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Ammonium nitrate

physical science Maturity 11-13 Next article →

Ammonium nitrate is a white crystalline salt with the chemical formula NH4NO3. It is composed of two specific ions: ammonium and nitrate. This compound is incredibly important in modern industry because it serves two very different roles. Primarily, it is used as a high-nitrogen fertilizer to help plants grow. Secondarily, it is a key ingredient in many industrial explosive mixtures. Because it can support combustion and release energy, it must be handled with extreme care.

Industrial production usually begins with an acid-base reaction. Workers combine ammonia (NH3) with nitric acid (HNO3) to create the salt. This specific chemical reaction is highly exothermic, meaning it releases a large amount of heat. Once the solution is formed, it typically reaches an 83% concentration. To increase this, the excess water is evaporated away. This process leaves a highly concentrated ammonium nitrate melt, ranging from 95% to 99.9% concentration. This melt is then processed into small beads called prills in a spray tower. Alternatively, it can be turned into granules by spraying and tumbling it in a rotating drum. These final products are often dried, cooled, and coated to prevent them from sticking together.

Ammonium nitrate exists in several different crystalline phases depending on the temperature. At room temperature, it often transitions between the beta-rhombic and alpha-rhombic forms. This transition happens at 32.3 °C. Because these two forms have a 3.6% difference in density, the material changes volume during the switch. This volume change can cause the solid to crack. For this reason, it cannot be used as a solid rocket motor propellant unless it is stabilized. Stabilized ammonium nitrate, or PSAN, uses metal halide stabilizers to prevent these density fluctuations.

Before modern chemistry, people had to mine ammonium nitrate from the earth. It occurs naturally as a mineral called gwihabaite in the Atacama Desert in Chile. In those extremely dry regions, it often appears as a crust on the ground. However, the invention of the Haber-Bosch process changed everything. This process allows scientists to synthesize nitrates from nitrogen in the atmosphere. As a result, mining the natural mineral became obsolete. Today, the world produces millions of tonnes of the synthetic version every year. Global production was estimated at 21.6 million tonnes in 2017, though it dropped to 16.7 million tonnes by 2021.

In agriculture, ammonium nitrate is a vital tool for food production. It has an NPK rating of 34-0-0, meaning it contains 34% nitrogen. While urea has a higher nitrogen concentration at 46%, ammonium nitrate is more stable. It does not lose its nitrogen to the atmosphere as quickly. In the mining and construction industries, it is used as a powerful explosive. A common example is ANFO, which is a mixture of 94% ammonium nitrate and 6% fuel oil. ANFO is widely used in coal mining and quarrying because it is inexpensive and easy to use. It accounts for 80% of the explosives used in North America.

Despite its uses, ammonium nitrate poses significant safety risks. It is a strong oxidizer, which means it helps other materials burn more intensely. If the salt is heated, it undergoes decomposition. At temperatures below 300 °C, it produces nitrous oxide and water. At higher temperatures, it decomposes into nitrogen, oxygen, and water. These reactions are also exothermic. If the decomposition happens too quickly, it can lead to a runaway reaction and an explosion. Large-scale disasters have occurred due to these risks. Notable examples include the Oppau explosion in 1921 and the Texas City disaster in 1947. More recently, massive explosions occurred in Tianjin, China, in 2015 and Beirut in 2020.

Beyond heavy industry, ammonium nitrate has several niche applications. It is used in instant cold packs because its dissolution in water is highly endothermic. This means it absorbs heat from its surroundings, making the pack feel cold. Scientists at King Abdullah University of Science and Technology are researching its use for off-grid cooling systems. It was also previously used as a cheaper component in car airbags. However, some versions were found to be unstable, leading to major recalls. Because of its potential for misuse, many countries have implemented strict regulations and licensing requirements for its sale.

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