Rare earth elements in coal fly ash- a possible resource? Presentation at 2015 GSA annual meeting

At the end of my June 19, 2015, blog post on some geologic highlights of a trip to Los Angeles/Las Vegas (focusing on the LaBrea Tar Pits), I mentioned driving by the Molycorp Mountain Pass REE (rare earth element) mine on Interstate 15, in Mountain Pass, California. The Mountain Pass mine had originally operated from 1950-2002; during part of this time, it was the world's major REE source. Operations resumed in 2012 and has operated off and on since then. The mine is one of two domestic deposits of rare earth elements (http://images.slideplayer.com/16/5108575/slides/slide_9.jpg). Bear Lodge in Wyoming is under development, but Mountain Pass has, since our drive-by in June, shut down due to a fall in REE prices.

I heard about the shutdown during a presentation by Allan Kolker on November 2, at the Geological Society of America annual meeting in Baltimore. Allan, a USGS scientist specializing in the inorganic chemistry of coal, was lead author on “Rare earth bearing trace phases in coal ash: Where are they?” (https://gsa.confex.com/gsa/2015AM/webprogram/Paper264228.html). Kolker summarized ongoing research, part of a larger National Energy Technology Laboratory (NETL; Department of Energy) program, looking at coal ash as a possible domestic source of REE, a commodity used in, among other things, fluorescent lights, glass, high-tech ceramic applications, hybrid engines, high-performance permanent magnets in defense systems and wind turbines (http://slideplayer.com/slide/5108575/ (Slides 4-6)).

 

Periodic table showing location of the rare earth elements

(Molycorp slide presentation; http://images.slideplayer.com/16/5108575/slides/slide_3.jpg)

What is coal ash? Ash is the uncombusted particulate residue of coal. That left in the bottom of the furnace or boiler is called bottom ash; what flies up the chimney or smoke stack is fly ash. Ideally, ash should be only non-combustible inorganic components, either original minerals, mineral reaction products, or melt glass. However, although modern power plants use pulverized coal to decrease particle volume and increase surface area to encourage complete fuel combustion, ash can also include uncombusted or partially-combusted coal and carbons. The Fly Ash (http://coalandcarbonatlas.siu.edu/fly-ash/fly-ash-tutorial.php) and Combustion Char (http://coalandcarbonatlas.siu.edu/combustion-chars/combustion-chars-tutorial.php) sections of Crelling’s Petrographic Atlas of Coals and Carbons have photomicrographs of both carbon and mineral matter combustion particles.

“Coal ash is the largest type of waste generated in the United States and in many other countries, with over 100 million tons produced in the USA every year.” (http://breakingenergy.com/2014/02/18/can-coal-fly-ash-waste-be-put-to-good-use/). Fly ash particulates are captured, to prevent release to the atmosphere, and usually stored in holding ponds at the power plant site. It can be used as a cement replacement in making bricks or possibly as a soil enhancement in agriculture. However, besides REE, and various elements useful for crop performance including K, Na, Ca, Mg, coal ash contains toxic elements, such as lead, arsenic, mercury and uranium, which are a serious concern in any application or disposal plan.

Kolker, with co-authors, wrote in their abstract that “During coal combustion, REE are strongly retained in the residual ash fraction so that it is typical for REE in fly ash to be enriched by a factor of ten over those in the respective coal.” They compared REE concentrations in Appalachian coal to the NIST 1633c fly ash standard and the North American Shale Composite (Gromet et al., 1984); fly ash REE concentrations were 2-3 times that in the shale. A Pittsburgh Post-Gazette PowerSource article (August 18, 2015) describes the NETL REE research program and the recent increase in interest for the REE-in-coal-and-fly-ash database of Jim Hower, University of Kentucky Center for Applied Energy Research, second author on the GSA abstract.            

Common REE-bearing trace minerals in coal, Kolker and others explained, include apatite, zircon, allanite, xenotime, and monazite. The melting points of xenotime and monazite much exceed the 1300-1700˚C range of various fluidized-bed or pulverized coal boilers, and these minerals are expected to be found intact (unmelted) in ash. However, REE in ash may occur in other forms, possibly including glasses or perhaps even nanoparticles in the ash.  Continuing research will be to further document location and concentration of REE-bearing ash constituents.

Why is it critical that we look for domestic alternatives to mined sources of REE? The blog post at http://thehill.com/blogs/congress-blog/homeland-security/253274-mountain-pass-mine-closure-puts-us-at-greater-risk (September 11, 2015) states that China controled 90% of the world REE market at that time. It has "REE dominance through a combination of overproduction and price manipulation to drive out competitors", and used an export ban to Japan as a political tool in a 2010 territorial dispute between the two countries. While we are very used to the politics of the global petroleum market, any commodity that is controlled by a single nation or consortium of nations can be used to manipulate international prices and leverage power.

REE supply concerns did not just appear in the last few months since the Mountain Pass closing, but ramped up in 2010, after the lifting of the Chinese export ban to Japan, when price increases on some elements went up ~650%. Major US REE manufacturing needs are not just in consumer products, but also various defense systems. During Geosciences Congressional Visits Day 2011 (September 21), GSA's DC Geoscience Policy office included in my Capitol Hill schedule a hearing of the House of Representatives Committee on Foreign Affairs, Subcommittee on Asia and the Pacific, on "China's monopoly on rare earths: Implications for U. S. foreign and security policy". Four witnesses providing testimony were the CEO of Molycorp, a manufacturer of REE permanent magnets, a manufacturer of pumps and valves that use such magnets, and an analyst specializing in natural resources in relation to national security. The most memorable points for me were 1) Molycorp was having a hard time filling mining and geologic engineering jobs, not because of the remote location, but the dearth of qualified applicants: they emphasized the need for federal support of STEM education; 2) the crisis in REE supply could have been foreseen and acted upon earlier since signs were present. The transcript of the hearing is available at http://www.gpo.gov/fdsys/pkg/CHRG-112hhrg68444/html/CHRG-112hhrg68444.htm).

So while re-opening Mountain Pass mine in 2012 seemed to be a solution for a domestic supply of REE, the United States in years since has sometimes been without an actively producing source of these elements. It's rather like the car/tree scene in Jurassic Park, "Well... we're back... in the car again." Further research on options, such as coal ash and coal waste, may provide alternatives.