Uranium

Kathy Feick

Uranium is a radioactive metal with a high specific gravity which was discovered in 1789 by Martin Klaproth. It was named after the planet Uranus.

Occurrence:

Uranium is concentrated on the surface of the Earth since its atoms don’t fit into the crystal structure of the minerals in the mantle. It is as common as tin, tungsten and molybdenumin the Earth’s crust. Uranium occurs naturally in low concentrations of a few parts per million in soil, rock, and water, all over the world. Over 150 uranium-bearing minerals have been identified. There are two main ways in which uranium may become concentrated enough in one area to be considered an ore.

Uranium in the form of pitchblende image source

Method 1 sedimentary deposition

Uranium gathers in areas where oxygen is absent, such as in rocks which are rich in organic matter. If oxidizing fluids move in, they mobilize the uranium and concentrate it in front of the moving fluid. This process may also occur under a thick layer of sedimentary rock, such as the case of the uranium deposit in Saskatchewan. In this case the uranium gathered between the older bedrock and the sedimentary layers above. Chemical activity and fluid flows concentrated the uranium into ore bodies reaching as much as 70% purity.

Method 2 granitic deposits

As large bodies of granite solidify, any trace amounts of uranium become concentrated in the last bits of fluid left. These may fracture and invade surrounding rocks with metal-bearing fluids, leaving veins with high concentrations of uranium. More episodes of tectonic activity can concentrate the uranium further. The world’s largest uranium deposit, a hematite breccia complex at Olympic Dam in South Australia, was formed in this way.

Affects of uranium on surrounding minerals:

The radioactivity of uranium affects the minerals around it. Signs may include

Blacked fluorite
Blue Celestite
Smoky quartz
Golden Beryl
Red-stained feldspar

Mining:

Uranium is detected due to its radioactivity. It can be mined by either open-pit methods or underground mining methods depending on the depth of the ore body. Sedimentary-type deposits are usually mined by in situ leaching, where an acidic or alkaline solution is pumped through the ore body to dissolve the uranium. Granitic deposits are mined by taking rocks with the ore from the ground.

The first step in processing the uranium is to crush and grind the ore to a fine size. Mixing these fine-sized particles with water produces a slurry of fine ore particles suspended in water. This slurry is then leached with either an acid or an alkali which causes the uranium to dissolve into the solution. This solution is filtered to remove any remaining solids and the uranium is recovered by techniques using solvent extraction, ion exchange or direct precipitation. Finally the uranium is recovered in a chemical precipitate, which is filtered and dried to produce a yellow powder known as yellowcake. If the yellowcake is heated to 700°C then it produces a dark grey-green uranium oxide powder concentrating more than 98% U₃O₈.

Uranium Yellowcake image source

The largest producers of uranium include:

Canada
Australia
Kazakhstan

Uses:

Fuel in nuclear power reactors to generate electricity
In the manufacture of radioisotopes
As a colorant in uranium glass
Used for tinting and shading in early photography
Tank armour and other removable vehicle armour can be hardened with depleted uranium
As a source of helium

Primary minerals:

Uranite (UO₂)

Uranite is the principal radioactive mineral. It forms black, gray or brown crystals and was once thought to be worthless until the value of uranium was realized. It is the most common and widespread uranium mineral, making it the best known uranium ore. It forms in granite and Syenite pegmatites, in high temperature hydrothermal veins and in quartz pebble conglomerates.

Pitchblende (a mixed oxide, usually U₂O₈ and UO₂)

Pitchblence is another primary mineral ore of uranium containing 50-80% uranium. Pitchblende’s name was derived from the word pitch (because of its black colour) and blende, a term used by German miners to denote minerals whose density suggested a metal content. It is a greenish brown to black uranium mineral and in its massive form it commonly has a grape-cluster appearance. It usually contains lead, thorium, rare-earth elements, and it is frequently associated with silver. It forms in granite and Syenite pegmatite’s, high temperature hydrothermal veins and in quartz pebble conglomerates.

Brannerite (UTi₂O₆)

Brannerite is used as a major ore of uranium and radium, and as a source of helium. It was named after Dr. John Casper Branner (1850-1922), who was formerly a Professor of Geology and the President of Stanford University. The mineral may be black but it is more often found in shades of yellow to brown to green. It sometimes forms tiny crystals, which may be vitreous. It may also fluoresce under a short-wave ultra-violet light. Brannerite is found in the same settings as gold, including placers, quartz veins and in granite pegmatites.

Coffinite (USiO₄)

Coffinite was first described in 1954 and was named in honour of Reuben Clare Coffin, Tusla, Oklahoma, who was a pioneer geologist in the study of uranium ore deposits of the Colorado Plateau. It is black in colour due to included organic matter. It contains over 72% uranium but it is extremely rare and is mainly for collectors.

Uranium rush (1950s to early 1960s):

During the 1950s to the early 1960s uranium was an incredibly valuable mineral. Everything radioactive was being processed in the name of the “arms race” (the time of the Cold War with Russia). The United States, Russia, and numerous other countries were getting ready for a possible war and untold numbers of uranium ore was processed to prepare. The world could not seem to find enough uranium to satisfy its needs. The value of uranium did not decrease until the threat of a war was no longer present.