# Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects
## 2 Analysis of Trends in Phosphogypsum Use
### Phosphogypsum Recycling in Various Industry Fields  
Theoretically, the construction materials industry's need for gypsum raw materials can be almost fully met by gypsum-containing waste (phosphogypsum). Various technologies for the production of construction and high strength gypsum from phosphogypsum have been developed, but they have not been sufficiently implemented in practice [23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. Figure 4: Comparison diagram behind the regions of research on phosphogypsum issues (Scopus Database). The technologies of phosphogypsum utilization entail transforming it into construction materials and products, and several variants of technological operations sequence are applied [33]. In the world, the construction sector accounts for over 40% of phosphogypsum usage [34]. Phosphogypsum preparation for the production of gypsum binders can be divided into four methods:  
* washing of phosphogypsum with water;
* washing in combination with neutralization and deposition of impurities in water suspension;
* the method of thermal decomposition of impurities;
* the introduction of additives that neutralize, mineralize, and regulate crystallization before and after firing. The first and second methods are associated with the generation of large amounts of contaminated water (2-5 m\({}^{3}\) per 1 ton of phosphogypsum) and the high costs of their removal and water treatment [35]. The disadvantages of the implementation of such engineering solution are as follows:  
* a complicated method of obtaining a hemihydrate binder that requires high-energy devices used for mechanical and chemical activation of raw materials;
* the need to use fresh phosphogypsum with stable humidity and the difficulty of calculating the required amount of calcium sulfate hemihydrate to ensure the humidity needed for pressing;
* the need for high-energy grinding and the use of superplasticizers, which significantly increases the cost of the binder. Engineering solutions have been developed for producing a gypsum binder from phosphogypsum, which has been aged for a long time in dumpsites [36]. The main disadvantages of the known technologies of phosphogypsum utilization for the processing into construction materials and products are significant consumption of thermal resources per unit of finished product, the presence of a high amount of gaseous emissions in the environment, and the mandatory process of adding water to wash phosphogypsum from water-soluble impurities. The generation of wastewater in the technological cycle has a negative side effect on the environment and requires additional construction of wastewater treatment facilities. Thus, the application of phosphogypsum as an additive in cement manufacturing is only possible when phosphogypsum is dried and granulated. The humidity of granulated phosphogypsum should not exceed 10-12%. Additionally, this technology, besides the high energy consumption, is burdened by the need for large-scale supplies of blast-furnace slag and additional raw materials for cement clinker production [34]. So, the large-scale consumption of phosphogypsum in the cement industry is constrained by the significant content of controlled for raw materials impurities such as water-soluble P\({}_{2}\)O\({}_{5}\) and fluorine compounds. The use of phosphogypsum to produce sulfuric acid and Portland cement is similar to the processing of natural anhydrite for these products through the Muller-Kuhne process, commonly used in Austria, South Africa, and Poland [37]. The method regenerates at least 90% of the sulphuric acid needed to decompose phosphate during the phosphoric acid production. However, this method makes economic sense only when the main method of sulfuric acid production (contact) is ineffective because of the absence or remoteness of traditional sources of sulfur-containing raw materials (elementary sulfur, sulfuric acid, off-gases containing sulfate anhydride).