ISR Mining

What is ISR Mining?

Conventional mining involves extracting ore from the ground and processing it to extract the target minerals. ISR (in situ leaching, also known as in situ recovery (ISR) or solution mining) involves using liquids (commonly referred to as “leaching liquors” or “lixivants”), which are pumped through the orebody while it is in the ground to recover the minerals from the ore by leaching. Consequently there is little surface disturbance and no tailings or waste rock are generated. However, the orebody needs to be permeable to the liquids used, and located so that the liquids do not contaminate groundwater away from the orebody.

ISR mining technology was developed independently in both the Soviet Union and the United States in the mid-1960’s. The method was developed to extract uranium from typical roll‐front type deposits (a sub‐type of sandstone uranium deposits) located in water‐saturated permeable sediments that were not suitable for economical conventional mining techniques. It was developed in both countries using similar approaches in engineering and technology.

However, an acid leach system was adopted in the Soviet Union while an alkaline, primarily carbonate‐based system was adopted in the United States. The specific approach is determined by deposit geology and groundwater conditions. If there is significant calcium in the ore zone, alkaline (carbonate‐based) leaching must be used.

While uranium production in Kazakhstan uses acid leaching agents, ISR mines in the US typically use alkaline leaching agents such as a combination of sodium bicarbonate and carbon dioxide.

ISR Wellfields

An ISR mine consists of well fields that are progressively established over the orebody as uranium is depleted from sections of the orebody after leaching. The well field consists of injection wells, which are used to inject the uranium leaching liquors (either acid or alkaline) into the orebody, and production wells which pump the “pregnant”, or uranium‐bearing, solution to the surface.

Typically, there are several injection wells to every production well. Well field patterns are usually configured as hexagons (with six injection wells surrounding each production well), or as parallel linear rows. The spacing between injection and production wells in a hexagon pattern typically ranges from 30 to 50 meters. A series of monitor wells are situated around each mineralized zone to detect any movement of mining fluids outside the mining area. The wells are cased to ensure that the leaching liquors only flow to and from the ore zone and do not affect any overlying aquifers.

Uranium Processing

The pregnant solution from the production wells is pumped to the treatment plant where the uranium is recovered either in a resin ion exchange (IX) or in a liquid ion exchange, also known as a solvent extraction (SX) system.

The uranium is then stripped from the ion exchange resin, and is precipitated chemically from the solution, usually with hydrogen peroxide (caustic soda is used at the Zarechnoye Mine and the Kharasan Mine in Kazakhstan). The resin is cleaned and returned to the IX columns. The uranium slurry is subsequently de-watered and dried to produce a hydrated uranium peroxide (UO4.2H2O) product. This is usually done by filtering at Uranium One’s projects in Kazakhstan.

The product also undergoes calcining (drying) to further purify it. The final product is then shipped to a converter for delivery to customers.

After the solution has been stripped, it is recharged with sulphuric acid to maintain the requisite level of acidity. In Kazakhstan, all of the solution is then returned to the injection wells and re-injected into the well field. At Uranium One’s properties in the United States, a very small flow (about 0.5 percent) is bled off to maintain a pressure gradient in the well field and this, with some solutions from surface processing, is treated as waste.

Waste contains various dissolved elements from the orebody and is re-injected into approved disposal wells pursuant to permits issued by local governmental agencies.

In the United States, the waste is injected into a different, much deeper aquifer from the one being mined. This bleeding of the process solution ensures that there is a steady flow into the well field from the surrounding aquifer and serves to restrict the flow of mining solutions away from the mining area. In Kazakhstan, minor quantities of waste sands from pregnant solutions may accumulate in the sand ponds over time but these are handled by cleaning the pond bottoms.

Environment & Health

After ISR mining is completed, the quality of the remaining groundwater must be restored to a baseline standard determined before the start of operations, so that any prior use can be resumed.

Upon decommissioning, wells are sealed or capped, process facilities removed and any evaporation pond re-vegetated, so that the land can revert to its previous uses.

The usual radiation safeguards are applied at an ISR mining operation, despite the fact that most of the orebody’s radioactivity remains deep underground and there is accordingly no ore dust and only a minimal increase in radon release.

The usual radiation safeguards are applied at an ISR mining operation, despite the fact that most of the orebody’s radioactivity remains deep underground and there is accordingly no ore dust and only a minimal increase in radon release.

Employees are monitored for alpha radiation contamination and personal dosimeters are worn to measure exposure to gamma radiation. Routine monitoring of air, dust and surface contamination are also undertaken.

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