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Author: Evan J. Granite, Elliot Roth, and Mary Anne Alvin
A Paradigm Shift for Coal Research
Coal is an important resource, both in the United States and around the world. The United States generates approximately 30 percent of its electricity through coal combustion and, at the current overall rate of consumption, has more than a 250-year supply of coal.
According to the US Energy Information Administration’s July 2016 Monthly Energy Review, approximately 1 Gt of coal has been mined annually in the United States over the past 25 years, although the 2015 total was 896 million tons and 2016 is projected to be less than 700 million tons. The recent decline in domestic coal production is due to the availability of inexpensive natural gas, regulatory uncertainty with regards to carbon dioxide emissions, and the retirement of older coal-fired power plants.
In the United States most coal is burned for power generation, but substantial quantities are also used in the manufacture of steel, chemicals, and activated carbons. Numerous industries—such as mining, power, rail transportation, manufacturing, chemical, steel, activated carbon, and fuels—are involved in the production, transportation, and use of coal.
Rare earth elements (REEs), which comprise the 15 lanthanide elements as well as scandium and yttrium, are present in the abundant coal and coal byproducts produced domestically and worldwide. Widely used in high-technology products such as catalysts, cell phones, hard drives, hybrid engines, lasers, fluorescent lamps, batteries, magnets, medical devices, and televisions, REEs are of significant value to US national security, energy independence, economic growth, and the country’s environmental future.
Everything that is in the Earth’s crust is also present in coal to some extent, and the challenge is to use coal in clean and environmentally friendly ways. Most of the common inorganic lanthanide compounds, such as the phosphates found in coal, have very high melting, boiling, and thermal decomposition temperatures, allowing them to concentrate in combustion and gasification byproducts.
Rare earths are commercially produced from ores containing monazite (rare earth phosphate mineral) or bastnäsite (rare earth carbonate-fluoride mineral) as well as from ion exchangeable clays. They are also found in coal; combustion byproducts such as ash, coal preparation residues, gasification slags, and mining byproducts; and the strata above and below some coal seams, thereby making every process in the mining and utilization of coal a potential source of rare earth elements.
Of further interest, some coal and coal byproducts have elevated concentrations of heavy rare earths (HREEs). These are lowest in supply, rank high in criticality and price, and are projected to increase in demand, making them potentially attractive targets for REE recovery despite their overall lower concentration.
The National Energy Technology Laboratory (NETL) Rare Earth EDX Database (https://edx.netl.doe.gov/ree/) is a resource for rare earth information related to coal and byproducts. The NETL Research and Innovation Center (NETL-RIC) recently initiated research to support the measurement of concentration, identification of rare earth compounds, and recovery of rare earths from abundant domestic coal byproducts. Many other research organizations have also initiated efforts for the analytical characterization and recovery of rare earths from unconventional sources such as coal byproducts.
Much of the recent research on coal use in the United States has focused on the capture of pollutants such as acid gases, particulates, mercury, and carbon dioxide. The possible recovery of rare earth and other critical elements from abundant coal and byproducts is an exciting new research area and represents a dramatic paradigm shift for coal.
The development of an economically competitive domestic supply of REEs will help to maintain the nation’s economic growth and national security. But additional research and data are needed on the rare earth contents of coals and byproducts in order to determine the most promising feed materials and extraction processes. Future work will focus on characterization of coals and byproducts and on separation methods for the recovery of rare earth elements.
This report was prepared as an account of work sponsored by an agency of the US government. The views and opinions expressed do not necessarily reflect those of the US government or any agency thereof.