2.3.3 In Situ Leaching (ISL) / Solution Mining

In situ leaching (also called solution mining or in situ recovery) was first developed in Eastern Europe and is now used extensively in Kazakhstan, the USA, Australia, and Uzbekistan. It is the fastest-growing extraction method.

Principle: A leaching solution (lixiviant) is pumped down into the ore body through injection wells. The solution dissolves the uranium from the surrounding rock. The uranium-bearing (“pregnant”) solution is then pumped back to the surface through extraction wells for processing.

Geological requirements (strict conditions must be met):

The deposit must be in a permeable formation (typically sandstone)
Surrounded by impermeable strata above and below
Must be below the water table to prevent contamination of surface water
Deposits are typically “roll-front” type, formed by lateral movement of oxidised groundwater through an aquifer

Leaching chemistry: The solution is typically either:

Acidic: Weak sulphuric acid plus oxygen (pH 2.5-3.0, similar to vinegar) — used in Australia and Kazakhstan
Alkaline: Sodium bicarbonate and carbon dioxide — used in the USA

ISL Well Field Layout:

The well field is arranged on a grid pattern with alternating injection and extraction wells:

        LEGEND:  I = Injection Well    E = Extraction Well
                 M = Monitoring Well   --- = Flow direction

     M ---- M ---- M ---- M ---- M      (Monitoring ring)
     |                             |
     M    I ----> E <---- I        M
     |    |       ^       |        |
     |    v       |       v        |
     M    E <---- I ----> E        M
     |    |       ^       |        |
     |    v       |       v        |
     M    I ----> E <---- I        M
     |                             |
     M ---- M ---- M ---- M ---- M      (Monitoring ring)

     Typical spacing: ~30 m between injection wells
     Each group of 4 injection wells has 1 central extraction well
     Monitoring wells surround the field to detect any seepage

The extraction wells contain submersible electric pumps that draw up the pregnant solution. A slight negative pressure gradient (extraction rate slightly exceeds injection rate, with ~0.5% “bleed”) ensures flow is always inward toward the well field, preventing leach solution from migrating into surrounding aquifers.

The uranium is recovered from the pregnant solution at a treatment plant using ion exchange (IX) resins. The uranium is stripped from the resin, precipitated with hydrogen peroxide, dewatered, and dried to produce hydrated uranium peroxide (UO₄.H₂O₂).

Advantages of ISL:

No large-scale excavation, no tailings, very little waste rock
Much lower cost than conventional mining
Greatly improved worker safety (no risk from explosives, cave-ins, or high radon exposure)
Minimal surface disturbance; land reverts to normal after operations cease

Disadvantages and risks:

Requires very specific geological conditions
Primary risk is contamination of groundwater — carefully controlled and monitored
Recovery rates typically around 70-80% (lower than conventional mining)

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