Thick-layer Self-Leveling Compound—Turning Phosphogypsum from Waste into a Valuable Resource
Release Time:
2020-05-25
Source:
Phosphogypsum is an industrial solid waste generated during the production of phosphoric acid, when sulfuric acid is used to treat phosphate rock; its main component is calcium sulfate dihydrate. China is the world’s largest producer of phosphate fertilizers and also the largest producer of phosphogypsum as a byproduct. According to statistics from the China Phosphate Fertilizer Industry Association, China’s cumulative historical stock of phosphogypsum has exceeded 500 million tonnes, with approximately 70 million tonnes being newly generated each year. However, the comprehensive utilization rate remains below 40%, and the stockpile continues to grow. The soluble phosphorus, heavy metals, fluorides, and other harmful substances contained in phosphogypsum are highly susceptible to leaching through wind erosion and rainfall, which not only consumes vast tracts of land but also poses significant safety hazards and environmental risks. As a result, phosphogypsum has become a major bottleneck hindering the sustainable development of the phosphate chemical industry. With increasingly stringent environmental regulations and the growing emphasis on green development placing ever higher demands on the industry, it has become imperative to implement vigorous measures for the effective treatment and disposal of phosphogypsum.
Phosphogypsum is a versatile raw material with applications across numerous fields. Comprehensive utilization of phosphogypsum not only addresses the issue of its stockpiling but also mitigates environmental pollution. The modification and application of phosphogypsum have become one of the current research hotspots. Phosphogypsum is primarily a solid waste generated during phosphoric acid production, resulting from the leaching of phosphate rock by sulfuric acid; it is a recyclable material that has found increasing use in various sectors, particularly in the context of today’s circular and sustainable development paradigm. Phosphogypsum is a fine, powdery solid, typically grayish-white or yellowish-white in color, occurring mainly in needle-like crystals, polycrystalline nuclei, monodisperse tabular crystals, and dense crystalline forms. The principal mineral constituent of phosphogypsum is gypsum, which accounts for about 96%–97% of the total material, followed by quartz and residues from beneficiation agents, with quartz comprising roughly 3%. The moisture content of phosphogypsum increases rapidly with rising temperature; its solubility is extremely low and decreases further as temperature rises. Interlayer permeability exceeds intra-layer permeability, and phosphogypsum exhibits strain-softening behavior. The main chemical components of phosphogypsum are calcium oxide and sulfur trioxide, along with a certain amount of water of crystallization and trace impurities such as sulfur dioxide, phosphorus, fluorine, and organic matter. Particle size distribution is generally normal and relatively narrow; the level of impurities is closely related to particle morphology—more regular morphologies correspond to larger particle sizes and lower impurity contents. Phosphogypsum boasts excellent high-temperature resistance and chemical corrosion resistance, with its most notable advantages being high strength and good toughness. Moreover, phosphogypsum exhibits strong affinity with polymers, facilitating surface treatment with polymer coatings and enabling the fabrication of polymer-based composites.
To achieve large-scale utilization of phosphogypsum, it is essential to break through barriers in high-value application technologies and develop high-added-value products. Over the past decade, the production of new gypsum-based building materials from phosphogypsum has made significant progress. The main products include paper-faced gypsum boards, gypsum blocks, high-strength gypsum powder, and gypsum-based dry-mix mortars; moreover, major breakthroughs have been achieved in gypsum-based self-leveling compounds and prefabricated construction technologies. The level of technology and equipment for the comprehensive utilization of industrial by-product gypsum has improved markedly, and China has developed process technologies and equipment with Chinese characteristics for the large-scale utilization of phosphogypsum. Core technological equipment—such as impurity removal, drying, and calcination—has reached a relatively advanced level and has been successfully commercialized. Significant improvements have also been made in terms of scale-up, system integration, automation, intelligentization, and energy conservation and consumption reduction. The industry for the comprehensive utilization of phosphogypsum resources has begun to exhibit a certain degree of industrial concentration, with clear trends toward agglomeration and the development of industrial parks. Phosphogypsum comprehensive utilization clusters have emerged in multiple regions, including Guizhou, Shandong, Hubei, Yunnan, and Sichuan. Guiyang was successfully selected as one of the first batch of demonstration bases for the comprehensive utilization of industrial resources by the Ministry of Industry and Information Technology. A number of leading enterprises in phosphogypsum comprehensive utilization have emerged across the country; for example, Lubei Group, Guizhou Wengfu, and Guizhou Kaiphi utilize phosphogypsum to produce cement and other building materials and have been recognized by the National Development and Reform Commission as national pilot units for the circular economy. Private enterprises are highly active, and a number of platform-based firms have emerged that provide comprehensive system solutions for the utilization of industrial by-product gypsum resources. These companies place great emphasis on technological innovation and business-model innovation and are eager to expand their scale of operations.
Xindadi’s technology for processing desulfurization gypsum or phosphogypsum into self-leveling materials provides a viable technical pathway for the large-scale, resource-based utilization of flue-gas desulfurization gypsum, thereby closing the ecological loop of flue-gas desulfurization and promoting the healthy development of this technology in China. On the one hand, this approach significantly reduces the land-use and storage costs associated with desulfurization gypsum; on the other hand, it helps to fill the gap in natural gypsum resources in certain regions, fostering the development of gypsum-based building materials in areas where natural gypsum is scarce, diversifying the range of gypsum products, and advancing the growth of self-leveling material applications. By converting desulfurization gypsum into self-leveling materials, we not only achieve resource recovery but also eliminate the risk of secondary pollution associated with desulfurization gypsum, embodying the principles of a circular economy. The primary raw materials for thick-layer self-leveling compounds include cementitious binders, aggregates, and admixtures. Cementitious binders are predominantly derived from industrial by-product desulfurization gypsum, while aggregates are sourced from solid waste materials such as steel slag, steel sludge, and tailings, thus conserving resources, turning waste into valuable resources, and delivering substantial economic and environmental benefits.
