Comprehensive Utilization of Fly Ash in Environmental Restoration

In the transformation and development path of coal resources, the comprehensive utilization of bulk solid waste and other methods can effectively reduce carbon emissions and realize efficient utilization of resources. Among them, the discharge of fly ash remains high, but its comprehensive utilization scale is also steadily increasing. The comprehensive utilization rate of fly ash in my country is about 69%. It is mainly used in the production of cement, road construction, agriculture and mineral extraction, etc. It has played an important role in realizing the goal of “reduction, resource utilization and harmless” of bulk solid waste in my country. effect. With the deepening of relevant research, scholars have found that fly ash has high value-added space for environmental restoration. Therefore, this article summarizes the comprehensive utilization and principles of fly ash in the fields of air purification, soil remediation, and wastewater treatment, aiming to provide a reference for the application of fly ash to remediate environmental pollution.

Physical and chemical properties of fly ash
Fly ash is a powdery solid waste that is discharged with flue gas after coal is melted at high temperature in a boiler. Due to differences in coal types, origins, and combustion conditions, the physical properties of fly ash fluctuate greatly. Fly ash is mostly gray and black spherical particles and amorphous particles, with a rough surface, a particle size range of 0.5μm~300.0μm, a density of 1.9g/cm3~2.9g/cm3, and a void rate of 40%~50%. The specific surface area is 300m2/kg~500m2/kg. The chemical composition of fly ash is mainly Al2O3 and SiO2, followed by Fe2O3, CaO, MgO and other oxides. The main phase is composed of porous glass and carbon particles. The crystal phase is mainly mullite, quartz, hematite, non The crystalline phase accounts for more than 60% in fly ash, and its main component is glass, so fly ash is a typical pozzolanic material.

Modification of fly ash
Fly ash contains a variety of trace elements and a large amount of active glass, and the particles are loose and porous, with high surface activity and adsorption, which can be used to adsorb pollutants in the environment. However, during the production process, part of the unescaped gas makes the particles form closed pores, which reduces the adsorption performance of fly ash to a certain extent. To this end, scholars have activated fly ash according to the target pollutants, including physical modification and chemical modification. The most common methods are alkali excitation, alkali modification and acid modification. Under the action of the alkali activator, the vitreous body in the fly ash dissolves, and a large amount of silicon and aluminum oxides are gradually activated, destroying the silicon-oxygen and aluminum-oxygen network and producing porous hydrated aluminosilicate and other products. Most of these products are fibrous, have high specific surface area and reactivity, and can effectively remove a variety of pollutants in the environment. Common alkali activators include NaOH and Ca(OH)2, etc., and can assist modification methods such as microwave and ultrasonic waves. In addition to mixing with a base activator, the complete synthesis of fly ash-based zeolites using base modifiers is a common approach. The chemical composition of fly ash is similar to zeolite, and it can be converted into zeolite with stronger adsorption capacity and clearer direction through a series of chemical reactions. On the other hand, zeolite has rich pores, large specific surface area and a large number of acid-base centers, which has better selective adsorption and recycling rate. Under the effect of acid modification, oxides such as Al2O3 and SiO2 in fly ash dissolve to form compounds such as Al2(SO4)3, H2SiO3, etc. These are compounds that are easy to form complexes or high molecular polymers, and are formed during adsorption. The bridge effect is helpful for the adsorption, flocculation and precipitation of suspended particles; pickling can also remove impurities on the surface of fly ash, increase the porosity, roughness and specific surface area, and help to adsorb pollutants.

Mechanism and Application of Fly Ash in Environmental Restoration
1 Atmospheric Repair
2 Soil remediation
3 Wastewater remediation

Fly ash has good adsorption capacity for some pollutants in the atmosphere, soil, and water environment, but compared with other adsorbents, it depends on the composition of the material and the treatment method. Using fly ash to synthesize zeolite and extracting unburned carbon to synthesize activated carbon can effectively improve the removal efficiency and adsorption capacity of fly ash. However, fly ash still has the technical limitation that it can only adsorb and treat pollutants. In order to completely degrade pollutants, the current hot research direction is to use fly ash as a carrier to load other degradation materials, or consider using other degradation technologies. But so far, considering the high cost of modification, the need to improve the adsorption efficiency, the need for research and development of related equipment, and safety issues, its high value-added utilization method has yet to be further improved, and there is still a long way to go to achieve industrial-scale application.