Lesson 2 ◆ 2.4 Processing Uranium Ore (Milling)

Syllabus Coverage: NFC1.1

The purpose of milling is to concentrate the uranium extracted from the mine into a tradeable product. Since mined ore typically contains only about 0.1-1 wt% uranium, the mill is almost always co-located with the mine to avoid transporting vast quantities of low-grade rock.

The final product of milling is uranium ore concentrate (UOC), commonly known as yellowcake due to the strong yellow colour of ammonium diuranate (ADU). Yellowcake is a traded commodity, priced in terms of $/lb U₃O₈.

The milling process involves several key steps, shown in the flow chart below:

Uranium Milling Process

Step 1: Stockpiling

Ore delivered from the mine is stockpiled in heaps to provide a buffer for the milling operation, ensuring a continuous and consistent feed.

Step 2: Comminution (Crushing and Grinding)

Comminution is the process of reducing the particle size of the ore to make it suitable for chemical leaching. This is typically a three-stage process:

  1. Primary crushing (jaw crusher or gyratory crusher): Reduces fragments to ~15 cm
  2. Fine crushing (cone crushers): Reduces to ~2 cm
  3. Fine grinding (semi-autogenous grinding, SAG mills): Reduces to sub-millimetre particles suitable for leaching

Step 3: Leaching

Leaching is the chemical process of dissolving uranium from the crushed ore while leaving the solid waste (called gangue). There are two main methods:

  • Acid leaching (most common): Uses sulphuric acid (H₂SO₄) to dissolve the uranium. Performed at ~50 degrees C for 3-10 hours in rubber-lined, agitated steel tanks. Achieves 90-98% uranium recovery.
  • Alkaline leaching: Uses sodium carbonate (Na₂CO₃). Used when the ore is rich in acid-consuming minerals such as calcite and dolomite. Slower kinetics but more selective.

Key Chemistry: Uranium naturally occurs in two oxidation states: U(IV) in UO₂ (insoluble) and U(VI) in UO₃ (soluble). The U(IV) must first be oxidised to U(VI) before it can be leached.

Step 4: Solid-Liquid Separation

The uranium-bearing liquid must be separated from the solid waste. The most common method is counter-current decantation (CCD), which achieves ~99% recovery in 4-6 stages. Horizontal belt filters are also used.

Step 5: Extraction (Purification and Concentration)

Either solvent extraction (SX) or ion exchange (IX) is used to recover and concentrate uranium from the leach solution:

  • Solvent extraction: The uranium-bearing aqueous solution flows counter-currently with an organic solvent (typically a tertiary amine — the “AMEX process”). Uranyl ions are selectively transferred to the organic phase, purified, then stripped back into a clean aqueous phase.
  • Ion exchange: Uranyl cations are selectively absorbed by a resin, then stripped using a salt solution.

Step 6: Precipitation, Filtering, and Drying

The purified uranium solution is treated to produce a solid product:

  1. The solution is limed (pH raised)
  2. Ammonia gas is bubbled through, precipitating ammonium diuranate (ADU): (NH₄)₂U₂O₇
  3. The ADU solid is separated by centrifuge and washed
  4. Dried (calcined) at approximately 200 degrees C to produce a free-flowing yellow powder
  5. Packed in simple steel drums with polythene liners for shipment

This dried product is yellowcake — the tradeable output of the milling process.