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T h e most extensively exploited deposits of phosphate in the world are in FIorida. In the processing of the phosphate matrix, modified hydraulic mining is used. This paper describes the mining and processing of these deposits and the production of soluble o r available phosphates. Treating the phosphate rock with acid converts the insoluble phosphorus to a soluble condition. Superphosphate manufacturers have found that for maximum recovery of available phosphorus pentoxide and for the production of a superphosphate that is in good physical condition, the best strength of sulfuric acid t o use is 55" or 56" Be. or 70 to 71%.
Processing Phosphate Use in gricultur T. L. WILKERSON AMERICAN CYANAMID COMPANY, NEW YORK 20,
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HE practice of using phosphatic material as 12 fcrt.ilizcr goes back so far that there is no record of when and where i t was first employed. For many years bones, fish, and guano were the world's chief source of phosphorus and phosphoric acid. It was not until 1840 ( 5 )that bones were dissolved wit,h sulfuric acid to render their phosphorus more soluble and available to plants. This discovery marked the beginning of the vast superphosphate industry which is the backbone of the fertilizer business in this count'ry. Phosphate rock (6) is the principal source of phosphoric acid for fertilizer and other industrial purposes. Its physical properties vary from flintlike masses to a soft plastic material resembling kaolin. I t s color may vary all the way from black massive seams of rock to white nodules and pebbles. The mineral phosphate rock, which has been adapted to the manufacture 01 superphosphate in the United States, varies widely in analysis, but usually contains from 70 to 78% calcium phosphate (bone phosphate of lime). I n other parts of the world, especially in Africa and Russia, a large proportion of the phosphate rock produced contains less than 70% calcium phosphate, while much of thc island phosphate rock contains more than 78%. The world's reserve of phosphate rock is estimat,ed to be 28,870,256,000 short tons of material, of which 14,885,763,000 tons ara located in the United States, and the remainder is distributed among nearly a score of foreign countries. The largest phosphat,e rock deposits outside the United States arc found in Russia, iT-here it is estimated that about 7.5 billion tons of natural phosphate deposits are located. While much of the phosphate rock mined in Russia is relatively low in phosphoric acid, over a million tons of apatite are mined yearly that contains 80% or more bone phosphate of lime. PROCESSING PHOSPHATE MATRIX The most extensively exploited deposits of phosphate in the world are in Florida and for the past 60 years the production of Florida phosphate rock has increased steadily (4).The land pebble district of Florida is about 30 miles in diameter. I n this area phosphate deposits are prevalent, but t,hc commercial deposits are scattered. Florida phosphate is mined by first removing the overburden of sand and clay which varies from 6 t o 40 feet in depth. The matrix lies underneat,h the overburden and varies in depth from 4 to 25 feet. In t,he processing of t,he phosphate niatris, modified hydraulic
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mining is used throughout the Florida field. The matrix, in rnosf cases, is mined with a dragline and deposited on the surface near a dredge pump. This matrix is then easily made into a slurry by high pressure water and pumped t o the washer where the clay balls are broken up, coarse phosphate i s screened out and recovered, and the fine phosphate is recovered by flotation. hpproximately 1400 gallons of water are used in processing to recover 1 ton of phosphate rock. Large quantities of low grade phosphate rock are lost in the mining operation, for at present there is no economical means of recovering this finely divided material. The expanding demand for phosphate has been accompanied by increasing mining facilities in the land pebble district of Florida Five new draglines have been purchased in the past 4 years a t a cost of millions of dollars. Many of these draglines will move overburden or matrix a t the rate of 1500 tons per hour. Two new dryers have been installed and the flotation equipment has been expanded. Furthermore, existing facilities have been improved on a large scale. The miners have installed n e v labor-saving machinery and have been ablr to increase production to supply the demand PRODUCTION OF SOLUBLE PHOSPHATES I n the fertilizer trade, it has long been the custom to express the quality of phosphate rock in terms of the tricalcium phosphate cquivalent of total phosphorus. Tricalcium phosphate i s seldom, if eve?, found in phosphate rock as such. The phosphate has the complex apatite (calcium-phosphate-fluoride) ( I ) structure, containing fluorine principally in the phosphat,e molecule. 13ccause of t,he chemiFal structure of the native phosphate rocli many methods have been suggested, patented, and employcd to convert insoluble phosphorus of the raw material to a soluble condition that can be used by plants. T o render phosphates more available for plant food, Lawes in 1842 took out a patent for the acidulat,ion of bones. This same treatment was later found applicable to phosphate rock and the discovery marked the bcginning of the superphosphate industry. When phosphate rock is treated wit,h an acid ( I ) , fluorine and other gases are driven off, and the tricalcium phosphate which the phosphate contains is converted into dicalcium and monocalcium phosphate. The amount of each product produced depends upoii the amount of acid employed. Nitric, hydrochloric, phosphoric, and sulfuric acids, as well a s Pome of the gases from burning sulfur
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July 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
or pyrites, have been used in treating phosphate rock in the production of soluble or available phosphates. Sulfuric acid has proved to be the most satisfactory; it is the cheapest and most readily manufactured acid, and when sulfuric acid reacts with tricalcium phosphate, calcium sulfate is produced, which greatly improves the physical condition of the superphosphate produced. In the year ending June 30, 1947, approximately 3,125,000 tons of sulfuric acid were used in the production of superphosphate alone. The ability of the superphosphate industry (3) to utilize spent sulfuric acid from other industries is highly important in obtaining supplies as well as in reducing the cost of producing phosphate rock. The four principal objectives for treating raw rock with sulfuric acid, or the manufacture of superphosphate are: to obtain a product with a maximum percentage of soluble or available phosphorus expressed as phosphorus pentoxide; to eliminate the maximum amount of gases and water whereby the percentage of phosphorus pentoxide is increased and the physical condition of the superphosphate is improved; to bring about the desired chemical reaction so that drying and conditioning of the superphosphate are accomplished in the shortest length of time; and to permit the use of the smallest amount of sulfuric acid to obtain the results enumerated above. TECHNICAL PROGRESS The superphosphate industry has met the increasing demands for phosphatic fertilizers by the construction of new plants and by large scale improvement of existing facilities. Since 1940, 45 new superphosphate plants have been built and a t least 80% of the old plants have installed new machinery to increase superphosphate production. By the end of 1948 there will be 191 superphosphate plants, with an annual capacity of 14,121,291 (a) tons of material, or 2,541,833 tons of phosphorus pentoxide, producing ordinary superphosphate. By early 1949 there will be 9 double or triple superphosphate plants in operation with an estimated annual production of 650,000 short tons of 45% material, or 292,000 tons of available phosphorus pentoxide. These 200 ordinary and triple superphosphate plants, with an estimated annual capacity of 2,833,833 short tons of available phosphorus pentoxide will increase the production of available phosphorus pentoxide in the United States by 1,125,181 tons (or 66%) over the amount produced in the year ending June 30, 1947, Although it is frequently said that, for many years, little or no technical progress has been made in the processing of phosphate rock, the facts do not justify this statement. For example, the average content of available phosphorus pentoxide in ordinary manufactured superphosphate has gradually increased from 11 % in 1880 to better than 19% in 1947. Some of this increase may be laid to the higher grade rock employed by the superphosphate manufacturer, but much of it has been brought about by finer grinding of the rock, the use of strongcr acid, and improved equipmcnt. In the early days of the superphosphate industry, the physical condition of ordinary superphosphate was, in general, notoriously poor. The product was usually damp, excessively high in free acid, required a longer curing peiiod, and had a tendency to cake or harden in the bag. These properties materially affected the physical condition of mixed fertilizers and made it difficult to distribute them uniformly in the field. Today, these undesirable properties have been overcome, and this has been done a t no additional cost to the user. Manufacturers have expended a great deal of effort in devising ways and means to reduce the amount of sulfuric acid used to acidulate phosphate rock; however, in order to obtain maximum recovery of available phosphorus pentoxide and to pro_duce superphosphate of good physical condition about 58.5 % as much basis 100% sulfuric acid as phosphate rock is required. The strength of the sulfuric acid used also has an important influence
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on the quality of superphosphate. Theoretically, there are many advantages t o be gained by the use of strong sulfuric acid, but many of these advantages are offset by operating difficulties in the plant. When a stronger acid is used, a smaller proportion of acid is required for a given amount of rock, which increases the phosphorus pentoxide content, and the resultant superphosphate is much drier. However, when an acid of over 56” BB. is used, the reaction is greatly slowed down, because the calcium sulfate which is formed is less soluble in strong than in weak sulfuric acid, and this tends to coat the rock particles and prevent complete reaction from taking place. Superphosphate manufacturers have found that for maximum recovery of available phosphorus pentoxide and for production of a superphosphate that is in good physical condition, the best strength of sulfuric acid to use is 55 O or 56” BB., or 70 to 71%. Finely ground rock plays an important part in the speed of chemical reaction when the rock and sulfuric acid are mixed together, and aids materially in obtaining the highest percentage of available phosphoric acid from a given grade of rock. Formerly, superphosphate stood several weeks for curing before it was ready t o ship, whereas today the time required for curing has been reduced. The longer time allowed for curing was necessary because the coarseness of the phosphate rock allowed only the surface of the large particles to be acted upon immediately by the sulfuric acid, and the acid had to permeate the layer of calcium sulfate formed around these particles. The extremely fine grinding now provides a greater contact surface for the acid, and allows a more thorough mixing, thus producing a material higher in available phosphorus pentoxide in a shorter length of time. Particularly notable in the development of the superphosphate industry is a substantial trend to step up production efficiency with modern mechanical equipment. Within the past 5 years many new pulverizing mills have been installed in new and old superphosphate plants. By the end of 1948, approximately 55% of the ordinary superphosphate plants in the continental United States will be equipped with mechanical den systems. Many plants are also installing mechanical equipment for unloading the rock when it comes into the plant, building rock storage bins, and mechanizing other operations, all of which makes the processing of phosphate rock more efficient. CONCLUSION For years, ordinary superphosphate, because of its simple and inexpensive method of production, has supplied most of the domestic needs of phosphorus pentoxide as a fertilizer. I n the calendar year 1947, it is estimated bhat the total production of soluble or available phosphorus pentoxide in the form of chemically processed fertilizers was distributed as follows: ordinary superphosphate 87%, double or triple superphosphate 9%, and other phosphates 4%. Even though ordinary superphosphate supplies most of the phosphorus for agriculture, other methods of processing phosphate rock offer promise and should be considered and further developed. LITERATURE CITED (1) Collings, G.H.,“Commercial Fertilizers,” 2nd ed., pp. 178-83, Philadelphia, Blakiston Co., 1938. (2) Jacob, K.D., Fertilizer Res., 23, 3-9, 19, 20 (1948). (3) Jacob, K.D., and Mehring, A. L., A g r . Chemicals, 2, No. 12, 21 (1947); 3,No. 1, 53 (1948). (4) Johnson, B. L.,and Tucker, E. M., reprint from Bureau of Mines
Yearbook, 1946. (5) Sauchelli, V., “Manual of Fertilizer Manufacture,” Baltimore, Davison Chemical Corp., 1946.
(6) Waggaman, W.H., and Eastervvood, H. F., “Phosphoric Acid, Phosphates, and Phosphatic Fertilizers,” A.C.S. Monograph 34, pp. 46-7, New York, Chemical Catalog Co., L927. RECEIVED August 26, 1048.
