Process chemistry
The process chemistry for manufacturing ammonium nitrate (AN) is simple, and the only reaction observed is the reaction between (anhydrous) ammonia and nitric acid:
NH3(g) + HNO3 (aq) → NH4NO3 + 145 kJ/gmol
The production is highly exothermic and proceeds with high speed. The concentration of the nitric acid used in the process may vary from 45% to 60%, depending on the process. The concentration of the resulting solution depends on the concentration of the nitric acid and temperature of the process.
Whereas in the United States a large amount of ammonium nitrate is used in liquid form, in many other countries ammonium nitrate solution is mostly concentrated, possibly mixed with other materials (limestone, dolomite, phosphates), and processed into prills or granules.
Prilling and granulation starting from an ammonium nitrate melt must take into consideration changes in the crystal phase and specific volume. For example, the design of cooling must accommodate the heat released during phase changes.
According to research results harder prills or granules can be produced when, during ammonium nitrate cooling, the changes in crystal phase occur with minimal volumetric and structural changes, for example, when crystals of phase II are transformed directly into crystals of phase IV. This type of transformation is possible when the moisture content of the melt is less than 0.1 % or special inorganic salts that prevent formation of phase III are added.
Changes in crystal specific volume may cause the granules to degrade during storage or handling. This is especially important in subtropical countries where the phase-change temperature of 32.2°C can be passed through often, with a specific volume change of 0.0215 m3/ tonne, eventually leading to full disintegration of the prills or granules.
Production technology
Solid ammonium nitrate is produced in the form of prills, granules, pastilles, or crystals. Large tonnages of ammonium nitrate are made in the form of highly concentrated (80 – 90 %) hot solutions that are used for the production of urea-ammonium nitrate solutions or are further concentrated for particulation (granulation, prilling, pastillation).
Ammonium nitrate production is composed of the following process unit operations:
- Neutralisation,
- Concentration,
- Addition of stabilisers,
- Finishing,
- Process condensate treatment,
- Vapour treatment with heat recovery.
Neutralisation
When sufficient steam to operate the plant is readily available from other processes or other low-cost sources, the use of an atmospheric type of neutraliser may be preferable since such units are relatively low in capital costs and simple to operate.
Alternatively, most or all of the steam needed to preheat the feeds and to concentrate the ammonium nitrate solution can be generated by neutralising acids containing more than 50 % HNO3 in a unit of the pressure type. At higher acid concentrations a specific amount of steam can be produced for each tonne of ammonia neutralised (for example in using a 64 % acid about 1 tonne of steam). In some plants, especially those designed to make a crystalline product, neutralisations are performed under vacuum in equipment similar to that used for ammonium sulphate manufacture.
In pressure neutralisation processes, the neutraliser usually operates at 4-5 atmosphere pressure and 175 – 180°C. Nitric acid is fed to the neutraliser in a concentration range of 50 – 60 %. In some cases, the nitric acid may be preheated with by-product steam. Ammonia is fed to the neutraliser in gaseous form. In case ammonia is available in liquid form, it is vaporised in a heat exchanger by steam or air. Liquid ammonia is not added to such a reactor containing nitric acid (to avoid the hazard associated with rapidly expanding and imploding gas bubbles). In case air is used to vaporise the ammonia, the cooled air may be used subsequently to cool the prilled or granulated ammonium nitrate particulates. The neutraliser may be operated at a low pH (3 – 4) to avoid ammonia loss, and more ammonia may be added later to adjust the pH to a value of around 7.
The concentration of the solution from the neutraliser is usually in the range of 80 – 87 % ammonium nitrate. This solution is evaporated further by using steam from the neutraliser to a concentration of 94 – 98 %. In many plants, a final evaporator-concentrator is used to increase the solution concentration to 99.5 – 99.8 %.
In the case of atmospheric pressure neutralisation, the temperature in the neutraliser is lower (about 145°C), and the steam generated is at a lower temperature and pressure. The waste steam can be used to vaporise ammonia or to evaporate the ammonium nitrate solution in a vacuum evaporator. Depending on the efficiency of utilisation, the heat of reaction, the nitric acid concentration, and other factors, the net steam requirement may range from 0.0 to 0.5 tonne/tonne of ammonium nitrate; in some cases, a small surplus of steam may be available for export. Manufacturers need to consider safety and economic aspects of the chosen pressure, temperature, and concentration conditions.
Concentration
For the concentration of the neutralised ammonium nitrate solutions and melts practically all types of evaporators and separators are used.
Examples are as follows:
- Natural circulation system,
- Falling film evaporator (under vacuum or air-swept),
- Forced circulation.
Stabilising agents
Most fertiliser-grade ammonium nitrate is treated with inorganic stabilisers to retard the 32°C crystal transition and to improve storage properties.
A most popular salt used to prevent disintegration from the crystal phase change is magnesium nitrate. It may be added as a solution to the hot ammonium nitrate solution prior to final concentration or in the form of the dry oxide (MgO) to the melt just before prilling. Final concentrations in the ammonium nitrate product are up to about 1.5 % magnesium nitrate or 0.4% as MgO.
Magnesium nitrate works only when ‘melt granulation’ or high-density prilling is done; in particular the melt concentration is about 99.5 % by weight or higher as done in, for example:
- High-density prilling.
- Nitram process,
- NH-pan granulation,
- Hydro Agri fluid bed granulation.
In fluid bed granulation the feed melt concentration is around 97 %, but the product leaving the granulator has only 0.25% moisture. The water content of the solidifying melt must be significantly lower than the possible water of hydration of the added magnesium nitrate (Mg(NO3)2.6-7 H2O). The additive thus serves as an internal desiccant agent and shifts the 32 °C crystal transition point to about 45°C.
Salts used for other finishing processes including ‘solution granulation,’ where the concentration ranges from 93 to 97 %; are aluminium and iron sulphates.
The addition of ammonium sulphate alone increases the hardness. ln some countries mixtures of either calcium nitrate and magnesium nitrate or ammonium sulphate and ammonium phosphate are commonly used. An effective but more expensive stabilising agent, as least used in previous years, is the mixture of boric acid, diammonium phosphate, and ammonium sulphate called ‘permalene-34’. This additive eliminates the phase transition point at 84°C and shifts the 32 °C transition to 43°C.
Vapour treatment
In vapour treatment the tendency is toward well-designed scrubbing columns, either packed columns for the recovery of ammonium nitrate and ammonia, or low pressure-drop trays for the recovery of ammonium nitrate and nitric acid.
Process condensate treatment
For process condensate treatment, the ion-exchange process is also considered to be an effective and economical method.
Caking
Stabilisation against thermal disintegration does not prevent caking but does reduce dust formation. The product remains hygroscopic, and caking is best prevented by protecting the product from direct contact with ambient air. For this aim the prills and granules are also coated with very small amounts of anticaking materials such as oil/amine mixtures. Some other materials are used as well.
Finishing Processes
Several finishing processes have been used including graining, flaking, granulation, crystallisation, pastillation, and prilling.
In low density prilling the ammonium nitrate solution is fed to the prill tower at about 95 % concentration, and the resulting prills are dried and cooled. The prills are somewhat porous. Low-density prilling is used to produce ammonium nitrate for use as a blasting agent. A porous prill that will absorb oil is preferred for this use.
The high-density prilling process, using 99+ % solution concentration, is used in many plants making straight ammonium nitrate for fertiliser use. High-density prilling requires expensive fume abatement equipment and has low flexibility for production of different N-content products; the product size is limited (usually < 2-4 mm) and the product less hard.
Granulation processes have an advantage over prilling regarding the choice of particle size range of the product. The most widely used granulation methods include rotary pan, rotary drum, and fluid-bed processes.
Industrial manufacturing processes
Over the past sixty and more years, a variety of industrial processes for the manufacturing of ammonium nitrate and ammonium nitrate-based fertilisers have been developed, operated and optimised.
Many of these processes are described in the Proceedings of the International Fertiliser Society. Most processes are described in the general literature, some use proprietary processes and/or equipment.
These processes are available from well-known companies, specialising in the design, construction and commissioning of these plants. The reader is advised to contact such engineering companies and study the information given also in view of the general information made available through the Society’s Proceedings.
In this context some characteristic features of some processes used in the past are described. However, details are to be studied separately, particularly as such manufacturing processes have been developed further taking into account new technological advances and innovations, improved (lower) energy requirements, and the need for new optimised products and improved product properties and qualities.
Industrial neutralisation processes:
– ThyssenKrupp (Uhde) neutralisation under vacuum or atmospheric pressure. This system has the lowest investment cost. The reactor is slightly pressurised to prevent ammonium nitrate solution from boiling since this impedes the absorption of ammonia. After it is heated by the reaction, the circulated ammonium nitrate solution is fed via a restriction orifice to the flash evaporator where part of the water is evaporated and the solution is cooled down. By using feed acid at an adequate concentration and preheating the feedstock, the ammonium nitrate solution may reach a concentration of about 95 %. The neutralisation pressure ranges between 0,3 and 1,2 bar and the reaction temperature between 130 to 145 °C; all of the equipment can be of normal stainless steel (304L). However, the concentration of the ammonium nitrate is slightly low for going directly to the finishing process, and a small final evaporator is needed.
– ThyssenKrupp (Uhde) neutralisation at 2 – 2,5 bar pressure.
This process was developed to balance the heat requirements but still can operate in normal stainless-steel equipment. The heat of neutralisation is used in a heat exchanger to concentrate ammonium nitrate solution recycled from the flash evaporator. The heat exchanger after flashing can act as a condenser-or reboiler. In the latter case, a steam of pressure of 4 bar is produced. The steam from the flash evaporator is condensed, and the AN solution is heated.
– Hydro Agri neutralisation at 4 – 5 bar pressure.
Neutralisation is carried out at 4 – 5 bar, but the operating temperatures (175 -185 °C) require highly resistant materials (special low carbon 304L and titanium). The heat of reaction is used for concentrating the AN solution to 95% and production of steam for export.
– CARNIT neutralisation at 7 – 8 bar pressure.
The CARNIT process operates at 185 °C and at a reactor output pressure of around 7 – 8 bar. The neutralisation is carried out in two steps:
• alkaline, where low carbon stainless steel can be applied.
• acidic, where a titanium reactor is installed.
The alkaline hot solution passes through a steam boiler and falling film evaporator made of low carbon stainless steel. Concentrated solution is recycled to the second reactor where additional nitric acid is added before flashing and scrubbing.
– AZF pipe reactor neutralisation
In the AZF-Grand Paroisse process the neutralisation is carried out in a pipe reactor using gaseous ammonia and preheated nitric acid to give an ammonium nitrate melt of up to 97 % concentration directly. The evolved steam is used to evaporate and superheat the feed ammonia and preheat the acid. The balance of the steam is condensed. The ammonia and acid, at pressure, are mixed at high velocity in the pipe reactor and are neutralised at a generated temperature of up to 200 °C. The solution flashes into the separator operating near atmospheric pressure, and the resulting steam passes overhead directly to the ammonia evaporator or alternatively to be scrubbed with dilute AN solution to reduce ammonia and AN in the downstream condensates. Concentrated 95 – 97 % ammonium nitrate solution overflows from the tank to the pump tank. If the AN solution is less than 96 %, it is first concentrated in a falling film evaporator.
Industrial finishing processes:
– AZF-Grand Paroisse prilling process
The AZF-Grand Paroisse prilling process can be used for fertiliser or for industrial grade ammonium nitrate. The ammonium nitrate solution from the pipe reactor neutralisation step is first concentrated to 99,8 % in an air swept falling film evaporator, with steam condensing in the shell. If porous ammonium nitrate prills suitable for industrial explosives are to be made, then 95 % solution is pumped directly to the mixing tank for re-dissolving recycled off-specification material. The solution concentration and pH are controlled by adding scrubbing liquor and gaseous ammonia respectively. The solution is pumped through sprays arranged in the top of the prilling tower. Fans located at the top ensure a counter current flow of cooling air against the falling droplets. Various options are available to clean the air before it is discharged to the atmosphere. The prills are collected at the bottom for discharge to the drying, cooling, and screening section.
– GIAP neutralisation and prilling technology
GIAP uses a high-density prilling process. A portion of the waste steam from the neutraliser is used to preheat the nitric acid to 70 – 90 °C; the remainder of the steam is fed to the prill tower air scrubber. The gaseous ammonia is preheated by a portion of the waste steam to 160 °C. The concentration of the solution from the atmospheric-pressure neutraliser is as high as 89 – 92%. The solution is further evaporated to a concentration of 99,7 – 99,8 % in the final evaporator. and is fed to the spray heads of special design to form ‘uniform’ prills. Such spray heads in conjunction with a fluidised cooling bed can provide a product with up to 85 % of the prills between 2 -3-mm diameter. There is a choice of stabilisers added to the AN solution before prilling. The prills are conditioned prior to bagging.
A distinguishing feature of this process is its simple operation and high reliability because most equipment, except for pumps and fans, is static and lacks moving parts. The high quality of the product (up to 85 % of 2 – 3 mm prills) allows it to be bagged without screening. The product is screened only when it is sent to the adjacent warehouse for lengthy bulk storage.
The company also offers a proprietary low-emission prilling process with a closed circuit for prill tower cooling air. An original method for the flow of cooling air through the system (without fans) is used in that process.
Granulation Processes:
Prilling has the disadvantages of difficult emission control, limited product size, and lower product hardness. Granulation processes are flexible and allow easier emission control. Granulation plants can produce other products, for example, calcium nitrate, ammonium nitrate sulphate, and NPK formulations. Granulation gives a coarser product than prilling, thereby minimising segregation of bulk blended fertilisers.
Production of straight granulated ammonium nitrate
Products with nitrogen contents in the range of 30 – 34.5 % are classified as straight ammonium nitrate. The following granulation processes can be used for straight ammonium nitrate production:
• Cold spherodisers,
• Drum granulation,
• Pugmill granulator,
• Pan granulation,
• Fluid bed granulation;
• Fluidised drum granulation.
These processes are used for products containing up to 34,5 % N. For products that contain only up to 33,5 % N, the pugmill, drum and cold spherodisers are also used. These processes operate in the same way as for calcium ammonium nitrate. However, the ammonium nitrate melt concentrations vary with the process. With all methods, additives are obligatory for granulation and for improving storage properties.
References
M. Voorwinden, J.-B. Peudpiece, and J.-F. Granger. (2003). Ammonium nitrate production using a pipe reactor: experience over 10 years, Proceedings International Fertiliser Society, 515.
Revamping the Azomures ammonium nitrate plant at Targu Mures, Arionex, Nitrogen & Methanol, No. 275, May – June 2005, pages 23 – 24.
H. Kiiski. (2006). Phase stabilisation of ammonium nitrate fertilisers, Proceedings International Fertiliser Society, 583.
C. Sjölin, The influence of moisture on the structure and quality of NH4NO3 – prills, J. Agr. Food Chem., Volume 19, No. 1, 1971, pages 83 – 95.
Links to Related IFS Proceedings
85, (1965), Ammonium Nitrate: Manufacture and Use, C H Solomon, K S Barclay
110, (1969), Production of Ammonium Nitrate including Handling and Safety, R W R Carter, A G Roberts
136, (1973), Ammonium Nitrate Production, G Drake
235, (1985), Fluid Bed Granulation of Ammonium Nitrate and Calcium Ammonium Nitrate, J P Bruynseels
320, (1992), Carnit Ammonium Nitrate Process, J L Bovens, F van Hecke
338, (1993), Cooling with a Bulk Flow Heat Exchanger, N P Jordison
414, (1998), Ammonium Nitrate Production – Environmental, Energy and Safety Aspects, R Antonus
415, (1998), Fluid Drum Granulation for Ammonium Nitrate, L Dall’Aglio
494, (2002), Off-spec and Reject Fertiliser: Management Guidelines, K D Shah, J A M van Balken
639, (2008), GHG Emissions and Energy Efficiency in European Nitrogen Fertiliser Production and Use, F Brentrup, C Pallière
672, (2010), Decommissioning and Demolition of an Ammonium Nitrate Fertiliser Complex, K D Shah, C Blackman
764, (2015), Ion-Exchange Treatment of Effluent from Ammonium Nitrate Plants for Nitrogen and Water Recovery, N Arion, G Mommaerts
767, (2015), Update on Fluidised Drum Granulation Technology and its Applicability for Different Fertilisers, S Valkov
768, (2015), Controlling Particulate Emissions from Ammonium Nitrate Prill Production, J H B Guimarães, S A Ziebold, J M Hanekom
800, (2017), NOx and SO2 abatement using gas-phase chlorine dioxide, R Richardson
805, (2019), The Carbon Footprint of Fertiliser Production: Regional Reference Values, A Hoxha, B Christensen
831, (2019), Innovative use of Thermal Imaging to Identify Flow Problems in an Ammonium Nitrate Neutraliser, R D Goncalves
853, (2021), Reducing Emissions from Ammonium Nitrate Based Fertiliser Operations, G. Cousland, M. Dean, R. Peddie
854, (2021), New Developments in Emissions Control for Ammonium Nitrate Based Plants, A. Gullà, S. Spreafico
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