Uranium
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Uranium Introduction Uranium (U) is a radioactive metal with a high specific gravity of 18.7. It was discovered in the mineral pitchblende by Martin Klaproth in 1789 and was named after the planet Uranus. Naturally occurring uranium consists of a mixture of three isotopes in the following proportions: U238 (99.28%), U235 (0.71%) and U234 (0.01%). U235 has an outstanding property in that it is the only naturally-occurring fissionable element. By interaction with neutrons, the nucleus of U235 may be split into two parts. This splitting - termed fission - releases energy and more neutrons, starting a nuclear chain reaction. The large amounts of heat released by this reaction are used in nuclear reactors to generate electricity. Uranium has two main valencies - U4+ when it occurs in a reducing medium, and U6+ in an oxidising medium, e.g. at the Earth’s surface. U6+ is more soluble than U4+ in natural waters. Streams carry a quantity of dissolved uranium depending on the geological characteristics of the region they drain. Sea water averages 0.003 parts per million U. Uranium oxide precipitates from groundwaters containing dissolved uranium when these waters enter a reducing environment. In terms of abundance in the Earth’s crust, uranium is about as common as tin, tungsten and molybdenum. Occurrence More than 150 uranium-bearing minerals have been identified. The main primary minerals are uraninite (UO2), pitchblende (a mixed oxide, usually U3O8), brannerite (a complex oxide of uranium, rare-earths, iron and titanium) and coffinite (uranium silicate). Most of the world’s uranium is produced from pitchblende ores. The most important secondary minerals are carnotite, autunite, davidite, gummite, torbernite and uranophane. Australia has some of the largest uranium deposits in the world and major uranium deposits occur in a number of distinct geological settings and rock types: Unconformity-related deposits - these deposits are in brecciated chlorite schists of Palaeoproterozoic age and they occur close to a major unconformity that separates these rocks from overlying sandstones, e.g. Ranger, Jabiluka, Koongarra and Nabarlek (NT); and Kintyre (WA). Breccia complex deposits - host rocks for this type of deposit are hematite-rich granite breccias and volcaniclastic rocks of Mesoproterozoic age, e.g. Olympic Dam (SA). In this deposit, uranium occurs together with copper, gold, silver, and rare earth elements. Sandstone-type deposits - occur in coarse-grained sandstones usually containing some organic (plant) matter. Deposits are often within buried palaeo-drainage channels, e.g. Beverley, Honeymoon (SA), Bigrlyi, Angela (NT), Westmoreland (Qld) and Mulga Rock (WA). Calcrete deposits - uranium occurs in surficial sand and clays, partly cemented by calcrete (calcium and magnesium carbonates) within recent drainage channels. These sediments formed in the arid areas of Western Australia and Northern Territory, e.g. Yeelirrie and Lake Way (WA). Other major uranium deposits are in metamorphic rocks, e.g. Mary Kathleen (Qld); and in acid volcanic rocks, e.g. Ben Lomond and Maureen (Qld). Australian Resources and Deposits The earliest uranium deposits mined in Australia were at Radium Hill and Mount Painter (SA). These deposits were worked from about 1910 to 1931 for radium, a radioactive daughter product of uranium, which was used mainly for medical purposes. Exploration for uranium in Australia began in 1944 at the request of the British Government. The Australian Government offered financial rewards and in 1949 the Rum Jungle deposit (NT) was discovered. Subsequently, the Mary Kathleen deposit and a number of smaller deposits in the South Alligator Valley (NT) were discovered. Between 1954 and 1971 the following deposits were mined: Rum Jungle (1954 to 1971), Radium Hill (1954 to 1962), Mary Kathleen (1958 to 1963) and South Alligator Valley (1959 to 1964). Uranium exploration declined during the late 1950s but increased again in the late 1960s, stimulated by the easing of the government’s export embargo and predictions of increased world demand for uranium in the early 1970s for generating electricity in nuclear power stations. Important deposits were discovered between 1969 and 1973 at Nabarlek, Ranger, Koongarra and Jabiluka in the Alligator Rivers area (NT), at Beverley and Honeymoon in the Lake Frome area (SA), at Yeelirrie and Lake Way (WA). The Olympic Dam (SA) and Kintyre (WA) deposits were discovered in 1975 and 1985 respectively. The Mary Kathleen mine recommenced production in 1975 and ceased operations in 1982. A total of 8894 tonnes U3O8 was produced from Mary Kathleen during its two periods as an operating mine. The Nabarlek deposit was mined and stockpiled in 1979. This stockpiled ore was processed from 1980 to mid-1988 for a total output of 10 858 tonnes U3O8. Australia currently has two uranium mining operations - Ranger and Olympic Dam. Ranger is a large unconformity-related deposit in the Alligator Rivers region (NT). The ore is mined by open-cut methods. Production commenced in 1981. The processing plant (acid leach and solvent extraction) has a production capability of 5800 tonnes U3O8 per year. Olympic Dam, is one of the world’s largest deposits of uranium (in terms of total reserves and resources of uranium). Mineralisation occurs in a hematite-rich granite breccia complex and is beneath approximately 300 metres of flat-lying sedimentary rocks of the Stuart Shelf geological province. The mine produces uranium, copper, gold and silver. The ore is mined by large-scale underground methods (long-hole open stoping). Production began in 1988. A major expansion of the Olympic Dam operation in 1999, increased copper production to 200 000 tonnes per annum. Uranium production, which is tied to copper production, is expected to increase to 4200 tonnes U3O8 in 2000. Australia has the world’s largest resources of low-cost uranium, with approximately 26% of world resources in this category. Other countries which have large low-cost resources include Kazakstan (19%), Canada (14%), South Africa (9%), Namibia (7%), and Brazil (7%). New Mine Developments Since the removal of the former Government’s ‘three mines policy’ in 1996, of new uranium mine developments have commenced at Jabiluka, Beverley and Honeymoon deposits. Jabiluka deposit, in the Alligator Rivers region, is approximately 20 km north of the Ranger mine. Both Jabiluka and Ranger are owned by Energy Resources of Australia Ltd (ERA). Jabiluka is one of the world’s largest unconformity-related deposits with mineable ore reserves containing 90,400 tonnes of U3O8 . The Jabiluka project has been subject to an exhaustive three-year environmental impact assessment process. This involved separate assessments of both the options to mill Jabiluka ore at the existing Ranger facility and the option of building a mill on-site at Jabiluka. Both options have received Commonwealth environmental clearance, subject to the company complying with certain requirements. Because the Traditional Aboriginal owners have not given approvals for trucking of ore from Jabiluka to the Ranger mill, ERA stated in October 1999 that it would focus its evaluation studies on developing the Jabiluka mill option. The mine decline and underground development to access the ore body were completed during 1999. Beverley project, in the Lake Frome area (SA), will be the first in situ leach (ISL) uranium mine in Australia. The deposit occurs in unconsolidated sands with inter-bedded clays (Upper Tertiary in age) that were deposited in a confined palaeochannel sequence. Mineralisation is at an average depth of 107 m below surface. In April 1999, Heathgate Resources Pty Ltd received Commonwealth and State environmental clearances to develop Beverley and production is scheduled to commence in mid-2000 at an annual rate of 1000 tonnes U3O8. Honeymoon deposit (south of Lake Frome) occurs in coarse-grained sands of Tertiary age and is between 100 m and 120 m below surface. It has a roll-front shape and occurs at an oxidation-reduction interface along the lateral margins of a buried palaeochannel. The impacts of the proposed Honeymoon ISL operations are currently being considered under an environmental impact assessment process. Subject to government approvals, production at annual rate of 1000 tonnes U3O8 is scheduled to commence in late 2000. Mining Uranium is usually mined by either open-cut methods (e.g. Ranger, Nabarlek, Mary Kathleen and Rum Jungle orebodies) or underground mining methods (e.g. Olympic Dam, Radium Hill and the South Alligator Valley orebodies), depending on the depth at which the orebody occurs. Sandstone-type deposits are usually mined by in situ leaching in which an acidic or alkaline solution is pumped through a permeable orebody to dissolve the uranium, which is then recovered from these solutions in a processing plant. Honeymoon and Beverley will be mined by in-situ leaching. Processing Initially, the uranium ore is crushed and then ground to a fine grain size. Grinding and mixing with water produces a slurry of fine ore particles suspended in water. This slurry is leached with either an acid or an alkali, depending on the metallurgical characteristics of the ore. Leaching causes uranium to dissolve in the solution. Most of the other minerals in the ore remain undissolved, and these solids, called ‘tailings’, are then separated from the uranium-rich liquid, usually by allowing them to settle out. The uranium-rich liquid is filtered to remove any remaining solids and the uranium is then recovered by techniques involving either solvent extraction, ion exchange or direct precipitation. The method used depends on the nature of the particular ore. Uranium is finally recovered in a chemical precipitate that is filtered and dried to produce a yellow powder known as ‘yellowcake’. The yellowcake is heated to about 700° C to produce a dark grey-green uranium oxide powder containing more than 98% U3O8, which is placed into 200 litre steel drums for export. For ISL operations, uranium is recovered in a processing plant using either ion exchange or solvent extraction technologies.
Uranium has two major peaceful uses: as the fuel in nuclear power reactors to generate electricity; and in the manufacture of radioisotopes. electricity generation In a nuclear reactor, the heat released during the fission of U235 is used to produce steam that drives turbines to generate electricity. Approximately 16% of the world’s electricity is currently generated by the use of uranium in nuclear reactors. As at mid-1999, 428 nuclear power reactors with a total net capacity of 345 gigawatts (electrical) were operating in 31 countries; a further 30 new reactors were under construction worldwide. A total of 16 countries generate more than 25% of their total electricity requirements from nuclear reactors. Radioisotopes Radioisotopes of a number of different elements are manufactured using relatively small, special purpose nuclear reactors. These radioisotopes have a wide range of uses including: diagnosis and treatment of certain illnesses; sterilisation of syringes and other medical equipment; examination of welds and the study of the rate of wear of metals; on-stream analysis as part of the metallurgical processing of various ores; preservation of foods; and production of high-yielding, disease-resistant varieties of food crops. Exports of Australian Uranium Australia has no significant national demand for uranium and all production is exported. Australia applies very stringent conditions to the export of uranium to ensure it is used only for peaceful purposes. These conditions - called nuclear safeguards - require customer countries to allow international inspectors from the International Atomic Energy Agency to verify that the uranium is not directed into weapons programs. In addition, Australia requires compliance with parallel conditions under treaties it has concluded with end customer countries. This compliance is monitored by the Australian Safeguards Office. Suggestions for further reading Battey, G.C., Miezitis, Y. and McKay, A.D. 1987 Australian Uranium Resources, Bureau of Mineral Resources, Resource Report 1. Australian Geological Survey Organisation 1999 Australia’s Identified Mineral Resources 1999, Australian Geological Survey Organisation, Canberra. Clark, I. and Cook, B. 1992 ITAM (Introduction to Australia’s Minerals) Uranium, Minerals Council of Australia, Canberra. Uranium Information Centre web site www.uic.com.au |
