Researchers at The Ohio State University have developed a novel process to clean coal mine drainage and extract rare-earth elements from it. In an open-access paper in the journal Environmental Engineering Science, the team reports that the process successfully cleaned clean coal mine drainage while producing rare-earth elements in samples from various rivers across Ohio, Pennsylvania, and West Virginia.
Coal mine drainage (CMD) impairs tens of thousands of kilometers of U.S. waterways each year, in part with the leaching of low concentrations of rare earth elements (REEs). REEs are essential for modern technologies, yet economically viable natural deposits are geospatially limited, thus engendering geopolitical concerns, and their mining is energy intense and environmentally destructive.
This work summarizes laboratory-scale experimental results of a trap-extract-precipitate (TEP) process and uses the mass and energy balances to estimate the economic costs and environmental impacts of the TEP. The TEP process uses the alkalinity and filtering capacity of stabilized flue gas desulfurization (sFGD) material or water treatment plant (WTP) sludge to remediate CMD waters and extract REEs.
Passive treatment systems that use WTP sludge are cheaper than those that use sFGD material ($89,300/year or $86/gT-REE vs. $89,800/year or $278/gT-REE) and have improved environmental performance across all indicators from two different impact assessment methods. These differences are largely attributable to the larger neutralizing capacity of WTP sludge in the treatment application.—Miranda et al.
The TEP process to produce a REE feedstock while neutralizing CMD. CMD, coal mine drainage; TEP, trap-extract-precipitate. Miranda et al.
Currently, coal mine drainage is treated using active treatment systems which employ chemicals to clean the water, or passive treatment systems, which often depend on bacterial activity or geochemical methods.
According to the study, passive approaches tend to require fewer resources and have fewer environmental impacts. The Ohio State team used a passive system employing a combination of alkaline industrial byproducts, including materials such as water treatment plant sludge, to neutralize the coal drainage and capture the rare earth elements.cThe average time it takes to rid water of waste often varies, because the process largely depends on how quickly water flows out from the mine.
The process captured a variety of metals used in modern technology, including terbium, neodymium and europium. The process is currently more costly than the current market price of rare metals, but further advances will bring the price down, said Jeff Bielicki, corresponding author.
One thing that surprised me was just how well our process cleans up the water. From an environmental standpoint, the major benefit of this work is that we’re successfully trapping and neutralizing so much pollution.—Jeff Bielicki
Bielicki said he hopes the research will inform future policy surrounding coal waste disposal and help the public to examine the environmental repercussions of mining outside of typical costs, such as its impact on human health and the ecosystem at large.
This work was supported by the Environmental Research Education Foundation and the US Department of Energy.
Marcos M. Miranda, Jeffrey M. Bielicki, Soomin Chun, and Chin-Min Cheng (2022) “Recovering Rare Earth Elements from Coal Mine Drainage Using Industrial Byproducts: Environmental and Economic Consequences” Environmental Engineering Science doi: 10.1089/ees.2021.0378