A tragic accident, perhaps compounded by carelessness, led to a fire and explosion in a fertilizer plant in West TX on April 17. (Just to clarify a point which was slightly confusing in the initial reports, ‘West’ is the actual name of the town.) The news was somewhat overshadowed by the bombings at the Boston Marathon on April 15, but the disaster killed 14 people, injured many more, and devastated the small town. The plant apparently had stores of anhydrous ammonia (NH3), a gas, and ammonium nitrate (NH4NO3), a solid. Both are very commonly used as components of fertilizers. Ammonia is a strong irritant, and a health hazard, but doesn’t burn in air except in very high concentrations (roughly 15-25%). Ammonium nitrate is also an irritant; however, it is also a powerful oxidizing agent, and can form explosive mixtures with many organic compounds.
In fact, ammonium nitrate has been used, mixed with fuel oil, to make bulk industrial explosives for routine use, because of its low cost. It has also been a popular ingredient for improvised explosive devices (IEDs) and vehicle bombs, such as the one set off at the Murrah Federal Building in Oklahoma City in 1995. Because of its potential for misuse, there are regulations concerning its storage and use, but these are apparently not always followed. (It appears that the plant in West did not report its February inventory of 270 tons to the Department of Homeland Security, as the law requires.)
An article at the Gizmag site reports that Kevin Fleming, an engineer from Sandia National Laboratory, has developed a technique for compounding ammonium nitrate so that it can’t be used to make fuel-based explosives.
Knowing that in ammonium nitrate the ammonium ion is weakly attracted to the nitrate ion, and that the right chemical reaction can pull them apart, Fleming decided to look for a compound they would rather cling to that could be added to the ammonium nitrate. He tried several materials, including iron sulfate, a readily available compound discarded by the ton from steel foundries.
If someone attempts to mix fuel into the ammonium nitrate / iron sulfate mixture, they will end up with ammonium sulfate and iron nitrate, neither of which will form an explosive mixture.
The addition of iron sulfate does not degrade the usefulness of the fertilizer; in fact, it probably makes it slightly better for environments with alkaline soils. Adding iron to the soil may also incrementally improve the iron content of vegetable crops.
Since iron sulfate is cheap — it’s a waste product from steel production — this technique might be an economical way to reduce the risk of explosions, accidental or otherwise.
Update Monday, 29 April, 22:16 EDT
Here is the original Sandia Labs information release. Their server appears to have been down last night.