Credit: AP | Coal acid mine drainage, which turns rivers and streams orange, may be a valuable source of rare-earth elements.
As global demand for rare-earth elements grows, the U.S. Department of Energy is investing millions of dollars to secure a potentially sustainable domestic source of these commercially critical elements from coal and coal waste products.The agency is funding more than 30 projects in this area, including work to characterize rare earths in various coal-based materials, such as coal ash, refuse rock from a coal preparation plant, acid mine drainage treatment sludge, and young lignitic coal. DOE is also funding work to develop separation and extraction processes for recovering rare earths from coal-based materials. So far, the results look promising from a technical standpoint, but researchers still need to demonstrate that the processes are economical when scaled up for commercial purposes.
The U.S. coal industry is facing tough times. Demand for coal is dwindling, and the industry is under fire for polluting streams and rivers with coal ash and acid mine drainage. But in that waste, and in some cases in the coal itself, are valuable rare-earth elements that may be readily recovered. If researchers can figure out how to extract them economically, sales of rare earths could help pay for some of the cleanup costs now borne by the coal industry and several coal-mining states.
Demand for rare-earth elements, which include the lanthanides plus scandium and yttrium, has ticked upward over the past few decades. The long list of high-tech products that rely on rare earths include cell phones, flat-screen TVs, electric cars, wind turbines, satellites, defense aircraft, and high-performance magnets.
In 2015, global demand for rare-earth elements was 149,000 metric tons per year, according to Mary Anne Alvin, technology manager of rare-earth elements at the Department of Energy’s National Energy Technology Laboratory (NETL). The U.S. consumes about 11%, or 16,000 metric tons per year.
Yet rare-earth elements “are a commodity that we do not domestically produce,” Alvin says. The U.S. gets its supply from China and elsewhere. “If there was a disturbance in the supply chain and we need these materials, especially for our national security, the question is, What do we do?” she says. For example, if the trade war between the U.S. and China escalates, China could cut off much of the U.S. supply of rare earths.
To prevent such a disruption, the Department of Energy (DOE) is investing millions of dollars in projects to develop a potentially sustainable domestic source from coal and coal waste products. Alvin oversees more than 30 DOE-funded projects in this area.
The research includes characterization of coal and coal by-products—including coal ash from power generation, refuse rock from coal preparation plants, acid mine drainage treatment sludge, and young lignitic coal from North Dakota—so that recovery efforts can focus on coal-based materials with the highest concentrations of rare earths. Other projects involve optimizing the separation and extraction of rare earths from various coal-related materials. All the feedstocks start with a minimum of 300 ppm total of rare-earth elements. For comparison, Chinese clay deposits contain 500 to 5,000 ppm total of rare earths. It is too early to tell whether the processes will be economical on a commercial scale, but DOE hopes to have economically viable prototype systems in place that produce 90 to 99% purity rare earths by 2020.
Seeking value in coal ash
“There are different types of coals and different types of ashes as a result,” says Prakash Joshi, manager of advanced systems technologies at Massachusetts-based Physical Sciences, a company that develops sensors and other technologies for defense and commercial applications. “Certain types of coal have fairly high concentrations of rare earths, on the order of hundreds of ppm,” he notes. But some coal deposits around the world can have rare-earth concentrations that reach the low thousands of parts per million, he says.
Joshi is leading a DOE-funded project at Physical Sciences, in collaboration with researchers at the University of Kentucky Center for Applied Energy Research and Pennsylvania-based Winner Water Services, a consulting firm that specializes in water treatment technologies, to develop and demonstrate a pilot-scale plant that produces high-purity rare-earth concentrates from coal ash.
“The good thing about using ash rather than coal itself is that combustion has the effect of concentrating rare earths in the ash by a factor of six to 10 relative to coal,” Joshi notes. Before doing any processing, his team collects samples and analyzes ash to ensure it contains enough rare earths to make recovery of the elements economical.
Then, rather than bringing the coal ash to the plant, Joshi and colleagues will take the plant to the ash source, whether that is a power plant, landfill, or other coal ash storage facility. “The plant will be modular,” Joshi says. “The physical processing part can fit on a flatbed, which can be moved from one location to another.”
Even though coal ash is more concentrated in rare earths than coal itself, the rare earths it contains are tightly bound in a glassy matrix. Pulling those elements out of the ash requires a combination of physical and chemical processes. Joshi’s team uses washing and other physical methods to separate unwanted materials from the ash and prepare it for further processing. The team then subjects the resulting ash, which is enriched in rare earths, to several chemical processing steps.
The chemical processing involves a preprocessing step that removes unwanted contaminants from the ash, rare-earth concentration steps, extraction or recovery steps, and post-processing steps to further enrich the product in rare earths, Joshi tells C&EN. In addition to rare earths, the process generates by-products of commercial value that enhance the process economics, such as ash that has been stripped of most hazardous substances and is marketable as a cement substitute in concrete, he says.
The pilot plant is expected to process 0.5 to 1.0 metric tons of coal ash per day, generating at least 50 g of concentrate containing more than 10% rare earths by weight. Eventually, the researchers hope to increase the plant’s daily production to 500 g of concentrate containing more than 20% rare earths by weight. Data obtained from the pilot-scale plant will be used to inform the design of a larger, commercial-scale plant. If all goes as planned, that information should be available by the end of 2020, Joshi says.
New life for rock refuse
When coal is mined, it typically contains a lot of unwanted rock material. That rock provides no heating value, so it is often removed by a coal-processing plant. Another research team, led by Rick Honaker, a professor of mining engineering at the University of Kentucky, is trying to extract rare earths from that refuse rock.
The team is building a mobile pilot-scale plant mounted on a trailer to carry out the extraction process. The plant is expected to process 0.25 metric tons per hour. When C&EN contacted Honaker, he was installing the plant at a coal-mining site in western Kentucky. He and his colleagues plan to demonstrate the plant at another location in eastern Kentucky later.
In contrast to coal ash, which starts out as a finely ground material with a particle size of about 100 µm, refuse from coal preparation plants is rocklike material that must be ground and crushed before it can be chemically processed. Once the particles are ground from about 50 to 75 mm in diameter to about 250 µm, the researchers send them through a flotation column to remove any excess coal. They can then sell that coal product.
The team processes the remaining particles, called tailings, through a column that removes calcium, a step that reduces reagent use later in the process. After removing the calcium minerals, the researchers isolate water from the tailings. They then subject the material to acid leaching and solvent extraction to recover the rare earths. So far, the team has been able to generate a final product containing about 80% total rare earths. After roasting that material, the concentration increases to about 98% or more rare-earth oxide, or 980,000 ppm, Honaker says.
Exploiting acid mine drainage
Honaker and colleagues are also trying to extract rare earths from acidic water, called acid mine drainage, which flows from coal mines containing high levels of pyrite, or iron sulfide. When pyrite is exposed to oxygen, it reacts to form sulfuric acid and ferrous (Fe2+) iron. When the mine drainage reaches a pH of 3 to 3.5, the ferrous iron reacts with oxygen and hydroxides to form ferric (Fe3+) hydroxides, which precipitate as orange solids. Those orange iron hydroxides are responsible for the discoloration seen in many streams and rivers polluted with acid mine drainage from coal mines that were abandoned before regulations were put in place in the 1970s. Acid mine drainage also typically contains high levels of aluminum and manganese.
Active coal mines are required to treat acid mine drainage before the water can be discharged into streams and rivers. They must remove iron, aluminum, and manganese and bring the pH up to 6 to 9. Many states are also treating acid mine drainage from coal mines that are no longer in business. If left untreated, acid mine drainage contaminates drinking water, harms aquatic organisms, and corrodes infrastructure such as bridges.
Treating acid mine drainage involves neutralizing water flowing out of coal mines with lime or another alkaline agent. The process creates tons of sludge containing iron, aluminum, and manganese hydroxides. The sludge is typically dried in large, open-air ponds or in dewatering containers and then disposed of, often underground. Sludge disposal costs contribute to about half the cost of treating acid mine drainage.
Honaker and colleagues are using the same pilot plant used for rock refuse to extract rare earths from acid mine drainage and the sludge that treating it leaves behind. Another group, led by Paul Ziemkiewicz, director of the West Virginia Water Research Institute at West Virginia University, is also optimizing a process to extract valuable rare earths from acid mine drainage sludge.
When DOE requested proposals for recovering rare-earth elements from coal products in 2015, Ziemkiewicz recalled a 1990s data set generated by a friend of his at the U.S. Geological Survey. The data showed that rare-earth elements are present in acid mine drainage but not in the water that is discharged from the treatment plant. He suspected that the rare earths must precipitate into the sludge during the treatment process.
He was right. After testing a few samples, the WVU team discovered that the sludge contains, on average, 2,600 times as much rare earths as the raw acid mine drainage. So far, the researchers have been able to produce an enriched solid that contains more than 90%, or 900,000 ppm, total rare earths using the sludge as their starting material.
The process includes an acid leaching step in which the sludge is digested in a large tank of acid, such as sulfuric acid. The acid solution, called the pregnant leach solution, enters an array of 100 columns called mixer-settlers. The first set of columns contains an organic phase, such as kerosene, and an extractant, which binds to rare-earth elements. This forms an emulsion containing rare earths in the organic phase. After removing unwanted metals from the aqueous phase, the researchers use a strong acid to get the rare earths back into the aqueous phase at a higher concentration than they started with. The team then raises the pH, allowing the rare earths to precipitate.
There is plenty of acid mine drainage sludge in West Virginia to keep the researchers busy. “We just completed a survey of the accessible acid mine drainage sources around the northern and central Appalachian coal basins,” Ziemkiewicz says. “We sampled 76 sites intensively. They contained 1,421 [metric] tons of rare earth elements in a form that we can run into our process here.” Ziemkiewicz estimates the value of those rare-earth elements at $337 million.
Although rare earths are much more concentrated in acid mine drainage sludge than in the acid mine drainage, it may be more economical to extract the rare earths from the acid water than the sludge. That is because the sludge contains high amounts of iron, aluminum, and manganese that must be removed. So Ziemkiewicz and colleagues are also planning to try various ways of preferentially precipitating out the rare earths directly from acid mine drainage sources before the water is treated.
“Mine acid water is naturally doing something that we are spending money to do on the refuse material,” Honaker says. It naturally leaches the refuse, “and the water ends up with a decent amount of rare-earth minerals.”
Young coal looks promising
Although most researchers are investigating coal waste material as a source of rare earths, some groups are also working to extract the elements from coal itself. Dan Laudal manages one such project focused on recovering rare earths from lignite at the University of North Dakota’s Institute for Energy Studies.
Lignite, which is found in North Dakota and Texas, is the youngest type of coal. Young coal contains a lot of organic acids called humic acids, which chelate metals and rare earths. But over time, the humic acid concentrations in coal drop, and the rare earths get incorporated into aluminosilicate clays. Once the rare earths are associated with the clays instead of humic acid, they are harder to extract.
“The reason we are using the lignite, the coal itself, is because the rare earths are tied up in such a way that they are easy to get out,” Laudal says. “We can use a mild solvent, and the rare earths will be released from the coal and go into the solvent phase,” he notes.
The researchers have found some lignite samples in North Dakota with concentrations of rare earths greater than 1,000 ppm, although most lignite typically contains rare earths in the range of hundreds of parts per million. Laudal and colleagues have demonstrated they can generate a product with 60 to 90% (600,000 to 900,000 ppm) rare earths from lignite. They are now scaling up their solvent extraction process to a small pilot-scale system. The plant is expected to process lignite at a rate of 10 to 20 kg per hour.
“Technically, things are looking really great,” Laudal says. But the researchers still have to demonstrate that the scaled-up process is economical. One of the added benefits of using lignite as the feedstock is that the process produces cleaner-burning, more-efficient lignite as a by-product, which can then be sold for fuel, Laudal notes.
Revealing the richest rare-earth sources
Laudal is also a coinvestigator on another DOE-funded project to analyze the rare-earth content in coal from western regions, including lignites from North Dakota and Texas and subbituminous coal from Montana and Wyoming. His team is partnering with researchers at the University of Kentucky, who are characterizing rare earths in coal in the Appalachian and Illinois Basin regions.
Three to four types of coal samples will be analyzed by eight to 10 commercial laboratories so the results can be compared to understand the reproducibility of the analytical methods, Laudal says.
For the most part, rare earths are analyzed by inductively coupled plasma mass spectrometry. The analysis is a “bit of a challenge because it is operator intensive,” Laudal says. The analysis involves acid digestion and other steps to bring the rare earths into solution. “It can take a few days if not more to get samples turned around,” Laudal notes.
DOE is funding several efforts to develop more-streamlined methods for detecting rare-earth elements, Alvin says. Many DOE scientists at NETL are also working in this area, as are researchers at several DOE national laboratories, she notes.
At NETL, for example, researchers have developed a fiber-optic sensor to detect rare earths in liquid samples. The sensor can be used on-site to characterize rare earths in liquid sources such as acid mine drainage, Alvin says. At Los Alamos National Laboratory, researchers are investigating whether the separation technology used to extract actinides from uranium can be transferred to extract and recover lanthanide products from coal, Alvin says. At Idaho National Laboratory, scientists are developing biosensors to detect rare earths in fluids, she adds.
DOE scientists at NETL are also spearheading an effort to detect rare earths using a technique called laser-induced breakdown spectroscopy (LIBS). The method is amenable to solids, liquids, and gases, says Tom Tarka, who oversees the in-house DOE work on recovering rare earths from coal products at NETL. The sensitivity of the LIBS technique is on the order of hundreds of parts per million, but the team is trying to optimize the method to detect lower concentrations of rare earths, Tarka notes. They are also developing a field-portable LIBS instrument, he says.
Another group of DOE scientists at NETL is using advanced microscopy to better understand where rare earths occur in coal—whether they are associated with the inorganic or organic components, Tarka says. In addition, NETL researchers are developing methods for predicting where rare earths will occur using geologic history, he notes. “We know that a lot of rare-earth elements are the result of volcanic deposition,” Tarka tells C&EN. By investigating where volcanoes erupted and where they deposited the rare earths, researchers can predict locations that likely have high concentrations of rare-earth elements, he explains. The DOE scientists are using the information to develop an assessment method “similar to what would have been developed for gold mining or uranium mining,” Tarka says.
Working on projects such as understanding the geology of where rare-earth elements turn up and the development of extraction methods, researchers have demonstrated that they can recover high percentages of rare earths to produce high-purity, salable products from a variety of coal-related sources. The question now becomes, Are the processes economical? “You can do the economic analysis up front, which we are on all of our projects,” Alvin notes. “But once we scale up the prototype system, that is where the true economics come into play.” There is also the question of how much can be produced. Will it be enough to meet demand? By 2020, DOE officials hope to have the answers to those questions.