Ammonium Nitrate

Ammonium Nitrate

HAZARDOUS MATERIALS

Chemical Data Notebook Series:

Usually a white solid that is very soluble in water, ammonium nitrate is principally used as a fertilizer—but, once involved in an incident, can present many serious hazards.

PROPERTIES/APPEARANCE

There are many grades of ammonium nitrate and, depending on the additives that may be present, the color can vary from off-white to brown.

The Department of Transportation (DOT) classifies ammonium nitrate as an oxidizer, and it is very efficient in supporting combustion. Ammonium nitrate is used to increase the brisance of some explosives, and is a major ingredient in blasting agents where it is mixed with combustible materials, usually fuel oil. When ammonium nitrate blasting agents are shipped, they are usually referred to as nitro-carbo-nitrates (NCN).

Ammonium nitrate is also used in the manufacture of solid rocket propellants, medicines, matches, fireworks, insecticides, herbicides, nitrous oxide, and several other materials.

IDENTIFICATION

The chemical formula for ammonium nitrate is NH4NO3, and its specific gravity is 1.72 (but remember, it is very soluble in water). Ammonium nitrate melts at 337.3°F and decomposes at 424.4°F. Its UN/NA designation is 2426, and one of its several Standard Transportation Commodity Code designators is 4918311. Its IMO (International Maritime Organization) designations are 1.1, explosive, and 5.1, oxidizing agent. It is shipped as a solid or dissolved in water.

All mixtures containing ammonium nitrate as the principal ingredient (except for explosives and blasting agents) will have ammonium nitrate as part of its name. Synonyms include Norway saltpeter, ammonium salt of nitric acid, and trade names such as Nitram and Varioform I.

HAZARDS

Reactivity

Although ammonium nitrate will not react with water, it will react with many other materials, particularly combustible materials. It will form powerful, explosive (unstable) mixtures with many powdered metals (particularly aluminum), with any flammable or combustible liquids, and with any combustible solids. Ammonium nitrate should not be allowed to mix with acids, reducing agents, and many other chemicals too numerous to mention.

When subjected to excessive heat, ammonium nitrate will decompose, liberating the reddishbrown nitrogen oxides (NOx), many of which are oxidizing agents and/or toxic. This decomposition is liable to convert to a detonation at any time. Ammonium nitrate does burn, even if it is the ammonia liberated by decomposition, and is supported by the oxygen and/or nitrogen oxides being produced during this exothermic reaction.

Ammonium nitrate has been responsible for several tragic detonations that have cost many lives. The most famous (or infamous) incident in the United States was the Texas City, TX, explosion on April 16, 1947, that took the lives of 468 people (mostly spectators to the fires), including the entire 27-man Texas City Fire Department.

A fire broke out on board the SS Grandcamp, a French freighter carrying 2.8-million pounds of ammonium nitrate. A fateful decision was made to stop fighting the fire with a handline and to batten down the hatches to flood the storage compartments with live steam, a common procedure that is successful on ordinary combustible materials. But ammonium nitrate, in addition to burning, is an oxidizing agent, which means it will actually support combustion. The tactic of using live steam never had a chance. The fire burned for several hours, liberating large quantities of toxic gases. It finally detonated. The first detonation wiped out the fire department and most of the bystanders, and started several more fires. The next day, it communicated to and detonated another ship, ending the incident.

The only chance firefighters had would have been to flood the Grandcamp or scuttle her, and even that might not have prevented the explosion.

Ammonium nitrate has not detonated every time a large quantity has burned, nor has it detonated every time researchers have tried to detonate it under circumstances similar to other detonations, but be assured that it will detonate as it has in the past, when least expected, and cause great loss of life.

Personal

Ammonium nitrate in its pure form is no great health hazard, but excessive exposure should be avoided, as it causes irritation of the eyes, nose, and skin. Inhalation of powdered ammonium nitrate can cause lung irritation. Flushing skin and eyes with water for 15 minutes will handle minor eye contact problems, while soap and water is all that is needed to wash material from the skin. If allergic reactions occur, seek medical attention immediately.

When ammonium nitrate decomposes, which it will do when subjected to excessive heat, it will liberate several gases belonging to one family, the nitrogen oxides (NOx). These gases, which may be reddish-brown in color, are highly toxic, but usually have delayed results.

One of the gases, nitrogen dioxide, NO2, will attack the lower respiratory function by dissolving in the moisture in the lungs, forming nitric acid, HNO3. Over a period of time, the nitric acid can destroy the alveoli, reducing lung capacity. Depending upon the exposure to these gases, irreversible lung damage can occur over a 12to 48hour period, causing shortness of breath, chest pains, a cold sweat, and pain radiating from the chest, all symptomatic of a heart attack. Death follows very quickly.

PROTECTIVE CLOTHING

Positive pressure self-contained breathing apparatus is required in any incident where ammonium nitrate may be subjected to excessive heat or fire. Where there is no excessive heat or fire, but dust may be a problem, an approved particulate respirator is required.

Additional protection will be needed in confronting released ammonium nitrate. Regular turnout gear that will protect the wearer from direct contact will be sufficient, if such gear includes rubber boots and gloves.

HANDLING

Ammonium nitrate is very soluble in water so that if it is threatened by an approaching fire, flooding amounts of water will dissolve most of the material and wet down the rest, preventing its ignition.

However, since ammonium nitrate is a powerful oxidizer, all runoff water must be contained or otherwise prevented from entering sewers or waterways, since sewer systems are usually loaded with organic material (which will burn). With the addition of an oxidizer, all that is needed is a source of sufficient energy to start a difficult-to-handle underground fire.

Should such a solution enter a waterway, all downstream users and processors of the water should be alerted. Diversion and/or damming techniques may be employed to prevent downstream contamination. Above all else, remember that under certain conditions, ammonium nitrate will detonate!

Since ammonium nitrate is a solid material shipped in granular or powder form, it is relatively easy to handle in a spill where no fire is involved, no water is used, and no fuel is present with which it can mix. It may be simply (but carefully) swept up and shoveled into a secure container. Care must be exercised to prevent the material from being blown around by the wind. A tarp or plastic sheet thrown over the spilled solid and anchored by weights will do the job. Covering the spilled product with sand or some other inert material will also prevent it from scattering, but will contaminate it.

If the material is being disturbed by the wind, a careful and gentle light soaking will wet down the material, and yet not dissolve it. The use of a water spray downwind will also remove ammonium nitrate dust from the air, but now runoff water will be a problem. In any event, the product may become contaminated and useless for the purpose for which it was intended.

Glossary

Blasting agent—A substance made up of a mixture of materials, none of which are classified as an explosive. Blasting agents are so stable that they require an explosion from a blasting cap (explosive) to detonate them.

Brlsance—The explosive power of an explosive or a material that explodes.

Exothermic—A chemical reaction in which heat is liberated.

Oxidizer—Any substance containing oxygen that gives it up readily, or in any way supports combustion.

Nitrogen oxides (NOx)—A group of gases that contain only nitrogen and oxygen. They include nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2). nitrogen tetroxide, (N2O4), nitrogen sesquioxide (N2O3), dinitrogen pentoxide (N2O5), and trinitrogen tetroxide (N3O4). Some of the gases are reddish-brown in color.

Your first responsibility is for your own safety, the safety of your co-workers, and the safety of other people who might be involved in one manner or another. However, if no life, property, or environmental concerns are in danger, there is no need to take steps that might ruin the product. Use your best judgement in handling the spill.

If water must be used at the incident, or rain begins to fall and you cannot cover the solid material, you must begin procedures that you would use if dealing with a liquid hazardous material. Containment of the solution becomes a major problem that must be handled.

Containment dikes are the simplest means of preventing a solution of ammonium nitrate and water from moving. You are concerned with prevention of runoff that might enter a sewer or contaminate a water supply, and/or the mixing of this solution with a fuel. You need not be concerned with evaporation of the water, since that is all that will evaporate; the ammonium nitrate will be left behind as a fine residue.

You may use soil, sand, or other sorbent materials to build the dike walls. All materials used will become contaminated with the nitrate, and will have to be disposed of properly, as will any soil into which the solution may have seeped.

A pit may be dug to hold the solution, remembering that soil at the bottom and sides of the pit will become contaminated as the runoff is allowed to collect. Trenches and ditches may be dug to lead the liquid from the spill to the pit. These areas will also become contaminated as the liquid seeps into the ground and walls of the trenches and pits. Once the liquid is contained, it may then be pumped out as a liquid and stored in secure containers until the ammonium nitrate may be reclaimed from the solution.

Once the dissolved material is contained, you may leave it in the pit or pond, but it will probably be better in the long run to pump it into tankers or other containers to prevent further contamination of the soil.

Once the incident has run its course, all contaminated soil must be removed (which may seem strange, since ammonium nitrate is a fertilizer, and is usually deliberately added to the soil).

Again, remember that the principle use of ammonium nitrate is as a fertilizer, and those users of the material may have no idea that its major hazard is that it is an oxidizer. Indeed, many (if not nearly all) of the users may not even know what an oxidizer is.

Originally ran in Volume 139, Issue 9.

ALSO

From the Fire Engineering Vault: Texas City Explosion

The Texas City Disaster

Vulnerability Of Buildings to Bombs: Additional Thoughts After Oklahoma City

Explosion in West, Texas: Lessons Learned from Multiple Deaths

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