Chemistry Reference
In-Depth Information
phosphate, and ammonium polyphosphate. The latter three examples also
provide nitrogen, another primary nutrient. Triple superphosphate is calcium
dihydrogen phosphate (CaH
2
PO
4
). Two processes, the dry process and
the wet process, are used to convert phosphate rock to phosphoric acid. In
the manufacture of phosphoric acid by the dry process, phosphate rock is
reduced in an electric furnace, and the resulting yellow phosphorus is burnt
into the oxide, P
4
O
10
, commonly referred to by its empirical formula, P
2
O
5
,
and therefore called phosphorous pentoxide. The phosphorous pentoxide is
hydrated to obtain phosphoric acid. This process has high energy costs. The
wet process is the more commonly used. In the wet process, phosphate rock
is decomposed with sulfuric acid, and the produced calcium sulfate is sepa-
rated to obtain dilute phosphoric acid, which is then concentrated into high
concentration acid [10]. Direct acidification with sulfuric acid is problematic
because the outside surface of the rock reacts with the sulfuric acid resulting
in a gypsum layer on the outside of the rock. Gypsum is calcium sulfate
dihydrate and is used for plaster and drywall. This gypsum layer limits the
reaction on the inside of the rock. To address this problem, the phosphate
rock is first acidified with phosphoric acid to make calcium dihydrogen
phosphate. Sulfuric acid is then added to make phosphoric acid [11].
9 Ca(H
2
PO
4
)
2
+ CaF
2
2 Ca
5
F(PO
4
)
3
+ 12 H
3
PO
4
18 H
3
PO
4
+ 2 HF
+ 10 CaSO
4
2H
2
O
9 Ca(H
2
PO
4
)
2
+ CaF
2
+ 10 H
2
SO
4
+ 20 H
2
O
This process requires that 12 of the 18 moles, or 67%, of phosphoric acid
produced be recycled to treat more phosphate rock. There are other metals in
the phosphate rock and upon acidification they become salts. The insoluble
salts remain with the calcium sulfate precipitate but the soluble salts such as
magnesium sulfate or iron sulfate remain in solution with the phosphoric acid.
Generally, high purity is not required for fertilizer grade phosphoric acid. The
crude phosphoric acid reacts, for example, with ammonia to make ammonium
dihydrogen phosphate.
NH
3
+ H
3
PO
4
(NH
4
)H
2
PO
4
If higher purity is required, such as for food applications, there are various
processes to improve the purity of wet process phosphoric acid. Generally
they involve a solvent which solubilizes the phosphoric acid, but not the impu-
rities [12]. Alternatively, dry process phosphoric acid can be used. However,
purification of wet process phosphoric acid is the dominant route because of
the higher costs associated with the thermal process.
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