Comprehensive Utilization of Desulfurization Gypsum

Comprehensive Utilization of Desulfurization Gypsum in the Building Materials Sector

　　Desulfurization gypsum is a byproduct of wet flue-gas desulfurization in coal-fired power plants and is primarily generated at these facilities. With the application and promotion of comprehensive utilization technologies for desulfurization gypsum, perceptions of this material have shifted: it is no longer viewed merely as a waste discharge from industrial plants but rather as a valuable resource, thereby attracting greater interest. At present, however, the added value derived from the comprehensive utilization of desulfurization gypsum remains relatively low, and the climatic and environmental conditions in southern regions further constrain the implementation and wider adoption of certain comprehensive utilization technologies. Consequently, it is necessary to review and summarize the progress made in the comprehensive utilization of desulfurization gypsum in the building-materials sector, so as to provide guidance for future research, development, and application of such technologies in this field.

1. Formation of desulfurization gypsum

　　The most widely used wet flue-gas desulfurization method today is the limestone–gypsum process, which primarily employs inexpensive and readily available limestone as the desulfurization absorbent. The limestone is crushed and ground into a fine powder, then mixed with water and stirred to form an absorbent slurry. The process system comprises a slurry preparation system, a flue-gas handling system, an SO₂ absorption system, a slurry discharge and recovery system, a gypsum dewatering system, a process-water system, a compressed-air system, and an electrical and control system. The absorption tower is the core equipment of the absorption system and is typically a counter-current spray tower. Sulfur-dioxide-containing flue gas enters the tower from the lower part of the spray zone, where it comes into thorough contact with the uniformly sprayed limestone absorbent slurry; mist droplets are removed, the gas is heated and warmed, and then discharged through the chimney. After sulfur removal, the absorbent slurry undergoes oxidation and dewatering to produce desulfurization gypsum. The oxidation step influences the formation of calcium sulfate crystals in the desulfurization gypsum, while the dewatering step affects the particle-size distribution of the gypsum—coarser, heavier particles are sent to secondary dewatering, whereas lighter, finer particles, including fly ash, are allowed to overflow. Both oxidation and dewatering processes demonstrate that the quality of desulfurization gypsum is closely linked to the choice of desulfurizing agent, SO₂ absorption efficiency, and process control; therefore, improving these key aspects can enhance the overall quality of the desulfurization gypsum.

　　Certain inherent properties of desulfurization gypsum pose challenges to its comprehensive utilization: ① high moisture content and strong stickiness, which easily lead to adhesion on loading, transportation, and lifting equipment, causing material blockages and disrupting normal production; ② the presence of fly ash and carbon particles alters the appearance and color of desulfurization gypsum, thereby limiting its application in decorative materials; ③ a narrow particle-size distribution can result in bleeding and stratification in cementitious materials or products made from desulfurization gypsum; ④ its high bulk density—reaching 1 kg/cm³—has a significant impact on storage, production, and product performance; ⑤ the presence of impurities such as calcium sulfite and fluctuations in composition further complicate efforts to achieve high-value-added applications.

2. Physicochemical Properties of Desulfurization Gypsum

　　Desulfurization gypsum is a wet powder containing 10%–15% adsorbed water; under normal conditions it is nearly white, but its color may range from yellowish-white to grayish-brown depending on the impurity content. The particle size of desulfurization gypsum generally does not exceed 90 μm, with more than 85% of the particles falling within the 30–60 μm range, resulting in a narrow particle-size distribution and less favorable gradation compared with that of naturally occurring gypsum after grinding. In naturally occurring gypsum after grinding, the coarse fraction consists mainly of impurities, while the fine fraction is predominantly gypsum; by contrast, in desulfurization gypsum, the coarse fraction is largely gypsum, whereas the fine fraction is predominantly impurities.

　　The chemical composition of desulfurization gypsum is similar to that of natural gypsum, with calcium sulfate dihydrate as the principal mineral; the CaSO4·2H2O content in desulfurization gypsum generally exceeds 90%. In addition to calcium sulfate dihydrate (which differs in crystal structure), desulfurization gypsum may also contain calcium sulfite, fly ash, carbon particles, sodium-containing components, unreacted CaCO3 particles, and soluble salts composed of sulfates or chlorides of potassium, magnesium, and other elements. By contrast, the impurities in natural gypsum are predominantly clay minerals.

3. Comprehensive Utilization of Desulfurization Gypsum

　　Research on desulfurization gypsum abroad began in the late 1970s and early 1980s, and to date a relatively comprehensive system for its research, development, and application has been established. Gaining an understanding of and learning from foreign technologies and experiences in the comprehensive utilization of desulfurization gypsum can provide valuable insights for the R&D and wider adoption of such technologies in China. In Japan, desulfurization gypsum is primarily used in the production of cement and gypsum board; owing to the presence of two molecules of water of crystallization in the gypsum, it offers excellent thermal insulation and flame-retardant properties in the event of a fire, which has spurred significant growth in the use of gypsum board as a fire-resistant partition for interior walls in residential buildings. In the European Union, desulfurization gypsum is utilized in applications such as gypsum board, self-leveling screed, castable gypsum, retarders, and gypsum blocks, with the majority of its use concentrated on manufacturing gypsum board. In the United States, desulfurization gypsum finds applications in gypsum board, cement and concrete, fillers and subgrade materials, mining, waste stabilization and solidification, and agriculture, with gypsum board production again accounting for the largest share. The progress achieved internationally in the application of desulfurization gypsum can be largely attributed to strong environmental awareness, supportive policy frameworks, and the development of high-value-added products.

　　Research on the utilization of desulfurization gypsum in China started relatively late. Currently, its main applications include use as a cement set regulator, production of building gypsum powder and fertilizers, and remediation of soda-saline-alkali soils. However, as the volume of desulfurization gypsum continues to increase, its application scope still needs to be expanded, and the value-added benefits of its utilization need to be enhanced. At present, most research and development efforts are focused on desulfurization gypsum discharged from thermal power plants; in reality, the quality of such gypsum is influenced by factors such as the type of coal used at the plant, the desulfurizing agent, the desulfurization process and equipment, and their operational control parameters. Yet systematic studies in this area remain scarce in China, highlighting the need for a shift in mindset, policy support, and broad societal engagement.