration and characteristics of fly ash.

Extract silicon. Extraction of silicon from fly ash usually involves co-extraction of silicon and aluminum, followed by separation of the product. W.Yan et al. extracted Al2O3 and SiO2 in fly ash by acid first and then alkali process. First, Al2O3 in fly ash was activated with concentrated acid, and then calcined to obtain Al2O3 with a purity of 99.91%. SiO2, and finally obtain SiO2 with a purity of 99.52%. The silicon and aluminum elements extracted from fly ash can be further synthesized into aluminum-silicon alloys, and silicon can be used to prepare silica or adsorbents.

Extract gallium, germanium, lithium and other scattered metal elements. At present, the methods of extracting Ga, Ge, Li and other elements from fly ash mainly include leaching method, extraction method and adsorption method. F. Arroyo et al. used the leaching method to extract Ga and Ge from fly ash, and the yield was about 70%. The leaching effect of this method was good, but it was not suitable for fly ash with high silicon and aluminum ratio.

A variety of elements are extracted together. L. Wang et al. used carbon chloride method to extract valuable elements from high-alumina fly ash, and the extraction rates of Al2O3, SiO2, CaO and TiO2 reached 84.3%, 72.7%, 68.9% and 87.3%, respectively. n (aluminum) was 4.5:1.

Physical extraction of high value substances in fly ash

Extract unburned charcoal. The unburned carbon extracted from fly ash has the characteristics of porosity and low iodine value. It can be used to prepare activated carbon and used as an adsorbent to remove harmful substances such as mercury in flue gas. It can also be used as a substitute for filler or carbon black.

Extract the hollow microbeads. Hollow microbeads are a new type of particulate material developed in the 1950s and 1960s, mainly including magnetic beads, floating beads and sinking beads.

Preparation of zeolite. Fly ash contains a large amount of SiO2 and Al2O3, which is a suitable raw material for the synthesis of zeolite.

Preparation of new glass materials. Fly ash contains a large amount of glass phase oxides (SiO2, Al2O3, CaO) and a small amount of nucleating agents (P2O5, TiO2, Fe2O3), which can be used as alternative raw materials.

For making cement, concrete, bricks and blocks. As a building material, fly ash has several characteristics: easy analysis of properties, existence of spherical particles, ability to match with concrete, large amount of utilization, wide utilization range, and high utilization degree.

Preparation of geopolymers. Geopolymers are a new class of building materials, which are synthesized by polycondensation of SiO2 and Al2O3 as precursors. Fly ash contains a large amount of silicon and aluminum elements and has strong processability.

Application in chemical industry. Fly ash contains a large amount of oxides such as SiO2, Al2O3, TiO2, etc., and is a commonly used catalyst carrier.

applications in agriculture. Studies have shown that although fly ash contains a certain amount of heavy metals such as chromium, lead, and mercury that are harmful to crops, when the application amount is less than 4.5kg/m2, it will not adversely affect the soil.

Applications in papermaking.

Application in ceramics. Fly ash contains elements such as Al, Si, Fe and Ca, which can be used as an excellent raw material for ceramics, and the particle size is finer, which can save the crushing and grinding process and save time.

other. In addition to the above materials, fly ash can also be used to prepare ceramic inorganic fibers or rubber-plastic composite materials.

Using reasonable technical means to “upgrade” fly ash has important practical significance for protecting the environment, improving economic value and promoting sustainable social development. Although the comprehensive utilization technology of fly ash in my country is constantly improving, it is still in the initial stage, and there is still a big gap compared with developed countries. Therefore, it is necessary to enhance the awareness of innovation, increase investment in scientific research, and focus on product transformation and development.