Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 111-1 17
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Two-Stage Process for Crystallization of Urea Phosphate for Production of Clear Polyphosphate Liquid Fertilizer Harry T. Lewls, Thomas M. Jones, and James R. Burnell' Tennessee Valley Authoriv, National Feriilizer Development Center, Muscle Shoals, Alabama 35660
I n the process described, relatively pure crystalline urea phosphate of about 17-44-0 grade is made from black, wet-process orthophosphoric acid of about 54% P205content. The process involves the addition of urea to the acid followed by cooling of the process materials to precipitate the relatively pure, crystalline urea phosphate. The product, which contains 80% of the feed urea and P2O5, is removed from the mother liquor by centrifugation, which leaves most of the impurity minerals, carbonaceous material, and water in the mother liquor. The crystalline urea phosphate can be pyrolyzed to produce a urea-ammonium polyphosphate melt that can be dissolved in water to produce relatively clear 15-28-0 grade solution liquid that contains 50% or more of its P205as polyphosphate. Byproduct mother liquor from the process, which contains about 20% of the urea and P2O5 and 85% of the impurities fed to the process, can be processed into suspension or solid fertilizers.
Production of high-quality liquid fertilizer from wetprocess acid involves numerous problems because of the impurity minerals (primarily AI, Mg, Fe, and F) and carbonaceous material present in the wet-process acid. TVA, however, has continued to work on these problems on various fronts. One process developed by TVA uses wet-process superphosphoric acid to produce a satisfactory liquid fertilizer with a grade of about 10-34-0 and with 60 to 70% P205as polyphosphate (Achorn and Kimbrough, 1974). This 10-34-0 grade liquid is the major polyphosphate liquid used in the United States today. The process, however, requires that the wet-process acid be concentrated by heating to about 70% P205with about 15 to 20% of the P205being converted to polyphosphate. Also, to obtain a liquid product with satisfactory clarity, carbonaceous material, which is present in most phosphate ores, must be removed by calcination before the acid is made, or else the acid must be purified, usually by solvent-extraction techniques (Slack, 1968). Otherwise, the acid and liquid product are black, and the product, if not specially clarified (Stinson et al., 1976), is not well accepted by the majority of liquid fertilizer users. Any of these options for obtaining a polyphosphate liquid with satisfactory clarity adds to the cost of the 10-34-0grade product which already is becoming too high because of the rising cost of energy required to concentrate the wet-process acid. One new process under development by TVA perhaps offers a viable solution to these problems. In this new process, relatively pure Crystalline urea phosphate of about 17-44-0 grade is made from black, wet-process, orthophosphoric acid of about 54% P205content. The crystallization process involves the reaction of the acid with urea to form urea phosphate [CO(NH2)2+ H3P04 CO(NH2)2.H3P04]in solution followed by cooling of the process materials to precipitate relatively pure, crystalline urea phosphate. The crystalline urea phosphate then is removed from the mother liquor by centrifugation, which leaves almost all of the impurity minerals, carbonaceous material, and water in the mother liquor. About 80% of the urea and P205is obtained in the crystalline urea phosphate product. The crystalline urea phosphate then can be pyrolyzed to produce a urea-ammonium polyphosphate melt that can be dissolved in water to produce a relatively clear 15-28-0grade liquid that contains 50% or more of its P205as polyphosphate. Some of the procedures involved in production of such a liquid have been
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reported in the literature (Gittenait, 1970, 1971, 1973; Keens, 1968; Koebner, 1970; McCullough et al., 1978; Nayar and Gopinath, 1970; Stinson et al., 1980). Byproduct mother liquor from the crystallization process, which contains about 20% of the urea and P205fed to the process, can be processed into suspension fertilizer or used in making solid fertilizers. Bench-scale development of this urea phosphate crystallization process, which is described in this article, is now complete, and pilot-plant development is nearing completion. The process appears to offer a promising alternative to the production of clarified 10-34-0 grade liquid. Informal cost estimates indicate that the cost of the 15-28-0 grade liquid would be about the same as that for clarified liquid made from black superphosphoric acid, and the 15-28-0 grade liquid also has significant advantages in long-term storage properties compared with the 10-34-0 grade product.
Experimental Equipment and Procedure As shown in Figure 1, feed materials to the first stage (reactor-crystallizer) were either urea melt (135-141 "C) or solid urea, merchant-grade (54% P205)wet-process orthophosphoric acid, and recycle mother liquor. Feed mole ratio of urea to phosphoric acid was 1.0. Recycle mother liquor was fed to the first stage to improve fluidity of the urea phosphate slurries in each stage and to facilitate separation of the product crystals from the mother liquor. In most of the work described in this article, prilled urea was fed to the first stage for convenience and accuracy of test methods. In some early test work, however, urea melt was fed to the first stage to simulate use of urea direct from a concentrator of a urea production plant. To obtain this melt for testing, unconditioned urea prills (46% N) were melted at a temperature of about 143 "C, about 11 O C above the melting point of urea. The process operated satisfactorily, but some decomposition of urea occurred in the melter. Decomposition of urea in a full-scale plant, however, would not be a problem. The first stage (reactor-crystallizer) and the secondstage crystallizer were similarly constructed. The first stage, however, was equipped with an egg-beater-typefoam breaker. Each stage was equipped with a jacket for cooling the urea phosphate slurries and each had a turbine-type agitator with a slanted-vane impeller. The impeller tip speed (1m/s) in each stage was sufficient to suspend the crystals in the slurries. The crystals, however, purposely
This article not subject to U.S. Copyright. Published 1983 by the American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 TURBINE
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TYPE (135-141 'C) OR
IWWRFED, WT -PROCESS 54% H3PO4 P*05'
SECOND STAGE (CRYSTALLIZER,
A PHOSPPATE SWRRY
UREA PHOSPHATE CRYSTALS ( 1 7 - 4 4 - 0 GRADE)
BY-PRODUCT MOTHER LIQUOR (9-22-0GRADE)
Figure 1. Two-stage process for production of crystalline urea phosphate by reaction of urea with wet-process phosphoric acid. Table I. Wet-ProcessOrthophosphoric Acids Used in Tests of Two-StageUrea Phosphate Crystallization Process composition, % acidno.
P,O,
A l 2 0,
Fe 2 0 3
1 2 3 4 5
53.7 53.8 53.0 52.7 53.6
1.6 1.3 1.2 1.1 1.4
1.6 1.2 1.2
6 7
52.8 53.6
0.6 0.6
1.3 1.4
1.4
1.4
MgO F Uncalcined Florida Ore 0.65 0.56 0.50 0.50 0.58
0.9 0.8 1.0 0.8 0.8
so4
3.1 3.2 3.6 3.7 3.1
Ca 0
C
W. L a solids
trace trace trace
0.34 0.23 0.31 0.24 0.23
1.8 0.9 0.1 0.9 1.6
trace trace
12 4-6 4-6 4-6 3/4-4 3i4 3i4
