Rare earth elements (REEs) are critical to advanced technology products like cell phones, airplanes, defense systems and many other applications. Because the U.S. imports most of the REEs it uses, research to reduce the cost of environmentally safe recovery processes from domestic sources is critical.
Acid mine drainage, which is a waste from coal mining, is a potential domestic source for REEs. Treatment of coalfield mine drainage to recover REEs can be accomplished with chemicals or through natural processes that concentrate the elements for recovery.
A team from NETL, the Department of Geology and Environmental Science at the University of Pittsburgh, and Hedin Environmental Inc. of Pittsburgh evaluated REE-enriched solids produced from domestic coal mine drainage treatment systems. In an article published in the International Journal of Coal Geology in May, the team reported that coalfield mine drainage treatment systems that use natural processes like limestone beds and flow ponds to solidify waste and then recover embedded REEs are more effective and environmentally friendly than conventional treatment systems that use chemicals.
Acid mine drainage originates from the oxidation of sulfide minerals resulting in a solid byproduct with embedded REEs. According to the article, “Contaminated mine drainage is a major global challenge facing the mining industry. In the Appalachian region of the eastern U.S. alone, over 5,400 km of streams are polluted with mine drainage due to centuries of coal mining.”
Mine drainage treatment technologies fall into two categories: “active treatment” or “passive treatment.” Active treatment approaches result in large quantities of waste solids that are costly to dispose of for treatment operators.
An active treatment also requires constant additions of chemicals to neutralize acidity and oxidize dissolved metals. Active systems require delivery, storage, and mixing procedures, routine maintenance of equipment, and electricity for pumps and aerators.
Meanwhile, passive treatment systems typically do not use electricity or chemicals and rely instead on gravity flow, natural geochemical processes and microbial activity and produce solids that can yield greater quantities of REEs.
The research team examined data from 17 passive and active coal mine drainage treatment systems to determine the conditions required for REE recovery from the solids produced from the systems.
“The results of this study will help inform future REE recovery efforts and aid in the construction of treatment systems specifically designed to remove and concentrate REEs,” the article reads. “The recovery of REEs from treatment solids is an opportunity to transform mine drainage, an environmental challenge and economic liability, into an asset, further spurring the treatment of polluted water.”