Phosphates, uranium deposits

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Dymkov, Y. M, Holliger, P Pagel, M., Gorshkov, A. Artyukhina, A. 1997. Characterization of a La-Ce-Sr-C aluminous hydroxy phosphate in nuclear zone 13 in the Oklo uranium deposit (Gabon). Mineralium Deposita, 32, 617-620. 

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). 

Uranium is a naturally-occurring element found at low levels in virtually all rock, soil, and water. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as uraninite in uranium-rich ores. Because uranium has such a long radioactive half-life (4.47x109 years for U-238), the total amount of it on earth stays almost the same. 

Uranium occurs in most phosphate rocks, but its concentration varies from deposit to deposit. Some sedimentary rocks show notably higher concentrations than most igneous rocks. The most uranium-rich rocks found to date are those of Florida (up to 300 ppmw U), Morocco (up to 230 ppmw U), and Jordan (up to 240 ppmw U) [32,33[. In comparison, conventional uranium deposits such as pitchblende typically have a uranium content of 1,000-3,000 ppmw. When the phosphate rock is acidulated, up to 80% or 90% of the ur ium passes into solution in the phosphoric add. The exact amount of PR acidulated depends on its characteristics and the parameters of the phosphoric acid process employed. Examples of the uranium contents of different phosphate rocks and of phosphoric acids produced from them are indicated in Table 11.29. 

Many phosphate rock deposits contain quantities of radioactive elements such as uranium and thorium. Selective leaching of uranium from raw phosphate ores is difficult because the U(V1) ion is incorporated into the crystal structure of apatite (Ca5(P04)3(0H,F,Cl)), rather than adsorbtively associated with it. Uranium is, therefore, typically recovered from phosphate rocks by recovering it from phosphoric acid produced by sulfuric acid leaching of phosphate ores. The radioactive species are also leached and must be removed during purification of the acid. Uranium in... 

In the earth s crust thorium is almost four times more common than uranium, and the radioactive decay of thorium generates a considerable part of the geothermal energy. The primary source of thorium is monazite (Figure M25), a yellow or reddish-brown rare-earth phosphate. Seashore deposits at Travancore in India contain very big quantities of monazite. The mineral formula is (Ce,La,Nd,Th)PO and the thorium content in monazite ores is 5-10%. In 2001 the production of monazite concentrate in the world was 5710 tonnes [52.13], of which India accounted for 5000 toimes or 88%. Other producers were Brazil and Malaysia. 

Furongite was described by the Hunan 230 Laboratory as a yellow, minutely crystalline deposit on carbonaceous shale in the oxidized zone of an illuvial-type uranium deposit. Walenta and Wimmenauer described hallimondite from Lahr, Baden. It is very similar to parsonsite. Pseudo-autunite was characterized by Sergeev from fenitized rocks of the exocontact zone of a massif of ultrabasic-alkaline rocks of northern Karelia, U.S.S.R. Walpurgite-(P), an unnamed phosphate analogue of walpurgite, was described in Soboleva and Pudovkina. ... 

Besides the conventional uranium resources, there are also the so-called unconventional uranium resources , which are defined as deposits with very low uranium content, from which uranium is typically only recoverable as a minor by-product. These unconventional uranium resources are obtained from the extraction of phosphates, non-ferrous ores and carbonatites, as well as black schist and lignite. It has to be noted that the distinction between conventional and unconventional resources is not entirely clear cut, but is, instead, somewhat transitional. 

Historically, phosphate deposits with an average uranium content of 0.01% are the only type of deposit from which significant quantities have been recovered, mostly in... 

Uranium is deposited widely in the Earth s crust, hence it has few ores, notably the oxides uraninite and pitchblende. The ores are leached with H2SO4 in the presence of an oxidizing agent such as NaClOs or Mn02, to oxidize all the uranium to the (+6) state as a sulfate or chloride complex. On neutralization with ammonia a precipitate of yellow cake , a yellow solid with the approximate composition (NH4)2U207 is formed. This is converted into UO3 on ignition at 300 °C. This can be purified further by conversion into uranyl nitrate, followed by solvent extraction using tributyl phosphate in kerosene as the extractant. 

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