We talked to North Americaís 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.
ISL EXTRACTION AND PROCESSING
During ISL mining, water is pumped to the surface from production wells that contain uranium in very low concentrations, on the order of parts per million concentrations. The next step in the ISL process is to extract the uranium dicarbonate. Extraction is done by chemically exchanging ions inside a processing facility. ìThe ion exchange process is very analogous to a home CulliganÆ water softener,î Anthony revealed. ìIt removes hardness or calcium from the water by replacing it with sodium, using ion exchange resins. If you go to Loweís or Home Depot, and buy a water softener, you basically have a home version of a uranium extraction plant.î The main difference is your water softener will have a cation exchanger. ìFor a uranium plant to function properly, you need to use an anion exchange resin, which is specifically designed to load uranium,î Anthony clarified.
And what is this magical ìion exchange resinî? The resin is comprised of little polymer beads, which are charged particles having an affinity for uranium anions. ìThere are literally millions of these small resin beads in a vessel, which can adsorb low concentration of uranium in solution,î said Anthony. Adsorption is when something is attracted to something else or clings to it, like static electricity.
Why do you have to process uranium like this? ìIn essence, the ion exchange process is a beneficiation (reduction) process that concentrates large volumes of low concentrate uranium solution into a much smaller volume containing a much higher concentration of uranium,î said Anthony. In other words, the beneficiation is just concentrating the uranium from the large volume of water in which it is mined into a more compact form. The preferred means is through an ion exchange.
Anthony gave a real-life example of the beneficiation process, ìThree million gallons of wellfield solution containing dilute concentrations of uranium, of 100 parts per million minus 0.10 grams/liter, is passed through a bed of ion exchange resin. This might take 24 hours to achieve if the solution is flowing at 2,500 gallons per minute. After this length of time, the resin becomes loaded with approximately 2,500 pounds of uranium.î
STRIPPING THE URANIUM
Stripping the uranium is called the elution process. This is done through a chemical exchange of positively and negatively charged ions. Resins are classified by the charge on the active sites. ìThe active sites on the resin are positively charged for anion resins and negatively charged for cation resins,î Norris enlightened us. ìThe resinís ability to extract chemical ions from a solution is derived from whatís called an active site,î he continued. ìIn our case, chloride ions obtained from ordinary tale salt are used to stabilize or temporarily neutralize this positively charged active site.î The negatively charged chloride ion sticks to the positively charged site, held in place by what Norris called ìelectrostatic forces.î When the negatively charged ions, such as uranyl dicarbonate, are placed in contact with the solution, it will kick off the chloride and replace that with the uranyl dicarbonate.
That was the chemistry lesson. Anthony summed it up in a nutshell, ìThey just displace it. Thereís a greater affinity for the chloride ion to the resin than there is for the uranium. So, the uranium is stripped from the resin bed.î The processing facility chemically strips the loaded uranium from the resin by soaking the entire package of uranium-laden resin in a salt bath solution. ìThe volume of salt solution is on the order of 10,000 gallons resulting in a solution concentration of 30 grams/liter uranium,î Anthony said, describing the process of how the uranium becomes concentrated. ìThe stripped uranium solution concentration is magnified 300 times more than the wellfield solution,î he informed us. ìThe concentration level can now be economically processed for recovery: precipitation, dewatering, drying and drumming for a nuclear facility.î
GETTING URANIUM INTO THE DRUM
After the uranium has been removed from the solution, it is precipitated. At this point in the processing stage, you have yellowcake slurry. Up close, it looks like a sort of yellowish and wet, runny cement mixture. The dewatering process does just that, it removes the water from the yellowcake mixture.
ìI use a filter press, a device that is designed to separate solids from solutions,î explained Anthony. Filter presses are extensively used in various types of food, chemical and drug processing across the world. ìThe uranium solids, now looking more like yellowcake, are retained in the filter press, where they can be washed and later air dried, before drying them to a powder with a low temperature vacuum dryer,î said Anthony taking us step by step through this process.
So what is the filter press and how do you end up with the finished yellowcake when youíre done? ìItís a series of plates and hollow frames, or it could be a series of recessed chambers,î Anthony answered. ìFilter cloth is draped over the plates or chalked in the recessed chambers. The yellowcake slurry is pumped through the filter allowing the liquid phase to pass through the filter cloth, trapping the uranium oxide inside the device.î Anthony likes to pack the filter press up with as much yellowcake as it can hold. ìIt is then washed with clean water to displace the chloride ions to a low level,î Anthony explained. If you donít remove the chloride concentrations to the acceptable level required by an uranium enrichment facility, a fine is assessed against that shipment.
The final steps include conveying the yellowcake to the vacuum dryer. The uranium oxideís color depends on how high or low a temperature is used to dry the ìyellowcake.î Patrick Drummond, the Smith-Highland Ranch plant superintendent, showed us pure uranium oxide dried at high temperatures. It was nearly black. After the drying process is complete, the uranium is packaged up in DOE-approved 55 gallon drums and transported to an enrichment facility. It is then when the enriched uranium can finally be used to power a nuclear reactor and provide an inexpensive source of electricity.
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Tag(s): Lance Braithwaite