Ammonium nitrate granulation

Ammonia from coal gasification has been used for fertilizer production at Sasol since the beginning of operations in 1955. In 1964 a dedicated coal-based ammonia synthesis plant was brought on stream. This plant has now been deactivated, and is being replaced with a new faciUty with three times the production capacity. Nitric acid is produced by oxidation and is converted with additional ammonia into ammonium nitrate fertilizers. The products are marketed either as a Hquid or in a soHd form known as Limestone Ammonium Nitrate. Also, two types of explosives are produced from ammonium nitrate. The first is a mixture of fuel oil and porous ammonium nitrate granules. The second type is produced by emulsifying small droplets of ammonium nitrate solution in oil. 

Ammonium Nitrate Granulated Commercial Explosives of Plant Manufacturing Granulites and Zemogranulites in the Collection Vzryvnoye Delo , No 49/6 (1962) 77a) L.G. 

Table 8.23. Ammonium Nitrate Granulation Process Requirements... 

Polymorphism and mechanical stability of ammonium nitrate granules the influence of additives, in D BIWIC 1993, p. 51, Delft + Bremen 1993... 

Ammonium nitrate granules and fertilizers based on these are coated with a layer consisting of sodium phosphates (trimetaphosphate and polyphosphate) and salts of alkylaryl sulfo-acids. These reagents may be added either both together to the ammonium nitrate melt or separately the sodium phosphates to the melt and the sodium salts of the alkylaryl sulfo-acids to the granules. The granules with added polyphosphates are then powdered with diatomite [538]. 

The urea produced is normally either prilled or granulated. In some countries there is a market for Hquid urea—ammonium nitrate solutions (32% N). In this case, a partial-recycle stripping process is the best and cheapest system. The unconverted NH coming from the stripped urea solution and the reactor off-gas is neutralized with nitric acid. The ammonium nitrate solution formed and the urea solution from the stripper bottom are mixed, resulting in a 32—35 wt % solution. This system drastically reduces investment costs as evaporation, finishing (priQ or granulation), and wastewater treatment are not required. 

Urea processes provide an aqueous solution containing 70—87% urea. This solution can be used directiy for nitrogen-fertilizer suspensions or solutions such as urea—ammonium nitrate solution, which has grown ia popularity recentiy (18). Urea solution can be concentrated by evaporation or crystallization for the preparation of granular compound fertilizers and other products. Concentrated urea is sohdified ia essentially pure form as prills, granules, flakes, or crystals. SoHd urea can be shipped, stored, distributed, and used mote economically than ia solution. Furthermore, ia the soHd form, urea is more stable and biuret formation less likely. 

Pan granulation of ammonium nitrate also has been developed commercially (16,17). Essentially water-free melt (<0.5%moisture) is sprayed onto a cascading bed of fines (usually cmshed recycle) ia a tilted, rotatiag pan. Production of 18—20 t/h is common. Product made ia this manner is said to require ao conditioning. The pan is an excellent size classifier, making it easy to produce granules of any desired size from 1 to 11 mm diameter. 

O. Skauh, "Pan Granulation of Ammonium Nitrate and Urea," paper presented at the 168th National Meeting of the Mmerican Chemical Society, Adantic City, N.J., 1974. 

Three steps are essential to ammonium nitrate manufacture neutralization of nitric acid with ammonia to produce a concentrated solution evaporation to give a melt and processing by prilling or granulation to give the commercial soHd product. 

Graining, flaking, and spraying have all been used to make soHd ammonium nitrate particles. Most plants have adopted various prilling or granulation processes. Crystallized ammonium nitrate has been produced occasionally in small quantities for use in specialty explosives. The Tennessee Valley Authority developed and operated a vacuum crystallization process (25), but the comparatively small crystals were not well received as a fertilizer. 

Many plants outside of North America pfill or granulate a mixture of ammonium nitrate and calcium carbonate. Production of this mixture, often called calcium ammonium nitrate, essentially removes any explosion hazard. In many cases calcium nitrate recovered from acidulation of phosphate rock (see Phosphoric acid and the phosphates) is reacted with ammonia and carbon dioxide to give a calcium carbonate—ammonium nitrate mixture containing 21 to 26% nitrogen (23). 

Ammonium Nitrate Plants - In ammonium nitrate plants, wet scrubbers can be considered for prill towers and the granulation plant. Particulate emissions of 0.5 kg/t of product for the prill tower and 0.25 kg/t of product for granulation should be the target. Similar loads for ammonia are appropriate. Other effluents that originate in a nitrogenous fertilizer complex include boiler blowdown, water treatment plant backwash, and cooling tower blowdown from the ammonia and nitric acid plants. 

Ammonium Nitrate Plant 7. Prill lower reduce microprill formation and reduce carryover of fines through entrainment. 8. Materials handling where feasible use covers and hoods on conveyors and transition points. Good cleanup practices must be in place to minimize contamination of stormwater mnoff from the plant property. 9. Granulators reduce dust emissions from the disintegration of granules. 

Frilling towers convert molten materials into droplets and allow them to solidify in contact with an air stream. Towers as high as 60 m are used. Economically the process becomes competitive with other granulation processes when a capacity of 200- 400 tons/day is reached. Ammonium nitrate prills, for example, are 1.6-3.5 mm dia in the 5-95% range. 

The reaction is carried out in aqueous phase using a slight excess of nitric acid. The heat of reaction is utilized to evaporate the water. Also, evaporation may be carried out under vacuum. Alternatively, solid ammonium nitrate is obtained by crystallization from a concentrated solution. The particle size of the dry product may be controlled by vacuum crystallization, granulation or... 

Ammonium nitrate is milled until granules with a particle size ranging from 5 to 500 n are obtained. Fineness of grinding governs the rate of burning of the mixture. 

Usually prilled ammonium nitrate is used as it facilitates detonation of the explosive and the porosity of the granules help to retain the liquid ingredient. 

The addition of fine-powdered inert coating substances which are adhering to the surface of the ammonium nitrate crystals or granules and are simultaneously capable of absorbing moisture, e.g. powdered kaolin, chalk, kieselguhr, silica gel, zinc carbonate or zinc oxide, in amounts from 0.1 to 5%. This method seems to be accepted in many countries. 

Adding inert coating and fuel oil to ammonium nitrate is more efficient when the latter is in granulated, prilled or flaked form. 

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