Thursday, May 6, 2010

Distribution of Uranium in world coals.

Jharkhand coal contains trace amount of Uranium in North Karanpura coal field.
by
Dr. Nitish Priyadarshi

Coal is largely composed of organic matter, but it is the inorganic matter in coal—minerals and trace elements— that have been cited as possible causes of health, environmental, and technological problems associated with the use of coal. Some trace elements in coal are naturally radioactive. These radioactive elements include uranium (U), thorium (Th), and their numerous decay products, including radium (Ra) and radon (Rn). Although these elements are less chemically toxic than other coal constituents such as arsenic, selenium, or mercury, questions have been raised concerning possible risk from radiation.


Uranium association with coal has a long history. There is a continuing interest in uranium in coal, because it is a source of radioactivity and because it may be an economic source of uranium. It is just 200 years since the discovery of uranium by M.H. Klaproth. The first detection in coal was by Berthoud (1875) who found up to 2% uranium in coal from near Denver, USA. The samples were collected from a mineralized section of the coal-bed. This mine was soon abandoned.

Subsequent field studies have proven several areas with high uranium coals, especially in the United States, mainly in the Dakotas, Wyoming, Montana, Colorado and New Mexico (Vine,1956). It seems that uranium is carried into the coal swamp in solution as carbonate complexes (Breger, et. al. 1955), which then release uranyl ions to form uranyl-organic complexes. In many coals, especially low-U coals, Uranium is predominantly organically bound.

After World War II, a very intensive uranium search was initiated. The measurement of coal radioactivity were performed in many countries; however only a few are documented. For example, in year 1967 scientists have measured uranium concentration in lignites from Spain (Huesca, Lerida, Ternel, Galicia, Murcia) and reported concentration values 20 to 1200 parts per million (ppm).

Gott (1952) has determined uranium distribution in lignites, shales, and limestones from throughout the US, and a possible mechanism for uranium accumulation in lignites was suggested. Highest uranium concentrations were prevalent in lignites from the Dakotas, Wyoming, and Montana (0.01%), and from high ash Nevada lignite which contained up to 0.05 % uranium. It was postulated that uranium was possibly concentrated in lignite by the action of percolating surface waters after having been leached from volcanic ash.

Uranium bearing coal in the Red Desert area in Wyoming has been studied by Masursky; his findings are documented in several reports. In the first report in year 1952, core and channel samples taken from the Red Desert area in Wyoming were used to investigate the origin of uranium in the coal of the region. Specific uraniferous zones examined included the Sourdough, Monument, Battle, and Luman zones. Areas which were topographically higher and in which coal was overlain by conglomerate showed the highest uranium concentration. Studies of core samples revealed that uranium concentration in the coal beds correlates well with the degree of permeability of adjacent rocks. Where coal beds are overlain or underlain by sandstone, the greatest concentrations of uranium occur at the top and / or bottom of the bed.

J.R. Gill and others in the year 1959 have studied uranium bearing lignite in South Dakota and Montana. They have reported some lignite deposits containing as much as 0.1% uranium.

Coal samples were analyzed for uranium concentration in the coals from the Western United States and approximately 300 coals from the Illinois Basin. In the majority of samples, concentrations of uranium fall in the range from slightly below 1 to 4 parts per million (ppm). Coals with more than 20 ppm uranium are rare in the United States (http://energy.er.usgs.gov/products/databases/ CoalQual/intro.htm).



Results for the uranium in world coals are as follows (Swaine,1990):

Australia- 0.01-4.5 ppm
Brazil- 2.7-19 ppm
Canada- 0.2-7.2 ppm
China- 0.16-21 ppm
Germany West- less than 1 – 13 ppm
India- 1.1-3.6 ppm.
New Zealand- 0.015-0.46 ppm
South Africa- 1.2- 7.3 ppm
Turkey- 1.4-6.4 ppm
UK- 1.1- 3.0 ppm.

Traces of uranium have been also found in the Permian coals of Jharkhand State of India. Areas are KDH, Dakra, Rohini, and Rai Bachra in the North Karanpura coalfield. Channeled samples were analyzed with the help of XRF instrument.

Occurrence of uranium in coals:

Three hypotheses advanced to explain the occurrence of uranium in some coals were described by Denson (1959) as follows.

1. Syngenetic: Uranium was deposited from surface waters by living plants or in dead organic matter in swamps prior to coalification.
2. Diagenetic: Uranium was introduced into the coal during coalification by waters bringing the uranium from areas marginal to the coal deposits or from the consolidating enclosing sediments.
3. Epigenetic: Uranium was introduced in the coal after coalification and after consolidation of the enclosing sediments by groundwater deriving uranium from hydrothermal sources or from unconformably overlying volcanic rocks.
Uranium is associated with clays, zircon and phosphates and may also be organically bound in coal. The accumulation of uranium in coal may vary markedly from place to place, and the occurrence of uranium in each deposit should be interpreted in relation to the geologic history of the region. Field evidence favors the epigenetic hypothesis of the origin of uranium in U.S. western coals. Secondary concentration of uranium in coal may occur when solution of small quantities of uranium by groundwater from overlying volcanic rocks is followed by downward percolation of these waters through previous strata until the uranium is taken up and retained by the highest of the underlying lignite beds. Application of this theory led to the discovery of uranium-bearing coal in Wyoming, Montana, Idaho and New Mexico.
Most thorium in coal is contained in common phosphate minerals such as monazite or apatite. In contrast, uranium is found in both the mineral and organic fractions of coal. Some uranium may be added slowly over geologic time because organic matter can extract dissolved uranium from ground water. In fly ash, the uranium is more concentrated in the finer sized particles. If during coal combustion some uranium is concentrated on ash surfaces as a condensate, then this surface-bound uranium is potentially more susceptible to leaching. However, no obvious evidence of surface enrichment of uranium has been found in the hundreds of fly ash particles examined by USGS researchers.
Most coal also contains potassium-40, lead-210, and radium-226. The total levels are generally about the same as in other rocks of the Earth's crust. Most emerge from a power station in the light flyash, which is fused and chemically stable, or the bottom ash. Some 99% of flyash is typically retained in a modern power station (90% is some older ones), and this is buried in an ash dam.
The amounts of radionuclides involved are noteworthy. In Victoria, 65 million tonnes of brown coal is burned annually for electricity production. This contains about 1.6 ppm uranium and 3.0-3.5 ppm thorium, hence about 100 tonnes of uranium and 200 tonnes of thorium is buried in landfill each year in the Latrobe Valley. Australia exports 235 Mt/yr of coal with 1 to 2 ppm uranium and about 3.5 ppm thorium in it, hence up to 400 tonnes of uranium and about 800 tonnes of thorium could conceivably be added to published export figures (http://www.world-nuclear.org/info/inf30.html).
Other coals are quoted as ranging up to 25 ppm U and 80 ppm Th. In the USA, ash from coal-fired power plants contains on average 1.3 ppm of uranium and 3.2 ppm of thorium, giving rise to 1200 tonnes of uranium and 3000 tonnes of thorium in ash each year (for 955 million tonnes of coal used for power generation). Applying these concentration figures to world coal consumption for power generation (7800 Mt/yr) gives 10,000 tonnes of uranium and 25,000 tonnes of thorium per year(http://www.world-nuclear.org/info/inf30.html).

Reference:
Berthoud, E.L. 1875. on the occurrence of uranium, silver, iron etc., in the Tertiary Formation of Colorado Territory. Proc. Nat. Acad. Sci., Philadelphia, 27, 363-365.
Breger, I.A., Deul, M. and Meyrowitz, R. 1955. Geochemistry and mineralogy of a uraniferous subbituminous coal. Econ. Geol., 50, 610-624.
Bouska, V. 1981. Geochemistry of Coal. Elsevier Scientific Publishing Company, New York.
Denson, N.M., 1959. Uranium in coal in the Western United States, U.S. Geological Survey Bull. 1055.
Gill, J.R. 1959. Reconnaissance for uranium in the Ekalaka Lignite field, Carter County, Montana. US Geological Survey, Bull. 1055.
Gott, G.B. 1952. Uranium in black shales, lignites and limestones in the United States. Selected papers on uranium deposits in the United States. U.S. Geological Survey, Circ.220,Washington. 31-35.
Swaine, D.J. 1990. Trace elements in coal. Butterworths, London.
Valkovic, V. 1983. Trace elements in coal. CRC Press, Inc. Florida.
Vine, J.D. 1956. Uranium-bearing coal in the United States. US Geol. Surv. Prof. Pap., No 300, 405-41.
Post a Comment