Flue gas desulfurization gypsum and its applications

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Flue Gas Desulfurization Gypsum and Its Applications by Zhao Yunlong (Office for Comprehensive Utilization of Desulfurization Gypsum, Solid Waste Treatment Center) Abstract: Flue gas desulfurization gypsum and natural gypsum originate from different sources, and they also differ in terms of physical properties as well as the level of impurities present. Therefore, the processing methods and equipment used for treating flue gas desulfurization gypsum are not exactly the same as those used for natural gypsum. Gypsum that has been treated using various methods can basically replace natural gypsum in the production of a wide range of products, such as construction gypsum, high-strength gypsum, plastering gypsum, gypsum blocks, and gypsum panels. It can also be used as a retarder in cement, in fireproof coatings for steel structures, as plaster for wall finishing, and in gypsum board production. Additionally, flue gas desulfurization gypsum can be utilized together with fly ash, another type of solid waste, to achieve comprehensive utilization in industrial applications. This approach brings about benefits in terms of electricity generation, environmental protection, and society as a whole, and is highly consistent with China’s strategy for sustainable development. Keywords: flue gas desulfurization gypsum, desulfurization gypsum, calcium sulfite, cement retarder, building gypsum, plastering gypsum, gypsum blocks, fly ash, comprehensive utilization. I. Overview 　With an increasing awareness of environmental protection, China has imposed stricter controls on the emission of sulfur dioxide in the flue gases from power plants, making desulfurization of these plants essential. 　　The production of flue gas desulfurization gypsum in our country has only been going on for a short time, and there is insufficient awareness of its application value and market potential. There is also a lack of detailed, systematic, and scientific fundamental research on the properties of this gypsum, as well as on the equipment and processing techniques used for its treatment. Currently, the processing methods and equipment used for desulfurization gypsum are not entirely identical to those used for natural gypsum, in aspects such as material transportation, dust control, and calcination processes. In terms of physical properties, desulfurization gypsum also differs from natural gypsum; it has a higher purity, more free water, and finer particle size ； The particle size distribution is uniform, but the gradation is poor; a large amount of water is required for setting, and it contains many water-soluble salts ； It is also worth noting that desulfurization gypsum and natural gypsum have different origins; in desulfurization gypsum, unreacted calcium carbonate is the main impurity, along with calcium sulfite that has not been fully oxidized, resulting in significant differences in the nature of these impurities. Furthermore, factors such as variations in the technical processes used in desulfurization, differences in the composition of the limestone raw material, and variations in the fineness of the processed limestone can all affect the quality of desulfurization gypsum. Therefore, when utilizing desulfurization gypsum in processing, it is essential to take into account specific practical conditions and conduct thorough research using scientific methods. 　　After testing various feasible options, desulfurized gypsum is widely used in the building materials industry. Through different treatment methods, it can essentially replace the vast majority of building materials products made from natural gypsum. II. Utilization of desulfurization gypsum The comprehensive treatment and application of desulfurization gypsum in China have already begun, and its potential for utilization holds great market opportunities. This is especially true in economically developed regions where natural gypsum is scarce but desulfurization gypsum is abundant. Practical applications have shown that desulfurization gypsum is a valuable resource of high quality, suitable for the production of gypsum-based building materials for various purposes. 　　Desulfurized gypsum differs to some extent from natural gypsum, but fundamentally, since their main components are the same and the content of the main component in desulfurized gypsum is high, the treated desulfurized gypsum is a better cementing material than natural gypsum. 　　⒈ Desulfurized gypsum is converted into plaster of Paris through drying and dehydration processes. Plaster of Paris can be classified according to its applications into: casting plaster, plaster for plastering, plaster used in the ceramics industry, plaster used in the foundry industry, plaster for dental uses, medical plaster for orthopedic bandages, plaster for strengthening mine shaft walls to prevent fires, and plaster for soil improvement, among others. The largest volume of plaster of Paris is used in construction (building plaster). In the author’s opinion, the dehydration temperature (temperature of the material) for construction-related desulfurized gypsum should be between 125°C and 145°C ； The calcination equipment and processes vary, and the calcination temperature also needs to be adjusted accordingly. From the perspective of phase composition, it is preferable for cooked gypsum to contain a higher amount of soluble anhydrous gypsum, as this substance has strong hydrophilicity; upon contact with humid air, it easily converts into hemihydrate gypsum. In actual production, cooked gypsum must be transported and stored, and it is impossible to keep it isolated from air. The soluble anhydrous gypsum present in cooked gypsum absorbs moisture from the air, gradually converting into hemihydrate gypsum. This conversion process increases the content of hemihydrate gypsum in the powder, stabilizing its phase transformation process. This improvement can enhance the physical and mechanical properties of cooked gypsum, which is of great significance for meeting the quality requirements needed in the production of gypsum products. The duration of the aging process for hydrated gypsum, as well as the quality of the aging effect, are directly related to the chosen aging method, the way in which it is piled up, and environmental humidity. Special attention must be paid to this factor in the calcination process of desulfurized gypsum.

2. Desulfurized gypsum can also be used to produce high-strength gypsum. This is achieved by turning desulfurized gypsum into a gypsum slurry of appropriate concentration, adding a certain amount of crystallization agent, and under conditions of stirring and pressure, controlling the temperature, time, and pressure during processing to form columnar gypsum crystals. These crystals are then dehydrated and dried to produce high-strength hydrated gypsum. In production, factors such as the type and amount of crystallization agent used, the heat treatment temperature, time, and pressure of the slurry, as well as the slurry concentration, are all important determinants of product quality.

3. A new type of binding material is made by using desulfurized gypsum together with fly ash or slag, or by combining desulfurized gypsum with fly ash and slag, along with an accelerator. There is extensive information on this type of binding material in Soviet-era literature. It allows desulfurized gypsum to be utilized in a wider range of applications, thereby improving both the water resistance and strength of hydrated gypsum. Over the past two years, the author has used construction-grade desulfurized gypsum produced by a power plant in Taiyuan, along with first-class fly ash, as well as finely ground slag produced by Taiyuan Delong Company, along with the reinforcing accelerator G developed by this research institute. The test results show that when desulfurized gypsum and fly ash, or desulfurized gypsum along with finely ground slag, are used in large quantities, it is possible to significantly enhance the hydraulic hydration products formed under the action of the accelerator, while maintaining the basic properties of gypsum products. This improves both the water resistance and strength of the binding material. Gypsum remains the main component that provides strength in binders, and the hydration reactions of fly ash and slag ground powders take place within the hardened gypsum matrix. This improves the later strength, increases density, reduces water absorption, and enhances frost resistance. The hardened matrix of the binding material contains both air-hardening hydration products and water-hardening hydration products; such a matrix exhibits low shrinkage upon drying and good volume stability. It is indeed a processing method that saves energy while simultaneously utilizing two types of industrial waste to produce building materials. 　　⒋ Plaster made from desulfurization gypsum possesses high strength, good workability, and low cost. In particular, plaster made from a mixture of desulfurization gypsum and fly ash exhibits better strength over time compared to plaster made from pure gypsum, and its water resistance is significantly improved. The author used desulfurization gypsum produced by a power plant in Taiyuan along with first-grade fly ash, and added special retarders, water-retaining agents, binders, and surfactants specific for Jinlongfeng gypsum to produce this plaster; the quality of this product meets standards that exceed those specified in the JC/T517-2003 industry standard for plaster. It is now being used in cities such as Shijiazhuang, Baoding, Luoyang, Tianjin, Beijing, Tangshan, and Taiyuan ； When using desulfurized gypsum to prepare plastering gypsum, the following points should be noted: ① Due to the fine particles of desulfurized gypsum and the large amount of water required, segregation and water separation can occur easily. 　②Since the moisture in desulfurization gypsum migrates easily within the slurry, it is readily absorbed by the wall surface after application, thereby affecting the normal hydration of the plaster layer. 　③The bond strength of all industrially produced gypsum by-products is inferior to that of natural gypsum, hence their adhesion to the substrate is poor. 　④Due to the high fineness of desulfurization gypsum, more dust is generated during the production of plastering gypsum compared to natural gypsum, and the transportation and mixing equipment also needs to be improved accordingly. 　　To this end, when preparing plastering gypsum, we use special water-reducing and retarding agents for gypsum to reduce the amount of water used in the mixture; at the same time, inorganic and organic materials that enhance water retention are added to improve the water-retention capacity of the plastering gypsum slurry. In addition, cementitious materials that help improve adhesion are required to increase its bonding strength. During the production of plaster for plastering, it is essential to pay close attention to ensuring that all materials are mixed evenly; neglecting the removal of material from the dead corners of the mixing equipment can have a direct impact on the quality of the plaster product. 　　⒌ The equipment and production processes for manufacturing gypsum blocks and gypsum boards from desulfurized gypsum are the same as those for natural gypsum. Due to the fine particles of desulfurized gypsum, the higher water requirement for achieving standard consistency, its high bulk density, rapid setting time, and high strength, the author recommends that, when producing gypsum blocks, efforts should be made to reduce water usage while ensuring the fluidity of the desulfurized gypsum (water reducers can also be used to achieve this). This approach helps to improve the strength of the gypsum blocks. Additionally, a small amount of foaming agent or other lightweight inorganic materials can be added to reduce the weight of the gypsum blocks, thereby minimizing internal stresses caused by other factors. Adding an appropriate amount of fly ash, ground slag powder, lime, etc., to desulfurized gypsum can promote the hydration of fly ash and slag, while ensuring the early strength and rapid setting properties of the gypsum binder. A hydraulic hydration product is formed, resulting in a significant improvement in water resistance and an increase in its softening coefficient. It is also possible to consider adding an appropriate amount of other inorganic materials to improve the particle size distribution of desulfurization gypsum, thereby enhancing the crystal structure of the gypsum blocks and increasing their shrinkage stability. Regarding the issue of hollow and solid blocks, we believe it is appropriate to have both types coexist, with priority given to solid blocks. The reasons for this are: they improve sound insulation, are easier to hang from or nail to (especially for moisture-resistant blocks in bathrooms and kitchens), facilitate the mechanical molding of blocks, and align with the trend in foreign countries where solid blocks are primarily used. Regarding blocks and slabs, the author believes that blocks should be given priority for development, for the reason that they are better suited to accommodate variations in building floor heights ； Walls constructed with tenon-and-mortise joints have good integrity, with no gaps at the joints; however, gaps between the slats need to be properly addressed ； Bricks are easy to handle and install, with a low rate of damage; solid bricks can be developed, whereas solid slabs have an excessively high weight per unit ； The blocks do not require reinforced fiberglass cloth, allowing for easy modification of the gypsum material while eliminating the cost of fiberglass reinforcement materials. 　　⒍ Desulfurized gypsum is used as a cement retarder. This product does not have a high added value nor substantial profits, but it remains one of the main methods for dealing with large quantities of desulfurized gypsum. In cement production, gypsum is generally added as a retarder in order to regulate and control the setting time of cement. Gypsum can also promote the hydration of tricalcium silicate and dicalcium silicate minerals in cement, thereby enhancing the early strength of cement as well as balancing the strengths at various ages. Based on its application in Japan, it has been proven that desulfurized gypsum can be used as an additive in cement without any issues regarding purity, particle size, or trace elements. It is entirely feasible to use it as a substitute for natural gypsum in the cement industry. Desulfurized gypsum is able to properly regulate the setting time of cement, allowing the cement’s properties to develop normally; indicators such as cement strength and setting time meet the relevant standards. 　　⒎ Desulfurized gypsum is used in fire-resistant coatings for steel structures. After the 9/11 attacks in the United States, fire protection for steel structures became an issue that received significant attention and research. One of the shortcomings of steel structures is their poor fire resistance; their strength drops rapidly during a fire (with a fire resistance limit of around 0.25 hours). Therefore, effective fire protection measures must be taken for steel structures. Thick-layer fire-resistant coatings made from desulfurized gypsum are typically composed of insulation materials and gypsum as fillers, along with various additives. These coatings exhibit excellent insulating properties, environmental friendliness, adhesion, and crack resistance. Desulfurized gypsum enhances the flame-retardant properties of the coatings as well as their applicability during construction, and this is due to the special physical and chemical properties of desulfurized gypsum. 　　⒏ The use of desulfurized gypsum in interior wall putties: Such putties are made by using desulfurized gypsum with a particle size of less than 15 nm and a specific surface area of more than 1000㎡/kg, along with additives such as water-retaining agents and adhesives. Several points need to be considered when preparing gypsum putties, namely: ① An appropriate setting time. 　②Excellent water retention. 　③Fineness: ④ It has high bonding strength and hardness. 　⑤Easy to use. 　　9. Application of desulfurized gypsum in gypsum board: Gypsum board is a major building material both domestically and internationally, and it is also the best type of gypsum-based building material. Desulfurized gypsum boasts advantages such as high purity, fine particle size, and low chloride content. In the application of gypsum board, it helps to ⑴ improve the fluidity of the slurry, thereby enhancing the overall performance and strength of the gypsum board and preventing hollow areas at the edges, which is highly beneficial for product quality. 　⑵Reduce the weight of gypsum board to lower costs. 　⑶Improving the quality of gypsum boards by adding a certain proportion of desulfurized gypsum to natural gypsum enhances the strength and hardness of the cardboard sheets. Japan and Germany have had large-scale production lines for manufacturing gypsum board using desulfurized gypsum for many years. The use of inexpensive desulfurized gypsum results in significantly lower production costs compared to using natural gypsum as a raw material. A high strength is one of the advantages of desulfurized building gypsum, but its adhesion to face paper is poor, and it is less adaptable to existing process parameters compared to natural building gypsum. Currently, it is used in mixtures at 50% each in production. Investigating the differences, reasons, and solutions related to the use of desulfurized building gypsum versus natural building gypsum in the production of gypsum board is a new challenge; resolving this issue would allow gypsum board to be produced entirely using desulfurized gypsum as raw material. 　　10. Application of mining backfill materials ； The cemented backfill mining method is a mining process with high operating costs; the backfilling cost accounts for about one-third of the total mining costs, and cement, which is used as the binding material for backfilling, makes up over 80% of that cost. The high expenses associated with cemented backfilling significantly hinder the application and development of this mining method. Developing new cementing materials to replace part or all of the cement without reducing the strength of the fillings is the main focus of filling technology. It can promote the development of the cemented backfill mining process in mines and reduce the costs associated with mine backfilling. By adding a certain amount of scale inhibitor during flue gas desulfurization, the desulfurized gypsum contains not only polymers but also a certain amount of surfactants. Polymers can fill the interfacial gaps, making the interface transition zone denser, as well as enhancing the bonding between the gypsum and the aggregates. Surfactants have the ability to emulsify air in water, thereby improving the homogeneity of the cemented backfill material and thus enhancing its quality to meet the required performance standards for backfill materials. 　　11. Applications in agriculture 　　① Used as a fertilizer; calcium is the fifth most essential nutrient for crops, after sulfur, and it enhances crops’ resistance to pests and diseases, resulting in thicker stems and leaves and fuller grains. If peanuts require calcium in second only to potassium, applying more calcium fertilizers can significantly improve their yield and quality. By utilizing the calcium ions in desulfurized gypsum, it is possible to reduce soil alkalinity and eliminate the toxicity of carbonates to crops. Meanwhile, these calcium ions can replace sodium ions on soil colloids, providing additional active calcium and enhancing the soil’s resistance to alkalinity. ② Improve the soil. The addition of desulfurized gypsum can reduce the content of exchangeable sodium, as the Ca2+ and Mg2+ in the gypsum replace the exchangeable sodium in the soil; at the same time, the highly alkaline condition of the soil is altered, thereby creating a better soil environment for the normal growth of crops. According to experiments conducted by Indian scientists, adding 0.5%–1.0% desulfurized gypsum to alkaline soils not only enables wheat to grow normally in such soils but also increases yields and profits. When planting corn in infertile, highly saline-alkaline soils, the addition of 0.5%–1.0% desulfurized gypsum results in excellent growth of the corn plants. III. Prospects and Recommendations 　　Based on the application experience of desulfurized gypsum in developed countries, and with the development of new materials as well as an increasing awareness of gypsum’s environmental benefits, gypsum-based building materials have seen rapid growth in recent years; products such as gypsum board, plastering gypsum, and gypsum blocks are all experiencing swift expansion. In addition, self-leveling gypsum for floors, gypsum wall putty, gypsum waterproof boards, fiber-reinforced gypsum boards, and gypsum used in ceramic molding will all see rapid development. With relevant policy support, the production of desulfurization gypsum will promote the rapid development of China’s gypsum industry. As a by-product of industrial processes, desulfurization gypsum can serve as a substitute for natural gypsum at a lower cost, which helps to reduce the costs of gypsum products and enhances their competitiveness and market share compared to similar products. Moreover, the production of desulfurization gypsum changes the uneven distribution of natural gypsum resources, helping to adjust and balance this distribution and enabling the overall balanced development of gypsum products on a wider scale. Furthermore, desulfurization gypsum can be comprehensively utilized together with fly ash, these two types of solid waste, to gradually be applied in industrial production. This approach will yield benefits in terms of power generation, the environment, and society, and is highly in line with China’s strategy for sustainable development. 　　At present, desulfurization gypsum has not been widely utilized. On the one hand, China’s abundant natural gypsum resources pose a barrier to the comprehensive use of desulfurization gypsum; on the other hand, the domestic technologies for the comprehensive utilization of this material are not yet mature, which also hinders its broader application. Therefore, it is necessary to analyze the gypsum product market in areas where desulfurization gypsum is being developed and used, and to develop products that are marketable. This is the first issue that needs to be addressed. In addition, while introducing foreign technologies and experiences related to the use of desulfurization gypsum, it is important to assimilate, improve upon them, and develop new products with local characteristics and independent intellectual property rights that suit China’s national conditions. It is also necessary to create a range of technologies for using desulfurization gypsum that reflect China’s unique features, as well as to explore new areas for its application. To do this, substantial technical documentation is required to strengthen market awareness and understanding. Policies, regulations, and laws should be used to protect natural gypsum resources, thereby supporting and ensuring the sustainable utilization of desulfurization gypsum resources. Each power plant must choose a feasible plan that achieves \"more, faster, better, and cheaper\" – that is, one with high consumption of gypsum products, rapid project construction, excellent product quality and market prospects, as well as low investment costs. 　　The comprehensive utilization of flue gas desulfurization gypsum has only just begun in China. Various new problems arise in practical applications, and it is not possible to simply apply the treatment methods used for natural gypsum in production and application processes. Instead, it is necessary to devote effort to researching and developing new methods, processes, and equipment that take into account the specific characteristics of desulfurization gypsum, in order to create systematic and practical solutions for its comprehensive utilization. 　References: Lin Fanghui, Peng Jiahui, Peng Zhihui, Ji Jianxin, Chongqing University of Architecture: Desulfurized gypsum and its applications in the building materials industry. Health ； Study on the properties of flue gas desulfurization gypsum and its comprehensive utilization in the building materials industry ； A collection on gypsum and gypsum products. Wu Xiaoqin. Wu Zhongbiao, Department of Environmental Engineering, Zhejiang University: Current Status and Prospects of the Resource Utilization of Flue Gas Desulfurization Gypsum. Gypsum Building Materials, Issue 1, 2004: Chen Yunnen, Liang Liming, Southern Metallurgical Institute – Research on the use of desulfurized gypsum as a binder in sand filling ； Shiyin Building Materials, Issue 1, 2004: Shen Shifu, Zhang Liansong, Sun Chuanyao – Research on the Comprehensive Utilization of Desulfurized Gypsum. Gypsum Building Materials, Issue 4, 2004: Chen Yunnen, Southern University of Metallurgy – The Resource Utilization of Flue Gas Desulfurization Gypsum. Gypsum building materials, February 2004