Enginnyring
Compound Fertilizers
Process development
FROM ROCK PHOSPHATE, NITRIC AND PHOSPHORIC ACIDS, AND AMMONIA I
E. C. HOUSTON, T.
P. HIGNETT,
AND
R. E. DUN”
TENNESSEE VALLEY AUTHORITY, WILSON DAM, ALA.
Pilot plant development of a fertilizer process in which rock phosphate is acidulated with nitric and phosphoric acids is described. The process, technical feasibility of which was demonstrated through operation of a 4-ton-perday pilot plant, involves acidulating rock with mixed acids [mole ratio (“08 -I-HsPOJ/CaO = 2.1 and CaO/Pz06 = 2.01, ammoniating the resultant solution to pH 3.7 to 4.0, drying, and granulating. Introduction of potassium chloride into the molten product from the dryer is optional to produce an N-PzOs-KzO fertilizer. The proportions of N-PzO5-KzO in the product are variant; compositions prepared include 12-33-0, 19-19-0, 11-22-11, and 15-18-10.
The proportion of water-soluble phosphorus pentoxide present ranges from about 7 to 45%. A major portion of the experimental product that was made was 17-22-0, containing 10% of the phosphorus pentoxide in water-soluble form and 95% or better in citrate-soluble form. The products showed up favorably in tests of crop response, storage, and drillability. Estimates indicate the process to be economically attractive. This is one of a group of related processes being investigated by the Tennessee Valley Authority in which nitric acid is used to convert the phosphorus pentoxide in rock phosphate into an available form.
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sulted in the conversion of increascd proportions of the phosphorus pentoxide to ammonium phosphate, which is watersoluble and therefore desirable for certain fertilizer usages. A flow sheet of the process as developed in the pilot plant is shown in Figure 1. The principal steps in the process are:
FUNDAMENTAL advantage of the use of nitric acid for decomposing rock phosphate in the manufacture of fertilizers is that the cost of the acid is offset by the value of the nitrogen in the fertilizer product. Furthermore, the requirement for-and the expense of-sulfuric acid, fuel, or electrical energy used for decomposing rock phosphate in other processes is eliminated or decreased, the products are more concentrated than the usual fertilizer mixes, and the products are homogeneous in the sense that each granule contains the several plant-food ingredients. Hignett ( 1 ) has described several processes involving decomposition of rock phosphate with nitric acid that a E being studied by the Tennessee Valley Authority. Study of these processes, some of which are similar to processes that have been used in Europe (8, 4 4 , was undertaken because very little detailed information was available with regard to methods, equipment, and suitability of types of.rock available in this country and because it appeared likely that significant improvements could be made in the processes. PILOT-PLANT OPERATION
Simple acidulation of rock phosphate with nitric acid does not yield a conveniently usable fertilizer, because the product contains calcium nitrate and monocalcium phosphate and, therefore, is extremely hygroscopic, corrosive, and too unstable to dry. In the process described in this paper, the presence of calcium nitrak in the product is avoided through the uee of phosphoric acid and ammonia. The proportion of phosphoric acid used together with nitric acid was such that the solution contained a t least 1 mole of phosphorus pentoxide (rock plus acid phosphorus pentoxide) per 2 moles of calcium oxide. This proportion of phosphoric acid ensured that, on subsequent ammoniation of the solution, all the calcium would be precipitated as dicalcium phosphate except a small proportion that formed calcium fluoride with the fluorine that was present. The nitrate then was present in the product as ammonium nitrate. The use of an excess of phosphoric acid rePresent address, Ethyl Corp., Baton Rouge, La.
1. The acidulation of raw rock phosphate by means of a mixture of nitric and phosphoric acids; the proportions of the acids and rock phosphate in most cases were approximately those indicated by the following equation: CaloFz(POa)e 20HN03 4H,PO, = 10Ca(N03jL 10H3P04 2EIF (1) 2. Ammoniating the slurry from acidulation, with anhydrous ammonia, to make ammonum nitrate and dicalcium phosphate according to the following equation:
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10Ca(N03)? 10H3PO1 2HF 21”s = CaFz 20”4N03 9CaHPOI NH,H2P04 ( 2 ) 3a. Drying the ammoniated slurry to obtain a nitrogenphosphate fertilizer or, alternatively, 3b. Drying and adding potassium chloride to obtain a homogeneous, three-component N-P20j-K20 fertilizer.
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4CIDULATION
Treatment of rock phosphate uith a mivture of nitric and phosphoric acids (Reaction 1) vias accomplished in two tanks in series; each tank was 1foot in diameter by 3 5 feet high and was equipped with a propeller agitator and a foam breaker, both of which were mounted on a motor-driven shaft. The tanks were intciconnected at the bottom, and the tops were connected to an evhaust manifold for fume removal. Rock phosphate was discharged from an elevated feed bin to a hopper and screw feeder mounted on scales. The screw feeder discharged the rock, a t n. rate of 120 to 150 pounds per hour, into a mixing funnel where the rock was wetted with a stream of premixed acids fed at a rate of 35 to 40 gallons per hour. The rock-acid mixture dropped from the funnel into the first extraction tank, flowed by gravity through both tanks, and was discharged through an overflow pipe in the side of the spcond tank. The position of the overflow pipe gave about 45% freeboard in each tank. This amount of freeboard and the use of foam breakers were required to prevent foam from overflowing the tanks. Calcination of the rock virtually eliminated foaming.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 43, No. 10
PHOSPHATE ROCK
(28-35% Pp.05
j
- 3 5 MESH)
CCWFW
N I T R I C ACID
( 4 2 % "03)
PHOSPHORIC ACID ( 8 0 % H3P04)
AC I D U L A T ION
I
AMMONIATION
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EVAPORATION 8 GRANULATION
PRODUCT Figure 1. Flow Diagram of Pilot Plant for Production of Compound Fertilizers
however, in most situations the use of raw rock and control of foam by freeboard and foam breakers should be the more economical procedure. Originally, the extractor included four tanks in series instead of txo. As tests showed that two tanks gave equally satisfactory results, the others were removed. The use of only one tank was not as satisfactory as was the use of two because foaming was more troublesome and extraction was not so complete. With two tanks, a retention time of about 15 minutes was adequate to solubilize 98% or more of the phosphorus pentoxide contained in the rock. Chemical analyscs and screen sizes of various rock phosphates used in the experimental work are shown in Table I. Florida rock caused more foaming than did Tennessee rock, but no other diffcrences in operation due to differences in grade or source of rock were observed in extraction. Most of the experimental work was done with pulverized Florida rock (sample 3, Table 1)because it happened to be conveniently available. However, sample 4,20mesh unground flotation concentrate, gave somewhat better results; it foamed less, dusted less, and worked more smoothly in the feeder. A11 the pilot-plant work was carried out with 42% nitric acid and 80% electric-furnace phosphoric acid, both of which were available from TVA operations. Both acids were used without dilution except when necessary t o correct for minor variations in concentration. The acidulates were sufficiently fluid to permit ammoniation without dilution with water. The concentration of most commercially produced nitric acid is in thc range of 50 to 60% HN03, n-hereas phosphoric acid produced by the wet process has a concentration of about 35% HSPO, (prior to the
customary evaporation step). Acids of these concentrations would produce slurries sufficiently fluid for ammoniation. Small scale test,s showed that the impurities present in wet-proccss phosphoric acid would not adversely affect processing or product, The ratio of phosphoric acid to nitric acid was varied according to the desired composition of the product. The minimum acidrock ratio is important in defining the minimum nitrogenphosphorus pentoxide ratio obtainable in the pinoduct without an increase in the proportion of phosphoric acid, which is the most expensive of the raw materials. The results of tests made in the laboratory and confirmed in the pilot plant showed that solubilization of the phosphorus pentoxide in the rock was substantially complete when t'he mole ratio ("03 H3P04)/Ca0 vas 2.1 or more. This applied to a range of test's in rhich the mole per cent of phosphoric acid was varied from 0 to 50% of the total acid, used. Grade of rock did not have a significant effect upon the minimum requirement for acidulation. During extraction substantially all the calcium oxide and Rn05 Tvere solubilized; about 30% of the fluorine in the rock wa6 volatilized and 1.5% or less of the nitrogen was evolved as fume or mist. In most of the work the acid mixture was fed to the extraction unit at room temperat'ure. When the acid mixture was. preheated to 130" F., the estimated maximum temperature of the acids if used directly from the acid plants, there \vas no significant increase in nitrogcn losses or in foaming. When acid mixture was used at room temperature, t'he slurry left the extraction unit a t 110" to 120" F. Its specific gravity was about 1.3, and it contained about 40% water. A typical analysis, expressed as grams per liter, is: PZO.;, 230; CaO, 165; N,92; Ra03, 10.3; and F;. 8.5.
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TABLE I. ROCKPHOSPHATES USEDIN EXPERIMEXTAL WORK Composition, % (Dry Basis) Source AcidNo. of Rookc PZOS CaO RzOs F insoluble Moisture 1 Florida 32.2 46.0 ... ... 2 Tennessee 28.0 39.1 . . . . . . ... 3 Florida 31.9 46.7 2.9 3.5 7.5 0.9 4 Florida 34.3 49.2 2.8 4.0 4.0 0.8 5 Tennessee 31.6 43.8 5.8 3.4 11.2 0.0 0.0 6 Florida 29.6 44.5 2.2 3.5 12.4 a Florida pebble and Tennessee brown rock from several commercial vendors.
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Particle Size,
% through Sieve No. 35 100 200 325 Q8.8 48.7 . . . . . . 100 . . . . . . 99.6 87.5 64.4 47.'7 81.2 7.3 . . . . . .
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100
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
October 1951
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The extraction slurry was very corrosive and abrasive, and attempts to pump it led to frequent failures of a variety of pumps. Subsequent arrangcment of the plant for gravity flow satisfactorily eliminated this problem. AMMONIATION
Four-stage continuous ammoniation was the most satisfactory method developed for carrying out the second step of the process (Reaction 2). Essential features of the pilot-plant ammoniation equipment are shown in Figure 2.
Four tanks, each 2 feet in diameter by 4 feet high, were arranged in series on the same elevation. The tanks were connected with pipes 2 inches in diameter which were on a 30-degree slope to avoid sanding out. The pipe inlet was about 6 inches below li uid level; the discharge end was about 16 inches below liquidyevcl and extended about 4 inches into the downstream tank to an area near the impeller. Anhydrous, gaseous ammonia was introduced to each tank through a 0.5-inch tube at a point near and slightly below the impeller (Figure 2) to ensure intimate mixing with the incoming slurry. Spargers with small multiple orifices were tested first, but these plugged rapidly and had no advantages over the open-end tubes. Agitation was obtained by means of a turbine impeller 10 inches in diameter, driven a t 200 r.p.m. Vigorous agitation was necessary, especially in the first two tanks where the slurry tended to gel. Without vigorous agitation, ammonia absorption and CLEAN-OUT
VAPOR OUT IA
SPARGER PIP
Figure 2.
Pilot Plant A m m o n i a t i n g T a n k
For first, second, and third stages Fourth-stage tank s a m e but slurry outlet at 2-foot level Material. A.I.S.I. Type 304 stainless steel
phosphorus pentoxide availability were poor. High-speed 4-inch propellers gave equally satisfactory agitation in the third and fourth stages. Foaming during ammoniation was about as serious as foaming
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MOLE RATIO NH3 X IOOIHNO3
Figure 3.
Course of Precipitation d u r i n g Batch Ammoniation
during extraction and was controlled satisfactorily by air-jet foam breakers and liberal freeboard. The beater-type foam breakers that were used in extraction were not tried because of the larger tank diameters and more slowly rotating impeller shafts. Slurries prepared from calcined rock did not foam. The flow of ammonia to each tank was measured with a rotameter and was controlled manually. The total amount of ammonia added was in the range 101 to 106% of that stoichiometrically required to convert all the nitric acid to ammonium nitrate. A satisfactory ammonia distribution through the system was found to be 57% in the first stage (pH
