Diammonium Phosphate
Diammonium phosphate (DAP; IUPAC name diammonium hydrogen phosphate; chemical
formula (NH4)2HPO4) is the world’s most widely used phosphorus fertilizer. It is one of a series
of water-soluble ammonium phosphate salts. It’s made from two common constituents in the
fertilizer industry, and its relatively high nutrient content and excellent physical properties make
it a popular choice in farming and other industries.
The compound occurs in the nature as the exceedingly rare mineral phosphammite. The related
dihydrogen compound occurs as the mineral biphosphammite. Both are related to guano
deposits.
Chemical properties
Production
Ammonium phosphate fertilizers first became available in the 1960s, and DAP rapidly became
the most popular in this class of products. It’s formulated in a controlled reaction of phosphoric
acid with ammonia, where the hot slurry is then cooled, granulated and sieved. DAP handles and
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UE Vehari Campus
stores well. Solid diammonium phosphate shows a dissociation pressure of ammonia as given by
the following expression and equation:
At 100 °C, the dissociation pressure of diammonium phosphate is approximately 5 mmHg.
2NH3 + H3PO4 ⇌(NH4)2HPO4
(NH4)2HPO4(s) ⇌ NH3(g) + (NH4)H2PO4(s)
The standard nutrient grade of DAP is relatively high, at 18-46-0, so fertilizer products with
lower nutrient content may not be labeled DAP. The inputs required to produce one ton of DAP
fertilizer are approximately 1.5 to 2 tons of phosphate rock, 0.4 tons of sulfur (S) to dissolve the
rock, and 0.2 tons of ammonia. Changes in the supply or price of any of these inputs will impact
DAP prices and availability. The high nutrient content of DAP helps reduce handling, freight and
application costs. DAP is produced in many locations in the world and is a widely traded
fertilizer commodity.
Process for producing granular diammonium phosphate
A process for the manufacture of granular diammonium phosphate (DAP) occurred by reacting
anhydrous ammonia with phosphoric acid in a reactor to form reaction product consisting of a
partially reacted slurry of monoammonium phosphate and diammonium phosphate. The slurry is
pumped to a rotary granulator-reactor where it is further reacted with anhydrous ammonia to
form a solid granular diammonium phosphate mixture having a range of particle sizes consisting
of undersize, oversize and product. After drying the diammonium phosphate mixture, a portion
of the dried granular diammonium phosphate mixture is diverted back to the granulator-reactor
prior to delivery to a classifying means. The portion of dried granular diammonium phosphate
mixture not diverted to the granulator-reactor, is introduced to a classifying means set to a
narrow separation to separate undersize and oversize granular particles from the desired product
granular particles. The oversize granular particles are milled and recycled to the granulatorreactor along with the undersize granular particles obtained during the classifying process. The
desired granular particles which have a size in the range of 2 mm to 4 mm are collected. About
90% of the granules collected by the classifying means as product are in a range of the desired
granule size of 2 mm to 4 mm.
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UE Vehari Campus
Process for producing purified diammonum phosphate from wet process phosphoric acid
The present procedure has a main constitution (1) and an embodiment (2) as follows:
(1) A process for producing purified diammonium phosphate from wet process phosphoric acid
which process comprises (1) reacting ammonia with wet process phosphoric acid in a reactor
while keeping the reaction temperature at 50' to 80 C. and the molar ratio of NH3/H3PO4 at 1.65
to 1.90; (2) separating the resulting slurry containing diammonium phosphate crystals and an
insoluble sludge into a slurry containing the portion of said crystals and a slurry containing the
portion of said insoluble sludge; (3) subjecting said slurry containing the portion of said crystals
to centrifugal separation to thereby recover the crystals, while returning the separated liquid to
said reactor; (4) heating said slurry containing the portion of said insoluble sludge to 60° to 95 C.
to dissolve diammonium phosphate contained therein, followed by subjecting the resulting liquid
to centrifugal separation and deposition into an insoluble sludge and a filtrate, while returning
the separated filtrate to said reactor; (5) concentrating the liquids returned to said reactor at the
steps (3) and (4) to form a slurry containing diammonium phosphate crystals; and (6) repeating
the steps (2)-(4).
(2) A process according to the above item (1) wherein said separated slurry containing the
portion of an insoluble sludge is mixed with 30 parts by weight or less of water based on 100
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UE Vehari Campus
parts by weight of said slurry in advance of subjecting said slurry to centrifugal separation and
deposition.
Uses
Agricultural uses
DAP is used as a fertilizer. When applied as plant food, it temporarily increases the soil pH, but
over a long term the treated ground becomes more acidic than before upon nitrification of the
ammonium. It is incompatible with alkaline chemicals because its ammonium ion is more likely
to convert to ammonia in a high-pH environment. The average pH in solution is 7.5–8. The
typical formulation is 18-46-0 (18% N, 46% P2O5, 0% K2O).
DAP fertilizer is an excellent source of P and nitrogen (N) for plant nutrition. It’s highly soluble
and thus dissolves quickly in soil to release plant-available phosphate and ammonium. A notable
property of DAP is the alkaline pH that develops around the dissolving granule.
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UE Vehari Campus
As dissolving DAP granules release ammonium, the seedlings and plant roots nearest the volatile
ammonia can be harmed. This potential damage more commonly occurs when the soil pH is
greater than 7, a condition that often exists around the dissolving DAP granule. To prevent such
damage, users should avoid placing high concentrations of DAP near germinating seeds.
The ammonium present in DAP is an excellent N source and will be gradually converted to
nitrate by soil bacteria, resulting in a subsequent drop in pH. Therefore, the rise in
soil pH surrounding DAP granules is a temporary effect. This initial rise in soil pH neighboring
DAP can influence the micro-site reactions of phosphate and soil organic matter.
Non-agricultural uses
DAP can be used as a fire retardant. It lowers the combustion temperature of the material,
decreases maximum weight loss rates, and causes an increase in the production of residue or
char. These are important effects in fighting wildfires as lowering the pyrolysis temperature and
increasing the amount of char formed reduces that amount of available fuel and can lead to the
formation of a firebreak. It is the largest component of some popular commercial firefighting
products.
DAP is also used as a yeast nutrient in winemaking and mead-making; as an additive in some
brands of cigarettes purportedly as a nicotine enhancer; to prevent afterglow in matches, in
purifying sugar; as a flux for soldering tin, copper, zinc and brass; and to control precipitation of
alkali-soluble and acid-insoluble colloidal dyes on wool. DAP is used in various industrial
processes, too, such as metal finishing. And, it’s commonly added to wine to sustain yeast
fermentation and to milk to produce cheese cultures.
Management practices
Differences in the initial chemical reaction between various commercial P fertilizers in soil
become minor over time (within weeks or months) and are minimal as far as plant nutrition is
concerned. Most field comparisons between DAP and monoammonium phosphate (MAP) show
only minor or no differences in plant growth and yield due to P source with proper management.
Dr. Nagina Naveed Riaz
UE Vehari Campus