This section on ammonium nitrate comprises these pages:

Ammonium nitrate – properties
Safety considerations and hazards related to ammonium nitrate
Ammonium nitrate production – technology and industrial processes

1. Introduction
2. Properties of ammonium nitrate
3. Crystalline forms
4. Density of ammonium nitrate-based fertilisers
5. Hygroscopicity and critical relative humidity of ammonium nitrate

Introduction

Ammonium nitrate (NH₄NO₃, AN) is the most popular form of nitrogen fertiliser in most European countries. In the northern parts of the United States and in Canada ammonium nitrate is increasingly valued but, in some areas, still is in competition with anhydrous ammonia and liquid fertilisers. Ammonium nitrate is becoming more available and appreciated in developing countries, where urea dominates the market.

It is applied as a straight material or in combination with calcium carbonate, limestone, or dolomite. The standard combination is called calcium ammonium nitrate (CAN); various trade names are used. Ammonium nitrate is also contained in compound fertilisers including in nitrophosphates. It is also a principal ingredient of most liquid nitrogen fertilisers.

The nitrogen in ammonium nitrate is more rapidly available to some crops than is the nitrogen in urea or ammonium sulphate; most crops take up nitrogen mainly in the nitrate form; thus, ammoniacal nitrogen must be converted to nitrate in the soil before it becomes effective. Many crops respond well to a mixture of ammonium and nitrate nitrogen. The nitrification process is rapid in warm soil, but is slower in cool soil (10 °C and below). Because urea may cause seedling damage due to volatilisation of ammonia, and as ammonium sulphate is strongly acid forming, for these – and other reasons – various forms of ammonium nitrate (for example AN, CAN) are effective and preferred fertilisers in zones with medium and low temperatures during the cropping period, especially in those with a short vegetation period.

Ammonium nitrate also shows some disadvantages:

• care must be taken in storage and handling as ammonium nitrate is quite hygroscopic (somewhat more than urea),
• ammonium nitrate is an oxidiser (see below) and there is a risk of enhancing a fire or even of explosions unless suitable precautions are taken,
• ammonium nitrate is less effective for flooded rice than urea or ammoniacal nitrogen fertilisers,
• ammonium nitrate is more prone to leaching immediately after application than ammoniacal products.

Although ammonium nitrate is principally used as fertiliser, it also has industrial uses, notably for production of explosives and for the production of laughing gas (for medical applications). Commercial plant capacities for ammonium nitrate or CAN fertiliser production generally range from 400 to 3.000 t/d.

Commercial ammonium nitrate is mainly manufactured either as fertiliser grade (FGAN) or as technical grade (TGAN). Fertiliser grade shows a high density (0,9 – 1,1 g/cm³) and a high resistance to detonation. Technical grade ammonium nitrate usually shows a much lower density (0,6 – 0,7 g/cm³).

Properties of ammonium nitrate

The molecular weight of ammonium nitrate is 80. The total N-content in pure ammonium nitrate is 35 %.

CAS number: 6484-52-2; EINECS number: 299-347-8.

Ammonium nitrate does not occur naturally in mineral form; it is produced synthetically. Pure ammonium nitrate is a white crystalline solid with a melting point of 169.6°C. There is no true ‘boiling point’ because decomposition begins to take place before the boiling condition is reached.

Technical data on solubilities, thermal conductivity, chemical properties, and pH of ammonium nitrate are well described in the relevant literature.

Commercial forms of ammonium nitrate

A large share of ammonium nitrate is supplied to domestic and international markets as prilled product although granulated material is preferred in many industrialised countries. The main difference is in the size and strength of the particles. The granules are usually coarser and harder than prills and are more suitable for bulk blending. For industrial use some quantities of crystalline product are produced.

Special provisions or requirements may be established for improving or accommodating the (weaker) strength of prills. The strength of ammonium nitrate prills depends very much on the stabilising additive that is used.

Both primary materials (prilled and granulated) usually incorporate additives (stabilisers) that improve the quality of the product. Prills and granules may be coated with anticaking agents. In a number of countries prills or granules are coated with surface-active materials such as amines by spraying the coating liquid onto the particles; this process is sometimes called ‘wet coating’. For severe climatic conditions and for long-time storage, a ‘dry coating’ by inert material such as kaolin or talcum may be used, sometimes also as an additional treatment to the spraying of a liquid coating agent.

When ammonium nitrate is used as straight material, the nitrogen content ranges from 30% to 34,5%. When mixed with (milled or ground) limestone, lime, dolomite, and/or calcium carbonate to make CAN, the nitrogen content is usually in the range of 25%-28%. Today, the most common grades for CAN are 26 or 27 % N.

Product handling

Ammonium nitrate may be stored in bulk although in most climates this requires air-conditioned facilities.

Storage facilities should have adequate ventilation to allow quick dispersion of heat and toxic gases in the event of fire. In some countries the storage area must be equipped with a high efficiency sprinkler system. Only large / copious amounts of water can extinguish a fire.

The commercial product is usually bagged in bags that should be ‘moisture proof’; at least one ply should be impermeable to moisture. This can be established, for example, by using an inner (impervious) polyethylene lining and an outer polypropylene bag for mechanical strength.

Bulk shipment is common in some countries using covered (roof can be opened) waggons that can be discharged on the bottom (usually on top of train rails). Before loading, the inside of the vehicle should be thoroughly cleaned.

It is important to prevent under all conditions of handling the contamination of ammonium nitrate with organic materials such as grease or other hydrocarbons, with chlorates, nitrates, and metal salts, which when ignited may support fire.

Crystalline forms

Ammonium nitrate occurs in five different stable crystalline forms or phases in the solid state, dependant on the ambient temperature. The main crystal forms are summarised in Table 1 (Shah and Roberts, 1985). These transitions are accompanied by volume changes as shown in Figure 1.

Of particular interest to the fertiliser industry is the transition at 32 ˚C, which is accompanied by a substantial volume increase (approximately 3.6%) as the temperature is raised. In storage situations where the temperature fluctuates across 32 ˚C and the material goes through cycles of temperature changes, the resulting repetitive density changes can cause the product to break down into fines. As a consequence, the bulk density is likely to reduce and bags may swell up. To prevent this, certain stabilisers can be added, for example magnesium nitrate or aluminium sulphate, which may shift the transition to a higher temperature. This is referred to as thermal stabilisation (Shah and Roberts, 1985; Laurent, 2002, Kiiski 2006).

The changes in crystalline form entail a change in the crystal density. The corresponding volume changes involved in the phase transitions are:

–          2.1 % (phase I to II),
+          1.3 % (phase II to III),
–          3.6 % (phase III to IV),
+          2.9 % (phase IV to V).

Density of ammonium nitrate-based fertilisers

The material density of solid AN (crystalline block) is 1725 kg/m³ at room temperature.

Prilled or granular fertilisers have loose bulk densities typically in the range of 850 to 1100 kg/m3, which depend on the source materials and the manufacturing process used.

Ammonium nitrate-based fertilisers such as AN, CAN, AN+CaSO₄, and NPK all show loose bulk density values in the range 900 – 1100 kg/m³.

The melting points measured for ammonium nitrate depend critically on the content of water, practically it is nearly impossible to obtain dry product under industrial conditions.

The solubilities of ammonium nitrate at various temperatures as well as the boiling points of its solutions are relevant for the engineering and design of commercial installations and are available in literature.

Hygroscopicity and critical relative humidity of ammonium nitrate

Ammonium nitrate and ammonium nitrate-based fertilisers, for example also calcium ammonium nitrate (CAN), are hygroscopic fertilisers.

This awkward property of nearly all ammonium nitrate containing fertilisers is well known to all stakeholders in the production, transport, and storage of these fertilisers and is extensively described in literature.

The hygroscopicity of a fertiliser is often described by means of its critical relative humidity. The critical relative humidity (CRH) of a substance is the humidity at a given temperature above which the substance will absorb moisture from the surrounding air. For prilled calcium ammonium nitrate the CRH at 30 °C is in the order of 55 %.

Consequently, when stored in a big heap, the product may absorb moisture from the atmosphere and, in some cases for example with calcium ammonium nitrate, a crust may form on the surface of the heap. Dependent on the temperatures of the product and of the ambient air, residual moisture in the product may migrate to the surface, amplifying such a crust formation. Fortunately for the players in the logistic chain, such a crust often acts as a rather impermeable layer and may minimise further moisture uptake significantly. In such cases of a moisture-uptake with crust formation, the crust can be removed and reworked / returned and the remainder of the material is usually still in fine condition.

Critical relative humidities of ammonium nitrate as a function of temperature are available in the literature.

Certain additives, for example magnesium nitrate and ammonium sulphate, (NH₄)₂SO₄, can slightly change the critical relative humidities.

References:

Ammonium nitrate and nitrochalk – Pollution Control in Fertilizer Production, R.E. Nitzschmann and J.G.A. Reuvers, edited by C.A. Hodge and N.N. Popovici, M. Dekker Inc., Chapter 9, pages 99 – 107, 1994.

Fertilizer Manual, edited by the United Nations Industrial Development Organization (UNIDO) and the International Fertilizer Development Center (IFDC), Kluwer Academic Publishers, 1998, page 484.

Fertilizers Europe. (2014). Guidance for compatibility of fertilizer blending materials.

Fertilizers Europe. (2022). Guidance for the storage, handling and transportation of solid mineral fertilizers, second edition.

IFDC. (2005). Fertilizer Dealer Handbook – Products, Storage, and Handling, Reference Manual IFDC-R-15, D. W. Rutland and J. Polo, IFDC, Muscle Shoals, Alabama, USA, October 2005, page 7.

International Fertiliser Society, compilation of all ammonium nitrate-based Proceedings, USB.

Properties of ammonium nitrate in nitric acid and fertilizer nitrates, K. D. Shah, and A. G. Roberts, C. Keleti (ed.), M. Dekker Inc., New York, 1985, ISBN 0-82477-332-2.

The influence of moisture on the structure and quality of NH₄NO₃ – prills, C. Sjölin, J. Agr. Food Chem., Volume 19, No. 1, 1971, pages 83 – 95.

Links to Related IFS Proceedings

257, (1987), Rationale for Mixed Ammonium Nitrate – Urea Fertilisers and Assessment of Granular Products, M K Garrett

265, (1988), Storage and Distribution of Straight Ammonium Nitrate Fertilisers in Bulk, W van Hijfte

296, (1990), Granular Ammonium Nitrate – A New Route, A M J Seto

583, (2006), Phase Stabilisation of Ammonium Nitrate Fertilisers, H Kiiski

627, (2008), Ammonium Nitrate Fertilisers: Analysis and Appraisal of Classification Categories, H Kiiski