SUPERPHOSPHATE PRODUCTION J
A Staff-IndustryCollaborative R e p o r t . R. L. DEMMERLE Associate Editor
WALTER J. SACKEW
in collaboration with
A . J . Sackett & Sons Company, Baltimore, Md.
P H O S P H Y .@ohnolo% the, basis of the largest segment of,,& pwsenkiay~ferthzermdustry, began with the inad&rtent.iwlation,qf phosphorus.in 1669 by Bran&, ,an alchemist, and with the discovery, 100 yearn later, by Gahn (4), that phosphorus is an essential constitutent of animal and ,human hones. In 1840, Liebig (16) suggested treating bones with SUIfuric acid to convert their calcium phosphate to a water-soluble form more easily absorbed by growing plants, but the &st patent for the production of superphosphate was granted to Lawes in 1842. Later in a suit agabst infringers, L a w s claimed he made superphosphate from bdnes, bone ash, bone dust, apatite, phosphorite, and other substances containing phosphoric acid. The defendants cited Liebig's priority with regard to bones, bone dust, and later bone ash aud compelled Lawes to conhe his claims to the mineral raw materials for superphosphate manufact-. This was not a setback to Lawes, however, for in the decade 1842 to 1852 emphasis had swung away from the we of ground bones to ground phosphorus;beariug minerals for direct application to the soil. Tbia shift was due to the discovery of medium and low grade mineral phosphates in both France and England and paved the way for agriculturists to accept a superphosphate made from mineral inatead of animal matxriala (24). The first complete superphosphate plant, including a sulfuric acid installation, was built in 1854 at Ipswich, England, by Edward Packard, an established processor of ground coprolites, a
medium grade of phosphste mineral found in Suffolk (87). In rapid succession, other countries began superphosphate production: Germany and France in 1855, America and Denmark in 1868, Sweden in 1870, and Belgium in 1880. By 1862, British production had reached an annual ontput of 200,000 tons ( 4 8 7 ) . The beginning of the superphosphete industry in the United States was coincident with the discovery and working of large phosphate beds in South Carolina in 1868. These deposits furnished the hulk of domestic as well as some European requirements for many yearn; a peak of 618,569 tons of rock was produced in 1893 (88). The Carolina operations declined and finally ceased in 1920 because of the increasing exploitation of higher grade and more cheaply mined deposits in Florida and Tennessee. The output of these two states constituted more than three quarters of the 7,000,000long tons produced in the United States in 1948. When geographic meas are considered, North Africa is found to he a larger producer than the United States because of the combined output of the large deposits in Tunisia, Algeria, and Egypt. Unitkd States reserves; however, are more plentiful; over 60% are relatively untapped deposits in Idaho, Montana, Ut&, and Wyoming. There are proved phosphate reserves also in Arkansas, Kentucky, North Carolina, Pennsylvania. and Virginia ( 8 1 0 ) . More than two thirds of the phosphate rock produced in the United States in m y year iS used in the manufacture of super-
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July 1949
phosphates for fertilizer. The remainder is employed in making other industrial phosphorus compounds, for direct application to the soil, as fertilizer filler, in stock and poultry feeds, and in miscellaneous uses (26).
PRODCCTION OF ~YORMAL AND CONCENTRATED SUPERPHOSPHATE I N THE UNITED STATESa-1880
1880 1890 1900 1910 1915 1920 1925 1930 1931 1932 1933 1934 1935
CHEMISTRY OF SUPERPHOSPHATES
At the outset of the superphosphate industry, all phosphates were classified broadly into two groups: water soluble and water insoluble. Water solubility, as the sole basis for predicting the availability of the phosphorus content of a fertilizer to plant needs,, .proved inadequate, however, and the concept of citrate solubility was proposed in 1871. This made use of the empirical fact that a neutral solution of ammonium citrate in water more nearly approximates the solvent properties of plant juices than water itself. At the first meeting of the Association of Official Agricultural Chemists in 1884, an ammonium citrate method of determining the availability of all water-insoluble phosphates, except basic slag, was adopted (23). By convention, the phosphorus content of any type of phosphatic fertilizer is expressed as phosphoric acid anhydride (PzOj), which the trade, somewhat confusingly, refers to as its phosphoric acid, phosphorus, or phosphates content. Most commonly, the term phosphoric acid is used to denote the P z O ~ content. The difference betwc,en the total P 2 0 5 content of a fertilizer and the fraction that is insoluble in ammonium citrate solution is the portion considered to be available to the use of the plant and hence is named available P20sor available phosphoric acid, symbolized by the abbreviation, APA (21). I n retrospect, the purpose of the superphosphate industry has been to increase the APA content of mineral phosphate materials in a manner commensurate with prevailing economic conditions. Improvements in manufacturing operations, and the use of, better grades of phosphate rock have enabled the superphosphate industry to increase the APA content of its product from l l . O l % , in 1880, to a current general level of 18 to 20% ( 1 1 ) . I n the desire for greater APA content, however, care had to be exercised to preserve certain desirable physical properties of superphosphate, propetties that affect the ease of its shipment and use and to some extent its behavior in the soil. There has always been some disagreement about the exact nature of the acidulation reaciion by which superphosphate is made. Recent studies lead to the belief that the mechanism may best be explained as consisting of three distinct and separate constituent reactions (62): 3Ca3(P04)2
+ GHzS04
=
4H3P04
+ Cas(POa)2
4H3P04
GCaS04
=
+ Ca8(P04)z + 6CaSO4 3CaH4 PO&
+ 3H20 = GCaS04.'/2H20
(1)
(2)
1307
a
Thousands of Short Tons (18% Pzos Basis) 205 480 1,505 2,595 2,545 5,130 4,040 4,415 2,655 1,705 2,575 2,825 2,950
1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1047 1948
TO
1948 Thousands of Short Tons (18% PzOfi Basis) 3,485 4,470 3,795 4,210 4,865 5,305 5,950 7,072 7,442 8,039 8,701 10,315 10,553
Data from National Fertilizer Association.
receptacle or den into which each batch of the acid-phosphate rock mixture is dumped after mixing. .4lthough the den processes differ from one another in the design and use of their constituent equipment, their basic operation is the same and may be described as consisting of four major steps:
1. Preparation or grinding of phosphate rock for acid treatment 2. Batch mixing of rock and acid 3. Removal from den and curing in bulk piles 4. Excavation from bulk piles after curing and bagging or compounding into mixed goods
A detailed account of the development of the various den processes would be beyond the scope of this article. Their history, however, shows a steady trend toward the mechanization of the operating steps and a subsequent minimization of manual labor. As such, it represents a century of evolution in the techniques of materials hahdling: haphazard proportioning of acid and phosphate rock has been replaced with accurate batch weighing methods; manual mixing of these reactants on open brick floors has given way to the use of large capacity, mechanical mixers; and the small kettle type of wooden den has beer1 suppjanted by multiton capacity concrete, brick,. or wooden dens,
(3b)
Domestic hosphate rocks, however, consist mainly of uorapatite, expressed as (CaF)Ca4(P04)s or CaFa.3Caa(P04)2, admixed with various proportions of other compounds of calcium, fluorine, iron, aluminum, and silicon. Since most of the phosphorus present is in the insoluble fluorapatite compound, its lattice must be destroyed and most of the fluorine eliminated before the phosphorus content is made available ( 2 ) . The following equation, therefore, is often used to describe the fundamental reaction in the manufacture of Superphosphate (25):
l
+ +
++1 7(CaSO4.2HzO) 7H~0 +
Z(CaF)Ca4(PO& 7H~S04 3CaH4(PO4)2.HZ0 2HF
Actually, in most cases, the hydrogen fluoride combines further with silica which is present and appears in the process effluent as SiF4 or HZSiF6. THE DEN PROCESSES
Until comparatively recently all superphosphate was made by a variety of den processes which take their name from the large
Acid Dildtion Pit Lead mixing chamber (center); small horizontal pipe carries diluent water into chamber, larger carries acid. Small vertical p p e introduces air for agitation. Large pipe (rear) supplies cooling water to outsi c of mixing chamber. Sampling pump (rear) sends constant sample of acid to control room
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INDUSTRIAL AND ENGINEERING CHEMISTRY
emptied by clamshell bucket shovpls or niechanical cutters and conveyers (6). The trend toward mechanization in the manufacture of superphosphate prompted a natural desire for the development of continuous processing methods, but in spite of the penchant of American chemical engineers for continuous operations, web processes for making superphosphate, with few exceptions, have had a better reception in Europe than in this country. This is mainly because batch type superphosphate operations have been developed to a fair degree of efficiencyin America and represent a considerable capital investment. The two raw materials of the superphosphate industry are sulfuric acid of 50“ to 56” BB. (approximately 62 to 71y0 I12SOg) and phosphate rock having a tricalcium phosphate or bone phosphate of lime (b.p.1.) content of 68 to 77%, depending on the source of the material and the degree of beneficiation or upgrading used. The first opcration in the den processes is the grinding of the tricalcium phosphate, usually to a fineness which allows 80 to 85% of the material to pass through a 100-mesh screen. The fineness of the rock has a direct effect on the reaction rate between the rock and acid, on the efficiency of the action of the acid, and on the grade and condition of the superphosphate (1, 26). The ratio of acid to rock in the acidulation reaction is about 5 parts ground rock to 4 parts acid, but i t will vary depending on the concentration of acid and the composition of the rock used. After weighing, the two ingredients are usually mixed in pan mixers, of up to %ton batch capacity, which will process 40 to 50 tons of material an hour. ,After a 2- or 3-minute mixing period, the contents of the mixer is discharged into the den, which in various plants has a capacity of 100 t o 300 tons. I n most plants, two dens are fed by a single mixer; thus one can be unloaded while the othei is being charged. The acidulation reaction p e s well toward completion during the 6 or more hours when the acid rock mixture is in the den. Carbon diouide, steam, and the gas,eous fluorine by-products epcape, and the mass, which is a t a temperature of 100’ to 120” C., undergoes a change from its original siiupy consistency to a product that is fairly dry and porous. The superphosphate is transferred from the den, by clamshell bucket, shovel, or other mechanical means, to storage piles where curing is completed and where the cooled material awaits packaging. This interval, “on the pile,” may be 4 to 12 weeks depending on the type of phosphate rock originally used. Generally superphosphate made from Florida pebble rock requires only 4 to 8 weeks of curing, whereas the material made froin Tennessee rock needs a curing period of up to 16 weeks ( 2 2 ) . During the curing interval a further conversion to available PzO5 niaterializes in the superphosphate. Because payment for superphosphate is based on its available PZOScontent (expressed to the nearest hundreth of l%),when shipped, i t is to the manufacturer’s advantage to permit adequate curing time before shipment. This practice often places a burden on the small producer of superphosphate who does not have the curing facilities for large quantities of material. He must either ship a “green” product and forego an important part of his profit or hold his superphoaphate for more complete curing and thereby tie up a considerable portion of his inventory. Granular Superphosphate, For a long time after the birth of the superphosphate industry little consideration was given to the control of the physical properties of the finished product which was often damp, high in free acid content, and susceptible to caking and hardening in the bag. It eventually became apparent that inasmuch as superphosphate constitutes about 60% of the composition of most mived fertilizers, the physical properties of this constituent are an important factor in determining the condition and therefore the ease of use of these materials. A notable stride in the improvement of the physical properties of superphosphate vas achieved by the development of the Oberphos process for the manufacture of a granular product (7, 17).
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Granulation imparts bett’er qualities of drillability arid caking resistance to the superphospha.t,e and greatly facilitates its uniform distribution in the field. Subsequent evidence has shown also that granulation improves t’he plant ut>ilizationof the phosphorus-fixing power ( 1 5 ) . Alt’hough the Oberphos process is still used in Canada, it was superseded in this country by the Davison process, another method of producing a granular superphosphate. The Davison process is essentially a den process that includes a two-stage treatment of the superphosphate after it leaves the den to produce a granular uniform material. The first stage consists of adjusting the moisture content’of the superphosphate, as it leaves the den, by the addition of a predetermined amount of classifier dust from a subsequent step and spraying the mixture in a rot,ating horizontal cylinder or classifier with water to cause a balling up of the material into small spheres. The form of these spheres is fixed in the second stage when the material is passed through :I rotary dryer and the excess moisture driven off. Before classifying and bagging the granular superphosphate is allowed t o cure for a period of only 10 days; the short curing time is possible because the heat of the granulation treatment hastens th.e completion of the acidulation reaction (25). Triple Superphosphate, Normal superphosphate (18 to 2074 P a 0 5 )of both the granular and regular types is without doubt i he predominant phosphatic fertilizer. However, in 1948 United States production of triple superphosphate (45 to 50% PDS) reached a new high of 469,000 tons (241, 12% of the total superphosphate output of the country. Triple superphosphate, sometimes known t i s double, treble, or concentrated superphosphate, is made by the acidulation of phosphate rock mit,h phosphoric instead of sulfuric acid and is often described by the following equation ( $ 5 ) : (C’aE‘)Car(POa)a
+ 7IIaP04 + 5HaO --+
+
5CaH%(P0&H20 H F
The use of phosphoric acid as the acidulating agent result,s in a pa,cking of the superphosphate with the additional phosphorus of the acid, but does so a t the expense of the calcium sulfate formed in the conventional sulfuric acid-rock production of superphosphate. It hss been established that the calcium sulfate portion (about half its weight) of normal superphosphate has secondary but valuable functions in t~hecomposition of H. phosphatic fertilizer (19). I n spite of this, freight rate considerat’ions have steadily focused increasing attention on triple superphosphat’e as an economical means of transporting available phosphorus to remote points of application. It is used also to a large extent as a supplemental source of highly concentrated available PzO6 in the prepatation of mixed fertilizers and 207, superphosphate. Ammoniation of Superphosphate. The amnioniation of supcrphosphate with anhydrous ammonia, aqueous a.mmonia, or h y the addit,ion of urea or nitrate solutions has been widely practiced by the fertilizer industry during recent years. The ammonia thus added reacts with arid neutralizes the unattached phosphoric acid, referred t o as the free acid, contained i n t.he superphosphate. This practice has the fourfold advantage of: improving the physical condition of tho superphosphate; adding a second essential plnnt nutrient from the most economical source; reducing the curing period prior to shipment,; and eliminat,ing one of the causes of bag deterioration. The use of free ammonia for this purpose has grown steadily since the development of the American synthetic ammonia industry in the late 1920’s and constitutes one of the largest consuming opernt,ions for rhe mat’erial. Urea and other carriers of organic nitrogen are often dissolved f
Plant Layout and Flow Sheet of Superphosphate Production Operations at Wisconsin Cooperative Farm Plant Foods, Inc., Prairie du Chien, Wis.
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
July 1949
CONTROLLER-RECORDER CID SAMPLE LINE
WASTEWATER
TRACK HOPPER
XIPERPHOSPHATE
c
-
-.
, ”
h K E T ELEVATORS
u
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1310
in the ammoniating solutions to increase still further the efficiency of the ammoniated superphosphate (20). The fixation of free ammonia in superphosphate depends partly on a reaction between it and the uncombined or free phosphoric acid in the fertilizer. The follov,-ing equations describe the ammoniation process ( 9 ) :
+ NIX3 = NH4HZPOI (1) Ca(H2P04)2.H20+ = CaHPOd + XH4H2PO4 + HzO (2) NH4H2P04+ CaS04 + NH3 = C a H P 0 4 + (SHp)nSO4 (3) 2CaHP04 + Cas04 + 2WH, = Caa(PO& + (SH&SO( (4) HJ'OI
SH3
SUPER FLO PROCESS
A nevi continuous method for making superphosphate was introduced on the American scene recently with the development of the Sackett Super Flo process. To date, there have been two installations of the process, one a t Indianapolis, Ind., and the other a t Prairie du Chien, Wis. Both were engineered and constructed last year by the inventors, The A. J. Sackett Rr Sons Company of Baltimore, Md. Except for a few standard items, all of the process equipment was specially fabricated. The plants are practically identical and the following account of the Wisconsin plant may be considered, lvith some slight differences, to be descriptive of the operations of the Indiana plant as well. The most striking features of the process are its rapid and continuous treatment of the process materials and the use of a suspended type of acidulation. The phosphate rock-acid reaction is carried out in an acidulating tower into which the reactants are introduced in a finely divided state: the ground rock as a dust stream and the acid as a n atomized spray. The reaction which immediately follows is very rapid due to the extreme intimacy of contact between the rock and acid particles. Because of this suspended acidulation, the superphosphate product has a hard, porous grain structure and possesses good physical proper ties for ammoniation, handling, and application. The centralized control and rapid continuity of this pi ocess make it possible for a three-man crew to operate a 45-ton-perhour plant. This represents a considerable saving in labor costs when compared with a den plant, of about equal capacity, which usually, requires a crew of 10 to 12. The continuity of operation also eliminates production losses incurred in the batch processes during excavation of the dens. Power cost also compares favorably with a typical den installation of equivalent capacity. Tne process descrjbed herein produces superphosphate a t the rate of 1.12 kw.-hr. per shrunk ton as compared to approximately 1.75 kw.-hr. using the batch den process. Inasmuch as the production of superphosphate is subject t o wide and frequent seasonal fluctuations, short and inexpensive shutdonm and start-up periods are important. A 45-ton-perhour plant can be activated or deactivated within a few minutes instead of the much longer intervals required in batch type den plants. PRAIRIE
nu
CHIEN P L A S T
The Prairie du Chieii plant of the Wisconsin Cooperative Farm Plant Foods, Inc., was built for the main purpose of supplying superphosphate, for the manufacture of mixed fertilizers, to an adjoining plant of the same organization. Construction of the fertilizer plant was begun in the fall of 1946 and a t the time of its completion, a year later, work was started on the superphosphate plant. This also took a year in building and superphosphate production began in October 1948. The erection of the plant was predicated on the need of the farm cooperative's mixed fertilizer plant for an assured and constant source of superphosphate, the principal constituent of its products. The decision of the cooperative's management to locate their superphosphate plant a t Prairie du Chien, however,
Vol. 41, No. 7
reflects the trend of the past several years on the part of superphosphate producers t.0 build their plants in the areas of consumption and near sources of sulfuric acid. Over 90% of t,he production of this plant is used within a 200-mile radius. It' has been said that to be in the superphosphate induvtry means to be in the sulfuric acid industry as well, and over one third of all the superphosphate plants in this country are supplied from captive acid plants. I n the case of the Prairie du Chien installation, a captive acid plant is not necessary because of the proximity of the production facilities of the Blgoiiquiri Chemical Company at Dubuque, Iowa. The superphosphat,e production units of the Wisconsin plant are housed in two steel frame buildings clad with asbestos-covered corrugat,ed steel panels. A-atural lighting is adinitt'ed through large skylights in the roofs of the buildings but t,here are also adequate provisions for artificial lighting. The smaller of the two structures (60 X 106 feet) contains all of the process equipment and is known as the acidulating building. Adjoining i t is a steel A-frame type building of adequate size to provide storage capacity for the superphosphat,e manufacturd. 'The main floors of b0t.h of the buildings are concrete, but parts of the second and third floors of the acidulating building arc of' safety type steel grillq-ork. A11 exposed steel surfaces of the equipment and plant st,ructure are protected by resin-based acid-reeking paint. I n spite of the fact that the plant area is situated on t,hc bank of the Mississippi river, i t was found more convenient to use a 120-foot well, driven a t the side of the acidulat'ing building, as the source of the cooling and scrubbing vater needed in the process. During the heavy production season (January to May), this well is used exclusively, but during the remainder of the year, cspecially for off-hour operstions, the city \mt'cr supply furnishes the wafer for short production runs. The t,wo process raw materials are Florida pebble rock and 66" BC. sulfuric acid; the rock is brought into the plant by railroad box and hopper cars, and the acid is received in railroad tank cars and tank trucks. About 75% of the acid come3 in b?- truck because of the lower cost of transportation on the short, haul from Dubuque: acid brought in by truck currently costs 52.88 per ton to transport vihereas the hauling charge by rail runs as high as $4.33 per ton. Rail transportation of acid is employcd only during the extreme n-int,er season when the roads in tha,t part of the country often become impassable. Rock Flow. The superphosphate area of the plant is served by t,wo railroad sidings, one capable of handling eight acid tank cars and the other of accommodating tnelve phosphate rock cars. The hopper cars, which have a capacity of 70 tons of rock, are unloaded through a hopper underneath the rock car siding which runs along one wall of the acidulat,ing building. An enclosed canopy extending from this structure protects the hopper and the car being unloaded from the weather. In unloading box cars, which normally carry only 50 tons of rock, a boom-operated drag shovel is used to pull the contents to the door of the car from which it drops to the track hoppc?:. Phosphate rock is obtained from Swift & Company and the International Minerals and Chemicals Corporation. Payinelit for the material ,is based on an average of the bone phosphate of lime contents (usually 72 to 75%) of the shipments received during the time interval being considered. Most of the material when received is capable of passing a %mesh screen, but there are no rigid specifications covering the particle size distribution of the rock. The steel grill over the track hopper has interstices of 1.5 X 3 inches, which serve t o screen out the few large pieces that might cause some difficulty in the subsequent grinding operation. The track hopper feeds a screw conveyer located beneath t,he tracks; the conveyer carries the rock into the acidulating building and discharges i t into a bucket elevator which carries the material from below the track level to a spout feeding a phosphate rock silo where i t is stored until ground. This silo, which is of re-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
inforced concrete construction, has a capacity of approximately 1100 tons. The unground rock is discharged from the silo through three adjustable bottom openings onto an inclined belt conveyer which serves the bucket elevator that fills a 20-ton steel supply hopper locabed over the grinding mill. The grinding operation is conducted in an air-swept Raymond pulverizer ( 1 8 ) which is fed by gravity from the rock mill supply hopper by a rotary feeder. The product of the mill, 85% of which is capable of passing a 100-mesh screen, is swept up an inclined conduit to a cyclonic separator where the ground rock settles out. The air is drawn back by another conduit t o the blower serving the mill. The ground rock discharges from the bottom of the cyclonic separator into the rock dust silo which is made of reinforced concrete. When the Super Flo process is started, the ground rock is discharged from the silo through two bottom openings to a screw conveyer which carries the material t o a bucket elevator. This elevator empties into a spout t h a t enters the weighing room of the plant and delivers ground rock t o the feeders that supply ground rock t o a continuous weighing machine. These feeders are especially designed to prsvide an air seal between the pulverized rock spout and the weighing machine thereby halting the fluid flow of the pulverized material. This feature is necessary t o obviate flushing, the intermittent spurting of the material onto the weighing machine, a condition that would make the accurate continuous weighing of the rock impossible. Pulverized phosphate rock is particularly difficult to handle in this respect, as i t fluidizes with air when in motion, taking on the properties of a liquid. The speed of operation of the feeders is controlled by a servomechanism which is in turn actuated by controlling devices located on the balance beam of the continuous scales. For average operation, the scales are set t o weigh out 25 tons of ground rock per hour. This, when acted on by approximately 20 tons of acid per hour, constitutes what is' rated as a 45-ton-per-hour operation. Actually 40.5 tons of superphosphate are netted from this rock-to-acid balance. The "shrink," as the industry calls it, averages about 10% of the gross weight of the ingredients and is due to losses of the volatile fluoride compounds liberated in the acidulation, loss of moisture, and general product losses in the handling of the material during curing, bagging, etc.
Air Swept Pulverizing Mill (center); Local Equipment Controls ( r i g h t )
1311
The scales are sufficiently sensitive t o keep the prescribed weight within 1% and are equipped with a Mercoid tripper device t o shut off both the scales and the acid pump supplying the acidulation tower should the scales fail to receive the proper feed of ground rock. After being weighed the ground rock drops into a hopper that feeds another bucket elevator which discharges into the top side of a horizontal steel air duct on the third floor of the building. Forced air, introduced in this duct from a blower a t its other end, carries the rock dust along and introduces i t tangentially into the upper side of the cylindrical rubber-lined steel acidulating tower. After it delivers the dust, the air is recirculated; enough new air is introduced t o keep vapors (fluorides, etc.) below undesired concentrations. Excess air leaves the system through the bottom of the tower and into the housing of the next process unit, the puddler, which is kept partially evacuated by an aspirator. The acid enters the acidulating tower by means of a Type 304 stainless steel atomizing nozzle located in the center of the top of the tower. For efficient acidulation, it is necessary that the rate of delivery of acid t o the tower, approximately 20 tons per hour, be carefully regulated by flowmetering. The temperature of the acid as it leaves the nozzle is kept a i t h i n a range of 115" to 135" F. by controlled cooling of the acid after its exothermic dilution from 66" to 56 BB. Acid Flow. After its receipt a t the plant, the 66" Be. acid is stored in two horizontal, mild steel tanks located along one of the inside walls of the acidulation building. Indoor storage facilities were provided because of the extreme weather conditions prevalent in that part of the country. Each of the tanks has a capacity of 200 tons of 66" B6. acid. The acid is diluted, as used, in a small room adjoining the acidulation building. The dilution equipment consists of a tapered lead mixing boot which is connected to one leg of an H-shaped dilution chamber also made of lead. The entire unit is sitfiated in a steel-lined concrete pit into which primary cooling water from the well is pumped. The acid, diluent water, and air for agitation are introduced into the mixing chamber through its removable cover. Although it is adjustable, the rate of the acid flow into the'mixer is usually held constant, but the rate of diluent water addition is automatically controlled by a specific gravity indicator and controller in the coptrol room located inside the acidulation building. A continous sample of the mixed acid is drawn into this device which actuates the water addition valve to produce the desired amount of dilution. The sampling and controlling instrument is adjusted t o compensate for aberrations in the specific gravity readings due to fluctuations in the temperature of the diluted acid, A secondary cooling of the diluted acid is made inside the acidulation building in an open, rectangular, lead-lined steel tank, I n this case, the cooling water is passed through a series of lead immersion coils by a manifold which is fed from the plant's water supply. Another manifold is provided for the exit of the secondary cooling water which is then piped into the concrete sump tank which is the reservoir for the aspirator that evacuates the puddler. As needed in the processing, the diluted acid, now a t a temperature within the 115" to 135" F. range, is drawn from the cooling tank by a stainless steel pump and delivered to the nozzle of the acidulator. A controlling and indicating flowmeter in the leadlined acid line, between the pump and the nozzle, regulates the flou of acid to any desired rate, Acidulated Material. The acidulation reaction is the main processing step. The atomized acid coming downward out of the nozzle in the top of the acidulating tower describes a spray cone having a 60" included angle. This spray penetrates the cyclonic and turbulent dust cloud within the tower t o produce a very rapid reaction between the acid mist and the finely divided rock dust particles. As the superphosphate is formed it drops
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INDUSTRIAL AND ENGINEERING CHEMISTRY
out of the suspended dust-acid mass and falls to the bottom of the tower in the form of a thin slurrylike material, T h e tower discharges directly into a puddler over which i t is situated, and the material begins its set. The puddler, which has a rubber-lined housing, is equipped with propelling flights or puddlers which knead the material and pass i t along to the solidifier. By the time the material leaves the puddler, it has a consistency that is just barely self-leveling. During its movement through the acidulating tower and the puddler, the material being processed undergoes the remainder of the acidulation reaction and emits hydrogen fluoride, silicon tetrafluoride, and other by-product gases. These fumes are flashed off and drawn out of the puddler by lowering the pressure within its housing by means of an aspirator with which i t is coqnected. At top operation, this aspirator recirculates over 500 gallons of water per minute, most of which has previously been used for cooling in the acid dilution steps. The mater used in the aspirator is drawn from and recycled to a concrete sump tank. As the water level in the sump tank rises i t spills over a weir and flows through a conduit into an outdoor evaporation basin. The latter is isolated from any other body of water, and peiiodic analysis of samples of the atmosphere and nearby stream waters reveals no harmful fluoride concentrations. The solidifier, which is fed by the puddler, is completely enclosed by a steel housing. I n a sense, the solidifier fulfills the function of the den in the older processes, the material leaving it
Vol. 41, No. 7
being ieady for storage and curing. I n structure, the solidifier might be described as a channellike pallet conveyer, made up of steel plates formed in a “-shape. The uprights of the U’s constitute the sides of the channel and the bottoms of the IT‘S the floor. The plates are sufficiently close to one another to prevent the viscous superphosphate in the solidifying stage from seeping through the cracks between them. A t the discharge end of the solidifier a rotary helical cutter removes the new solid superphosphate from the plates, A vent over the cutter exhausts the water vapor released fiom the superphosphate during its removal from the solidifier. As the material is excavated from the solidifier, i t drops into a spout feeding a bucket elevator that delivers to a belt conveyer. The. belt carries the superphosphate from the acidulation building into the middle of the storage building. At this point, a selfpropelled reversible shuttle belt conveyer is used to distrihutc. the material to any desired storage area within the building. The daily chemical analyses of the superphosphate as it vti< produced a t this plant during a typical week’s operation were: Per cent Moisture Available phosphoric acid Insoluble phosphoric acid Total phosphoric acid Free acid (H2S04)
7.60 19.86 1.18 21.04 1.42
7.40 19.94 1.05 20.99 1.36
7.90 19.72 1.46 21.18 1.70
7.70 19.78 1.28 21.06 1.60
7.60 19.70 1.32 21.02 1.65
As the superphosphate is removed from storage each day a
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INDUSTRIAL AND ENGINEERING CHEMISTRY
sample is drawn and tested. Chemical analyses of these samples covering a typical week’s shipping operation follow : Par n n n +
After the required curing period has elapsed, the superphosphate is removed from the storage area b y a large Diesel-powered tractor equipped with a hydraulically-operated scoop, which drops i t into a grated floor hopper located in the middle of the storage building. This hopper feeds a belt conveyer which transports the superphosphate through a subground-level tunnel to the mixed fertilizer plant. Upon entrv into the mixed fertilizer plant, the belt conveyer delivers to a bucket elevator that carries the material to a milling and screening unit. The superphosphate enters this unit through a double-decked, inclined vibrating screen. The upper screen is 5 mesh and the lower 8 mesh. Matcrial rejected by the upper screen is recycled for further milling through a mill especially designed for this purpose. Material passing the 5 mesh but retained on the %mesh screen is classified as granular material; that passing the 8-mesh screen as fines. The granular thus Classified represents about 40% of the plant’s production. This specially designed milling and double-screening unit permits the fines to go directly t o one of the compartments of a multiple-hopper batching unit without a second handling of the product. From that point they are compounded with other ingredients, including ammonia solutions, into fertilizer mixed goods. Depending on the immediate demand the two size fractions of the superphosphate may be handled in any of five ways:
1. Total production of both fractions recombined and bagged 2. Granular fraction bagged, fines used in mixed fertilizers 3. Total production recombined and shipped in bulk 4. Granular shipped in bulk, fines used in mixed fertilizers 5 . Granular and fines shipped separately, Box car and truck loaders are used to facilitate the handling of bulk shipments. Although a full year’s production figures are not as yet available i t is estimated that about 40% of the total production of superphosphate is eventually used in the-compounding of mixed fertilizers. About three quarters of this amount is compounded a t the Prairie du Chien mixed fertilizer plant. A continuous ammoniating system is now being installed in the fertilizer mixing plant. When completed, the superphosphate fines, granular product, or compounded solid ingredients for complete mixed fertilizers will be delivered from the screening operation to an 8-ton steel hopper by means of a bucket elevator. This hopper will supply feed to a continuous weighing machine having a top capacity of 60 tons of superphosphate per hour. The scales will discharge the weighed material directly into one end of a horizontally-mounted steel ammoniating machine of open-top design with an externally-driven steel impeller that transverses its length. As the material cascades off the scales, i t will be sprayed with an accurately controlled flow of an ammonia solution from a nozzle located underneath the scales. The designers (The A. J. Sackett & Sons Company) point out that anhydrous ammonia may be used with the same equipment by modifying its design to withstand the higher working pressures experienced with this material. Accurate and controllccl proportioning of the superphosphate and ammonia solution will prevent the possibility of significant amounts of ammonia fumes escaping into the plant. Experience with previous installations of open-top continuous ammoniating equipment in fertilizpr plants substantiates this conclusion. The ammoniation reaction will be complete by the time the material leaves the ammoniator. On being discharged the ammoniated material will be fed onto a
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belt conveyer system which will deliver it to storage to await bagging or further blending operations. FUTURE TRENDS IN THE SUPERPHOSPHATE INDUSTRY
The superphosphate industry has somewhat belatedly, but definitely, been struck by the wave of interest in continuous processing that has swept other chemical process industries during recent years. Just as in these other industries, continuous processing in the superphosphate field offers inherent production, economies and product control not afforded by batch type methods. These advantages are sufficiently important in some instances to warrant rcplacing serviceable, yet obsolete, batch type dens with continuous processing equipment. The Super Flo process, because of its high production rate, the improved physical properties of its product, and its ability to produce at normal acidulating cost a classified granular material suitable for direct use, is in a strong position to bid for attention whenever such process replacement is contemplated. Although i t will undoubtedly t,ake many years before the output from den installations declines to a small fraction of the total annuaj production, the recent return of a more normal balance between superphosphate supply and demand should impel more and more producers to consider continuous processing as ’one promising means of improving their position for the competitive period the superphosphate industry is likely to face in the near future. ACKNOWLEDGMENT
The authors wish to express their appreciation for the cooperation and suggestions given them in the preparation of the introductory part of this article by Vincent Sauchelli of The Davison Chemical Corporation. BIBLIOGRAPHY
1) Collins, S. 13.. “Chemical Fertilizers and Parasiticides,” pp.
135-43, New York, D. Van Nostrand Co., 1920. (2) Elmore, K. L., Huffman, E. O., Wolf, W.W., IND. EKG.CHEM., 34, 40 (1942). (3) Gray, A. N., “Phosphates and Superphosphate,” Val. 1, pp. 76-9, New York, Interscience Publishers, 1947. (4) Ibid., pp. 104. (5) Ibid., p 106. (6) Ibid., pp. 111-22. (7) I b i d . , pp. 124-6. ( 8 ) I b i d . pp. 145-8. (9) Hardesty, J. O., and Ross, W. H., IND.ERG.CHEM.,29, 1283-6 (1937). (10) Mansfield, G. R., Ibid., 34, 9-12 (1942). (11) Mehring, A. L., FertiZizerRev., 14, No. 1, pp. 3, 11-13 (1939). (12) . , Molinari. E.. “Treatise on General and Inorganic Chemistrv.” Val. 1, p. 643, Philadelphia, P. Blakiston’s Son & Co., 1920.‘ (13) I b i d . p. 645. (14) National Fertilizer Association, Washington, D. C . , Vol. XXIII, Service Letter 47 (1949). (15) Nordengren, S., and Lehrecke, H., Am. Fertilizer, 93, No. 1 , 5-7, 24,26; NO.2 , 10-11, 22 (1940). (16) Parrish, P., and Ogilvie, A,, “Calcium Superphosphate and Compound Fertilisers,” p. 19, London, Hutchinson’s Scientifip and Technical Publications, 1939. (17) Ibid., pp. 101-7. (181 Raymond Pulverizer Co., Roller Mill Catalog 61, 1948. (19) Sauchelli, V., “Manual on Fertilizer Manufacture,” pp. 31-3, Baltimore, Davison Chemical Corp., 1946. (20) Ibid.. nn. 37-9. izij ~m.l 53. (22) Ibid., pp. 57-58. (23) Sauchelli, Vincent, “Manual on Phosphates,” p. 13, I b i d . , 1942. (24) Ibid., p. 9. (25) Shreve, It. N., “Chemical Process Industries,” p. 333-6, New York, McGraw-Hill Book Company, 1945. (26) U. S. Dept. Interior, “Minerals Yearbook 1943,” pp. 1429-50, Washington, D. C., U. S. Govt. Printing Office, 1945. (27) Waggaman, W. H., and Easterwood, H. W., “Phosphoric Acid, Phosphates, and Phosphatic Fertilizers,” A.C.S. Monograph 34, pp. 16-19, New York, Chemical Catalog Co., 1927. (28) Ibid., pp. 81-2. RECEIVED May 12, 1949.
