The mining technique of In Situ Leaching or ISL involves the circulation of solutions through an orebody to dissolve the mineral of interest in the ground, from where it is pumped to the surface and recovered in a processing plant. The process is also known as Solution Mining.

The ISL technique is generally suitable for small or low grade ore deposits that would otherwise be uneconomic to extract using conventional open cut or underground mining techniques. However, before ISL can be considered, the deposit must be within an aquifer or groundwater table and this aquifer is effectively isolated from other aquifers or groundwater users.

The process of ISL is typically applied to uranium and copper deposits.

Groundwater Mining

Groundwater is simply water found underground within the free space of materials like sands, gravels or sandstones. As such, the occurrence of water within a region is primarily determined by it's geology and soil and rock types. Where water occurs, the strata is known as an aquifer.

Uranium can be found in aquifers where it may form a coating of mineralisation around the sand particles. In such deposits, it becomes possible to use the groundwater as the primary uranium extraction facility. This is achieved by injecting corrosive chemicals into the aquifer to dissolve the uranium into the groundwater and the resulting solution is then pumped back to the surface to remove the uranium in a processing plant. Typical chemicals that are used to dissolve the uranium include sulphuric acid and oxygen or bicarbonate and hydrogen peroxide, that is - highly acidic or strongly alkaline leaching chemistry.

A conceptual cross-section of the ISL technique.⁽¹⁾

This process of leaching the uranium in the ground or in situ is therefore known as In Situ Leaching or ISL.

Controlling the Movement of Solutions

A typical ISL wellfield will use a series of injection and extraction wells to control the movement of solutions. A pattern known as a "5-spot" is most often used, where one extraction well is surrounded by 4 injection wells. The distance between these is usually 15 to 30 m.

In order to ensure an economic uranium recovery rate is achieved and to prevent the escape of the highly corrosive and toxic solutions into the wider environment, it is necessary to have very tight controls on the flow of the solutions through the aquifer. There are two primary ways this is achieved - vertical restriction of water flow (see above diagram) and always extracting more groundwater out than the solutions pumped in (thereby ensuring flow towards the extraction well).

Vertical Restriction - this is achieved through the orebody layer being bounded by clay above and below.

Horizontal Restriction - on average, excess pumping over injection (the "bleed") of up to 5% is typical.

A typical 5-spot ISL wellfield pattern.⁽²⁾

The excess pumping or bleed solutions creates large volumes of waste water to dispose of.

The Good, The Bad, The . . . . .

The following table lists the main advantages and disadvantages of ISL :

Advantages	Disadvantages
- cheaper infrastructure requirements - no large-scale tailings dams - no large open cut or underground mine to rehabilitate - lower occupational health and safety : accidents, dust and radiation - reduced workforce requirements	- significant risks of contaminating groundwater systems outside the mining zone - inherent difficulties in the hydraulic and geochemical behaviour of the deposit - difficult to restore groundwater to pre-mining quality - large volumes of waste water and solutions to dispose of

Post-ISL Rehabilitation of Groundwater

After the completion of mining by the ISL technique, it is expected by the general community and regulatory agencies that the groundwater will be chemically treated to restore the pre-mining water quality. Despite some decades of operating experience at numerous mines worldwide, this still remains a significant technical challenge with many old ISL mine sites being allowed relaxed restoration standards.

In the same way that chemicals are injected into the aquifer deposit to dissolve the mineral of interest, groundwater quality restoration involves the injection of further but different chemicals to precipitate out the unwanted minerals and elements and thereby immobilise their transport in groundwater. The typical chemicals used to achieve this are hydrogen sulphide or sodium sulphide. This process is also carried out with extensive pumping of the former leaching zone to ensure fresh groundwater moves into the area and the new groundwater chemistry is established.

However, it should be pointed out that even rigorous pumping and chemical treatment after leaching does not guarantee successful restoration - many elements, such as radium and selenium, remain mobile under the chemical conditions induced by the addition of sulphides to groundwater. It is extremely difficult to obtain a chemical balance between all contaminants of concern - some will inevitably remain dissolved and able to migrate away from the former mining zone to a potential user of the groundwater.

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Page developed by Gavin Mudd for SEA-US Inc.

Refs :
¹ - ENDAUM, 1997, Why Navajos Resist New Uranium Mining, Eastern Navajo-Diné Against Uranium Mining, The Workbook, Vol. 22 No. 2, Summer 1997.
² - Brunt, D A, 1998, ISL Uranium Mining - The Key Elements, Paper presented at the 2^ND Australian Uranium Summit, February 11-13, 1998, Adelaide, SA, 11 pages.

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Page last updated July 19, 1998.

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