What is ISL Mining?
Conventional mining involves the removal of ore from the ground, breaking it up and treating it to remove the minerals being sought. In situ leaching (ISL), also known as solution mining, involves leaving the ore where it is in the ground, and using liquids which are pumped through it to recover the minerals out of the ore by leaching. Consequently there is little surface disturbance and no tailings or waste rock generated. However, the orebody needs to be permeable to the liquids used, and located so that they do not contaminate groundwater away from the orebody.
ISL mining technology was developed independently in both the USSR and USA in the mid 1970's. The method was conceived for extracting uranium from typical roll-front type deposits (a sub-type of Sandstone Uranium Deposits) located in water saturated permeable rocks that were not suitable for conventional mining techniques. It was developed in both countries using similar approaches in engineering and technology. However, the Soviets adopted the acid leach system, while the US specialists employed an alkaline, primarily carbonate-based system. 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.
Techniques for ISL have evolved to the point where it is a controllable, safe, and environmentally benign method of mining which can operate under strict environmental controls and which often has cost advantages.
ISL Wellfield
The mine consists of wellfields which are progressively established over the orebody as uranium is depleted from sections of the orebody after leaching. Wellfield patterns are typically configured as hexagons and sometimes as parallel linear rows. The spacing between injection and production wells is in a hexagon pattern is about 45 to 50 metres at the Akdala mine. The spacing between rows in a linear pattern is 60 metres, with 30 metre spacing between wells in each row.
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 liquors only flow to and from the ore zone and do not affect any overlying aquifers. Wells are pressure tested before use.
While uranium production in Kazakhstan and Australia uses acid leaching of the crushed ore, ISL in other countries normally uses alkaline leaching agents such as a combination of sodium bicarbonate and carbon dioxide. At Beverley and Honeymoon in South Australia, the process is acid leaching with weak sulfuric acid plus oxygen. The leach solution is at a pH of 2.5 - 3.0, about the same pH as vinegar.
At Akdala in Kazakhstan, the life of an individual ISL well pattern is typically three to five years. Most of the uranium is recovered during the first 6 months of the operation. The most successful operations have achieved a total overall recovery of between 70% and 90% of the ore.
Uranium Processing
The submersible pumps initially extract native groundwater from the host aquifer prior to the addition of uranium complexing reagents (acid or alkaline) and an oxidant (hydrogen peroxide or oxygen) before injection into the wellfield. The leach liquors pass through the ore to oxidize and dissolve the uranium minerals in situ.
With some ores, and notably with ISL in the USA, carbonate leaching is used to form a soluble uranyl tricarbonate ion. This can then be precipitated with an alkali, for example as sodium or magnesium diuranate. Alkaline leaching or treatment of ores is not undertaken in Kazakhstan, Australia or Canada at present.
The pregnant solution from the production wells is pumped to the treatment plant where the uranium is recovered in a resin ion exchange (“IX”) or liquid ion exchange, also known as solvent extraction (“SX”) system. The choice between IX and SX is largely determined by the salinity of the groundwater. SX is better with high salinity, as at Honeymoon (17-20,000 ppm), while IX is most effective below 3,000 ppm chloride, as presently experienced in Kazakhstan and at Beverley in Australia.
The uranium is then stripped from the ion exchange resin, and precipitated chemically, usually with hydrogen peroxide. The uranium slurry is subsequently dewatered and dried to give hydrated uranium peroxide (UO4.2H2O) product.
Before the remaining process solution is reinjected, it is oxygenated and, if necessary, recharged with sulfuric acid to maintain a pH of about 2.5 to 2.8. Most of the solution is returned to the injection wells, but a very small flow (about 0.5%) is bled off to maintain a pressure gradient in the wellfield and this, with some solutions from surface processing, is treated as waste. Waste contains various dissolved minerals such as radium, arsenic and iron from the orebody and is reinjected into approved disposal wells in a depleted portion of the orebody. This bleed of process solution ensures that there is a steady flow into the wellfield from the surrounding aquifer, and serves to restrict the flow of mining solutions away from the mining area.
Environment & Health
After ISL mining is completed, the quality of the remaining groundwater must be restored to a baseline standard determined before the start of the operation, so that any prior use can be resumed. Contaminated water drawn from the aquifer is either evaporated or treated before reinjection.
Upon decommissioning, wells are sealed or capped, process facilities removed, any evaporation pond revegetated, and the land can readily revert to its previous uses.
The usual radiation safeguards are applied at an ISL mining operation, despite the fact that most of the orebody's radioactivity remains deep underground and there is hence minimal increase in radon release and no ore dust. 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 undertaken.