Ammonium nitrate is a white, crystalline solid widely used in agriculture and industry. It is chemically a salt, formed from the reaction between ammonia and nitric acid, and is stable and safe to handle at room temperature. Its high nitrogen content made it a popular and affordable fertilizer, accounting for a large portion of its global consumption. Historically, its properties were also recognized for their potential in explosives, and it remains a primary ingredient in industrial blasting agents. Ammonium nitrate is not flammable, meaning it cannot sustain a fire by itself, but it is a powerful oxidizer that can dramatically intensify a fire or lead to a massive explosion under specific, rare conditions.
The Critical Distinction: Oxidizer vs. Flammable Material
A flammable material, or fuel, is a substance like wood or gasoline that reacts with oxygen from the air to produce heat, light, and fire. Ammonium nitrate is not a fuel and does not burn in the traditional sense, classifying it as noncombustible. It is instead classified as a strong oxidizer, a substance that supplies the oxygen necessary for a fuel to burn.
The chemical source of this oxidizing power lies in the nitrate ion component of the compound. This ion contains oxygen atoms that are readily available to participate in combustion reactions. When ammonium nitrate is introduced to a fire involving organic material, the oxygen it supplies accelerates the burning process, making the fire far more intense than it would be otherwise.
The presence of this internal oxygen source means that a fire involving ammonium nitrate and a fuel can proceed even in an environment with limited external air. This contrasts sharply with a normal fire, which is dependent on atmospheric oxygen to continue.
The Conditions Required for Decomposition and Explosion
Pure ammonium nitrate is very stable, but it can transition from a benign oxidizer to a devastating explosive when three specific conditions are met: high heat, confinement, and contamination.
High Heat
The first condition is exposure to an external heat source, such as a fire, which causes the material to melt at temperatures around 170°C. When heated, the solid begins thermal decomposition, initially breaking down non-explosively into gases like nitrous oxide and water vapor. If the heat continues to rise, typically to a range between 260°C and 300°C, the decomposition reaction shifts. This higher-temperature reaction is exothermic, meaning it generates its own heat, and rapidly produces large volumes of gases, including nitrogen, oxygen, and water vapor. The rapid, near-instantaneous generation of these gases is the source of the explosive force.
Confinement
Physical confinement is crucial for the reaction to escalate from a high-speed burn, or deflagration, to a full-scale detonation. Confinement, such as that provided by a large stockpile, a storage silo, or a closed container, traps the gases being produced. This pressure buildup increases the temperature and promotes a self-accelerating reaction that results in a supersonic shockwave characteristic of a detonation.
Contamination
The third factor involves contamination, which significantly lowers the temperature and energy needed to trigger the explosive reaction. Mixing ammonium nitrate with incompatible materials like carbonaceous substances (e.g., fuel oil, wood, or charcoal), certain metals (like copper or zinc), or acidic compounds increases its sensitivity to shock and heat. These contaminants can act as fuels or catalysts, making the decomposition process much easier to initiate and control.
Industrial Applications and Safe Handling Guidelines
Ammonium nitrate is predominantly used in two major applications: as a high-nitrogen agricultural fertilizer and as a foundational component in commercial explosives. Its use in mining and construction is extensive, where it is commonly mixed with fuel oil to create Ammonium Nitrate Fuel Oil (ANFO), a bulk industrial explosive that is comparatively inexpensive and stable. The ANFO mixture relies on the ammonium nitrate acting as the oxidizer and the fuel oil as the fuel to create a controlled explosion.
Due to the chemical’s hazardous potential, the storage and handling of large quantities are subject to stringent regulations. The primary safety guideline is the absolute separation of ammonium nitrate from all combustible materials, including fuels, organic compounds, and certain incompatible metals.
Storage facilities must be designed to avoid any possibility of confinement, particularly in the event of a fire. This includes:
- Using noncombustible flooring without pits or drains where molten ammonium nitrate could pool and become trapped.
- Ensuring proper ventilation to prevent the buildup of heat and pressure.
- Maintaining a low storage temperature.
- Preventing the material from becoming contaminated.