Production of DAP
- Mohamed Lembaid
- 7 days ago
- 3 min read
In this article, we’ll explore the DAP production process, focusing on its chemistry, the importance of mole ratio, and the raw materials involved.
Introduction
Diammonium Phosphate (DAP) is produced from phosphoric acid neutralized with ammonia.
The resulting solid is granulated and shipped either in bulk or in bags.
DAP is one the biggest commodity phosphate fertilizer being traded on the world market. DAP is a concentrated phosphate fertilizer with an additional content of nitrogen.
DAP popularity is mainly due to the very high nutrient content which minimizes the transport cost per kg of nutrient.
Chemistry
In most parts of the world, phosphate is commonly measured as phosphorus pentoxide, or P2O5, although P2O5 is not actually an occurring chemical species in fertilizer.
The international recognized chemical quality of DAP is 18-46-00, 18% nitrogen, 46% P2O5, and no Potash.
The main reactions of the DAP process can be expressed in two chemical equations :
Reaction 1 : NH3 + H3PO4 = NH4H2PO4
Reaction 2 : NH3 + NH4H2PO4 = (NH4)2HPO4
Phosphoric acid used for the fertilizer production contains generally some sulphuric acid. The most important side reaction of sulphuric acid with ammonia is :
Reaction 3 : 2NH3 + H2SO4 = (NH4)2SO4
Sulphuric acid being a stronger acid than phosphoric acid, reaction 3 will go to completion before the ammonia will react with phosphoric acid.
All reactions 1,2,3 are exothermic.
Normally, in DAP granulation processes, reaction 2 is not taken to completion, and therefore fertilizer grade DAP (18-46-00) consists of a mixture of MAP and DAP.
N:P mole ratio
The degree to which the reactions are completed is expressed as N:P mole ratio:
A ratio of 1:1 corresponds to Mono Ammonium Phosphate (MAP)
A ratio of 2:1 corresponds to Di Ammonium Phosphate (DAP)
The design ratio required to achieve the 18-46-00 product analysis is a function of the phosphoric acid purity and will normally be between 1.8:1 and 1.9:1
The ratio is strictly referring to the relationship between ammonia and P2O5 in the ammonium phosphate, any ammonia associated with sulphuric acid is excluded.
Solubility of ammonium phosphate as a function of mole ratio
The production process for DAP is designed to overcome the challenges presented by the solubility, or lack of solubility of ammonium phosphate.
At mole ratio of 1:1 (MAP) and above 1.8:1 (DAP), it is practically insoluble, whereas it is quite soluble below N:P mole ration 0.4 and around N:P mole ratio 1.4-1.5:1
DAP cannot be easily pumped without using a large proportion of water.
The control of water content and the N:P mole ratio is therefore major objective in the design and operation of an ammonium phosphate reactor.
Raw materials - Phosphoric acid
The phosphoric acid for DAP production must have a P2O5 content of at least 40% P2O5, although 50% is preferable to avoid a liquid effluent from the scrubbers.
Some integrated phosphoric acid - DAP plants use a mix of 28% acid and 54%.
Wet process phosphoric acid contains a number of impurities which have an impact on the chemistry of the process and the product quality.
Iron, Aluminium and Magnesium react to produce metal ammonium phosphate salts, such as MgNH4PO4, AlNH4HPO4F2, FeNH4(HPO4)2.
Some phosphate component of these complexes are water insoluble but remain citrate soluble, and therefore can be counted as available P2O5.
Some other metal phosphates are however citrate insoluble and are "lost" as salable P2O5.
The reactions from which these metal phosphates are formed are generally slow, and the formation can be limited by minimizing the residence time in the reactor.
There are other unfavorable effects of a high content of impurities in the phosphoric acid. For example, high magnesium content in the phosphoric acid tends to make the DAP reactor slurry very viscous, and difficult to handle.
Raw materials - Ammonia
Ammonia can be supplied to the DAP process in gaseous or liquid form.
In a Tank Reactor process, the gaseous ammonia is generally used. The cooling duty of evaporating ammonia can then be used to condition the air to the final product cooler.
In a Pipe Reactor process, liquid ammonia is generally used.
Raw materials - Filler
If the phosphoric acid contains little impurities, the nutrient content in the product may be come too high.
In that case, a filler material can be added to dilute the product to the correct grade.
Conclusion
If you have any questions about the DAP production process, raw materials, or related challenges, feel free to reach out—we’re always happy to discuss and exchange ideas!
