Acid baking of coal fly ash for recovery of rare earth elements

Abstract

This disclosure provides processes for the recovery of one or more rare earth elements from coal fly ash, as well as compositions, devices, and articles including one or more rare earth elements recovered by the processes described herein.

Description

[0001] Attorney Docket No. 10046-644W01

[0002] ACID BAKING OF COAL FLY ASH FOR RECOVERY OF RARE EARTH ELEMENTS

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 692,416, filed September 9, 2024, which is incorporated by reference herein in its entirety.

[0005] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0006] This invention was made with government support under Grant No. CBET2145374 awarded by the National Science Foundation. The government has certain rights in the invention.

[0007] BACKGROUND

[0008] Rare earth elements (REEs), comprised of the lanthanide group along with Sc and Y, are key strategic metals used for the manufacturing of advanced devices and clean energy infrastructure. (See Balaram, V. Geoscience Frontiers 2019, 10 (4), 1285-1303; Vidal, O., et al. Nat Geosci 2013, 894—896; Alonso, E., et al. Environ Sci Technol 2012, 46 (6), 3406- 3414; Dutta, T., et al. Environ Res 2016, 150, 182-190; and Imholte, D. D., et al. Energy Policy 2018, 113, 294—305). Increasing efforts to combat climate change have created an unprecedented need for REEs, with projections indicating that by 2040, REEs demand will surge by three to seven times more than current levels. (See The Role of Critical World Energy Outlook Special Report Minerals in Clean Energy Transition). To address rising demand and strengthen the supply chain, alternative sources of REEs must be identified. One promising unconventional source of REEs is coal fly ash, an abundant industrial waste generated in coal- fired power plants that hosts considerable concentrations of REEs. (See Mardon, S. M., et al. Int J Coal Geol 2004, 59, 153-169; and Seredin, V. V., et al. International Journal of Coal Geology. May 1, 2012, pp 67-93). In the United States, ~28 million tons of fly ash are generated annually. (See American Coal Ash Association. 2022 Production and Use Survey Results', 2022). This ash is typically disposed of in active landfills and surface impoundments, occupying ~ 74,000 acres of land, creating an environmental burden and posing public health risks. (See Frequent Questions about the 2015 Coal Ash Disposal Rule I US EPA'. Gopinathan, P., et al. Chemosphere 2022, 307, 135710; Catalano, J. G., et al. Environ Sci Attorney Docket No. 10046-644W01

[0009] Technol 2012, 46 (21), 11804—11812; Harkness, J. S., et al. Environ Sci Technol 2016, 50 (12), 6583-6592; Wu, J., et al. Environ. Sci. Technol 2021, 55, 6654; and Chen, Y., et al. A Ecotoxicol Environ Saf lA, 269, 115905). Concurrently, U.S. fly ashes host an average REE concentration of 300 to 600 ppm, equivalent to an annual REEs output of ~ 12,000 tons, which exceeds the current U.S. consumption rate of REEs. (See Taggart, R. K., et al. Environ Sci Technol 2016, 50 (11), 5919-5926; Scott, C., et al. FS 2019-3048: Rare Earth Elements in Coal and Coal Fly Ash. USGS Fact Sheet 2019, 3048'. and Geological Survey, U. Mineral Commodity Summaries 2024', 2024). Therefore, leveraging fly ash for the recovery of REEs provides a sustainable waste management solution and contributes to securing the supply of critical metals.

[0010] Previous studies have suggested that ash feedstocks hosting higher concentrations of calcium are more amenable to milder and less reagent-intensive REEs extraction techniques. (See King, J. F., et al. Int J Coal Geol 2018, 195, 75-83; and Liu, P., et al. Environ Sci Technol 2019, 53 (9), 5369-5377). Methods such as low-molarity inorganic acid leaching, organic acid leaching, and direct ionic liquid extraction have demonstrated REEs recovery efficiencies of 70-100% from calcium-rich fly ashes. (See Liu, P., et al. Environ Sci Technol 2023, 57 (13), 5414-5423; and Stoy, L., et al. Environ Sci Technol 2021, 55 (13), 9209- 9220). Calcium-poor ashes, in contrast, typically yield recoveries of only 10-40%. The efficiency of mild, leach-based REE extraction methods is primarily influenced by the elemental composition of the ash matrix, particularly its content of alkali and alkaline-earth metals. (See Middleton, A., et al. Int J Coal Geol 2020, 227, 103532). In coal fly ash, REEs are predominantly found embedded in microscopic amorphous aluminosilicate particles. To access and extract REEs via leaching, metal content in the ash matrix must be sufficiently high to facilitate cation release and reagent transport. (See Gerardo, S., et al. Environ Sci Technol Lett 2023; and Gerardo, S., et al. Environ Sci Technol 2022, 56 (22), 16200-16208). Fly ash samples with high calcium concentrations are thus better candidate feedstocks for leaching methods that employ milder reagents and less harsh conditions.

[0011] Leaching of REEs from amenable calcium-rich ashes, under both very acidic conditions and milder pH environments, is accompanied by secondary phase precipitation for low liquidsolid ratios. Secondary phase precipitation often impacts metal extraction processes, as the morphology and structure of the precipitates (e.g., amorphous flakes, crystalline particles, condensed phases) can hinder reagent transport and accessibility to the targeted phase. For instance, the formation of passivating secondary phases that reduce extraction efficiencies Attorney Docket No. 10046-644W01 has been observed for various metal leaching processes. (See He, S., et al. Hydrometallurgy 2010, 104 (2), 235-240). Similarly, gel-like phases reported in coal fly ash processing are hypothesized to have a passivating effect on REEs accessibility. (See Anand, R. K., et al. Cleaner Chemical Engineering 2022, 4, 100078; and Mokoena, B. K., et al. hit J Coal Geol 2022, 259, 104037). The formed precipitates can also increase the viscosity of the leaching fluid, reducing leachate filterability and precluding the scalability of leaching operations. (See Queneau, P. B., et al. Canadian Metallurgical Quarterly 1986, 25 (3), 201-209).

[0012] There is a clear need for methods of recovery of rare earth elements from coal fly ash that avoid secondary phase precipitation. This disclosure addresses this as well as other needs.

[0013] SUMMARY

[0014] The present disclosure provides processes for the recovery of one or more rare earth elements, as well as compositions, devices, and articles including one or more rare earth elements recovered by processes described herein.

[0015] In one aspect, a process for the recovery of one or more rare earth elements from coal fly ash is provided. In some aspects, the process includes baking a first mixture including the coal fly ash and an acid solution, wherein the first mixture forms a baked solid. In some aspects, the process further includes contacting the baked solid with a solvent to form a leach phase and a precipitate phase, wherein at least a portion of the one or more rare earth elements is dissolved in the leach phase. In some aspects, the process further includes separating the leach phase and the precipitate phase.

[0016] In another aspect, a composition, device, or article is provided including one or more rare earth elements recovered by a process described herein.

[0017] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description, the drawings, and the claims.

[0018] BRIEF DESCRIPTION OF DRAWINGS

[0019] FIGs. 1A-1C depict characterization of gels precipitated during 6M HNO3 leaching of the FA-3 ash sample as described in the examples. FIG. 1A shows microscopic characterization of the dried gels collected using SEM. FIG. IB shows XRD spectra of the dried gel. Shaded region denoted as “Am” corresponds to the amorphous phase, and peak denoted as B Attorney Docket No. 10046-644W01 corresponds to barite that likely originated from insufficient precipitate wash. FIG. 1C shows Raman spectra of gels in their native hydrated state. The region corresponding to fivemembered silica rings is shaded in dark blue, and region corresponding to four-membered silica rings is shaded in light blue. Molecular representations have been added depicting the approximate structure of the silica rings, where white circles represent oxygen, and red circles represent silicon.

[0020] FIGs. 2A-2C depict further characterization of gels precipitated during 6M HNO3 leaching of the FA-3 ash sample as described in the examples. FIG. 2A shows concentration measurements, in ppm, of Si, Ca, Al, and Fe in solution. Si concentrations decrease by two orders of magnitude from 5 minutes to 240 min, indicating precipitation of Si-rich phases. FIG. 2B shows recovery efficiencies of major cations in solution. FIG. 2C shows calculated saturation indices for key mineral phases, including amorphous silica (SiO2(a>), kaolinite, goethite, gibbsite, and portlandite. The positive saturation index of amorphous silica indicates that the solution is oversaturated, and precipitation of this phase is thermodynamically favorable.

[0021] FIGs. 3A-3B depict composition data and REEs recovery efficiencies from 6M leaching of ash samples as described in the examples. FIG. 3A shows major cation composition data of the leachates originated from 6M leaching of ash samples. Initial calcium concentrations (reported here as oxide equivalent wt.%) are shown below the ash sample labels and increase from left to right. For the non-log figure and the tabulated data, refer to FIGs. 16A-16B, TABLES 7-9, and TABLES 10-12. FIG. 3B shows average REEs recovery efficiencies for each ash sample, plotted as a function of calcium oxide concentration in the ash. Higher REEs recovery efficiencies are observed in ashes with greater calcium oxide content.

[0022] FIGs. 4A-4B depict concentration profiles and REEs recovery as described in the examples. FIG. 4A shows concentration profiles across the gel resulting from solute transport experiments performed for 24 hours and 52 hours. The calculated average DCu in the gelled leachate was ~ 5.9 x 10-6 cm2 / s, which is within the same order of magnitude as DCu in aqueous solution. These results suggest that the gel phase does not hinder cation transport. FIG. 4B shows REEs recovery over time using a 6M HNO3 leaching solution highlights that REEs extraction plateaus to ~ 95 % recovery in less than 1 hour. Inset plot shows the individual REEs recovery efficiencies at t = 240 min. Attorney Docket No. 10046-644W01

[0023] FIG. 5 depicts XRD data of the leaching residue as described in the examples, which includes both the newly formed products and remaining ash solids. The shaded gray “Am” region denotes the amorphous phase. Peaks corresponding to the unreacted quartz (Q) and mullite (M) phases present in the ash are also present. Inset image corresponds to an SEM micrograph of the flocs that precipitated during low acid concentration leaching.

[0024] FIGs. 6A-6B depict characterization of flocculants formed at low molarity leaching conditions (I M HNO3) as described in the examples. FIG. 6A shows time-resolved fluid concentration data of major cations (Si, Ca, Al, and Fe) found in the FA-3 fly ash showing slowed reactions after ~ 1 hour. FIG. 6B shows Saturation Indices calculated for key phases, including amorphous silica, kaolinite, goethite, gibbsite, and portlandite. For 1 M HNO3 leaching, SI calculations suggest the thermodynamically favorable (SI > 0) precipitation of kaolinite, gibbsite (Al(0H)3), amorphous silica (SiCh), and goethite (Fe(OH)3). Recovery efficiency of major cations is not shown here since most major cations appear to precipitate.

[0025] FIGs. 7A-7C depict further characterization of flocculants formed at low molarity leaching conditions (1 M HNO3) as described in the examples. FIG. 7A shows major cation concentration data for samples FA-2, FA-3, FA-4 after a 4-hour leaching period. Sample FA- 1 was excluded since it yielded lower recoveries in prior experiments under high acid concentrations. Initial calcium concentrations (reported here as oxide equivalent wt.%) are shown below the ash sample labels and increase from left to right. FIG. 7B shows REEs recovery efficiencies for samples FA-2, FA-3, and FA-4. FIG. 7C shows REEs recovery efficiency for sample FA-3 during IM HN03 leaching showing slowed extraction kinetics after ~ 1 hour.

[0026] FIGs. 8A-8C depict analysis of leachates processed using liquid-solid ratios of 10 and 100 as described in the examples. FIG. 8A shows the elemental concentration of Si, Al, Ca, and Fe. FIG. 8B shows average REEs recovery efficiencies. FIG. 8C shows total REEs present in the leachates.

[0027] FIGs. 9A-9D depict further leaching recovery and kinetics as described in the examples. FIG. 9A shows elemental concentration of Si, Al, Ca, and Fe. FIG. 9B shows calculated saturation indices for amorphous silica, gibbsite, goethite, kaolinite, and portlandite. FIG. 9C shows recovery efficiency of Al, Ca, and Fe in fly ash. FIG. 9D shows REEs recovery efficiency, showcasing the REEs leaching kinetics profile to up to 4 hours. Attorney Docket No. 10046-644W01

[0028] FIG. 10 depicts silica condensation rate as a function of pH. At the isoelectric point of silica, silica surface is neutral and the condensation rate reaches a minimum as described in the examples. At pH values below the isoelectric point, silica surfaces are primarily positively charged ([SiOH2+] / [SiOH] increases with pH decrease), and at pH values above the isoelectric point, silica surfaces are primarily negatively charged ([SiO"] / [SiOH] increases with pH increase). The green shaded region corresponds to the metastable region, whereas the gray shaded region corresponds to the rapid aggregation region. Adapted from Iler, R. The Chemistry of Silica Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry 1979.

[0029] FIGs. 11A-11C depict acid baking experiments performed using different acid concentrations (2M, 6M, and 15M HNO3) for the acid baking step, followed by water leaching as described in the examples. FIG. HA shows the concentration, in ppm, of the major cations (Si, Ca, Al, and Fe) present in the final leachate solutions. FIG. 11B shows REEs recovery efficiency. FIG. 11C shows total REEs concentration in the final leachate in ppm. Data marked by the symbol (*) denotes that the elemental concentration was below detection limit. For Si, this suggests concentrations below 10 ppm.

[0030] FIGs. 12A-12B depict the effectiveness of different precipitate mitigation approaches as described in the examples. (FIG. 12A) Concentrations of major cations present in the final leachates. (FIG. 12B) REEs concentrations in the final leachate, in ppm, and average REEs recovery efficiency for each extraction method.

[0031] FIG. 13 depicts initial mineralogy of ash samples used in the examples characterized via XRD as described in the examples. The main phases found in fly ash include quartz, mullite, iron oxide phases (i.e., hematite, maghemite), and basic compounds and salts (i.e., anhydrite, lime, periclase, calcium silicate).

[0032] FIG. 14 depicts UV-Vis normal calibration curve for copper (II) nitrate solution, measured at kabs= 800 nm as described in the examples.

[0033] FIGs. 15A-15B depict SEM-EDS compositional data for the solid residues obtained from 6M HNO3 leaching conditions as described in the examples. FIG. 15A shows an SEM micrograph of amorphous flocculants precipitated during leaching. FIG. 15B shows an EDS spectrum and semi-quantitative elemental composition of the precipitates.

[0034] FIGs. 16A-16B depict composition data and REEs recovery efficiencies from 6M leaching of ash samples as described in the examples. FIG. 16A shows major cation composition data Attorney Docket No. 10046-644W01 of the leachates originated from 6M leaching of ash samples. Initial calcium concentrations (reported here as oxide equivalent wt.%) are shown below the ash sample labels and increase from left to right. FIG. 16B shows average REEs recovery efficiencies for each ash sample, plotted as a function of calcium oxide concentration in the ash. Higher REEs recovery efficiencies are observed in ashes with greater calcium oxide content.

[0035] FIG. 17 depicts pH measurements for FA-3 ash sample during 2M HNO3 leaching as described in the examples. The pH of the leachate increases from < 0 at initial time to 1.1 after 4 hours. Measurements were performed in duplicate samples, and average values were used for plotting.

[0036] FIGs. 18A-18B depict SEM-EDS compositional data for the solid residues obtained from IM HNO3 leaching conditions as described in the examples. FIG. 18A shows an SEM micrograph of amorphous flocculants precipitated during leaching. FIG. 18B shows an EDS spectrum and semi-quantitative elemental composition of the precipitates.

[0037] FIG. 19 depicts pH measurements for fly ash samples FA-2 and FA-4 after acid leaching for 4 hours at a temperature of 80°C as described in the examples. Experiments were performed in duplicate, and average values are reported in the plot.

[0038] FIG. 20 depicts pH measurements for FA-3 ash sample during IM HNO3 leaching as described in the examples. The final pH of the solution after a 4-hour leaching was 3.4.

[0039] DETAILED DESCRIPTION

[0040] The following description of the disclosure is provided as an enabling teaching of the disclosure in its best, currently known aspects. Many modifications and other aspects disclosed herein will come to mind to one skilled in the art to which the disclosed compositions and methods pertain, benefiting from the teachings presented in the descriptions herein and the associated drawings. Therefore, it is understood that the disclosures are not limited to the specific aspects disclosed and that modifications and other aspects are intended to be included within the scope of the appended claims. The skilled artisan will recognize many variants and adaptations of the aspects described herein. These variants and adaptations are intended to be included in the teachings of this disclosure and to be encompassed by the claims herein.

[0041] Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Attorney Docket No. 10046-644W01

[0042] As is apparent to those of skill in the art upon reading this disclosure, each of the individual aspects described and illustrated herein has discrete components and features that may be readily separated from or combined with the features of any of the other several aspects without departing from the scope or spirit of the present disclosure.

[0043] Any recited method can be carried out in the order of events recited or any other order that is logically possible. Unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not explicitly state in the claims or descriptions that the steps are to be limited to a particular order, it is in no way intended that an order be inferred in any respect. This holds for any possible non-express basis for interpretation, including logic concerning the arrangement of steps or operational flow, meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.

[0044] All publications mentioned herein are incorporated by reference to disclose and describe the methods or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure before the filing date of the present application. Further, the dates of publication provided herein can be different from the actual publication dates, which can require independent confirmation.

[0045] It is also to be understood that the terminology herein describes particular aspects only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed compositions and methods belong. It can be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0046] Before describing the various aspects of the present disclosure, the following definitions are provided and should be used unless otherwise indicated. Additional terms may be defined elsewhere in the present disclosure.

[0047] Definitions

[0048] As used herein, “comprising” is interpreted as specifying the presence of the stated features, integers, steps, or components, but does not preclude the presence or addition of one or more Attorney Docket No. 10046-644W01 features, integers, steps, components, or groups thereof. Moreover, each of the terms “by,” “comprising,” “comprises,” “comprised of,” “including,” “includes,” “included,” “involving,” “involves,” “involved,” and “such as” is used in their open, non-limiting sense and may be used interchangeably. Further, the term “comprising” is intended to include examples and aspects encompassed by the terms “consisting essentially of’ and “consisting of.” Similarly, “consisting essentially of’ is intended to include examples encompassed by the term “consisting of.”

[0049] As used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context dictates otherwise.

[0050] Ratios, concentrations, amounts, and other numerical data can be expressed herein in a range format. Further, the endpoints of each of the ranges are significant both in relation to the other endpoint and independently of the other endpoint. There are many values disclosed herein, and each value is also disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. Ranges can be expressed herein as from “about” one particular value and to “about” another particular value. Similarly, when values are expressed as approximations, using the antecedent “about,” the particular value forms a further aspect. For example, if the value “about 10” is disclosed, then “10” is also disclosed.

[0051] When a range is expressed, a further aspect includes from the one particular value and to the other particular value. For example, where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure, e.g., the phrase “x to y” includes the range from ‘x’ to ‘y’ as well as the range greater than ‘x’ and less than ‘y’. The range can also be expressed as an upper limit, e.g. ‘about x, y, z, or less’ and should be interpreted to include the specific ranges of ‘about x,’ ‘about y,’ and ‘about z’ as well as the ranges of Tess than x,’ Tess than y.’ and Tess than z.’ Likewise, the phrase ‘about x, y, z, or greater’ should be interpreted to include the specific ranges of ‘about x,’ ‘about y,’ and ‘about z’ as well as the ranges of ‘greater than x,’ greater than y,’ and ‘greater than z.’ In addition, the phrase “about ‘x’ to ‘y’,” where ‘x’ and ‘y’ are numerical values, includes “about ‘x’ to about ‘y’.”

[0052] Such a range format is used for convenience and brevity and thus, should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range but also to include all the individual numerical values or sub-ranges encompassed within that Attorney Docket No. 10046-644W01 range as if each numerical value and sub-range is explicitly recited. To illustrate, a numerical range of “about 0.1% to 5%” should be interpreted to include not only the explicitly recited values of about 0.1% to about 5%, but also include individual values (e.g., about 1%, about 2%, about 3%, and about 4%) and the sub-ranges (e.g., about 0.5% to about 1.1%; about 5% to about 2.4%; about 0.5% to about 3.2%, and about 0.5% to about 4.4%, and other possible sub-ranges) within the indicated range.

[0053] As used herein, the terms “about,” “approximate,” “at or about,” and “substantially” mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact but may be approximate, larger or smaller, as desired, reflecting tolerances, conversion factors, rounding, measurement error, and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In such cases, as used herein, “about” and “at or about” mean the nominal value indicated ±10% variation unless otherwise indicated or inferred. In general, an amount, size, formulation, parameter, or other quantity or characteristic is “about,” “approximate,” or “at or about,” whether or not expressly stated to be such. Where “about,” “approximate,” or “at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself unless expressly stated otherwise.

[0054] As used herein, “optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur. The description includes instances where said event or circumstance occurs and those where it does not.

[0055] As used herein, the term "substantially" means that the subsequently described event or circumstance completely occurs or that the subsequently described event or circumstance generally, typically, or approximately occurs.

[0056] Still further, the term “substantially” can, in some aspects, refer to at least about 90 %, at least about 91 %, at least about 92 %, at least about 93 %, at least about 94 %, at least about 95 %, at least about 96 %, at least about 97 %, at least about 98 %, at least about 99 %, or about 100 % of the stated property, component, composition, or other condition for which substantially is used to characterize or otherwise quantify an amount. Attorney Docket No. 10046-644W01

[0057] As used herein, the term “substantially,” in, for example, the context “substantially identical” or “substantially similar,” refers to a method or a system, or a component that is at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, or about 100% by similar to the method, system, or the component it is compared to.

[0058] Compounds are described using standard nomenclature. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this disclosure belongs.

[0059] As used herein, “rare earth element” refers to a metal or a salt thereof selected from the lanthanides, scandium, and yttrium. In particular, a rare earth element refers to one or more of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), or salts thereof. Typical salts of rare earth elements include those in combination with phosphate, carbonate-fluoride, and oxide anions, but other salts are contemplated herein.

[0060] As used herein, “coal fly ash” is a coal combustion product that is composed of particulates that are driven out of coal-fired boilers together with the flue gases. Coal fly ash is captured after coal combustion by filters (e.g., bag houses), electrostatic precipitators, or other air pollution control devices. The main chemical components of coal fly ash include S iCL. AI2O3, Fe2O3, and CaO. Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash. In general, Class F fly ash contains less than 7% CaO, while Class C fly ash contains greater than 20% CaO, each by weight based on the total weight of the fly ash.

[0061] Processes for Recovery of Rare Earth Elements

[0062] The present disclosure provides processes for the recovery of one or more rare earth elements from coal fly ash. The disclosed processes provide advantages over those previously described by avoiding the requirement for any pretreatment or leaching as compared to previously described methods used for rare earth element recovery. Further, the disclosed processes avoid secondary precipitation (typically from silicates that form gels or flocculants) that may occur otherwise with low pH acid-based dissolution and leaching processes; this secondary precipitation both prohibits large-scale implementation and also may prevent sufficient leaching of the desired rare earth elements. Attorney Docket No. 10046-644W01

[0063] In some aspects, the process includes baking a first mixture including the coal fly ash and an acid solution. In such aspects, the first mixture forms a baked solid. In some aspects, the coal fly ash may be derived from bituminous coal, subbituminous coal, lignite coal, or combinations thereof. In some aspects, the coal fly ash is derived from bituminous coal. In some aspects, the coal fly ash is derived from subbituminous coal. In some aspects, the coal fly ash is derived from lignite coal.

[0064] In some aspects, the coal fly ash can have a SiCh content from 20% to 60% by weight based on the total weight of the coal fly ash. For example, in some aspects, the coal fly ash can have a SiCh content by weight based on the total weight of the coal fly ash of 20%, 25%, 30%, 35%, 50%, 45%, 50%, 55%, 60%, or any subrange formed from the above exemplary values.

[0065] In some aspects, the coal fly ash can have a SiCh content of 20% or more by weight based on the total weight of the coal fly ash (e.g., 25% or more, 30% or more, 35% or more, 40% or more, 45% or more, 50% or more, 55% or more, or 60% or more). In some aspects, the coal fly ash can have a SiCF content of 60% or less by weight based on the total weight of the coal fly ash (e.g., 55% or less, 50% or less, 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, or 20% or less).

[0066] The coal fly ash can have a SiCh content ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the coal fly ash can have a SiCh content from 20% to 60% by weight based on the total weight of the coal fly ash (e.g., from 25% to 55%, from 30% to 50%, from 35% to 45%, from 20% to 40%, from 25% to 35%, from 40% to 60%, or from 45% to 55%).

[0067] In some aspects, the coal fly ash can have an AI2O3 content from 5% to 35% by weight based on the total weight of the coal fly ash. For example, in some aspects, the coal fly ash can have an AI2O3 content by weight based on the total weight of the coal fly ash of 5%, 10%, 15%, 20%, 25%, 30%, 35%, or any subrange formed from the above exemplary values.

[0068] In some aspects, the coal fly ash can have an AI2O3 content of 5% or more by weight based on the total weight of the coal fly ash (e.g., 10% or more, 15% or more, 20% or more, 25% or more, 30% or more, or 35% or more). In some aspects, the coal fly ash can have an AI2O3 content of 35% or less by weight based on the total weight of the coal fly ash (e.g., 30% or less, 25% or less, 20% or less, 15% or less, 10% or less, or 5% or less).

[0069] The coal fly ash can have an AI2O3 content ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the coal Attorney Docket No. 10046-644W01 fly ash can have an AI2O3 content from 5% to 35% by weight based on the total weight of the coal fly ash (e.g., from 10% to 30%, from 15% to 25%, from 5% to 20%, from 10% to 15%, from 20% to 35%, or from 25% to 30%).

[0070] In some aspects, the coal fly ash can have a Fe2O3 content from 4% to 40% by weight based on the total weight of the coal fly ash. For example, in some aspects, the coal fly ash can have a Fe2O3 content by weight based on the total weight of the coal fly ash of 4%, 6%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or any subrange formed from the above exemplary values.

[0071] In some aspects, the coal fly ash can have a Fe2O3 content of 4% or more by weight based on the total weight of the coal fly ash (e.g., 6% or more, 8% or more, 10% or more, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, or 40% or more). In some aspects, the coal fly ash can have a Fe2O3 content of 40% or less by weight based on the total weight of the coal fly ash (e.g., 35% or less, 30% or less, 25% or less, 20% or less, 15% or less, 10% or less, 8% or less, 6% or less, or 4% or less).

[0072] The coal fly ash can have a Fe2O3 content ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the coal fly ash can have a Fe2O3 content from 4% to 40% by weight based on the total weight of the coal fly ash (e.g., from 6% to 35%, from 8% to 30%, from 10% to 25%, from 15% to 20%, from 4% to 20%, from 6% to 15%, from 8% to 10%, from 15% to 40%, from 20% to 35%, or from 25% to 30%).

[0073] In some aspects, the coal fly ash can have a CaO content from 1 % to 40% by weight based on the total weight of the coal fly ash. For example, in some aspects, the coal fly ash can have a CaO content by weight based on the total weight of the coal fly ash of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 12%, 14%, 15%, 20%, 25%, 30%, 35%, 40%, or any subrange formed from the above exemplary values.

[0074] In some aspects, the coal fly ash can have a CaO content of 1 % or more by weight based on the total weight of the coal fly ash (e.g., 2% or more, 3% or more, 4% or more, 5% or more, 6% or more, 7% or more, 8% or more, 9% or more, 10% or more, 12% or more, 14% or more, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, or 40% or more). In some aspects, the coal fly ash can have a CaO content of 40% or less by weight based on the total weight of the coal fly ash (e.g., 35% or less, 30% or less, 25% or less, 20% or less, 15% or less, 14% or less, 12% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less). Attorney Docket No. 10046-644W01

[0075] The coal fly ash can have a CaO content ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the coal fly ash can have a CaO content from 1% to 40% by weight based on the total weight of the coal fly ash (e.g., from 2% to 35%, from 3% to 30%, from 4% to 25%, from 5% to 20%, from 6% to 15%, from 7% to 14%, from 8% to 12%, from 9% to 10%, from 1% to 10%, from 2% to 9%, from 3% to 8%, from 4% to 7%, from 5% to 6%, from 9% to 40%, from 10% to 35%, from 12% to 30%, from 14% to 25%, or from 15% to 20%).

[0076] In some aspects, the coal fly ash can have a carbon content as measured by loss on ignition (LOI) from 0% to 15% by weight based on the total weight of the coal fly ash. For example, in some aspects, the coal fly ash can have a carbon content as measured by LOI by weight based on the total weight of the coal fly ash of 0%, 1%, 3%, 5%, 7%, 9%, 11%, 13%, 15% or any subrange formed from the above exemplary values.

[0077] In some aspects, the coal fly ash can have a carbon content as measured by LOI by weight based on the total weight of the coal fly ash of 0% or more (e.g., 1% or more, 3% or more, 5% or more, 7% or more, 9% or more, 11% or more, 13% or more, or 15% or more). In some aspects, the coal fly ash can have a carbon content as measured by LOI by weight based on the total weight of the coal fly ash of 15% or less (e.g., 13% or less, 11% or less, 9% or less, 7% or less, 5% or less, 3% or less, or 1% or less).

[0078] The coal fly ash can have a carbon content as measured by LOI ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the coal fly ash can have a carbon content as measured by LOI by weight based on the total weight of the coal fly ash from 0% to 15% (e.g., from 1% to 13%, from 3% to 11%, from 5% to 9%, from 0% to 7%, from 1% to 5%, from 7% to 15%, or from 9% to 13%).

[0079] In some aspects, the coal fly ash includes a SiO2 content from 20% to 60% by weight, an AI2O3 content from 5% to 35% by weight, a Fe2O3 content from 10% to 40% by weight, a CaO content from 1% to 12% by weight, and a carbon content (as determined by LOI) from 0% to 15% by weight, each based on the total weight of the coal fly ash.

[0080] In some aspects, the coal fly ash includes a SiO2 content from 40% to 60% by weight, an AI2O3 content from 20% to 30% by weight, a Fe2O3 content from 4% to 10% by weight, a CaO content from 5% to 30% by weight, and a carbon content (as determined by LOI) from 0% to 3% by weight, each based on the total weight of the coal fly ash. Attorney Docket No. 10046-644W01

[0081] In some aspects, the coal fly ash includes a S1O2 content from 15% to 45% by weight, an AI2O3 content from 20% to 25% by weight, a Fe2O3 content from 4% to 15% by weight, a CaO content from 15% to 40% by weight, and a carbon content (as determined by LOI) from 0% to 5% by weight, each based on the total weight of the coal fly ash.

[0082] In some aspects, the coal fly ash can have a calcium content from 10% to 15% by weight based on the total weight of the coal fly ash, for example 10%, 11%, 12%, 13%, 14%, 15%, or any subrange formed from the above exemplary values.

[0083] In some aspects, the coal fly ash can have a calcium content of 10% or more by weight based on the total weight of the coal fly ash (e.g., 11% or more, 12% or more, 13% or more, 14% or more, or 15% or more). In some aspects, the coal fly ash can have a calcium content of 15% or less by weight based on the total weight of the coal fly ash (e.g., 14% or less, 13% or less, 12% or less, 11% or less, or 10% or less).

[0084] The coal fly ash can have a calcium content ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the coal fly ash can have a calcium content from 10% to 15% by weight based on the total weight of the coal fly ash (e.g., from 11% to 14%, from 12% to 13%, from 10% to 13%, from 11% to 12%, from 12% to 15%, or from 13% to 14%).

[0085] In some aspects, the acid solution may include one or more mineral acids. In some aspects, the acid solution may include one or more of hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, boric acid, hydrofluoric acid, hydrobromic acid, perchloric acid, or hydroiodic acid. In some aspects, the acid solution may include one or more strong acids, for example, hydrochloric acid, hydrobromic acid, hydroiodic acid, triflic acid, perchloric acid, nitric acid, sulfuric acid, or combinations thereof. In some aspects, the acid solution may include nitric acid. In some aspects, the acid solution may include other suitable acids, such as any suitable acid known in the art (such as a sulfonic acid). In some aspects, the acid solution may include water as a solvent; however, any other suitable solvent or mixture of solvents may be used as would be understood by one of ordinary skill in the art.

[0086] In some aspects, the acid solution may include a concentration of an acid from 0.1 molar to full strength. As used herein, a “full strength” of an acid refers to a maximum available concentration of a specific acid as used in the present disclosure. For example, and without limitations, the “full strength” of hydrochloric acid is about 12 molar, of nitric acid is about 15 molar, of hydrofluoric acid is about 29 molar, etc. In exemplary aspects, the acid solution Attorney Docket No. 10046-644W01 can have a concentration of an acid of 0.1 molar, 0.5 molar, 1 molar, 2 molar, 3 molar, 4 molar, 5 molar, 6 molar, 7 molar, 8 molar, 9 molar, 10 molar, 11 molar, 12 molar, 13 molar, 14 molar, 15 molar, 16 molar, 17 molar, 18 molar, 19 molar, 20 molar, 21 molar, 22 molar, 23 molar, 24 molar, 25 molar, 26 molar, 27 molar, 28 molar, 29 molar, 30 molar, or any subrange formed from the above exemplary values. It is understood, however, that the full strength of the acid depends on the type of acid, and therefore, the values provided above are only exemplary.

[0087] In some aspects, the acid solution can have a concentration of an acid of 0.1 molar or more (e.g., 0.5 molar or more, 1 molar or more, 2 molar or more, 3 molar or more, 4 molar or more, 5 molar or more, 6 molar or more, 7 molar or more, 8 molar or more, 9 molar or more, 10 molar or more, 11 molar or more, 12 molar or more, 13 molar or more, 14 molar or more, 15 molar or more, 16 molar or more, 17 molar or more, 18 molar or more, 19 molar or more, 20 molar or more, 21 molar or more, 22 molar or more, 23 molar or more, 24 molar or more, 25 molar or more, 26 molar or more, 27 molar or more, 28 molar or more, 29 molar or more, or 30 molar or more). In some aspects, the acid solution can have a concentration of an acid of 30 molar or less (e.g., 29 molar or less, 28 molar or less, 27 molar or less, 26 molar or less, 25 molar or less, 24 molar or less, 23 molar or less, 22 molar or less, 21 molar or less, 20 molar or less, 19 molar or less, 18 molar or less, 17 molar or less, 16 molar or less, 15 molar or less, 14 molar or less, 13 molar or less, 12 molar or less, 11 molar or less, 10 molar or less,

[0088] 9 molar or less, 8 molar or less, 7 molar or less, 6 molar or less, 5 molar or less, 4 molar or less, 3 molar or less, 2 molar or less, 1 molar or less, 0.5 molar or less, or 0.1 molar or less).

[0089] The acid solution can have a concentration of an acid ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the acid solution can have a concentration of an acid from 0.1 molar to 30 molar (e.g., from 0.5 molar to 29 molar, from 1 molar to 28 molar, from 2 molar to 27 molar, from 3 molar to 26 molar, from 4 molar to 25 molar, from 5 molar to 24 molar, from 6 molar to 23 molar, from 7 molar to 22 molar, from 8 molar to 21 molar, from 9 molar to 20 molar, from

[0090] 10 molar to 19 molar, from 11 molar to 18 molar, from 12 molar to 17 molar, from 13 molar to 16 molar, from 14 molar to 15 molar, from 0.1 molar to 15 molar, from 0.5 molar to 14 molar, from 1 molar to 13 molar, from 2 molar to 12 molar, from 3 molar to 11 molar, from 4 molar to 10 molar, from 5 molar to 9 molar, from 6 molar to 8 molar, from 14 molar to 30 molar, from 15 molar to 29 molar, from 16 molar to 28 molar, from 17 molar to 27 molar, Attorney Docket No. 10046-644W01 from 18 molar to 26 molar, from 19 molar to 25 molar, from 20 molar to 24 molar, or from 21 molar to 23 molar).

[0091] In some aspects, the acid solution includes nitric acid having a concentration from 6 molar to 15 molar. In some aspects, the acid solution includes nitric acid having a concentration of 15 molar.

[0092] Upon baking the first mixture including the acid solution and coal fly ash, the solvent (e.g., water) is evaporated, leaving behind a baked solid. Without wishing to be bound by any one theory, the baked solid may include a mixture of dissolved salts (formed via reaction of the acid solution and the one or more rare earth elements), which are water soluble, and a recalcitrant phase including secondary species (e.g., silicates), which are not readily water soluble and thus remain precipitated.

[0093] In some aspects, baking the first mixture may occur at a temperature from 100 degrees Celsius to 800 degrees Celsius or more. For example, in some aspects, baking the first mixture may occur at a temperature of 100 degrees Celsius, 125 degrees Celsius, 150 degrees Celsius, 175 degrees Celsius, 200 degrees Celsius, 250 degrees Celsius, 300 degrees Celsius, 350 degrees Celsius, 400 degrees Celsius, 450 degrees Celsius, 500 degrees Celsius, 550 degrees Celsius degrees Celsius, 600 degrees Celsius, 650 degrees Celsius, 700 degrees Celsius, 750 degrees Celsius, 800 degrees Celsius, or any subrange formed from the above exemplary values.

[0094] In some aspects, baking the fist mixture may occur at a temperature of 100 degrees Celsius or more (e.g., 125 degrees Celsius or more, 150 degrees Celsius or more, 175 degrees Celsius or more, 200 degrees Celsius or more, 250 degrees Celsius or more, 300 degrees Celsius or more, 350 degrees Celsius or more, 400 degrees Celsius or more, 450 degrees Celsius or more, 500 degrees Celsius or more, 550 degrees Celsius or more, 600 degrees Celsius or more, 650 degrees Celsius or more, 700 degrees Celsius or more, 750 degrees Celsius or more, or 800 degrees Celsius or more). In some aspects, baking the first mixture may occur at a temperature of 800 degrees Celsius or less (e.g., 750 degrees Celsius or less, 700 degrees Celsius or less, 650 degrees Celsius or less, 600 degrees Celsius or less, 550 degrees Celsius or less, 500 degrees Celsius or less, 450 degrees Celsius or less, 400 degrees Celsius or less,

[0095] 350 degrees Celsius or less, 300 degrees Celsius or less, 250 degrees Celsius or less, 200 degrees Celsius or less, 175 degrees Celsius or less, 150 degrees Celsius or less, 125 degrees Celsius or less, or 100 degrees Celsius or less). Attorney Docket No. 10046-644W01

[0096] Baking the first mixture may occur at any temperature ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, baking the first mixture may occur at a temperature from 100 degrees Celsius to 800 degrees Celsius (e.g., from 125 degrees Celsius to 750 degrees Celsius, from 150 degrees Celsius to 700 degrees Celsius, from 175 degrees Celsius to 650 degrees Celsius, from 200 degrees Celsius to 600 degrees Celsius, from 250 degrees Celsius to 550 degrees Celsius, from 300 degrees Celsius to 500 degrees Celsius, from 350 degrees Celsius to 450 degrees Celsius, from 100 degrees Celsius to 400 degrees Celsius, from 125 degrees Celsius to 350 degrees Celsius, from 150 degrees Celsius to 300 degrees Celsius, from 175 degrees Celsius to 250 degrees Celsius, from 400 degrees Celsius to 800 degrees Celsius, from 450 degrees Celsius to 750 degrees Celsius, from 500 degrees Celsius to 700 degrees Celsius, or from 550 degrees Celsius to 650 degrees Celsius).

[0097] In some aspects, baking the first mixture may occur at a temperature from 100 degrees Celsius to 300 degrees Celsius. In some aspects, baking the first mixture may occur at a temperature of 125 degrees Celsius. The baking of the first mixture may use any suitable heat source as known in the art. For example, in some aspects, baking of the first mixture may occur by heating using waste heat generated by a power plant.

[0098] In some aspects, the process further includes contacting the baked solid with a solvent to form a leach phase and a precipitate phase. In some aspects, at least a portion of the one or more rare earth elements is dissolved in the leach phase. In some aspects, the one or more rare earth elements are substantially dissolved in the leach phase.

[0099] Any suitable solvent or mixture of solvents may be used for contacting the baked solid as would be recognized by one of ordinary skill in the art. In particular aspects, the solvent includes water. In other particular aspects, the solvent may include water and one or more organic solvents. In other particular aspects, the solvent may include an organic solvent or a mixture of organic solvents.

[0100] In some aspects, the portion of the one or more rare earth elements is dissolved in the leach phase as one or more salts of the one or more rare earth elements. In some aspects, the one or more rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or combinations thereof. In some aspects, the one or more rare earth elements include scandium. In some aspects, the one or more rare earth Attorney Docket No. 10046-644W01 elements include yttrium. In some aspects, the one or more rare earth elements include lanthanum. In some aspects, the one or more rare earth elements include cerium. In some aspects, the one or more rare earth elements include praseodymium. In some aspects, the one or more rare earth elements include neodymium. In some aspects, the one or more rare earth elements include promethium. In some aspects, the one or more rare earth elements include samarium. In some aspects, the one or more rare earth elements include europium. In some aspects, the one or more rare earth elements include gadolinium. In some aspects, the one or more rare earth elements include terbium. In some aspects, the one or more rare earth elements include dysprosium. In some aspects, the one or more rare earth elements include holmium. In some aspects, the one or more rare earth elements include erbium. In some aspects, the one or more rare earth elements include thulium. In some aspects, the one or more rare earth elements include ytterbium. In some aspects, the one or more rare earth elements include lutetium.

[0101] In some aspects, the leach phase includes one or more salts of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or combinations thereof. In some aspects, the leach phase includes one or more scandium salts (e.g., scandium nitrates, scandium halides, or scandium sulfates). In some aspects, the leach phase includes one or more yttrium salts (e.g., yttrium nitrates, yttrium halides, or yttrium sulfates). In some aspects, the leach phase includes one or more lanthanum salts (e.g., lanthanum nitrates, lanthanum halides, or lanthanum sulfates). In some aspects, the leach phase includes one or more cerium salts (e.g., cerium nitrates, cerium halides, or cerium sulfates). In some aspects, the leach phase includes praseodymium salts (e.g., praseodymium nitrates, praseodymium halides, or praseodymium sulfates). In some aspects, the leach phase includes one or more promethium salts (e.g., promethium nitrates, promethium halides, or promethium sulfates). In some aspects, the leach phase includes one or more samarium salts (e.g., samarium nitrates, samarium halides, or samarium sulfates). In some aspects, the leach phase includes one or more gadolinium salts (e.g., gadolinium nitrates, gadolinium halides, or gadolinium sulfates). In some aspects, the leach phase includes one or more terbium salts (e.g., terbium nitrates, terbium halides, or terbium sulfates). In some aspects, the leach phase includes one or more dysprosium salts (e.g., dysprosium nitrates, dysprosium halides, or dysprosium sulfates). In some aspects, the leach phase includes one or more holmium salts (e.g., holmium nitrates, holmium halides, or holmium sulfates). In some aspects, the leach phase includes one or more Attorney Docket No. 10046-644W01 erbium salts (e.g., erbium nitrates, erbium halides, or erbium sulfates). In some aspects, the leach phase includes one or more thulium salts (e.g., thulium nitrates, thulium halides, or thulium sulfates). In some aspects, the leach phase includes one or more ytterbium salts (e.g., ytterbium nitrates, ytterbium halides, or ytterbium sulfates). In some aspects, the leach phase includes one or more lutetium salts (e.g., lutetium nitrates, lutetium halides, or lutetium sulfates).

[0102] In some aspects, the process further includes separating the leach phase and the precipitate phase. Suitable methods for separating the leach phase and precipitate phase would be recognized by those of ordinary skill in the art. In one exemplary aspect, the leach phase and precipitate phase may be separated via centrifugation of a mixture of the leach phase and the precipitate phase that results after contacting the baked solid with a solvent. In another exemplary aspect, the leach phase and the precipitate phase can be separated by filtration of the precipitate phase from the leach phase. In a further exemplary aspect, the leach phase and the precipitate phase may be separated via settling of the precipitate phase from a mixture of the leach phase and the precipitate phase that results after contacting the baked solid with a solvent.

[0103] In some aspects, the process may further include repeating the baking, contacting, and separating steps on a second mixture including the precipitate phase and an acid solution. More particularly, the process may further include: baking a mixture of the precipitate phase and an acid solution to form a second baked solid; contacting the second baked solid with a solvent to form a second leach phase and a second precipitate phase; and separating the second leach phase and the second precipitate phase.

[0104] In some aspects, the process may further include isolating the one or more rare earth elements from the leach phase. Suitable methods for isolating the one or more rare earth elements from the leach phase are known in the art. Exemplary, but non-limiting methods that can be used include solvent extraction, ion exchange, adsorption, membrane separation, or combinations thereof.

[0105] In some aspects, the processes disclosed herein may include recovery of greater than 0% to 100%, including exemplary values of 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 99% of the one or more rare earth elements from the coal fly ash. Attorney Docket No. 10046-644W01

[0106] In some aspects, the processes disclosed herein may include recovery of greater than 0% of the one or more rare earth elements from the coal fly ash (e.g., 1% or more, 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, 45% or more, 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 95% or more, 99% or more, 100%). In some aspects, the processes disclosed herein may include recovery of 100% or less of the one or more rare earth elements from the coal fly ash (e.g., 99% or less, 95% or less, 90% or less,

[0107] 85% or less, 80% or less, 75% or less, 70% or less, 65% or less, 60% or less, 55% or less,

[0108] 50% or less, 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, 20% or less,

[0109] 15% or less, 10% or less, 5% or less, 1% or less).

[0110] The processes disclosed herein may include recovery of the one or more rare earth elements from the coal fly ash ranging from any of the minimum values described above to any of the maximum values described above. For example, in some aspects, the processes disclosed herein may include recovery of greater than 0% to 100% of the one or more rare earth elements from the coal fly ash (e.g., from 1% to 99%, from 5% to 95%, from 10% to 90%, from 15% to 85%, from 20% to 80%, from 25% to 75%, from 30% to 70%, from 35% to 65%, from 40% to 60%, from 45% to 55%, greater than 0% to 50%, from 1% to 45%, from 5% to 40%, from 10% to 35%, from 15% to 30%, from 20% to 25%, from 50% to 100%, from 55% to 99%, from 60% to 95%, from 65% to 90%, from 70% to 85%, from 75% to 80%).

[0111] In some alternative aspects, the processes described herein may be performed on other secondary sources than coal fly ash. A “secondary source,” as described herein, is any source containing at least some amount of the one or more rare earth elements that is not an ore source. Representative examples of such secondary sources include, but are not limited to, manufacturing waste, materials to be recycled, industry byproducts, and the like. In some aspects, the source is an ash. Ash is a known pollutant that can be obtained from combustion of materials, for example, coal, wood, or municipal waste. Many industries are required to collect ash produced as a byproduct of their processes. Thus, in some aspects, the ash may include a coal ash other than coal fly ash, for example, a bottom ash, a boiler slag, a flue gas desulfurization material, or combinations thereof (including combinations with coal fly ash).

[0112] The one or more rare earth elements as recovered by the processes herein may find use in many applications. In another aspect, a composition, device, or article is provided including one or more rare earth elements recovered by the processes described herein. Non-limiting Attorney Docket No. 10046-644W01 exemplary uses include catalysts, magnets, alloys, glasses, electronics, batteries, ceramics, in polishing, as phosphors and pigments, medical imaging and tracers, fertilizers, and water treatment. Further uses of the rare earth elements as recovered by the processes described herein would be readily apparent to those of skill in the art.

[0113] Additional Aspects

[0114] In view of the described processes, compositions, devices, and articles, certain more particular aspects of the disclosure are described below. These particularly recited aspects should not, however, be interpreted to have any limiting effect on any different claims containing different or more general teachings described herein, or that the “particular” aspects are somehow limited in some way other than the inherent meanings of the language literally used therein.

[0115] Aspect 1. A process for recovery of one or more rare earth elements from coal fly ash, the process including: baking a first mixture including the coal fly ash and an acid solution, wherein the first mixture forms a baked solid; contacting the baked solid with a solvent to form a leach phase and a precipitate phase, wherein at least a portion of the one or more rare earth elements is dissolved in the leach phase; and separating the leach phase and the precipitate phase.

[0116] Aspect 2. The process of any aspect herein, such as aspect 1, wherein the coal fly ash is derived from bituminous coal, subbituminous coal, lignite coal, or combinations thereof.

[0117] Aspect 3. The process of any aspect herein, such as aspect 1 or aspect 2, wherein the coal fly ash has a SiCh content from 20% to 60% by weight based on a total weight of the coal fly ash.

[0118] Aspect 4. The process of any aspect herein, such as any one of aspects 1-3, wherein the coal fly ash has an AI2O3 content from 5% to 35% by weight based on a total weight of the coal fly ash.

[0119] Aspect 5. The process of any aspect herein, such as any one of aspects 1-4, wherein the coal fly ash has a Fe2O3 content from 4% to 40% by weight based on a total weight of the coal fly ash. Attorney Docket No. 10046-644W01

[0120] Aspect 6. The process of any aspect herein, such as any one of aspects 1-5, wherein the coal fly ash has a CaO content from 1 % to 40% by weight based on a total weight of the coal fly ash.

[0121] Aspect 7. The process of any aspect herein, such as any one of aspects 1-6, wherein the coal fly ash has a carbon content as measured by loss on ignition (LOI) from 0% to 15% by weight based on a total weight of the coal fly ash.

[0122] Aspect 8. The process of any aspect herein, such as any one of aspects 1-7, wherein the coal fly ash includes a calcium content of 10% to 40% by weight based on a total weight of the coal fly ash.

[0123] Aspect 9. The process of any aspect herein, such as any one of aspects 1-8, wherein the coal fly ash includes a calcium content of 10% to 15% by weight based on a total weight of the coal fly ash.

[0124] Aspect 10. The process of any aspect herein, such as any one of aspects 1-9, wherein the acid solution includes a mineral acid.

[0125] Aspect 11. The process of any aspect herein, such as any one of aspects 1-10, wherein the acid solution includes hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, boric acid, hydrofluoric acid, hydrobromic acid, perchloric acid, triflic acid, hydroiodic acid, or combinations thereof.

[0126] Aspect 12. The process of any aspect herein, such as any one of aspects 1-11, wherein the acid solution includes nitric acid.

[0127] Aspect 13. The process of any aspect herein, such as aspect 12, wherein the nitric acid has a concentration within the acid solution from 6 molar to 15 molar.

[0128] Aspect 14. The process of any aspect herein, such as aspect 12 or aspect 13, wherein the nitric acid has a concentration within the acid solution of 15 molar.

[0129] Aspect 16. The process of any aspect herein, such as any one of aspects 1-14, wherein baking the first mixture occurs at a temperature from 100 degrees Celsius to 800 degrees Celsius.

[0130] Aspect 16. The process of any aspect herein, such as any one of aspects 1-15, wherein the solvent includes water. Attorney Docket No. 10046-644W01

[0131] Aspect 17. The process of any aspect herein, such as any one of aspects 1-16, wherein the portion of the one or more rare earth elements is dissolved in the leach phase as one or more salts of the one or more rare earth elements.

[0132] Aspect 18. The process of any aspect herein, such as any one of aspects 1-17, wherein the one or more salts of the one or more rare earth elements comprise nitrate salts, halide salts, sulfate salts, or combinations thereof.

[0133] Aspect 19. The process of any aspect herein, such as any one of aspects 1-18, wherein the one or more rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or combinations thereof.

[0134] Aspect 20. The process of any aspect herein, such as any one of aspects 1-19, wherein separating the leach phase and the precipitate phase includes centrifugation of a mixture of the leach phase and the precipitate phase.

[0135] Aspect 21. The process of any aspect herein, such as any one of aspects 1-19, wherein separating the leach phase and the precipitate phase includes filtration of the precipitate phase from the leach phase.

[0136] Aspect 22. The process of any aspect herein, such as any one of aspects 1-19, wherein separating the leach phase and the precipitate phase includes settling of the precipitate phase.

[0137] Aspect 23. The process of any aspect herein, such as any one of aspects 1-22, further including repeating the baking, contacting, and separating steps on a second mixture including the precipitate phase and an acid solution.

[0138] Aspect 24. The process of any aspect herein, such as any one of aspects 1-23, further including isolating the one or more rare earth elements from the leach phase.

[0139] Aspect 25. The process of any aspect herein, such as aspect 24, wherein isolating the one or more rare earth elements from the leach phase includes solvent extraction, ion exchange, adsorption, membrane separation, or combinations thereof.

[0140] Aspect 26. The process of any aspect herein, such as aspect 1-25, wherein the process recovers greater than 0% to 100% of the one or more rare earth elements from the coal fly ash.

[0141] Aspect 27. A composition including one or more rare earth elements recovered by the process of any aspect herein, such as any one of aspects 1-25. Attorney Docket No. 10046-644W01

[0142] Aspect 28. A device or article including one or more rare earth elements recovered by the process of any aspect herein, such as any one of aspects 1-25.

[0143] Aspect 29. A catalyst, magnet, allow, glass, electronic, battery, ceramic, polishing agent, phosphor, pigment, medical imaging composition or device, tracer, fertilizer, or water treatment composition including one or more rare earth elements recovered by the process of any aspect herein, such as any one of aspects 1-25.

[0144] A number of aspects of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other aspects are within the scope of the following claims.

[0145] By way of non-limiting illustration, examples of certain aspects of the present disclosure are given below.

[0146] EXAMPLES

[0147] The following examples are set forth below to illustrate the methods disclosed and claimed herein, along with associated methods and results according to the disclosed subject matter. These examples are not intended to be inclusive of all aspects of the subject matter disclosed herein, but rather to illustrate representative methods and results. These examples are not intended to exclude equivalents and variations of the present disclosure, which are apparent to one skilled in the art.

[0148] Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, temperatures, pressures, and other reaction ranges and conditions that can be used to optimize the product purity and yield obtained from the described process. Only reasonable and routine experimentation will be required to optimize such process conditions.

[0149] Example 1: Rare Earths Recovery from Calcium-Rich Fly Ash: Formation of Secondary Phases and Approaches to Minimize Precipitation

[0150] Understanding the conditions that govern precipitate formation in coal fly ash leaching methods assists to ensuring the viability and scalability of coal fly ash as a source of REEs. The objectives of this example are to (i) elucidate the formation of secondary phases during Attorney Docket No. 10046-644W01 acid leaching of calcium-rich coal fly ash, (ii) determine the impact of precipitation on REEs recovery, and (iii) evaluate the feasibility of various methods for precipitate mitigation. Acid leaching experiments were conducted using IM HNO3 and 6M HNO3 solutions, and the composition and structure of the resulting secondary phases were characterized using scanning electron microscopy (SEM-EDS) and x-ray diffraction (XRD). These results show that leaching with 6M HNO3 yields a silica-rich gelatinous phase resulting from soluble silica polymerization and water entrainment. Conversely, leaching with a IM HNO3 solution yields aluminosilicate aggregated flocculants that remain suspended in solution. The gel-forming phases in their native state were further characterized using Raman spectroscopy to evaluate their chemical composition, and using solute transport studies, to assess potential transportlimiting effects induced by the gels. Time-resolved leachate compositions are measured via inductively coupled plasma mass spectrometry (ICP-MS), providing insights into the kinetics of phase precipitation and REEs extraction.

[0151] Furthermore, this example describes and discusses three REE extraction approaches that inhibit secondary phase formation while achieving high REE recovery efficiencies: greater liquid-solid ratios, intermediate acid concentration leaching, and an acid baking method. These results indicate that acid baking is a promising method for REEs extraction from fly ash, yielding high extraction efficiencies with lower solubilized concentrations of major cations when compared to conventional direct leaching. Moreover, although acid baking is a standard hydrometallurgical process used for large-scale REEs extraction, few studies have explored its applicability for coal byproducts. (See Pilkington, E. S., et al. Production of Rare Earth and Thorium Compounds from Monazite. Part I. Journal of the Society of Chemical Industry 1947, 66 (11), 387-394; Demol, J., et al. Hydrometallurgy 2018, 179, 254—267; Rivera, R. M., et al. Miner Eng 2018, 119, 82-92; Nawab, A., et al. International Journal of Coal Preparation and Utilization 2023; and Nawab, A., et al. Miner Eng 2022, 184, 107610). This example constitutes one of the first to report its application to coal fly ash feedstocks. The findings presented in this example, thus, advance understanding of processes governing REEs extraction from coal fly ash, and enable the development of more efficient extraction methods.

[0152] Materials and Methods

[0153] Coal Fly Ash Samples: Four coal fly ash samples with varying calcium concentrations were used in this example. Ash aliquots were sieved using a no. 60 mesh (250 um diameter) to remove clastic contaminants and stored in closed containers. To characterize the elemental Attorney Docket No. 10046-644W01 composition of each ash sample, full acid digestion was carried out using an HF-HN03 microwave-assisted procedure. Specifically, ~25 mg of ash was loaded into PTFE vials along with 3 mL of optima hydrofluoric acid, and 1 mL of concentrated nitric acid. The ash aliquots were digested for 2 hours using a microwave digestion system (Anton-Parr Multiwave). The resulting effluents were transferred to clean PFA vials, dried down, and re-digested using 3 mL of aqua-regia to ensure complete digestion. The final effluents were dried down, redissolved using dilute nitric acid, and their elemental composition was determined using a quadrupole inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500ce). Total REEs concentrations range from 313 to 491 ppm, calcium concentrations (reported as oxide equivalent) vary from 2.9 to 27.9 wt. %, and iron content (reported as oxide equivalent) are typically ~5 wt. %, except for FA-1, where Fe(III) oxide was estimated to be 22.1 wt. %.

[0154] Bulk phase composition was determined using a powder X-Ray diffractometer (XRD, Rigaku Miniflex 600 Difractometer) with a CuKa radiation source operated at a voltage of 40kV, current of 15 mA, and a scan speed of 17minute. Major phases present in the ash samples included amorphous phase, quartz, mullite, anhydrite, periclase, and lime.

[0155] Bulk Leaching Experiments: Leaching experiments were performed using nitric acid solutions with molarities ranging from 1 M to 6 M. First, 2.5 mL of acid solution was transferred to Savillex PFA vials, and pre-heated for 45 minutes at 80° C using a hot plate equipped with a heating block and temperature control. Fly ash aliquots were then added to the pre-heated, mixed vigorously, and allowed to react for a time period ranging from 5 min to 4 hours. The final leachates were transferred into clean polypropylene tubes and, when possible, decanted into separate vials to distinguish the portion hosting the residual ash from the portion hosting the secondary phases. Leachates were then centrifuged, filtered using a 0.45 um PTFE syringe filter to remove any flocculant solids, and the resulting effluents were transferred into clean polypropylene tubes for further analysis. Remaining solid materials were triple washed using deionized (DI) water, air dried for ~ 3 days, and reserved for further characterization. Leaching experiments were performed in quadruplicates, where the resulting effluents of two runs were diluted immediately and reserved for ICP-MS elemental analysis, and the resulting effluents of the remaining runs were used for pH measurements. Liquid-solid ratios of 10 (weight / weight basis) were used for the leaching experiments, unless noted otherwise. Trace metal grade nitric acid (HNO3, Fisher Scientific) was used for all leaching experiments. Attorney Docket No. 10046-644W01

[0156] Precipitates and Secondary Phases Characterization: The physicochemical properties of the precipitated phases were evaluated using scanning electron microscopy (SEM-EDS), XRD, micro-Raman spectroscopy, and solute transport experiments.

[0157] First, phase and elemental characterization of the precipitates formed during acid leaching were carried out using XRD and SEM-EDS (Scios 2HiVac), respectively. The dried precipitates were homogenized using a pestle and mortar prior to analysis. XRD samples were loaded in a zero-background silicon holder and analyzed using a voltage of 40 kV, current of 15 mA, and scan speed of 1.5° / min. For the microscopic characterization, the precipitates were mounted in an aluminum stub using double-sided carbon tape and lightly sputtered (< 10 seconds, Au-Pd EMS Sputter coater) to minimize charge accumulation and improve EDS data collection.

[0158] Second, the chemical properties of gel-forming secondary phases were further evaluated in their original hydrated state using micro-Raman Spectroscopy. Specifically, the chemistry of gelled leachates was characterized using an optical microscope (Nikon Eclipse Lvidia-N) equipped with a Raman spectrometer (Horiba i320) and 532-nm laser source. The resulting spectrum was corrected for baseline shifts and normalized using the water band peak at 1645 cm'1(OH bending vibration mode) as the internal standard. (See Pelletier, M. J. Appl Spectrosc 2003, 57 (1)). While characteristic broad band peak for amorphous phases of interest lies in the 200-600 cm'1wavenumber range, this analysis focused primarily in the 400-600 cm'1region since Rayleigh Scattering interference precluded the analysis of lower wavenumber spectral bands. (See Geisler, T., et al. Nat Mater 2019, 18 (4), 342-348). Relevant band peaks used in this example include the 400-460 cm'1region, indicative of fivefold or larger Si-O-Si rings, and the band defect around 488-495 cm1, characteristic of 4- membered Si-O-Si rings. (See Lenting, C., et al. Corrosion of Ternary Borosilicate Glass in Acidic Solution Studied in Operando by Fluid-Cell Raman Spectroscopy. NPJ). The degree of polymerization of the precipitate was qualitatively assessed using an integrated intensity ratio Rn, defined as the ratio of the integrated intensity of the five-membered ring peak to the integrated intensity of the entire amorphous broad band.

[0159] Third, to determine the transport properties across gelled leachates, solute transport studies were performed using copper nitrate solutions. Copper was selected as the solute for its distinctive color in solutions, enabling visual tracking of the solute front position, and direct colorimetric concentration measurements. Gel leachates were prepared in a 50 mL polypropylene test tube using 6M HNO3, and a liquid-solid ratio of 10, weight / weight basis. Attorney Docket No. 10046-644W01

[0160] Approximately 3 mL of 0.5 M Cu2+solution was then placed on top of the gel, and Cu2+cations were allowed to diffuse through the gel phase.

[0161] The gel phase was sectioned along its length, each gel section was centrifuged, and the concentration of the resulting effluents were measured using an Agilent Cary 60 UV-Vis Spectrometer at 800 nm. Leachate concentrations were determined using a calibration curve generated using solutions with Cu2+concentrations that range between 0.005 M and 0.075 M. The absorbance of the initial leachate was also obtained to enable the subtraction of the background absorbance. Lastly, spatially-resolved concentration profiles were used to model the diffusion of Cu2+cations across the gelled phase. For simplicity, the system was modeled as a one-dimensional unsteady-state diffusion process incluidng a finite source of solute, enabling a first approximation of the diffusive properties of cations across the gelled leachate.

[0162] Acid Baking Experiments: Acid baking experiments were performed using the FA-3 ash sample. First, ~ 200 mg of ash sample were transferred into PFA vials, and 800 mg of fullstrength nitric acid were added to the vial (acid-solid ratio of 4:1, w / w basis). The mixture was dried down at 120°C for 2 hours using a hot plate equipped with a heating block and temperature control. Following the digestion, the resulting solids were grinded using a pestle and mortar, transferred into clean vials, and 2 mL of DI water were added for post-baking leaching. The DI leaching proceeded for 6 hours at room temperature. Samples were periodically agitated during leaching using a Vortex mixer at 1200 rpm. The resulting leachates were centrifuged to separate the remaining solid phases, further diluted and analyzed for their elemental composition using ICP-MS.

[0163] Results and Discussion

[0164] The properties of secondary phases formed in highly acidic and mildly acidic leaching environments, their impact on REE extraction operations, and precipitate mitigation experiments are described in this section.

[0165] The results demonstrate that in highly acidic conditions (pH < 0), a silica-rich gelatinous phase forms, primarily composed of amorphous silica. This gel phase does not hinder the transport of metals. Gelation, however, complicates large-scale operations, creating a highly viscous fluid that precludes solid-liquid separations, and reduces material transport rates at scale.

[0166] In mildly acidic conditions (pH ~ 3), leaching produces amorphous aluminosilicate phases rather than continuous gels. Notably, these results show that low acid concentration conditions Attorney Docket No. 10046-644W01 result in final leachates with significantly lower REEs recoveries, suggesting REEs adsorption onto the precipitated phases.

[0167] Lastly, this example describes and discusses experiments focused on mitigating silica precipitation while maintaining high REEs recovery. The methods include adjusting the liquid-solid ratio and acid concentration, as well as using an acid baking technique followed by DI water leaching. Acid baking followed by water leaching emerges as a promising method, achieving high REEs recovery with minimal silica content in the final solution. This approach addresses the precipitation challenges associated with direct leaching techniques by employing a pre-processing digestion step that effectively breaks down the feedstock into distinct insoluble Si products, and soluble metal salts.

[0168] Secondary Phases in Highly Acidic Solutions (pH < 0): In leaching experiments performed using a 6M HNO3 solution, the final leachate transforms into a silica-rich gelatinous phase (FIGS. 1A-1C). Semi-quantitative elemental characterization using SEM-EDS shows that the gel structural framework is primarily composed of silicon and oxygen (~38 and 45 wt. %, respectively). XRD analysis of the solid phase shows an amorphous hump, indicating that the precipitates in the gel primarily include amorphous silica (FIG. 1A). Similarly, Raman spectroscopy of the gelatinous leachate exhibits broad Raman band peaks in the 400-600 cm" characteristic of amorphous silica (FIGS. 2A-2C). The calculated Raman integrated intensity ratio was 0.22, suggesting a higher prevalence of four-membered silica rings in the precipitated silica. It is noted, though, that the four-membered silica rings structures are reflective of the chemistry of the nm-sized silica nuclei that form under acidic conditions. The gelled phase includes nm-sized silica polymeric aggregates that form at low pH and condense into three-dimensional chain-like structures, trapping water through capillary effects. (See Lazaro, A., et al. Langmuir 2017, 33 (51), 14618-14626; and Iler, R. The Chemistry of Silica Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry 1979).

[0169] Elemental characterization of the fluid entrained in the gel, along with saturation index calculations, indicates that the formed silica gel structure does not incorporate other major cations dissolved during leaching (FIGS. 3A-3B). High recovery efficiencies for the major cations present in the FA-3 ash sample (Fe, Ca, and Al) are achieved rapidly, with 60-95% recovered after 5 minutes and no concentration decreases indicative of product formation (FIGS. 3A-3B). Similarly, thermodynamic calculations indicate that leachates remain undersaturated with respect to Fe, Ca, and Al hydroxide phases, as well as aluminosilicate clay phases (FIG. 3B). In contrast, Si concentrations show a 10-fold reduction as the leaching Attorney Docket No. 10046-644W01 progresses from 5 minutes ([Si]t=5min: ~ 940 ppm, FIG. 3A) to 15 minutes ([Si]t=i5mini ~ 40 ppm, FIG. 3A), and the saturation index of silica was found to be positive (FIG. 3B). Therefore, other than Si, major cations do not precipitate under the acidic conditions imposed here.

[0170] For ash samples with varying compositions, similar susceptibility to secondary silica precipitates is observed (FIG. 2A), although macroscopic gel formation was exclusively noted in the calcium-rich samples (i.e., FA-2, FA-3, and FA-4). Fluid composition measurements show that the silica concentrations across all ash samples range from 9 ppm to 85 ppm (FIG. 2A), proximate to the Si equilibrium concentration for nitric acid solutions of comparable strength. (See Elmer, T. H., et al. Journal of the American Ceramic Society 1958, 41 (12), 517-520). The calculated saturation indices for silica were positive for all three ash samples (TABLE 1), further corroborating the tendency of the studied fly ashes to precipitate silica phases. The observed variations in macroscopic gel formation may therefore be attributed to the composition of the final leachate. Specifically, the metal salts have been shown to lead to higher silica nuclei growth rates and higher collision efficiency rates in acidic solutions. (See Gorrepati, E. A., et al. Langmuir 2010, 26 (13), 10467-10474). Leachates with higher concentrations of major cations therefore are hypothesized to exhibit faster precipitation rates than those with lower-cation concentration leachates.

[0171] TABLE 1. Saturation indices calculated for ash samples FA-1, FA-2, FA-3, FA-4, after a 4- hour 6M HNO3 leaching.

[0172] Another potential explanation for accelerated gel formation observed in calcium rich ashes could be the increased availability of Si in solution resulting from greater destabilization of the alkali-earth enriched ash matrix. Most inorganic acids are not capable of breaking down the Si-0 bonds directly. Secondary cations found dispersed in the ash matrix, conversely, have been shown to be released from ash particulates under very acidic conditions. Silica removal from the ash solid matrix during acid leaching therefore can be surmised to occur through the destabilization of SiO groups during the leaching of major cations co-occurring in the Attorney Docket No. 10046-644W01 aluminosilicate matrix. Calcium-rich ash matrices, in this case, would be predicted to release greater quantities of secondary cations, and, subsequently, larger quantities of silica may be destabilized and co-extracted compared to calcium-poor ashes. It is noted though that the quantity of silica release and re-reprecipitation during acid leaching is predicted to be small, owing to a lack of microscopic morphological changes observed in partially leached ash particulates.

[0173] The impact of gels on REEs recovery was evaluated via solute transport experiments and kinetics study of gel formation (FIG. 4A). Solute transport experiments showed that diffusivity coefficients for Cu2+species across the gels were calculated to be ~ 6.13 (IO6) cm2 / s for 24 hours, and 5.69 (IO-6) cm2 / s for 52 hours. Notably, the modeled transport properties of Cu2+in the silica gels reported here are similar to the reported values for Cu transport in aqueous solutions (Dcuin water: 6.6 (IO-6) to 7.3 (IO-6) cm2 / s). (See Emanuel, A., et al. Diffusion Coefficients of Copper Sulfate in Water and Water in N-Butyl Alcohol). The precipitate framework of the gel phase, therefore, does not appear to create a transport barrier that would hinder metals transport during acid leaching. Moreover, the kinetics of gel formation with respect to REEs recovery further indicates that recoverability is not impacted by the gel. For sample FA-3, the fluid composition shows that ~95 % of REEs are recovered within the first ~5 minutes of leaching, and no loss of REEs is observed over extended leaching periods (FIG. 4B). In contrast, complete leachate gelation requires approximately 10-15 minutes indicating that the kinetics of REEs solubilization are much faster than the rate of gel polymerization. REEs recovery therefore is predicted to occur irrespective of gel formation, based on its faster kinetics, and low impact of gels on cationic transport. While these results demonstrate that gelation of the leachate does not limit high REEs recoveries, gel formation has significant implications for large scale hydrometallurgical operations. Specifically, the filterability of the leachate is severely compromised due to fluid retainment within the gelatinous phase, leading to leachate losses and reduced metal recoveries. (See Abkhoshk, E., et al. Hydrometallurgy 2014, 149, 153-167). Even at low concentrations, silica gels can also raise difficulties for downstream processes, creating stable emulsions in solvent extraction systems that lead to reagent losses, and poorer separation efficiencies. Gel formation, therefore, should be minimized to enable effective and scalable REEs extraction processes.

[0174] Precipitation in Leaching Experiments Using Low Molarity Acid: Low acid concentration leaching of calcium-rich ash generated a precipitate that primarily includes amorphous Attorney Docket No. 10046-644W01 aluminosilicate phases (FIG. 5). Microscopic elemental analysis using SEM-EDS of sample FA-3 shows that the main elemental composition of the formed secondary phases includes Al (~ 15 wt. %), Si (~20 wt. %), and O (~ 45 wt. %). Mineral phase characterization using XRD shows both a hump characteristic of amorphous phases as well as residual quartz and mullite phases since the newly precipitated phases could not be separated from the residual ash (FIG. 5). Interestingly, the macroscopic distribution in solution of the formed products resembles voluminous unsettled colloidal agglomerates rather than the continuous gel phase observed in high acid conditions. Nonetheless, the flocculated particles do not entrap water via capillarity and, thus, are predicted to enable better filterability of final leachates.

[0175] Time-resolved fluid concentration profiles indicate that, except for Ca, other major elements (Si, Fe and Al) present in the FA-3 ash sample re-precipitate into secondary products (FIGS. 7A-7C). As leaching progressed from 5 min to 4 hours, Si concentrations decreased from 910 ppm to 100 ppm, and Fe concentrations decreased from 140 ppm to 3 ppm, constituting nearly a 10-fold concentration reduction for both elements (FIG. 6A). For Al, a 3-fold concentration reduction is observed, with the concentration of Al decreasing from -2700 ppm at 5 minutes, to 900 ppm after 4 hours (FIG. 6A). Thermodynamic modeling corroborates that, given the final solution pH and metal concentrations in the solution, precipitation of amorphous silica, gibbsite, goethite, and clay phases is favorable during low molarity leaching conditions (FIG. 6B).

[0176] Notably, leaching experiments performed using IM HN03 leaching solutions resulted in the formation of secondary phases for samples FA-3 and FA-4, while no precipitates were observed for sample FA-2 (FIG. 7A, TABLE 2). Solution pH measurements indicated a significant pH increase from an initial value of 0, to - 3.1 and - 3.3 for samples FA-3 and FA-4, respectively. The rapid pH rise results from the dissolution of basic compounds (calcium aluminum silicate, periclase, lime) found in calcium-rich ashes. In contrast, sample FA-2 exhibited a final leachate pH of 0.5 and, while the calculated saturation index indicated that the solution was oversaturated with respect to silica (TABLE 2), macroscopic precipitates were not observed during leaching. The composition of the initial ash feedstock is therefore a determinant of precipitate formation in batch leaching conditions.

[0177] TABLE 2. Saturation indices calculated for ash samples FA-1, FA-2, FA-3, FA-4, after a 4- hour IM HNO3 leaching. Attorney Docket No. 10046-644W01

[0178] Moreover, low-molarity acid leaching resulted in insignificantly lower REEs recoveries, suggesting that the formed precipitates reduce the concentrations of REEs in solution. Leaching samples FA-3 and FA-4 using IM HNO3 achieved REEs recovery efficiencies of -55% and 27%, respectively (FIG. 7B). These values are significantly lower compared to those achieved with 6M HNO3 leaching, where final extraction efficiencies of -95 % and -85% were reached for samples FA-3 and FA-4, respectively (FIGS. 3A-3B). Importantly, while the final leachate pH promotes major cation precipitation in the form of amorphous materials, REEs hydroxide formation is not thermodynamically favorable for low-acid molarity leachates (TABLE 2). A plausible explanation for the lower REEs recovery efficiencies is the adsorption of REEs cations onto the formed precipitates. Experiments focused on the pH modification of ash leachates done by Middleton et al (2020) reported a decrease in REEs concentrations before reaching their solubility limit, suggesting REEs adsorption onto the precipitate phases. Minimizing precipitate formation is therefore desired to ensure that high recoveries are achieved.

[0179] Approaches to Mitigate Precipitate Formation While Maintaining High REEs Recoveries:

[0180] Tuning Leachate Conditions: Liquid-Solid Ratio and Optimal Acid Concentration

[0181] Higher Liquid-Solid Ratios'. To mitigate silica precipitation during the extraction of rare earth elements (REEs) via acid leaching, a direct and effective approach involves employing higher liquid-solid ratios (FIGS. 8A-8C). This strategy reduces the silica concentration and, consequently, prevents Si oversaturation. Leaching experiments performed with a liquid-solid ratio of 100, compared to a ratio of 10, yielded REEs extraction efficiencies of -95%, with no visible secondary phases at a macroscopic scale (FIG. 8B). Analysis of the leachte composition revealed Si concentrations below the detection limit, indicating a final Si concentration of less than 10 ppm (FIG. 8A). It is important to note, though, that the resulting total REEs concentration in solution was only -3 ppm, a remarkably low concentration that would require extensive downstream processing to achieve a concentrated final REE product (FIG. 8C). Attorney Docket No. 10046-644W01

[0182] Intermediate Acid Concentration Leaching: The molarity of the acid used is another crucial parameter that can be adjusted to modify the leachate pH and, thus, inhibit the formation of secondary phases. Leaching experiments performed using 2M HNO3 solution yielded REEs recovery efficiencies of -80%, with final total REE concentrations of - 28 ppm (FIG. 9D). Notably, although the final leachate was oversaturated with respect to silica phases (FIGS. 9A-9C), hosting Si concentrations in solution of - 3800 ppm after a 4-hour leaching period, no precipitates or gels were observed at a macroscopic scale.

[0183] Elevated Si concentrations are facilitated by the final leachate pH of - 1, falling within a metastable region for silica condensation. Specifically, Si polymerization rate has been shown to be minimized for pH values near, or at the isoelectric point for colloidal silica (FIG. 10). Leaching using a 2M HNO3 solution, therefore, maintains larger quantities of silicic acid in solution, while reducing their propensity to polymerize. It is important to note, however, that while the rate of gelation was severely reduced, gelation will still occur. Time -resolved fluid concentrations for 2M leaching conditions showed a decrease in Si concentration from - 7900 ppm at 15 minutes, to -3800 ppm after 4 hours (FIG. 9D). Long-term colloidal stability is not anticipated and, for at-scale extraction processes, the accumulation of precipitates over time would present a costly challenge for REEs extraction operations.

[0184] Acid Baking Followed by DI Leaching: For REEs extraction from coal fly ash, acid baking followed by DI leaching achieves high REEs recoveries, with no visible macroscopic gels, and low final silica content in solution (FIGS. 11A-11C). Acid baking experiments performed with 6M HNO3 and 15M HNO3 yielded equally high REE extraction efficiencies of 71 and 74%, respectively, equivalent to - 22 ppm (FIGS. 11B-11C). Interestingly, an inverse relationship is observed between the acid concentration and the recoverability of major cations present in fly ash. Specifically, as the concentration of the acid used during baking is increased from 6M to 15 M, Al concentration in the leachate is reduced from -5900 ppm to 2200 ppm, Fe concentrations are reduced from -530 ppm to -280 ppm, and Si concentration is reduced from 58 ppm to less than 10 ppm (FIG. HA). More concentrated acid solutions during the baking step therefore yielded lower concentrations of unsought secondary cations in solution. Lower acid concentrations of 2M were also tested for acid baking (FIGS. 11A- 11C). Their extraction efficiency was, however, only -6% with similarly negligible recoveries of most major cations with the exception of Ca. Most of the 2 M HNO3 acid is, thus, consumed in the neutralization of the basic Ca components found in the ash. Attorney Docket No. 10046-644W01

[0185] The acid baking process promotes silica precipitation by creating a water- limited environment that promotes silica growth and aggregation into large silica clusters that are not soluble in water. (See VoBenkaul, D., et al. Journal of Sustainable Metallurgy 2017, 3 (1), 79-89; and Vaccarezza, V., et al. Beneficiation and Leaching Study of Norra Karr Eudialyte Mineral. Minerals, Metals and Materials Series 2018, Part F5, 39-51). The metals present in the feedstock, in contrast, are solubilized during acid baking and re-precipitated as water-soluble metal salts. (See Davris, P., et al. Miner Eng 2017, 108, 115-122). Valuable metals are thus re-dissolved using water while the silica product remains in the residual solid phase. In coal fly ash, the acid baking step proceeds by first breaking down the aluminosilicate phase into silicic acid and soluble metals, followed by the precipitation of amorphous Si phases and the formation of metal nitrate salts. Interestingly, the lower recoveries of other major cations found in fly ash suggests that the amorphous product also contains Al and Fe phases. Additional studies are therefore required to fully elucidate the mechanisms controlling phase conversion during acid baking.

[0186] Comparing the Effectiveness of the Three Mitigation Methods'. While all three mitigation approaches described in this section yielded high recovery efficiencies, acid baking provides the most favorable approach to mitigate unsought precipitates while ensuring concentrated REE solutions (FIGS. 12A-12B). Both direct leaching at or near the isoelectric point and acid baking provided attractive alternatives to mitigate the formation of amorphous phases while yielding a solution that is relatively more concentrated in REEs (FIG. 12B). The final Si concentration in solution was, however, much lower for acid baked samples ([Si] < 10 ppm) compared to 2M HNO3 leaching effluents ([Si] ~ 3800 ppm, FIG. 12A). Therefore, acid baked samples are predicted to have a lower susceptibility to Si phase precipitations than direct leaching methods. Moreover, the acid baking approach showed some degree of selectivity, yielding final leachates that host less undesired major cations. When compared to 2M HNO3 acid leaching, acid baking leachates yielded 80% less Al, 85% less Fe, and 25% less Ca in solution (FIG. 12A). Since the acid baking process yields nitrate salts, known for their high solubility in water, the low major cation recoveries observed here are puzzling, and require further investigation to fully elucidate the processes that would yield lower secondary cations with high REEs extraction. Nonetheless, the acid baking method provides a promising alternative for REEs extraction, yielding low silica content solutions, and high REEs recovery efficiencies than direct leaching approaches evaluated in this example.

[0187] Conclusion Attorney Docket No. 10046-644W01

[0188] The results presented in this example delineate the secondary phases that precipitate during acid leaching of calcium-rich fly ash, implications for REEs recoveries and, lastly, three methods to enable precipitate-free final leachates. Under the highly acidic conditions (pH < 0) created by a 6M HNO3 leaching solution, silica precipitation and condensation created a gelatinous leachate. Although a passivating effect was not observed for the formed gel, water entrainment by the secondary precipitate precludes at scale operations under these conditions. For IM HNO3 leaching conditions, most secondary cations present in calcium-rich ashes (Fe, Al, Si) were susceptible to precipitation, forming amorphous flocculant phases. Notably, final REEs recoveries for IM leaching were significantly lower, suggesting that the formed precipitates adsorb some of the REEs released into solution. Lastly, the study evaluated the efficacy of three different precipitate mitigation approaches: higher liquid-solid ratios, intermediate acid concentration leaching, and acid baking followed by DI leaching. These results indicate that acid baking followed by DI leaching generated the most favorable final leachate, with a final fluid composition characterized by high concentrations of REEs and low concentrations of precipitate-forming cations in solution. These findings clarify phenomena that take place during acid leaching of coal fly ash and enable the development of scalable and deploy able REEs extraction processes.

[0189] Example 2: Rare Earths Recovery from Calcium-Rich Coal Fly Ash: Secondary Phase Formation and Mitigation Approaches

[0190] In recent years, legacy coal ash has been identified as a promising alternative source of rare earth elements (REEs) that enables simultaneous waste remediation and valorization. Specifically, ashes enriched in basic compounds (e.g., CaO) lend well to effective REEs extraction using mild reagents but are prone to the formation of precipitates that renders upscaled implementation infeasible. This example characterizes the secondary phases formed during acid leaching and the controls on their formation, evaluates their impact on REEs extraction, and develops an approach that circumvents their precipitation while maintaining effective REEs recovery. Specifically, precipitation is mapped for a range of aqueous pHs: silica gels condense readily under very acidic conditions (pH < 0), while aluminosilicate flocs are ubiquitous at mild pHs (~ 3.5). Solute transport studies and temporally-resolved fluid compositions show that REEs recoveries of ~ 90% are achievable even with the formation of secondary gels, and suggest a minimal impact of gels on cation transport and REEs leaching from the ash matrix. The presence of gels, however, alters the rheologic character of the leachate via its transformation to a semi-condensed phase, and is problematic in upscaled Attorney Docket No. 10046-644W01 operations. Higher pHs (~ 3.5) circumvents the precipitation of gels but are plagued by ubiquitous formation of aluminosilicate flocs and lower REEs recoveries (~ 30 to 55 %). To minimize precipitate formation, this example evaluated alternative extraction methods, including an acid baking (i.e., dry digestion) approach that circumvents secondary precipitates while yielding high REEs extractability of ~ 74% and low co-extraction of secondary elements. Together, the results here provide insights for controlling acid leaching of ash feedstocks and provides alternative methods for REEs extraction that bypass gel and floc precipitation to enable effective REEs recovery from coal ash wastes.

[0191] Introduction

[0192] Rare earth elements (REEs), including the lanthanide series along with Sc and Y, hold unique magnetic and catalytic properties critical across modern technologies. (See Balaram, V. Geoscience Frontiers 2019, 10, 1285-1303; Vidal, O., et al. Nature Geoscience 2013, 894- 896; Alonso, E., et al. Environmental Science and Technology 2012, 46, 3406-3414; Dutta, T., et al. Environmental Research 2016, 150, 182-190; and Imholte, D. D., et al. Energy Policy 2018, 113, 294-305). Increased use of electronics, sustainable energy, defense, and medical infrastructure is projected to raise REEs demand three- to seven-fold by 2040. (See International Energy Agency (IEA). The Role of Critical World Energy Outlook Special Report Minerals in Clean Energy Transitions 2022). Satisfying this rapid demand will require alternative unconventional REEs resources. Coal fly ash, an abundant legacy waste generated in coal-fired power plants, contain up to ~ 1,500 ppm of REEs and presents an exciting opportunity as an abundant and ubiquitous resource. (See Mardon, S. M., et al. International Journal of Coal Geology 2004, 59, 153-169; and Seredin, V.V., et al. International Journal of Coal Geology 2012, 94, 67-93). In the United States alone, fly ash could yield ~ 12,000 tons of REEs annually, surpassing current U.S. consumption. (See Taggart, R.K., et al. Environmental Science and Technology 2016, 50, 5919-5926; Scott, C., et al. FS 2019-3048: Rare Earth Elements in Coal and Coal Fly Ash. USGS Fact Sheet 2019, 3048; and U.S. Geological Survey. Mineral Commodity Summaries 2024 2024). Extracting REEs from fly ash repositories, many of which are piled in unlined landfills, provides an exciting opportunity to simultaneously remediate and valorize toxic environmental wastes.

[0193] The efficacy of REEs extraction, however, hinges on the elemental and mineralogic composition of the ash feedstock. (See Gerardo, S., et al. Environmental Science and Technology 2022, 56, 16200-16208; and Gerardo, S., et al. Environmental Science and Technology Letters 2023, 10, 943-948). In coal fly ash, discrete REE-bearing minerals (e.g., Attorney Docket No. 10046-644W01 phosphates, oxides, fluorocarbonates, etc. that incorporate the rare earth elements into its lattice structure) (see Dai, S., et al. Earth-Science Reviews 2022, 222; and Thomas, B. S., et al. Journal of Environmental Chemical Engineering 2024, 12) exist in one of three modes: ~ 80 % encapsulated in dense amorphous ash particles without internal porosity or pore connectivity necessary for interfacial reaction and reagent transport, ~ 10 % encapsulated in porous ash particles, and the remainder as discrete free-floating REE minerals or bound to the surface of an ash particle accessible directly to reagents. (See Gerardo, S., et al. Environmental Science and Technology 2022, 56, 16200-16208; and Gerardo, S., et al. Environmental Science and Technology Letters 2023, 10, 943-948). While the latter two are amenable to interfacial REE mineral dissolution reactions, effective extraction of REEs should unlock pathways for REE mineral dissolution from dense amorphous ash matrices.

[0194] Recent advances show that effective REEs extraction from dense amorphous ash particulates is possible, where glassy matrices bearing high concentrations of alkali and alkali-earth metals enable solid-state intermolecular diffusion and metal-oxide hydrolysis without bulk dissolution. (See Gerardo, S., et al. Environmental Science and Technology 2022, 56, 16200- 16208; Gerardo, S., et al. Environmental Science and Technology Letters 2023, 10, 943-948; and Middleton, A., et al. International Journal of Coal Geology 2020, 227). Specifically, loosely-held metal cations (e.g., Ca, Al, Fe, etc.) in the glassy matrix are released readily upon exposure to protons (i.e., cation exchange). The liberated metal cations diffuse outward through the vacated molecular interstices while protons diffuse into the glassy ash matrix for further exchange reactions. (See Gerardo, S., et al. Environmental Science and Technology 2022, 56, 16200-16208; and Gerardo, S., et al. Environmental Science and Technology Letters 2023, 10, 943-948). Notably, ashes rich in calcium (i.e., class C ash, CaO wt. > 18%) (see Suraneni, P., et al. ASTM C618 Fly Ash Specification: Comparison with Other Specifications, Shortcomings, and Solutions. ACI Materials 2021). are particularly amenable to this solid-state reaction-diffusion process, where milder and less reagent-intensive approaches (e.g., nitric acid, hydrochloric acid, etc.) have achieved ~ 70 to 100% recovery of the total REEs available in the ash. (See Taggart, R. K., et al. Environmental Science and Technology 2016, 50, 5919-5926; King, J. F., et al. International Journal of Coal Geology 2018, 195, 75-83; Liu, P., et al. Environmental Science and Technology 2019, 53, 5369- 5377; and Stoy, L., et al. Environmental Science and Technology 2021, 55, 9209-9220). By contrast, Ca-poor ashes (class F ash, CaO wt. < 18%) (see Suraneni, P., et al. ASTM C618 Fly Ash Specification: Comparison with Other Specifications, Shortcomings, and Solutions. Attorney Docket No. 10046-644W01

[0195] ACI Materials 2021) yielded ~ 10 to 40 % recovery using the same approach. (See Taggart, R. K., et al. Environmental Science and Technology 2016, 50, 5919-5926; King, J. F., et al. International Journal of Coal Geology 2018, 195, 75-83; Liu, P., et al. Environmental Science and Technology 2019, 53, 5369-5377; and Stoy, L., et al. Environmental Science and Technology 2021, 55, 9209-9220). Ca-rich ashes, therefore, are particularly attractive for REEs extraction due to their high REEs extractability and minimal reagent requirements.

[0196] REEs leaching from Ca-rich ash, however, is coupled frequently with secondary precipitates. (See Middleton, A., et al. International Journal of Coal Geology 2020, 227; and King, J. F., et al. International Journal of Coal Geology 2018, 195, 75-83). Under both mild pHs (e.g., pH ~ 4) and strongly acidic cases (pH < 0), secondary phases (e.g., particles, gels, etc.) precipitate readily that undermine industrial operations. (See Middleton, A., et al. International Journal of Coal Geology 2020, 227; and King, J. F., et al. International Journal of Coal Geology 2018, 195, 75-83). The drawbacks of secondary phase formations in hydrometallurgical operations, including the acidic leaching of REEs from coal ash, are understood from the point of view of upscaled operations and downstream processing. Specifically, precipitates increase the viscosity of the REE-impregnated leachate, reduce solution filterability, and ultimately limits the economic feasibility of upscaled hydrometallurgical operations. (See Queneau, P. B., et al. Canadian Metallurgical Quarterly 1986, 25, 201-209). To inform upscaled industrial practice, the impact of secondary phases on REEs recovery from coal fly ash and approaches to circumvent their formation - so far elusive - should be elucidated.

[0197] This example (i) characterizes the secondary phases formed during acid leaching of calcium- rich coal fly ash, (ii) determines the impact of precipitation on REEs recovery, and (iii) develops approaches to minimize precipitation while maintaining economic rates of REEs extraction. Precipitate formation was probed using nitric acid (HNO3, 1 to 6M), in keeping with the high REEs recoveries demonstrated previously in nitric environments (see Taggart, R. K., et al. Environmental Science and Technology 2016, 50, 5919-5926), and liquid-solid ratios of 10 that result in favorable leaching efficacies for a range of acidic media. (See Middleton, A., et al. International Journal of Coal Geology 2020, 227; and King, J. F., et al. International Journal of Coal Geology 2018, 195, 75-83). The physicochemical properties of the reaction products were probed using scanning electron microscopy (SEM-EDS), x-ray diffraction (XRD), and Raman spectroscopy. Product formation was corroborated by fluid chemical analyses and thermodynamic calculations. Leaching with 6M HNO3 produced a Attorney Docket No. 10046-644W01 silica-rich gelatinous phase, while a 1 M HNO3 solution precipitated aluminosilicate-rich flocculants. Potential transport-limiting effects induced by gelation were evaluated using solute transport experiments, and the overall impact of precipitates on REEs recovery were investigated using time-resolved fluid analysis. Lastly, the study investigated three REE extraction approaches that circumvent secondary phase formation while achieving high REE recovery efficiencies: greater liquid-solid ratios, intermediate acid concentration leaching, and an acid baking method. These results show that acid baking holds promise for REEs extraction from fly ash and yield high extraction efficiencies with low dissolved concentrations of major cations when compared to state-of-the-art direct leaching. It is noted that although acid baking has been used in monazite ore processing (see Demol, J., et al. Hydrometallurgy 2019, 188, 123-139), only recently has this method been tested in unconventional REE resources (see Davris, P., et al. Minerals Engineering 2017, 108, 115— 122; Rivera, R. M., et al. Minerals Engineering 2018, 119, 82-92; Nawab, A., et al. International Journal of Coal Preparation and Utilization 2023; and Nawab, A., et al. Minerals Engineering 2022, 184, 107610), and this is the first report of its application to circumvent gelation during REEs extraction from coal fly ash. The findings reported here advance understanding of processes governing REEs extraction from ash to enable the development of economically feasible and environmentally-benign approaches to simultaneously valorize and remediate ash wastes.

[0198] Materials and Methods

[0199] Coal Fly Ash Samples: Four coal fly ash samples with varying calcium concentrations were used in this example. Ash aliquots were sieved using a no. 60 mesh (250 pm diameter) to remove clastic contaminants and stored in closed containers. It is noted here that the mesh size is substantially larger than the ash particles (< 100 pm) to separate rock fragments without discarding the target ash phase. To characterize the elemental composition of each ash sample, full acid digestion was carried out using an HF-HNO3 microwave-assisted procedure. Specifically, ~ 25 mg of ash was loaded into PTFE vials along with 3 mL of optima hydrofluoric acid and 1 mL of concentrated nitric acid. The ash aliquots were digested for 1 hour at 280 °C and 115 bar using a microwave digestion system (Anton-Parr Multiwave 7000). The resulting effluents were transferred to clean PFA vials, dried down, and re-digested using 3 mL of aqua-regia to ensure a solids-free digested effluent. The final effluents were dried down, re-dissolved using dilute nitric acid, and their elemental compositions were determined using a quadrupole inductively coupled plasma mass spectrometry (ICP-MS, Attorney Docket No. 10046-644W01

[0200] Agilent 7500ce). Total REEs concentrations in the ashes range from 313 to 622 ppm, with calcium concentrations (reported as oxide equivalent) of 2.6 to 27.9 wt. % and iron contents (reported as oxide equivalent) typically ~ 5 wt. % (TABLES 3-4, TABLES 5-6). Sample FA-1 was an exception, where iron oxide was 20.7 wt. % (TABLES 3-4). TABLE 3. Major cation composition of the fly ash samples used in this example, measured via ICP-MS analysis of full digestion samples. Concentrations shown here were converted from elemental ppm to oxide wt%, and represent the average values obtained from duplicate runs. TABLE 4. Major cation composition of the fly ash samples used in this example, measured via ICP-MS analysis of full digestion samples. Concentrations shown here were converted from elemental ppm to oxide wt%, and represent the average values obtained from duplicate runs. *Estimated via mass balance due to Si volatilization and loss during HF-HNO3 sample digestion.

[0201] TABLE 5. Average REEs concentrations in the fly ash samples used in this example. Concentrations were measured via ICP-MS analysis on duplicate runs of full digestion samples. Attorney Docket No. 10046-644W01

[0202] TABLE 6. Average REEs concentrations in the fly ash samples used in this study. Concentrations were measured via ICP-MS analysis on duplicate runs of full digestion samples.

[0203] Bulk phase composition was determined using a powder X-Ray diffractometer (XRD, Rigaku Miniflex 600 Difractometer) with a CuKa radiation source operated at a voltage of 40kV, current of 15 mA, and a scan speed of 17minute. Major phases present in the ash samples included an amorphous aluminosilicate phase, quartz (SiCh), mullite (SAhCh^SiCh), anhydrite (CaSCfl), periclase (MgO), lime (CaO), calcium silicate (CaSiCL), hematite (a- Fe2O3), and maghemite (y-Fc2O3) (FIG. 13).

[0204] Bulk Leaching Experiments and Solids Separation: Leaching experiments were performed using nitric acid solutions with molarities ranging from 1 M to 6 M. First, 2.5 mL of acid solution was transferred to Savillex perfluoroalkoxyl (PFA) vials and pre-heated for 45 minutes at 80° C using a hot plate equipped with a heating block and temperature control. Fly ash aliquots were then added to the pre-heated solution, mixed, and reacted for up to 4 hours to elucidate the kinetics of the acid leaching reaction with ash samples. Previous research shows that 4 hours is sufficient time to reach reaction equilibrium here.

[0205] The final leachates were transferred into clean polypropylene (PPE) tubes and, when possible, decanted into separate vials to differentiate the portion hosting the residual ash from the portion hosting the secondary precipitate phases. Specifically, physical separation via decantation was performed to isolate the newly formed phases from the residual ash particulates. This approach separates the ash residues from the precipitates by leveraging their differential densities. Here, the newly formed phases suspended in the leachate are decanted into a new PPE tube while the heavier residual ash are retained in the original vessel. Leachates were then centrifuged, filtered using a 0.45 pm PTFE syringe filter to remove any particulates, and the resulting effluents were transferred into clean PPE tubes for further Attorney Docket No. 10046-644W01 analysis. The remaining solid materials were triple washed using deionized (DI) water, air dried for ~ 3 days, and reserved for further characterization. Leaching experiments were performed in quadruplicates, where the resulting effluents of two runs were diluted immediately and reserved for ICP-MS elemental analysis, and the resulting effluents of the remaining runs were used for pH measurements. Liquid-solid ratios of 10 (weight / weight basis) were used for the leaching experiments unless noted otherwise. Trace metal grade nitric acid (HNO3, Fisher Scientific) was used in all leaching experiments.

[0206] To better understand the precipitation behavior of different leachates, thermodynamic calculations were performed using PHREEQC, an open-source geochemical modeling tool, along with the Minteq thermodynamic database. Additional solubility products of mineral phases of interest (i.e., REEs hydroxides) were compiled (see Martell, A. E., et al. Critical Stability Constants: Inorganic Complexes 2004, 4; Shiery, R. C., et al. Inorganic Chemistry 2021, 60, 3117-3130; and Diakonov, I. I., et al. Radiochimica Acta 1998, 81, 107-116 and added to the database to evaluate potential REEs losses through precipitation. The input files for the calculations included the leachate pH measurements along with the measured concentrations of major cations and REEs in solution. Nitrate (NO3 ) concentration was also included based on the molarity of the initial acid solution used. As an output, PHREEQC provides the aqueous speciation data and calculated saturation indices (SI). The output saturation indices were used to evaluate whether the precipitation of a particular mineral is likely to occur. Specifically, a positive saturation index (SI > 0) indicates that the solution is oversaturated with respect to a mineral and, thus, precipitation is favorable, while negative a saturation index (SI < 0) indicates that the solution is undersaturated and, therefore, species will remain in the aqueous solution (i.e., precipitation will not occur).

[0207] Precipitates and Secondary Phases Characterization: The physicochemical properties of the precipitated phases were evaluated using scanning electron microscopy (SEM-EDS), XRD, micro-Raman spectroscopy, and solute transport experiments.

[0208] First, phase and elemental characterization of the precipitates formed during acid leaching were carried out using XRD and SEM-EDS (Scios 2HiVac), respectively. The dried precipitates were homogenized using a mortar and pestle prior to analysis. XRD samples were loaded in a zero-background silicon holder and analyzed using a voltage of 40 kV, current of 15 mA, and scan speed of 1.5 7min. For microscopic characterization, the precipitates were mounted on an aluminum stub using double-sided carbon tape and lightly sputtered (< 10 Attorney Docket No. 10046-644W01 seconds, Au-Pd EMS Sputter coater) to minimize charge accumulation and to improve EDS data collection.

[0209] Second, the chemical properties of secondary phases were further evaluated in their original hydrated state using micro-Raman spectroscopy. Specifically, gelled leachates were characterized using an optical microscope (Nikon Eclipse Lvidia-N) equipped with a Raman spectrometer (Horiba i320) and a 532 nm laser source. The resulting spectrum was corrected for baseline shifts and normalized using the water band peak at 1645 cm'1(OH bending vibration mode) as the internal standard. (See Pelletier, M. J. Applied Spectroscopy 2003, 57).While characteristic broad band peaks for amorphous phases of interest lie in the 250 to 600 cm'1wavenumber range (see Geisler, T., et al. Nature Materials 2019, 18, 342-348), this analysis focused primarily in the 400 to 600 cm'1region since Rayleigh scattering interference impedes the analysis of lower wavenumber spectral bands. Relevant band peaks used in this study include those in the 400 to 460 cm'1region indicative of five-fold or larger Si-O-Si rings, and the band at 488 to 495 cm'1characteristic of 4-membered Si-O-Si rings. (See Lenting, C., et al. NPJ Materials Degradation 2021). The degree of polymerization of the precipitate was qualitatively assessed using an integrated intensity ratio, Rn(see Geisler, T., et al. Nature Materials 2019, 18, 342-348; and Lenting, C., et al. NPJ Materials Degradation 2021), defined as the ratio of the integrated intensity of the five-membered ring peak to the integrated intensity of the entire amorphous broad band (Rn= Idv).

[0210] Third, to determine the transport properties of the gels, solute transport studies were performed using copper nitrate (CuNOs, Sigma Aldrich). Copper was selected as the solute for its distinctive color in solutions that enables visual tracking of the solute front position and direct colorimetric concentration measurement. Gel columns were prepared by mixing FA-3 with 6 M HNO3 at a liquid-solid ratio of 10 (weight / weight basis). The acid-ash mixture was prepared in a 50-mL PPE test tube in the upright position to ensure that the ash settled at the bottom, while the leachate remained at the top of the column. The leachate was allowed to gel overnight at room temperature. Approximately 3 mL of 0.5 M CuNO? solution was then placed on top of the gel and allowed to diffuse through the gel phase.

[0211] Following the CuNCL diffusion (e.g., after 24 h and 52 h), the gel phase was sectioned along its length to evaluate the impact of gel on cation (i.e., Cu2+) transport. Each gel section was centrifuged, and the concentration of the fluids entrained in the gel were measured using an Agilent Cary 60 UV-Vis Spectrometer at 800 nm. The data were cast on a calibration curve Attorney Docket No. 10046-644W01 generated using standard Cu2+solutions with concentrations that range between 0.005 M and 0.075 M (FIG. 14). The absorbance of the initial leachate was also obtained to subtract background signals. Lastly, spatiotemporal concentration profiles were used to model the diffusion of Cu2+cations across the gelled phase. For simplicity, the system was modeled as a one-dimensional unsteady diffusion process: dC _ d2C dt Qx2 where C is the solute concentration, x is the position along the gel column, t is the time, and D is the diffusion coefficient. A thin source of solute in a semi-infinite medium is assumed (C(x -> oo, t) = 0), constrained by the mass conservation of the solute (f C(x, t)dx = N, where N is the amount of solute in the system), and zero concentration in the gel at initial time C(x, t = 0) = 0. The analytical solution was used to determine the diffusion coefficient, enabling a first-order approximation of the diffusive properties of cations across the gelled leachate:

[0212] Acid Baking Experiments: Acid baking experiments were performed using the FA-3 ash sample. First, ~ 200 mg of ash sample was transferred into PFA vials, and 800 mg of 15 M nitric acid was added to the vial (acid-solid ratio of 4:1, weight / weight basis). The mixture was dried down at 120 °C for 2 hours using a hot plate equipped with a heating block and temperature control. Following digestion, the resulting solids were grinded using a mortar and pestle, transferred into clean vials, and 2 mL of DI water were added for post-baking leaching. The water leaching step proceeded for 6 hours at room temperature. Samples were agitated periodically during leaching using a Vortex mixer at 1200 rpm (30 s pulses each hour). The resulting leachates were centrifuged to separate the remaining solid phases, further diluted using 2% HNO3, and analyzed for their elemental composition using ICP-MS.

[0213] Results and Discussion

[0214] Secondary Phases in Highly Acidic Solutions (pH < 0):

[0215] Precipitate Characteristics: Ash leaching using a 6M HNO3 solution precipitated a gelatinous phase (FIGS. 1A-1C, FIGS. 2A-2C). Semi-quantitative elemental characterization using SEM-EDS shows that the gel structural framework is composed primarily of silica (~ 36 wt.% Si and ~ 43 wt. % O, FIG. 1A, FIGs. 15A-15B). Minor elements include Al, Ca, and P. XRD Attorney Docket No. 10046-644W01 analysis of the dehydrated gel shows an amorphous hump (see Chancey, R. T., et al. Cement and Concrete Research 2010, 40, 146-156) and, along with the elemental data collected, indicates that the precipitates in the gel primarily include of amorphous silica (FIG. IB). Raman spectroscopy of the native hydrated gel further corroborates the in situ composition of the precipitated phase, with broad Raman band peaks in the 400 to 600 cm'1characteristic of amorphous silica (FIG. 1C). The calculated Raman integrated intensity ratio was Rn~ 0.22, suggesting a higher prevalence of four-membered silica rings that are polymerized into nanometer-sized nuclei (~ 2 to 4 nm). (See Iler, R. The Chemistry of Silica Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry 1979). These molecular nuclei further condense into three-dimensional chain-like structures that trap water through capillary effects and produce a hydrated gel. (See Iler, R. The Chemistry of Silica Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry 1979; and Lazaro, A. et al. Langmuir 2017, 33, 14618-14626).

[0216] Elemental characterization of the fluid entrained in the gel along with saturation index calculations further corroborate that other than Si, the gel structure does not incorporate other major cations dissolved during leaching (FIGS. 2A-2C). In the FA-3 ash sample, 6 M HNO3 leaching extracted ~ 60 to 95% of the Fe, Ca, and Al present in the ash within the first 5 minutes (FIGS. 2A-2B). Importantly, no Fe, Ca, and Al concentration decreases, indicative of product formation, were measured over time (FIG. 2A). Thermodynamic calculations agree, with negative saturation indices for the Fe, Ca, and Al hydroxide phases as well as aluminosilicate clays (FIG. 2C), confirming that precipitation of these phases is not favorable. In contrast, dissolved Si concentrations decrease ~ 10-fold as the leaching progresses from 5 minutes ([Si]t=5min: ~ 940 ppm, FIG. 2A) to 15 minutes ([Si]t=i5mini ~ 40 ppm, FIG. 2A), during which time gels precipitate ubiquitously. Throughout the leaching process, the calculated saturation index of silica was positive, and precipitation is predicted (FIG. 2C).

[0217] When 6M leaching experiments were performed using ash feedstocks of different compositions, it was noted that silica precipitation was consistently favorable, although gelation was only observed macroscopically in ashes FA-2, FA-3, and FA-4. Dissolved Si concentrations ranging between ~ 9 ppm and ~85 ppm were measured in the leachates of all ash samples (FIG. 3A, FIGs. 16A-16B, TABLES 7-9, TABLES 10-12), comparable to Si solubility in concentrated HNO3 solutions ([Si] ~ 45 ppm). (See Elmer, T, et al. Journal of the American Ceramic Society 1958, 41, 517-520). Positive saturation indices for silica in all Attorney Docket No. 10046-644W01 ash leachates further corroborate their tendency to precipitate silica phases (TABLE 13). Kinetically favorable conditions for precipitation, however, appear to only be present in calcium-rich ashes (i.e., FA-2, FA-3, and FA-4). A first explanation for the differences in precipitation behavior is the composition of the final leachate. Specifically, metal cations in solution have been reported to accelerate silica precipitation (see Gorrepati, E. A., et al. Langmuir 2010, 26, 10467-10474) and, therefore, ash feedstocks that release more secondary cations (i.e., major cationic species present in the ash that are not of extractive interest, including Al, Ca, Fe, Na, Mg) are predicted to precipitate at faster rates than those releasing fewer cations in solution.

[0218] TABLE 7. Average elemental concentrations for 4-hour 6M leaching experiments using S :L ratio of 10.

[0219] TABLE 8. Average elemental concentrations for 4-hour 6M leaching experiments using S :L ratio of 10.

[0220] TABLE 9. Average elemental concentrations for 4-hour 6M leaching experiments using S :L ratio of 10.

[0221] TABLE 10. Average recovery efficiencies for 4-hour 6M leaching experiments using S:L ratio of 10. Attorney Docket No. 10046-644W01

[0222] TABLE 11. Average recovery efficiencies for 4-hour 6M leaching experiments using S:L ratio of 10. TABLE 12. Average recovery efficiencies for 4-hour 6M leaching experiments using S:L ratio of 10.

[0223] TABLE 13. Summary of relevant saturation indices calculated for ash samples FA-1, FA-2,

[0224] FA-3, FA-4, after a 4-hour 6 M HNO3 leaching.

[0225] Another explanation for the accelerated gel formation in calcium-rich ashes is an increased destabilization and release of Si-0 groups from the bulk ash matrix during cation exchange (i.e., M-O-Si + H+— > H-O-Si + M+). The release of metal cations from amorphous ash matrices create molecular interstices that enable the solid-state interdiffusion of metal cations outward and solution protons inward. (See Gerardo, S., et al. Environmental Science and Attorney Docket No. 10046-644W01

[0226] Technology 2022, 56, 16200-16208; and Gerardo, S., et al. Environmental Science and Technology Letters 2023, 10, 943-948). Si mobilization during leaching, however, can only occur after a certain degree of matrix framework destabilization during cationic exchange. Specifically, the removal of secondary cations from the ash matrix can generate Si-0 tetrahedra with partial to no connectedness to the bulk matrix network, and consequently promote Si-0 release into solution by reducing or eliminating the number of protonation steps needed to hydrolyze an Si-0 tetrahedron. (See Brantley. Kinetics of Water Rock Interactions). Therefore, owing to their higher cation concentrations in the ash matrices, calcium-rich samples experience more extensive cationic exchange and, potentially, generate larger quantities of silica that oversaturate the fluid and promote rapid gel formation.

[0227] Impact of Gels on REEs Recovery: To understand the impact of gels on REEs recovery, the study evaluated the transport properties of the gel and assessed whether precipitation adversely affects REEs recovery rates (FIG. 4A). Specifically, solute transport experiments were performed for Cu2+across the gels for 24 and 52 hours. Here, the transport of Cu2+in the gels followed Fickian diffusion closely, where diffusion coefficients, calculated using spatially-resolved Cu2+concentrations in the interstitial fluids, were consistent across time (~ 6.13 x 10’6cm2 / s for 24 hours, and 5.69 x 10’6cm2 / s for 52 hours).

[0228] Interestingly, the transport properties of Cu2+in the silica gels measured here are similar to values reported for Cu2+transport in aqueous solutions (6.6 x 10"6to 7.3 x 10"6cm2 / s). (See Emanuel, A., et al. Diffusion Coefficients of Copper Sulfate in Water and Water in N -Butyl Alcohol). The gel framework, therefore, does not significantly bar the transport of cations and is not anticipated to hinder metals diffusion during acid leaching. Moreover, a comparison between the kinetics of gel formation and the kinetics of REEs recovery indicates that proton accessibility to the REEs is not impacted by the gel. For sample FA-3, fluid analyses show that more than 90 % of REEs are recovered within the first ~ 5 minutes of leaching, with no loss of dissolved REEs observed over extended leaching periods (i.e., no REE precipitation, FIG. 4B). In contrast, complete leachate gelation requires approximately 10 to 15 minutes, suggesting that the kinetics of REEs extraction from the ash are much faster than the rate of silica transport and polymerization. REEs recovery therefore is predicted to occur irrespective of gel formation based on its faster kinetics and the minimal impact of gels on cationic transport.

[0229] When compared to other gel-prone ash samples (FA-2 and FA-4, FIG. 3B), it is noted that REEs recovery appears to follow primarily the composition of feedstock material rather than Attorney Docket No. 10046-644W01 variations in gel characteristics (e.g., degree of crosslinking, viscosity). For instance, the FA- 4 ash has a higher alkali content and thus is predicted to destabilize more Si-0 tetrahedra during leaching and generate a more condensed (i.e., tightly packed) gel. The REEs recovery for sample FA-4, however, is equally as high as sample FA-3 (FIG. 3B). Higher concentrations of major cations in solution provide further evidence of greater leaching performance of feedstocks characterized by high REEs extraction efficiency (TABLES 7-9, TABLES 10-12). Thus, although the potential impact of gel characteristics cannot be fully decoupled from the intraparticle events controlling REEs release, the results here suggest that gel condensation differences may have limited impact on recovery.

[0230] While these results demonstrate that gelation of the leachate does not hinder REEs release from the ash particles, gel formation does have significant implications for large-scale hydrometallurgical operations. Specifically, the filterability of the leachate is compromised due to fluid retainment within the gelatinous phase, leading to leachate loss and reduced metal recovery. (See Rivera, R. M., et al. Minerals Engineering 2018, 119, 82-92; and Abkhoshk, E., et al. Hydrometallurgy 2014, 149, 153-167). The gelled leachate is also a highly viscous phase that reduces mass transfer at scale, strains pumping equipment, and can create a cemented phase that clogs processing equipment. (See Queneau, P. B., et al. Canadian Metallurgical Quarterly 1986, 25, 201-209). Even at low concentrations, silica gels complicate downstream processes by creating stable emulsions in solvent extraction systems that lead to reagent losses and poorer separation efficiencies. (See Abkhoshk, E., et al. Hydrometallurgy 2014, 149, 153-167). Gel formation, therefore, should be minimized to enable effective and scalable REEs extraction processes.

[0231] Low Molarity Acid Leaching: Low molarity solutions (1 and 2 M HNO3) were evaluated to minimize and when possible, inhibit, gel formation. Concentrations of 1 M and 2 M HNO3 were tested using an L:S ratio of 10 to evaluate REEs recovery efficiency and potential for gel mitigation.

[0232] 2 M HNO3 leaching: Leaching with 2 M HNO3 recovers ~ 82% of the REEs present without visible gel formation within the timeframes for leaching (~ hours), however gelation remains favorable over long periods (~ weeks). Leachate samples aged over the course of 2 weeks condensed into gelatinous phases similar to higher leaching molarity cases. Although no precipitates or gels were observed at a macroscopic scale, time-resolved fluid concentrations for 2 M leaching conditions showed a decrease in Si concentration from ~ 7900 ppm at 15 minutes to ~ 3800 ppm after 4 hours (FIG. 9A). Thermodynamic modeling indicates that the Attorney Docket No. 10046-644W01 final leachate remained oversaturated with respect to silica phases ([Si] ~ 3800 ppm) after the 4 h leaching and, thus, further precipitation and polymerization is predicted to take place. Here, Si oversaturation is enabled by the solution pH of ~ 1 (FIG. 17), which falls within the metastable region for Si condensation where gelation rates are minimized for pH values near or at the silica isoelectric point (PHIEP ~ 1 to 3, FIG. 10). (See Queneau, P. B., et al. Canadian Metallurgical Quarterly 1986, 25, 201-209). Leaching using a 2 M HNO3 solution, therefore, maintains higher concentrations of silicic acid in solution while reducing the rate at which silicate species polymerize. Notably, while the rate of condensation (Si-O-Si formation) was reduced drastically, gelation remains favorable and progresses slowly. Therefore, the precipitation of silica species is still predicted to take place, and its accumulation would present a costly challenge over longer operation periods and cycles.

[0233] 1 M HNO3 leaching: Leachate gelation during REEs extraction was avoided successfully using 1 M HNO3. Floc precipitation, however, occurred for the Ca-rich ashes, and lower REEs recoveries were observed across all tested ashes (FIG. 7B). For the calcium-rich FA-3 ash, low acid concentration leaching generated a precipitate that primarily includes amorphous flocculants (FIG. 5). Microscopic elemental analysis using SEM-EDS of flocs from sample FA-3 shows that the main elemental composition of the formed flocculants primarily includes Al (~ 14 wt. %), Si (~ 17 wt. %), and O (~ 45 wt. %) (FIGs. 18A-18B). Mineral phase characterization of the total residue (i.e., reacted ash and precipitate mixture) using XRD shows both a hump characteristic of amorphous phases as well as residual quartz and mullite phases from the residual ash (FIG. 5).

[0234] Silica polymerization is speculated to limit metal diffusion from and to oxide surfaces, and secondary hydroxide / aluminosilicate phases are speculated to provide additional surface for re-adsorption of soluble metals. (See Middleton, A., et al. International Journal of Coal Geology 2020, 227). Understanding how precipitate formation during coal fly ash leaching impacts metals transport assists to better elucidate the mechanisms controlling REEs release from fly ash.

[0235] Solution pH measurements indicated a significant pH increase from an initial value of 0 to ~ 3.1 and ~ 3.3 for samples FA-3 and FA-4, respectively, within the first 5 minutes (FIG. 19, FIG. 20). The rapid pH rise results from the dissolution of basic compounds (calcium silicate, periclase, lime) in the calcium-rich ashes. In contrast, sample FA-2 exhibited a final leachate pH of 0.5 (FIG. 19), and while the calculated saturation index indicated that the solution was oversaturated with respect to silica (TABLE 14), no macroscopic precipitation was observed. Attorney Docket No. 10046-644W01

[0236] Time-resolved fluid concentration profiles indicate that, except for Ca, the major elements (Si, Fe and Al) present in the FA-3 ash sample re-precipitate into flocculants. Over the course of leaching (t = 5 min to 4 hours), dissolved Si and Fe concentrations decreased - 10-fold ([Si] - 910 ppm to 100 ppm, [Fe] - 140 ppm to 3 ppm, FIG. 6A). For Al, a 3-fold concentration reduction is observed, with the concentration of Al decreasing from -2700 ppm at 5 minutes to 900 ppm after 4 hours (FIG. 6A). Saturation index calculations corroborate that given the final solution pH and metal concentrations in solution, the precipitation of amorphous silica, gibbsite, goethite, and clay phases is thermodynamically favorable during low molarity leaching (FIG. 6B).

[0237] TABLE 14. Summary of relevant saturation indices calculated for ash samples FA-2, FA-3, FA-4, after a 4-hour IM HNO3 leaching.

[0238] Leaching samples FA-3 and FA-4 using IM HNO3 yielded REEs recovery efficiencies of only - 55 % and 27 %, respectively (FIG. 7A, FIG. 7C). These values are significantly lower compared to those achieved with 6M HNO3 leaching, where final extraction efficiencies of - 90 % and - 85 % were reached for samples FA-3 and FA-4, respectively (FIGS. 3A-3B). REEs hydroxide formation is not thermodynamically favorable for low-acid molarity leachates (TABLE 14). A possible explanation for the lower REEs recovery efficiencies is the adsorption of REEs cations onto the precipitated flocs. Experiments focused on the pH modification of ash leachates by Middleton et al (2020) reported a decrease in REEs concentrations before reaching their solubility limit, suggesting REEs adsorption onto the precipitate phases. (See Middleton, A., et al. International Journal of Coal Geology 2020, 227). It is noted, however, that stoichiometric calculations suggest that it is possible that full REEs recovery is not achieved during 1 M HNO3 leaching owing to fewer available protons. Therefore, although adsorption is possible, acid limitations may also contribute to the decreased ultimate REEs recoveries under these conditions.

[0239] Mitigating Gel Formation: Acid Baking Followed by Water Leaching: Acid baking (120 °C, 2 h, acid molarity: 15 M) followed by water leaching, excitingly, recovered REEs with high efficacy, no visible macroscopic gels or flocs, and low final silica content in solution Attorney Docket No. 10046-644W01

[0240] (FIGS. 11A-11C). REEs recovery efficiencies of up to ~ 74 % were achieved, with the final REEs concentration in solution of up to 24 ppm and amenable to separations downstream (FIGS. 11B-11C). Importantly, the acid baking approach yielded lower concentrations of unsought secondary cations in the final solution and reduces the load for separation. Compared to direct leaching, the full acid strength (15 M) acid baking pretreatment reduced the concentrations of Al, Ca, and Fe in solution from 11,600 ppm to 2,200 ppm, 17,530 ppm to 12,160 ppm, and 3,500 ppm to 280 ppm, respectively, and yielded undetectable silica concentrations in solution with no visible suspended precipitates (FIG. 2A vs. FIG. 11A).

[0241] These compelling results are enabled by the acid baking process, which generates an acidic environment that breaks down metal-silica bonds while limiting water availability and promotes the rapid reprecipitation of dissolved species (e.g., amorphous silica clusters, salts). (See VoBenkaul, D., et al. Journal of Sustainable Metallurgy 2017, 3, 79-89; and Vaccarezza, V. & Anderson, C. Beneficiation and leaching study of Norra Karr eudialyte mineral. Minerals, Metals and Materials Series Part F5 2018, 39-51). Specifically, metal cations are precipitated as water-soluble salts (see Davris, P., et al. Minerals Engineering 2017, 108, 115— 122; and VoBenkaul, D., et al. Journal of Sustainable Metallurgy 2017, 3, 79-89) that are amenable to water leaching while the silica product remains insoluble. In the coal fly ash experiments, the acid baking pretreatment breaks down the amorphous aluminosilicate matrix into silicic acid and soluble metals. With decreasing water concentration, amorphous insoluble Si phases and soluble metal salts precipitate. Interestingly, while high REEs recoveries found in this example suggest the formation of water-soluble REE compounds during acid baking, secondary cation concentrations in the water leachate are low (FIG. 11A) and suggest the incorporation of Al, Fe, and Ca into the insoluble product. Additional studies should be performed to fully elucidate the possible phase conversions during acid baking.

[0242] To evaluate the impact of acid concentration on the effectiveness of the baking step, baking experiments were also performed using a 2 M and 6 M HNO3 solutions. Acid baking using 2 M yielded the lowest REEs extraction efficiency, recovering only ~ 6 % of the total REEs (FIG. 11B). Similarly, negligible recoveries were recorded for most major cations, with the exception of Ca (FIG. 11A). Acid baking experiments performed with 6 M HNO3, in contrast, yielded favorable REE extraction efficiencies of ~ 71 %, equivalent to ~ 22 ppm dissolved REEs in solution (FIGS. 11B-11C). Interestingly, for effective baking conditions (i.e., 6M and 15M solutions), an inverse relationship between the acid concentration used and the purity of the final leachate was observed. Specifically, when comparing the composition of Attorney Docket No. 10046-644W01 the leachate generated following a 6 M and a 15 M baking step, it was noted that the Al concentration in the leachate is reduced from - 5900 ppm to 2200 ppm, Fe concentrations are reduced from -530 ppm to -280 ppm, and Si concentration is reduced from 58 ppm to less than 10 ppm, respectively (FIG. 11A). More concentrated acid solutions during the baking step (e.g., 15 M HNO3) therefore yielded lower concentrations of unsought secondary cations in solution. The mechanisms that lead to a reduction in the solubility of secondary cations, however, remain unclear and should be investigated further.

[0243] Environmental Implications: Coal fly ash accounts for - 70% of solid waste generated in coal-fired power plants and poses a human health and ecologic hazard to local communities and to the environment. (See Chen, Y., et al. Ecotoxicology and Environmental Safety 2024, 269, 115905; and Wu, J., et al. Environmental Science and Technology 2021, 55, 6654). Recently, REEs extraction has been proposed as an alternative waste management approach to traditional ash disposal. Fundamental phenomena that take place during REEs extraction from coal fly ash, however, remain poorly understood and should be further explored to ensure the scalability of this remediation-valorization method.

[0244] This example advances understanding of REEs extraction from coal fly ash by evaluating the impact of precipitate formation on the effectiveness of leach-based extractive approaches. Specifically, this example delineates the secondary phases that precipitate during acid leaching of calcium-rich fly ash, describes the implications of precipitates on the recovery of REEs, and, lastly, provides methods to minimize precipitates in the final leachates to enable upscaling. Under the highly acidic conditions (pH < 0) created by a 6M HNO3 leaching solution that recovers - 95% of the REEs in 1 hour, silica precipitation and condensation created a gelatinous leachate. Although gels did not passivate the extraction of REEs from ash matrices, water entrainment within the gels impedes at-scale operations under these conditions. For IM HNO3 leaching, most major elements present in calcium-rich ashes (Fe, Al, Si) were susceptible to precipitation and form flocculants. Notably, final REEs recoveries for IM leaching were significantly lower, suggesting either incomplete extraction or that the flocculants adsorb some of the REEs released into solution. Lastly, the study evaluated the efficacy of acid baking followed by water leaching approach to circumvent gel formation. These results show that acid baking followed by water leaching achieves high REEs recovery as well as high REEs concentrations and low secondary cation concentrations in the final solution while avoiding the formation of secondary phases (e.g., gels, flocs, etc.). Overall, the findings presented here clarify the impact of phenomena that take place during acid leaching Attorney Docket No. 10046-644W01 of coal fly ash to enable the development of scalable and deployable REEs extraction processes.

[0245] References for Experimental Examples

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[0302] The compositions and methods of the appended claims are not limited in scope by the specific compositions and methods described herein, which are intended as illustrations of a few aspects of the claims and any compositions and methods that are functionally equivalent are intended to fall within the scope of the claims. Various modifications of the compositions and methods in addition to those shown and described herein are intended to fall within the scope of the appended claims. Further, while only certain representative compositions and method steps disclosed herein are specifically described, other combinations of the compositions and method steps also are intended to fall within the scope of the appended claims, even if not specifically recited. Thus, a combination of steps, elements, components, or constituents may be explicitly mentioned herein; however, other combinations of steps, elements, components, and constituents are included, even though not explicitly stated.

Claims

Attorney Docket No. 10046-644W01WHAT IS CLAIMED IS:

1. A process for recovery of one or more rare earth elements from coal fly ash, the process comprising: baking a first mixture comprising the coal fly ash and an acid solution, wherein the first mixture forms a baked solid; contacting the baked solid with a solvent to form a leach phase and a precipitate phase, wherein at least a portion of the one or more rare earth elements is dissolved in the leach phase; and separating the leach phase and the precipitate phase.

2. The process of claim 1, wherein the coal fly ash comprises a calcium content of 10% to 40% by weight based on a total weight of the coal fly ash.

3. The process of claim 1 or claim 2, wherein the coal fly ash comprises a calcium content of 10% to 15% by weight based on a total weight of the coal fly ash.

4. The process of any one of claims 1-3, wherein the acid solution comprises a mineral acid.

5. The process of any one of claims 1-4, wherein the acid solution comprises nitric acid.

6. The process of claim 5, wherein the nitric acid has a concentration within the acid solution from 6 molar to 15 molar.

7. The process of claim 5 or claim 6, wherein the nitric acid has a concentration within the acid solution of 15 molar.

8. The process of any one of claims 1-7, wherein baking the first mixture occurs at a temperature from 100 degrees Celsius to 800 degrees Celsius.

9. The process of any one of claims 1-8, wherein the solvent comprises water.

10. The process of any one of claims 1-9, wherein the portion of the one or more rare earth elements is dissolved in the leach phase as one or more salts of the one or more rare earth elements.Attorney Docket No. 10046-644W0111. The process of any one of claims 1-10, wherein the one or more rare earth elements comprise scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or combinations thereof.

12. The process of any one of claims 1-11, wherein separating the leach phase and the precipitate phase comprises centrifugation of a mixture of the leach phase and the precipitate phase.

13. The process of any one of claims 1-11, wherein separating the leach phase and the precipitate phase comprises filtration of the precipitate phase from the leach phase.

14. The process of any one of claims 1-11, wherein separating the leach phase and the precipitate phase comprises settling of the precipitate phase.

15. The process of any one of claims 1-14, further comprising repeating the baking, contacting, and separating steps on a second mixture comprising the precipitate phase and an acid solution.

16. The process of any one of claims 1-15, further comprising isolating the one or more rare earth elements from the leach phase.

17. The process of claim 16, wherein isolating the one or more rare earth elements from the leach phase comprises solvent extraction, ion exchange, adsorption, membrane separation, or combinations thereof.

18. A composition comprising one or more rare earth elements recovered by the process of any one of claims 1-17.

19. A device or article comprising one or more rare earth elements recovered by the process of any one of claims 1-17.

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