The Calcium Sulfate co-product from a phosphoric plant is either hemihydrate (half a molecule of crystal water) or dihydrate (two molecules of crystal water) also named gypsum. Some processes include only one crystallization stage while the others consist of a conversion of hemihydrate to dihydrate or vice versa.

The dihydrate process is the most widely used to produce phosphoric acid and Prayon Technologies has worldwide references on various types of rock phosphate.

Back in the seventies, the local political and economic context promoted the development of other processes. As a pioneer of innovation in the phosphorus chemistry, the Prayon company operates a double crystallisation process (dihydrate – hemihydrate, with two filtration stages) that produces marketable phosphogypsum for the cement and plaster industry. In the same way, Prayon Technologies developed and patented the DA-HF process (dihydrate – hemihydrate, with one filtration stage) to produce valuable gypsum while reducing rock and steam consumption with a minimum capital cost investment.

During the oil crisis, the price of energy became so critical that some producers were attracted by the hemihydrate process routes. As the P₂O₅ content in the product phosphoric acid is higher, the energy consumption to evaporate the water and reach merchant grade is reduced. That process – also licensed by Prayon Technologies – enabled projects in remote desert locations where water availability is critical. The range of processes was completed with the Prayon HDH process (hemihydrate – dihydrate, with two filtration stages) that adds high-recovery efficiency to the benefits of the Hemihydrate technology mentioned above. In a context of soaring energy prices, these processes are more relevant than ever to enable producers to reduce their energy costs and carbon footprint.

Read Hadrien Leruth’ s recent article in World Fertilizer that provides an overview of these two categories of processes – where hemihydrate calcium sulfate is generated. It includes tips and techniques to guide future producers in the selection of the best process for their projects.