Uranium in phosphate rock

Woodis TC Jr., Trimm JR, Holmes JH, et al. 1980. Determination of uranium in phosphate rock and wet-process phosphoric acid by argon-plasma emission spectrometry. J Assoc Off Anal Chem 63 208-210. 

Phosphate rock deposits contain uranium (U), radium (Ra), thorium (Th), and other radionuclides as contaminants. Uranium in phosphate rock deposits throughout the world range from 3 to 400 mg kg (Guimond, 1978). It has been estimated that 1000 kg of Florida phosphate rock contains about 100 pCi each of" U and Ra and 4 pCi of °Th (Menzel, 1968). Some of these elements are retained in the HjPO and the remainder are transferred to the by-products during fertilizer manufacture. For instance it is estimated that 60% of the radioactivity in mined Florida phosphate rock remains with slime and sand tailings during beneficiation (Guimond and Windham, 1975). 

Cathcart, J.B., Uranium in phosphate rock. Geological survey Professional paper. No. 988-A, Washington, D.C. Geological Survey. 

Tetravalent uranium can be precipitated from aqueous solution as the insoluble oxalate, fluoride, or phosphate. UF4 precipitated from aqueous solution contains water of crystallization. When this compound is heated to drive off the water, it is partially hydrolyzed to an oxyfluoride. The phosphate U3(P04)4 is soluble in hot, concentrated phosphoric acid and appears in this form when uranium in phosphate rock, Ca3(P04)2, is dissolved in sulfuric acid. 

Andreou, G., Efstathiou, M., and PashaUdis, I. (2012). A simplified determination of uranium in phosphate rock and phosphogypsum by alpha spectroscopy after its separation by liquid-extraction, J. Radioanal. Chem. 291, 865-867. 

Uranium, not as rare as once thought, is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources. 

Uranium is present in small (50—200 ppm) amounts in phosphate rock and it can be economically feasible to separate the uranium as a by-product from the cmde black acid (30% phosphoric acid) obtained from the leaching of phosphate for fertilizers (qv). The development and design of processes to produce 500 t U Og per year at Ereeport, Louisiana have been detailed (272). 

Lanthanides are also found as minor components in other ores, particularly in association with uranium or in phosphate rock. These are often coextracted with the major product and can be economically recovered from the waste streams resulting from the uranium or phosphoric acid extraction. 

The concentration of uranium contained in phosphate rocks (50 200 ppm) is higher than that in seawater (see section 12.3.5). Even though economic recovery of uranium from phosphate rock is difficult, several phosphoric acid plants include operation of uranium recovery facilities. 

Vanadium is not found in its pure state. Small amounts of vanadium can be found in phosphate rocks and some iron ores. Most of it is recovered from two minerals vanadinite, which is a compound of lead and chlorine plus some vanadium oxide, and carnotite, a mineral containing uranium, potassium, and an oxide of vanadium. Because of its four oxidation states and its ability to act as both a metal and a nonmetal, vanadium is known to chemically combine with over 55 different elements. 

Our study of sedimentary apatite from Israel proved that laser-induced time-resolved luminescence is a perspective tool for evaluation of sedimentary phosphate ores with high dolomite content (Gaft et al. 1993b). The idea was based on the fact that natural apatite contains several characteristic luminescence centers, which enables us to differentiate it from dolomite. The most widespread characteristic luminescence center in sedimentary apatite is uranyl (U02) with a typical vibrational green band luminescence under nitrogen laser excitation (Fig. 8.13a,b). Nevertheless, it appears that such luminescence is absent in phosphate rock samples from Florida, evidently because of extremely low uranium concentration (Fig. 8.13c,d). hi order to find potential liuninescence centers, ICP-MS analyses of Florida phosphates was accompHshed. From discovered REE, theoretically Dy + is the best candidate... 

There are wide variations from the values presented in the table, particularly in areas where uranium minerals are more concentrated. Concentrations of uranium in Louisiana soils ranged from 2.35 to 3.98 pg/g (1.6-2.7 pCi/g) (Meriwether et al. 1988), while uranium concentrations in phosphate rock in north and central Florida ranged from 4.5 to 83.4 pCi/g (6.8-124 pg/g) (EPA 1985J). 

Komura, K., Sakanone, M., Yanagisawa, M. and Sakurai, J., Uranium, Thorium and Potassium contents and radioactive equilibrium states of the U and Th series in phosphate rock and phosphate fertilizers (in Japanese). Radioisotopes (Tokyo) 34 (1985) 529-536. 

Menzel, R.G., Uranium, Radium and Thorium content in phosphate rocks and their possible radiation hazard. J. Agric. Food Chem., 16 (1968) 231-234. 

Apatite has a greater geochemical affinity for uranium than most other rocks. The trace quantities (50-100 ppm) of the element fonnd in phosphate rock probably replace Ca in the crystal lattice. 

F. Habashi, Recovery of uranium from phosphate rock, in (9). 

Uranium is also found at enhanced concentrations in phosphate rock. In 80 years the fertilizer industry in the state of Florida has strip mined and processed some 2X10 tonnes of the rock [6]. 

As the carbonate content of the ore increases, the substitution of carbonate in the francolite tends to increase. It is common for trace quantities of heavy metals, toxic elements such as cadmium, and radionuclides such as uranium to exist in phosphate rock owing to substitution in the crystal lattice (see Table 10.5). Usually the quantities of these elements are insufficient to be of concern. 

In the fertilizer manufacturing scheme, the wet process phosphoric acid most commonly ensues from dissolution of sedimentary phosphate rock in sulfuric acid. Such acid solution contains around 1 g 1 1 uranium which is recovered as the byproduct. This task is accomplished by three well-proven extraction processes, some salient details of which are presented in Table 5.10. 

Today, phosphoric acid for use in the pharmaceutical or food industry must be free from uranium and other harmful metals. In the future, the processing of phosphate rock for production of fertilizers may be an important source of uranium because uranium removal may become compulsory to avoid its dissemination into the environment from the use of phosphate-based fertilizers. 

Wet Process Phosphoric Acid. A production process flow diagram is shown in Figure 8. Insoluble phosphate rock is changed to water-soluble phosphoric acid by solubilizing the phosphate rock with an acid, generally sulfuric or nitric. The phosphoric acid produced from the nitric acid process is blended with other ingredients to produce a fertilizer, whereas the phosphoric acid produced from the sulfuric acid process must be concentrated before further use. Minor quantities of fluorine, iron, aluminum, sUica, and uranium are usually the most serious waste effluent problems. 

There are two useful side products. The H2Sip6 is shipped as a 20-25 % aqueous solution for fluoridation of drinking water. Fluorosilicate salts find use in ceramics, pesticides, wood preservatives, and concrete hardeners. Uranium, which occurs in many phosphate rocks in the range of 0.005-0.03% of UsOg, can be extracted from the dilute phosphoric acid after the filtration step, but this is not a primary source of the radioactive substance. The extraction plants are expensive and can only be justified when uranium prices are high. 

---