The Casale superphosphate process allows all possible superphosphates (SSP, TSP, and USP) to be produced as ROP products in the same plant:
- The main equipment (phosphate rock mill, mixer, drum den, scrubbing) items are identical.
- USP production requires an additional ‘urea sulphuric acid salt’ reagent preparation stage.
- The gas treatment section has to be designed provide the required performance whichever product is being manufactured.
- The mixer is fed with phosphate rock and either sulphuric acid or phosphoric acid or ‘urea sulphuric acid salt’ reagent, depending on the grade to be produced.
- The final product is in powder form.
If a granular SSP, TSP, USP product is desired, the powder product can be granulated (alone or with other raw materials) in a conventional granular complex fertilizer process unit.
Because the entire P2O5 content of SSP and USP, and one third of the P2O5 content of TSP, comes directly from the phosphate rock, the unit P2O5 production cost is much lower than for fertilizers such as ammonium phosphates, in which the entire P2O5 content comes from phosphoric acid (phosphoric acid route).
Rock phosphate is ground to the required degree of fineness and stored in a surge bin, from which it is fed into a horizontal paddle mixer developed by Casale via a weighing belt conveyor. The Casale mixer is of a very special design, with different reaction zones, and is fabricated in a special material of construction selected for its resistance to acid corrosion.
Acid (sulphuric acid for SSP or phosphoric acid for TSP) is fed at a metered rate into the mixer. An automatic controller linked to the weighing conveyor keeps the acid flow constantly in the correct proportion to the phosphate rock input.
For USP, the acid flow is replaced by a eutectic reagent obtained by mixing concentrated sulphuric acid and solid urea.
The retention time in the mixer is set so that the initial reaction goes as close to completion as possible.
Slurry is discharged into a rotary drum den, whose speed is adjusted in order to control the retention time at the required value. The Casale drum den is rubber-lined to prevent build-up of material, and a scraper is not required. That minimises maintenance costs.
The powder superphosphate is sent to storage for curing.
The very efficient and proven scrubbing system recovers fluorine compounds released during the acidulation process and recycles them into the final product, making the process environment-friendly.