We talked to North America’s leading In Situ Leach (ISL) uranium mining engineers, and had them explain exactly how ISL worked. Most of the significant ISL operations in the United States were designed and/or constructed by these engineers. They explained how ISL mining is really just reversing the process of Mother Nature.
“Blossom” is what underground uranium miners called the crystals forming on the tunnel walls. Because the ore was in contact with air inside an underground mine, and as ground water moved slowly against the mine’s walls, a visible crust of uranium crystals would precipitate, or blossom along those walls. Making the uranium soluble doesn’t require a lot of oxygen and water because oxidization is a natural process. Adding more oxygen to the groundwater found in, and around, a uranium-mineralized orebody is the principle upon which present-day In Situ Leach (ISL) uranium mining is based.
Eons ago, the uranium was soluble and moved, on or below the surface, with the ground water. “In roll front uranium deposits the uranium was transported into the area through the natural groundwater system and precipitated from solution due to some reducing environment,” explained Harry Anthony, Chief Operating Officer of Uranium Energy Corp. Often, the reducing agent was something organic, such as coal, deep-seated oil and gas deposits, or hydrogen sulfide gases. In its reduced form, the uranium crystals are insoluble. “It will precipitate as a coating on the existing sand grains of the sandstone,” added Anthony. “As more water containing uranium sweeps through this area, and encounters this reducing environment, more uranium is precipitated until there is a sufficient concentration to make it a commercial deposit.”
After the geological team has delineated a company’s uranium “roll front” deposit and determined it is of economic value, the company must turn to its ISL design engineers to complete the “mining” process. While it takes stellar geologists such as David Miller of Strathmore Minerals, Bill Sheriff of Energy Metals, or William Boberg of UR-Energy to accumulate large, proven uranium-mineralized holdings, as they have done in Wyoming, New Mexico, Texas or elsewhere, each must turn to their engineers to extract the uranium from those sand grains and process them to produce an economic quantity of uranium oxide, or U3O8. The overwhelming majority of ISL facilities, designed in the United States, were engineered by Harry Anthony, Doug Norris and Dennis Stover.
Trained as a mechanical engineer, Harry Anthony has been involved with more than ten ISL uranium operations from Union Carbide’s Palangana in 1976 to Uranium Resources’ Bruni, Benavides, North Platte, Kingsville Dome and Rosita ISL projects. Anthony’s consulting work has taken him to ISL projects in Kazakhstan, Uzbekistan and the Czech Republic. Dennis Stover is best remembered for designing Smith Ranch in Wyoming, now owned by Cameco Corp. With a PhD in chemical engineering from the University of Michigan, Dr. Stover helped develop the first commercial alkaline ISL project in south Texas for Atlantic Richfield and helped develop an additional five small ISL operations in south Texas. Also a chemical engineer by training, Doug Norris’s paths have crossed with both Stover and Anthony. He helped build the Highland and Smith Ranch ISL operations in Wyoming, and designed Mestena’s Alta Mesa ISL operation in south Texas.
HOW DOES ISL MINING REVERSE MOTHER NATURE?
“In its natural, reduced environment, uranium exists as a solid in the +4 valence,” Anthony explained. “In the mining stage, we are reversing Mother Nature’s process by adding oxygen, oxidizing the uranium from a valence of +4 to a valence of +6.” The uranium was oxidized at one time, but then reduced by Mother Nature. By drilling wells into the ore zone, circulating the water and adding oxygen to it, the uranium is made soluble again.
Is it really this simple? Yes and no. Energy Metals Chief Operating Officer Dennis Stover outlined the process, “You’re simply adding, into the injection well, gaseous oxygen, just pure oxygen, but you’re doing it under the water level in the well. The natural pressure, created by that column of water above the injection point, allows the oxygen to dissolve into the water so that there’s no free gas being put into the well.”
Stover compared the oxygen dissolved in the liquid to the carbon dioxide dissolved in a bottle of soda. The soda remains clear, dissolved in the liquid, when stationery. “But when you shake it up, the gas will break out,” added Stover. “The pressure that’s available that lets you dissolve the oxygen is determined by the amount of naturally occurring water pressure that’s on the uranium deposit.” Stover explained that if the deposit is 100 feet below the water table, you can dissolve a certain amount of oxygen. “If the uranium deposit is 200 feet below the water table, or twice as deep, you can dissolve twice as much oxygen.”
Historically, ISL mining evolved from acid leaching to leaching with sodium bicarbonate or sodium carbonate. “Most people add only carbon dioxide in dissolved oxygen at this point,” Stover explained. “There’s a chemical relationship between carbon dioxide gas, bicarbonate, and the carbonate ion. The host rock typically contains calcium carbonate or sodium carbonate minerals.” By adding the carbon dioxide, Stover said, “It will lower the PH of the solution just slightly.” That enhances the solubility of the naturally occurring calcium carbonate.” According to Stover and the other experts, the addition of carbon dioxide is an effective replacement for the previously added bicarbonate ion.
The goal is to get the uranium out of the sandstone and soluble. “We’re accelerating Mother Nature and making the uranium soluble again,” said Doug Norris, engineering manager for Uranium Energy. “When it’s soluble, we can just pump it out of the ground. But it is dissolved in the water like salt in sea water. You can’t see it, but it’s there.”
“MINING” THE URANIUM
ISL “mining” and processing the uranium is a very simple process. It’s a water treatment plant with hundreds of water wells. There are two types of wells: injection and production. The water plus reagent (oxygen, carbon dioxide) is injected into the ground via water wells. Outside the United States, where environmental regulations may be less restrictive, an ISL’s aquifer may be bombarded with harsh acid leaching. On Harry Anthony’s engineering services website, he describes the process he observed in the Czech Republic, “Over 4,100,000 tons of H2SO4 (sulfuric acid), 270,000 tons of HNO3 (nitric acid), 100,000 tons of NH3 (ammonia), and 25,000 tons of HF (hydrofloric acid) were consumed by the mine.”
It would be nearly impossible to get an ISL project permitted in the United States using these chemicals to leach the uranium. The water quality division, within a state’s Department of Environmental Quality (DQE), demands restoration to background, which is about where the groundwater was before ISL mining began. “The less things you add, the less you have to reclaim at the end of the process,” Doug Norris pointed out. “The more stuff you add trying to get it out of the ground, the more you have to clean up.”
Dennis Stover explained how the fluids presently used came about, “Historically, most ISL operations had a great deal of difficulty with plugging or fouling of their injection wells due to the precipitation of excessive amounts of salts.” He pointed out that the chemistry miners were using in conventional milling operations didn’t work in ISL mining. “Because they had very high concentrated salt solutions, they were trying to accelerate everything,” Stover told us. “When you take those concentrated solutions and put them underground, Mother Nature is not always happy. Other salts that were present in the rock would dissolve, solutions would become supersaturated and they would precipitate out. The wells would plug up.”
Some of the early U.S. operations tried to enhance their production, for example, by using ammonia to enhance the pH of their water. “They forgot that ammonia is easily locked up by clay and almost impossible to get back to background,” explained Norris. “It’s pretty reactive and doesn’t occur that much in nature.” Norris would give anyone using ammonia during the mining procedure, “a 95 percent chance of having a very bad time.” Why, we asked? Norris responded, “It’s bad from the fact that nobody has been able to successfully clean up a site that has used ammonia.”
Norris explained that sometimes you have to add a carbonate source, such as carbon dioxide “to stabilize the dissolved uranium as uranyl dicarbonate.” Norris said, “The uranium is in a solid state in the ore, as Mother Nature left it. We oxidize it and turn it into uranyl dicarbonate.” What goes to the processing plant is called lixiviate, the dissolved uranium in its ionic form. According to Anthony, “Today, most ISL mining operates at neutral pH, and the uranium is complexed as a dicarbonate.”
Water is circulated through the injection wells with the expressed purpose of separating the uranium coating the sandstone. Each time you circulate the water through the orebody, you are capturing some of the uranium. Each pass through is called a pore volume. “It’s like filling up a bucket of sand with water,” explained Anthony. “Once you have the bucket full of sand, you can still pour in water. The amount of water you can pour in until you just bring it up to the top of the sand is termed a ‘pore volume.’ Pore volume is the interspatial volume.”
In Anthony’s models for operating an economic ISL plant, he calculates 20 pore volumes (PV). Porosity, or the spaces in between the sand particles, where the water can travel (permeability), helps determine how much uranium can be recovered. “It takes about 20 PV to 30PV to recover the highest percentage,” said David Miller, who was Cogema’s chief ISL geologist in the United States, before becoming President of Strathmore Minerals. “But, as the price of uranium keeps going higher, it may be economic to recover a higher percentage of the orebody. Maybe 40PV to 50PV will be possible with the direction the prices are moving. Of course, your average processed grade will go down. A few years ago, you would want to shut wells off at 15 parts per million (ppm), but now you might want to run them at 10ppm. At $50/pound uranium, you may be able to run at 7 or 8ppm.”
Typically, an ISL operation should recover about 70 percent of the uranium in the ore, under the 20PV to 30PV scenario. However, in the case of the Czech Republic’s Diamo project, once Europe’s largest uranium mining operation, only 55 percent was recovered. Clearly, the more uranium recovered with the least number of pore volumes, the lower the operating costs. Trying to recover more uranium is only possible if you have the plant capacity. Because of the rising price of uranium, we would expect more companies to attempt to recover a higher percentage of uranium. Miller warns, however, “You will not make your production quota if your plant is ‘sized’ at a certain gallons per minutes at a certain grade to meet your annual production. If you lower the average grade and fail to increase your flow rate, your annual production will decrease.”