-Questions-

Question: What happened in Toulouse on Friday 21 September 2001?

As of Monday 24 September the following is known:

In one of France's worst industrial accidents, at least 29 people were killed and more than 2000 injured by a huge blast at the Grande Paroisse factory owned by Atofina, part of the TotalFinaElf oil and chemicals group. Several more people are still missing, and 400 of the wounded were still in hospital on Sunday.

French officials said it appeared that a store of ammonium nitrate had exploded. Some 200-300 tonnes of pure ammonium nitrate product is believed to have been involved. The shock wave damaged dozens of schools, churches and other buildings, destroyed passing cars and smashed windows in the town centre. It damaged 3,000 government-subsidised homes for the needy, 400 of them seriously. The blast left a crater 50 metres across.

It is not known what initiated the event. Ammonium nitrate may detonate under extreme conditions. However, ammonium nitrate is considered a very stable salt when it is handled according to regulations. It is normally classified as an oxidizing agent. Pure ammonium nitrate is difficult to detonate. Neither spark, flame nor friction cause detonation, and it is relatively insensitive to shock. Mixtures of ammonium nitrate dust and air do not present an explosion hazard. Pure ammonium nitrate does not decompose self-sustainingly. To get an explosion in pure ammonium nitrate due to high pressure impact, high explosives such as TNT are needed. There is no reason to believe that falling objects or collisions will lead to an explosion. However, this is not valid for ammonium nitrate with reduced stability due to contamination. Here much smaller loads can trigger an explosion. Also, ammonium nitrate is vulnerable in case of a fire and decomposition in a confined space, whereby high pressure can build up.

Learn more about agriculture and fertilizers.

Question: What is ammonium nitrate used for?

Ammonium nitrate is basically used for two purposes:
(1) as a fertilizer (fertilizer grade), and
(2) as an explosive (technical grade).
The fertilizer grade is produced to specifications which make it safe from explosions. The technical grade is not explosive in itself, but becomes an explosive when adding organic materials, like diesel fuel. For an explosion to occur there must be a blast initiator, like a detonation from using dynamite.

Question: What kind of accidents have happened before with ammonium nitrate and nitrate containing fertilizers?

Some accidents are listed below. A more complete overview of accidents are given in The International Fertiliser Society Proceedings No. 384: Safety of Ammonium Nitrate Fertilisers.

Following a period of serious accidents up to the late 1950's, extensive research was carried out to improve the safety of ammonium nitrate fertilizers. This has translated into stringent regulations for the manufacturing, storage and transportation of ammonium nitrate and nitrate based fertilizers.

LOCATION	ACCIDENT
Gibbstown, New Jersey, USA 1916	An evaporating pan containing 1800 kg of ammonium nitrate exploded at a temperature between 150'C-165'C when heated by steam coils operating at 7 barg. The enquiry concluded that hot ammonium nitrate had been forced back into a pipe and confined. The pipe explosion initiated the larger mass to explode leaving a crater 2 m deep.
Oppau, Germany 1921	An estimated 400 tonnes out of a pile of 4,000 tonnes ammonium sulphate/nitrate exploded when conventional explosives were used to break up a cake of material; it was reported that this operation had been carried out about 30,000 times before.
Texas City, USA 1947	SS 'Grandcamp' carrying 2,230 tonnes of ammonium nitrate caught fire and exploded after 1 hr. The SS 'Highflyer' carrying 960 tonnes of similar material and 2,000 tonnes of bulk sulphur caught fire subsequently and exploded 14 hrs later. The fertilizer was coated with wax and clay and contained in paper bags supported on wooden dunnage. Over 600 killed and 3,000 injured.
France 1972	A contractor's tanker carrying hot 95% ammonium nitrate solution caught fire and exploded. The tanker had also been used for carrying organic material and the fire is thought to have occurred in lagging impregnated with both organic material and ammonium nitrate. Fire, but without explosion, subsequently occurred on two similar tankers. 2 killed.
Delaware City, USA 1977	N₂O was being prepared by heating an 83% solution of ammonium nitrate to about 270'C in a steel reactor. The incident occurred because the temperature at which the decomposition reaction was being run allowed the rate of production of decomposition products (N₂O + H₂O) to exceed the flow capacity of the line to the downstream processing equipment, resulting in an increase in pressure. As the pressure increased, the temperature increased. The resulting blast shattered the concrete floor and produced a significant crater.
Iowa, USA 1994	At Terra, trapped ammonium nitrate decomposed leading to a violent rupture of the sparger, detonating more ammonium nitrate within the neutraliser and setting off a secondary explosion in a nearby storage tank. The plant was shut down at the time of the incident. High levels of chloride, a neutraliser with contents below pH 1 and injection of steam at temperature above 220'C for a period of over 9 hrs were significant factors in the sensitisation and decomposition of the ammonium nitrate. 4 killed and 3,000 evacuated.

Question: How is ammonium nitrate manufactured, and what are the risks involved?

Ammonium nitrate is made from ammonia and nitric acid in an exothermic process. The resulting solution is concentrated by removing water in an evaporation step. The final ammonium nitrate products are particulated (granulation, prilling, crystallisation). In the particulation process characteristics of the final products can be adjusted depending on the use (fertilizer or technical use).

Ammonium nitrate has been known and produced for many years and there is a lot of experience on safe operation. Ammonium nitrate may decompose or explode under certain conditions (contamination, low pH, or high temperature). Strong focus is therefore in the production process on ensuring correct operating parameters and safe conditions. Hydro's technical standard for production of ammonium nitrate sets strict limits with regard to operating conditions, contaminants and equipment to ensure safe production.

Question: What is the difference between 'tehnical' ammonium nitrate and 'fertilizer' ammonium nitrate?

In the solid form ammonium nitrate is commercially available as granules, prills or crystalline material. Both fertilizer and technical grade can be available in any of these forms. Generally, fertilizer grade has a larger particle size than technical grade. The technical grade also have a high nitrogen content (>34.5%) whereas fertilizer grade normally contains a filler to reduce the nitrogen (33.5% or lower) which makes it safer to handle and store.

The most common use of technical grade is for explosive purposes (ANFO) which requires a porous product. The technical grade used for ANFO blasting, requires a porosity of minimum 6% (typically 7-10%) measured as oil absorption. The fertilizer grade is classified as non-porous (<2-3% oil absorption). The technical grade is prillled, has a mean particle size around 1.5 mm whereas fertilizer grade often has 90% of the particles between 2-4 mm. The porosity and size are important for the detonation properties of the technical grade (often referred to as porous grade or low density).