Triple
(Concentrated) Superphosphate. The first important use of phosphoric acid in
fertilizer processing was in the production of triple superphosphate (TSP),
sometimes called concentrated superphosphate. Basically, the production process
for this material is the same as that for normal superphosphate, except that
the reactants are phosphate rock and phosphoric acid instead of phosphate rock
and sulfuric acid. The phosphoric acid, like sulfuric acid, solubilizes the
rock and, in addition, contributes its own content of soluble phosphorus. The
result is triple superphosphate of 45- 47% P2O5 content
as compared to 16- 20% P2O5 in normal
superphosphate. Although triple superphosphate has been known almost as long as
normal
superphosphate, it did not reach
commercial importance until. the late 1940s, when commercial supply of acid
became available.
Simplicity of production, high
analysis, and excellent agronomic quality are reasons for the sustained high
production and consumption of TSP. A contributing factor is that manufacture of
the triple superphosphate has been an outlet for so-called sludge acid, the
highly impure phosphoric acid obtained as a by-product of normal acid
purification.
Chemistry and Properties: TSP is essentially impure monocalcium phosphate
monohydrate, Ca(H2PO4)2 ·H20, made by
acidulating phosphate rock with phosphoric acid according to:
The complete chemistry of TSP
production has been studied and reported in great detail. As in the production
of NSP also known as Single Superphosphate (SSP) there are also reactions with
impurity minerals. The range of constituents in commercial TSP from wet-process
acid and phosphate rocks are typically: Ca(H2PO4 )2.H2O,
63-73%; CaSO4, 3- 6%; CaHPO4 and Fe and Al
phosphates, 13- 18%; silica, fluoro silicates, unreacted rock, and organic
matter, 5-10%; and free moisture, 1-4%. The average citrate solubility of the P2O5 in
best quality TSP is 98-99%, but products with citrate solubility values a few
percentage points lower are not uncommon. The P205 citrate
solubility of TSP made by a quick-cure process is a ca 96%. Other common
properties of TSP are bulk density, non-granular, 879 kg/m3;
granular, 1040-1200 kg/m3; and critical relative humidity at 30°C,
94%.
Production
Technology: Phosphate rock
and wet-process phosphoric acid are the only raw materials required for
manufacturing TSP. The grade of rock can be a little lower than that needed for
NSP production. Over the years, a large number of process modifications, both
batch and continuous, have been used. For the production of nongranular TSP,
52-54% P2O5 acid is used without dilution or
heating. The P2O5:CaO mole ratio, including the P2O5 in
the rock, is 0.92-0.95. The rock is ground to 70% <74.urn (200 mesh). Pile
curing for a few weeks is typical, as for NSP.
Granular TSP (-6
+ 16 mesh (1.19 to 3.35 mm dia)) is preferred for direct application and is
used in bulk blend fertilizers. A widely used slurry granulation process is the
Dorr-Oliver process. The ground rock is mixed with 38-49% P2O5 acid
in a series of reaction vessels. Slurry from the reaction train then is mixed
with a large proportion of recycled undersize granules and crushed oversize in
either a pugmill or drum granulator. The granules are dried and screened. The
product-size material is about 1 part in 13, thus the recycle ratio is 12:1.
Other processes involve granulation of previously prepared nongranular
TSP.
Economics: In
contrast to NSP, the high nutrient content of TSP makes shipment of the
finished product preferable to shipping of the raw materials. Plants,
therefore, are located at or near the rock source. The phosphoric acid used,
and the sulfuric acid required for its manufacture, usually are produced at the
site of the TSP plant. As in the case of NSF, the cost of raw materials
accounts for more than 90% of the total cost. Most of this is the cost of acid.
Since about
1968, triple superphosphate has been far outdistanced by Diammonium Phosphate
as the principal phosphate fertilizer, both in the United States and worldwide.
However, production of triple superphosphate is expected to persist at a
moderate level for two reasons: (1) at the location of a phosphoric
acid-diammonium phosphate complex, production of triple superphosphate is a
convenient way of using sludge acid that is too impure for diammonium phosphate
production; and (2) the absence of nitrogen in triple superphosphate makes it
the preferred source of phosphorus for the no-nitrogen bulk-blend fertilizers
that frequently are prescribed for leguminous crops such as soy beans, alfalfa,
and clover.