Acid mine drainage may offer rare earths
New process could turn enviro concern into producing asset Metal Tech News Weekly Edition August 12, 2020
Last updated 8/26/2020 at 4:13am
Penn State scientists have developed a process that could transform acid mine drainage from an environmental liability that is costly to an asset that produces rare earths and other valuable minerals.
"We are currently incurring costs just to treat the water, and in many cases, we are not even collecting all these minerals," said Sarma Pisupati, professor of energy and mineral engineering at Penn State. "Now we are able to turn what had been considered a waste product into a valuable resource."
Acid mine drainage occurs when mining exposes sulfide containing minerals, which reacts to air and water to form sulfuric acid. While this process sometimes occurs naturally, it can be more pronounced after mining if the exposed sulfides are not cut off from sources of water and air. The acidic runoff from such mines often carry excessive quantities of metals, many of which are toxic to plants and wildlife.
"Acid mine drainage has been a significant environmental concern for many decades," said Mohammad Rezaee, assistant professor of mining engineering in the College of Earth and Mineral Sciences at Penn State.
Rezaee and his colleagues have developed a two-stage treatment process that can recover more of the metals out of acid mine drainage, while using less chemicals than earlier techniques.
"This research shows we can modify existing treatment processes in a way that not only addresses environmental concerns, but at the same time recovers valuable elements and actually decreases the cost of treatment," Rezaee added.
The traditional process involves collecting acid mine drainage in ponds and adding chemicals to neutralize the pH, which causes the dissolved metals to solidify and drop out of the water. The Penn State researchers said that about 70% of rare earths can be extracted as a sludge using this process.
This could provide a valuable domestic source of these rare earths used in an increasing number of high-tech, renewable energy and other applications.
"This technique represents an efficient, low-cost and environmentally friendly method to extract these valuable minerals that are used in a wide variety of consumer and industrial products," said Pisupati, who is also director of the Center for Critical Minerals at Penn State.
By first injecting carbon dioxide into acid mine drainage, a process that produces a carbon mineral called carbonatites, the Penn State team found it could recover more metals at higher pH values. This is because rare earths and other metals latch onto the carbonatites and more readily settle out of the water.
While transforming carbon dioxide to rocks is an emerging technique for removing carbon dioxide from the atmosphere, the Penn State research is the first time carbon dioxide mineralization has been applied to treating acid mine drainage.
This could be particularly useful at legacy coal mines in the Appalachia region of eastern U.S., some of which are oozing acid mine drainage carrying rare earths.
Using the carbon dioxide method developed by the Penn State scientists, 90% of aluminum was recovered at a pH of 5, which is still mildly acidic, and 85% of rare earth elements were recovered by pH 7, which is neutral.
To get the same recoveries with other methods would require raising the pH even higher, which translates to more chemicals and costs.
"With a simple modification of existing treatment processes, industry could use less chemicals and get more value out of AMD waste," Rezaee said. "This is the beauty of this research."