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Processes with an initial dihydrate reaction

Over the past few years, Prayon licensing activities have been based mainly on the DPP Mark 4 dihydrate (*) phosphoric acid production process. This process is one of the best on the market — it can be used with different types of phosphate, is flexible, is easy to use and is inexpensive to maintain.

Processes with an initial dihydrate reactionThis traditional process took on major innovations every ten years or so until the current Mark 4 process was developed. This process is favored by producers because it is reliable, easy to use and is performed with tried and tested equipment.

It has the following main characteristics :
  • low investment and operation costs;
  • can be used with all types of phosphates (sedimentary or igneous);
  • phosphate can be supplied in slurry form (wet grinding);
  • easy to control the water balance (recycle pond water, reduction/elimination of liquid effluents);
  • dihydrate process;
  • good P2O5 yield.
Recommended for sites with a :
  • low to average phosphate cost;
  • low energy cost (steam);
  • gypsum discharge facility.

At our production plant in Engis, Belgium, we produce and sell calcium sulfate, which must have a high level of purity and be of excellent quality: We use the dihydrate hemihydrate CPP or Central Prayon Process.(**)

Dihydrate-hemihydrate process The Central Prayon Process has been developed to produce a calcium sulfate (phosphogypsum) that can replace natural gypsum. This is a dihydrate-hemihydrate process. During the first reaction stage, a pulp containing dihydrate is formed. The amount equivalent to the acid to be produced is filtered. The acid is then sent for storage, while the gypsum and the remaining pulp are sent to the conversion tank. In this reactor, sulfuric acid and steam are added to transform the dihydrate crystals into hemihydrate ones. The pulp produced is then filtered and the filter cake washed. All the filtrates from the second filter are recycled in the attack tank, while the solids can be neutralized and used in other processes.

The main characteristics of this process are as follows :
  • it produces acid with a higher P2O5 content than that produced in the DPP process, and a higher P2O5 yield;
  • it can be used with all types of phosphates (sedimentary or igneous);
  • it produces hemihydrate (CaSO4.1/2H2O) which is self-drying due to its chemical properties, is purer that the dihydrate, and can therefore be used directly as a material of merchantable quality (the choice of phosphate can be important).
Recommended for sites with :
  • a low to average phosphate cost;
  • high energy costs (steam);
  • limited gypsum discharge capacity, and/or a potential market for this purer gypsum.

This high-yield process (approximately 98.5%) produces a relatively strong acid (approx. 36% of P2O5) and a dry gypsum (less than 8% humidity) by natural rehydration of hemihydrate and dihydrate. Rehydration in this way substantially reduces the energy costs involved in drying the gypsum.