Optimizing The Mining Of Uranium From Coal Ash And Seawater

Of all the elements that make up the Earth’s crust, uranium is reasonably abundant, coming in at 49th place, ahead of elements such as tin, tungsten and silver. Ever since humankind began to exploit uranium for its fissile properties in energy production, this abundance has also translated into widespread availability for mining. As of 2019, Kazakhstan, Canada and Australia formed the world’s main producers, accounting for about 68% of output.

Considering the enormous energy density of uranium when used as fuel in a nuclear fission reactor, the demand for uranium is relatively low, especially combined with the long (two years on average) refueling cycles of commercial reactors. The effect is that even with the very inefficient once-through fuel cycle – which only uses a fraction of the uranium fuel’s potential energy – uranium market prices have remained relatively low and stable even amidst geopolitical crises.

Despite this, the gradual rise in uranium market prices ($10/lb in 2003, $49/lb in 2022), as well as the rapid construction of new reactors is driving new exploration. Here recent innovations may make uranium fuel even more accessible to all nations, by unlocking the billions of tons of uranium found in plain seawater as well as the many tons of fly ash produced by coal plants every single day.

It’s The Economy, Silly

The primary reason why most nuclear plant operators opt for a once-through fuel cycle that only uses a fraction of the fissile U-235, is that the fuel costs for a nuclear plant are so incredibly low. While the up-front costs for a new, GW-level nuclear power plant are considerable, the operating costs over its 40-100 year lifespan are very low, which is where the added cost that reprocessing spent fuel, to remove the newly formed actinides and transuranics, makes little economic sense.

This economic angle is also one of the factors that has made the development of fast neutron reactors (FNRs) unattractive. While these FNRs can breed their own fuel from fertile isotopes through neutron capture, they are more complicated and expensive than a light water reactor — virtually all commercial nuclear plants in operation today. These factors all play a role in whether certain uranium resources are economical to extract from the ground or other sources, while making it quite obvious why extracting uranium from fly ash and seawater wasn’t economical before.

Even though the Earth’s seas and oceans contain an estimated 4.6 billion tons of uranium, the dilution of it into these vast waters means that that you have to filter out a substance of which there are only a few molecules per billion. Similarly for fly ash, separating the uranium from the other components in the ash has to be done in a way that is efficient enough to compete with existing mining methods, such as in-situ leaching (ISL).

However, another potential benefit of extracting uranium from seawater and fly ash is that it can fully avoid the environmental impact of traditional mining, while potentially helping with addressing the environmental hazard posed by fly ash.

Coal Waste Problem

Coal ash consists out of the ash that remains at the bottom of the boiler along with the fine ash, or fly ash, that is captured by electrostatic precipitators or similar equipment installed in or near the coal plant’s chimneys. It contains some left-over carbon, along with large amounts of silicon dioxide, aluminium oxide and calcium dioxide. In addition, it contains trace elements of many heavy metals and similarly problematic substances, including arsenic, cadmium, hexavalent chromium, lead and mercury.

Finding a way to recycle or process fly ash has been an increasingly problematic topic, especially now that fly ash is generally not released into the atmosphere any more, but instead stored in massive coal ash ponds. These ponds form a potential hazard, as illustrated by the 2008 Kingston Fossil Fuel Plant slurry spill, that released 4.2 million cubic meters (1.1 billion US gallons) of fly ash mixed with water into the adjacent Emory River. The subsequent clean-up cost the lives of approximately 40 workers from exposure to the hazardous chemicals.

While fly ash is increasingly mixed into everything from concrete to tarmac and toothpaste, the sheer amount produced means that a significant amount of the approximately 34.7×10⁶ tons of fly ash produced by coal plants in the US alone each year ends up in landfills. Mining fly ash for useful elements like uranium could help with reducing its total volume, while potentially making it easier to use the left-over material.

As early as 2007, Sparton Resources from Canada reported producing yellowcake (mostly U₃O₈) from fly ash from a Chinese coal-fired plant. Uranium levels were found to be about 160 ppm, corresponding to about 0.2 kg of yellowcake from a ton of ash. As noted by The Economist in a 2010 article, this compares to the 1,000 ppm or more in uranium ore. Using a process involving sulfuric and hydrochloric acid that is reminiscent of in-situ leeching, the uranium and other dissolved elements are then filtered out and precipitated using ammonium carbonate.

At that point in 2010, Sparton claimed to be able to extract a kilogram of uranium this way for $77, with the uranium spot price market then being $90. Most of the R&D on this topic appears to occur in China, where it is considered a viable source of uranium fuel, especially in light of the more than hundred new nuclear reactors China has planned or is in the process of constructing, and its focus on self-reliance.

Sun et al. (2016) describe the extraction of uranium from bottom ash, and the high concentration of uranium (374 mg/kg) in the bottom ash remaining after high-germanium containing coal was burned. Although uranium mined from CFA isn’t a big industry yet, ongoing research and small-scale trials as described show the viability of this approach.

Mining The Oceans

The concept of filtering uranium out of sea- and ocean waters is a simple one: essentially it comes down to pumping as much water as possible through a filter system that will retain the elements we are interested in, after which the uranyl molecules can be further processed into reactor fuel. Where things get tricky is that although the oceans contain hundreds of times more easily accessible uranium than dry land, the low density of a few particles per billion (ppb) makes it essential to have a highly efficient filtering method.

As noted by a 2017 Stanford article, the approach used at that point involved sticking plastic fibers containing a compound called amidoxime into the water and waiting for the fibers to become saturated with uranyl, which can then be processed. In 2018, PNNL reported recovering a whole gram of yellowcake from seawater this way.

In a study published last year in Nature, Yang et al. (2021), demonstrate an alternative approach where seawater is led through a hierarchical porous membrane, not unlike the branching network of a lung or blood vessels. By coating the insides of the thus formed channels with amidoxime and forcing uranium-containing water through them, they demonstrated a significantly improved extraction rate over previous attempts, mostly due to the massively increased surface area, and the increased flow of water across the amidoxime-coated material.

Once saturation has been reached, the uranium can be dissolved using hydrochloric acid, following the usual processing approach into fuel for reactors. The thus cleaned membrane can then be reused many times, making it a potentially economical option, and a way to extract uranium for one’s own needs for any nation that has access to a body of sea- or ocean water.

Not Running Out Soon

Unlike fossil fuels, which are beginning to see some significant production drops as many of the best oil, coal and gas fields have been exploited to the point of exhaustion, fission reactors using only uranium as fuel can sustain human society at its current energy levels for thousands of years, even without significant investment in the reprocessing of spent fuel, or in fast neutron reactors. Perhaps even more tantalizing is that it adds the cleaning up of the toxic waste created by coal plants as a feature of this transition.

In addition to exploiting fly ash for uranium, some researchers are trying to determine whether rare-earth elements can perhaps also be economically recovered from these ashes. In this way the pollution of yesterday may conceivably help power the future, in an ironic twist.

Banner image: Chien Wai of PNNL holding up vial with 5 grams of yellowcake recovered from seawater.