Fourteen people died, more than 100 hundred were injured and around 150 buildings were either destroyed or badly damaged after an explosion in a fertiliser storage plant containing ammonium nitrate (NH4NO3) in West (Texas). But this is not the only incident of its type; in the last ten years there have been several significant accidents involving ammonia nitrate as can be seen in the table below.
So, what needs to happen to ammonium nitrate to cause an explosion?
Ammonium nitrate is not combustible on its own, it is a strong oxidizing agent with a melting point of 170 0C and decomposes at 210 0C. As an oxidizing agent it will assist other materials or contaminants, such as powdered metals, urea, chromium etc to burn. In a fire the ammonium nitrate will melt and flow into nearby drains and culverts where it will accumulate and continue burning. The resulting separation of the NH4 and NO3 components in ammonium nitrate precipitates an explosion. The liberation of oxygen in the explosion fuels the fire and continues the reaction until all the ammonium nitrate is consumed. This makes this type of incident extremely difficult to control once a fire has started in or near to an ammonium nitrate storage facility. Adding water to ammonium nitrate during a fire generates heat and increases the danger of further explosions. This was graphically brought home to people in Texas in 1947 when fire broke out on a ship loading ammonium nitrate and the resulting explosion killed over 580 people with thousands injured. It still is the worst loss of life in a single industrial accident in the history of the United States.
Why are we still having industrial accidents involving the storage of ammonium nitrate? Can these types of accidents not be prevented or there impact reduced?
The storage of ammonium nitrate or fertiliser compounds that contain more than 28% nitrogen are high hazard materials with significant risk of explosion. Certain minimum precautions need to be taken for safe storage:
- Store ammonium nitrate in single storey, well ventilated, dedicated buildings and away from potential sources of heat, contamination, fire or explosive risk.
- Avoid storage above or near drains where, in the event of a fire, molten Ammonium Nitrate can accumulate with consequent risk of explosion.
- Limit stack quantities to 300 tonnes and separate stacks by a minimum of one metre.
- Keep a minimum of one metre distance from the wall, roof, services pipework and cables.
- All storage areas should be no smoking areas.
- Store outdoors.
- Avoid spillages from bagged products and clear spillages promptly.
- Ensure storage areas are secure to minimize the risk from vandalism and arson.
To conclude, while many people regard ammonium nitrate as a “fertiliser” it is a dangerous and, under certain circumstances, an unstable chemical which must be managed carefully and stored under the right conditions to avoid accidents and potentially significant loss of life and facilities.
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