HDH Process

04 Hdh Process 05

The Prayon HDH Process

The Prayon HDH Process is a clever technology that maximizes the P2O5 strength in the acid and produces a high-quality gypsum.

It consists of an attack of the phosphate rock under hemihydrate conditions. Hemihydrate slurry is filtered on a Prayon tilting pan filter to recover the phosphoric acid. Then, the hemihydrate slurry is hydrated to dihydrate under specific process conditions of temperature and acidity defined by Prayon. Dihydrate slurry is filtered on a Prayon tilting pan filter to recover a high-quality calcium sulfate.

Depending on the rock processed, the ex-filter acid strength is typically 40-45% P2O5 and the typical P2O5 recovery efficiency is 98-99%.


  • highest P2O5 recovery
  • lowest P2O5 content in the phosphogypsum suitable for further applications after drying
  • limited grinding of the rock
  • high strength phosphoric acid


  • where medium- to high-cost rock is accessible
  • where low-cost fuel oil or natural gas is available to dry the gypsum
  • where gypsum disposal is limited and/or where a potential market for gypsum has been identified (cement or plaster applications)
  • where water and steam supply are a challenge