Phosphate is a naturally occurring form of the element phosphorus, present in all living cells. It is found in the fossilised remains of ancient marine creatures, and in large build-ups of bird droppings (guano), which form sandy deposits known as rock phosphate.
Making superphosphate involves chemically treating rock phosphate, quarried in the US, Egypt, Tunisia and elsewhere.
The production process
- One of the basic materials needed to make superphosphate is sulfuric acid. This is produced by burning sulfur and dissolving the fumes in water.
- Rock phosphate is then dissolved in the acid.
- The chemical reaction (which takes about half an hour) produces phosphoric acid and calcium sulfate. These are the main components of superphosphate. The ratio of acid to rock phosphate must be carefully judged: too much sulfuric acid produces excess phosphoric acid.
- The steaming product is usually carried on a conveyor belt to storage for maturing. During storage, ‘free’ phosphoric acid continues to react with residual rock phosphate.
Too much liquid can cause the superphosphate to become like putty under mechanical pressure (often referred to as ‘green’ super). This is difficult to use in farm machinery. The problem can be overcome by artificial drying, but this is prohibitively expensive.
Purity of rock phosphate
Rock phosphates can range in purity from 62% to 88% tri-calcium phosphate content. This affects the amount of acid needed. The main impurities are silicates, oxides of iron and aluminium, fluoride, and carbonates. These may all complicate the process, or form undesirable emissions.