Prilling

In most cases air is used as the cooling medium but other gases or liquids may also be used. In the manufacture of lead shot, molten lead is prilled into water. The prilling of calcium nitrate into mineral oil has been described [5], Prilling into liquid medium requires shorter towers because of the improved cooling by liquids but an extra step to remove the liquid cooling medium is necessary. 

Some typical prillable materials. (Courtesy HPD Incorporated.) 

Pentachlorophenol Petroleum wax Phenolic resins, Novalak resin Pine rosin Polyethylene resins Polystyrene resins Polypropylene-maleic anhydride Potassium nitrate Resins 

Sodium nitrate Sodium nitrite Sodium sulphate Solid glycols Stearic acid Stearyl alcohol Substituted amides Sulphur 

Urea and additives Urea-sulphur mix Wax-resin blends Substituted aliphatics 

In the process a molten material is disintegrated into droplets which are allowed to fall and to solidify in contact with an air stream. The 

Dimensional and some operating data for prilling of urea and ammonium nitrate also are in Table 12.18. Towers as high as 60 m have been installed. Because of the expense of towers, prilling is not competitive with other granulation processes until capacities of 200-400 tons/day are reached. 

The density of the prills is reduced substantially when much evaporation occurs with 0.2-0.5% water in the feed, ammonium nitrate prills have a specific gravity of 0.95, but with 3-5% water it falls to 0.75. Prilled granules usually are less dense than those made by layering growth in drum or fluidized bed granulators. The latter processes also can make larger prills economically. To make large prills, a tall tower is needed to ensure solidification before the bottom is reached. The size distribution depends very much on the character of the atomization but can be made moderately uniform. Some commercial data of cumulative % less than size are  

Cooling of the prills can be accomplished more economically in either rotary drums of fluidized beds than in additional tower height. Fluidized bed coolers are cheaper and better because more easily dust controllable, and also because they can be incorporated in the lower section of the tower. After cooling, the product is screened, and the fines are returned to the melter and recycled. 

Initial particle size distribution often is in the range of 50-250 / m. The product of Table 12.19(a) is 0.7-2.4 mm dia. 

Evaporators in commercial use include circulatory systems, shell and tube heat exchangers and falling film-type evaporators. The falling film evaporator has the advantages of a small working volume and a short residence time. All evaporators produce steam that must be purified, and some of the techniques mentioned above can also be applied to this steam103. 

The droplets crystallize and condense into hard, spherical prills that are dried, cooled, and sized for shipment. If calcium ammonium nitrate (CAN or nitro chalk) is made, ground calcium carbonate (limestone or dolomite) is added to the melt prior to the formation of the droplets when CAN is being made103 10S. 

Sasol in South Africa produces a porous, prilled ammonium nitrate (PPAN) that finds its widest application in a mix with fuel oil. This mixture is used as an explosive and is commonly known as ANFO (Ammonium Nitrate Fuel Oil). Standard PPAN contains randomly distributed closed pores of an uncontrolled variable size and quantity. Sasol also makes EXPAN by using a patented process where polymeric microspheres are entrained uniformly in individual prills. Surfactants are added prior to the prilling process to ensure that the microspheres are evenly distributed in the prill. The addition of these microspheres (or encapsulated gas bubbles) reduces and controls prill density to desired levels. This improves the sensitivity and performance of the explosive while retaining the desirable properties of the standard prills (mechanical strength, oil absorption and free-flowability)106. 

SIDE VIEW SIDE VIEW SDEWEW C IRC UL AR CROSS- SECTIONS - 

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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. 

The urea solution is evaporated in a two-stage system (99.8%) if the final product is prills, and a single-stage system (+95%) if granules are to be provided. 

The urea solution is then evaporated to 99.8% for prilling (2 stages) or plus 95% for granulation (1 stage). 

The urea solution stream is then fed to the vacuum concentrator unit which operates at 17.3 kPa (130 mm Hg abs) and produces 88.7 wt % urea. It then goes to either two-stage evaporators if prills are made, or a single-stage unit for granule production. 

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. 

Prilling. The manufacture of ptiUs is rapidly decreasiag owiag to both environmental problems and product quaUty as compared to granules. 

Lipids present in the diet may become rancid. When fed at high (>4-6%) levels, Hpids may decrease diet acceptabiUty, increase handling problems, result in poor pellet quaUty, cause diarrhea, reduce feed intake, and decrease fiber digestion in the mmen (5). To alleviate the fiber digestion problem, calcium soaps or prilled free fatty acids have been developed to escape mminal fermentation. These fatty acids then are available for absorption from the small intestine (5). Feeding whole oilseeds also has alleviated some of the problems caused by feeding Hpids. A detailed discussion of Hpid metaboHsm by mminants can be found (16). 

Fig. 2. Fertilizers in the prilled or granular form (a), prilled ammonium nitrate (b), prilled urea (c), granular urea (d), granular diammonium phosphate ...

Nitrophosphates are made by acidulating phosphate rock with nitric acid followed by ammoniation, addition of potash as desired, and granulation or prilling of the slurry. The acidulate, prior to ammoniation, contains calcium nitrate and phosphoric acid or monocalcium phosphate according to the foUowiag equations ... 

Iodine is a bluish black, crystalline soHd having a metallic luster. It is obtained in shiny flakes or prills that can be easily cmshed to powder. Iodine crystallines in rhomboidal plates belonging to the triclinic system. 

Up to 0.4 g/L of the iodine stays in solution and the rest precipitates as crystallized iodine, which is removed by flotation (qv). This operation does not require a flotation agent, owing to the hydrophobic character of the crystallized element. From the flotation cell a heavy pulp, which is water-washed and submitted to a second flotation step, is obtained. The washed pulp is introduced into a heat exchanger where it is heated under pressure up to 120°C to melt the iodine that flows into a first reactor for decantation. From there the melt flows into a second reactor for sulfuric acid drying. The refined iodine is either flaked or prilled, and packed in 50- and 25-kg plastic-lined fiber dmms. 

The crystallized iodine is decanted and transferred into a fusion kettie. The melted product is contacted with strong sulfuric acid to remove organic impurities and humidity. Finally the iodine is flaked or prilled and packed. 

The cmde lead and discard slag from the reduction furnace are tapped continuously through a single taphole into molds. The discard slags have a 1 to 2% lead oxide content and 1 to 2% of lead metal prills (pellets). 

Uses. A soluble form of magnesium nitrate is used as a fertilizer ia states such as Florida where drainage through the porous, sandy soil depletes the magnesium (see Fertilizers). Magnesium nitrate is also used as a prilling aid in the manufacture of ammonium nitrate. A 0.25—0.50% addition of magnesium nitrate to the process improves the stabHity of the prills and also improves durabHity and abrasion resistance. 

Urea is soluble in water, and the crystalline solid is somewhat hygroscopic, tending to cake when exposed to a humid atmosphere. For this reason, urea is frequendy pelletized or prilled (formed into litde beads) to avoid caking and making it easy to handle. 

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. 

Concentration. Evaporation procedures depend on the concentration of the solution produced during neutralization and the water content required for the subsequent production of soHd product. Neutralizer solutions can contain as Httle as 2% and as much as 25% water feeds to drum granulators can contain 5% water, prill towers 0.3 to 0.5% water. 

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. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

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