The decision as to which mining method to use for a particular uranium deposit is governed by the nature of the orebody, safety and economic considerations. Both excavation and in situ techniques are used to recover uranium ore.
In the case of underground uranium mines, special precautions (consisting primarily of increased ventilation) are required to protect against airborne radiation exposure. In general, open pit mining is used where deposits are close to the surface and underground mining is used for deep deposits, typically greater than 100 metres deep. Open pit mines require large holes on the surface, larger than the size of the ore deposit, since the walls of the pit must be sloped to prevent collapse. As a result, the quantity of material that must be removed in order to access the ore may be large. Underground mines have relatively small surface disturbance and the quantity of material that must be removed to access the ore is considerably less than in the case of an open pit mine.
Nevertheless, an increasing proportion of the world’s uranium now comes from in situ leaching (ISL) – sometimes today referred to as in situ recovery (ISR) or solution mining. This 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 ground water away from the orebody.
ISL mining was first tried on an experimental basis in Wyoming during the early 1960s and the first commercial mine began operating in 1974. Uranium deposits suitable for ISL occur in permeable sand or sandstones, confined above and below by impermeable strata, and which are below the water table. There are two operating regimes for ISL, determined by the geology and groundwater. If there is significant calcium in the orebody (as limestone or gypsum), alkaline (carbonate) leaching must be used. Otherwise, acid (sulphate) leaching is generally better. 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.
Mining methods have been changing. In 1990, 55% of world production came from underground mines, but this shrunk dramatically to only 33% by 2000. Since then, the new
Canadian mines have pushed it up again. By-production of uranium (where uranium is produced in combination with other metals such as copper and gold) has been fairly constant over time at around 10% of output and today mainly reflects the production levels at the Olympic Dam mine in South Australia (copper and gold) rather than South Africa (gold). The share of ISL production has been rising, reaching a quarter in 2006 and almost one third today. This increase is largely explained by rapidly-rising output of uranium in Kazakhstan, which may approach 14,000 tonnes in 2009.
From the environmental standpoint, in many respects uranium mining is much the same as any other mining. Projects must have approvals prior to commencing, and must comply with all environmental, safety and occupational health conditions applicable. Increasingly, these are governed by international standards, with external audits, sometimes performed by major uranium customers who are responsible to their own shareholders.
At an early stage of a mine feasibility study, environmental studies of the site begin. These escalate in detail and progressively focus on issues of concern in relation to the proposal, in consultation with the governmental authorities. Depending on the government jurisdiction, an environmental effects or impact statement is published and may be made available for public comment. After consideration of comments and in the light of judgements by a wide range of authorities, approval may then be given for the project to proceed.
It is important to note that uranium minerals are always associated with more radioactive elements such as radium and radon in the ore. Therefore, although uranium itself is not very radioactive, the ore which is mined, especially if it is very high-grade such as in some Canadian mines, is handled with some care for occupational health and safety reasons. Mining methods, tailings and run-off management and land rehabilitation are subject to Government regulation and inspection. Mining operations are performed under relevant national health and radiation protection codes of practice. These set strict health standards for exposure to gamma radiation and radon gas. Standards apply to both workers and members of the public.
Uranium itself is not strongly radioactive; the major isotope U-238 having a half-life equal to the age of the earth. U-235 has a half life one sixth of this and emits gamma rays as well as alpha particles. Hence a lump of pure uranium would give off some gamma rays, but less than those from a lump of granite. Its alpha radioactivity in practical terms depends on whether it is as a lump (or in rock as ore), or as a dry powder. In the latter case the alpha radioactivity is a potential, though not major, hazard. It is also toxic chemically, being comparable with lead. Gloves are a sufficient precaution to handle uranium metal safely. Uranium concentrate needs to be handled and contained to ensure that people do not inhale or ingest it.
At any mine, designated employees (those likely to be exposed to radiation or radioactive materials) 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 is undertaken. If uranium oxide is ingested it has a chemical toxicity similar to that of lead oxide. Similar hygiene precautions to those in a lead smelter are therefore taken when handling it in the drying and packing areas of the mill. The usual radiation safeguards are applied at an ISL mining operation, despite the fact that most of the radioactivity remains well underground and there is hence minimal increase in radon release and no ore dust.
Milling, which is generally carried out close to a uranium mine, extracts the uranium from the ore. Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines. Milling produces a uranium oxide concentrate which is shipped from the mill. It is sometimes referred to as ‘yellowcake’ and generally contains more than 80% uranium, while the original ore may contain as little as 0.1% uranium.
In a mill, uranium is extracted from the crushed and ground-up ore by leaching, in which either a strong acid or a strong alkaline solution is used to dissolve the uranium. The uranium is then removed from this solution and precipitated. After drying and usually heating it is packed in 200-litre drums as a concentrate. The remainder of the ore, containing most of the radioactivity and nearly all the rock material, becomes tailings, which are placed in engineered facilities near the mine (often in mined out pit). Tailings contain long-lived radioactive materials in low concentrations and toxic materials such as heavy metals; however, the total quantity of radioactive elements is less than in the original ore, and their collective radioactivity will be much shorter-lived. These materials need to be isolated from the environment.
Uranium mine tailings contain all the radium present in the original ore. At an underground mine they may be first cycloned to separate the coarse fraction which is used for underground fill. The balance is pumped as a slurry to a tailings dam, which may often be a worked-out pit.
When radium undergoes natural radioactive decay one of the products is radon gas. Radon occurs in most rocks and traces of it are in the air we all breathe. However, at high concentrations it is a health hazard. Because radon and its decay products (daughters) are radioactive and because the tailings are now on the surface, measures are taken to minimise the emission of radon gas. During the operational life of a mine the material in the tailings dam is usually covered by water to reduce surface radioactivity and radon emission (though with lower-grade ores neither pose a hazard at these levels).
Run-off from the mine stockpiles and waste liquors from the milling operation are collected in secure retention ponds for isolation and recovery of any heavy metals or other contaminants. The liquid portion is disposed of either by natural evaporation or recirculation to the milling operation. Process water discharged from the mill contains traces of radium and some other metals which would be undesirable in biological systems downstream. This water is evaporated and the contained metals are retained in secure storage.
Generally in situ leach (ISL) operations keep the orebody in the ground and recover uranium by circulating oxygenated and acidified groundwater through it, using injection and recovery wells. The main environmental consideration with ISL is avoiding pollution of any groundwater away from the orebody, and leaving the immediate groundwater unaffected by the mining process.
At the conclusion of open-pit mining, tailings are covered permanently with enough clay and soil to reduce both gamma radiation levels and radon emanation rates to levels near those naturally occurring in the region, and enough rock to resist erosion. A vegetation cover is then established. Apart from groundwater considerations discussed above, rehabilitation of ISL mines is very straightforward, making this a technique with remarkably low environmental impact. Upon decommissioning, wells are sealed or capped, process facilities removed, any evaporation pond revegetated, and the land can readily be returned to its previous uses.
The increasing share of ISL techniques in world uranium mining is tending to lessen some of the environmental concerns generated by the shifting of huge quantities of ore in conventional mining and the subsequent establishment of large tailings dams. Provided that the quality of groundwater is not harmed, ISL provides an excellent solution to exploiting amenable low grade ores. It is likely, however, that there will also be significant investment over the next 20 years in new underground and open pit operations, as demand for uranium increases.
Steve Kidd is Director of Strategy & Research at the World Nuclear Association, where he has worked since 1995 (when it was the Uranium Institute). Any views expressed are not necessarily those of the World Nuclear Association and/or its members.Related ArticlesEngineering is the key for nuclear waste disposal Nuclear waste disposal - what's the problem?
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