Figure 2—Gypsum stacks in Florida.
PHOSPHATE PRODUCERS COULD ELIMINATE FLOTATION PLANTS AND WASTE BY ADOPTINGWELL-ESTABLISHED HYDROMETALLURGICAL PROCESSES AND USING DILUTE NITRIC OR HYDROCHLORIC ACID TO LEACH PHOSPHATE FROM ROCK
By Dr. Fathi Habashi
Great effort is spent by the phosphate industry to beneficiate the rock to obtain a concentrate containing 30%-31% P2O5.[1] During this process, much of the phosphate values are lost to the tailings stream. For example, to obtain 1 ton of rock containing 31% P2O5, about 6.5 tons of waste containing 1.69% P2O5 must be discarded to the tailings impoundment (Figure 1). The industry is also burdened with importing elemental sulphur, manufacturing sulphuric acid, then throwing it away in the form of gypsum (Figure 2). In addition, leaching equipment is expensive, needs regular maintenance and broken agitators are a common occurrence.
The phosphate fertilizer industry is at present based mainly on the use of sulphuric acid. Phosphoric acid produced is treated with ammonia to produce ammonium phosphate:
The Hydrometallurgical Option
preparation of heap.
Applying hydrometallurgical techniques such as in situ leaching, using injection and recovery wells; vat leaching; or heap leaching (Figure 3) to phosphate processing could solve the problems faced in the sulphuric acid route, i.e., the disposal problem of gypsum and the erosion of the agitators in the reaction vessel. These techniques are well established in the copper, gold and uranium industries. [2]
These methods require nitric or hydrochloric acid. In the case of nitric acid, a 20% acid solution should be used, and for hydrochloric acid, a 10% acid solution must be used. [3] The leach solution is monocalcium phosphate and calcium nitrate or calcium chloride.
Higher concentration will produce phosphoric acid at the top of the bed, which reacts further with apatite on its descent to form insoluble dicalcium phosphate, and the flow of solution will be blocked (Figures 4 and 5).
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When this technology is used, monocalcium phosphate is obtained. When the solution is evaporated, double salts are obtained:
– Nitric acid: Ca(NO3)2.Ca(H2PO4)2.2H2O [or CaNO3.H2PO4.H2O], known as calcium nitrate phosphate;
– Hydrochloric acid: CaCl2.Ca(H2PO4)2.2H2O [or CaCl2.H2PO4.H2O], known as calcium chloride phosphate.
These can be crystallized and decomposed at low temperature (200°C-250°C) to form dicalcium phosphate and acid vapors:
using dilute nitric acid and hydrometallurgical techniques.
The acid vapors can be condensed or washed with water for recycle. The residue, which typically analyzes 40% P2O5, is insoluble in water but soluble in citric acid and can be marketed as a fertilizer or as a high-grade phosphate product. Since calcium chloride is a waste product while calcium nitrate is a fertilizer, nitric acid is preferable as a leaching agent (Figure 6).
Uranium, Rare Earths, Radium, Fluorine
Methods have been devised to recover uranium and rare earths by extraction with organic solvents [4] while radium can be co-precipitated with barium sulphate. [5] Under the mild leaching conditions used in this process, HF in solution reacts with silica to form fluorosilicic acid:
which can be precipitated with sodium nitrate to form sodium hexaflurosilicate: [6]
Production of Dicalcium Phosphate
Instead of evaporating the leach solution of monocalcium phosphate, limestone is added to precipitate finely divided dicalcium phosphate:
of dicalcium phosphate + ammonium phosphate
fertilizer by hydrometallurgical technique.
Dicalcium phosphate (40% P2O5) obtained as a final product is insoluble in water but soluble in citric acid and is an excellent fertilizer. It can be used also as animal feed. It can be shipped safely as a high-grade phosphate for direct use or for further treatment to phosphoric acid.
Using Phosphoric Acid
If phosphoric acid is needed, two processes can be used. The leach solution is acidified with nitric or hydrochloric acid and cooled to crystallize calcium nitrate or calcium chloride:
Dicalcium phosphate is similarly treated with acids then cooled to crystallize calcium nitrate or calcium chloride:
Ammoniation can be used to produce different grades of ammonium phosphates containing ammonium nitrate and calcium hydroxide (Figure 7). The reaction of monocalcium phosphate with ammonia:
The reaction of calcium nitrate with ammonia:
Summary
Dilute nitric acid [20%] or dilute hydrochloric acid [10%] can be used to one’s advantage for leaching phosphate rock. This permits the use of standard hydrometallurgical processes such as in situ, vat and heap leaching. The reaction is rapid and does not need agitated reactors.
Nitric or hydrochloric acids used in the process are recycled and recovered. Makeup acid is added to compensate for that consumed to react with impurities.
If phosphoric acid is needed, then the leach solution must be treated with HNO3 or HCl then cooled to crystallize thecorresponding calcium salts or dicalcium phosphate.
Dr. Fathi Habashi is a professor at Laval University’s Department of Mining, Metallurgical & Materials Engineering, in Quebec City, Canada. He can be reached at: Fathi.Habashi@arul.ulaval.ca.
References
1. S. K. Kawatra and J.T. Carlson, Beneficiation of Phosphate Ore, Society for Mining, Metallurgy & Exploration, Englewood, Colorado 2014.
2. F. Habashi, A Textbook of Hydrometallurgy, Métallurgie Extractive Québec, Québec City, Canada 1993, second edi- tion 1999. Distributed by Laval University Bookstore www.zone.ul.ca.
3. F. Habashi, In-situ and Dump Leaching Technology: Application to Phosphate Rock, Fertilizer Research 18, 275-279 (1989).
