DOE Promotes Innovative Approach to Rare Earth Element Production from Coal Fly Ash
Abstract
Rare Earth Elements (REEs) play a critical role in advanced technologies, including consumer electronics, renewable wind powered energy systems, and strategic defense applications such as advanced radar systems. However, the global supply chain for these materials remains vulnerable, as approximately 90% of REE processing occurs in a single country—China. To mitigate this dependency and ensure supply security, the U.S. Department of Energy (DOE) and other federal agencies are actively pursuing alternative sources and innovative production methods. One promising avenue identified by the DOE involves recovering REEs from coal combustion residuals, a byproduct of power generation. Significant concentrations of REEs have been detected in coal fly ash, prompting DOE initiatives to fund and accelerate the development of extraction and purification technologies aimed at establishing a sustainable domestic supply of these essential materials.
Ancient Forests to Fossil Fuel
It took millions of years to make the coal that is used today to light our homes and power the internet. 300 million years ago, when the oxygen content of the atmosphere was 10% higher, dense forests and swamps covered much of the Earth. During this period known as the Carboniferous Period, when plants died, they fell into swampy waterlogged environments. Because of the lack of oxygen in the swamps, the plant material did not fully decompose and instead formed a peat material, which is a soft spongy accumulation of decaying vegetation. Over time, mud and sand accumulated on top of the peat. The weight of layers of sediment compressed the peat expelling the water and compacted the material. Over millions of years as the peat was buried deeper, it was subjected to increasing heat and pressure causing chemical and physical changes. Moisture and volatile solids were removed and the carbon content increased. This was the formation of coal. There are different types ranking in carbon content depending on the depth, temperature, pressure, and time of incubation. It is called a fossil fuel because ancient forests that never completely decomposed are the raw ingredient. All of the vast amounts of coal that exist are a fixed amount and may never be generated again.
Status of Coal-Fired Power
Estimates vary, but 205 to 227 coal-fired power plants are currently in operation in the United States. With DOE emergency orders, about 16 coal plants are operationally idle and reserved for emergency use. A power plant might have have 2-6 power generating coal fired units which are individual boiler-turbine-generator systems. Since 2000, there have been 790 individual coal-fired units retired. Between 2010 and 2019, 290 coal-fired power plants were closed. In 2024, 4.74 GW of coal generation power was taken offline. In 2023, the amount of coal power taken out was 12.4 GW. This power was replaced by other types of power generation while power grids across the country have struggled to keep up with expanding power demand.
When coal is burned to make heat to generate steam, several elements are oxidized and become gases. Coal is mostly carbon, but it also contains hydrogen, sulfur, and nitrogen. These become CO2 carbon dioxide, H20 water vapor, SO2 sulfur dioxide, and NOx nitrogen oxides. Minerals are left behind as an ash residue. Fly ash is the light fine ash powder that exhausts with the gases. Bottom ash are residuals that collect on the base of the combustion chamber. The small particulates that rise with the flue gas exhaust, or fly ash, are collected with electrostatic precipitators and then stored on the power plant property. While it is being stored rainwater will percolate through the ash collecting various pollutants. This leachate is collected and stored in large ponds that should be lined to prevent seepage and groundwater contamination.
Coal Combustion Residuals
Table: Estimated Production of Coal Combustion Residuals (CCR) Including Fly Ash
Estimates are based on CCR yield of 15% from total Coal Consumed by Country
In 2023, 66.7 million tons of coal combustion products were produced domestically. Of that, 11.9 million tons were used a concreate additive. Globally, roughly 600 million tons was produced in 2022. Coal fly ash can contain particles from 0.1 µm to 100 µm. The leachate from fly ash piles can contain arsenic, mercury, lead, cadmium, high levels of sulfate, and other heavy metals.
Unlike any other power producing source of energy, the residual from coal combustion has value. Fly ash is primarily used as an additive in concrete improving its durability and strength. Because fly ash particles are very fine, they fill microscopic voids that make concrete more dense and less permeable. This means better resistance to chloride and sulfate attack which are the main causes of deterioration of concrete. The addition of fly ash also improves the strength by reacting with calcium hydroxide forming an additional calcium silicate hydrate compounds which are the main strength giving ingredient in cured concrete. Fly ash also lowers the alkali content which reduces the risk of alkali-silica reactions that cause cracking.
There are many other beneficial uses. It can be used as an asphalt filler and for stabilizing road beds for highways. It reduces the shrink-swell behavior of clay soils. Because it acts like Portland cement it can be made into bricks that are lighter than clay bricks. It can be used for waste stabilization for hazardous sludge materials by making it a solids that can be encapsulated and safely disposed of. It is also used for mine reclamation and structural fill material. It is estimated that the global market for fly ash is $13.5B USD. However, most fly ash is not reused and is simply sent to a landfill.
DOE Identifies Additional Value-Added Minerals
This has been the long-standing value and use. While there is some value, beneficial uses have really been a way to economically dispose of the material that accumulate at a power plant. However, recently, the Department of Energy (DOE) estimates that there are several million tons of Rare Earth Elements (REE) that could be recovered from fly ash and coal mine residues. The total value of REEs could exceed $50 billion USD. The DOE has issued a report and is funding several projects to determine feasibility. There are many challenges to overcome including extraction efficiency, purity levels, environmentally safe practices, and variations in the REE present in different regions of coal.
The DOE has already demonstrated methods for extracting these metals at its Battelle facility. Acid leaching is used to dissolve metals present in the ash. Aluminum and Zinc are often found with rare earth metals and their presence can interfere with REE purification processes. Aluminum is easy to remove, but Zinc requires a special extractant. The result of this demonstration was to produce 90% pure Lanthanum Oxide.
As part of the national security concern about critical minerals, the DOE is enabling early commercial production of REEs from unconventional sources. Several universities are currently working on this process development. The DOE has emphasized the need for more R&D to work on the technical challenges and the need for integration of advanced technologies for efficient and scalable mineral recovery.
The Importance of Rare Earth Metals
Rare earth elements are a group of 17 chemically similar metallic elements. They have many unique magnetic, luminescent, and electrochemical properties. For example, Neodymium can be magnetized to be the strongest magnet of any element. Europium is used in phosphors for LED and fluorescent lighting. Cerium is the main catalyst in automotive catalytic converters. REE metals are used in cell phones, wind turbines, electric vehicle motors, computers, and flat screen TVS. They also have strategic uses for defense being used in guidance systems, lasers, and radar technology. Many of these products could not be made without these unique elements.
China controls nearly all of the world’s processing capabilities. It produces about 69% of the world’s supply of REE through mining operations. After the REE metals are mined, they need to be refined and processed into high purity metals. China accounts for 90% of the world’s processing capacity; There are significant concerns about supply chains and the one source method of obtaining these materials. The DOE and the US government have taken action to encourage mining and processing of REE outside of China to insure secure supplies of these critical minerals in the future. Because China has a basic monopoly on REE production, they can manipulate the price to make it impossible for new companies to enter the market.
Rare Earth Elements are Found in Coal Fly Ash
The REEs commonly found in coal byproducts include: Lanthanum, Cerium, Neodymium, Praseodymium, Samarium, Gadolinium, Yttrium, Dysprosium, Europium, and Terbium. These will vary by region. Coal fly ash typically has ~400 mg/kg of REE which is 5-10 times higher than the content in the coal feedstock. REEs are retained and encapsulated in aluminosilicate glass phases that form during combustion. Coal fly ash has much lower concentrations than found in typical ore bodies that are mine. However, it is very abundant and is already a waste product. So the key to recovering this value will be efficient refinement. The operation of mining and crushing is removed and so this would just compete with typical REE refinement and processing.
The northern Appalachian coal basin fly ash contains Neodymium, Dysprosium, and Yttrium. Central Appalachia contains Praseodymium, Samarium, and Gadolinium. Rocky mountain coal contains a mixture of REE. Even though they are called “Rare” Earth Elements, they are plentiful in the Earth’s crust. The problem is that they are found in low concentrations. REEs are typically sparingly soluble and will not leach out of the fly ash. They are also encapsulated in the aluminosilicate glass phase and for this reason, less than 1% will end up in fly ash leachate.
While studies are currently being done on various extraction methods, rare earth metals are already mined and extracted using acid leaching. So this process is well known. The REE found in fly ash leachate are present as metal oxides that are strongly bound to the glass phase formed during combustion. But, the DOE has shown that these can be recovered using acid leaching. The key to unlocking this hidden wealth will be developing efficient advanced technologies to enhance separation and purification. These critical minerals have been hibernating for 300 million years and unlocking them could provide a new sustainable source of strategic materials.
About the Author – Greg Johnson has been the CEO of New Logic Research since 2000. New Logic Research has been a trusted provider of water treatment solutions for fly ash leachate and other effluents for the power industry since 1987. www.vsep.com