Ammonium nitrate melt production and concentration system

01

Match the melt concentration to the product

EPA's AP-42 chapter on ammonium nitrate says every plant makes an aqueous solution by reacting ammonia and nitric acid in a neutraliser. To make a solid product, the solution is concentrated in an evaporator or concentrator. The resulting melt contains about 95 to 99.8 percent ammonium nitrate at approximately 149 °C (300 °F). Plants making only solutions use solution formation, blending and bulk shipping alone. [1]

The melt concentration sets the product. Low-density prills (specific gravity around 1.29) come from a 95 to 97.5 percent melt and are porous enough to absorb oil for blasting agents. High-density prills (around 1.65) come from a 99.5 to 99.8 percent melt and are used as fertiliser. Rotary drum granulators spray a 99.0 to 99.8 percent melt onto seed particles. Low-density prills need drying before cooling because of the lower melt concentration. [1]

AP-42 notes that an additive such as magnesium nitrate or magnesium oxide may be injected into the melt stream, prefacing this with “Although not widely used”. The additive raises the crystalline transition temperature of the product, acts as a desiccant against caking, and lowers the freezing point so that solidification can occur at a lower temperature. Such additives may make coating unnecessary. [1]

Fertilizers Europe's BAT booklet on ammonium nitrate gives the water side of the same choice: the acceptable water content “is normally below 1% for a prilled product”, while a water content up to 8% is required for the feed to some granulation processes. Those granulation processes still have to remove the water, though at a lower temperature. Plants for ammonium nitrate and calcium ammonium nitrate (CAN) produce from a few hundred up to 3,600 t/d. [3]

Editorial calculation: a 1% water limit corresponds to a melt of at least 99% ammonium nitrate, and 8% water to about 92%, if water is the only other component.

Give the supplier the target products and their split, the capacity, the required melt concentration and temperature, the ammonia and nitric acid feed specifications, the evaporator steam and cooling utilities, any melt additive, the emission limits and the codes that apply.

02

Choose the neutraliser: stages, pressure and reactor type

The EU reference document on best available techniques for large volume inorganic chemicals (LVIC-AAF BREF, 2007) describes ammonium nitrate production as neutralising 50–70 wt-% aqueous nitric acid with gaseous ammonia in a highly exothermic reaction, followed by evaporation and solidification. Neutralisers may be free-boiling vessels, circulating systems or pipe reactors, in one or two stages. Pressure, temperature and concentration are linked by the boiling-point behaviour of the solution, with only two of them independent: at 4 bar and 76% w/w solution, the temperature is 180 ºC. [4]

A two-stage neutraliser runs at low pH in the first stage and neutral pH in the second. Fertilizers Europe writes that a two-stage neutraliser “produces most of the boil-off steam in the first stage and most of the ammonia emission from the second stage”, which reduces total ammonia emission, and that a single-stage unit is simpler and cheaper. [3] Neutralising at elevated pressure gives hotter steam and a stronger solution, which can then drive evaporation and drying. The BREF cites a two-stage design with a first stage pressurised to 2 bar, a closed plant that ran its first stage at pH 2 and its second at pH 6, and a single-stage neutraliser at 3.5 bar giving 72–75% solution at 168–171 ºC. [4]

In a pipe reactor the acid and ammonia mix and react in a pipe, and the pressure falls from about 4–7 bar at the reactor head to about 1 bar in the separator, where solution and process steam part. The BREF reports that this design can run on 63% nitric acid preheated to 60 ºC and yield a 97% w/w solution without further evaporation. [4]

Fertilizers Europe says at least 10 different types and designs of neutraliser are in use in Europe. All installations must include pH and temperature control with reliable equipment that “must be tested on a routine basis”, alarms and automatic independent trips; such safety systems may drench the neutraliser contents with clean water on a significant temperature rise. Steam leaving the neutraliser carries ammonia and ammonium nitrate at up to a few thousand ppm each, which careful design can reduce to a few hundred ppm. [3]

Ask each supplier for the neutraliser type and number of stages, the operating pressure, temperature and outlet concentration at your nitric acid strength, the pH and temperature control and trip philosophy, and how heat of reaction is used to preheat acid or vaporise ammonia.

03

Size the evaporator and treat its steam and condensate

Fertilizers Europe states that evaporation is always performed with steam, from the neutraliser or from a site steam supply, and that saturated steam at an appropriate temperature avoids contributing to decomposition. Evaporation may run at substantially atmospheric pressure or under vacuum; vacuum allows reuse of neutraliser steam but needs more capital. Circulatory systems, shell-and-tube exchangers and falling-film types are in commercial use, and the falling-film evaporator has a small working volume and a short residence time. [3]

Some ammonia is lost from the solution during evaporation and “must normally be replaced prior to solidification”, and the evaporator must deliver solution at the required concentration and at a temperature that avoids crystallisation. Its boil-off steam carries ammonia and ammonium nitrate droplets and must be purified before discharge. The booklet lists knitted wire-mesh demister pads, wave-plate separators and PTFE fibre-pad separators for droplets, and packed columns, venturi scrubbers and irrigated sieve plates for scrubbing, with nitric acid added to the scrubbers to neutralise free ammonia. Contaminated condensate can be stripped with air or steam, distilled or treated by reverse osmosis; ion exchange raises safety concerns because resin must not be recycled to the process or become nitrated. [3]

On utilities, the booklet gives electricity of 25 to 60 kWh per tonne of product for a new solid ammonium nitrate facility, with up to 70 kWh/t more where existing plants are retrofitted to BAT, and about 5 kWh/t for solution only. Steam demand can range from zero to 50 kg/t of product, and a plant making only solution can export steam at up to 170 kg/t. Nitric acid feed is around 60% strength, and the water it brings leaves the process in one form or another. [3]

The BREF's BAT conclusions for this stage are to use the heat of reaction to preheat the acid or vaporise ammonia, to neutralise at elevated pressure and export steam, to use the generated steam for evaporation and condensate treatment, to recover residual heat, and to control pH, flow and temperature effectively and reliably. It records low-temperature process steam of 125–130 ºC as a residual heat source for absorption chilling. [4]

04

Plan emission control against published levels

AP-42 states that neutralisers and evaporators emit mainly ammonia and nitric acid, and that the neutraliser vapour is mainly steam with some ammonia and ammonium nitrate particulate. Its uncontrolled emission factors per megagram of product are 0.045–4.3 kg particulate and 0.43–18.0 kg ammonia for the neutraliser, and 0.26 kg particulate and 0.27–16.7 kg ammonia for evaporation and concentration. Controlled factors assume a 95% wet-scrubber efficiency on neutralisers. [1]

Fertilizers Europe gives achievable levels for new ammonium nitrate plants with no insoluble solids present: 30 mg/Nm3 particulates and 50 mg/Nm3 ammonia for individual emission points other than prill towers and granulators, and in total no more than 0.5 kg particulates and 0.2 kg ammonia per tonne of product. Water discharges are 100 mg N/l (0.2 kg N/t product). Without BAT, emissions to air can reach 200 mg/Nm3 of particulates and of ammonia (2 kg/t each), and unabated water emissions up to 5,000 mg AN-N/l and 2,500 mg NH3-N/l. [3]

The 2007 BREF reproduces these Fertilizers Europe levels in its emission tables, but states that because of an insufficient data basis no BAT conclusions could be drawn for emissions to air from neutralisation, evaporation, granulation, prilling, drying, cooling and conditioning. Its water conclusion is to recycle process water and treat the remainder biologically or by an equivalent technique. Ask the supplier which permit limits it will guarantee rather than assuming a BREF value. [4]

05

Control impurities, temperature and confinement

The ILO International Chemical Safety Card for ammonium nitrate (ICSC 0216) states that it is not combustible but enhances combustion of other substances. There is a risk of fire and explosion on confinement, on exposure to high temperatures or when it is contaminated with other materials. It is a strong oxidant that reacts with acids and with combustible and reducing materials. The card gives a melting point of 170 °C and decomposition above 210 °C, and says to store it separated from combustible substances and reducing agents. [2]

Fertilizers Europe gives 169 °C as the melting point of pure solid ammonium nitrate. On further heating it decomposes mainly to N2O and water up to about 250 °C, and above 300 °C reactions producing N2, NO and NO2 become significant. Endothermic dissociation into nitric acid and ammonia vapour limits temperature only while the gases can escape; under confinement a run-away decomposition can develop. Acids, chlorides, organic materials, chromates, dichromates, salts of manganese, copper and nickel and metals such as zinc, copper and lead catalyse decomposition, while alkaline conditions suppress it, so ammonia addition is a major safeguard. Explosions can occur when ammonium nitrate is heated under confinement in pumps, through no or insufficient flow, incorrect design, poor maintenance or contamination. Hot solutions are in the range 120–180 °C. [3]

The BREF names the contaminants to keep out of the neutraliser: organic material below 100 ppm total carbon, chloride below 300 ppm, heavy metals (Cu, Zn, Mn, Fe, Cr) below 50 ppm and nitrite below 200 ppm, all relative to 100% ammonium nitrate. It notes that acidic solution is more unstable than alkaline solution, and that some manufacturers do not recycle screenings to the neutraliser, especially where organic anticaking additive may be present. [4]

Ask the supplier for the hazard study, neutraliser trips and drench system, pump low-flow and high-temperature trips, the impurity control plan for feeds and recycles, and the materials of construction for hot acid and solution service.

06

Trace what each part of the system does

This is a functional reference built from the sources cited; it is not one maker's specification.

07

Fill the specification fields with values for your plant

A melt system quotation needs values for your plant. The table explains each field, gives published information where a reviewed source states it, and says what to ask the supplier for.

08

Send the product, capacity and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Products: [low/high-density prills; granules; solution]
Capacity: [t/day]
Melt: [concentration; water %; temperature]
Feeds: [ammonia; nitric acid strength]
Neutraliser: [stages; pressure]
Evaporator: [type; atmospheric or vacuum]
Utilities: [steam; electricity; cooling water]
Emissions: [air and water limits]
Codes: [applicable]

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