Ammonium nitrate (NH4NO3) is a chemical compound composed of ammonium ions (NH4+) and nitrate ions (NO3–). It is a colorless, crystalline substance widely used in various applications, including agriculture, industry, and explosives.
Ammonium nitrate is an important chemical compound. Its primary production method involves the reaction between nitric acid and ammonia and it is used mainly in high-quality fertilizers. As a direct fertilizer this compound contributes to approximately 24% of global nitrogen fertilizer consumption.
Ammonium nitrate is an essential ingredient in various blended and complex fertilizer formulations, thus playing an important role in addressing the nutritional needs of the world’s populace.
Apart from its agricultural use, ammonium nitrate is used as an oxidizing agent and forms an integral part of numerous explosive compositions.
Table of Contents
1. Physical and Chemical Properties of Ammonium nitrate
Ammonium nitrate, with a molecular weight of 80.05 g/mol, presents as a colorless salt. Its density at 20 degrees Celsius is 1.725 g/cm³, and it possesses a specific heat capacity of 1.70 J/g·K within the temperature range of 0 to 31 degrees Celsius.
Its melting point is 169.6-170 degrees Celsius. The process by which ammonium nitrate is synthesized from ammonia and nitric acid is characterized by its highly exothermic nature, as depicted in the reaction:
NH3 + HNO3 → NH4NO3 ∆H = -146 kJ/mol
The phase transition occurring at 32.3 °C holds significant implications for the storage of fertilizers containing ammonium nitrate. Repeated exposure to this transition can lead to the deterioration of fertilizer granules due to differing densities, ultimately resulting in their disintegration.
Ammonium nitrate is very soluble in water and it displays hygroscopic behavior. Consequently, precautions are necessary to prevent moisture absorption during transportation and storage.
When ammonium nitrate dissolves in water, it absorbs heat, rendering it useful in freezing mixtures, such as those involving sodium chloride and ice.
Ammonium nitrate is solubile in various nonaqueous solvents. Liquid NH3 serves as a solvent in which the salt dissolves and readily absorbs NH3, producing solutions known as Divers liquid.
Aqueous ammonium nitrate solutions, with a concentration ranging from 50 to 70% by weight, vigorously absorb NH3 and serve as agents for NH3 stripping from gases. These solutions also find utility in the ammoniation of superphosphate.
In methanol, ammonium nitrate dissolves to create solutions of approximately 20% at 30 °C and about 40% at 60 °C. Its solubility in ethanol is around 4% at 20 °C, while in acetone is less soluble. Ammonium nitrate is insoluble in ether.
As a potent oxidizing agent, ammonium nitrate remains stable under standard temperature and pressure conditions. However, by heating above 170 °C, it decomposes into gases. This decomposition is accelerated by small quantities of chlorine or free acid.
2. Production of Ammonium nitrate
Ammonium nitrate is produced by the reaction of ammonia with nitric acid. Also, ammonium nitrate is generated during the manufacturing process of nitrogen-phosphorus (NP) and nitrogen-phosphorus-potassium (NPK) fertilizers.
This formation occurs by the decomposition of crude phosphate with nitric acid, with the resultant salt becoming a component of the fertilizers.
In the European region, the reaction of calcium nitrate, NH3, and CO2 is used to produce NH4NO3.
2.1. From Ammonia and Nitric Acid
Ammonium nitrate is formed by the reaction between gaseous ammonia and nitric acid and is characterized by heat release, ranging from 100 to 115 J per mole of NH4NO3. In various production processes, this exothermicity is harnessed for the partial or complete evaporation of water.
Depending on the pressure conditions and the concentration of nitric acid involved, it becomes feasible to yield ammonium nitrate solutions of 95 – 97%.
During the neutralization process, it is imperative to ensure swift and thorough mixing of the reactants within the reactor to prevent localized overheating, nitrogen losses, and the breakdown of ammonium nitrate.
Conventional installations use methods like the Uhde process and the SBA (Société Belge de l’Azote) process which are characterized by their lower reaction temperatures and reduced susceptibility to corrosion.
Exploiting the heat of reaction optimally is achieved through pressure-based neutralization.
The UCB process (Figure 1) features a heat exchanger within the pressure reactor, utilizing a portion of the heat of reaction to produce steam. Preheated ammonia and 52 – 63% HNO3 are injected into the reactor’s sump, operating at a pressure of about 0.45 MPa (4.5 bar) and temperatures of 170 – 180 °C.
The resulting 75 – 80% NH4NO3 solution is then concentrated to 95% by a falling film evaporator. Here, the heat of reaction facilitates the generation of process steam from the evaporating water within the nitric acid.
This process steam serves to preheat boiler feed water and nitric acid, along with operating the falling film evaporator. Furthermore, a portion of the heat of reaction creates pure steam, which can be channeled into the steam pool for alternate uses.
Sustaining a pH range of 3 – 5 mitigates nitrogen losses into the process steam, and operational conditions are finely tuned to prevent excess process steam accumulation.
Another pressurized process is the Stamicarbon process (Figure 2) employing a loop reactor that opens into a separator. The circulation of the reaction solution is maintained through the heat generated within the system.
a) Neutralizer; b) Intermediate tank; c) Surplus steam condenser; d) Ammonia scrubber; e) Condenser; f) Dilute ammonia solution tank; g) Condensate tank; h) Cooler; i) Evaporator; k) Separator; l) Seal pot; m) Storage tank for 95 % ammonium nitrate solution
At the lower end of the loop, preheated nitric acid (60 wt %), ammonia, and a small quantity of sulfuric acid are introduced. Operating at 0.4 MPa (4 bar) and 178 °C, the reactor yields an ammonium nitrate solution with a concentration of 78%.
The steam removed from the separator’s top is used for concentrating the NH4NO3 solution to 95% in a vacuum evaporator. Additionally, excess steam is condensed, and the recovered ammonia is recycled to the reactor.
Further concentration to 98 – 99.5% is done in a subsequent evaporator, using fresh steam, while the temperature of the ammonium nitrate solution is meticulously maintained below 180 °C throughout neutralization and evaporation.
The NSM/Norsk Hydro pressure process (Figure 3) is characterized by the use of preheated ammonia and nitric acid. Operating at a pressure between 0.4 and 0.5 MPa (approximately 4.5 bar) and temperatures ranging from 170 to 180 °C, this method results in a solution concentration of 70 – 80%.
a) Ammonia evaporator/superheater; b) Nitric acid preheater; c) Boiler; d) Reactor; e) Reactor separator; f) Scrubber; g) Flashtank; h) Evaporator; i) Separator; k) Condenser; l) Ejector; m) Tank
Forced circulation, coupled with a thermal siphon effect, drives the solution through the reactor. A portion of the heat of reaction contributes to generating pure steam within an external boiler, while some vaporizes water within the reactor, generating process steam used for concentrating the ammonium nitrate solution to 95%.
Minimizing ammonia losses is achieved by the washing of process steam with nitric acid, added to a circulating ammonium nitrate solution. The final stage of concentration, up to 99.5%, is achieved via steam in a specialized vacuum evaporator.
In the United States, the Stengel process allows the direct production of anhydrous ammonium nitrate. Preheated ammonia and approximately 58% nitric acid are introduced into a packed vertical tube reactor, operating at 0.35 MPa (3.5 bar) and 240 °C.
Following expansion into a vacuum within a centrifugal separator, the resulting mixture of NH4NO3 and steam undergoes stripping with hot air, leading to the discharge of a 99.8% NH4NO3 melt.
This melt is subsequently solidified on a cooled steel belt and further processed by breaking or granulation. The removal of steam occurs at the top of this process.
In all these processes, the careful maintenance of the desired pH range is important. For reaction temperatures below 170 °C, maintaining a pH between 2.4 and 4 serves to minimize nitrogen losses.
In pressure-based neutralizers, where higher temperatures and increased potential for decomposition are encountered, a slightly elevated pH range of 4.6 to 5.4 becomes necessary.
2.2. Conversion of Calcium Nitrate Tetrahydrate
In the Odda process, nitrophosphate fertilizers are produced by the digestion of crude phosphate with nitric acid forming calcium nitrate tetrahydrate, Ca(NO3)2 · 4 H2O, as a byproduct in substantial quantities.
While the production of nitrophosphates is witnessing an increase, the demand for calcium nitrate is experiencing a decline. A method that was introduced several years ago includs the treatment of calcium nitrate tetrahydrate with ammonia and carbon dioxide, resulting in the formation of ammonium nitrate and calcium carbonate by the following chemical reaction:
Ca(NO3)2 • 4 H2O + 2 NH3 + CO2 → 2 NH4NO3 + CaCO3 + 3 H2O ⇒ ΔH = -126 kJ/mol
The heat released during this reaction is sufficiently substantial to facilitate the complete evaporation of all associated water. However, a direct approach to this procedure is unfeasible due to the unfavorable equilibrium conditions that prevail at elevated temperatures.
The BASF process separates the heat removal from the reaction involving Ca(NO3)2 and (NH4)2CO3. In this method, NH3 and CO2 are dissolved within a circulating NH4NO3 solution, and the generated heat is efficiently managed.
Simultaneously, calcium nitrate tetrahydrate is dissolved in a distinct NH4NO3 solution. Subsequently, these two solutions are combined and reacted at approximately 50 °C, resulting in minimal heat generation.
The size of the calcite precipitated grains can be manipulated through the manner in which the reactants are introduced. Following the reaction, the resulting NH4NO3 solution, possessing an approximate concentration of 65%, is separated from CaCO3 using a belt filter, subsequently concentrated by evaporation.
The residual CaCO3 may still contain trace quantities of ammonium compounds and phosphate, rendering it especially suitable for use in the production of calcium ammonium nitrate. By preparing the calcium nitrate prior to its conversion, the generation of relatively pure CaCO3 becomes a viable outcome.
In a distinct approach that facilitates the direct conversion of calcium nitrate while concurrently addressing the heat released during the reaction, Hoechst developed a specialized vertical reactor.
In this reactor design, gaseous CO2 is introduced at the lower section, and the introduction of ammonia occurs within three distinct zones, each of which is actively cooled by water circulation.
3. Uses of Ammonium Nitrate
Ammonium nitrate (AN) is used primarily as a fertilizer, often in its pure form, diluted, or as a component within multinutrient mixtures. It is found in liquid fertilizers alongside urea, which is important in regions such as the United States, Eastern Europe, and France.
In the agricultural sectors of the United States, the United Kingdom, and France, ammonium nitrate having nitrogen content of 33.5% or higher is widely employed. The United States also utilizes a fertilizer comprising 32.5% N.
In the Federal Republic of Germany, ammonium nitrate is integrated into mixtures alongside lime, dolomite, ammonium sulfate, or potash, with a particular emphasis on calcium ammonium nitrate (CAN).
3.1. Calcium Ammonium Nitrate
A solution of ammonium nitrate (approximately 95 – 97%) can be transformed into granules by combining it with finely divided calcium carbonate derived from crushed limestone or obtained through the conversion of calcium nitrate.
Subsequent steps involve drying, cooling, screening, and applying treatments to prevent caking. The formation of hygroscopic calcium nitrate resulting from the reaction between ammonium nitrate and limestone is prevented by the addition of additives like (NH4)2SO4, MgSO4, and FeSO4.
In Germany, the nitrogen content of calcium ammonium nitrate has been gradually increased from the original 20.5% N to the current level of 27.5% N, adhering to the upper limit defined by the regulations of the European Economic Community at 28% N.
3.2. Ammonium Sulfate Nitrate
This mixed sulfate-nitrate fertilizer is created by adding ammonium sulfate into a solution of approximately 95% ammonium nitrate, or by neutralizing HNO3-H2SO4 mixtures with ammonia.
The granulated product, slightly hygroscopic in nature, is a mixture of the double salt 2 NH4NO3 · (NH4)2SO4 and a minor quantity of ammonium sulfate, possessing a nitrogen content of 26% N for < 45% ammonium nitrate.
Over time, this mixture may tend to harden due to further reactions. Such hardening can be avoided by introducing salts of Mg, Fe, or Al.
3.3. Potassium Ammonium Nitrate
Potassium ammonium nitrate is synthesized in a similar manner as ammonium sulfate nitrate, with the addition of a potassium salt (either chloride or sulfate) to yield fertilizers such as 20–0–20 (N–P2O5–K2O).
3.4. Nitromagnesia
Nitromagnesia is a fertilizer that is derived from ammonium nitrate, ammonium sulfate, and magnesium compounds like dolomite, magnesium carbonate, or magnesium sulfate. One such formulation may contain approximately 20% N, 8% MgO, and typically 0.2% Cu.
3.5. Other Applications
Ammonium nitrate serves as a key ingredient in safety explosives used in mining due to its comparatively low explosion temperature. Combining it with NaCl lowers the explosion temperature, mitigating the risk of igniting fire damp.
In some cases, safety explosives are formulated based on the complementary salt pair NaNO3 + NH4Cl → NH4NO3 + NaCl.
For greater explosive power in applications like mining (rock explosives), porous prilled ammonium nitrate containing around 6% diesel oil is used.
In small quantities, ammonium nitrate is involved in the production of dinitrogen monoxide. The salt must be free from organic substances, iron, chlorides, and sulfates and highly pure at 99.5% NH4NO3.
4. Safety
Ammonium nitrate, while stable under usual circumstances, undergoes distinct decomposition reactions at elevated temperatures such as:
1. An endothermic dissociation and pH reduction above 169 °C:
NH4NO3 → HNO3 + NH3 ΔH = +175 kJ/mol
2. Exothermic elimination of N2O by careful heating at 200 °C:
NH4NO3 → N2O + 2 H2O ΔH = -37 kJ/mol
3. Exothermic elimination of N2 and NO2 above 230 °C:
4 NH4NO3 → 3 N2 + 2 NO2 + 8 H2O ΔH = -102 kJ/mol
4. Exothermic elimination of nitrogen and oxygen, leading to detonation:
NH4NO3 → N2 + 1/2 O2 + 2 H2O ΔH = -118.5 kJ/mol
Pure ammonium nitrate, concentrated hot solutions, certain mixtures, and non-stabilized fertilizer forms can all be explosives susceptible to detonation by shock waves. Though the heat released is relatively small compared to compounds like hexogen, storing significant quantities presents serious risks. Factors such as hydrogen ions, chlorides, and heavy metals can catalyze decomposition.
Heating contaminated or compacted ammonium nitrate is particularly hazardous. Following incidents like the Brest and Texas City disasters in 1947, where wax-coated fertilizer ammonium nitrate exploded due to fires, regulations now limit flammable substances to 0.2% or 0.4%.
In Germany, the Working Materials Act outlines protocols for ammonium nitrate storage, loading, and transport. For instance, storage of potentially detonatable ammonium nitrate is restricted to small quantities in specially equipped facilities.
Similar rules apply in other European countries, while the United States, France, Norway, and England permit relatively larger storage under certain conditions.
In Germany, inert materials like limestone powder or dolomite are added to ammonium nitrate for fertilizer use (calcium ammonium nitrate). These fertilizers, containing up to 80% ammonium nitrate, a maximum of 0.4% combustible components, and at least 18% magnesium or calcium carbonate, are considered non-detonatable.
safety tips are as follow:
- Don’t blast solidified products containing ammonium nitrate during storage.
- Store ammonium nitrate products away from oxidizable or flammable materials.
- When heating ammonium nitrate, use small amounts and avoid catalysts.
Reference
- Ammonium Compounds; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a02_243
FAQ
The molar mass of ammonium nitrate is approximately 80.04 grams per mole (g/mol).
Ammonium nitrate is commonly produced by neutralizing nitric acid (HNO3) with ammonia (NH3). This exothermic reaction forms ammonium nitrate and water:
HNO3 + NH3 → NH4NO3 + H2O
Ammonium nitrate has diverse applications. In agriculture, it is a key ingredient in fertilizers, promoting plant growth by providing essential nitrogen. It is also used in explosives, where its controlled decomposition releases gases that cause explosions. Additionally, it finds use in various industrial processes, cold packs, and as an oxidizing agent.
The pH of a 1% solution of ammonium nitrate in water is approximately 5.5 to 6.0, making it slightly acidic. The presence of ammonium ions contributes to this slightly acidic nature.
Calcium ammonium nitrate (CAN) is a type of fertilizer that includes both ammonium nitrate and calcium carbonate. It is used to provide plants with nitrogen and calcium nutrients, enhancing their growth and development.
Ammonium nitrate can become explosive under certain conditions due to its ability to undergo rapid decomposition, releasing gases like nitrogen oxides and water vapor.
When ammonium nitrate is mixed with water, it dissolves readily, forming an aqueous solution. This dissolution process is endothermic, meaning it absorbs heat from its surroundings, resulting in a cooling effect.
