Tikalon Blog by Dev Gualtieri

Rare Earths from Coal Waste

April 18, 2022

The rare earth elements were the focus of my early research career. The research topics were permanent magnets, hydrogen storage materials, magnetic bubble memory materials, phosphors, and solid state lasers. In today's world, you can find rare earths in permanent magnets, in motors and actuators, especially those for automobile applications such as window openers, windshield wipers, and door locks, in headphones and earpieces, and in position actuators for computer hard drive read-write heads. The rare earth elements are also important as additions to catalysts. I've worked with every one of the rare earth elements, except for the unstable element Promethium (⁶¹Pm).

The Rare Earth Elements
Atomic Number	Symbol	Element	ppm in Earth's Crust
21	Sc	Scandium	22
39	Y	Yttrium	33
57	La	Lanthanum	39
58	Ce	Cerium	66.5
59	Pr	Praseodymium	9.2
60	Nd	Neodymium	41.5
61	Pm	Promethium	1x10^-15
62	Sm	Samarium	7.05
63	Eu	Europium	2
64	Gd	Gadolinium	6.2
65	Tb	Terbium	1.2
66	Dy	Dysprosium	5.2
67	Ho	Holmium	1.3
68	Er	Erbium	3.5
69	Tm	Thulium	0.52
70	Yb	Ytterbium	3.2
71	Lu	Lutetium	0.8

These elements are called rare since they exist in very low concentrations in Earth's crust and this leads to high prices. For example, the current price of neodymium metal, an important component of permanent magnets, is about $65,000 per US ton, as compared with the price of iron, $425/US ton. In most cases, this is a consequence of the fact that elements of high atomic number are less abundant in the universe than those of low atomic number. The distribution of these elements among countries is not uniform. As the following chart shows, China is the major producer of rare earths, a fact that is problematic for other countries, and there have been efforts to extract rare earths from unusual sources, such as the seafloor.[1]

Rare earth production by country in 2021.

China produced 168,000 US tons of rare earth oxide equivalent in 2021, up from 140,000 the previous year.

(Chart created using Gnumeric from data in ref. 2.[2] Click for larger image.)

Coal has been used extensively as an energy source, and one consequence of that usage is the enormous amount of ash produced by coal combustion (see table). Coal ash contains mostly silica (silicon dioxide, SiO₂, 20-40 wt-%), aluminum oxide (Al₂O₃, 10-35 wt-%), iron oxide (Fe₂O₃, 5-35 wt-%), and calcium oxide (CaO, 1-20 wt-%), along with smaller quantities of the toxic elements, mercury (Hg, 0.10 ppm), arsenic (As, 1.4–71 ppm), and selenium (Se, 3 ppm). Coal ash also contains rare earths, particularly scandium and neodymium, in concentrations up to 100 times higher than the concentration in Earth's crust.

Carbon and Ash Content in Coal
Coal Type	Carbon (wt-%)	Ash (wt-%)
Lignite	65-72	18-35
Subbituminous	72-76	14-18
Bituminous	76-90	10-14
Anthracite	90-95	5-10

To quantify the potential for rare earth recovery from coal, we need to examine how much coal is produced annually, even under the threat of global warming. The following table lists the amount of coal produced by the world's top ten producer countries.

Top Coal Producer Countries
Coal production in 2020 in million metric tons (1.102 US tons)
Country	2020 Production (million metric tons)
China	3,902.0
India	756.5
Indonesia	562.5
United States	484.7
Australia	476.7
Russia	399.8
South Africa	248.3
Kazakhstan	113.2
Germany	107.4
Poland	100.7

Coal ash consists of two types. There's the heavier bottom ash which my grandfather shoveled from his coal furnace when I was a child. Bottom ash is just ten percent of the ash produced by coal combustion. The remainder, fly ash, is fine particulates that are carried away by the flue gases. In the past, fly ash would fly into the atmosphere, but today's modern coal-fired power plants have electrostatic precipitators for capture. In an earlier article (Rare Earth Metals from Fly Ash, July 7, 2016), I reviewed one process for recovery of rare earths from fly ash. Research on this topic is still ongoing.[3-4]

Coal combustion was used to generate about 30 percent of US electric power in 2017.[4] Some of the vast quantity of fly ash produced is used to create construction materials, but the remainder is stored in landfills and coal slurry impoundments.[4] The American Coal Ash Association reported that 118.4 million tons of coal ash, enough to bury Kentucky in three feet of solid waste, became fill from 2000-2017.[5] More than 3-4 billion tons of coal ash are stored at about 1,400 US sites.[5]

The entire United States production of rare earth elements comes from a single mine, the Mountain Pass mine in California, and this mine produced just 15.8 percent of global production in 2020.[4] About seventy percent of global rare earth element production comes from China.[4] The International Energy Agency has estimated that by 2040, the demand for rare earths for just the sustainable energy industry could be three to seven times greater than it is today.[7] About five metric tons of coal ash are needed to produce one kilogram of rare earth elements.[5] Economics are working against recovery from coal ash, since the average rare earth concentration in coal ash is 500 parts per million, compared with forty times that in mined ore.[5]

The US state of Pennsylvania has been a major part of coal mining, and estimates suggest that 300,000 tons of rare earth elements exist in coal ash in Pennsylvania.[6] The United States Department of Energy has funded pilot projects for mineral recovery from various coal mine residues in Pennsylvania, West Virginia, Kentucky and North Dakota.[6-7] Another source of recovery material is acid mine drainage that forms when when iron pyrite (iron sulfide) reacts with water and air to oxidize and create sulfuric acid.[8-9] This acid dissolves surrounding rocks to dissolve metals into the water.[8-9] In the Appalachian region alone, an estimated 6,000 metric tons of rare earths are created annually in acid mine drainage sites.[7] Acid mine drainage in Appalachia, which is presently being collected and treated for environmental reasons, is a promising source of rare earth elements.[8] Appalachian acid mine drainage might produce 400-1,700 tons of rare earth elements each year.[6]

A map of the US Appalachian region showing estimated quantities of rare earth elements in coal.

The estimates are shown as metric tons of rare earth elements per block, a block being a section a square kilometer in surface area and one meter in depth.

(Figure 5b of ref. 3.[3] Click for larger image.)

Rare earth recovery, as combined with such efforts to clean up a coal mines, would generate revenue from the rare earth recovery to help pay for such remediation.[7] The United States Environmental Protection Agency performs remediation on acid mine drainage, and other entities operate more than 300 treatment systems on their own.[9]

Scientists at Pennsylvania State University have developed a process for recovery of high concentrations of rare earth elements using small amounts of chemicals.[8] They add carbon dioxide to acid mine waste to produce carbonates, and the rare earth elements bond with the carbonates to precipitate out of the water at lower pH values.[8-9] This carbon dioxide mineralization process also removes excess carbon dioxide from the atmosphere.[8-9]

Rare Earths from Coal Waste

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