Wednesday, February 27, 2008

Uranium ore, Yellow cake and Uranium:


Uranium ore, Yellow cake and Uranium:

An ore mineral is a mineral which may be used for extraction of one or more than one metals. A uranium ore mineral is therefore a mineral possessing such physical and chemical properties and occurring in a deposit in such concentrations that it may be used for the profitable extraction of uranium, either alone or together with one or more other metals.

Pitchblende and Uraninite are used for extraction of uranium, contain theoretically up to 85 per cent uranium (average from 50 to 80%). Other minerals such as carnotite, torbernite, tyuyamunite, autunite, uranophane, and brannerite, contain 45 to 60 per cent uranium. The majorities of uranium-bearing minerals, however, contain uranium in small or trace amounts as an accessory to other major constituents.

In uranium ore, the presence of uranium is in combinations which are extremely difficult to break down chemically in order to recover the uranium. These minerals also usually occur scattered sparsely throughout the deposit so that recovery difficult and expensive. Primary uranium minerals have been found most commonly in veins or pegmatites, although in recent years extensive, flat-lying deposits of pitchblende in sedimentary rocks have also been discovered. The primary uranium minerals are generally black or dark brown, noticeably heavy, and often have a shiny or pitch-like luster. At the present time, there are only three known primary uranium ore minerals, and the most important of these, uraninite and pitchblende, are really varieties of the same mineral.

Uraninite is a naturally occurring uranium oxide with cubic or octahedral crystal form. It has a specific gravity of 8-10.5 (iron = 7.85), a grayish-black color and a hardness of 5-6, about the same as steel.

Pitchblende is the massive variety of uraninite, without apparent crystal form, that occurs most abundantly in the rich primary vein deposits of uranium. It is the chief constituent of nearly all high-grade uranium ores and has provided the largest part of all uranium produced throughout the world.

Yellowcake is milled uranium oxide, known to chemists as U3O8. When uranium ore comes out of the mine, it actually contains fairly little of the precious radioactive element. The milling process gets rid of the useless minerals that dominate the ore. First, raw ore is passed through a series of industrial-sized crushers and grinders. The resulting "pulped" ore is then bathed in sulfuric acid (H2SO4), a process which leaches out the uranium. After some drying and filtering, the end product is yellowcake: a coarse, oxidized powder that is often yellow in color but can also have a red or gray tint, depending on the number and type of impurities that may remain. Yellowcake is a first step toward enriched uranium. At the refinery, the yellow cake is dissolved in nitric acid. The initial separation and refining processes generate large volumes of acid and organic waste.

It is necessary to enrich the U-235 isotope concentration from its natural composition of 0.7% for use in either reactors or bombs. Reactor grade uranium contains from 3.5 to 4.0% U-235, while the Hiroshima uranium bomb contained more than 80% of the lighter U-235. The process used for enrichment involves ‘gaseous diffusion’ and thus the uranium must be converted to a gaseous compound, uranium hexafluoride (UF6).

No comments:

Loading...