Control ammonium nitrate fertiliser

This control measure should be read in conjunction with Hazardous materials – Substance identification.

Ammonium nitrate technical grades that are classified as an explosive and assigned to ‘Class 1 explosive’ of the UN transportation classification system are subject to controls under explosives legislation. Responders should apply the control measures for the control measure Cordons controls: Explosives in all such incidents.

Ammonium nitrate is often present in mixed fertilizers (sometimes called balanced mixtures and known as N:P:K fertilisers). A code will be found on the bag, e.g., 6-12-12.

The numbers, in order, refer to:

• N % of nitrogen
• P % of phosphorus as if it were phosphorus pentoxide (P2O5)
• K % potassium as if it were potassium oxide (K2O).

N.B. the first number is the only one of interest to fire and rescue services. The higher it is, the greater the amount of nitrates or ammonium compounds, and thus the greater the danger from the mixed fertilizer if it is near, or becomes involved in fire.

Historically, any N% figure of 20 or more (maximum 35%) has been regarded as a potential explosive risk when the fertilizer is close to or directly involved in, any fire. However, it is possible for fertilisers with less than 20% nitrogen to explode if they have been contaminated and/or pressurised before being heated.

White/brown fumes coming from the fertiliser are a sign of potential explosion. In a fire, all types of ammonium nitrate may melt and decompose with the release of toxic fumes (mainly oxides of nitrogen), which may be white, yellow or brown.

Ammonium nitrate fertilisers that are slowly decomposing or smouldering can be managed by:

• Removing the heat source and extinguishing the fire or decomposition, if possible
• Recognising that decomposition is indicated by the release of white/brownish fumes from the fertilizer mass.

If an area of the ammonium nitrate-based fertilizer is slowly decomposing or smouldering, the following steps should be taken immediately:

• Identify the source of heat and, if found, turn it off or remove it
• If the area of decomposing material is still small and easily accessible, try to remove it from the main heap of the fertilizer using picks, shovels or a ship’s grab, and cool it down by localised quenching with water
• If it is impossible to remove the decomposing mass, soak the fertilizer involved as rapidly as possible with a large quantity of water, preferably directed against the centre of the decomposition through high pressure jets. This may cause the additional problem of large quantities of contaminated water run-off.
• Fighting the decomposition by other means, such as foam, carbon dioxide, steam or covering with sand, is useless and may even promote the decomposition
• If fumes are present, use self-contained breathing apparatus (SCBA)
• If suppressing the slow decomposition is impractical or unsuccessful use defensive tactics and consider evacuation
• In ships or containerised stores, open doors, hatches, etc. immediately to maximise ventilation if it is safe to do so.
• Use defensive firefighting and evacuation if explosive decomposition is suspected

For further information see National Operational Guidance: Fires and firefighting.

The risk of fire or explosion is greatly increased if ammonium nitrate is mixed with combustible or incompatible materials, such as powdered metals, alkali metals, urea, chromium or copper salts, organic and carbonaceous materials, sulphur, nitrites, alkalis, acids, chlorates and reducing agents. Fertilisers that contain 28% or less nitrogen (see the label or safety data sheets (SDS) for the percentage of nitrogen present) do not normally present an explosion hazard and therefore, in agriculture, ammonium nitrate-based fertilisers can be divided into two groups to identify the precautions required:

• Fertilisers that contain more than 28% nitrogen – most of these are straight ammonium nitrate types, although they include a small number of compound fertilisers
• Fertilisers that contain 28% or less nitrogen – compound fertilisers form the major proportion of this group; the straight nitrogen types are usually a mixture of ammonium nitrate with limestone or similar inert materials

The Ammonium Nitrate Materials (High Nitrogen Content) Safety Regulations 2003 prohibit the importation and supply of relevant ammonium nitrate with more than 28% nitrogen without a supporting ‘Resistance to Detonation certificate’.

Where a site contains 25 tonnes or more of dangerous substances, the Dangerous Substances (Notification and Marking of Sites) Regulations 1990 (NAMOS) require the person in control of the site to notify the fire authority and the enforcing authority of certain details. For ‘relevant ammonium nitrate mixtures’ this threshold limit is 150 tonnes. Within the regulations ‘relevant ammonium nitrate mixtures’ are defined as ammonium nitrate and mixtures containing ammonium nitrate, where the nitrogen content exceeds 15.75% of the mixture by weight.