Patent Application
20250171879
Rare earth elements (REEs), including lanthanide elements and yttrium, are a valuable component in a diverse set of industries, ranging from consumer electronics to chemical catalysts. Amid unprecedented demand for these elements in recent decades, the global production of REEs has largely stagnated. Current commercial production of REEs occurs from the environmentally-deleterious mining of rare ores such as bastnasite, monazite, and xenotime. Almost half of these deposits are located in a few regions, including China, Russia, the Commonwealth of Independent States, Brazil, and Australia, while more than 90% of the world's REE-production is controlled solely by China.
There has been recent interest in obtaining REEs from alternative sources. REEs are also relatively plentiful in small concentrations in much of the earth's crust. Among these other sources, waste ash byproducts, such as coal fly ash (CFA) and municipal solid waste incineration (MSWI) ash, have been explored as an alternative source largely due to its prevalence. However, current techniques of REE extraction from waste ashes use highly corrosive and expensive solutions, which leave a considerable amount of hazardous secondary waste to be disposed of.
There is a benefit to having systems and methods that can efficiently extract REEs and other chemicals from readily available waste ash sources without producing large amounts of toxic byproducts.
Exemplary systems and methods are disclosed for the simultaneous recovery of rare earth elements (REE) and zeolite formation from coal fly ash and industrial/municipal solid waste incineration ash. The systems and methods include a leaching and precipitation process that leaches REE from coal ash using a chelating agent and then precipitates the leached REE as solid products using a precipitation agent. The output includes REEs and other trace metals that can be further processed in downstream purification and production, e.g., REE compounds. The residual streams from the REE recovery process can be combined to synthesize zeolite, a marketable industrial catalyst, and adsorbent, thereby reducing a large amount of secondary waste.
Advantageously, the exemplary systems and methods can use inexpensive and environmentally-sustainable chemicals that are low in acidity and non-toxic to extract both REE and zeolite as marketable products. The process is high efficiency, has high scalability, and generates minimal waste production.
In an aspect, a method is disclosed for recovering rare earth elements (REE) from a waste ash source. The method includes contacting an REE-containing waste ash source (e.g., municipal solid waste incineration ash; coal fly ash) with a chelating agent to form a leachate comprising one or more REE and a first residual material; contacting a precipitation agent with the leachate to form a second residual material and a solid precipitate comprising an amount of the one or more REE; separating the solid precipitate from the second residual material; and hydrothermally treating (e.g., using recycled heat or new heat) the first and/or second residual materials under conditions effective to yield a processed material comprising zeolites.
In some embodiments, the REE-containing waste ash source comprises coal fly ash (CFA), municipal solid waste incineration (MSWI) ash, industrial solid waste incineration ash, or a combination thereof.
In some embodiments, the REE-containing waste ash source comprises coal fly ash (CFA) (e.g., Class C and/or Class F CFA).
In various embodiments, the REE-containing waste ash source comprises 5% or more SiO2 by weight (e.g., 10 wt % or more SiO2, 20 wt % or more SiO2, 30 wt % or more SiO2, 40 w % or more SiO2, 50 wt % or more SiO2, 60 wt % or more SiO2, 70 wt % or more SiO2, or 80 wt % or more SiO2).
In some embodiments, the chelating agent comprises an acid (e.g., an organic acid) or salt thereof. In some embodiments, the chelating agent comprises citric acid or a salt thereof (e.g., microbially synthesized citrate or citric acid). In some embodiments, the citric acid or salt thereof is contacted with the REE-containing waste ash source at a concentration from 25 mM to 150 mM (e.g., from 25 mM to 100 mM, from 25 mM to 50 mM, from 50 mM to 100 mM, or about 50 mM). In some embodiments, the REE-containing waste ash source is contacted with the chelating agent at a pH of 7.0 or less (e.g., 6.0 or less, 5.0 or less, 4.0 or less, 3.0 or less, 2.0 or less, or 1.0 or less).
In some embodiments, the precipitation agent comprises an organic ligand (e.g., oxalic acid or a salt thereof or a microbially synthesized oxalic acid or oxalate). In some embodiments, the solid precipitate comprises an REE-oxalate (e.g., an REE-containing Ca-oxalate).
In some embodiments, the first residual material is contacted with the second residual material prior to or during hydrothermal treatment. In some embodiments, the first and/or second residual material are contacted with an alkaline solution (e.g., NaOH) prior to or during hydrothermal treatment. In various examples, the hydrothermal treating of the first and/or second residual material occurs at a temperature from 50° C. to 300° C. (e.g., from 100° C. to 300° C., 150° C. to 300° C., 200° C. to 300° C., from 50° C. to 250° C., from 50° C. to 200° C., from 100° C. to 200° C., or from 100° C. to 150° C.).
In some embodiments, the method further comprises substantially separating the zeolites from the processed material to form a zeolite-rich product and a depleted waste material.
In some embodiments, the zeolite-rich product comprises 75% or more zeolites by weight (e.g., 80% or more by weight, 85% or more by weight, 90% or more by weight, 95% or more by weight, 97% or more by weight, or 99% or more by weight).
In some embodiments, the method further includes substantially separating a purified REE-product from the solid precipitate.
In some embodiments, the step of substantially separating the purified REE-product comprises dissolving the solid precipitate comprising the amount of one or more REE in a solvent (e.g., nitric acid) and recovering the purified REE-product (e.g., by adsorption/desorption).
In some embodiments, the purified REE-product is adsorbed using functionalized magnetic mesoporous silica particles.
In some embodiments, the method is performed continuously or semi-continuously. In some embodiments, the method is performed batch-wise.
In another aspect, a method is disclosed for processing residual waste ash material. The method includes hydrothermally treating a residual waste ash material (e.g., a solid-liquid heterogeneous mixture) comprising a concentration of aluminosilicates under conditions effective to yield a processed waste ash material comprising zeolites, wherein the residual waste ash material was collected as a byproduct from the removal (e.g., by extraction or leaching) of a heavy metal (e.g., rare earth elements) from a prior process; and substantially separating the zeolites from the processed waste material, thereby forming a zeolite-rich product and a depleted waste material.
In another aspect, a method is disclosed for recovering rare earth elements (REE) from a waste ash source. The method includes contacting an REE-containing waste ash source with a chelating agent to form leachate comprising one or more REE and a first residual material (e.g., a solid material); adding a precipitation agent to the leachate to form a second residual material and a solid precipitate comprising an amount of the one or more REE; separating the solid precipitate from the second residual material (e.g., a liquid material); and using the first and/or second residual materials to produce porous metal oxide particles (e.g., zeolites).
Also disclosed herein is a system comprising components to perform the method described above.
In yet another aspect, a system is disclosed for recovering rare earth elements from an REE-containing waste source. The system includes a first reactor (e.g., a continuous stirred-tank reactor) configured to receive the REE-containing waste ash source and a chelating agent, and wherein the first reactor is configured to produce leachate comprising one or more REE and a first residual material (e.g., a solid material); and a second reactor (e.g., a continuous stirred-tank reactor) configured to receive the leachate and a precipitation agent, wherein the second reactor is configured to produce a second residual material (e.g., a liquid material) and a solid precipitate comprising an amount of the one or more REE.
In some examples, the system includes a third reactor (e.g., an autoclave or high-pressure chemical reactor) configured to receive the first and/or second residual materials, wherein the third reactor operates under conditions effective to produce a processed waste material comprising zeolites.
In some embodiments, the REE-containing waste ash source comprises coal fly ash (CFA), municipal solid waste incineration (MSWI) ash, or a combination thereof. In some examples, the REE-containing waste ash source comprises coal fly ash (CFA) (e.g., Class C and/or Class F CFA).
In some embodiments, the REE-containing waste ash source comprises 5% or more SiO2 by weight (e.g., 10 wt % or more SiO2, 20 wt % or more SiO2, 30 wt % or more SiO2, 40 wt % or more SiO2, 50 wt % or more SiO2, 60 wt % or more SiO2, 70 wt % or more SiO2, or 80 wt % or more SiO2).
In some embodiments, the chelating agent comprises an acid (e.g., an organic acid). In some examples, the chelating agent comprises citric acid or a salt thereof (e.g., microbially synthesized citric acid or citrate). In some embodiments, a concentration of citric acid or salt thereof in the first reactor is from 25 mM to 150 mM, such as from 25 mM to 100 mM, from 25 mM to 50 mM, from 50 mM to 100 mM, or about 50 mM.
In some embodiments, the first reactor is configured to operate at a pH of 7.0 or less (e.g., 6.0 or less, 5.0 or less, 4.0 or less, 3.0 or less, 2.0 or less, or 1.0 or less).
In some embodiments, the precipitation agent comprises an organic ligand (e.g., oxalic acid or a salt thereof or a microbially synthesized oxalic acid or oxalate).
In some embodiments, the solid precipitate comprises an REE-oxalate (e.g., an REE-containing Ca-oxalate).
In some embodiments, the third reactor is configured to operate at a temperature of 50° C. to 300° C. (e.g., from 100° C. to 300° C., 150° C. to 300° C., 200° C. to 300° C., from 50° C. to 250° C., from 50° C. to 200° C., from 100° C. to 200° C., or from 100° C. to 150° C.).
In some embodiments, the system further includes a separation unit configured to receive the solid precipitate and a solvent (e.g., nitric acid), wherein the separation unit produces a purified REE-product and a third residual material. In some examples, the separation unit comprises an adsorption column or reactor comprising an adsorbent (e.g., functionalized magnetic mesoporous silica particles).
In some embodiments, the third reactor is configured to receive the first, second, and third residual materials.
In some embodiments, the system is configured to operate continuously or semi-continuously. In some embodiments, the system is configured to operate batch-wise.
The skilled person in the art will understand that the drawings described below are for illustration purposes only.
To facilitate an understanding of the principles and features of various embodiments of the present invention, they are explained hereinafter with reference to their implementation in illustrative embodiments.
As used herein, the term coal fly ash (CFA) or fly ash means a fly ash resulting from burning coal. Reference to a specific class of CFA (e.g., Class C or Class F) is intended to refer to the chemical compositions as defined in ASTM C618-12. For example, these classes generally differ in the amount of calcium, silica, alumina, and iron content in the ash. Class F fly ash typically contains less than 20% lime (CaO), while Class C fly ash generally contains greater than 20% CaO. In one embodiment, either Class F or Class C fly ash can be used.
As used herein, the term “zeolite” refers to a family of micro-porous hydrated aluminosilicate minerals. More than 150 zeolite types have been synthesized, and 48 naturally occurring zeolites are known. Zeolites have an “open” structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others. Some exemplary zeolites include Amicite, Analcime, Barrerite, Bellbergite, Bikitaite, Boggsite, Brewsterite, Chabazite, Clinoptilolite, Cowlesite, Dachiardite, Edingtonite, Epistilbite, Erionite, Faujasite, Ferrierite, Garronite, Gismondine, Gmelinite, Gobbinsite, Gonnardite, Goosecreekite, Harmotome, Herschelite, Heulandite, Laumontite, Levyne, Maricopaite, Mazzite, Merlinoite, Mesolite, Montesommaite, Mordenite, Natrolite, Offretite, Paranatrolite, Paulingite, Pentasil, Perlialite, Phillipsite, Pollucite, Scolecite, Sodium Dachiardite, Stellerite, Stilbite, Tetranatrolite, Thomsonite, Tschernichite, Wairakite, Wellsite, Willhendersonite, and Yugawaralite, among others.
As used herein, the term “chelating agent” refers to compounds capable of selectively removing a metal ion, such as a rare earth element, from a material. Exemplary chelating agents include oxalic acid, Ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), N-(hydroxyethyl)-ethylenediaminetetraacetic acid (HEDTA), nitrilotriacetic acid (NTA), citric acid, and ascorbic acid.
As used herein, the term “precipitation agent” refers to a compound or solution of a compound that is capable of causing a solid REE-containing precipitate to form as the precipitation agent contacts the leachate.
The term “extraction” refers to material removed from a substrate (e.g., a waste ash material) by introducing a solvent. As used herein, the term “leachate” refers to a liquid that has passed through and/or around matter, such as a waste ash source, and has extracted therefrom or otherwise contains soluble or suspended solids or any other component or aspect of the matter to which it was exposed, whether suspended or dissolved.
As used herein, the term “REE-containing waste ash material” and the like is intended to refer to any ash material comprising rare earth elements that were obtained as a waste byproduct. Examples of REE-containing waste ash materials include industrial and municipal solid waste incineration ash and coal fly ash.
The system 100 shown in
In
In some embodiments, the REE-containing waste ash source comprises coal fly ash (CFA), municipal solid waste incineration (MSWI) ash, or a combination thereof. In some examples, the REE-containing waste ash source comprises coal fly ash (CFA) (e.g., Class C and/or Class F CFA). In some embodiments, the REE-containing waste ash source comprises 5% or more SiO2 by weight (e.g., 10 wt % or more SiO2, 20 wt % or more SiO2, 30 wt % or more SiO-2, 40 wt % or more SiO2, 50 wt % or more SiO2, 60 wt % or more SiO2, 70 wt % or more SiO2, or 80 wt % or more SiO2).
In some embodiments, the first reactor is configured to operate at a pH of 7.0 or less (e.g., 6.0 or less, 5.0 or less, 4.0 or less, 3.0 or less, 2.0 or less, or 1.0 or less).
In some embodiments, the chelating agent comprises an acid (e.g., an organic acid). In some examples, the chelating agent comprises citric acid or a salt thereof (e.g., microbially synthesized citric acid or citrate). In some embodiments, a concentration of citric acid or salt thereof in the first reactor is from 25 mM to 150 mM, such as from 25 mM to 100 mM, from 25 mM to 50 mM, from 50 mM to 100 mM, or about 50 mM.
In some embodiments, the precipitation agent comprises an organic ligand (e.g., oxalic acid or a salt thereof or a microbially synthesized oxalic acid or oxalate). In some embodiments, the solid precipitate comprises an REE-oxalate (e.g., an REE-containing Ca-oxalate).
The system 300a shown in
As used herein, the term “conditions effective to” refers to conditions to which a material or materials are subjected that result in the formation of a desired zeolite product. Conditions may include temperature, pressure, reaction time, and the like, which are conditions known to those of ordinary skill in the art with the benefit of this disclosure.
Referring to
In some embodiments, the separation unit comprises an adsorption column or reactor comprising an adsorbent. The term “adsorption column” refers to a mass transfer device that enables a suitable adsorbent to selectively adsorb a contaminant, i.e., adsorbate, from a fluid containing one or more other contaminants. In some examples, adsorbent comprises functionalized magnetic mesoporous silica particles. Exemplary synthesis schemes of functionalized magnetic mesoporous silica include, for example, those described by Zhao et al. (2014), Zhang et al. (2012), Lin et al. (2009), and Chen et al. (2009), each of which is hereby incorporated by reference in its entirety.
In
Referring now to
As shown in
Referring to
In some embodiments, the REE-containing waste ash source comprises coal fly ash (CFA), municipal solid waste incineration (MSWI) ash, industrial solid waste incineration ash, or a combination thereof. In some embodiments, the REE-containing waste ash source comprises coal fly ash (CFA). In various embodiments, the REE-containing waste ash source comprises 5% or more SiO2 by weight.
In some embodiments, the chelating agent comprises an acid (e.g., an organic acid) or salt thereof. In some embodiments, the chelating agent comprises citric acid or a salt thereof (e.g., microbially synthesized citrate or citric acid). For example, the citric acid or salt thereof can be contacted with the REE-containing waste ash source at a concentration of from 25 mM to 150 mM (e.g., from 25 mM to 100 mM, from 25 mM to 50 mM, from 50 mM to 100 mM, or about 50 mM). In some examples, the REE-containing waste ash source is contacted with the chelating agent at a pH of 7.0 or less (e.g., 6.0 or less, 5.0 or less, 4.0 or less, 3.0 or less, 2.0 or less, or 1.0 or less).
Method 400a further includes contacting (404a) a precipitation agent with the leachate to form a second residual material and a solid precipitate comprising an amount of one or more REE.
In some embodiments, the precipitation agent comprises an organic ligand (e.g., oxalic acid or a salt thereof or a microbially synthesized oxalic acid or oxalate). In embodiments where the precipitation agent includes oxalic acid or a salt thereof, the solid precipitate can comprise an REE-oxalate (e.g., an REE-containing Ca-oxalate).
Method 400a further includes separating (406a) the solid precipitate from the second residual material. This separation of the solid precipitate and the second residual waste can be done using various separation techniques. Exemplary separation techniques include, for example, centrifugation, membrane filtration, and membrane filter pressing (plate and frame filter press with squeezing membranes). In some embodiments, the method can further comprise collecting the separated solid precipitate.
Method 400a can be performed, e.g., in systems (e.g., 100, 300a, 300b, 300d, 300e) described in relation to any one of
Referring to
Method 400b further includes hydrothermally treating (408b) (e.g., using recycled heat or new heat) the first and/or second residual materials under conditions effective to yield a processed material comprising zeolites.
As used herein, the term “conditions effective to” refers to conditions to which a material or materials are subjected that result in the formation of a desired zeolite product. Conditions may include temperature, pressure, reaction time, and the like, which are conditions known to those of ordinary skill in the art with the benefit of this disclosure.
In some embodiments, the method comprises contacting the first residual material with the second residual material prior to or during hydrothermal treatment. In some examples, the first and/or second residual materials are contacted with an alkaline solution prior to or during hydrothermal treatment. In some embodiments, the hydrothermal treating of the first and/or second residual material occurs at a temperature from 50° C. to 300° C.
In some embodiments, zeolites from the processed material are separated to form a zeolite-rich product and a depleted waste material. In some examples, the zeolite-rich product comprises 75% or more zeolites by weight.
Method 400b can be performed, e.g., in systems (e.g., 100, 300a, 300b, 300c, 300d, 300e) described in relation to any one of
Method 400c can be performed, e.g., in systems (e.g., 100, 300d) described in relation to any one of
The exemplary systems and methods can be used to extract rare earth elements (REE) from coal ash or other solid wastes containing high Si contents, such as municipal solid waste incineration ash and industrial solid waste incineration ash. Municipal solid waste incineration can include, for example, ash derived from domestic household waste, sewage sludge, medical or hospital waste, furniture, tires, textiles, plastics, rubber, cartons, and the like. Industrial solid waste incineration can include, for example, ash derived from industrial sludge, paper pulp sludge, wastepaper, waste paperboard, furniture, textiles, plastics, rubber, cartons, and tannery waste.
REE is widely used in a range of high-technology applications. Due to the growing demands of REE and the vulnerability to a potential supply disruption, there has been increasing interest in exploring alternative REE resources and recovery of REE from waste streams. Coal ash has been recently studied as a promising resource for REE recovery. Coal ash is a sizeable industrial waste stream in the U.S., with massive reserves in legacy disposal sites plus ˜40 million tons of newly-produced coal ash every year. The annual value of REE derived from coal ash is estimated to be $4.3 billion. In addition, with stricter governmental regulations, decreasing land space, and increasing costs of coal ash disposal, the management of coal ash poses significant environmental and financial burdens. Thus, recovering REE from coal ash can be employed to address REE scarcity crisis and an opportunity to address the solid waste management problem.
Coal ash is typically a low-grade REE feedstock. The total REE concentration in coal ash generally ranges from 250 to 800 ppm, well below the cutoff grade of 1,000 ppm expressed as rare earth oxide). Previous studies generally focus on REE recovery and utilize highly corrosive acids/bases in combination with high temperatures to leach REEs from coal ash. These leaching processes are chemical- and energy-intensive and are not economically and environmentally viable. Lowering the sintering temperature or using milder mineral acid typically results in significant decreases in REE leaching efficiency.
In addition, leachate from the acid leaching process usually has complex solution chemistry with low REE concentration (e.g., total REEs 30 mg/L) and high concentration of interfering elements (e.g., Na, Al, Ca, and Fe, at 1,000-14,000 mg/L). Multiple techniques have been proposed for downstream REE separation from the leachate, such as solvent extraction, ionic liquid, liquid membrane, and biosorption. Solvent extraction is widely used in REE separation, but working with organic liquids (e.g., kerosene) might be hazardous and unsafe due to harmful, flammable vapors. Efficient REE leaching and separation might be achieved using ionic liquids such as betainium bis(trifluoromethylsulfonyl)imide, but the synthesis of ionic liquids is currently not cost-effective, and the high viscosity of ionic liquids might slow down mass transport in large-scale processes. Organic ligands such as citrate can chelate with REEs and facilitate REE leaching from REE-bearing minerals, coal fine refuse, or coal coarse refuse. Few studies have examined the efficiency of REE leaching from coal ash using organic ligands.
Additionally, REEs or REE-bearing phases only account for a minor fraction of coal ash, and a considerable amount of coal ash solid residues remains after upstream REE leaching. Coal ash solid residues and wastewater production during REE separation necessitate additional treatment steps from a waste management perspective. However, few studies have addressed the fate of secondary wastes after REE leaching and separation.
This method employed an integrated system and method for concurrent REE recovery and waste reduction of coal ash. The system includes, in some embodiments, three modules, which can be implemented alone or in combination with each other or other modules. In module “I,” the module leaches REE from coal ash using sodium citrate. In module “II,” the module separates REE from other trace metals and precipitates as an REE-oxalate product. In module “III,” the module combines the solid residue and wastewater from Modules I and II to synthesize zeolite, a common industrial sorbent, as an additional saleable product. This system and associated method are characterized by the selective recovery of REEs, production of REE-rich products and zeolite, and minimal waste production.
The exemplary systems and methods recover rare earth elements (REE) from coal ash or other solid wastes containing high Si and REE contents, such as municipal solid waste incineration ash. Coal ash and municipal solid waste incineration ash are emerging waste streams that, only until recently, have been studied as promising REE resources. The exemplary method first uses sodium citrate to leach REE from coal ash. The leaching solution is combined with sodium oxalate to precipitate REE and separate from other impurities, such as other trace metals.
These two steps separate REE from other impurities and enrich REE into solid products that can be marketed for downstream processing. Additionally, the produced solid residue and liquid wastewater after REE extraction and precipitation steps are combined to synthesize zeolite, a marketable industrial catalyst, and adsorbent, as an additional product.
Previously published papers and patents utilized highly corrosive mineral acids (e.g., HCl) in combination with high temperatures to leach REE from coal ash (e.g., U.S. Pat. Nos. 8,968,688 and 9,394,586). Such leaching processes are chemical- and energy-intensive and pose environmental and health hazards. In contrast, the exemplary method generally uses organic ligands such as citrate under mild conditions (pH 4-7) and room temperature, which is more environmentally friendly, has a lower energy cost, and is readily scalable.
After REE leaching, previous methods use solvent extraction (e.g., U.S. Pat. No. 8,968,688) or cation exchange (e.g., U.S. Ser. No. 11/186,894) to enrich and separate REE from other impurities. The product is REE-containing solutions. In contrast, the exemplary method may use oxalate to precipitate REE and separate REE from other impurities. The product is an REE-rich solid phase.
Previous methods only focused on REE recovery and only produced REE-containing products. In contrast, the exemplary method may also produce zeolite as a marketable product.
Previous methods produce secondary solid/liquid wastes after REE recovery, which are not treated and cause secondary waste management problems. In contrast, the exemplary method may up-cycle the solid residue and liquid wastewater to synthesize zeolite products, which can minimize the waste volume and eliminate the production of secondary wastes.
The mineral acids used in previous methods for REE extraction cannot be recycled. In the exemplary method, after the zeolite synthesis step, the residual process water may contain citrate and can be reused for REE extraction. Given the growing economic demands of REE and the large annual production of coal ash, the need for recycling REE from coal ash is warranted for both sustainability and environmental considerations.
The citation and/or discussion of such references is provided merely to clarify the description of the present disclosure and is not an admission that any such reference is “prior art” to any aspects of the present disclosure described herein. In terms of notation, “[n]” corresponds to the nth reference in the list. All references cited and discussed in this specification are incorporated herein by reference and to the same extent as if each reference was individually incorporated by reference.
Background Rare earth elements (REEs, including lanthanide elements and yttrium) are widely used in a range of high-tech applications [1]. Due to the growing demand of REEs and the vulnerability to a potential supply disruption, the United States (U. S.) has labeled REEs as “critical minerals” [2]. As a result, there has been increasing interest and research to explore alternative REE resources and recovery of REEs from waste streams. For example, the U.S. Department of Energy has initiated programs to examine methods of recovering REEs from coal-related wastes [3], and coal fly ash (CFA) has been proposed as a promising resource for REE recovery [4], [5], [6], [7], [8]. CFA is a sizeable industrial waste stream in the U.S., with massive reserves in legacy disposal sites plus ¬40 million tons of newly-produced CFA every year [9]. The annual value of REEs derived from CFA is estimated to be $4.3 billion [6]. In addition, with stricter government regulations [10] and increasing economic costs on CFA disposal, the management of CFA poses significant environmental and financial burdens. Thus, recovering REEs from CFA is a promising solution to the REE scarcity crisis and an opportunity to address the solid waste management problem.
Although REE-rich CFA was previously identified (e.g., 17,026 ppm [4]), CFA is typically a low-grade REE feedstock. The total REE concentration in CFA generally ranges from 250 to 800 ppm [6], well below the cutoff grade of 1,000 ppm (expressed as rare earth oxide) suggested by Seredin and Dai [4]. Previous studies generally focus on REE recovery and utilized highly corrosive solutions to leach REEs from CFA to yield a high REE leaching efficiency [11], [12], [13], [14], [15], [16]. For example, Taggart et al. (2018) sintered CFA with a 1:1 NaOH-ash ratio at 450° C., followed by leaching with 2 M HNO3, achieving >70% REEs leached from different types of CFA [11]. Such leaching processes are chemical- and energy-intensive and are not economically and environmentally viable. Lowering the sintering temperature or using milder mineral acid resulted in a significant decrease in REE leaching efficiency [11].
Leachate from the acid leaching process usually has complex solution chemistry with low REE concentration (e.g., total REEs <30 mg/L) and high concentration of interfering elements (e.g., Na, Al, Ca, and Fe, at 1,000-14,000 mg/L) [15]. Multiple techniques have been proposed for downstream REE separation from the leachate, such as solvent extraction [12], [13], ionic liquid [17], [18], liquid membrane [13], and biosorption [19]. Solvent extraction is widely used in REE separation [12], [13], but working with organic liquids (e.g., kerosene) might be hazardous and unsafe due to harmful, flammable vapors [12]. Stoy et al. (2021) achieved efficient REE leaching and separation based on the unique thermomorphic behavior of ionic liquid betainium bis(trifluoromethylsulfonyl)imide with water [17]. However, the synthesis of ionic liquids is currently not cost-effective, and the high viscosity of ionic liquids might slow down mass transport in large-scale processes [18]. As reviewed by Opare et al. (2021), a cost-effective and environmentally friendly approach to REE separation is yet to be achieved [20]; thus, the opportunities remain for exploring alternative separation technologies.
Regarding REE leaching and separation, organic ligands (especially low molecular weight organic acids) might be an environmentally friendly option. A previous study demonstrated that the dominant REE-bearing phases in CFA include REE oxides, REE phosphates (e.g., monazite and xenotime), apatite, etc. [21], [22]. Organic ligands such as citrate can chelate with REEs and facilitate REE leaching from REE-bearing minerals [23]. Yang et al. (2021) tested REE leaching from coal fine refuse using citric acid and suggested that citric acid was not a competitive option compared to mineral acid [24]. In contrast, Ji et al. (2020) examined REE leaching from coal coarse refuse using a set of organic acids, including citric acid, which showed high leaching efficiency comparable to HCl at the same pH [25]. Yet, few studies have examined the efficiency of REE leaching from CFA using organic ligands.
As for REE separation, oxalate is an organic ligand that can be used to precipitate REEs from acidic solutions due to the low solubility of REE oxalate precipitates (e.g., Ksp=10−29.2 for La2(CO)) [26]. Oxalate is typically added at the later stage of REE purification with a relatively high concentration of REEs and a low concentration of interfering ions (e.g., Ca2+) Zhang et al. (2020) modeled the interaction of REEs (0.1 mM) with oxalate in the presence of interference ions (e.g., Fe3+, Al3+, and Ca2+ at 0.1-1 mM) at pH 1-5 and showed that REE recovery via REE-oxalate precipitation was thermodynamic favorable [27]. However, results from Zhang et al. (2020) might not be directly applicable to acidic leachate from REE leaching, which has a much lower REE concentration, and experimental results might be different from thermodynamic calculations due to kinetic limitations. The efficacy of selective recovery of REEs from CFA leachate using oxalate requires further investigation.
Additionally, REEs or REE-bearing phases only account for a minor fraction of CFA, and a considerable amount of CFA solid residues remains after upstream REE leaching [15], [21], [28]. CFA solid residues and wastewater production during REE separation necessitate additional treatment steps from a waste management perspective. However, few studies have addressed the fate of secondary wastes after REE leaching and separation. CFA solid residues might be used to recover other valuable metals (e.g., Cu and Zn) [29] or synthesize porous materials (e.g., zeolite) [30], which might minimize waste production and add extra economic benefits.
The goal of this study is to develop an integrated system for concurrent REE recovery and waste reduction of CFA. Specifically, the system disclosed herein includes three modules. In module I, REE leaching from CFA using sodium citrate was investigated. Citrate was selected due to its high chelating ability with REEs. For example, the stability constant of the Y-citrate complex is 109.4[31].
In module II, REE separation by directly adding oxalate in leachate was examined. Behaviors of other valuable metals (e.g., Cu and Zn) were monitored. To better understand metal speciation and behavior in modules I and II, thermodynamic calculation of aqueous speciation was conducted using PHREEQC [32]. In module III, in order to minimize liquid and solid waste production, the solid residue and wastewater from modules I and II were combined to synthesize zeolite, a common industrial sorbent, as an additional saleable product. Two representative CFA samples, including a Class F (SiO2+Al2O3+Fe2O3≥70 wt %) and a Class C (50 wt %≤SiO2+Al2O3+Fe2O3≤70 wt %) CFA sample were evaluated. This treatment system is advantageously characterized with the selective recovery of REEs, production of REE-rich products and zeolite, and minimal waste production.
Materials and method—CFA Samples. Class F and a Class C CFA samples were selected for this study, which were collected from coal-fired power plants located in the southeastern U.S. The composition of these two raw samples was well-characterized in earlier studies [21], [28] and are referred to as samples F-1 and C-1, respectively. The major element concentrations were measured using X-ray fluorescence (XRF) [33]. The concentrations of trace metals, including REEs, were measured by inductively coupled plasma mass spectrometry (ICP-MS) after total digestion [21], [22].
Citrate leaching. Unless otherwise specified, chemicals used in this study are all ACS grade or higher. To leach REEs, CFA samples were mixed with sodium citrate under continuous stirring (200 rpm) at room temperature. The solution was maintained at desired pH using dilute HCl and NaOH. Effects of pH (2-7), citrate concentration (50-200 mM), and liquid-to-solid ratio (50-200 mL/g) on REE leaching were examined. After leaching, solids and leachate were separated by vacuum filtration using 0.2 μm filters. Solids were rinsed with deionized water (18.2 MΩ/cm), dried at 45° C., and weighted. The citrate leachate was analyzed for metal concentrations. The leaching efficiency of elements was calculated as:
where V (mL) is the volume of the citrate leachate; M (g) is the mass of the CFA sample; C1 and C2 (ppm) are the element concentrations in the CFA sample and citrate leachate, respectively.
Oxalate precipitation. To separate REEs from citrate leachate after leaching, sodium oxalate was gradually added. After each addition, the reaction was allowed to proceed for 30 min under stirring (200 rpm) at room temperature. Solution aliquots were collected for concentration analysis. Metals that remained in the leachate after oxalate addition was calculated as:
where C0 and C* (ppm) are the concentration of metals in the initial citrate leachate and after oxalate addition, respectively.
Oxalate precipitates were harvested at the end of the experiment, rinsed, and dried at 45° C. The element concentration of the oxalate precipitates was measured by ICP-MS after acid digestion. The enrichment factor of elements in oxalate products as compared to raw CFA samples was calculated as:
where CCFA and Coxalate_product (ppm) are element concentrations in the raw CFA sample and oxalate products, respectively.
Thermodynamic modeling. To better understand element speciation in citrate leachate and element behavior during oxalate precipitation, thermodynamic calculations of aqueous speciation were conducted using the program PHREEQC [32]. The minteq.v4.dat database was used. The stability constants of REE-ligand complexes (e.g., Cl−, CO32−, citrate, oxalate, etc.) and solubility products of relevant REE mineral phases were compiled from the literature [31], [34]-[41]. Citrate solution containing major elements (Na, Mg, Ca, and Al, ranging from 10 to 1000 mg kg−1 water) and trace metals (Cr, Co, Ni, and REEs, ranging from 1 to 1000 μg kg−1 water) was included in the solution phase to simulate the composition of the citrate leachate from samples F-1 and C-1. Details of the considered metal-ligand interactions and leachate chemistry are summarized in Table 1.
Zeolite synthesis. To reduce the production of secondary residues, zeolite was synthesized in hydrothermal reactors (Parr Instrument) using the solid residues and wastewater after REE separation. 0.2 g CFA solid residues from the citrate leaching step were mixed with 10 mL wastewater from the oxalate precipitation step. 5 M NaOH was used as an activation agent. The hydrothermal reactors were sealed and heated at 100 or 150° C. under autogenous pressure for 24 h. The synthesized solids were collected by vacuum filtration, rinsed, and dried at 45° C. for composition, structure, and morphology analysis.
Analytical methods—Inductively coupled plasma mass spectrometry (ICP-MS). Element concentration in the solution during citrate leaching and oxalate precipitation was measured using ICP-MS (Agilent 7500a). All solution aliquots were diluted with HNO3 solution and spiked with 10 ppb of indium (In) as an internal standard to monitor instrument drift. A series of calibration standards (0-200 ppb) was prepared using standards from SPEX (CertiPrep) and Sigma-Aldrich (TraceCert). Each calibration standard was spiked with 10 ppb In. The mass spectrometer was tuned for high sensitivity, low isobaric interference (CeO+/Ce+<1%), and low doubly charged ions (<2%). 53Cr, 59Co, 60Ni, 63Cu, 66Zn, 72Y, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb, and 175Lu were measured. Calibration standards were measured after every 20 samples to ensure accuracy.
X-ray diffraction (XRD). CFA samples and solid products were analyzed using a Panalytical Empyrean multipurpose diffractometer with Cu Ku radiation and a PIXcel 3D-Medipi×3 1×1 detector. XRD patterns were recorded over 5-50° 2θ with a step size of 0.03° 2θ and a contact time of 15 s/step at 45 kV and 40 mA.
Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX). The morphology of CFA samples, oxalate products, and zeolite products was examined using Hitachi SU8230. Samples were gently ground before being dusted onto carbon tape. SEM images were taken at 5 kV and 10 ρA with a working distance of 5 mm. EDX spectra were obtained at 15 kV and 30 ρA with a working distance of 15 mm using an Oxford X-MaxN EDX detector.
CFA samples. The chemical composition of samples F-1 and C-1 is summarized in Table 2. Sample F-1 is enriched in SiO2 (54.3%), Al2O3 (25.2%), and Fe2O3 (11.9%), while sample C-1 is relatively abundant in CaO (28.1%). The total REE content in sample F-1 is similar to that of sample C-1 (˜315 ppm). Among all REEs, Ce shows the highest concentration at ˜110 ppm. The total concentrations of light REEs (LREEs, from La to Sm) and heavy REEs (HREEs, from Eu to Lu plus Y)1 in both samples are around 230 ppm and 80 ppm, respectively. Notably, the fraction of critical REEs (Nd, Eu, Tb, Dy, Y, and Er) [4] in total REEs for sample F-1 (35.5%) is slightly higher than that of sample C-1 (32.1%). The concentrations of other trace metals (e.g., Cr, Cu, and Zn) range from 20 to 200 ppm. Sample F-1 contains more Cr, Co, Ni, and Zn as compared to sample C-1, except for Cu.
aMajor element information measured by XRF or EDX.
bConcentration of trace metal elements in raw coal fly ash is from Liu et al. (2019) and Liu et al. (2020).
XRD patterns of the CFA samples are shown in
Citrate leaching. The leaching kinetics were first investigated using 50 mM citrate at pH 4 and a liquid-to-solid ratio of 200 mL/g. All metals of interest in samples F-1 and C-1 reached a steady state after 3 h and ˜4 h, respectively. Based on this data, the following leaching experiments were conducted for 4 h.
To examine the effect of citrate concentration on metal leaching from CFA samples, citrate concentration was varied (0-100 mM), while the pH and liquid-to-solid ratio were fixed at 4 and 200 mL/g, respectively. In the absence of citrate, only ˜5% of REEs are leached from sample F-1, while sample C-1 is characterized by higher REE leaching efficiency at ˜20% (
In the presence of citrate, metal leaching from both CFA samples is remarkably enhanced (
Solid residues of the CFA samples after citrate leaching were characterized by SEM and XRD. Although SEM did not observe noticeable morphology changes, XRD analysis shows mineral dissolution after metal leaching (
Further increasing citrate concentration from 50 to 100 mM does not lead to a notable increase in metal leaching efficiency, likely reaching a maximal leaching efficiency at pH 4 (
Effect of pH. The effect of pH on citrate leaching was investigated with 10 mM citrate and a liquid-to-solid ratio of 200 mL/g. REE leaching efficiency increases significantly as pH decreases (
Effect of liquid-to-solid ratio. To explore the influence of liquid-to-solid ratio on metal leaching efficiency, pH and citrate concentration were fixed at 4.0 and 50 mM, respectively (
Oxalate precipitation. Following citrate leaching (module I), the study conducted an oxalate precipitation step (Module II) to separate REEs from the dilute citrate leachate. Citrate leachate was collected from experiments with 50 mM citrate at pH 4 and a liquid-to-solid ratio of 200 mL/g. The study selected this reaction condition for the oxalate precipitation experiment because metal leaching efficiency is relatively high for both CFA samples at this condition and the leachate pH is not too low.
Precipitates gradually formed in the citrate leachate upon the addition of oxalate. The fraction of metals remaining in the solution as a function of added oxalate concentration is plotted in
XRD analysis of the solid precipitates (
Metal concentrations in the oxalate precipitates are summarized in Table 2, and the enrichment factors of those metals are plotted in
Thermodynamic calculations were conducted to obtain a better understanding of the metal selectivity in the presence of oxalate. Calcium oxalate is the only phase oversaturated with the addition of oxalate (i.e., saturation index, SI>1). In contrast, the precipitation of REE-oxalate and other metal (e.g., Mg, Sr, Cu, and Zn) oxalate phases are not thermodynamically favorable (i.e., SI<1). However, the decrease of Ca concentration in F-1 and C-1 leachate showed a similar trend as the decreases of REEs (
Zeolite synthesis. Zeolite is a group of crystalline aluminosilicate minerals that has a three-dimensional framework of Si/Al tetrahedrons with lots of voids and open spaces. Additionally, the substitution of Si(V) by Al(III) results in permanent negative changes in zeolite and, consequently, a high cation exchange capacity (CEC) of zeolite [48]. Because of those unique properties, zeolite has a wide range of industrial applications (e.g., waste absorbent and molecule sieve) [49]. Hydrothermal synthesis of zeolite from CFA has been extensively studied. Up to 15 types of zeolite (e.g., zeolite NaP1, A, and X) can be synthesized from CFA depending on CFA chemical composition (e.g., SiO2/Al2O3 ratio), temperature (e.g., 80-200° C.), alkaline solution concentration (e.g., 0.5-5 M NaOH), liquid-to-solid ratio (1-50 mL/g), and reaction time (3-48 h) [48], [49].
To demonstrate that CFA solid residues after citrate leaching can be used for zeolite synthesis, the solid residues after module I (citrate leaching) were reacted with the waste leachate after module II and 5 M NaOH at 100 or 150° C. At 100° C., reflection peaks of quartz and mullite decrease significantly, and the broad hump at 20-30° 2θ is flattened, suggesting the disappearance of amorphous aluminosilicates. On the other hand, new reflections of hydroxy-sodalite [Na1.08Al2S11.68O7.44·1.8H2O] and halite [NaCl] appeared in both samples. The presence of halite in samples might be due to insufficient rinse of the products. At 150° C., quartz and mullite completely disappeared, and more hydroxy-sodalite formed in both products (
To minimize NaOH consumption and wastewater production during zeolite synthesis, the alkaline solution after one round of hydrothermal synthesis at 100° C. was tested for another round of zeolite synthesis without further addition of NaOH. It is found that zeolite yields are the same as the first round based on XRD analysis (
System analysis and environmental significance. The simple but effective system reported in this study combines citrate leaching of CFA, preferential precipitation of REEs as oxalate products, and zeolite synthesis using the solid and liquid wastes from upstream steps. Compared to raw CFA samples, the oxalate products are 2-4 times more enriched in REEs (especially critical REEs) and contain fewer impurity elements such as Cr, Co, and Cu. Therefore, oxalate products could be regarded as a more promising REE feedstock for downstream REE purification to yield single-REE products. As for zeolite, previous studies showed that zeolite yields from CFA varied widely at 40-75%, depending on the glass content of CFA, non-reactive phases (e.g., hematite and lime), and resistant silicates (e.g., quartz and mullite)48. This study produces near-pure zeolite products at 150° C., given that halite can be easily removed by washing. The citrate leaching experiment might serve as a pre-treatment process to remove non-reactive phases (such as hematite, lime, and periclase) prior to hydrothermal synthesis [51], and thus resulted in the high yields of zeolite in this study. Importantly, as all CFA solid residues are converted to zeolite, no solid residues will be produced from this system from the perspective of solid waste management. Wastewater from zeolite synthesis could be reused for synthesis to minimize wastewater production (
The system can be optimized to maximize economic and environmental benefits. For example, citrate concentration might be tailored for class F vs. C CFA to minimize citrate consumption. The remaining leachate after oxalate precipitation in module II can be recovered for other valuable metals (e.g., Cr, Co, Ni, Cu, and Zn). Cu and Zn could be preferentially precipitated by adding NaS2 and adjusting pH [52]. In some embodiments, microbially produced citrate and oxalate may be used to replace chemicals and reduce operation costs. For example, Aspergillus niger is capable of producing citric acid or oxalic acid depending on the pH, Mn availability, and nitrogen limitation of the culture medium [53], [54], [55].
This system can be employed at various production and operational scales, including large-scale operations. Overall, the exemplary system and method address both resource recovery and solid waste management challenges with CFA. From the resource recovery aspect, this system can be characterized by the production of REE-rich oxalate products and zeolite. From the solid waste management aspect, the system can achieve maximum waste volume reduction and minimal production of wastewater.
Although example embodiments of the present disclosure are explained in some instances in detail herein, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the present disclosure be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or carried out in various ways.
It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” or “5 approximately” one particular value and/or to “about” or “approximately” another particular value. When such a range is expressed, other exemplary embodiments include from the one particular value and/or to the other particular value.
By “comprising” or “containing” or “including” is meant that at least the name compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named.
In describing example embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. It is also to be understood that the mention of one or more steps of a method does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Steps of a method may be performed in a different order than those described herein without departing from the scope of the present disclosure. Similarly, it is also to be understood that the mention of one or more components in a device or system does not preclude the presence of additional components or intervening components between those components expressly identified.
The term “about,” as used herein, means approximately, in the region of, roughly, or around. When the term “about” is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. In general, the term “about” is used herein to modify a numerical value above and below the stated value by a variance of 10%. In one aspect, the term “about” means plus or minus 10% of the numerical value of the number with which it is being used. Therefore, about 50% means in the range of 45%-55%. Numerical ranges recited herein by endpoints include all numbers and fractions subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.90, 4, 4.24, and 5).
Similarly, numerical ranges recited herein by endpoints include subranges subsumed within that range (e.g., 1 to 5 includes 1-1.5, 1.5-2, 2-2.75, 2.75-3, 3-3.90, 3.90-4, 4-4.24, 4.24-5, 2-5, 3-5, 1-4, and 2-4). It is also to be understood that all numbers and fractions thereof are presumed to be modified by the term “about.”
The following patents, applications and publications as listed below and throughout this document are hereby incorporated by reference in their entirety herein.
This PCT application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 63/307,568 filed Feb. 7, 2022, which is incorporated by reference herein in its entirety.
This invention was made with government support under contract no. DE-FE0031739 awarded by the National Energy Technology Laboratory (Department of Energy) and under contract no. DE-AR0001394 awarded by the Advanced Research Projects Agency—Energy (Department of Energy). The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2023/012429 | 2/6/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63307568 | Feb 2022 | US |
