Ammonium nitrate

Ammonium nitrate or ammonium nitrate is a salt formed by nitrate and ammonium ions. Its formula is NH₄NO₃. It is a colorless and hygroscopic compound, highly soluble in water. Ammonium nitrate is an unstable compound and is often used as a fertilizer. Global production is estimated to have been 21.6 million tons in 2017.

Summary

Ammonium nitrate is obtained by neutralizing nitric acid with ammonium hydroxide after evaporation of water:

$${displaystyle {ce {NH4OH + HNO3) NH4NO3 + H2O}}$$

In the laboratory it can be obtained by double decomposition between ammonium sulfate (NH₄)₂SO₄ and strontium nitrate [Sr (NO₃)₂], in solution. After precipitating the strontium sulfate and filtering the solution, which is then evaporated, ammonium nitrate is obtained in white crystals or powder.

$${displaystyle {ce {(NH4)2SO4{}_{(aq)}}+ SrNO3{}_{(aq)}- 2NH4NO3{}_{(aq)}}+ SrSO4 v (blanco)}}}}}$$

Applications

Ammonium nitrate is mainly used as a fertilizer due to its good nitrogen content. The nitrate is used directly by the plants while the ammonium is oxidized by the microorganisms present in the soil to nitrite (nitrosomonas) or nitrate (nitrobacter) and serves as a longer lasting fertilizer.

A part of the production is dedicated to the production of nitrous oxide (N₂O) through controlled thermolysis:

${displaystyle {ce {NH4NO3 - 2005 2 H2O + N2O}}}}$

This reaction is exothermic and can be explosive if carried out in a closed container or heated too quickly. In 2000, EFMA produced an eight-volume compendium presenting the "Best available industrial procedures for production prevention and control in the European fertilizer industry& #34;, in response to European and Spanish regulations.

According to EFMA, there are currently around ten different methods for the industrial production of ammonium nitrate in its different richnesses in Europe, there is no single procedure that can be considered the most advantageous compared to the rest, mainly due to two reasons:

• Trade considerations will influence the choice of a process or another.
• The same product can be obtained with similar characteristics by using different methods.

For this reason, first of all, a general impact will be made on each of the steps of the process, then establishing the best solutions that exist to solve the problems raised.

In Mexico, it is a product regulated by the Secretary of National Defense (SEDENA) with clear and rigorous measures under the "Federal Law on Firearms and Explosives" as well as its Regulations.

Fertilizer

Ammonium nitrate is a major fertilizer labeled NPK code 34-0-0 (34% in nitrogen). It is less concentrated than urea (46-0-0), giving ammonium nitrate a small disadvantage in transportation. As an advantage, ammonium nitrate is more stable than urea and does not rapidly lose nitrogen to the atmosphere. It is served in compact granules when used as a fertilizer, which improves its stability. Most of the production is used for this purpose. The salt resulting from ammonium nitrate when dissolved in water can be easily absorbed by plants.

Explosives

Ammonium nitrate is an ingredient in certain explosives. Examples of explosives containing ammonium nitrate include:

• Astrolite (amonic nitrate and hydrazine)
• Amatol (amonic nitrate and TNT)
• Ammonal (amonic nitrate and aluminum powder)
• Amatex (amonic nitrate, TNT and RDX)
• ANFO (amonic nitrate and fuel oil)
• DBX (amonic nitrate, RDX, TNT and aluminum powder)
• Tovex (amonic nitrate and aluminium nitrate)
• Minol (amonic nitrate, TNT and aluminum powder)
• Goma-2 (Amonic nitrate, nitroglycol, nitrocellulose, dibutyl phthalate and fuel)

In its pure state it is also an explosive, although quite insensitive until it reaches high temperatures. Mixed powdered aluminum adds energy to the shock wave, but with some lessening of the blast wave.

Mixture with fuel oil

ANFO is an explosive resulting from mixing 94% ammonium nitrate ("AN") and 6% fuel oil ("FO") widely used as an industrial explosive. used in coal mining, open pit mining, metal mining and civil works where the advantages of ANFO's low cost and ease of use outweigh the benefits offered by conventional industrial explosives, such as water resistance, oxygen balance, detonation velocity and its ability to work in small diameters.

Chemical industry

Ammonium nitrate is also used in:

• Treatment of titanium minerals
• Preparation of nitrous oxide
• In survival kits as mixed with powdered zinc and ammonium chloride will be lit in contact with water
• To produce anhydric ammonia, a chemical of frequent use in amphetamine.

Specialized uses

Ammonium nitrate is used in some instant cold packs since diluted in water it is highly endothermic. It is also used, in combination with explosives "fuels" such as guanidine nitrate as a cheap (but less stable) alternative to 5-aminotetrazole, an airbag inflator produced by the Takata Corporation. It was later found to be unsafe killing 14 people.

A solution of ammonium nitrate with nitric acid known as Cavea-b became a promising mixture for use as rocket fuel, being more energetic than hydrazine. After some tests carried out in the 1960s the substance was not adopted by NASA.

Manufacturing processes

The reaction between ammonia and nitric acid is irreversible, complete, instantaneous, exothermic, and admits any thermodynamic or kinetic discussion. The heat of reaction depends on the concentration of nitric acid used and the ammonium nitrate solution produced, since the more concentrated the solution is, the higher the heat of reaction. Said heat of reaction can be used to cause evaporation of water from the ammonium nitrate solution and also to produce steam.

Pure ammonium nitrate undergoes endothermic decomposition at 169°C and has a boiling point of 230°C. The concentration of nitric acid normally used is 55 to 65%, while its boiling point at atmospheric pressure is 120 °C, therefore lower than the solution produced from ammonium nitrate, highly concentrated solutions show high boiling points and of freezing. The first can cause high temperatures and therefore dangerous operations and the second blockage of the pipes.

Ammonium nitrate stored at 100 °C for an extended period of time undergoes thermal decomposition to ammonia and nitric acid, which decomposition above 185 °C can produce a dangerous explosion. The solubility of ammonia in water decreases rapidly with increasing temperature and the high volatility of the components and the decomposition of the produced salt easily lead to environmental losses and corrosion problems. The control of the reaction variables (temperature, pressure, heat used and concentrations of nitric acid and ammonium nitrate) and the construction details, achieve the use of maximum heat, generating a molten mixture without the addition of external heat that at the same time time ensures conditions, all with the same equipment and energy consumption, in which the highest possible production and high product quality are achieved.

The process for obtaining ammonium nitrate basically consists of the following steps:

1. The neutralization of ammonia with nitric acid.
2. Evaporation of the neutralized solution.
3. Control of the size of particles in crystallization and the characteristics of the dry product.

Neutralization

It is an instantaneous and highly exothermic reaction, as seen above, with an unstable reaction product, but we can obtain a good industrial performance when the following conditions are met:

1. Excellent mix of reagents.
2. Strict control of the pH, modern systems use automatic control of the pH, using two automated valves, the theoretical proportion we need of ammonia and nitric acid in the reactor is controlled.
3. Temperature control in the reactor, to avoid local overheating because the higher the temperature in the reactor, the more important it is to maintain the constant pH value and to avoid the introduction in the same of chlorides, heavy metals and organic compounds, as there is a risk of explosion. It must also be controlled to:
   • Avoid losses in the reagents, as both especially the ammonia are considerably volatile and could therefore escape along with the water vapor generated if the temperature rises unduly.
   • Prevent the risk of decomposition of the product.

The reaction temperature is controlled by means of the proper regulation of the addition of the reagents, by removing the heat generated and in extreme cases, adding water (condensates) to the content of the neutralizer. Although acid losses can be virtually eliminated through reaction temperature control alone, the same is not the case for ammonia losses, due to its higher volatility. Therefore, it is necessary to take additional measures. In some processes, a slight excess of acid over the stoichiometrically required amount is added for this purpose. In others, the neutralizer works completely filled with liquid, which makes it feasible to maintain a pressure of several atmospheres in it, well above the vapor pressure of the solution.

In practice, commercial processes differ in two main points, in the mixture and in temperature control, this being the most important characteristic. The parameters of the reaction and the construction adopted in the neutralization define a whole production line: preheated acid, evaporation of ammonia and evaporation of the remaining water (partially or totally) can be carried out by means of the heat recovered in the neutralization.

Types of neutralizers

Depending on the temperature of the reaction zone

The neutralizers are divided into three groups according to the temperature of the reaction zone, which can work:

1. Below the atmospheric boiling point.
2. Atmospheric boiling point.
3. On the boiling point of the solutions of ammonium nitrate.

Neutralizers that work below the atmospheric boiling point, are low temperature methods and have advantages such as:

• Low temperature causes lower corrosion problems.
• Material loss is lower and operational security is good.

They also have some drawbacks, such as:

• The flash vacuum complicates the equipment and depending on its complexity, increases investment and energy consumption.
• The use of reaction heat is necessary because the operating temperature is very low.

Neutralizers that work at the atmospheric boiling point, do not use recirculation of the ammonium nitrate solution, therefore the reaction will be less controlled as it is very exothermic and abrupt, if the solution is recirculated it absorbs part of the heat and this suddenness is controlled, avoiding the losses of nitrogen that could originate. Although its temperature is higher than that of the previous neutralizers, around 150 and 200 °C, it has advantages such as:

• Good chemical efficiency.
• Low-material losses.

The main drawback is the contamination of the process steam with ammonia and nitric acid, which requires stainless steel equipment. Neutralizers above the atmospheric boiling point are the most suitable for a good production process.

The neutralizers that work above the atmospheric boiling point, the common feature of all designs in this group is that the applied pressure generally between 2 and 6 bar is used to raise the temperature in the neutralizer up to approximately 180 °C. At higher pressures and temperatures, greater losses and more corrosion are caused, requiring special equipment.

According to the recovery of heat of reaction

The following types of neutralizers are distinguished:

1. Processes without heat use.
2. Processes using heat, where reaction heat is used to bring the reactive mixture to the boiling point and partially evaporate the water introduced with weak acid.
3. Processes with double heat use, reaction heat is used to partially evaporate water introduced with nitric acid and to produce steam. The latent heat of such steam will be used later to preheat the reagents and for the preconcentration of the ammonium nitrate solution.

The first two cases are not used in modern plants, that is, at least a part of the vapors produced are used in processes in the same plant.

Depending on the pressure of the vapors produced in the neutralizer

Since the determining factor in heat recovery is the neutralizer, the operating conditions of the neutralizer will define the pressure of the vapors in it and therefore its condensation temperature, which is the parameter used in the previous classification. Therefore it seems more appropriate to group the processes according to the pressure of the vapors produced in the neutralizer, so they will exist:

1. Vacuum flash processes:
   • Simpler, with the least possible heat recovery, such as the Process Udhe IG Farbenindustrie.
   • More complex, with maximum heat recovery, like the Process Kestner.
2. Processes with atmospheric pressure neutralization:
   • Process ICI.
   • Process Kaltenbach Nitrablock.
3. Processes with pressure neutralization:
   • Process Fauser.
   • Process Stamicarbon.
   • Process Kaltenbach high concentration.
   • Process SBA.
   • Process UCB.
   • Process Stengel.

Types of neutralizations

At pressure below atmospheric (in vacuum)

In this type of neutralizers, when ammonia and nitric acid react, the heat of reaction begins to increase increasing the temperature of the mixture towards its boiling point, where evaporation will begin and the temperature will continue its increase until the point where the water present evaporates consuming the heat of the reaction left over from heating the mixture.

To work around this point, all processes use recirculation systems, where a part of the ammonium nitrate produced is cooled and recirculated to the neutralizer, thus causing finer control of the temperature in the neutralizer. Said cooling and the recirculation ratio will define the temperature in the neutralizer. This type of neutralizers maintains the temperature around 100 and 120 °C, but it is necessary to use the heat of the reaction to evaporate part of the water contained in the product, that is, low product concentrations are obtained. This type of neutralizers are usually of the vacuum flash or vacuum neutralizers type, and can be carried out in one or several stages, thus they can be distinguished:

• Vacuum neutralization in one step: ammonia, nitric acid and recirculated ammonium nitrate are fed to a neutralizer that works atmospheric pressure, where the good distribution, mixture and control of pH is controlled. The formed product passes to a post-neutralizer or flash evaporator, where more exhaustive control of pH takes place. Part of the heat of the reaction contained in the recirculated solution is used for the partial evaporation of the water contained in the ammonium nitrate produced, in turn cooling the recirculated current. The concentration of the resulting current will depend on the concentration of acid and the warming of raw materials.
• Multi-step vacuum neutralization: similar to the previous one, except because several series flash evaporators are available, obtaining concentration solutions around 98% w of ammonium nitrate.

Advantages and disadvantages

• Reduced material corrosion problems are presented, resulting in the reduction of material losses and increased security. In contrast they are bulky equipment and therefore expensive.
• The use of reaction heat is very low, basically used in the preheating of nitric acid, so energy efficiency will be small.
• Improve heat recovery is only possible by means of more sophisticated equipment, such as multi-pass neutralizers, although there will be greater corrosion problems since the temperature will increase around 160 or 170 °C.
• The depuration systems of steam removed from the neutralizer (which is always contaminated with ammonia and with fine ammonium nitrate particles) are also very bulky and therefore expensive.

At atmospheric pressure

This equipment is simpler than the previous ones, they work at higher temperatures (around 150 and 200 °C) they will produce a stream of steam that will contain most of the water introduced by the nitric acid, which will be used for the preheating of raw materials.

With concentrations of nitric acid around 60% w, concentrations of around 98% w of ammonium nitrate can be achieved, although a small evaporator is usually used after the neutralizer. To achieve better pH control, two neutralizers are used in series, the second being smaller than the first, to achieve a finer adjustment.

Advantages and disadvantages

• It works at moderate temperature, so materials can be less demanding and there is less risk of decomposition of the ammonium nitrate than overpressure.
• The depuration systems of the detached steam of the neutralizer (which is always accompanied with ammonia and fine particles of ammonium nitrate) are also very bulky and therefore expensive, for the same reason will be necessary stainless steel heat exchangers.
• The low temperature of the steam restricts its use in other applications, so heat is used only for the warming of the raw materials, so energy performance is very low, requiring an external heat input, to reach the final concentrations of work.

Overpressure

Two types of overpressure processes can be distinguished:

• Medium-pressure neutralizers (up to 4 absolute atm), these processes are the most used in the industry, since their reaction temperature is not so high that it poses danger, and allow the use of reaction steam for the concentration, at least partial, of nitrate liqueur. Some of these reactors are provided with external recirculation of the reactionary mass, in order to increase the homogeneity of nitric acid in the mass, so that its reaction with the ammonia occurs evenly and within a significant volume of liquor that acts of tampon. Other reactors are provided with an exchanger-caldera that is placed in the back of the reactionary mass, and that inside it is fed by water that evaporates, producing clean steam in exchange for a lower concentration of the resulting liquor.
• High-pressure neutralizers (upper to 4 absolute atm),

They are usually carried out between 4 and 6 atm, depending on the industrial process. The pressure serves to increase the temperature in it to around 200 °C. Within this group the processes Fauser and Stengel can be displayed.

Advantages and disadvantages

• The use of high-pressure neutralizers, such as the previous two, has advantages in terms of investment costs, but presents problems in controlling the process of neutralization and explosion hazards when operating at such high temperatures.
• The main advantage they present will be the possible use of the vapors of the neutralizer (from 4 to 5 atm), both for the preheating of raw materials and the evaporator, so there will be greater energy efficiency.
• The main problem is that increased pressure and temperature will cause greater corrosion and greater losses of both nitrogen and ammonium nitrate, so the cost of materials will be higher.

Evaporation

The different procedures differ from the water content of the reagents (therefore from the concentration of ammonium nitrate that comes out of the neutralization section), from the amount of water required in the following solidification processes of the final product and of temperature control.

In methods used until 1945, the neutralized ammonium nitrate solution was evaporated to a high concentration, followed by consecutive cooling and product formation. Other methods carried out evaporation to a lower concentration and completed it by means of crystallization or continuous evaporation in devices designed for this purpose, said evaporation was also done using wiped film evaporators, which had the advantage of contain very low weights of matter being treated.

After 1965, effective vacuum-operated evaporators have been used in new factories, these modern units have higher thermal efficiency and can be precisely controlled. The part of the unit where the concentration is greater than 99% w ammonium nitrate is designed to retain only small amounts of concentrated solution for safety reasons. These precautions are necessary to avoid contamination of the solution by organic matter and its possible explosion.

Ammonium nitrate solutions can vary between 78 and 98% w, and solidification processes can work with molasses from 5% w of water (in drum granulators) to 0.3 to 0, 5% w of water (in prilling towers), which is why there are hundreds of evaporators in the industry, each one adjusted as much as possible to the needs imposed by the required product.

Handling and storage

• Manage: provide adequate ventilation. Use eye and hands protection.
• Storage: place tanks away from fuel storage. Protect the corrosion tanks and physical damage. Check the pH of the solution daily. If the pH of the solution to 10% is below 4.5 add ammonia gas until this pH is reached. The material suitable for containers is austenitic stainless steel. No smoking. Use protected lamps in storage areas.

Security measures

• Recommended exposure limits: there are no specified official limits. (1995-96)
• Precautionary measures and mechanical equipment: avoid exposure to steam and provide the necessary ventilation facility. Install lava-ey equipment and safety showers anywhere where contact with eyes and skin can occur.
• Personal protection: in emergencies, use appropriate breathing equipment. Use heat-resistant gloves and protective clothing. Use chemical safety glasses or face screen.

Transport information

• UN Classification: Class 5, Division 5.1 —Comburent — UN No. 2426.
• RD. 1254/1999: Control of the risks inherent in serious accidents.
• RD. 145/1989 National Regulation on Admission, Manipulation and Storage of Dangerous Subjects in Ports.

Disasters

1940s

• 1942: Tessenderlo, Belgium under Nazi occupation, an accidental explosion of 150 tons of Ammonium Nitrate caused 189 deaths and 900 wounded.
• 1947: Texas, the United States, considered as the most deadly accidental explosion in history, occurred in the industrial port of Waco, a French ship loaded with this substance exploded causing 581 dead and 3500 wounded, parts of the structure of the ship fell into the city, five hundred newly manufactured cars were destroyed.

Tianjin Explosions

On August 12, 2015, an explosion occurred at a warehouse in the port of the city of Tianjin. The explosion left 173 dead, 28 missing, and 797 injured. Chinese authorities determined that the explosion occurred from the burning of hazardous materials, including ammonium nitrate.

Beirut Explosions

Red smoke from the ammonium nitrate that came out in the second explosion, seen on the sky of Lebanon.

On August 4, 2020, an explosion occurred in a warehouse containing 2,750 tons of ammonium nitrate that had been confiscated by the government and stored in the Port of Beirut. At least 158 people have been confirmed dead, 6,000 injured and several missing. The explosion was equivalent to a few hundred tons of TNT.

Terrorism

In 1995 in Oklahoma City, an American named Timothy McVeigh filled a truck with two tons of ammonium nitrate and blew it up in front of a city government building, killing 168 and injuring 600.

In 2001 Anders Breivik also blew up ammonium nitrate in Oslo, the same day he carried out the Utoya island bombings.