This facility will extract rare earths from coal wasteEngineering360 News Desk | May 30, 2019
Editor's Note: News reports in late May said that China may consider reducing its supply of rare earths materials to the United States as part of an escalating trade disagreement. China is the source of more than 75% of the world's rare earths, which are used for electronics and electric vehicles, among other uses. The following article first was published at Engineering360 in July 2018 and remains relevant with the recent news developments.
Researchers at West Virginia University are opening a new facility to capture rare earth metals from acid mine drainage from coal mining.
Through an R&D program with the Energy Department's National Energy Technology Laboratory, WVU is opening the Rare Earth Extraction Facility.
The university is partnering with Rockwell Automation to facilitate market readiness through use of the company's sensor and control technologies.
The facility follows on an earlier project to study acid mine drainage as feedstock for rare-earth extraction. The goal of the pilot facility is to test the technical and economic feasibility of scaling up the technology to commercialize the separation and extraction process.
In addition, the team will be working to define a U.S.-based supply chain including the sludges created during acid mine drainage treatment and upstream to the acid-mine drainage source.
In December 2017, high concentrations of rare earth elements (REEs) were reported to have been documented in coal samples collected from several American coal basins. Analyses conducted by researchers from the NETL demonstrated REE concentrations exceeding 300 parts per million (ppm) in coal from the Illinois, Northern Appalachian, Central Appalachian and Rocky Mountain coal basins, and from the Pennsylvania Anthracite region.
Concentrations of rare earths at 300 ppm are vital to the commercial viability of extracting REEs from coal and coal byproducts, the earlier research showed.
The name “rare earth elements” is a misnomer for chemical elements that are actually neither rare nor earths, the researchers say.
A collection of 16 elements that hang off the bottom of the periodic table, they are moderately abundant but well dispersed in the Earth’s crust. They are identified as rare because it is unusual to find them in large concentrations.
The elements are all metals that carry similar properties. In rare cases they are found in deposits together. Unlike an element such as gold, natural rare earth deposits never occur as pure metals, but are bonded in low-value minerals, making extraction challenging.
Conventional rare-earth recovery methods typically generate large volumes of contaminated waste. China has been able to provide a low-cost supply of rare earths using these methods, and dominates the global market.
The conventional mining and extraction processes require mining ore from mineral deposits in rock, which is crushed into a powder, dissolved in chemical solutions and filtered. The process is repeated multiple times to retrieve rare earth oxides. Additional processing and refining separates the oxides from their tight bonds and further groups them into light rare earths and heavy rare earths.
In usable form, these elements are necessary components of modern technologies. They are used in cellular phones, computers, televisions, magnets, batteries, catalytic converters, defense applications and many more segments of modern society.
Two-Step Extraction Process
Paul Ziemkiewicz, director of the West Virginia Water Research Institute and principal investigator on the project, says that acid mine drainage, a byproduct of coal mining, “naturally” concentrates rare earths. Active coal mines, and in many cases state agencies, are required to treat the waste, which in turn, yields solids that are enriched in rare earth elements.
“Acid mine drainage from abandoned mines is the biggest industrial pollution source in Appalachian streams, and it turns out that these huge volumes of waste are essentially pre-processed and serve as good rare earth feedstock,” Ziemkiewicz says. “Coal contains all of the rare earth elements, but it has a substantial amount of the heavy rare earths that are particularly valuable.”
Studies show that the Appalachian basin could produce 800 tons of rare earth elements per year, approximately the amount the U.S. defense industry would need.
The researchers are using a two-step process to separate the rare earths from acid mine drainage: acid leaching and solvent extraction, which they call ALSX.
Researchers will dissolve the sludge in an acid. That solution will then be transferred to glass mixers and settlers that will make an emulsion that allows the oil phase and its extractant chemical to grab rare earths from the water, leaving the non-rare earth base metals like iron in the water
When that process is completed, the rare-earth-laden organic liquid enters another series of mixers and settlers that will strip the rare earths out as a concentrated solution and precipitate the rare earths as a solid, creating a concentrated rare earth oxide that can then be refined and further concentrated into pure rare earth metals to supply the metal refining industry.
The goal of the project is to produce three grams of rare earth concentrate per hour.
For example, scandium, one of the rare earths, is worth about $4,500 per kilogram as an oxide, the form that it will leave the facility, researchers say. After refining, it would be worth $15,000 per kilogram.
Unused materials would be returned to the acid mine drainage treatment plant’s disposal system, resulting in what researchers say is a negligible environmental footprint.
“This process uses an existing waste product that is abundant in our region,” Ziemkiewicz said. “It is also much easier to extract and requires much milder acids and has negligible waste materials when compared to conventional rare-earth recovery methods.”