(763d) Brief Overview of Rare Earth Research at NETL R&ic

Everything that is in the Earth’s crust is also present in coal to some extent, and the challenge is always to use abundant domestic coal in clean and environmentally friendly ways. Trace elements present within the coal such as mercury, arsenic, selenium, cadmium, antimony, phosphorus, and chlorine, as well as sulfur and nitrogen, have long been of interest due to their roles in producing pollution emissions from combustion or gasification plants. Critical elements, such as some of the rare earths present within the coal, have recently been of great interest due to their importance in the world’s economy.

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, approximately 1 Gt of coal has been mined annually in the United States over the past twenty-five years (1990-2014), although the 2015 total was 896 million tons, and 2016 was 739 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 lanthanide elements, as well as scandium and yttrium, are present in the abundant coal and coal by-products 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.

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 by-products. Rare earths are commercially produced from ores containing monazite (rare earth phosphate mineral) or bastnasite (rare earth carbonate-fluoride mineral), as well as from ion exchangeable clays. The lanthanides are found in coal as well as combustion by-products such as ash, coal preparation residues, gasification slags, and mining by-products. Rare earths have also been found in interesting concentrations in 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.

Also of interest, some coal and coal by-products have elevated concentrations of heavy rare earths (HREEs)—which 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’s 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 by-products. The NETL Rare Earth EDX Database (https://edx.netl.doe.gov/ree/) is a resource for rare earth information related to coal and by-products. Many other research organizations have also initiated efforts for the analytical characterization and recovery of rare earths from unconventional sources such as coal by-products.

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 by-products is an exciting new research area and represents a dramatic paradigm shift for coal. Some of the early exciting results on the occurrence and recovery of lanthanides from abundant coal by-products will be discussed.