Reversing Mother Nature, Part One
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.
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