Nitrogen+Syngas 384 Jul-Aug 2023
31 July 2023
Ammonium nitrate and fertilizer safety
AMMONIUM NITRATE
Ammonium nitrate and fertilizer safety
A new book by leading industry expert Kish Shah looks at all aspects of ammonium nitrate production.
In a career spanning more than thirty years at major producers of ammonium nitrate and ammonium nitrate-based products, Kish Shah has come to be regarded as a leading international expert on ammonium nitrate safety. He has represented the fertilizer industry at the relevant UN committees on transport of dangerous goods for many years. Now he has literally written the book on ammonium nitrate (AN), distilling that experience into a 300page volume: Ammonium Nitrate and Fertilizer Safety, with forewords by leading industry figures including Jacob Hansen and Atoine Hoxha of Fertilizers Europe, and Johannes Reivers, a leading light of the AN-NA Conference committee. This is a brief overview of the book and its contents.
History
Ammonium nitrate was first synthesised in 1659 by German chemist Glauber, but its history as an industrial product of course dates from the commercialisation of the Haber-Bosch process after the First World War, leading to commercial production of nitric acid from ammonia, and ammonium nitrate from nitric acid, as a potential cheaper alternative to the coke oven gas derived ammonium sulphate, or nitrates mined from guano deposits on Chilean islands. However, AN’s hygroscopic nature meant that it caked badly, to the point of being unusable, and so tended to be used in a mixture with ammonium sulphate to try and ameliorate this somewhat. Even so, cargoes caked in longer storage, and it became common practise to break caked cargoes up using blasting charges from left over war munitions. At least until 11st September 1921, when an AN cargo being blasted at Oppau in Germany detonated, killing more than 500 people.
AN was used for its explosive properties in the inter-war period and during World War II, but a second attempt at its use at a fertilizer happened after the war, by coating the granules in wax to prevent them agglomerating. Again however a major industrial accident, this time at Texas City in 1947 when two vessels carrying AN caught fire and exploded. A similar incident occurred just months later in Brest, France. It was not until the development of the AN prilling process that the compound was able to achieve rapid growth as a fertilizer in the 1960s, replacing ammonium sulphate from caprolactam and other industrial processes. The ammonium nitrate industry and related products (calcium ammonium nitrate, urea ammonium nitrate etc) now amounts to around 50 million t/a of AN.
The 1950s also saw the development of ammonium nitrate/fuel oil (ANFO) as a commercial blasting explosive, using porous AN prills. AN has now become the main material for the commercial explosives industry, accounting for around 90% of solid explosives. The book also covers AN’s more minor uses in propellants, as a source for nitrous oxide gas for anaesthesia etc, and in commercial cooling packs.
The third chapter moves on to a brief history of major incidents and their investigation and the consequent evolution of regulation. The US Bureau of Mines, in particular, conducted a great deal of testing and studies into AN properties in response to some industrial accidents. There was also considerable effort put into overcoming the caking properties that had resulted in the Oppau and Texas City accidents. Several additives were proposed for the purpose, with the most popular becoming magnesium oxide or carbonate to achieve the incorporation of magnesium nitrate, which also pushes the AN phase change to a higher temperature.
Major AN accidents in the 1970s at Seveso in Italy and Flixborough in the UK led to a major EU industrial accident directive in 1982, listing AN as technical/explosive grade or fertilizer grade, but the 2001 Toulouse explosion led to the addition of two new categories; fertilizers capable of self-sustaining decomposition; and ‘offspec’ material and fertilizers which did not fulfil the detonation test.
Properties
Chapter 4 concerns the physical and chemical properties of AN. One of its unique properties is several different crystal structure phase changes in the solid form. One of these occurs at 32°C, and causes a substantial volume change. In hot conditions, cycling above and below 32°C can lead to the AN breaking up and forming very fine particles. As noted above, the addition of magnesium nitrate can increase this phase change to around 50°C.
AN is highly soluble in water, and absorbs atmospheric moisture at ambient temperatures, depending on humidity. Addition of an internal dessicant such as MgNO3 can reduce moisture ingress to a top layer of 2-3 prills in a heap, but control of the condition of the air or the heap may be needed to be covered with sheeting. AN notionally melts at 169.6°C, but it tends to decompose before it boils. But its low thermal conductivity means that heating or cooling can be limited to a surface layer in bulk. The chapter also gives pH curves and tables of specific heat capacity and enthalpy.
As regards chemical properties, the decomposition reactions are covered. Nitric acid has a catalytic effect on decomposition, as do chloride ions. AN is of course a major oxidising agent. It does not present any major toxicity hazards.
“The main risk comes where the AN is mixed with flammable material.”
Hazards
Chapter 5-7 covers hazards, beginning with fire. AN is not flammable, but it is an oxidising agent – fires therefore depend upon the presence of other flammable materials. The main risk comes where the AN is mixed with flammable material such as when contaminated with liquid fuel, or when it is in contact with large combustible items such as wooden structural supports and beams. Decomposition, however, presents another hazard with hot AN. As well as normal thermal decomposition, AN can be subject under the right conditions and with sufficient heat input to self-heating, or self-sustaining decomposition (‘cigar burning’), and under confinement can even lead to rapidly accelerating decomposition. This latter takes us into the chapter on AN’s explosion hazard, both via detonation and deflagration. AN is not classified as an explosive because it requires very energetic initiating impulses. But it can become sensitised by mixing with organic/combustible material, chlorides etc. Kish presents comprehensive models of AN explosive initiation and the results of work carried out by various bodies from the US Bureau of Mines and Queens University Canada to Dutch research organisation TNO under a variety of conditions.
Compounds
Chapters 8-11 cover mixtures of AN, beginning with urea, and ranging across ammonium sulphate nitrate, calcium ammonium nitrate and compound NPK fertilizers, in each case looking at how their properties such as fire, explosion and other hazards differ from bulk AN.
Safe plant operation
The next section of the books is devoted to production of AN, and particularly safety aspects of manufacture. As AN has both oxidising and corrosive properties, choice of materials for plant construction is important, and the book ranges across steels, concrete, asphalt, wood, process vessels and pipes, conveyor belts, insulation, instruments and control systems. It also describes the main process flowsheets for AN production, including the ICI two stage process, the Uhde AN neutralisation process, the AZF pipe reactor process, and the nitrophosphate or Odda process. Given AN’s propensity to absorb moisture and cake, there is a section on granulation and finishing, and also pollution control. The chapter on plant safety covers all of the major hazards than can be encountered in AN plant operation, and gives a checklist of good safety practises.
Storage and handling
Chapters 16-19 cover storage and handling. Different options for storage are detailed, and good principles for the design and maintenance of storage areas, particularly if other materials may be present nearby. Special attention is paid to off-spec and other reject fertilizer materials, and options for handling these safely, as these can be more sensitive to detonation or decomposition, as occurred at Toulouse in 2001. There are also chapters on hot ammonium nitrate solution and design and location of storage tanks, as well as liquid fertilizer such as UAN and its attendant corrosion issues.
After a brief review of AN-based explosives, there are then chapters covering transportation, by road, rail and sea, and security. This section finishes with chapters on consequence analysis of AN incidents and emergency planning and management.
Regulations
The last section of the book details the various tests for measuring oxidising potential, decomposition and reactivity and self-heating tests, and resistance to detonation and deflagration. There is also a comprehensive listing of regulatory instruments covering AN, including the UN harmonised system of classification and labelling of chemicals, and sections on EU and US legislation.
Appendices
The book concludes with a complete listing of the most significant AN accidents from 1920-1950 and all major reported AN accidents from 1950 to the present, by type (fire, explosion, decomposition etc) and with likely causes and key contributing factors. Serious fire or explosion accidents have mainly occurred with solid AN products. The main factors for accidents prior to 1950 were the high detonation sensitivity of some products and the use of unsafe practices to break up caked AN. Since 1950, just four accidents (in Iran 2004, North Korea 2004, China 2015 and Beirut 2020) have accounted for 85% of all 992 fatalities recorded. A high proportion of the explosion accidents involved fires preceding them. The prevention of fires and control or use of the presence of combustible materials where AN is stored or handled remains paramount.
Conclusions
There is a considerable amount of literature on ammonium nitrate hazards and safety, but as far as I am aware there has been no attempt prior to this to draw all of this together into one publication. This book serves as a comprehensive overview of AN manufacture, handling and especially safety and would be a useful primer for anyone working in the industry as well as a reference for those seeking pointers to where more information can be found. It is a hardback with 312 pages, 42 figures, 34 tables and 234 references and can be ordered via www.fertilizerandnitratesafety.com.
