superphosphate: A commercial phosphate mixture consisting mainly of monocalcium phosphate. Singlesuperphosphate is made by treating phosphate rock with sulphuric acid; the product contains 16–20% ‘available’ P2O5: Ca10(PO4)6F2 + 7H2SO4 = 3Ca(H2PO4)2 + 7CaSO4 + 2HF Triple-superphosphate is made by using phosphoric(V) acid in place of sulphuric acid; the product contains 45–50% ‘available’ P2O5: Ca10(PO4)6F2 + 14H3PO4 = 10Ca(H2PO4)2 + 2HF
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Superphosphate is obtained by treating phosphate rock
with sulphuric acid, phosphoric acid, or their mixture.
Rock phosphate mixed (a) with sulphuric acid (H2SO4)
gives single superphosphate (9% phosphorus), and
(b) with phosphoric acid gives triple superphosphate
(20 to 22 % phosphorus).
Equal weights of finely ground rock phosphate and
sulphuric acid are mixed in cast iron pans, equipped with
a stirring mechanism. The fluid mixture is dropped into a
'hot den' where it solidifies. 15 to 30 minutes later, it can
be used for making different grades of superphosphates.
The main reaction between sulphuric acid and rock
phosphate is as follows:
A guaranteed percentage of the available phosphoric
acid determines the type of superphosphate produced; for
instance, '20% superphosphate' or '45 to 50%
superphosphate', which are single superphosphate (SSP)
and triple superphosphate (TSP), respectively. The
percentages indicate the amount of available phosphorus (as P2O5/sub>).
Boronated single superphosphate is a modified form
of single superphosphate. It contains 0.18% borax and is
mostly available in western countries as a fertilizer.
Superphosphates are neutral fertilizers in that they
have no appreciable effect on soil pH. Superphosphate
(also called ordinary or single superphosphate) contains
about 8 to 10% sulphur as calcium sulphate.
Superphosphate is also used in the production of mixed
fertilizers in which other dry, powdered or finelygranular
materials are blended to effect a product that
contains nitrogen, phosphorus and potassium. It is also
applied directly. Ammoniated superphosphates are prepared by reacting anhydrous or aqua ammonia with
ordinary or triple superphosphate. Ammoniation of
superphosphates offers the advantage of inexpensive
nitrogen but decreases the amount of water-soluble
phosphorus in the product. The water solubility is greater
in superphosphate than in triple superphosphate.
