Raw graded Perlite is used in the production of phosphoric acid with the so called “Wet” process (Wet process Phosphoric Acid). Perlite is a naturally occurring siliceous rock the main constituent of which is amorphous silica (70-76% SiO2), while it contains lower quantities of Al2O3 (12-13%), potassium, sodium, calcium, iron and magnesium. The fact that the SiO2 which is present in perlite is amorphous makes it very reactive compared to other crystalline silica sources (like clays, quartz, etc.). Perlite may be pre-mixed to the phosphate rock prior to the chemical attack or introduced into the reactor with the other raw materials. Addition of perlite varies between 1,0% and 3,0% w/w. Typically the size of raw perlite grades used by fertilizers producers is between 1,2mm and 0,075 mm or between 0,6mm and 0,075 mm.
The main advantages of the perlite addition are:
- Increased efficiency in the filtration and washing stages of the H3PO4 production. Due to the size (large spreading of size distribution), shape (formation of needles) and crystallinity of the gypsum particles produced, the filtration rate of the CaSO4/H3PO4 slurry is very low. The addition of perlite changes the morphology of the gypsum and its particle size-distribution. It enhances the thickening of the gypsum crystals by increasing their growth to all dimensions resulting to more homogeneous spherical particles and thus improving filtration and sedimentation rates
- Reduction in overall plant corrosion. During the reaction, the HF, which is released, is bound by the active amorphous silica of perlite (6HF + SiO2 Þ H2SiF6 + 2H2O) and precipitates as Na2SiF6, KSiF6 or NaKSiF6. Perlite’s reactivity value exceeds 90%
- Reduction of the HF emission to the atmosphere (SiF4 is retained in the scrubbers)
- Less P2O5 residues left in the filtering cake due to the facilitation of the gypsum crystals washing