Ammonium Nitrate - nitrogen fertilizer and deadly explosive

 Ammonium nitrate has the chemical formula NH₄NO₃. Produced as small porous pellets, or “prills”, it’s one of the world’s most widely used fertilisers, referred to by its NPK formula of 34- 0-0. It is made by combining ammonia gas with liquid nitric acid, which itself is made from ammonia.

It's use as a fertilizer in Canada is declining as urea (46-0-0) and anhydrous ammonia (84-0-0) are more economical and so more popular.

Ammonium nitrate is also the main component in many types of mining explosives, where it’s mixed with fuel oil and detonated by an explosive charge. I first heard of ammonium nitrate as an explosive when I was living in Ontario in the early 1970s. Highway construction in many parts of the province requires a great deal of blasting of solid rock to create a road bed in the Canadian Shield.

Highway construction crews used big trucks with compartments of ammonium nitrate and diesel fuel. Holes were drilled in the rock to be blasted, then the ammonium nitrate and diesel fuel were pumped into the holes, mixed while pumping. Large anti-blast mats made of tires were dragged over the spot and the explosive mixture detonated. 

Ammonium nitrate is classified as dangerous goods and all aspects of its use are tightly regulated because it can be used in the making of bombs. Two tonnes of Ammonium nitrate mixed with diesel fuel were used by Timothy McVeigh in the Oklahoma city bombing in 1995 which killed 168 people, injured at least 600 others and destroyed or damaged 324 building and 86 cars within a 16 block radius.  

The amount of ammonium nitrate which exploded in Beirut was over 10 times that, making the blast the equivalent of several hundred tonnes of TNT.

Ammonium nitrate does not burn on its own. Instead, it acts as a source of oxygen that can accelerate the combustion (burning) of other materials. In order for an accidental explosion to occur several things have to go wrong.

For combustion to occur, oxygen must be present. Ammonium nitrate prills provide a much more concentrated supply of oxygen than the air around us. This is why it is effective in mining explosives, where it’s mixed with oil and other fuels.

At high enough temperatures, however, ammonium nitrate can violently decompose on its own. This process creates gases including nitrogen oxides and water vapour. It is this rapid release of gases that causes an explosion.

Ammonium nitrate decomposition can be set off if an explosion occurs where it’s stored, if there is an intense fire nearby. The latter is what happened in the 2015 Tianjin explosion, which killed 173 people after flammable chemicals and ammonium nitrate were stored together at a chemicals factory in eastern China.

https://www.scientificamerican.com/article/what-is-ammonium-nitrate-the-chemical-that-exploded-in-beirut

The following is a true story. The names have been redacted to protect the guilty. About 30 years ago, long before Oklahoma made it impossible to buy this stuff across the counter, a friend of mine decided he needed a dugout in a coulee a good distance east of his farm site to water his cattle when the grazed there. He had heard of using ammonium nitrate and diesel fuel as an explosive. With that very limited knowledge (not that it had ever stopped him before) he bought a 25 kg bag of 34-0-0 and with a can of diesel fuel went down into the coulee. He set the bag where he wanted the dugout, poured the diesel into it, added a dynamite cap and 30 minutes of fuse, then covered it with mud.  He then retreated about 750 meters (1/2 mile) away on the highway where he could watch the action.

And action he got. Far more than he bargained on. The explosion rattled windows in the town a few km away, as well as in his own yard. His truck was showered with stones and dirt. The RCMP showed up in minutes and chewed him a new one. But he was happy. He had a dugout, and learned something new while having fun.