Phosphogypsum is an industrial waste residue produced in the production of wet phosphoric acid, and it is one of the largest solid wastes discharged in the chemical industry. China began to develop the phosphate compound fertilizer industry in the 1950s, but the accumulation of phosphogypsum waste residues discharged from production continued to increase. Usually 1 ton of phosphoric acid (P2O5) can produce 4.5-5.0 tons of phosphogypsum by-products. With the rapid development of the phosphate fertilizer industry, China has become the world’s largest producer of phosphate fertilizer and the largest by-product of phosphogypsum.

At present, the global stockpile of phosphogypsum has reached 6 billion tons, and it is still increasing at a rate of 150 million tons every year. China is the world’s largest producer of phosphate fertilizers and the largest by-product of phosphogypsum. The stockpile of phosphogypsum has reached 400 million tons, which is still increasing at a rate of about 50 million tons every year. The accumulation of huge amounts of phosphogypsum not only occupies a large area of arable land, but also causes pollution to groundwater, soil, vegetation and atmospheric environment, which seriously restricts the healthy development of phosphorus chemical enterprises. Therefore, how to use phosphogypsum in a low-cost and high-value plan is an urgent problem to be solved at present.

According to the survey, the global stockpile of phosphogypsum and the annual production and discharge are huge, but its development and utilization degree is very low, and the comprehensive utilization rate is less than 10%.

In Japan, about 90% of the by-product phosphogypsum from wet phosphoric acid production is used, most of which (about 74%) are used for gypsum board and wall powder, and a small part is used for cement admixture. Due to the lack of natural gypsum resources in Japan, there are few by-products of phosphogypsum. The utilization rate of phosphogypsum is close to 100%. Food, medical and other industries; some other countries such as the United States, Canada, Belgium, etc. all adopt the method of stockpiling; some countries such as Morocco, Tunisia, the Netherlands, the United Kingdom, etc. mainly adopt the method of discharging into the ocean.

In the United States, gypsum resources and sulfur resources are abundant, and the production cost of gypsum and sulfuric acid is very low, and the use of phosphogypsum to co-produce sulfuric acid and cement or use it as a cement retarder will increase the cost, so the phosphoric acid enterprises in the United States prefer to pile phosphogypsum The deposit is also unwilling to proceed further. In addition, the United States has stricter solid waste management, coupled with the radioactivity of phosphogypsum and the acidic wastewater produced by phosphogypsum heaps, the US Department of Environmental Protection defines phosphogypsum as a technology-enhanced natural radioactive substance, and explicitly prohibits the use of radioactive substances exceeding 370B·kg- Therefore, most of the phosphogypsum produced by the U.S. phosphate fertilizer industry can only be temporarily stored and not used for other purposes.

In China, the treatment methods of phosphogypsum are mainly divided into two types: recycling and comprehensive utilization and storage treatment. But at present, stockpiling is still the main disposal method of by-product phosphogypsum in China. A large number of phosphogypsum storage yards are located along the banks of the river, and the risk of water pollution is high, which can easily cause the total phosphorus content of the surrounding water to exceed the standard and become eutrophic.

Resource utilization of phosphogypsum

￭ Utilization of cement industry

Phosphogypsum can be used as cement retarder and sulfuric acid co-production cement in cement industry. The use of phosphogypsum as a cement retarder instead of natural gypsum is the field with the largest consumption. In the process of cement hydration, SO42- dissolved in gypsum reacts with calcium aluminate hydrate to form calcium sulfoaluminate hydrate crystals, which precipitate and cover On the surface of the cement clinker particles, the contact area between the cement clinker and the water is reduced, and the hydration of the cement clinker particles is delayed, so as to achieve the purpose of retarding setting.

The phosphogypsum-to-sulfuric acid co-production cement is to decompose calcium sulfate (CaSO4) in phosphogypsum at high temperature, and the SO2 generated by the decomposition is used to produce sulfuric acid, and CaO is used to produce cement.

￭ Utilization of building materials

The resource utilization of phosphogypsum building materials is mainly concentrated in the production of gypsum powder, gypsum board, gypsum block, gypsum brick and other gypsum products. Among them, gypsum board has the largest market demand. It is a new type of building decorative lightweight board with light weight, sound insulation, heat insulation, shock resistance, low shrinkage, high strength, automatic fine-tuning of indoor humidity, strong processing performance and The advantage of simple construction method.

￭ Utilization of basic materials in construction projects

The basic materials of construction projects are used as road base materials and filling aggregates. The use of phosphogypsum in road construction can not only relieve environmental pressure, but also reduce construction costs. Phosphogypsum is used as filling aggregate to be backfilled to the goaf of the phosphate mine, which can not only realize the large-scale reuse of phosphogypsum, but also avoid the problems of landslides and surface deformation caused by the goaf.

￭ Agricultural utilization

The use of phosphogypsum in agriculture is saline-alkali soil amendment, sulfur, calcium, and silicon fertilizer. Phosphogypsum contains essential nutrients such as phosphorus, sulfur, calcium, and silicon, which can promote crop growth, improve crop quality, and increase crop yield.

￭Utilization in fertilizer industry

The application of phosphogypsum in the fertilizer industry is in the production of ammonium sulfate and potassium sulfate.

The principle of producing ammonium sulfate from phosphogypsum is that phosphogypsum reacts with ammonium carbonate to generate ammonium sulfate, and calcium carbonate is produced as a by-product. The process technology is mature, the production equipment is common, and the process conditions are easy to control, but the nitrogen content of the ammonium sulfate product is low, measured in units of Calculated in terms of quality nitrogen, the production cost is much higher than that of urea and ammonium nitrate.

￭ Extraction of Associated Rare Earth Elements

Phosphate deposits are often associated with a large amount of rare earth elements, which is a potential rare earth resource, and its comprehensive recovery is extremely important. The total reserves of phosphate rock in the world are about 100 billion tons, and the associated rare earth reserves are about 50 million tons. China’s rare earth reserves associated with phosphate rock ranks among the top three in the world, mainly in Guizhou, Hebei and Yunnan.