Compound Fertilizers FROM ROCK PYOSPHATE, NITRIC AND SULFURIC ACIDS, AND AMMONIA 31. M. STRIPLIN, JR., DAVID McKNIGHT, AND T. P. EIIGNETT Tennessee Valley A u t h o r i t y , Wilson Dum, Ala.
T
generally available and because its direct use might be preferable VA began work several years ago to develop methods for the to its intermediate use to produce phosphoric acid by the wet production of compound fertilizers by the nitric acid acidulaprocess. The requirement of sulfuric acid for making products tion of rock phosphate. The large expansion in nitric acid prowith a nitrogen to phosphorus pentoxide ratioof l.Owas onlyabout duction and the current sulfur shortage have stimulated interest 600/, of that required for production of an equivalent amount in use of nitric acid for this purpose. Hignett (3) has described of available phosphorus pentoxide in ordinary superphosphate. several such processes that are being studied by TVA. I n all The process steps are similar regardless of whether phosphoric these processes the acidulated mixtures are ammoniated to yield or sulfuric acid is used together with nitric acid, and pilot plant products that consist predominantly of dicalcium phosphate and work showed that the same equipment could be used for acidulaammonium nitrate; the processes differ chiefly in the manner in tion and ammoniation in either procees. However, the drying which the calcium in the rock in excess of the amount required t o method which, in the case of the nitric-phosphoric process, deform dicalcium phosphate is treated to avoid the presence of pended on discharging the dehydrated material from the dryer in a calcium nitrate in the product. Houston, Hignett, and Dunn semifluid state was not apelicable in the nitric-sulfuric process. have published (4) the results of the pilot plant development of The presence of calcium sulfate and the smaller proportion of amone of these processes in which two- or three-component fertilizers monium nitrate caused thickening of the slurry and this inter(N-P205 or N-P2O5-K2O) of high plant-food content were profered with passage of material through the dryer and caused duced from rock phcsphate, nitric and phosphoric acids, amlocalized overheating. monia, and, optionally, potassium chloride. A simplified flow sheet of the process and equations that repThe present paper describes briefly the development, through resent the acidulation and ammoniation reactions are shown in the pilot plant stage, of a similar process in which sulfuric acid inFigure I. The acidustead of phosphoric lation reaction proacidis used t o react duces calcium nitrate, with the excess cal~ H oROCK spHATE phosphoric acid, and cium. A process N I T R I C ACID S U L F U R I C ACID calcium sulfate. Amsimilar in many remoniation results in spects to this one has C O , ~ IPO4)e F ~ + 12 "03 + 4 HzSOa,. t h e f o r m a t i o n of been reported to be in 6 H3P04 + 6 C a ( N O 3 I 2 t 4 C a S O , 2HF ACIDULATION ammonium nitrate, commercial producdicalcium phosphate, tion in France (6). and some monoamT h e nitric-phosnionium phosphate. phoric acid process 6 H3P04 + 6 C a I N O S I Z + 4 Ca S O 4 i 2 HF + I3 NH3 = Potassium c h l o r i d e N H ~ N O + ~5 C O H P O ~+ N H , H ~ P O ~c ~ C O S O ,+ C E F ~ was studied first beGASEOUS may be added during cause both acids are the drying step to ohproduced in the current TVA fertilizer tain a homogeneous, three-component operations and could POTASSIUM be used t o demonCHLOR IO€ GRANULATION (N-P206-K20) ferti(OPTIONAL) lizer. strate the process on a reasonably large scale. The use of sulACIDULATION furic instead of phosphoric acid was inPRODUCT Unground Florida vestigated, b e c a u s e flotation concentrate Figure 1. Simplified Diagram of Process that was screened to sulfuric acid is more
I
236
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952
TABLE I. TYPICAL CHEMICAL AND SCREENANALYSES OF ROCKPHOSPHATES USED PILOT PLANTAND SMALL SCALETESTS PzOs
CaO
RzO:
Composition, % Acid Ignition F insoluble lass
Screen Anal sis 7 of Size Fraction Indioatei (b. Series No.)
8.
Moisture
Pulverized Florida Pebble
32.2 47.2
2.0
3.5
7.6
5.5
-40 -100 -200 +lo0 +200 +325 -325 12.1 23.1 16.7 47.7
+40 0.6
0.4
Screened Florida Flotation Concentrate
34.8
49.6
28.0 39.4
4.3
5.2
0.8
-8 +12 5.0
-12 +20 7.9
2.0
3.8
5.3
Pulverized Tennessee Phosphate Sand (Washed) -100 +lo0 +140 3.2 20.0 3.2 1.0 1.4 5.8
remove the small amount of plus &mesh material present was used almost exclusively in the pilot plant work. Pulverized Florida and Tennessee rock phosphates were tested either in pilot plant or small scale operations and were found to be suitable for use in the process. Nitric acid of 42% "08 concentration and 93% sulfuric acid were used in the tests. The acids were mixed in an acid feed tank in specified proportions, and a small amount of water was added to give a slurry of satisfactdry fluidity for ammoniation. When Florida phosphate was used, it was necessary that the slurry from the acidulation step contain about 40% of water. With the Tennessee phosphate, it was necessary that the slurry contain somewhat more water because i t thickened more during ammoniation. Also, it was necessary to use a higher degree of acidulation on the basis of the calcium oxide content of the phosphate with Tennessee than with Florida phosphate to obtain complete extraction of the phosphorus pentoxide. Chemical and screen analyses of some of the rock phosphates used are given in Table I. In the pilot plant work the proportion of sulfuric acid used was a t least that required to form calcium sulfate with the calcium in the rock in excess of that required to form dicalcium phosphate with the phosphorus pentoxide. The proportion of nitric acid used was determined by the total acid required to solubilize the phosphorus pentoxide in the rock phosphate and by the composition desired for the product. It was demonstrated that the acid required to solubilize all the phosphorus pentoxide in the Florida phosphate was about 90% of that indicated by the first equation in Figure 1-namely, an acidulation ratio of
H2Soc = 0.9. With sulfuric acid in the amount CaO specified above and an acidulation ratio of 0.9,the nitrogen to phosphorus pentoxide ratio of the final product was about 0.8. However, most of the pilot plant work was done at an acidulation ratio of 1.1, by using an excess of nitric acid, to obtain a product with a nitrogen to phosphorus pentoxide ratio of 1.0. A typical mixed acid for the acidulation of Florida phosphate to yiela such a product contained 15%sulfuric acid, 29% nitric acid, and 56% water. Continuous acidulation was carried out in the equipment described previously for the nitric-phosphoric acid acidulation (4). The acidulation section of the pilot plant consisted of tw9 1-foot diameter by 3.5-foot high tanks which were equipped with mechanical agitators and foam breakers. The rock phosphate was fed into the frat acidulation tank a t a rate of 150 to 160 pounds per hour, and the acid mixture was fed simultaneously into the same tank a t a rate of about 38 gallons per hour. The acid mixture was used a t room temperature, and the temperature of the slurry was about 160" F. because of the heat generated by the reaction. The specific gravity of the slurry a t this temperature was about 1.5. The acidulation step was simple in operation and loss of nitrogen averaged only about 0.2%.
-20 -50 +50 +lo0 34.1 43.4
-140 +200 6.1
-200 +230 6.0
-100 9.6
-230 80.7
AMMONIATION
As was found in the work with mixtures of nitric and phosphoric acids, four-stage continuous ammoniation was the most satisfactory method for carrying out the second stepof the process. The ammoniators previously described (8) were used in this work, but a second impeller was added to each turbine mixer. The mixers were driven a t 200 r.p.m. by 3.5-hp. motors. The slurry lines between the ammoniators were increased from 2 to 4 inches in diameter and were set at a slope of 45 degrees instead of 30 degrees. These changes were desirable because the slurries were considerably thicker than those that had been obtained with the mixture of nitric and.phosphoric acid. The modified ammoniators were 2 feet in diameter by 4 feet high; one of them is shown in Figure 2, and a photograph of the four ammoniators is shown in Figure 3. Slurry from the acidulation step was fed to the first ammoniator a t 45 gallons per hour and anhydrous, gaseous ammonia was introduced into each tank a t a predetermined rate through a 0.5-inch open pipe a t a point near and slightly below the lower impeller.
t AMMONIA
INLET PIPE I/.. OIA.
Oe5"Oa
STATIC LlPWR
SLURR IN
Y Figure 2.
Ammoniation Tank
The flow of ammonia to each tank was controlled manually and was measured by means of a rotameter. The total amount of ammonia added to the system was 105 to 107% of that stoichiometrically required to convert all the nitric acid to ammonium nitrate. Distribution of ammonia in the various stages was not studied extensively, but it was shown that more than about 57% in the first stage resulted in loss of ammonia and more than &.or
238
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 3.
Vol. 44, No. 1
Four-Stage Continuous Ammoniation Unit
570 in the last stage resulted in lower availability of the phosphorus pentoxide in the product. Results of typical tests on the effect of ammonia distribution, in a four-stage system, on the phosphorus pentoxide availability in the product are shown in Table 11. The most satisfactory distribution of ammonia, from standpoints of good phosphorus pentoxide availability and minimum ammonia losses, was 57% in the first stage (pH 0.7 to OB), 26% in the second stage (pR 1.4 to 1.7), 13y0in the third stage (pH 2.4 to 2.7), and 4% in the last stage (pH 3.3 to 3.7). I n continuous ammoniation the availability of the phosphorus pentoxide in the product decreased as the final pH increased over about 3.7. The p H values were determined with indicator test paper. Attempts to record pH automatically were unsuccessful because of deposition of solids and failure of electrodes at high temperatures. The slurry temperature was highest in the first stage (about 220' F.) and decreased to about 190' F. in the fourth stage. The slurry discharged from the fourth ammoniator averaged 30 to 32% water as compared with 40% water in the extract slurry. Approximately 25y0 of the original water was evaporated during ammoniation. The vapors, largely steam, were vented to the atmosphere. I n good operation ammonia losses were lese than 1 %. I n tests of batch ammoniation the slurry was thicker than in continuous ammoniation, and i t occasionally was necessary to
stop the flow of ammonia and allow the agitators to thin the slurry; this slurry also was more difficult t o handle in subsequent operations of pumping and drying. Although the phosphorus pentoxide availability of the batch-ammoniated material was 98% or more, the difficulties in operation made this procedure unattractive when compared with continuous ammoniation. The four-stage continuous system proved dependable and required little attention in operation. Pilot plant tests as long as 5 days in duration were made without interruption in flow of ammonia. DRYING
The ammoniated slurry consisted of a solution of ammonium nitrate which contained precipitated dicalcium phosphate and calcium sulfate, some soluble phosphates, and acid-insoluble material from t h e rock. The approximate composition of this slurry was calculated from chemical analysis to be as follows: 29.0yo NH4N03, 18.0% C a H P 0 4 , 16.0% CaS04, 2.6% acid insoluble, 1.6% ammonium phosphates, 0.7% CaF2, 0.1% Ca8(P0&,.and 32.0y0 water. The development of a satisfactory drying method for this slurry proved to be the most difficult problem in the pilot plant work. The high temperature rotary drying method that was employed satisfactorily in the phosphoric acid modification of t h e process ( 4 ) proved unsuitable for complete drying of this slurry which thickened and built u p on the wall of t h e dryer where it became overheated with the result that ammonium nitrate was volatilized. A satisfactory two-stage drying procedure was developed for TABLE 11. EFFECTOF AMMONIA DISTRIB~TION O N PRODUCT PH~SPHORUS PENTOXIDE AVAILABILITY [N FOUR-STAGE this process. This procedure consisted in (1) partially drying the CONTINUOUS AMMOKIATION ammoniated slurry to a water content of 12 to 15ojo in a rotary Ammonia. Distribution, '%/Stage Availahilitya of PzOs, dryer operated with inlet gas a t 2200" F. and exit gas a t 700" F. 1 2 3 4 7% (product temperature about, 230" F.); (2) mixing the partially 03 29 29 29 13 dried material with recycled dried product and, optionally, potasna 48 33 13 6 616 16 15 5 95 sium chloride in a double-shaft paddle mixer; (3) drying the 95 57 26 13 4 mixture, which contained about 5 to 6% water, in a rotary dryer a Per cent of total PzO6 soluble in atninoniiim citrate arcording to 4 . O . A . C . method ( I ) . operated with inlet gas a t 500" F. and exit gas a t 235" to 290" F.; b Detectable ammonia losses alien this amount of ammonia was added in and (4) cooling, screening, and bagging the product. The product thq first stage. contained about 1% moisture.
January 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
The first-stage dryer was 1.5 feet in diameter and 10 feet long.
It was operated a t 18 r.p.m. with a slope of about 0.3 inch per foot of dryer length and with cocurrent flow of feed and gas. The flow of slurry to the dryer was 90 to 100 gallons per hour and was regulated by means of a constant-head feeding device. T h e unit was tested with cight full-length flights, with flights in the feed half and a scraper blade in the discharge half, and without flights but with. a scraper blade that extended throughout its entire length. Best operation was obtained with the full-length scraper blade, which was effective in removing material that stuck t o the walls. The minimum moisture content of the partially dried slurry that could be attained without excessive loss of nitrogen was about 12%. When the moisture content was 12% or more, loss of nitrogen ranged from 1 to 1.5% of the total amount of nitrogen charged t o the process. D r y product was recycled continuously to the paddle mixer and there was mixed into the partially dried slurry in a ratio of about 2:l to obtain a granular material containing 5 to 6% moisture; this material could be dried without difficulty. I n the production of a three-component (N-P206-K20) fertilizer, potassium chloride was added to the hot product from the first-stage dryer to ensure homogeneity of the final product before recycle product was added. It was not practical to add the potassium chloride to t h e ammoniated slurry before it entered the first-stage dryer because inadvertent overheating of the material, which would contain chloride and ammonium citrate, would result in slow, propagated decomposition of the prcduct. Pilot plant tests showed that, in the production of an 11-11-11 material, homogeneous incorporation of potassium chloride required a retention time G f 12 to 15 minutes in the paddle mixer and that 5 to 7 minutes of mixing was sufficient for addition of recycle and subsequent granulation. Because the pilot plant mixer was small, the two operations were carried out to simulate two-stage operation: the potassium chloride was added during the first pass through the mixer and the recycle was added during the second pass. I n large scale operation. multiple units probably would be required because of design limitations. Figure 4 shows the first-stage dryer and the paddle mixer with feeders for controlling the flow of potassium chloride and recycled dry product to the paddle mixer. Second-stage drying was carried out in a conventional rotary dryer or in a Roto-Louvre dryer. These dryers were on hand and were of larger capacity than other units of the pilot plant that had been constructed to test this procesk Consequently, the dryers were operated intermittently for several-hour periods as sufficient feed material was accumulated. The rotary dryer was 3 feet in diameter and 24 feet long. It was a direct, gas-fired unit, equipped with eight flights, and was operated a t about 5 r.p.m. with a slope of 0.17 inch per foot. Operation was with cocurrent flow of feed and heating gases to permit the use of a higher inlet-gas temperature without danger of overheating the material in the dryer than was possible with countercurrent flow. The product temperature was maintained below 290' F. to minimize loss of nitrogen. Satisfactory operation was obtained with an inlet-gas temperature of 500' F., an exit-gas temperature of 290' F., and a gas velocity of about 5.5 feet per second. Under these conditions a retention time of 20 minutes was required t o dry material from 6 t o 1% moisture a t a rate of 2000 pounds of product per hour. The solids occupied about 12% of the volume of the dryer. Loss of nitrogen in the second-stage drying ranged from 1 to 1.5% of the total nitrogen charged to the process. Cooling of the product was not carried out 8s an integrated part of the pilot plant operation. However, tests showed that the product from the second-stage dryer could be cooled to 120' to 140' F. without excessive size reduction and dust loss in rotary-type equipment. About 40% of the product from the cooler was of the desired size ( - 6 to +50 mesh) and was obtained by screening. The amount of oversize and undersize material was about equal to that required for recycling and, therefwe, crushing of the product was
239
unnecessary. The material to be recycled was conveyed pneumatically from the screens to a hopper from which it was discharged to a belt feeder that regulated the flow of recycle material to the paddle mixer. Dust from the second-stage dryer was collected with a cyclone dust catcher and recycled. Loss of dust from the cyclone amounted to less than 0.5%of the feed to the dryer. The Roto-Louvre dryer had a n inside diameter of 1.5 feet a t the feed end and 2.0 feet a t the discharge end; the shell had an outside diameter of 2 feet 7 inches and was 10 feet long. It was operated a t about the same temperatures used with the conventional rotary dryer and loss of nitrogen was the same. However, closer control of feed-particle size was necessary since there was very little breakdown of material in the dryer and large particles were not dried sufficiently. T h e dryer had a capacity of about 1500 pounds of product per hour and was operated with about 35% loading and a retention time of 23 minutes. Data obtained in typical two-stage drying carried out during this study are shown in Table 111.
TABLE 111. RESULTS OF TWO-STAGE DRYING First Stage" Second Stageb Feed rate lb /hour 1150 2095 CO gas bhrned, cu. ft./hour 2840 2000 Temperature, F. Inlet gas 2200 .500 700 290 Exit gas Product 230 275 Moisture content, % Feed 31.1 5.5 Discharge 14.6 1 .o Input heat, million B.t.u./ton final productc 1.65 1.63d a An 18-inch b y IO-foot rotary dryer with scraper blade (no flights). b A 3 X 24 foot rotary dryer with eight flights. c For total heat required per ton final product add columns 1 and 2. d Based on a weight ratio of recycle dry product to final product of 1 . 7 : l .
A one-stage drying method also was studied. Ammoniated slurry, to which potassium chloride had been added in the desired proportion, was mixed with recycled dry product to make a pelletized mixture suitable for low temperature drying. A rotary tumbler, 3 feet in diameter and 6 feet long, was employed as a means for continuously mixing the slurry and the recycled material. T h e slurry was sprayed onto the recycled, dry material in the tumbler in such proportions that the mixture discharged as pellets that contained 5 to 7% moisture. However, this method of drying was not as satisfactory as two-stage drying because it was necessary to use a recycle t o slurry solids ratio of about 6: 1, and 40% more heat was required for drying the material. VARIATION OF PRODUCT COMPOSITION
It was demonstrated in small scale work that the nitrogen to phosphorus pentoxide ratio in the product could be varied by varying the proportion of nitric acid to sulfuric acid. Although most of the pilot plant work was directed toward the production of material with a nitrogen to phosphorus pentoxide ratio of 1:1, some runs were made for the production of material with this ratio a t 1:2. The operation for the production of these materials was the same as that for the production of materials having a nitrogen to phosphorus pentoxide ratio of 1 :1 except that (1) the slurry from the acidulation step had to contain 45 instead of 4Oy0water to maintain satisfactory fluidity during ammoniation, and (2) the temperature of the product from the second-stage dryer had to be held below 150' F. to avoid excessive loss of nitrogen. For this purpose a typical mixed acid contained 24y0 H,SO,, 12% HNO,, and 64% H20. The increase in proportion of sulfuric to nitric acid required for the production of material having a nitrogen to phosphorus pentoxide ratio of 1:2 resulted in less calcium oxide being available for the formation of dicalcium phosphate and in the formation of a larger amount of ammonium phosphate. Typical chem-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4.
Vol. 44, No. 1
First-Stage Dryer and Paddle Mixer
Potassium chloride and recycled dry product ure inwrporated into partially dried slurry
ical analyses of tlie materials produced on a pilot plant scale arc shown in Table IV. T h e high proportions of water-soluble phosphorus pentoxide in t h e 1:2 ratio materials result from thc. prescnw o f larger amounts o f ztmmonium phosphate. OPERATION OF PLANT AND COLLECTION OF DATA
The pilot plant had a capacity o f 4 tons of product pcr d:~yiri continuous operation, and most tests comprised continuous 3-duy periods of operation. An operating crew consisted of an engineer, three trained operators, and two laborers. T h e cngineer had general supervision of the plant and responsibility for the niaintenance o f t h e desired conditions and for the proper recording of data. One operator prepared the mixed acid and observed and controlled the operation of t h e ammoniation unit, t h e second operalor controlled the feed of rock phosphate and mixed acid to the acidulation unit, and the third operator controlled thc dryers. T h e laborers moved materials between the dryers and screened and bagged t h e product. Data were taken during pilot plant operation in such a manner as to ensure the maintenance of test conditions and to supply information for material balances. T h e weight o f rock phosphate and the volume of mixed acid fed were recorded every half hour; the rock was sampled each time t h e feeder hopper was filled, and a composite rock sample for the test run was prepared for chemical analysis; t h e mixed acid was prepared in batches of 450 t o 500 gallons, and a sample was taken from each batch for analysis.
TABLE
N 14 11 8 7 6 a
Nomina Grade“ 1 P2Os Kr0 14 0 11 11 16 0 14 7 12 12
Total N 14.5 11.6 8.2 7.2 6.4
1v.
NHr N
Total PtOs
7.6 6.1 5.1 4.5
14.9 12.0 16.9 14.8 13.2
4.0
The late of flov of nininonia to each ammoniator W:LR recorded each half hour. The slurry in each amnionintor \\x. tested with iiidiwtor paper every half hour, or more otten if necebsary, :LS a guide to contiol of amnioniatioll, :ind the indiratcd pI-1 \\as re(-orded. T h e effluent slurry from the fourth :tninioniai or was saniplcd hourly and the samples \\ere combined into :L daily sample for cheniical analysis : in atdditioii, :L moisture dcternlination was made on an efflurrit sample every 4 hours. The feed of slurry to the first-stage dryer m d of p o t , i ~ ~ i uchloride m :rnd recycled product to the piiddle mixer neie bet a t tlic dcsired lates lxfore the beginning of a period of dryer operation. T h e secoiidstage dryer was fed nmiiutdly and tlie rate of fced WLS iegulated by the operators to give a desired product rttte. T h e :mounts of materials fed and produced \\ere recorded a t suitahle intervals. T h e rate of flow of fucl gas to each dryer : i d the feinperatures of the products and of the gases at hey points in the dryer system were recorded every half hour. Products from the first-stage dryer, from the paddle mixer, and from the serond-stage dryer were sampled for moisture determinations at periods of 30 minutes or 1 hour, depending on operating conditions, :~ndshift composite samples for chemical analysis were prcpared from periodic samplings o f the second-stage drycr product mid (Jftlic h:igged product. DEMONSTRATION RUN AND PROCESS REQUIREMENTS
Figure 5 is a quantitative flow sheet showing the conditions during operation of the plant for the production o f 11-11-11
COMPOSITION OF PRODUCTS
Watersoluble
PzOs
x 2.7
2.2 7.6 6.6
13.0
Product Analysis, % ’ Availability, % Available of total
Pro6
P-@s
KzO
14.6 11.8
98.0 98.3 97.0 97.3 97.0
0 11.9
16.4 14.4 12.8
CaO
21.6 17.3 0 24.0 7.2 21.2 1 3 . 0 18.8
SOs 14.3 11.5 22.6 19.7 1.5.5
H?O 0.9 1.0 1.2 1.1 1.0
Filler could be added to product t o obtain specified grade without excessive overage in plant food content.
241
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January 1952 ROCK
NITRIC
PHOSPHATE 77% B P L o 358 T
SULFURIC
!sT,E,R
POTASSIUM CHLORIDE
AMMONIA
60%K20 0203T
HANDLING LOSS
0 001 T N H 3 LOSS 0 0 0 4 T "03 LOSS 0 . 2 3 1 T H e 0 LOSS
0004T
t 0 . 3 5 4T
KCI HANDLINQ LOSS 0.003T. 02OOT
FIRST-STAGE
AIR
- 36,630 F T 3
GO GAS- 5,490 FT3 H 2 0 ' LOSS
ROCK VOLATILE LOSS 0 Ol8T
0 136T
0.001 T NH3 LOSS 0 0 0 4 T HNO3 LOSS 0 . l S O T H e 0 LOSS
+&\;;;;;
0957
T
9 MIXER
I
2,11Tv J RECYCLE FINES AND O V E R S I Z E I689T
120.F
SECOND-STAGE
230.F 14 6% H 2 0
E
AIR- 2 0 0 5 4 0 FT3 COOAS- 5 : 4 2 0 FT3
T7'lT
1 0 % tip0
PRODUCT HANDLING LOSS
AIR- 7 5 , 6 8 0 f T 3
0022T
PRODUCT
IOOOT
CHEMICAL ANALYSIS: P2 0, 1 3 4 . 5 % 00 0 = 5 0 . 5 % Hp 0 * 2 0 %
11.7% N 11.7% P2O5 K20
Figure 3.
Quantitative Flow Diagram for Pilot Plant
fertilizer. The material balance shows an over-all nitrogen recovery of about 95% and phosphorus pentoxide recovery (aa available phosphorus pentoxide) of about 95%. Table V gives raw material and heat requirements for the production of several grades of fertilizers produced by this process. DISCUSSION
The work,doscribed in this paper demonstrated the feasibility of producing N-P~OSand N-P206-KzO fertilizers from rock phosphate, nitric and sulfuric acids, ammonia, and potassium chloride. Only about 60% of the amount of sulfuric acid required in the production of normal superphosphate was required for the production of an equal amount of available phosphorus pentoxide in products having a nitrogen to phosphorus pentoxide ratio of 1:1. The operating procedure and types of equipment used in the pilot plant tests should be adaptable to large scale production of fertilizers by this process. Cost estimates indicated the process to be economically attractive. Field tests of the products are under way in ten states, extend-
TABLE V.
ing as far west as Colorado. Results that have been reported on tests with cotton, corn, and s d l grain on the acid soils of the southeastern statea demonstrate that the products are as effective as commercial-type mixtures or aa ammonium nitrate, concentrated superphosphate, and potassium chloride when these materials were used so as to supp[y equivalent amounts of nitrogen, phosphorus pentoxide, and potassium oxide. No significant difference was shown between the products of the nitric-sulfuric process and those of the nitric-phosphoric process (4) except in a few tests on cotton in Alabama, Mississippi, and Tennessee where a slightly greater crop response t o product of the nitric-sulfuric process was ascribed to a need for sulfur in the soil. Tests of the effectiveness of the products on the alkaline soils of the Midwest and West are not complete. The storage properties of the products from the pilot plant weI'e determined by storing them in 1Wpound multiwall paper bags. The products were dry and free flowing after 12 months in sixply bags having two asphalt-laminated plies and after 6 months in five-ply bags having one asphalt-laminated ply. Products in six-ply paper bags without a moisture barrier were caked, and
ESTIMATED PROCESS REQ~REMENTS FOR VARIOUS GRADESOF FERTILIZERS
Ton8 per Ton of Product Florida phosphate rock (77% Nitr$ Sulfuric Potaseium Heat Re uirements. bone phosacid, acid' Anhydrous chloride Million %.t.u./Ton Grade phaCe of lime) (100% "01) (100% &SO41 ammonia (60% KtO) Fillers Product 14-14-0 0.429 0.323 0.173 0.093 0.041 4.02 11-11-11 0.336 0.254 0.135 0.073 o:iii 0.060 3.08 8-16-0 0.490 0.143 0.278 0.064 0.059 4.24 7-14-7 0.429 0.126 0.243 0.057 o:iii 0.054 3.68 6-12-12 0.368 0.108 0.209 0.049 0.208 0.083 3.13 a In pilot plsnt operation 42% "01 and 93% HtSO4 were used and sufficient water was added to result in 40 ,, water in the slurry for the firat two grades shown and 4.5% for the other three grades. ?Filler required to obtain product of exact grade shown in first' column. Note:
Power requirements for all grades estimated at 60 kw.-hr./ton of product.
INDUSTRIAL AND ENGINEERING CHEMISTRY
242
t h e bags had broken or had weakened seriously within 6 months. Since the products contain ammonium nitrate, they are hygroscopic and will absorb moisture when exposed under humid conditions. Drillability tests showed the products to have satisfactory drilling characteristics before exposure and after exposure, for a t least 48 hours in open trays, t o air at 80" F. and 80% relative humidity. The relative drillability with a screw-type fertilizer distributor (John Blue No. 30) b y a procedure used in previous tests on conditioned, grained ammonium nitrate (6) was 90% after 48 hours of exposure as compared t o only 38y0 for conditioned, grained ammonium nitrate. Explosibility tests carried out according to a procedure developed by the Underwriters' Laboratories (7) showed that the products were not explosive. However, since they contain ammonium salts and some contain chloride, they should not be exposed to unusually high temperature; propagated slow decomposition occurs when the product is heated to 380' to 400' F. Some of t h e problems encountered in ammonium nitrate production are encountered in this process and should be considered in plant design. The product is hygroscopic and when damp is corrosive. Therefore, the equipment should be as dust-tight as practical, and all motors should be of the totally enclosed type. No copper-bearing metals should be used in any exposed parts of equipment, and the floor in areas subject to contact with the slurry should be constructed of acidproof material. The pilot plant acidulation and ammoniation tanks, the mixer impellers and shafts, the piping and valves for mixed acid and slurry, and the first-stage dryer were constructed of A.I.S.I. Type 304 stainless steel, which was satisfactory. Corrosiqn tests on specimens suspended in the slurry in the first acidulation tank and in t h e first ammoniation tank and in the vapors above the f i s t ammoniation tank during pilot plant operation showed that A.I.S.I. Types 304, 316, and 430 stainless steels had acceptably low corrosion rates (less than 13 mils penetration per year). Neither mild steel nor cast iron was a satisfactory material of construction for parts of the equipment in contact with slurry, but mild steel was satisfactory for the paddle mixer, t h e low temperature dryer, and all other equipment in contact with the solid product. Other considerations which should enter into the design of a plant are that gravity flow should be utilized wherever possible since maintenance of slurry pumps is costly, and accurate proportioning of raw materials is necessary to ensure uniform product composition. SWERPHQSPHATETYPE PROCESS
After completion of the pilot plant work on the slurry-type process described above, an investigation was begun to determine whether t h e process could be modified so that it could be carried out in t h e type equipment used for acidulating and ammoniating in plants that produce normal superphosphate. It was believed t h a t if rock phosphate were treated with a mixture of nitric and sulfuric acids in proportions to convert the phosphorus pentoxide in the rock t o monocalcium phosphate rather than to phosphoric acid the acidulate would set u p into a friable solid suitable for solid-state ammoniation. The acidulation reaction can be represented by the equation CaloF*(PO&
+ 6HN03 + 4H~S04= 3Ca(HzPO& 43Ca(NO& + 4CaSOd + 2HF
(1)
and t h e ammoniation reaction by the equation
+ +
3Ca(HaPOa)z 3Ca(NO& 5CaHPOd NH4HzPO4
+ 4CaS04 + 2HF + 7NH3 = + 4CaS04 + 6NH4N03 + CaFZ
(2)
T h e equations indicate that the process would require the same amount of sulfuric acid and half the amount of nitric acid required for the acidulation of rock phosphate by the slurry-type
Vol. 44, No. 1
process and that the product would have a nitrogen to phosphorus pentoxide ratio of about 0.4: 1. T h e modified process has been investigated in exploratory tests in small scale and pilot plant scale equipment and has been found to be feasible. Ground rock phosphate was acidulated with mixed acid in the proportions given b y Equation 1; the sulfuric acid used contained 91% HzSOd and the nitric acid contained 41 to 5901, " 0 3 . Promising results were obtained when using batchwise acidulation with brief, vigorous agitation followed by a storage period t o simulate the use of a pan mixer and a den in the manufacture of normal superphosphate and when using a funnel mixer (8that discharged into a den. The acidulatcs set u p into friable solids which were ammoniated in a revolving drum of the type commonly used for ammoniating normal superphosphate. The final products contained 6 t o 770 nitrogen and 16 to 18% phosphorus pentoxide, of which over 96% was available. A study to determine optimum operating conditions and to obtain data necessary for further evaluation of the process is continuing. ACKNQWLEDGMENT
Pilot plant operation was under the immediate supervision o f J. L. Graham. J. F. Anderson and R. D. Young participated in the experimental work and contributed significantly to the preparation of the manuscript. F. A. Faulkinberry prepared preliminary plant design data and cost estimates used in evaluating the process. Acknowledgment also is made to R. M. Dasher and V. J. Jones, who participated in the development of this process. A significant portion of the analytical work was done by 1,. R. Randles and P. C. Gwinn. Agronomic tests were conducted by the state agricultural experiment stations under contractual arrangement with TVA. These tests were planned jointly b y the station agronomists and H. T. Rogers of TVA's division of agricultural relations, Knoxville, Tenn., who also coordinated the work of the several states. LITERATURE CITED
(1) Association of Official Agricultural Chemists, "Official Methods
of Analysis," 7th ed., pp. 10-11, 1950. (2) Harvey, S. A., and Bridger, G. L., U. S. Patent 2,528,514 (Nov. 7, 1950). (3) Hignett, T. P., Chem. Eng., 58, 166-9 (May 1951). (4) Houston, E. C., Hignett, T. P., and Dunn, R . E., ~ N D .ENO. CHEM.,43, 2413-18 (October 1951). (5) Miller. Philip, Lenaeus, G. A., Seaman, W. C., and Dokken, M. N., Ibid., 38,709-18 (July 1946). (6) Quanquin, M., Zndustrie chimique, 34, 165-7 (1947). (7) Underwriters' Laboratories, Inc., Bull. Research No. 39. Chicago (August 1947).
RECEIVED June
4. 1951.
ERRATA REPRINTS Reprint sheets of INDUSTRIAL AND ENGINEERING CHEMISTRY errata for 1951 are available, free of charge, by requests addressed to: Reprint Dept., 1 1 5 6 1 6 t h St., N.W., Washington 6, D. C.
