How phosphoric acid is made

Most phosphoric acid is made by dissolving rock in sulfuric acid, and the waste is a mountain of gypsum.

stations: 5
things upstream: 8
raw sources: 6
steps from the ground: 2

The short version

In the wet process, ground phosphate rock is treated with sulfuric acid. That makes a dilute phosphoric acid and a solid by-product, gypsum, which is filtered out. The acid is then concentrated by evaporation. A pricier electric-furnace route gives the purer acid used in food and drink.

Grind the phosphate rock

Phosphate minerals such as fluorapatite are ground fine so the acid can reach them.

Goes in
Phosphate rock
Digest it in sulfuric acid

Sulfuric acid is added to the rock. It reacts to make phosphoric acid, calcium sulfate and some hydrogen fluoride gas.

Goes in
Sulfuric acid
Filter off the gypsum

The calcium sulfate, better known as gypsum, is filtered off. Phosphate gypsum can carry traces of radium, so it often cannot be used in construction.
Boil off water to concentrate

The crude acid is 32 to 46 percent H3PO4. Evaporation lifts it to merchant grade at 75 to 85 percent.

75 to 85% H3PO4
Or reduce rock in a furnace

For food-grade acid, phosphate ore is reduced with coke in an electric arc furnace to give elemental phosphorus. The phosphorus is burned and dissolved in water to make a purer acid.

Goes in
Coke Silica sand

End of the linePhosphoric acid Next it goes into cola, soda.

Worth repeating

Phosphoric acid is rarely sold stronger than 85 percent, a grade that freezes at 21 °C.
The wet process is the most common way to make phosphoric acid for fertilizer.
The hydrogen fluoride by-product can be scrubbed with water to make hydrofluoric acid.