# Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects
## 2 Analysis of Trends in Phosphogypsum Use
### Phosphogypsum Recycling in Various Industry Fields
There is a method of deep processing of phosphogypsum into sulfate acid with subsequent production of ammonium nitrate using ion-exchange technology by gas-liquid-phase conversion of gypsum with carbon dioxide and ammonia water [37]. e [PERSON] et al. [37] propose to carry out the regeneration of used ion nitric acid with the production of ammonium nitrate; this requires an additional source of production or input of nitric acid. Furthermore, the formed phosphate acid contains impurities of phosphorus pentoxide and can only be used in phosphate fertilizer production processes. Therefore, this method can only be used in complex fertilizer production facilities [1; 37]. Rare-earth elements from phosphate raw materials convert up to 90-95% in to phosphogypsum in the production of phosphorus-containing products, and the degree of their extraction as chlorides is about 95%. Up to 4 kg of cerium (III) chloride can be produced from 1 ton of phosphogypsum [38]. The important attribute of the old phosphogypsum dumps in Ukraine, originated after processing of Khibiny apatite, is the absence of radioactive elements in the composition, unlike other sources of rare-earth metals. This means that the obtained rare-earth elements are not radioactive and do not require decontamination, which makes their production more profitable. Sulphogypsum or nitric acid treatment methods are used to extract rare-earth elements from phosphogypsum. These methods convert rare-earth elements into solution with further processing with ammonia, alkalis, and fluorides for their extraction [39; 40]. Depending on the solution composition, extraction or sedimentation methods are used to remove rare-earth elements [41]. There is a method [42] that involves the leaching of rare-earth metals from phosphogypsum by 1-5% sulphuric acid solution, sorption of rare-earth metals from the leaching solution by cationic, desorption of rare-earth metals, deposition of rare-earth metal concentrate from desorbate, obtaining rare-earth metal concentrate, and mother liquor used for desorption of rare-earth metals. Notably, the implementation of such engineering solutions generates significant amounts of wastewater, which also needs further treatment and disposal. The process is energy-intensive and requires significant investments in chemical reagents. At the same time, these methods of rare-earth elements extraction from phosphogypsum do not always allow high results (the level of extraction is variable from 25 to 80%). Besides, the described technologies require large volumes of equipment for sulphate acid leaching and filtration equipment. Despite a large number of studies [41; 42; 43; 44], the main drawbacks of the known methods are the low recovery of rare-earth elements from phosphogypsum (30-50%); the need to use acids with a high concentration (40-50%); and the formation of secondary waste, which requires recycling [43]. The disadvantages of gypsum plasterboard include the following qualities [45]: low water resistance; high hygroscopic; and high creep under load, especially when humidifying, so the products based on gypsum are used in rooms with a relative air humidity of no more than 60%, because, when humidifying, there is a significant decrease in strength and britteness--gypsum plasterboard sheets cannot be exposed to shock loads, as dents form on them. To eliminate the disadvantages of the gypsum plasterboard, several studies [45; 46] have proposed technology on a composite binder containing phosphogypsum and sulfur-containing materials; for instance, as in the study of [PERSON] et al. [46], where dehydrated phosphogypsum was granulated, pressed, and hydrated with periodic pressing into paperless and fibreless gypsum plasterboards with a flexural strength of 14.7 MPa. The researchers were processing phosphogypsum into gypsum plasterboard. However, the main disadvantage of gypsum plasterboard production using phosphogypsum is the presence of various impurities in it, which requires additional operations: neutralization, enrichment, and drying.