Ind. Eng. Chem. Res. 1989,28, 329-334
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Urea-Nitric Phosphate Granular Fertilizer Ronald E. Edwards,* Oscar E. Moore, and Charles A. Hodge Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, Alabama 35660
A process development program was initiated for a new type of granular fertilizer based on the acidulation of phosphate rock with nitric acid followed by the formation of urea adducts. The initial investigation of this urea-nitric phosphate (UNP) process was performed using 17 lb/h bench-scale equipment t o test continuous processing and various raw materials. On the basis of data from the bench-scale plant, a 350 lb/h granular U N P pilot plant was built for larger scale testing of the main process steps-acidulation, urea adduct formation, granulation, and drying. T h e success of these tests proved t h a t a new nitric phosphate fertilizer can be produced without the use of a filtration and crystallization unit, with high P205efficiency, and without waste phosphogypsum. Standard fertilizer properties of the product, initial storage, and application testing were satisfactory for the U N P products. According to the United Nations Industrial Development Organization (UNIDO) study (Nielson, 1987),world fertilizer production is expected to continue to increase, with a tendency toward higher N:Pz05 weight ratios. Recent trends in fertilizer technology are aimed at higher material and energy efficiencies, manufacture of multicomponent products, reducing nutrient losses by modifying the fertilizer properties, and pollution abatement. At present, the fertilizer industry is the world’s major consumer of sulfur, with nearly 50% of this sulfur being processed to waste phosphogypsum in wet-process phosphoric acid production (Becker, 1983). Nearly 60% of all the PzOj contained in fertilizer is derived from wet-process phosphoric acid. Also, according to UNIDO/FAO prognosis (International Fertilizer Development Center, 1979), future PzOj production will increase with the construction of new phosphoric acid, monoammonium phosphate, and diammonium phosphate (DAP) facilities and probably with the development of new nitric phosphate technologies. These technologies offer a possibility of sulfur savings and of eliminating the production of waste phosphogypsum. According to published data, nitric phosphate processes are used in 80 plants with a total capacity of about 5 X lo6tons of P205, and several large, new plants have been built recently. The main problem associated with the nitric phosphate technology is the poor physical properties (high hygroscopicity) of calcium nitrate. The present solution to this problem involves the addition of ammonium sulfate to the decomposition liquor to effect low-temperature (about 25 O F ) crystallization of CaS04-2H20.The calcium sulfate is reacted with ammonia and carbon dioxide to regenerate the ammonium sulfate; and calcium carbonate is precipitated, filtered, and dried (Slack, 1966). The calcium ion also can be removed by the addition of carbonate or phosphate compounds (Slack, 1966). These nitric phosphate plants have large capital expenses (for filters, heat exchangers, buffer tanks, etc.) and high maintenance expense. The new TVA process differs from the usual nitric phosphate processes, and the products have unique compositions and promising potential as a fertilizer. This process utilizes the system HN03-Ca0-P205-CO(NH2)2-H20 to produce granular urea-nitric phosphate (UNP) fertilizer and eliminate the need for a Ca(N03)2 removal step. The technology consists of the acidulation of phosphate rock, urea adduct formation, dewatering, granulation, and drying. Addition of urea to the acidulate gives a product with high-water-soluble P205(over 95% of total P205)containing essentially no dicalcium phosphate. The urea stabilizes the acidulate by the formation
of adducts. The products are acidic; thus, ammonia losses from the urea are decreased (Achorn, 1984).
Previous Research Investigation of the system HN03-Ca0-Pz0 j-CO(NH2)2-H20and application of its properties to solid fertilizer production have been carried out for nearly 40 years. The formation of urea adducts with nitrate, phosphate, and calcium compounds during the reaction of nitric acid with phosphate rock in the presence of urea provides the chance of producing N-P fertilizers containing calcium urea nitrate, urea nitrate, urea phosphate, and dicalcium phosphate. Thermodynamic and X-ray analyses indicate a number of retrograde equilibria in the system, which occur as water is removed to produce a solid product. According to the equilibrium information, the addition of water immediately results in the liberation of nitric acid, which combines with calcium to liberate a phosphate group from calcium diphosphate. This explains the increased watersoluble Pz05 in solid products using this equilibrium system. The results of TVA fundamental investigations provided new concepts for practical applications using the properties of this system. According to these results, the physicochemical properties of these UNP products are strongly correlated with the moles of HNO, used to decompose the phosphate rock and to the urea-to-Ca0 mole ratio in the urea adduct formation (Sullivan et al., 1984). X-ray analysis determined the presence of a urea-monocalcium phosphate discovered previously by TVA (Frasier et al., 1967), a calcium urea nitrate, urea nitrate, and a new fertilizer salt, Ca(H2P04)(N03)CO(NHz)2.The presence of this new fertilizer salt and other urea adducts, as opposed to free calcium nitrate, significantly increases the critical relative humidity (CRH) of the UNP products (Sullivan et al., 1985). Physicochemical analysis of the properties of the salts formed in the wide range of concentrations in this system (Sullivan et al., 1984) and statistical evaluation of the results of a multidimensional experiment (Sullivan et al., 1985) indicated the compositional parameters that would guarantee good properties of the UNP products. Description of Process and Equipment Based on the TVA fundamental investigations, development of this UNP process progressed to bench-scale (17 lb/h) and then to pilot-plant (350 lb/h) studies in an effort to better characterize the desirable physical, chemical, and agronomic properties associated with the UNP products. Figure 1 is a flow sheet of the granular UNP fertilizer process which consists of an acidulation-reaction stage, a
This article not subject to U S . Copyright. Published 1989 by the American Chemical Society
330 Ind. Eng. Chem. Res., Vol. 28, No. 3, 1989 GRANUAR uKA.HTRK: PHOSPHATE PILOT PLANT
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Figure 1. Process flow diagram. Table I. Dimensions of the Main Equipment Used in Bench Scale and Pilot Plant dimensions (dia X length), in. equipment bench scale pilot plant 24.0 X 42.0 acidulator 8.0 X 14.0 4.0 X 6.0 10.0 X 26.0 urea mixer 24.0 X 36.0 dehydrator 4.2 X 16.0 24.0 X 48.0 granulator drum 12.0 X 24.0 36.0 X 144.0 dryer 12.0 X 38.4 vibrating screen 16.0 (dia.) 20.0 X 37.0 none 12.0 X 72.0 cooler
urea adduct formation stage, and a granulation section. The UNP bench-scale test unit and pilot plant consist of three reactor-type vessels (acidulator, urea mixer, and dehydrator), three rotary drums (granulator, dryer, and cooler), a crusher, a vibrating screen, and other auxiliary equipment. The dimensions of the main apparatus are presented in Table I. In the acidulator vessel, phosphate rock is digested with nitric acid a t an HN0,:CaO molar ratio of 2.0 using nitric acid concentrations ranging from 55% to 70% HNO,: 20HN03 + CaloF2(P0,)6 6H3P04+ 10Ca(N03)2+ 2HF (1)
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Phosphate rock is acidulated at temperatures of 150-155 for approximately 85 min to ensure a high degree of phosphate rock digestion (about 99%). Foaming in the acidulator is controlled by the addition of an antifoaming agent. Slurry from the acidulator overflows into a steamjacketed urea mixer where urea is added as prills, granules, or melt. A ratio of 3.2 mol of CO(NH2),per mole of CaO is used. Urea adducts form at temperatures of 212-215 O F and a retention time of approximately 10 min. The following reactions are involved in the urea adduct formation: H3P04+ Ca(N03)2+ 2CO(NH2), Ca(H2P04)N03CO(NH2)2 + CO(NH2)2-HN03(2) O F
the liquid for granulation. The amount of water to be removed depends on the amount of water introduced to the acidulation stage. The dehydrator agitator is equipped with side and bottom scraper blades, and the vessel has a shrouded gravity overflow. Retention time in the vessel is 30 min at 212-220 O F . This slurry along with recycled fines and crushed oversize are introduced into a nonflighted drum granulator. The slurry is sprayed onto the rolling bed of granules in two locations by directing compressed air across two streams of slurry. Recycle is fed to the granulator at a recycle-to-product weight ratio of approximately 5 to 1. Granules discharged from the granulator fall into the dryer where heated air is blown cocurrent to the flow of granules. The dried granules contain about 1.0% water. The dried granules drop to a vibrating screen deck which uses two screens to separate the onsize product from the undersize and oversize products. The oversize is crushed and returned with the undersize as recycle. The onsize product is cooled and transferred to storage. Other N-P grade UNP granular fertilizers (such as 24-12-0) can be produced by adding supplemental P205 to the acidulation stage or to the urea adduct formation stage as either merchant-grade (53% P205)wet-process phosphoric acid or superphosphoric acid (68% P205). To produce a 23-7-7 grade NPK fertilizer, solid potassium chloride must be added directly to the granulator with the entering recycle and cogranulated.
Raw Materials The following materials were used in the bench-scale plant and pilot-plant tests: nitric acid, 57% HNO,, produced by TVA, Muscle Shoals, AL; 60% HN03 and 70% HNO,, solution of chemical-grade HNO,; phosphoric acid, 53% P205, blend of acid from several central Florida sources; superphosphoric acid, 68% P205,central Florida; urea, urea prills, -7 to +10 Tyler mesh, conditioned with formaldehyde; Urea LS (TVA trademark for urea conditioned with calcium lignosulfonate, produced by TVA, Muscle Shoals, AL); potassium chloride, white solution grade, 63.1% K20, 98% -12 Tyler mesh; antifoam agent, sulfonated oleic acid; phosphate rocks, unground Florida phosphate flotation concentrate; unground Florida phosphate flotation concentrate; ground North Carolina phosphate rock; ground Idaho phosphate rock; ground Florida phosphate flotation concentrate. Chemical analyses and physical properties of the phosphate rocks are given in Table 11.
Research Procedures Chemical Analysis. Chemical analyses of the raw materials, intermediate products, and granular products were determined by using the Association of Official Analytical Chemist (AOAC) procedures. Chemical analysis of all samples included fertilizer nutrients (P205,N, K20, and CaO) and the various forms of these nutrients (e.g., water-soluble P205,available P205,nitrate nitrogen, ammonia nitrogen, urea nitrogen, and biuret). 4CO(NH2)2+ Ca(N03), Ca(N03)2.4CO(NH2)z (3) Physical Properties. The TVA standard testing procedures (Hoffmeister, 1979) were used for measuring the CO(NH2)2 + H3P04 CO(NH2)2*H3PO, (4) physical properties of the fertilizer products. A description Ca(H2P04)2+ Ca(N03)2+ 2CO(NH2I2 of each of these tests is as follows: CRH at 86 O F , sample ~ C ~ ( H Z P ~ ~ ) ( N ~ ~ ) .(5) C O ( Nexposure H Z ) ~ method under controlled humidity and temperature; melting point, determined by using electroth2HF + Ca(N03)2+ 2CO(NH2), ermal melting point apparatus; moisture, determined by CaF, + 2CO(NH2),-HNO3 (6) AOAC classification method; granule hardness, crushing strength of individual granules (-7 to +8 Tyler mesh), Slurry from the urea mixer is pumped to a steam-jacketed dehydrator where excess water is removed to prepare measured by compression tester; abrasion resistance, de-
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Ind. Eng. Chem. Res., Vol. 28, No. 3, 1989 331 Table 11. Chemical a n d Wet-Screen Analysis of t h e P h o s p h a t e Rock Processed by Using t h e U N P Process vhosvhate rocka chem. anal. A B C D wt % 32.3 32.6 30.1 33.0 Pz06(total) 47.7 49.8 48.0 45.7 CaO 1.8 2.4 0.7 0.6 FeZ03 1.6 1.3 1.4 0.5 A1Z03 12.6 4.2 3.7 2.9 SiOz 0.5 0.4 0.6 0.5 MgO 1.5 1.6 1.4 0.1 H20 3.8 3.9 3.6 3.3 F 0.4 1.3 1.3 2.9 C (total) 0.17 0.11 0.12 0.48 KzO 0.57 1.19 1.94 0.50 NazO 0.3 1.0 1.1 1.7 COP PPm c1 37 ' 88
