Mining And Refining: Uranium And Plutonium | Hackaday

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When I was a kid we used to go to a place we just called "The Book Barn." It was pretty descriptive, as it was just a barn filled with old books. It smelled pretty much like you’d expect a barn filled with old books to smell, and it was a fantastic place to browse — all of the charm of an old library with none of the organization. On one visit I found a stack of old magazines, including a couple of Popular Mechanics from the late 1940s. The cover art always looked like pulp science fiction, with a pipe-smoking father coming home from work to his suburban home in a flying car.

But the issue that caught my eye had a cover showing a couple of rugged men in a Jeep, bouncing around the desert with a Geiger counter. "Build your own uranium detector," the caption implored, suggesting that the next gold rush was underway and that anyone could get in on the action. The world was a much more optimistic place back then, looking forward as it was to a nuclear-powered future with electricity "too cheap to meter." The fact that sudden death in an expanding ball of radioactive plasma was potentially the other side of that coin never seemed to matter that much; one tends to abstract away realities that are too big to comprehend.

Things are more complicated now, but uranium remains important. Not only is it needed to build new nuclear weapons and maintain the existing stockpile, it’s also an important part of the mix of non-fossil-fuel electricity options we’re going to need going forward. And getting it out of the ground and turned into useful materials, including its radioactive offspring plutonium, is anything but easy.

Lixiviants and Leachates

Despite its rarity in everyday life, uranium is surprisingly abundant. It’s literally as common as dirt; stick a shovel into the ground almost anywhere on Earth and you’ll probably come up with a detectable amount of uranium. The same goes for seawater, which has about 3.3 micrograms of uranium dissolved in every liter, on average. But as with most elements, uranium isn’t evenly distributed, resulting in deposits that are far easier to exploit commercially than others. Australia is the winner of this atomic lottery, with over 2 million tonnes of proven reserves, followed by Kazakhstan with almost a million tonnes, and Canada with 873,000.

While most of the attention uranium garners has to do with the properties of its large, barely stable nucleus, the element also participates in a lot of chemical reactions, thanks to its 92 electrons. The most common uranium compounds are oxides like uranium (IV) oxide, or uranium dioxide (UO2), the main mineral in the ore uranite, also known as pitchblende. Uranite also contains some triuranium octoxide (U3O8), which forms when UO2 reacts with atmospheric oxygen. The oxides make up the bulk of commercially significant ores, with at least a dozen other minerals including uranium silicates, titanates, phosphates, and vanadates being mined somewhere in the world.

Getting uranium out of the ground used to be accomplished through traditional hard-rock mining techniques, where ore is harvested from open-pit mines or via shafts and tunnels running into concentrated seams. The ore is then put through the usual methods of extraction that we’ve seen before in this series, such as crushing and grinding followed by physical separation steps like centrifugation, froth flotation, and filtration. However, the unique chemical properties of uranium, especially its ready solubility, make in situ leaching (ISL) an attractive alternative to traditional extraction.

ISL is a hydrometallurgical process that has become the predominant extraction method for uranium. ISL begins by drilling boreholes into an ore-bearing seam, either from drill rigs on the surface or via tunnels and shafts dug by traditional mining methods. The boreholes are then connected to injection wells that pump a chemical leaching agent or lixiviant into the holes. For uranium, the lixiviant is based on the minerals in the ore and the surrounding rock, and is generally something like a dilute sulfuric acid or an aqueous solution of sodium bicarbonate. Oxygen is often added to the solution, either via the addition of hydrogen peroxide or by bubbling air through the lixivant. The solution reacts with and solubilizes the uranium minerals in the ore seam.

ISL offers huge advantages compared to conventional mining. Although uranium is abundant, it’s still only a small percentage of the volume of the rock bearing it, and conventional mining requires massive amounts of material to be drilled and blasted out of the ground and transported to the surface for processing. ISL, on the other hand, gets the uranium into aqueous solution while it’s still in the ground, meaning it can be pumped to the processing plant. This makes ISL a more continuous flow process, as opposed to the more batch-wise processing methods of conventional...