Fertilizers granular processing

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. 

Essentially all the ammonium sulfate fertilizer used in the United States is by-product material. By-product from the acid scmbbing of coke oven gas is one source. A larger source is as by-product ammonium sulfate solution from the production of caprolactam (qv) and acrylonitrile, (qv) which are synthetic fiber intermediates. A third but lesser source is from the ammoniation of spent sulfuric acid from other processes. In the recovery of by-product crystals from each of these sources, the crystallization usually is carried out in steam-heated sa turator—crystallizers. Characteristically, crystallizer product is of a particle size about 90% finer than 16 mesh (ca 1 mm dia), which is too small for satisfactory dry blending with granular fertilizer materials. Crystals of this size are suitable, however, as a feed material to mixed fertilizer granulation plants, and this is the main fertilizer outlet for by-product ammonium sulfate. 

The routes by which mineral phosphates are processed into finished fertilizers are outlined in Eigure 7. World and U.S. trends in the types of products produced are shown in Eigures 8 and 9, respectively. Most notable in both instances is the large, steady increase in the importance of monoammonium and diammonium phosphates as finished phosphate fertilizers at the expense of ordinary superphosphate, and to some extent at the expense of triple superphosphate. In the United States, about 65% of the total phosphate appHed is now in the form of granular ammonium phosphates, and additional amounts of ammonium phosphates are appHed as integral parts of granulated mixtures and fluid fertilizers. 

Granular TSP (—6 + 16 mesh (1.19 to 3.35 mm dia)) is preferred for direct appHcation and is used in bulk blend fertilizers. A widely used slurry granulation process, the Dorr-OHver process, is illustrated in Figure 11. The ground rock is mixed with 38—49% P2 5 series of reaction vessels. 

Fig. 16. Steam granulation process for production of granular mixed fertilizers from dry, pulverized feed materials (7). A granulator producing 12 t/h would...

Fig. 18. TVA-type cogranulation process with preneutralizer, as used for production of granular mixed fertilizers. Feed materials such as ammonium sulfate, ammonium nitrate, urea, superphosphates, sulfuric acid, and potash are used.

Granulation processes offer a number of important advantages. The most significant are decreased pollution problems and the abiUty to produce granules of almost any reasonable size allowing close size matching with granular ammonium phosphates and potassium chloride in the preparation of NPK fertilizers (26). 

Oberphos A version of the superphosphate process for making a fertilizer by treating phosphate rock with sulfuric acid, which yields a granular product. Used in the United States and Canada, but superseded in the United States by the Davison process. 

Figure 12. Hydro Fertilizer s granular DAP process. (Reproduced by permission of Wiley-VCH)...

Fig. 3.22. SAI-R granular fertilizer process reaction, granulation and drying in one vessel [38].

Because of the low phosphate analysis of SSP (20% P2Os), economics favor shipping the rock (32% P2Os) to local plants and producing the superphosphate there. A majority of the producers use the locally produced SSP in formulations for granular NPK fertilizers made in an ammoniation-granulation process. The maximum amount of SSP possible is... 

Fig. 24.12. Slurry process for the manufacture of granular triple superphosphate.

Fig. 24.13. TVA process for production of granular diammonium phosphate. (Source Fertilizer Manual.)...

Standards of Performance for the Phosphate Fertilizer Industry Wet-Process Phosphoric Acid Plants Standards of Performance for the Phosphate Fertilizer Industry Superphosphoric Acid Plants Standards of Performance for the Phosphate Fertilizer Industry Diammonium Phosphate Plants Standards of Performance for the Phosphate Fertilizer Industry Triple Superphosphate Plants Standards of Performance for the Phosphate Fertilizer Industry Granular Triple Superphosphate Storage Facilities... 

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