Uranium Ore Mill Tailings Management
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Uranium Ore Mill Tailings Management Radioactive Waste Management and Disposal NUCP 2311 Lecture Materials contributed by Dr. John Poston.
Objectives • Review general approaches to uranium mining. • Provide a general overview of mill tailings management. • Provide some general understanding of the constituents of mill tailings.
Uranium Mill Tailings • The residual wastes from milled ore after the uranium has been extracted. • May result from an acid leach process or an alkaline leach process. • Mills in the U.S. are designed to use the acid leach process. • Tailings consist of slurries of sands and claylike particles (called “slimes”).
Uranium Milling • The starting point in the nuclear fuel cycle whether U-cycle or Th-cycle. • Uranium is ubiquitous at low concentrations. • In the past, uranium was a waste product in mining for other materials. • Uranium ores are normally classified as high, medium, and low grade.
Uranium Material Production - Manhattan Project
Crustal Abundance of Selected Elements
“Fun Facts” • Uranium is a common substance which is found throughout the earth’s crust. As it is present virtually everywhere, it contributes to what is called natural background radiation. • A typical backyard (in Canada), with dimensions of 10 metres by 10 metres (about 33 feet by 33 feet) and a soil depth of one metre (slightly more than three feet), contains approximately 300 grams (0.7 pounds) of uranium. • There are only a few places in the world where major uranium deposits have been found and where it is mined—Australia, Canada, Kazakhstan, Namibia, and the United States.
Uranium Exploration • Uranium is one of the more common elements in the earth’s crust – it is more common than tin – ~40 times more common than silver – ~500 times more common than gold.
• More than 200 minerals which contain uranium – Uraninite is the most common. – Uranium concentrations vary from substance to substance and place to place.
• Photo: A geologist examines core samples for uranium at an exploration site in northern Saskatchewan.
High Grade Ores • Contain a few percent of uranium (1-4%), in unusual cases, up to 10% • Typically in the form of uraninite (largely UO2), or pitchblende. • These ores are found primarily in central Africa (Zaire) and in Canada (Big Bear Lake).
Medium Grade Ores • Contain 0.1 to 1.0% uranium • Found on the Colorado plateau region (Colorado, Utah, New Mexico and Arizona), also found in California, Nevada, Texas, and Washington • Found in Canada, Australia, and Czechoslovakia • Typically carnotite, thorianite, phosphates, and carbonates
Medium Grade Ores • Assume a typical ore with a concentration of 0.25% – Concentration = 0.0021 g U/g ore, which means ~ 0.0021 g 238U/g ore – All daughters in equilibrium with both the 238U and 235U – Activity = 26 Bq/g ore – Activity = 700 pCi/g ore
Typical 0.1 % Ore (low grade) • • • • • • •
1000 tons of ore Uranium content – 1 ton U-238 activity – 0.3 Ci Ra-226 activity – 0.3 Ci Ra-226 mass – 0.3 g Found in many parts of the world. Have found some commercial feasibility in recent years. • However, generally ores less than 0.1% may not be processed efficiently.
U.S. Uranium Processing • Since 1978, concentration in ores has ranged from 0.112% to 0.531% U3O8 • Recovery rate from ores has ranged from 87% to 97% • Total mass of tailings produced is about 1.9 x 108 tons • Total volume of tailings produced is about 1.2 x 108 m3
Uranium Mines and Mills
Uranium Mining and Milling • At the end of 1996, there were no uranium mills in the U.S. that were operational. • Six mills have been put into a standby status. • Twenty-one mills were either decommissioned or scheduled for decommissioning. • Primary source of tailings is activities called “nonconventional” production .
Uranium Mining Techniques • Open pit mines • Hard-rock mining • In situ leaching
An Underground Mine
In-Situ Leach Mining
A Typical Uranium Mill Operation
General Approach – Grind to powder to increase surface area (Comminution) – Oxidize and dissolve U-oxide in acid (Leaching) [U(IV) to U(VI)]
– Separate liquids and solids and concentrate U solution (multiple steps) – Reduce with a strong base and precipitate (precipitation) [U(VI) to U(IV)]
– Dry Product for further processing.
Uranium Milling • Many mined ores in the U.S. are in the range of 0.04 to 0.42%. • About 2 kg (~5 lbs.) of uranium is obtained from a ton of ore. • The remainder is called “tailings” or “mill tailings.” • Transferred as a slurry into “tailings ponds.”
Uranium Mill Product (U3O8)
Waste Characteristics • Dry weight of tailings about equal to dry weight of the ore processed. • Dry tailings contain 70 to 80 wt% sand-sized particles and 20 to 30 wt% finer-sized particles. • Waste liquid accompanying tailings to ponds is about 1.5 times the weight of the processed ore.
Characteristics of Sands • Particle size range is 75 to 500 m. • Typically SiO2 • Contains