Date of Award
Master of Science
Robert M. Boarts
[From the Introduction]
The practice or using phosphatic materials as fertilizers goes back so far there is no record of when and where they were first employed. The dung of birds was used over 200 years B.C. The use of bones is also an ancient practice. These materials continued to be the main sources of phosphorus and phosphoric acid until after the middle of the nineteenth century, when the Napoleonic battlefields, which had been a source of bone, could no longer be worked profitably.
The treatment of phosphatic materials with sulfuric acid was first suggested by Liebig in 1840. In 1842 Lawes took out the first English patent for acidulation of bones and later the same process was applied to rock phosphate.
Mineral phosphates known as coprolites were discovered in England in 1845 and in France seven years later. Rock phosphate was first developed in South Carolina in 1867. Since that time the United States has been the foremost phosphate producing nation in the world. The chief deposits of rock phosphate in this country are located in Florida, Tennessee, Montana, Utah, Wyoming, and Idaho. Other important deposits have been found in Belgium, Germany, Spain, Russia, Algeria, Tunis, Egypt, and Morocco.
The most extensively exploited deposits of phosphate in the world are those of Florida. The phosphate deposits of this country which rank next to Florida in commercial importance are those of Tennessee. Tennessee is well situated for the distribution of fertilizer material to the southern and middle western states. Its phosphate deposits occur in the central basin of Tennessee and in the western part of the Highland Rim. The area covers approximately 7000 square miles. Most of the phosphate rock marketed in 1936 came from the brown-rock fields of Maury, Sumner, Giles, Davidson, and Hickman counties. Considerable blue rock was shipped from Lewis and Hickman counties, and a little white rock from Perry County.
There are three economically important types of rock phosphate in Tennessee, namely, the brown, blue, and white phosphate. The deposits of brown rock phosphate are generally conceded to be formed from phosphatic limestone by the washing out of the more soluble carbonate of lime. The blue rock is a conglomerate deposit derived in part from the underlying Ordovician limestone and partly from remains of later marine life.
The white phosphate has been divided into three classes by Hayles, namely, the stony, breccia, and lamellar varieties. The latter is the richest and most plentiful. The breccia variety consists of chert fragments imbedded in a matrix of high grade phosphate. The stony phosphate consists of silicious skeletons formerly filled with carbonate of lime but now containing phosphate.
The two commercially important acids used in acidulation of rock phosphate today are sulfuric and phosphoric acids. Phosphoric acid is derived from the rock itself. The two distinct methods of producing phosphoric acid are, first, the sulfuric acid process and, second, the volatilization process. ln the sulfuric acid process phosphoric acid is produced from the tricalcium phosphate of the rock as shown in the following reaction: Ca₃(PO₄)₂+3H₂SO₄ = 3CaSO₄+2H₃PO₄.
The term, leaching, as used throughout this paper will refer to the process in which the above reaction takes place.
In the volatilization process silica and coke are added to the rock and the mixture heated to about 1500°C either by burning part of the coke in a blast furnace or by heating with an electric arc. The general course of this reaction is usually represented as follows: Ca₃(PO₄)₂+3SiO₂+5C = 3CaSiO₃+P₂+5CO.
To produce phosphoric acid from the phosphorus volatilized in the above reaction the phosphorus is oxidized to phosphorus pentoxide and this oxide hydrolyzed to phosphoric acid.
Much of the phosphate rock in Tennessee is of such low grade that if it were simply acidulted with sulfuric acid the resulting superphosphate would be too low in plant food value to warrant extensive distribution.
The volatilization method has not yet been developed so that it can use the low grade rock satisfactorily. The sulfuric acid process is probably best adapted for the utilization of low grade rock phosphate. For this reason, an investigation on the leaching properties of three low grade rock phosphates was made. The low grade rock phosphates chosen were from Hickman, Johnson, and Ferry Counties of Tennessee.
Reed, David Weldon, "An Investigation on the Sulfuric Acid Leaching of Low Grade Rock Phosphate. " Master's Thesis, University of Tennessee, 1938.