Following the recent events in the Port of Beirut on the 4th August we at LAW felt it was worthwhile reminding ourselves of the risks posed by the substance.
It is not in itself combustible but, as it is an oxidising agent, it can assist other materials to burn, even if air is excluded. Ammonium nitrate will not explode due to the friction and impact found in normal handling, but it can be detonated under heat and confinement or severe shock. For example, in a fire, pools of molten ammonium nitrate may be formed and if the molten mass becomes confined (e.g. in drains, pipes, plant or machinery) it could explode, particularly if it becomes contaminated.
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 yellow or brown. Most types do not continue to decompose once the fire has been extinguished. However, when some types of ammonium nitrate fertilisers (cigar burners) are heated they undergo a smouldering (self-sustaining) decomposition that can spread throughout the mass to give substantial toxic fumes, even when the initial heat source is removed.
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 (consult data sheets to establish if a substance has reducing properties).
The National Counter Terrorism Security Office (NaCTSO) hosts the Secure Your Fertiliser website, which aims to increase the awareness in the farming and growing community and provide practical guidance about securing fertilisers.
The HSE has produced the following leaflet A Guide to the Security of Ammonium Nitrate used in Blasting (PDF) - Portable Document Format offers some practical help and guidance for sites, such as quarries, that use ammonium nitrate for blasting.
LAW’s Health & Safety At Work team are here to support employers, 24/7.