What is ISL / ISR Mining?
Conventional mining involves extracting ore from the ground and processing it to extract the minerals being sought. ISR (in situ recovery, also known as in situ leaching (“ISL”) 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 out of 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.
ISL mining technology was developed independently in both the U.S.S.R. and the U.S.A. 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 economical conventional mining techniques. It was developed in both countries using similar approaches in engineering and technology. However, the acid leach system was adopted in the U.S.S.R., while an alkaline, primarily carbonate‐based system was adopted in the U.S.A. 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 and Australia uses acid leaching agents, ISL mines in the U.S.A. normally use alkaline leaching agents such as a combination of sodium bicarbonate and carbon dioxide. At the Honeymoon Project in South Australia, the process involves acid leaching with weak sulfuric acid supplemented by sodium perchlorate.The leach solution is at a pH of approximately 2.0, about the same pH as vinegar. The concentration of acid used in Kazakhstan is higher, but the pH achieved is the same as in Australia.
The following diagram illustrates a typical ISL mining operation based on the methods used on the Uranium One's material properties in Kazakhstan.
An ISL mine consists of wellfields that are progressively established over the orebody as uranium is depleted from sections of the orebody after leaching. The wellfield 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. Wellfield patterns are typically 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 metres. 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.
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 choice between IX and SX is largely determined by the salinity of the groundwater. SX is better with high salinity, as at the Honeymoon Project (17‐20,000 ppm), while IX is most effective below 3,000 ppm chloride, as presently experienced in Kazakhstan. The uranium is then stripped from the ion exchange resin (Uranium One’s operations usually use ammonium nitrate for this purpose), and is precipitated chemically from the solution, usually with hydrogen peroxide (caustic soda is used at the Zarechnoye Mine and the Kharasan Mine).The resin is cleaned and returned to the IX reactors. The uranium slurry is subsequently dewatered and dried to give hydrated uranium peroxide (UO4.2H2O) product. This is usually done by filtering at Uranium One's projects in Kazakhstan. At the South Inkai Mine and the Karatau Mine (the production from the Akbastau Mine is also processed there) the product also undergoes calcining (drying) to further purify it. The final product is then shipped to a converter for delivery to customers. One converter in Russia requires product to be at a higher grade of purity, so shipments to that converter first have to be sent to a refinery for further processing before delivery to the converter.
After the solution has been stripped, it is oxygenated and, if necessary, recharged with sulfuric acid to maintain the requisite level of acidity. In Kazakhstan, all of the solution (as well as the small amount of waste water generated by the processing plant) is then returned to the injection wells and reinjected into the wellfield, but at Uranium One's properties in Australia and the United States 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 elements such as radium, arsenic and iron from the orebody and is reinjected into approved disposal wells. At the United States properties, the aquifers are usually very deep and the waste is injected into a different aquifer from the one being mined. At the Australian properties, the waste is injected into the same aquifer that is being mined, but at a location some distance away from the area being mined. This bleeding of the 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. In Kazakhstan, minor quantities of waste sands may accumulate in the sand ponds over time, but these are handled by cleaning the pond bottoms and taking the waste material to an approved disposal site.
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.