Page 115 - Chemical and process design handbook

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AMMONIUM PHOSPHATE

There are three possible ammonium orthophosphates, only two of which
are manufactured on any scale.
Monoammonium phosphate (NH H PO , white crystals, density: 1.803)
4 2 4
is readily made by reacting ammonia with phosphoric acid, centrifuging,
and drying in a rotary dryer. It is used in quick-dissolving fertilizers and as
a fire-retarding agent for wood, paper, and cloth. Diammonium phosphate
[(NH ) HPO , white crystals, density: 1.619] requires a two-stage reactor
4 2 4
system in order to prevent loss of ammonia. A granulation process follows
with completion of the reaction in a rotary drum. Both compounds are sol-
uble in water and insoluble in alcohol or ether.
A third compound, triammonium phosphate [(NH ) PO ] does not exist
4 3 4
under normal conditions because, upon formation, it immediately decom-
poses, losing ammonia (NH ) and reverts to one of the less alkaline forms.
3
Ammonium phosphates usually are manufactured by neutralizing phos-
phoric acid with ammonia in which control of the pH (acidity/alkalinity)
determines which of the ammonium phosphates will be produced.
Pure grades of a particular ammonium phosphate can be produced by
crystallization of solutions obtained from furnace-grade phosphoric acid.
There has been a trend toward the production of ammonium phosphates
in powder form by a process in which concentrated phosphoric acid is neu-
tralized under pressure, and the heat of neutralization is used to remove the
water in a spray tower. The powdered product then is collected at the bot-
tom of the tower. Ammonium nitrate/ammonium phosphate combination
products can be obtained either by neutralizing mixed nitric acid and phos-
phoric acid or by the addition of ammonium phosphate to an ammonium
nitrate melt.
Large quantities of the ammonium phosphates are used as fertilizers and in
fertilizer formulations as well as fire retardants in wood building materials,
paper and fabric products, and in matches to prevent afterglow. Solutions of
the ammonium phosphates sometimes are air-dropped to retard forest fires,
serving the double purpose of fire fighting and fertilizing the soil to acceler-
ate new plant growth.

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