DAP, diammonium phosphate, is one of the most widely traded phosphate fertilizers in the world, and its production process is one of the more chemically specific in the fertilizer industry. Unlike NPK granulation by steam or slurry, which blends nutrients by controlling a physical process, DAP production requires managing an exothermic chemical reaction in real time, and the properties of the resulting ammonium phosphate slurry- its density, viscosity, temperature, and neutralization ratio- directly determine whether the granulation downstream works or doesn’t.
We’ll walk through the full process chain from raw material intake through to screened product, covering the chemistry and the engineering decisions at each stage that determine plant performance.
Raw Material Preparation
The two primary raw materials in DAP production are phosphoric acid and anhydrous (or aqueous) ammonia. The quality and concentration of the phosphoric acid is one of the most significant variables in the entire process, because it propagates through every downstream step.
Phosphoric acid used in fertilizer-grade DAP production is typically wet-process acid (WPA), produced by reacting phosphate rock with sulfuric acid. Merchant-grade WPA is typically supplied at a P2O5 concentration of around 40–54% by weight, depending on the source and the degree of concentration applied. Impurities in WPA, particularly iron, aluminium, magnesium, and fluorine compounds carried over from the phosphate rock, affect the slurry properties, the granulation behaviour, and the final product analysis. Plants receiving WPA should establish the acid’s impurity profile and verify compatibility with the plant’s design parameters before commissioning.
Ammonia is typically stored as a liquid under pressure (anhydrous ammonia) and vaporised or metered as liquid into the reaction system. Aqueous ammonia (ammonium hydroxide, typically 20–25% NH3 by mass) is an alternative that avoids pressurised liquid storage but introduces significant additional water into the process, increasing the drying load downstream. The choice between anhydrous and aqueous ammonia depends on site logistics, available infrastructure, and the cost of drying energy versus ammonia handling complexity.
Feed Metering and Ratio Control
Phosphoric acid and ammonia are metered independently into the reaction system. Maintaining the correct molar ratio between the two feeds is the single most important control task in DAP production; it determines the product chemistry, the slurry properties, and the stability of the granulation loop. The feed metering system should be gravimetric and accurate, capable of holding the target ratio within tight tolerances even as acid concentration or ammonia supply pressure varies. Feed metering accuracy requirements are covered in more detail in the companion article on neutralization ratio control.
The Reaction Stage
The central chemical step is the exothermic neutralization of phosphoric acid with ammonia:
2NH3 + H3PO4 → (NH4)2HPO4
This reaction is highly exothermic. The heat released raises the temperature of the reacting slurry and drives evaporation of water, which concentrates the product and reduces the downstream drying load , but only if the heat is managed correctly. Too much heat loss from the reaction stage, or too much dilution from low-acid concentration or water additions, reduces the benefit of the reaction heat and increases the thermal load on the dryer.
Industrial DAP plants can use a preneutralizer, a pipe reactor, or a combination of both to carry out the reaction.
In the preneutralizer route, phosphoric acid and ammonia are continuously fed into an agitated reactor. The preneutralizer typically operates at a neutralization ratio, the molar ratio of ammonia to phosphoric acid (N/P) , below the stoichiometric requirement for pure DAP (which is 2.0 mol/mol). The preneutralizer N/P ratio in conventional DAP production is often in the range of 1.4–1.8, depending on process design and the properties of the acid. The slurry leaving the preneutralizer at this intermediate ratio is pumpable and is transferred to the ammoniator-granulator, where additional ammonia is introduced to complete the reaction and reach the target product composition. Operating the preneutralizer below full stoichiometry keeps the slurry in a manageable physical state, avoiding premature crystallisation or excessive viscosity.
In the pipe reactor route, phosphoric acid and ammonia react rapidly inside a compact tubular reactor. The reaction heat is used very effectively in this configuration; the residence time in the reactor is short, the temperature rises sharply, and the resulting slurry or melt discharged into the granulator carries a significant heat load that can reduce the moisture entering granulation and lower drying requirements compared to the preneutralizer route. Pipe reactor systems are well-suited to plants designed for high throughput and low energy intensity, but they require more careful control of the reaction conditions to prevent reactor plugging.
Combined configurations, where both a preneutralizer and a pipe reactor are used with the feed distributed between them, allow additional flexibility in managing heat balance and slurry properties.
Granulation
The ammonium phosphate slurry from the reaction stage is fed into a rotary drum ammoniator-granulator. This machine performs two functions simultaneously: it provides the mechanical environment for granule formation (cascading, tumbling, and layered growth), and it allows additional ammonia to be introduced into the bed to complete the neutralization reaction that began in the preneutralizer.
Granule formation in DAP follows the same layered growth mechanism as other drum granulation processes; fines and recycled product act as seed particles, slurry is distributed onto the bed through spray nozzles or the pipe reactor discharge, and the granules grow through successive slurry layers as they cascade through the drum. The recycle ratio, the ratio of recycled off-spec material to on-spec product, is typically higher in DAP granulation than in some NPK processes, often in the range of 3:1 to 5:1, depending on the slurry properties and granulation parameters. This higher recycle load reflects the narrower operating window in DAP granulation and the importance of maintaining a well-seeded bed.
Key parameters controlled during granulation include the slurry feed rate, slurry temperature and density, ammonia addition rate, drum rotation speed and filling factor, and the total circulating load. The interactions between these parameters are coupled; a change in slurry density affects the bed moisture and therefore the optimal ammonia addition rate, for example, and maintaining stable granulation requires a control philosophy that accounts for these interactions rather than managing each parameter independently.
Drying
Wet granules leaving the granulator contain residual moisture that must be removed to ensure product stability in storage and handling. Drying is carried out in a rotary drum dryer. For DAP products, the target outlet moisture is typically below 1% by weight, though tighter specifications may apply for export-grade product or for DAP intended for use in further blending operations where surface moisture can affect mixing performance.
DAP granule drying requires careful temperature management. The decomposition temperature of diammonium phosphate begins to become relevant above approximately 70°C for the product itself, though the inlet gas temperatures in the dryer can be substantially higher because the gas contacts wet product at the feed end where evaporative cooling limits the product temperature. The choice of co-current versus counter-current dryer configuration has a direct bearing on product temperature control: co-current drying, where the hot gas and the wet material enter at the same end, provides natural protection against product overheating because the evaporative cooling at the feed end limits granule temperature even at high gas inlet temperatures.
Cooling
Dried granules leave the dryer at elevated temperature and must be cooled to near-ambient before handling, storage, and shipment. Cooling is typically carried out in a rotary drum cooler or a fluidised-bed cooler. The cooled product should be at a temperature sufficiently below the ambient dew point to avoid surface condensation and caking during subsequent handling.
Screening and Size Classification
The cooled product passes through a multi-deck vibrating screen that separates it into three fractions: oversize (above the upper cutpoint, typically around 4–5 mm for granular DAP depending on the target specification), on-spec product (typically 2–4 mm for standard granular grades, though product specifications vary by market), and undersize fines below the lower cutpoint. Both the oversize and the undersize fractions are recycled back to the granulator as seed material, the oversize after passing through a crusher that reduces it to the target seed size range. The on-spec fraction proceeds to product storage, coating (if applicable), and dispatch.
The recycle fractions from the screen, combined with the fresh slurry, create the material balance of the granulation loop. Maintaining this balance within the design parameters is the operational core of the DAP production process, and the primary measure of whether the plant is running well or not.
Ceylan Machine & Process designs and delivers turnkey DAP and MAP fertilizer production facilities covering the full process from reaction through granulation, drying, and screening. For technical enquiries or to discuss your project requirements, contact our engineering team.
