Fire Clay coal releases rare earth elements in two distinct ways: some are easily leached from organic associations, while others require extreme acidity to dissolve the tiny minerals dispersed through the coal. Research and development efforts began in 2014 to investigate rare earth recovery from coal, coal refuse, acid coal mine drainage, and coal fly ash.

Since then, an increasing number of studies have assessed technologies for rare earth recovery from coal-related materials, with many encouraging results reported. Examples include over 80% leaching from coal refuse after calcination, around 89% leaching from coal fly ash after roasting and acid leaching, and a mixed rare earth oxide with 94% purity from acid mine drainage.

Leaching has been investigated most frequently because it dissolves rare earths from solids into solution for further hydrometallurgical purification and high-purity product generation. Rare earth leachability depends on leaching conditions, mineralogy, particle size, and associations with dominant mineral particles.

Fire Clay coal has high rare earth content, primarily because of a tonstein layer formed from volcanic ash deposition. The major rare earth minerals reported in Fire Clay coal were monazite, zircon, iron-nickel-chromium spinels, and apatite, and most have stable crystal structures that are difficult to dissolve under strongly acidic conditions.

Yet acid leaching of inorganic Fire Clay material produced around 80% recovery, while similar tests on coal refuse recovered less than 40%, indicating unique rare earth characteristics in the inorganic matrix. The study aimed to achieve an in-depth understanding of the rare earth elements in Fire Clay coal.

A clean coal fraction was treated with a low-temperature plasma system to remove organic matter, and the resulting mineral matter underwent pH-dependent leaching tests. Scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses examined untreated mineral matter and the remaining mineral matter after leaching.

SEM images and EDS data from hundreds of rare earth-bearing particles were analyzed to explain the leaching characteristics. A representative Fire Clay seam coal sample was collected at a coal preparation plant in eastern Kentucky using a belt-sweep sampler. The sample was air-dried, dry-sieved with a ten-millimeter aperture, and separated by float-sink treatment in a dense medium bath with a specific gravity of one point four.

The float fraction was rinsed, dried, and pulverized to a top particle size of zero point eighteen millimeters; proximate analysis showed an ash content of five point thirty-one percent. Low-temperature plasma removed organic matter while preserving the crystal structure of associated mineral matter, and nitric acid, hydrochloric acid, hydrofluoric acid, and deionized water were used in the tests.

The pH-dependent leaching tests evaluated how pH affects elemental recovery, while mineral matter before leaching and solid residues after leaching helped infer rare earth mineralogy. Each test used a three-neck round-bottom flask with a maximum capacity of one hundred milliliters, a condenser, and a water bath for temperature control.

Fifty milliliters of deionized water and zero point five grams of solid were used at seventy-five degrees Celsius, with continuous magnetic stirring. The slurry pH was measured continuously, concentrated hydrochloric acid maintained the target pH, and samples were centrifuged after stabilization.

Supernatants and digested solid residues were analyzed by inductively coupled plasma mass spectrometry, and recoveries were calculated from their rare earth contents. SEM-EDS analyses examined the low-temperature plasma ash to understand rare earth mineralogy and identify a large number of rare earth-bearing particles.

Potential particles were first identified with Map mode at low magnification, then rare earths were confirmed with Point and ID mode at high magnification. SEM images, EDS spectra, and elemental compositions were collected and used to determine rare earth mineralogy.

The pH-dependent tests used mineral matter from the low-temperature plasma treatment of the one point four specific-gravity float fraction to evaluate rare earth extractability. At the initial slurry pH of around three point three, around nineteen percent of total rare earths, ten percent of light rare earths, and forty percent of heavy rare earths were leached.

This showed that a portion of the rare earths, particularly heavy rare earths, could be readily leached under weakly acidic conditions. Recovery increased only slightly as pH decreased at first, but after pH reached around one point two, both light and heavy rare earth recovery increased dramatically with further pH decreases.

Rare earth occurrence in coal can be divided into organic association and inorganic association. Organic association means rare earths are chemically bound to organic matter or atomically dispersed within the organic matrix. Low-temperature plasma eliminated most organic matter, releasing organically associated rare earths into the remaining mineral matter.

The released rare earths likely transformed into rare earth oxides or hydroxides rather than crystallizing into mineral species, and these oxides or hydroxides are typically leachable under mildly acidic conditions. Below pH one point two, both light and heavy rare earth recovery increased, but the improvement in light rare earth recovery was more significant.

At the final pH of negative zero point zero five, light rare earth recovery exceeded heavy rare earth recovery. This behavior cannot be explained by organically associated rare earths alone and is more likely determined by dissolution of rare earth-bearing minerals dispersed within the organic matrix.

Scandium behaved differently: around forty-five percent was leached at pH three point three, increasing to eighty-seven percent as pH fell to one point eight, suggesting that most scandium originated from organic associations. In panel b, the scandium curve stays near the top through the acidic range, while the individual light rare earth curves cluster much lower by the rightmost pH point.

In panel c, the heavy rare earth curves remain broadly spread, from roughly twenty to just over fifty percent at that same point, rather than forming a single uniform response. The leaching results suggested that both organically and inorganically associated rare earths contributed to recovery from the low-temperature plasma ash.

SEM-EDS was used on mineral matter after low-temperature treatment to better understand the rare earth mineralogy. SEM-EDS has limitations: nanoscale rare earth-bearing particles can fall below its detection limit, and it cannot provide direct information about crystal structure.

Compared with techniques such as TEM-EDS, SEM-EDS is more cost-effective and does not require particles to be extracted and milled into thin slices. SEM-EDS evaluated rare earth-bearing particles in the one point four specific-gravity float fraction after low-temperature plasma treatment, using EDS elemental compositions to assess mineralogy.

A total of ninety-four rare earth-bearing particles, labeled FC1-1 through FC1-94, were identified. The particles divided into group H and group L based on elemental composition. Group H particles were predominantly heavy rare earth-rich with negligible major light rare earths, whereas group L particles were predominantly light rare earth-rich with negligible major heavy rare earths.

All group L particles contained phosphorus, and total rare earth content increased as phosphorus content increased, indicating that the rare earths likely occurred as rare earth phosphate minerals. Group L particles divided into subgroups L1 and L2, with L1 having higher total rare earth content and a larger total-rare-earth-to-phosphorus ratio at similar phosphorus content.

A total-rare-earth-to-phosphorus ratio of two was used to separate the L1 and L2 subgroups, following the distinction between monazite and crandallite-group minerals. The elemental patterns supported this division: L1 aluminum decreased with increasing phosphorus, while L2 aluminum generally remained high, and several L2 particles contained over two point five percent strontium.

In panel a, the black L1 points follow a relatively tight rising dashed fit, while the red L2 points remain clustered near the bottom with a much shallower dotted fit. Panel b shows the same separation: black points occupy a broad band mostly above the horizontal reference at two, whereas red points lie predominantly below it.

In the BSE image, particle FC1-88 is the bright, arrow-marked grain near the center. The corresponding maps show phosphorus and cerium concentrated in that same grain, while the silicon and aluminum signals are more broadly distributed across the surrounding matrix.

SEM-EDS was also performed on the final leaching residues obtained after gradually reducing slurry pH to negative zero point zero five. The residue contained ninety-four rare earth-bearing particles, labeled FC2-1 through FC2-94, divided into heavy-rare-earth-rich and light-rare-earth-rich groups and five subgroups.

The five subgroups were H1 with high zirconium, H2 with high heavy rare earths and phosphorus, H3 without phosphorus, L1 with a total-rare-earth-to-phosphorus ratio greater than two, and L2 with a ratio less than two. Figure eight relates phosphorus content to total rare earth elements, titanium, and iron in H2-group particles from the Fire Clay coal leaching residue.

Panel a shows a general increase in total rare earth content as phosphorus rises, supporting the authors’ interpretation that these elements mainly occur in rare earth phosphate minerals. The labeled particles also highlight unusual compositions: FC2-31 has elevated total rare earths, FC2-20 and FC2-32 have elevated titanium, and FC2-30 has elevated iron relative to particles with similar phosphorus contents.

SEM-EDS results from raw and leached low-temperature plasma ashes revealed complex rare earth mineralogy in Fire Clay coal. The particles formed two major groups: group H was rich in heavy rare earths, and group L was rich in light rare earths. Heavy rare earth occurrence included zirconium-bearing minerals, heavy rare earth phosphate minerals, and minerals rich in titanium, niobium, or tantalum.

Group L particles were divided by their total-rare-earth-to-phosphorus ratios: greater than two for L1 and less than two for L2, indicating different rare earth mineralogies. The earlier assignment of ratios greater than two to monazite and ratios less than two to crandallite-group minerals partially applied to the group L particles formed by low-temperature plasma treatment.

Figure ten plots total rare-earth content against phosphorus for L-group particles in the leaching residue. Panel a separates L1 and L2 particles along lines with different slopes, while panel b shows their TREE-to-phosphorus ratios relative to the dashed ratio-of-two line.

The authors use this distribution to support classifying residue particles with a TREE/P ratio of two and to suggest that many L-group particles contain rare-earth phosphate minerals; FC2-67 is a notable phosphorus-free exception. The rare earth-bearing particles in both raw and leached materials were very fine, with most particle sizes around one micrometer.

Acid leaching did not significantly alter the size of the rare earth-bearing particles. At pH negative zero point zero five, light rare earth recovery was higher than heavy rare earth recovery, and decreasing pH from three point three to negative zero point zero five increased light rare earth recovery by around sixty-seven percent versus around thirty percent for heavy rare earth recovery.

The leaching results were therefore consistent with changes in subgroup proportions caused by leaching. Figure fifteen compares subgroup proportions in raw and leached low-temperature plasma ash. The chart shows that leaching changes the balance among H1, H2, H3, L1, and L2 particles, with the combined H subgroups occupying a larger share after leaching and L1 decreasing substantially.

This matters because the authors use these shifts as evidence that leaching dissolves some REE-bearing minerals and alters the mineralogical distribution of the particles. The one point four specific-gravity float fraction was treated by low-temperature plasma, and its resulting mineral matter underwent pH-dependent leaching tests.

At pH three point three, around nineteen percent of total rare earths, ten percent of light rare earths, and forty percent of heavy rare earths were leached; at pH negative zero point zero five, recovery increased to seventy-five, seventy-seven, and seventy percent, respectively.

At high pH, individual rare earth recoveries followed an order similar to binding constants with organic acids, so recovery at pH three point three was ascribed to dissolution of organically associated rare earths. The major recovery increases between pH one point two and negative zero point zero five, particularly for light rare earths, were explained by dissolution of rare earth-bearing minerals dispersed within the organic matrix before plasma treatment.

SEM images showed that all rare earth-bearing particles were very fine, with most having particle sizes around one micrometer. Comparing raw and leached materials showed that light-rare-earth-bearing particles were preferentially leached relative to heavy-rare-earth-bearing particles.

This matched the pH-dependent results: heavy rare earths were more readily leached under relatively mild acidic conditions, while remaining heavy rare earths required stronger acidity for dissolution. Light rare earth recovery remained low until pH decreased below zero, a contrast explained by the authigenic origin and low-crystallinity nature of the rare earth-bearing minerals.

The study established correlations between rare earth occurrence modes in Fire Clay coal's micro-dispersed mineral matter and its leaching behavior. Figure seventeen links phase transformation to leaching behavior. In panel a, diffraction patterns show Ce-phosphate hydrate peaks in the as-synthesized material, while monazite peaks appear after calcination at six hundred degrees Celsius and become more pronounced at higher temperatures.

Panel b plots Ce recovery against pH for the as-synthesized and calcined products, showing that recovery changes with both calcination temperature and acidity, supporting the authors’ explanation for altered rare-earth leachability. The study links leaching behavior to mineral occurrence: organically associated rare earths dissolve under mild acidity, while low-crystallinity dispersed minerals, especially those carrying light rare earths, drive the major recovery increase at very low pH.