Ultrafast flash vaporization joule heating synthesis method and system for implementing the same

Abstract

Ultrafast flash evaporation joule heating synthesis methods and systems, more particularly, ultrafast synthesis methods for recovering metals from ores, fly ash, and bauxite residue (red mud).

Description

[0001] Cross-reference of related patent applications

[0002] This application claims priority to U.S. Patent Application Serial No. 63 / 082,592, filed September 24, 2020, entitled "Ultrafast Flash Joule Heating Synthesis Methods and Systems For Performing Same," which is jointly owned by the owners of this invention. The entire contents of this patent application are hereby incorporated by reference. Technical Field

[0003] This invention relates to an ultrafast flash joule heating synthesis method and system, and more particularly to an ultrafast synthesis method for recovering metals from ores, fly ash and bauxite residues (red mud). Background Technology

[0004] Efficient and low-cost synthesis of nanomaterials is a prerequisite for their commercial application.

[0005] carbide

[0006] Nanoscale transition metal carbides (TMCs) have been widely used as precursors for superhard and superstrong ceramics [Zou 2013; Zhang 2019; Reddy 2012], high-performance electrochemical catalysts (due to their platinum-like electronic structure) [Li 2018; Zhong 2016; Gao 2019; Gong 2016; Han 2018], and catalyst supports (due to their strong metal-substrate interactions) [Lin 2017; Yao 2017]. Conventional methods for the synthesis of bulk carbides include carburizing of metal precursors with gaseous carbon precursors or sintering of metal precursors with graphitic carbon at high temperatures [Rosa 1983]. These procedures can be problematic because they lead to coking of the carbide surface due to an excessive supply of carbon source, as well as large particle sizes and low surface areas detrimental to catalytic performance [Chen 2013; Zeng 2015].

[0007] Significant efforts have been invested to synthesize carbides with fine particle sizes, including temperature programmed reduction [Oyama 1992], carbothermal reduction of metal precursors [Wu 2020; Wang K 2019], laser spray pyrolysis of metal complexes [Kolel-Veetil 2005], and solution-based precipitation and carburization [Wan 2014]. The TPR method is versatile for high surface area metal carbide synthesis, but requires a well-optimized reaction window [Claridge 2000]. Carbothermal reduction of metal precursors in a furnace is prevalent in the synthesis of TMCs [Wu 2020]; however, extended high-temperature conditions are necessary to compensate for the slow solid-solid reaction kinetics, which inevitably leads to sintering or agglomeration [Wang K 2019].

[0008] To avoid severe agglomeration, a microwave combustion method was developed for the rapid synthesis of Mo2C and WC nanodots in 2 minutes [Wan 2019]. Pyrolysis of metal complexes involves the use of expensive and toxic metal-organic compounds, such as Cp2Mo2(CO)6 for the synthesis of Mo2C [Kolel-Veetil 2005; Wolden 2011] and W(CO)6 for the synthesis of WC [Pol 2009].

[0009] The type of carbide is also limited by the availability of volatile metal compounds. Solution-based precipitation and carburization require long annealing times to achieve full conversion. For example, the synthesis of MoC using ammonium heptamolybdate ((NH4)6Mo7O 24 ·4H2O) as a precursor requires annealing at 850 °C for 12 to 24 hours [Wan 2014].

[0010] Recently, several unconventional electrothermal methods have been developed to target high-energy efficient high-temperature synthesis [Wang 2020; Giorgi 2018; Yan 2018]. The thermal shock (CTS) process uses short current pulses to synthesize high-entropy alloy nanoparticles on carbon supports at ~2000 K [Yan 2018]. Ultra-high temperature sintering (UHS) based on current-induced heating was proposed for sintering and screening ceramics in 10 seconds [Wang 2020]. Spark flash sintering (SPS) applies current for 10 minutes for the reactive carbothermal synthesis of zirconium carbide (ZrC) [Giorgi 2018]. However, these methods target sintered bulk ceramics and lack the ability to synthesize fine nanocrystals.

[0011] Furthermore, the phase and the crystal surface structure of carbides play an important role in their carbide behavior, such as in their hydrogen adsorption / desorption energy [Gong 2016; Politi 2013]. However, the procedure to selectively design the phase and the crystal surface of carbides for maximum performance is rarely explored [Gong 2016; Wan 2014].

[0012] Electrocatalytic hydrogen evolution (HER) reaction depends on the availability of low-cost electrocatalysts. TMCs are very promising in HER due to their similar electronic structure to platinum [Gao 2019]. However, the existing techniques for synthesizing metal carbide nanoparticles have limitations of high cost and low productivity [Gong 2016]. Crucially, most methods are too specific and lack generality, and also it is very difficult to perform phase control [Wan 2014].

[0013] Corundum

[0014] High-surface-area corundum nanoparticles (a-Al203NPs) have a wide range of applications. For example, corundum is widely used in ceramics for prosthetic implants [De Aza 2002] and high-speed cutting tools [Kumar 2003]. a-Al203NP precursors provide a route to obtain nanocrystalline alumina ceramics with significantly improved fracture toughness [Ighodaro 2008], wear resistance [Krell 1996], and high density at reduced sintering temperatures [Guo 2016]. Even though g-Al203NPs are mainly used as catalyst supports [Peterson 2014] due to their high surface area, a-Al203NPs are also used as catalyst supports, and they have higher mechanical stability in automotive exhaust Pt-Mo-Co catalytic converters [Frank 1998], as well as enhanced Ru catalyst activity for ammonia synthesis [Lin 2019].

[0015] Many efforts have been made to improve the synthesis of a-Al203, but almost no method provides high-surface-area NPs due to inherent thermodynamic limitations [Guo 2016; McHale 1997; Amrute 2019]. Even though corundum is the thermodynamically stable phase of coarse crystalline alumina (Al203), the synthesis of nanocrystalline Al203 usually results in g-Al203 because its surface energy is lower than that of a-Al203 when the surface area is greater than 125 m2g 2 g -1 [McHale 1997].

[0016] Another reason is the phase transition of ~485 kJ mol -1high activation energy barrier [Steiner 1971]. As a result, thermal treatment typically requires temperatures >1470 K and prolonged annealing times of 10 to 20 hours to promote the transformation [Steiner 1971; Levin 1998]. Due to the large mass transfer, high energy input and prolonged high temperature annealing lead to surface areas <10 m 2 g -1 [Amrute 2019]. Furthermore, the polymorphic forms of Al2O3further increase the complexity during phase transformation and can lead to mixed transition (t)-alumina with undesirable δ- and θ-Al2O3[Steiner 1971; Chang 2001; Laine 2006]. Representative methods for corundum nanoparticles are rather time- and energy-consuming, e.g. annealing of γ-Al2O3at 1473-1673 K for 10-20 hours [Lodziana 2004] and hydrothermal reaction of γ-AlOOH at 723 K and 1200 bar for 35 days [McHale 1997; Loffler 2003].

[0017] Thus, the fabrication of α-Al2O3by phase transformation from the cubic close-packed γ phase (γ-Al2O3) is generally hindered by a high activation energy barrier (~485 kJ mol -1 ) which requires prolonged high-temperature thermal annealing (~1500 K, 10 to 20 hours) and is plagued by severe agglomeration. Therefore, the development of ultrafast and energy-efficient methods is of paramount importance for the widespread application of α-Al2O3nanoparticles.

[0018] E-waste

[0019] The recovery of valuable metals from waste is important for the circular economy and is also crucial for addressing environmental issues. Specifically, electronic waste (e-waste) contains abundant valuable elements.

[0020] E-waste comes from discarded electrical or electronic devices. The recovery of precious metals from electronic waste, known as "urban mining", is important for the circular economy. Current methods for urban mining, mainly smelting and leaching, are plagued by lengthy purification processes and adverse environmental impacts.

[0021] More than 40 million tons of electronic waste (e-waste) is generated globally each year [Zhang 2012; Zeng 2018], and is the fastest growing component of solid waste due to the rapid upgrade of personal electrical and electronic equipment [Ogunseitan 2009; Wang 2016]. Most e-waste is landfilled, with only about 20% being recycled [Ghosh 2015], which can lead to adverse environmental impacts due to the widespread use of heavy metals in electronic devices [Leung 2008; Julander 2014; Awasthi 2019].

[0022] E-waste can be a sustainable resource due to its rich content of valuable metals [Kaya 2016]. The concentration of some precious metals in e-waste is higher than in ores [Zhang 2012]. Recycling precious metals from e-waste, i.e. urban mining, becomes more cost-effective than primary mining [Zeng 2018], and is important for a circular economy [Awasthi 2019].

[0023] Similarly, e-waste can lead to significant health risks and adverse environmental impacts due to the widespread use of heavy metals in electronic devices, including Cd, Co, Cu, Ni, Pb, and Zn [Leung 2008; Julander 2014; Awasthi 2019]. Heavy metal leakage due to improper landfilling leads to environmental damage [Zhang 2012; Awashthi 2019]. The release of hazardous ingredients in dust or fume form during recycling [Leung 2008] makes the health of recycling workers and local residents worse. For example, significantly higher concentrations of Pb were found in the blood of e-waste workers [Julander 2014; Popoola 2019].

[0024] A lack of high-yield and environmentally friendly recycling processes is a major obstacle for urban mining [Kaya 2016]. Traditional methods of e-waste recycling are based on pyrometallurgical processes [Hall 2007] in which metals are melted by heating at high temperatures. Pyrometallurgy is energy-intensive, lacks selectivity, and requires high-grade precursors [Cui 2008]. Pyrometallurgical processes also produce hazardous fumes containing heavy metals, especially for those with low melting points such as Hg, Cd, and Pb [Kaya 2016]. Hydrometallurgical processes are more selective and proceed by leaching metals using acids, bases, or cyanide [Sun Z 2017]. Leaching kinetics are generally slow. The use of highly concentrated leaching agents makes hydrometallurgical processes difficult to apply on a large scale and produces large amounts of liquid waste and sludge, which can lead to secondary pollution [Jafhav 2015]. Bio-metallurgy can be highly selective and environmentally sustainable, but it is still in its early stages [Zhuang 2015]. Separating valuable metals from various material matrices including plastics, glass, and ceramics is based on differences in their physical or chemical properties. For example, gravity separation techniques rely on different specific gravities [Sarvar 2015]. Magnetic separation is used to separate magnetic metals from non-ferrous metal waste [Yamane 2011]. Hydrometallurgical separation is based on the chemical reactivity of metals with leaching agents [Sethurajan 2019].

[0025] Electronic components contain potentially very harmful materials, including lead (Pd), cadmium (Cd), beryllium (Be), and chromium (Cr). If released into the environment, these harmful materials can cause many waterborne or even airborne diseases. At the same time, circuit boards contain many precious metals, such as gold (Au), silver (Ag), and platinum (Pt), as well as rare elemental metals, including neodymium (Nd) and dysprosium (Dy), which are difficult to mine and are considered key elements for electronic device manufacturing and electric motors. However, less than 20% of e-waste is recycled, with 80% being landfilled. One method of e-waste recycling is by melting circuit boards and leaching valuable metals [Sthiannopkao 2013]. Traditional recycling methods expose workers to dangerous and carcinogenic substances. Therefore, there is a great need for ultra-clean and efficient methods of recovering valuable metals from e-waste.

[0026] Ore, fly ash, and bauxite residue (red mud)

[0027] Similar situations involve ores, fly ash, and red mud (red mud has recently been referred to as bauxite residue), also because rare earth elements (REEs) are strategic resources in the modern electronics, clean energy, and automotive industries [Cheisson 2019]. Leaching of REE minerals with concentrated aqueous acid solutions followed by two-phase solvent extraction is the mainstream approach for large-scale production of REEs [Cheisson 2019]. However, resource- and pollution-intensive production has a large environmental footprint, with a degradation environmental cost of $14.8 billion in 2015, necessitating the search for sustainable solutions [Lee 2018]. As easily accessible REE minerals decrease, extraction of REEs from industrial waste has gained more attention [Jyothi 2020]. Applicable secondary waste wastes include coal fly ash (CFA) [Taggart 2016; Smith 2019; Zhang 2020; Liu 2019; Sahoo 2016; Middleton 2020], bauxite residue (BR, also known as red mud) from bauxite ore processing for aluminum production [Deady 2016; Rivera 2018; Reid 2017], and electronic waste (e-waste) from consumer electronics and electric vehicles [Maroufi 2018; Deshmane 2020; Peelman 2018]. The annual production of alumina in 2018 was about 160 million tons. Red mud is a highly alkaline waste, consisting mainly of oxides, including Fe2O3, Al2O3, TiO2, CaO, SiO2, and Na2O. In addition, red mud contains valuable rare earth elements, including La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y [Deady 2016]. Thus, similar situations to those discussed above regarding the need to recover metals from e-waste also involve ores, fly ash, and bauxite residue (red mud).

[0028] Recycling of these wastes further reduces the environmental burden of disposing of them [Sahoo 2016]. However, the REE content in these secondary waste wastes is typically less than that in REE minerals, and the recycling yield remains extremely low, exacerbating the need to establish a circular economy plan [Taggart 2016].

[0029] As an example, CFA is a byproduct of coal combustion with an annual production rate of ~ 750 million tons worldwide [Sahoo 2016]. The average total REE content of CFA is ~ 500 ppm, which can vary depending on the geological origin of the feed coal [Taggart 2016; Middleton 2020]. However, the acid-extractable REE content is typically much smaller and highly dependent on the CFA feedstock. For example, Taggart 2016 reported HNO3 extractability of REEs from 1.6% to 93.2%, with a median of ~ 30% from major U.S. power plants, or 7.4 ppm to 372 ppm, with a median of ~ 127 ppm. The extractability of REEs in CFA depends on the REE species, such as oxides, phosphates (monazite, xenotime, bastnaesite, etc.), apatite, zircon, and glass phase [Liu 2019]. The low REE extractability in most CFA resources is attributed to a large fraction of insoluble REE species, such as REE phosphates, zircon, and glass phase [Liu 2019].

[0030] Optimizing the acid leaching process can improve the extractability to some extent, with 70% extractability obtained using highly concentrated inorganic acids such as 15 M HNO3 at 85-90 °C [Taggart 2016], and 35-100% extractability obtained using 12 M HC1 at 85 °C, depending on the feedstock [King 2018]. However, the use of concentrated acids inevitably increases the extraction cost and disposal burden. Chemical or thermal pretreatment of CFA prior to acid leaching helps achieve high REE recovery [Wang Z 2019; Taggart 2018]. For example, by NaOH hydrothermal treatment followed by acid leaching, a total REE recovery of 88% was achieved [Wang Z 2019]. Alkaline roasting using NaOH resulted in > 90% recovery [Taggart 2018]. However, these pretreatment processes are often lengthy and energy-intensive, which greatly reduces the profit margin and incentive.

[0031] Furthermore, there are environmental hazards in the discharge of these materials. The discharge of red mud is very harmful to the environment due to its alkalinity. In October 2010, approximately one million cubic meters of red mud was accidentally released into a Hungarian village, killing ten people and contaminating the surrounding area. Indeed, given the large amount of acid used, developed methods for the separation and recovery of rare earth elements, such as leaching and cation exchange chromatography [Ochsenkuhn-Petropulu 1995] can lead to secondary pollution.

[0032] Thus, the existing methods for REE recovery are plagued by long purification times, low extraction, and high wastewater streams. Therefore, there remains a need for a fast and energy-efficient pretreatment to recover REEs from ores, fly ash, and bauxite residue (red mud). This further necessitates the development of a "dry" method to directly recover rare earth elements from ores, fly ash, and bauxite residue (red mud). SUMMARY

[0033] The present invention relates to an ultrafast flash Joule heating synthesis method, and more particularly, embodiments of the present invention include an ultrafast synthesis method for recovering metals from ores, fly ash, and bauxite residue (red mud).

[0034] Such a solvent-free method based on flash Joule heating can provide an ultrafast synthesis for activating ores, fly ash, and bauxite residue (red mud) to improve REE extractability. The FJH process thermally degrades or reduces the insoluble REE species into components with high thermodynamic solubility, resulting in ~2-fold increase in leachability content and high recovery using dilute acid (e.g., 0.1 M HC1). The activation can be used for various wastes including fly ash and bauxite residue (red mud). The fast FJH process is energy-efficient with low electrical energy consumption of 600 kWh / ton, achieving more than 10-fold profit growth.

[0035] Generally, in another embodiment, the present invention features a method of recovering a metal. The method includes mixing a material with an electrically conductive additive to form a mixture. The material is prepared from an ore, fly ash, and / or bauxite residue. The method further includes applying a voltage across the mixture to recover the metal from the material. The voltage is applied in one or more voltage pulses. A duration of each of the one or more voltage pulses is a time period. The method further includes collecting the recovered metal. The recovering and collecting of the metal includes a leaching process after applying the voltage across the mixture.

[0036] Implementations of the present invention can include one or more of the following features:

[0037] The electrically conductive additive can be a carbon source.

[0038] The material can be prepared from an ore.

[0039] The material can be prepared from fly ash.

[0040] The material can be prepared from bauxite residue.

[0041] The material can be prepared by performing a mechanical process to transform the material into a fine powder.

[0042] The mechanical process can be selected from the group consisting of cutting the material into small pieces, pulverizing the material, grinding the material, milling the material, and combinations thereof.

[0043] This fine powder can be micron-sized.

[0044] The conductive additive can be selected from elemental carbon, carbon black, graphene, flash graphene, coal, anthracite, coke, metallurgical coke, calcined coke, activated carbon, biochar, natural gas carbon that has had its hydrogen atoms removed, activated carbon, sub-graphite, plastic waste, carbon derived from plastic waste, food waste, carbon derived from food waste, biomass, carbon derived from biomass, hydrocarbon gases and mixtures thereof.

[0045] The conductive additive can be carbon black.

[0046] This conductive additive can be primarily elemental carbon.

[0047] The material and the conductive additive can be mixed in a weight ratio ranging from 1:2 to 25:1.

[0048] The applied voltage can be in the range of 15V to 300V.

[0049] The mass of the mixture to which the voltage is applied can exceed 1 kg. The applied voltage can range from 100V to 100,000V.

[0050] The mass of the mixture to which voltage is applied can exceed 100 kg.

[0051] The mass of the mixture to which the voltage is applied can exceed 1 kg. The applied current can be from 1,000 amps to 30,000 amps.

[0052] The mass of the mixture to which voltage is applied can exceed 100 kg.

[0053] When a voltage is applied, the mixture can have a resistance of 0.1 ohms to 25 ohms.

[0054] The duration of each of the one or more voltage pulses can be from 1 microsecond to 25 seconds.

[0055] The duration of each of the one or more voltage pulses can be from 1 microsecond to 10 seconds.

[0056] The duration of each of the one or more voltage pulses can be from 1 microsecond to 1 second.

[0057] The duration of each of the one or more voltage pulses can be from 100 microseconds to 500 microseconds.

[0058] One or more voltage pulses can be from 2 voltage pulses to 100 voltage pulses.

[0059] This voltage pulse can be generated using direct current (DC).

[0060] The method can be performed using pulsed direct current (PDC) joule heating.

[0061] The voltage pulses can be performed using alternating current (AC).

[0062] The voltage pulses can be performed using direct current (DC) and alternating current (AC).

[0063] The method can switch back and forth between using direct current (DC) and alternating current (AC).

[0064] The method can use direct current (DC) and alternating current (AC) simultaneously.

[0065] The one or more voltage pulses can increase the temperature of the mixture to at least 3000 K.

[0066] The metal can include a rare earth element.

[0067] The metal can include a noble metal.

[0068] The material can include a metal oxide. The step of applying a voltage across the mixture can cause a carbothermal reduction of the metal oxide to recover the metal.

[0069] The voltage can be applied across the mixture to recover the metal from the material at a pressure of 0.001 to 25 atmospheres.

[0070] The pressure can be about 1 atmosphere.

[0071] The pressure can be at least 2 atmospheres.

[0072] The pressure can be at least 10 atmospheres.

[0073] The pressure can be at least 20 atmospheres.

[0074] The method can be performed using a pressurized cell.

[0075] Applying a voltage across the mixture to recover the metal from the material can cause a majority of the metal to remain with the graphene produced by the method.

[0076] The collecting step can include collecting a gas stream comprising volatilized products produced by the application of the voltage across the mixture.

[0077] The collecting step can further include cooling the gas stream.

[0078] The leachability of the metals in the mixture after the voltage is applied across the mixture can be more than twice the leachability content of the metals in the mixture before the voltage is applied across the mixture when treated using the same pH and the same volume of water.

[0079] The leaching process can be performed using a dilute acid.

[0080] The dilute acid can be an acid of at most 1 M.

[0081] The dilute acid can be an acid of at most 0.1 M.

[0082] The dilute acid can be an acid of at least 1 M.

[0083] The method can be performed in a continuous process or an automated process.

[0084] Generally, in another embodiment, the present application features a system for a method of recovering metals using at least one of the above methods. The system includes a source of a mixture of the material and a conductive additive. The system further includes a cell operably connected to the source, such that the mixture can flow into the cell and be held under compression. The system further includes an electrode operably connected to the pressure cell. The system further includes a flash power source for applying a voltage across the mixture to recover metals from the material.

[0085] Embodiments of the application can include one or more of the following features:

[0086] The cell can be a pressure cell. The system can further include a gas supply for pressurizing the pressure cell.

[0087] The system can further include an adjustable safety valve.

[0088] The system can further include a particle collector.

[0089] The system can further include a gas collector.

[0090] The system can be operable to perform a continuous process or an automated process.

[0091] SUMMARY

[0092] FIGS. 1A-1E Ultrafast synthesis of carbides by flash joule heating (FJH) is shown. FIG. 1A Schematic of FJH synthesis of carbides, where route (i) shows the high-temperature FJH process of one embodiment of the present application, and route (ii) shows a conventional carburization process. FIG. 1B Current measurements during the FJH process are shown. FIG. 1CReal-time spectral radiance is shown at wavelengths from 640-1000 nm. Inset are photographs of the sample before FJH, during FJH, and during rapid cooling. FIG. 1D Real-time temperature measurement is shown by fitting the blackbody radiation from the sample during FJH. FIG. 1E Temperature-vapor pressure relationships are shown for various metal precursors and carbon.

[0093] FIGS. 2A-2H Phase-controlled synthesis of molybdenum carbides is shown. FIG. 2A are X-ray diffraction (XRD) patterns of β-Mo2C, α-MoC 1-x , and η-MoC 1-x synthesized at voltages (V) of 30 V, 60 V, and 120 V, respectively. The respective PDF reference cards are β-Mo2C, 35-0787; α-MoC 1-x , 65-8092; and η-MoC 1-x , 08-0384. FIG. 2B are crystal structures of the three phases of molybdenum carbides. β-Mo2C is hexagonal with ABAB stacking, α-MoC 1-x is cubic, and η-MoC 1-x is hexagonal with ABCABC stacking. FIG. 2C are X-ray photoelectron spectroscopy (XPS) spectra of the three phases of molybdenum carbides. FIG. 2D are bright-field transmission electron microscopy (BF-TEM) images of β-Mo2C nanocrystals supported on graphene. 0.339 nm corresponds to the interplanar spacing (d) of graphene. FIG. 2E are high-resolution transmission electron microscopy (HRTEM) images of β-Mo2C and the corresponding fast Fourier transform (FFT) patterns. FIG. 2F are high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of β-Mo2C and energy dispersive X-ray spectroscopy (EDS) elemental maps. FIG. 2G are HRTEM images of α-MoC 1-x and the corresponding FFT patterns. FIG. 2H are HRTEM images of η-MoC 1-x and the corresponding FFT patterns.

[0094] FIGS. 3A-3B Phase transformation processes of molybdenum carbides are shown, which are revealed by density functional theory (DFT) calculations. FIG. 3A Formation energies of β-Mo2C with different carbon contents versus α-MoC 1-x and η-MoC 1-x are shown. FIG. 3B are formation energies of β-Mo2C, α-MoC 1-x(x = 1 / 2), a-MoC 1-x (x = 3 / 8), and η-MoC 1-x Calculated crystal structure of (x = 3 / 8) Mo2C (dashed circles represent carbon vacancies).

[0095] FIGS. 4A-4F Dependence of the hydrogen evolution reaction (HER) performance on the phase of molybdenum carbide is shown. FIG. 4A Polarization curves of the three phases of molybdenum carbide are shown. Pt / C and pure flash graphene (FG) were used as controls. The performance was normalized to the same mass loading of molybdenum carbide. FIG. 4B Tafel curves of the three phases of molybdenum carbide are shown. FIG. 4C Alternating current (AC) impedance of the three phases of molybdenum carbide is shown. FIG. 4D Durability of molybdenum carbide is shown. a-MoC 1-x Polarization curves of the 1st cycle and 1000th cycle of a-MoC FIG. 4D The inset is the change in overpotential of the three phases of molybdenum carbide. FIG. 4E Free energy diagrams of the HER on β-Mo2C (001), a-MoC 1-x (110), and η-MoC 1-x (001) at a monolayer hydrogen adsorption coverage are shown. FIG. 4F Calculated partial density of states of Mo and C in β-Mo2C (001), a-MoC 1-x (110), and η-MoC 1-x (001) are shown. The dashed line indicates the location of the Fermi level.

[0096] FIGS. 5A-5D A general strategy for carbide synthesis is shown. FIG. 5A is the carbothermic reduction temperature of oxides derived from the Ellingham diagram. FIG. 5B are X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy (HRTEM) images of group IVB metal carbides. The PDF reference cards are TiC, 65-7994; ZrC, 65-8834; and HfC, 65-7326. FIG. 5C are XRD patterns and HRTEM images of group VB metal carbides. The respective PDF reference cards are VC, 65-8825; NbC, 65-8780; and TaC: 65-0282. FIG. 5D are XRD patterns and HRTEM images of group VIB metal carbides. The respective PDF reference cards are Cr3C2, 65-0897; Mo2C, 35-0787; and W2C, 20-1315 FIGS. 5B-5D The scale bar in is 5 nm.

[0097] FIGS. 6A-6D A flash joule heating (FJH) setup is shown. FIG. 6A is an electrical schematic of the FJH system. Ten aluminum electrolytic capacitors (450 V, 6 mF, Mouser #80-PEH200YX460BQU2) were used for charging, with a total capacitance of 60 mF. Additional details of the electrical components can be found in this publication [Luong, 2020]. FIG. 6B is a photo of the FJH setup. FIG. 6C is a photo of the reaction stage. FIG. 6D is a photo of the reaction chamber.

[0098] FIGS. 7-12 Alumina ultrafast phase transition by pulsed direct current joule heating is shown. FIG. 7 Schematic showing pulsed direct current joule heating and resistive hot spot effect. FIG. 8 Representative method for the phase transition from γ- to α- Al2O3 is shown. FIG. 9 is the XRD pattern of the γ- Al2O3 and calcined α- Al2O3 products after different PDC durations. FIG. 10 Crystal structures of alumina phases: γ- Al2O3 (crystal system: cubic; space group: Fd-3m), δ ′ - Al2O3 (crystal system: orthorhombic; space group: P222), and α- Al2O3 (crystal system: trigonal; space group: R-3c). For γ- Al2O3, all Al positions are depicted to show the crystal structure, while not all sites are occupied in the actual structure. FIG. 11 Phase mass ratio of alumina polymorphs as a function of PDC duration is shown. FIG. 12 Raman spectra of the synthesized α- Al2O3 / CB mixture and purified α- Al2O3 NPs by calcination in air are shown.

[0099] FIGS. 13A-13B PDC joule heating system is shown. FIG. 13A is the electrical diagram of the system. FIG. 13B Pulse voltage generation that can be used in the system to generate PDC is shown.

[0100] FIG. 13C Raman spectra of CB precursor and products after PDC joule heating at 60 V for 0.8 s are shown.

[0101] FIGS. 14A-14F Characterization of α- Al2O3 NPs is shown. FIG. 14A is a BF-TEM image of α- Al2O3 NPs. FIG. 14B is a HRTEM image of α- Al2O3 NPs. FIG. 14CThis is a bar chart and distribution of α-Al2O3 NP particle size determined by TEM. FIG. 14D The pore width distribution determined by applying the DFT model is shown. FIG. 14E These are the Fourier transform infrared spectra of the γ-Al₂O₃ NP precursor and the α-Al₂O₃ NP product. FIG. 14F These are the fine XPS spectra of Al and O in α-Al₂O₃ NP.

[0102] FIGS. 15A-15F The resistive hotspot effect in the PDC method is shown. FIG. 15A These are XRD patterns of γ-Al2O3 / CB with different mass ratios after the PDC method. FIG. 15B It is the phase mass ratio of the product after PDC method, which varies with the volume fraction f(γ-Al2O3). FIG. 15C It is a graph of conductivity versus temperature as a function of f(γ-Al2O3). FIGS. 15D-15F The current density diagrams are for samples in the PDC process with different γ-Al2O3 volume fractions of f = 0.41, f = 0.73 and f = 0.78, respectively.

[0103] FIG. 16 The current density is shown in the body region and the hot spot region.

[0104] FIGS. 17A-17D The topological phase transition process revealed by DFT calculations is shown. FIG. 17A The bulk cohesive energy (μ, eV / Al2O3) and surface formation energy of the three Al2O3 phases are shown. FIG. 17B The free energies of the three phases of Al2O3 nanocrystals are shown relative to their specific surface area. FIGS. 17C-17D It is from the top view ( FIG. 17C ) and side view ( FIG. 17D ) of γ-Al2O3(100), δ ′ Contour plot of partial charge density at the highest energy band (0.3 eV below the Fermi level) of the surface states of -Al2O3(100) and α-Al2O3(001).

[0105] FIGS. 18A-18C The ultrafast alternating current sintering (ACS) system and sample holder are shown. FIG. 18A This is the electrical diagram of the ACS system. FIGS. 18B-18C These are top and side views of the carbon paper holder used for sintering.

[0106] FIGS. 19A-19H The ultrafast ACS of alumina ceramics was demonstrated. FIG. 19APhotos of carbon paper during heating, sintering, and cooling are shown. FIG. 19B Real-time temperature measurements during the ACS process are shown. FIG. 19C Images of sintered ceramic granules loaded on carbon paper are shown. FIG. 19D XRD patterns of alumina ceramic using either a- AI2O3 NPs or commercially available a- AI2O3 nanopowder as precursors are shown. FIG. 19E SEM images of ceramic by using a- AI2O3 NPs as precursors are shown.

[0107] FIG. 19F Particle size distribution of alumina ceramic is shown. FIG. 19G Statistics of Young's modulus of alumina ceramic using a- AI2O3 NPs precursors are shown. FIG. 19H Statistics of Young's modulus of alumina ceramic using commercially available a- AI2O3 precursors are shown.

[0108] FIGS. 20A-20B and FIGS. 21A-21B Scalability of the PDC process is shown. FIG. 20A Photo of a sample with a mass of 700 milligrams synthesized using a tube (D = 15 mm) and a PDC voltage of 60 V. FIG. 20B is FIG. 20A XRD pattern of the product shown in FIG. 21A Photo of a sample with a mass of 1.4 grams synthesized using a tube (D = 15 mm) and a PDC voltage of 120 V. FIG. 21B is FIG. 21A XRD pattern of the product shown in

[0109] FIGS. 22-28 Recovery of precious metals by flash joule heating (FJH) is shown. FIG. 22 is a schematic of the FJH and vaporization separation system. FIG. 23 Photo of a printed circuit board (PCB) (scale, 5 centimeters), inset shows a mixture of carbon black (CB) with PCB powder (scale, 2 centimeters). FIG. 24 Precious metal concentration in PCBs as determined by inductively coupled plasma mass spectrometry (ICP-MS) is shown. FIG. 25 Current vs. time recorded at different FJH voltages is shown. FIG. 26 Real-time temperature measurements at different FJH voltages by fitting the blackbody radiation emitted from the sample are shown. FIG. 27 Vapor pressure-temperature relationship of precious metals and carbon is shown. FIG. 28 Precious metal recovery by condensing vaporized gas components is shown.

[0110] FIGS. 29A-29E Photo of the system to collect vaporized metal vapor. FIG. 29AThis is a photo of an evaporation collection system. FIGS. 29B-29C These are photographs of the vacuum gauge before and after flash Joule heating (FJH). FIGS. 29D-29E These are photos of the condensate containers before and after the FJH reaction.

[0111] FIG. 30 This is a circuit diagram of the flash Joule heating (FJH) system used in the system shown in Figure 29.

[0112] FIGS. 31A-31G Halogen-assisted improvements show recovery rates. FIGS. 31A-31F They respectively showed the use of ( FIG. 31A NaF, ( FIG. 31B PTFE, ( FIG. 31C NaCl, ( FIG. 31D CPVC, ( FIG. 31E )NaI and ( FIG. 31F The recovery rate of precious metals using a mixture of NaF, NaCl, and NaI as additives. Y0 and Y represent the recovery rates of precious metals with and without additives, respectively. The dashed line indicates that Y / Y0 = 1, meaning that if Y / Y0 ≤ 1, there is no advantage of the additive. FIG. 31G These are scanning transmission electron microscope (STEM) images of the collected solids, and energy dispersive X-ray spectra (EDS) of Rh, Pd, Ag, and Au in rectangular regions (scale bar in STEM images, 0.5 μm; scale bar in EDS images, 100 nm).

[0113] FIGS. 32A-32F The study demonstrates the recovery of precious metals via flash joule heating (FJH) and calcination. FIG. 32A Different methods for recovering precious metals from printed circuit boards (PCBs) are shown. FIG. 32B The thermogravimetric analysis (TGA) curves of the PCB after FJH (PCB-flash evaporation) in air are shown. The inset shows photographs of the PCB after FJH and calcination (PCB-flash evaporation-calcination) and the PCB itself. FIG. 32C It is the TGA curve of the PCB. FIG. 32D The X-ray photoelectron emission spectra (XPS) of PCB, PCB-flash evaporation, and PCB-flash evaporation-calcination are shown. FIG. 32E The concentration of precious metals in the PCB after calcination (PCB-calcination) is shown. FIG. 32F The results show that calcination improves the leaching yield. Y0 and Y represent the recovery rates obtained by leaching PCB and PCB-calcination, respectively.

[0114] FIGS. 33A-33F This demonstrates how flash joule heating (FJH) can improve the leaching efficiency of precious metals. FIG. 33AA schematic diagram of the pressurization apparatus of FJH is shown. FIG. 33B A gas flow simulation at different pressures is shown. The internal pressure (P0) in the FJH process is calculated to be ~ 5 atm. P out FJH at positive pressure corresponding to vacuum, atmospheric pressure, and 3 atm. FIG. 33C Improvement in noble metal concentration and recovery by FJH is shown. FIG. 33D Improvement in noble metal concentration and recovery by FJH and calcination is shown. FIG. 33E Improvement in recovery with FJH voltage variation at atmospheric pressure is shown. FIG. 33F Improvement in recovery with pressure variation is shown. For FIGS. 33E-33F Recovery of Rh, Pd, and Ag is calculated from PCB-Flash, and recovery of Au is calculated from PCB-Flash-Calcination.

[0115] FIGS. 34A-34E A mechanism for improving leaching efficiency by flash Joule heating (FJH) is shown. FIG. 34A A schematic diagram of the laminated configuration of several types of electronic devices is shown. FIG. 34B is a scanning electron microscope (SEM) image of printed circuit board (PCB) powder. FIG. 34C is a SEM image of PCB-Flash. FIG. 34D is a SEM image of PCB-Flash-Calcination. FIG. 34E A schematic diagram of the morphological and structural changes of PCB during FJH and calcination is shown.

[0116] FIGS. 35A-35F Removal of heavy metals from electronic waste by flash Joule heating (FJH) method is shown. FIG. 35A Vapor pressure-temperature relationship of toxic heavy metals and carbon is shown. FIG. 35B Concentration of toxic heavy metals in printed circuit board (PCB) is shown. FIG. 35C Concentration of toxic heavy metals in PCB after FJH is shown. FIG. 35D Removal efficiency and collection yield of heavy metals is shown. FIG. 35E Concentration of Hg in the residue after multiple FJH reactions is shown. FIG. 35F Concentration of Cd in the residue after multiple FJH reactions is shown. FIGS. 35E-35F The dashed line in indicates the starting content and the World Health Organization (WHO) level of the approved agricultural soil safety limit.

[0117] FIG. 36 is a graph showing the theoretical separation factor of evaporation separation method.

[0118] FIGS. 37A-37F Carbonthermal reaction for the recovery of metals from metal oxides is shown.FIG. 37A is an XRD pattern of Al recovered from AI2O3. FIG. 37B is an XRD pattern of Fe recovered from Fe2O3. FIG. 37C is an XRD of Cu recovered from CuSO4. FIG. 37D is an XRD of Ni recovered from NiSO4. FIG. 37E is an XRD of Mn recovered from MnO2. FIG. 37F is an XRD of Pb recovered from PbNO3. This is as would occur in bauxite residue (red mud).

[0119] FIG. 38 is a schematic of a flash Joule heating pressure and gas collection system that can be used in embodiments of the present application.

[0120] FIGS. 39A-39D shows a scale up of the flash Joule heating (FJH) process. FIG. 39A are photographs of treated samples treated with the following conditions: m0 = 0.2 grams, V0 = 150 V and C0 = 0.06 F (left), ml = 2 grams, VI = 150 V and CI = 0.6 F (middle), m2 = 4 grams, V2 = 300 V and C2 = 0.6 F (right). FIGS. 39B-39D is a real-time temperature profile of a sample.

[0121] FIGS. 40A-40B is a diagram of a continuous flash Joule heating (FJH) reactor.

[0122] FIGS. 41A-41C shows a FJH system for fly ash. FIG. 41A shows an electrical diagram of a FJH system. FIGS. 41B-41C are photographs of a FJH clamp that connects the sample to the FJH system for 200 milligrams and 2 grams, respectively.

[0123] FIG. 42 are photographs of CFA-C and CFA-F. Scale bar is 4 cm.

[0124] FIGS. 43A-43G shows the acid extractable REE content in CFA. FIG. 43A is an XRD pattern of CFA-F and CFA-C. FIG. 43B is an XPS full spectrum of CFA-F and CFA-C. FIG. 43C is the concentration of total REE in CFA-F and CFA-C obtained by HNO3 leaching (15 M, 85 °C), HC1 leaching (1 M, 85 °C) and total quantification. FIG. 43D is a SEM image of CFA-F (scale bar, 2 pm). FIG. 43EREE content extractable by HCI (1 M, 85 °C) and total quantification of REE in CFA-F, and recovery of REE are shown. FIG. 43F is a SEM image of CFA-C (scale bar, 5 μm). FIG. 43G REE content extractable by HCI (1 M, 85 °C) and total quantification of REE in CFA-C, and recovery of REE are shown (all error bars represent standard deviation, where N = 3).

[0125] FIGS. 44A-44H REE recovery from CFA improved by electrothermal activation is shown. FIG. 44A is a graph of FJH of CFA. FIG. 44B is a current profile under conditions of 120 V and 1 s. FIG. 44C Real-time temperature measurements are shown. FIG. 44D Relationship between HCI leachable REE content (1 M, 85 °C), improvement in recovery, and FJH voltage from CFA-F is shown. FIG. 44E pH dependent REE leachability from CFA-F feedstock and activated CFA-F is shown. FIG. 44F pH dependent REE leachability from CFA-C feedstock and activated CFA-C is shown. FIG. 44G REE content extractable by HCI (1 M, 85 °C) and improvement in recovery from activated CFA-F is shown. FIG. 44H REE content extractable by HCI (1 M, 85 °C) and improvement in recovery from activated CFA-C is shown (Y0 represents REE recovery by HCI leaching of CFA feedstock, Y represents REE recovery by HCI leaching of activated CFA. All error bars show standard deviation, where N = 3).

[0126] FIG. 45 is a flowchart of REE recovery from secondary waste by electrothermal activation.

[0127] FIGS. 46A-46G Mechanism of improved REE extractability by electrothermal activation is shown. FIG. 46A is an XRD pattern of YPO4 (bottom) with reference PDF (YPO4, #11-0254) and YPO4 after FJH (top) with reference PDF (Y2O3, #43-0661).

[0128] FIG. 46B is an XRD pattern of LaPO4 (bottom) with reference PDF (LaPO4, #35-0731) and LaPO4 after FJH (top) with reference PDF (La2O3, #05-0602). FIG. 46Cis the calculated dissolution profile of Y2O3, YPO4, La2O3, and LaPO4 in 100 mL solution with a mass of 1 g. Cl - to balance the charge. FIG. 46D is the Ellingham diagram for carbon monoxide and REE oxides. The vertical dashed line indicates the temperature at which Sc2O3 is reduced. FIG. 46E is the XPS fine spectrum of Y2O3 after FJH. FIG. 46F is the XPS fine spectrum of La2O3 after FJH. FIG. 46G is the change in Gibbs free energy for the dissolution reaction of REE oxides and REE metals.

[0129] FIGS. 47A-47C shows the recovery of REE from BR. FIG. 47A is a photograph of BR (scale bar is 5 cm). FIG. 47B is the XRD pattern of BR. FIG. 47C is the acid leachable REE content (0.5 M HNO3) from BR feedstock and 120 V FJH activated BR, and the improvement in recovery rate (Y0 represents the REE recovery rate from the feedstock by direct leaching, Y represents the REE recovery rate from the activated material by leaching). All error bars show standard deviation, where N = 3.

[0130] FIGS. 48A-48B shows the FJH voltage dependent REE recovery rate from BR. FIG. 48A is the acid leachable REE content (0.5 M HNO3) from BR, and the improvement in REE yield as a function of FJH voltage. FIG. 48B is the acid leachable REE content (0.5 M HNO3), and the improvement in recovery rate at 120 V FJH. (Y0 represents the REE recovery rate from the BR feedstock by direct leaching, Y represents the REE recovery rate from the activated BR by leaching. Error bars represent standard deviation, where N = 3).

[0131] FIGS. 49A-49C shows the recovery of REE from e-waste. FIG. 49A is a photograph of e-waste ground into a powder, with a scale bar of 5 cm. FIG. 49B is the XRD pattern of e-waste. FIG. 49C is the acid leachable REE content (1 M HCl) from e-waste feedstock and 50 V FJH activated e-waste, and the improvement in recovery rate. (Y0 represents the REE recovery rate from the feedstock by acid leaching, Y represents the REE recovery rate from the activated material by acid leaching. All error bars show standard deviation, where N = 3).

[0132] FIGS. 50A-50B shows the improvement in REE recovery from e-waste by FJH activation.FIG. 50A is the acid leachable content of total REEs (1 M HC1) from e-waste, and the increase in REE recovery as a function of FJH voltage. FIG. 50B is the acid leachable content of total REEs (1 M HC1), and the increase in REE recovery at 50 V FJH (Y0 represents the REE recovery by direct leaching of e-waste feedstock, and Y represents the REE recovery by leaching of activated e-waste. Error bars show standard deviation, where N=3). DETAILED DESCRIPTION

[0134] The present invention relates to ultrafast flash Joule heating synthesis methods, and more particularly, embodiments of the present invention include ultrafast synthesis methods of carbides, ultrafast synthesis methods of corundum nanoparticles, ultrafast synthesis methods of recovering precious metals from electronic waste (e-waste), and ultrafast synthesis methods of recovering metals from ores, fly ash, and bauxite residue (red mud).

[0135] Ultrafast synthesis of carbides

[0136] Synthesis methods

[0137] The present invention includes ultrafast methods for synthesizing metal carbide nanoparticles via flash Joule heating [see Luong 2020; Stanford 2020; Tour PCT’000 application]. Metal carbides are synthesized in seconds, which is hundreds of times faster than previous methods [Gong 2016; Wan 2014; Ma 2015]. Thus, in some embodiments, the present invention provides phase-controlled synthesis of transition metal carbide nanocrystals via ultrafast flash Joule heating.

[0138] Such flash Joule heating-based solvent-free methods can provide ultrafast synthesis of coke-free carbide nanocrystals in 1 second. Millisecond current pulses can be passed through the precursor, bringing the sample to ultra-high temperatures (>3000 K), which are then rapidly cooled to room temperature (>10 4 K·s -1 ). Carbides of thirteen elements can be synthesized, including interstitial TMCs of TiC, ZrC, HfC, VC, NbC, TaC, Cr2C3, MoC, and W2C, and covalent carbides of B4C and SiC, which provides excellent versatility. Furthermore, by controlling the FJH pulse voltage, pure phase molybdenum carbides including β-Mo2C and metastable α-MoC 1-x and η-MoC 1-x can be selectively synthesized, showing the phase design capability of the synergistic electrothermal process. Phase-dependent HER performance of molybdenum carbides was also discovered; β-Mo2C exhibits the best HER performance (overpotential of -220 mV, Tafel slope of 68 mV dec-1 and good durability.

[0139] FIG. 1A is a schematic of the FJH synthesis of carbides using various precursors. Route (i) 101 illustrates the ultra-high temperature FJH process described herein, where carbon black and metal oxides form metal carbides and graphene. The graphene can be subsequently removed by a post-synthesis purification process (not shown). This can be referred to as a reverse gas-solid reaction interface. Route (ii) 102 shows the traditional carburization process, which is referred to as a solid-gas reaction interface.

[0140] The method for ultrafast synthesis of carbides can include the following.

[0141] The reaction precursors (or precursors) are selected and mixed with a conductive carbon additive such as carbon black. Alternatively, the conductive additive can be other carbon sources (in combination with or in place of carbon black, as these temperatures will convert any carbon source at these temperatures to nearly pure carbon). Carbon black can be replaced with graphene, flash graphene, coal, anthracite, coke, metallurgical coke, calcined coke, activated carbon, biochar, natural gas carbon that has had its hydrogen atoms removed, activated carbon, subgraphite, plastic waste, carbon char derived from plastic waste, food waste, carbon char derived from food waste, biomass, carbon char derived from biomass, hydrocarbon gases, and mixtures thereof. The use of carbon black as described herein represents a conductive additive that can be used in the present invention. In certain embodiments of the present invention, the weight ratio of the precursors to the conductive additive is 1 :2 to 15: 1, and in other embodiments, the weight ratio of the precursors to the conductive additive is 1 :2 to 2: 1.

[0142] As shown in FIG. 1A General precursors, including elemental metals and metal components such as metal oxides, metal chlorides, and metal hydroxides, can all be used as precursors. The carbon black (or other conductive additive) and metal precursors are thoroughly mixed by using hand grinding or ball milling.

[0143] The mixture of carbon black (or other conductive additive) and metal precursors is flash joule heated. As shown in FIG. 1AAs shown, the mixture can be packed into a quartz tube and compressed to have a resistance of 0.5 to 20 ohms. Two copper or graphite rods are placed on either side as electrodes. A high voltage of 30V to 150V is applied through a capacitor bank (this can also be done using AC, sometimes even with an advantage). However, the voltage will be much higher, up to several thousand volts (and possibly exceeding 10,000 amperes), if the reaction scale is larger, because as the scale increases, the voltage (and current) must increase to induce the same reaction temperature. For example, if the reaction scale is several kilograms or hundreds of kilograms of material, the voltage can be 10,000 volts, and even up to 100,000 volts (and the current can be 10,000 amperes, and up to 30,000 amperes).

[0144] In one embodiment, a mixture of a metal precursor and commercially available carbon black is slightly compressed within a quartz tube between two graphite electrodes. FIG. 1A The widely applicable metal precursors can be elemental metals (M) and metal oxides (MO). x ), chloride (MCl) x ) and hydroxide (M(OH) x Carbon black is used simultaneously as a carbon source for carbothermal reduction and as a conductive additive. Two electrodes are connected to a capacitor bank, which is first charged by a power source and then discharged at a high voltage to bring the precursor to a high temperature. In a typical FJH process with a voltage of 100V and a sample resistance of 1Ω, the current passing through the sample within a ~50ms discharge time was recorded to be ~100A. FIG. 1B ).

[0145] Fast light emission was observed during the FJH process (see [link]). FIG. 1C (Photos 110-112). Temperature was measured by fitting the blackbody radiation spectrum of the sample. FIG. 1C The highest estimated temperatures obtained at 80V and 100V FJH are ~2700K and ~3000K, respectively (e.g. FIG. 1D (As shown in curves 121-122 in the figure).

[0146] The cooling rate is extremely fast, and it is 10. 4 K·s -1 The magnitude of the temperature distribution was measured using the finite element method (FEM) to simulate the temperature distribution of the sample, which further provided insights into the influence of FJH parameters on the achievable temperature. It was found that higher temperature values ​​could be obtained by applying a larger FJH voltage and an appropriate sample conductivity. In contrast, the higher thermal conductivity of the sample resulted in lower temperatures due to faster heat dissipation. The temperature plots showed that the temperature distribution was uniform throughout the sample, demonstrating the uniform heating characteristics of the FJH process.

[0147] Sample FJH was volatilized of most non-carbon components at such high temperatures (~3000 K). According to the temperature-vapor pressure relationship ( FIG. 1E ), all representative metal precursors, including elemental metals and metal oxides and chlorides, have higher vapor pressures than carbon sublimated at ~3900 K [Abrahamson 1974]. As a result, the metal precursors are volatile components, and the carbon source remains solid during the reaction. In this case, the metal precursor vapor reacts with carbon to form metal carbide, which is referred to herein as the reverse gas-solid reaction interface ( FIG. 1A , route (i) 101). In contrast, in the traditional carburization process [Rosa 1983], a gaseous hydrocarbon such as methane (CH4) is introduced into a solid metal precursor. Carbon diffusion through the solid-gas interface is usually fast, and a coked carbide surface ( FIG. 1A , route (ii) 102) results from the excess supply of carbon source, which passivates the catalytic activity of the final product [Gong 2016].

[0148] Phase-controlled synthesis of molybdenum carbide nanocrystals

[0149] Molybdenum carbides that are attractive for catalysts [Yao 2017; Wan 2014; Li 2016; Ma 2015] were synthesized using embodiments of the present invention. The phases of molybdenum carbides are complex due to their temperature, composition, and vacancy-dependent stabilities [Hugosson 1999]. Different phases have different geometric and electronic structures [Politi 2103; Baek 2019], with the catalytically relevant phases being hexagonal β-Mo2C [Wan 2014; Ma 2015; Fan 2017], cubic α-MoC 1-x [Yao 2017; Baek 2019; Song 2019] and hexagonal η-MoC 1-x 34 [Song 2019].

[0150] MoCl3was chosen as the precursor ( FIG. 1E ) due to its high vapor pressure. It was found that by adjusting the FJH voltage, three pure phases of molybdenum carbide could be selectively synthesized ( FIGS. 2A-2B ). According to X-ray diffraction (XRD), the β-Mo2C phase was produced at a 30 V voltage ( FIG. 2A , bottom); when the voltage was increased to 60 V, a pure α-MoC 1-x phase was obtained ( FIG. 2A , middle); further increasing the voltage to 120 V yielded η-MoC 1-x ( FIG. 2A , top). Note that the diffraction peak at ~26° (indicated by the star) is attributed to the graphene support.

[0151] From hexagonal β-Mo2C to cubic α-MoC 1-x To hexagonal η-MoC 1-x The phase transition from hexagonal β-Mo2C to cubic α-MoC 1-x to β-Mo2C upon annealing at 850 °C for ~24 h, while η-MoC 1-x is stabilized at higher temperatures only by using a NiI2 additive.

[0152] To investigate the electronic structure, Mo 3d core level X-ray photoelectron spectroscopy (XPS) spectra FIG. 2C ) were collected. Mo 3d spectra were deconvoluted into 3d 3 / 2 and 3d 5 / 2 peaks. Peak fitting revealed four chemical states of Mo in the molybdenum carbides, including Mo 0 , Mo 2+ , Mo 4+ , and Mo 6+ . The dominant Mo 0 peak and the smaller Mo 2+ peak are attributed to the molybdenum carbides due to the coexistence of Mo-Mo and Mo-C bonds in the molybdenum carbides [Wan 2014]. Mo 4+ and Mo 6+ are assigned to MoO2 and MoO3, respectively, due to surface oxidation of the molybdenum carbides upon exposure to air [Wan 2014; Ma 2015]. Quantitative analysis of the Mo chemical state ratios indicates that the high oxidation states (Mo 4+ and Mo 6+ ) in η-MoC 1-x are greater than those in β-Mo2C and α-MoC 1-x , indicating that β-Mo2C is the most oxidation-resistant phase, followed by α-MoC 1-x .

[0153] Morphological characterization by scanning electron microscopy (SEM) shows fine powder characteristics for all three carbide phases. Energy dispersive spectroscopy (EDS) mapping images show uniform distribution of Mo and C.

[0154] Transmission electron microscopy (TEM) and XRD were used to characterize the size and crystallinity of the molybdenum carbides. The particle size of the molybdenum carbide phases was determined by FJH voltage. β-Mo2C synthesized at the lowest voltage has the largest average size of ~26.4 nm, followed by α-MoC 1-x (~21.2 nm) and η-MoC 1-x(smaller particle size values obtained at higher voltages can be attributed to faster nucleation kinetics at higher temperatures [Jang 1995]).

[0155] Particle size values determined by TEM match well with the crystal sizes determined by XRD using the Halder-Wagner method (see Table I), indicating the single crystalline character of the synthesized carbide particles.

[0156] Table I

[0157] Parameters of carbide synthesis

[0158]

[0159] A typical bright-field TEM (BF-TEM) image of β-Mo2C nanocrystals shows regular hexagonal nanoplates of ~20 nm in lateral size (depicted by hexagons 201) supported on carbon FIG. 2D ). High-resolution TEM (HRTEM) images show lattice fringes FIG. 2E (top), with a 0.26 nm interplanar spacing (d) corresponding to the (300) plane of β-Mo2C. From the atomic resolution image and the corresponding fast Fourier transform (FFT) pattern FIG. 2E (bottom), the nanoplate orientation is assigned to be β-Mo2C (001). High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images in STEM mode and EDS elemental maps reveal a uniform spatial distribution of Mo, C, and O FIG. 2F . Note that O is attributed to surface contamination, consistent with the XPS results FIG. 2C . HRTEM images and corresponding FFT patterns of α-MoC 1-x ( FIG. 2G ) and η-MoC 1-x ( FIG. 2H ) are also obtained in the orientation of α-MoC 1-x (110) and η-MoC 1-x (116) for specific samples. However, no preferred orientation of these carbide nanocrystals is observed according to the XRD results FIG. 2A .

[0160] Phase transition process of molybdenum carbides

[0161] To explain the voltage-dependent phase formation, the current and temperature across the sample at different FJH voltages were first recorded. Higher voltages result in higher temperatures and energy input. The highest temperatures at FJH voltages of 30 V, 60 V, and 120 V were measured to be 839 K, 1468 K, and 3242 K, respectively.

[0162] Formation energies of β-Mo2C, α-MoC 1-x and η-MoC 1-x as a function of carbon content were calculated by first-principles density functional theory (DFT). It was found that the β-Mo2C phase is the most stable phase with the lowest formation energy; thus, β-Mo2C forms at relatively low voltage and temperature (point 301). FIG. 3A

[0163] In contrast, α-MoC 1-x and η-MoC 1-x are metastable phases [Hugosson 1999] and form and stabilize at higher temperatures according to the Mo-C phase diagram. The α-MoC 1-x (x = 1 / 2) structure has a slightly higher formation energy and the same stoichiometric composition as β-Mo2C FIG. 3B . Thus, a topological transition from β-Mo2C to α-MoC 1-x is expected when the carbon content is slightly increased (see line 304, which shows the expected phase transition path). As more carbon is introduced into the Mo-C system, the α-MoC 1-x formation energy continues to increase (curve 302), and this energy curve intersects with the energy curve of η-MoC 1- x (curve 303).

[0164] This η-MoC 1-x phase becomes a relatively stable phase near x = 3 / 8 FIG. 3B and continues to be stable up to higher carbon content. This result indicates that carbon vacancies dominate the energy landscape of the Mo-C system and act as the driving force for the topological transition path from β-Mo2C to α-MoC 1-x and then to η-MoC 1-x phases.

[0165] The FJH process, with its wide range of adjustable energy input, allows for the attainment of metastable phases with higher formation energies than the thermodynamically stable phases; subsequently, the ultrafast cooling rate (> 10 4 K s -1 of the FJH process helps to kinetically retain metastable phases (including α-MoC 1-x and η-MoC 1-x phases) to room temperature. As a control, synthesis using a conventional tube furnace with its slow cooling rate of ~ 10 K min 1-x only produces the thermodynamically stable β-Mo2C phase at the same temperature at which the metastable α-MoC -1 phase is produced by the FJH process. This clearly shows the role of the ultrafast cooling rate of the FJH process in kinetically obtaining metastable phases. ​

[0166] Phase-dependent HER performance of molybdenum carbides

[0167] Parallel electrochemical comparison of the three phases of molybdenum carbide reveals the influence of phase control on their respective intrinsic properties and catalytic behavior. To demonstrate their catalytic performance, the HER performance of the three molybdenum carbide phases was measured using a standard three-electrode setup in 0.5 M H2SO4. FIG. 4A Linear sweep voltammetry (LSV) curves of different electrocatalysts as well as Pt / C benchmark (where Pt / C, β-Mo2C, α-MoC 1-x , η-MoC 1-x and flash graphene (FG) are curves 401-405) are shown in

[0168] Phase-dependent HER activity of molybdenum carbides was observed. For β-Mo2C, α-MoC 1-x and η-MoC 1-x , the overpotential (η) with respect to the reversible hydrogen electrode (RHE) at a geometric current density of 10 mA cm -2 was -220 mV, -310 mV and -510 mV, respectively FIG. 4A . The Tafel slopes (b) of β-Mo2C, α-MoC 1-x and η-MoC 1-x were calculated to be 68 mV dec -1 , 84 mV dec -1 and 113 mV dec -1 , respectively FIG. 4B (curves 411-413) show the phase-dependent HER reaction kinetics.

[0169] According to electrochemical impedance measurements, the fast electrode kinetics of the β-Mo2C phase is reflected in a small charge transfer resistance of ~60 Ω at a potential of -0.5 V vs. RHE (see FIG. 4C AC impedance of β-Mo2C, α-MoC 1-x and η-MoC 1-x are shown in curves 421-423, respectively.

[0170] The durability of the three molybdenum carbide phases was evaluated by scanning the electrocatalysts 1000 cycles using cyclic voltammetry. LSV curves of the 1stand 1000thcycle of the three phases of molybdenum carbide (curves 431-432, respectively) are shown in FIG. 4D No significant current drop was observed for all three phases at a geometric current density of 10 mA cm -2The overpotential of β-Mo2C(001) hardly dropped (curve 433), showing excellent long-term stability.

[0171] DFT calculations were performed to explain the phase-dependent HER performance. The Gibbs free energy of hydrogen adsorption (AG H ) has become a descriptor for the selection of HER electrocatalysts [Mavrikakis 2006], and according to the Sabatier principle, the best catalyst has AG H [Greenley 2006] close to 0 eV. The AG 1-x of β-Mo2C(001), a-MoC 1-x (110), and η-MoC H (001) were calculated to be 0.48 eV, 0.71 eV, and 1.09 eV, respectively ( FIG. 4E ). These results show that β-Mo2C and a-MoC 1-x have smaller hydrogen adsorption energy compared to η-MoC 1-x , consistent with previous reports [Fan 2017; Matanovic 2018]. In addition to AG H , the electronic structure provides valuable insights into the metallic character of the carbide phases [Politi 2013].

[0172] FIG. 4F The partial density of states (DOS) of Mo and C in molybdenum carbides is shown. The DOS of β-Mo2C near the Fermi level is significantly larger than that of a-MoC 1-x and η-MoC 1-x . The higher Mo content in β-Mo2C leads to a higher carrier density and enhanced metallic character, which is beneficial for charge transfer during the electrochemical reaction ( FIG. 4C ). The larger surface area of β-Mo2C compared to the other two phases also contributes to the greater current density, as measured by the Brunauer-Emmett-Teller (BET) method. The observed optimal HER performance of β-Mo2C is a combined effect of the relatively small hydrogen adsorption energy, enhanced metallic character, and high surface area. In addition, the flash graphene support provides conductive pathways and prevents the aggregation of carbide nanocrystals, which is beneficial for improving the HER performance [Li 2019].

[0173] General strategy for carbide nanocrystal synthesis

[0174] Due to the ultra-high available temperature of the FJH process, a variety of TMCs can be easily synthesized regardless of the availability of metal precursors with high vapor pressure. A series of carbide nanocrystals from the IVB, VB, and VIB transition groups were successfully synthesized ( Figures 5A-5D). The uniform temperature distribution allows for pure phase synthesis throughout the sample (the peak at ~26° (star) is attributed to the graphene support).

[0175] According to the Ellingham diagram, the reduction temperature of metal oxides is calculated, which serves as a reference value for carbide formation since the reaction of metals with carbon is exothermic ( Figure 5A ). The ultra-high temperature of the FJH process (~3000 K) enables the reduction of all listed oxides to the elemental metal, including the most challenging Hf02at temperatures up to ~2510 K. Almost all low-cost metals or metal compounds, including oxides, hydroxides, and chlorides, can be used as precursors, making FJH a promising low-cost production method compared to previous methods that rely on the availability of volatile compounds [Kolel-Veetil 2005; Wolden 2011; Pol 2009].

[0176] Group IVB carbides only have a stable rock-salt crystal structure, including TiC, ZrC, and HfC, which are easily synthesized ( Figure 5B ). The particle sizes of TiC, ZrC, and HfC were measured to be ~30.4 nm, ~38.6 nm, and ~30.6 nm, respectively. These values match well with the crystal sizes determined by XRD (see Table I above), proving that the synthesized carbide nanoparticles are mainly single crystals. For Group VB carbides, the competing M2C (M = V, Nb, and Ta) phase can exist at lower C contents [Hugosson 1999]. However, by using a large C / M molar ratio, pure phases of VC, NbC, and TaC nanocrystals were successfully synthesized, which have a cubic structure and particle sizes of ~20 to ~30 nm ( Figure 5C ). In contrast, Group VIB carbides (Cr, Mo, and W) are much more complex in their phases [Hugosson 2001]. Here, the orthorhombic Cr3C2 phase and the hexagonal W2C phase were synthesized, with particle sizes of ~14.2 nm and ~18.7 nm, respectively ( Figure 5D ). According to the W-C phase diagram, W2C is not thermodynamically favored over WC below 1250 °C [Kurlov 2006]. The successful synthesis of metastable W2C is attributed to the high energy input and ultrafast cooling rate of the ultrafast electrothermal reaction, once again proving the excellent phase design capability of the FJH process. In addition to TMCs, covalent carbides of B4C and SiC were also synthesized, proving the universality of the FJH process.

[0177] System and synthesis process

[0178] Thus, for the synthesis of metal carbides, the present invention provides, inter alia, (i) ultrafast synthesis that is thousands of times faster than previously reported methods; (ii) phase control capabilities that are difficult to achieve by other methods; (iii) universality, as evidenced by the synthesis of up to 13 carbides, which is not achievable by any other method.

[0179] The metal carbides, particularly molybdenum and tungsten carbides, obtained from the present invention can be used as electrocatalysts, such as for hydrogen evolution, which is crucial for the application of fuel cells in clean energy. In addition, nanoscale carbides are important precursors for the fabrication of high-performance carbide ceramics.

[0180] Figures 6A-6B Exemplary systems and processes used are shown in FIGS. 1-3, including circuit diagrams and the setup of the FJH system. (Additional details of the electrical components can be found in Luong 2020). A capacitor bank with a total capacitance of 60 mF was used as the power source. The metal precursors and carbon black with specific weight ratios (Table I) were mixed by grinding using a mortar and pestle. The reactants (-50 mg) were loaded into a quartz tube with an inner diameter (ID) of 4 mm and an outer diameter (OD) of 8 mm. When scaling up the process, an ID of 8 mm and an OD of 12 mm were used for ~200 mg samples, and an ID of 16 mm and an OD of 20 mm were used for ~1 g samples. Scaling up the mass further to the kilogram level would require a container without quartz. Graphite rods were used as electrodes at both ends of the quartz tube. The electrodes were loosely fitted in the quartz tube to allow outgassing. The resistance was controlled by the compression force of the electrodes on the sample. The tube was subsequently loaded on a reaction stage (FIG. 2) Figure 6C ) inside a sealed reaction chamber, which was evacuated to a moderate vacuum (-10 mmHg) to accommodate outgassing and avoid sample oxidation Figure 6D ). The reaction stage was then connected to the FJH system.

[0181] The capacitor bank was charged by a direct current (DC) power supply that can reach up to 400 V. A relay with programmable millisecond delay time was used to control the discharge time. The charging, flash Joule heating, and discharging were automatically controlled by using a National Instruments Multifunction I / O (NIUSB-6009) in conjunction with a custom LabView program. After the FJH reaction, the equipment cooled down to room temperature rapidly by itself. Before removing the sample, the capacitor bank was ensured to be fully discharged. The detailed conditions for the synthesis of various carbides are listed in Table I.

[0182] Features and applications

[0183] In embodiments, the synthesized carbide nanocrystals are supported on flash graphene. The necessity of graphene and carbide separation depends on the further application. For the application of nanocrystalline carbides in electrocatalysts, the graphene support is beneficial for improving performance by providing electrical conductivity and preventing particle aggregation. For another major application of nanocrystalline carbides as super-strong ceramic precursors, the removal of excess carbon is necessary.

[0184] It has been recognized that carbides can be effectively purified by post-synthesis processes, including simple calcination in air for SiC; Ca metal etching for TiC, ZrC, HfC, VC, NbC, TaC, Cr3C2, β-Mo2C, and W2C [Dyjak 2013]; and liquid density purification methods for metastable molybdenum carbides α-MoC 1-x and η-MoC 1-x Furthermore, by using controlled feeding during synthesis, a greatly improved purity of B4C is shown.

[0185] Due to the ultrafast heating / cooling rate, direct sampling heating feature, and short reaction duration within 1 second, the FJH process for carbide synthesis is highly energy efficient compared to traditional furnace heating where a large amount of energy is used to maintain the furnace chamber temperature. In terms of electrical energy, only 2.2 to 8.6 kJ g -1 Synthesis of carbide nanocrystals. The FJH synthesis has good scalability, with constant temperature values and uniformity on different mass scales can be obtained by adjusting the discharge voltage and / or capacitance.

[0186] By increasing the FJH voltage, the synthesis of carbide nanocrystals up to the gram scale is demonstrated. This FJH process is scalable to synthesize carbide alloys [Sarker 2018], heteroatom-modified carbides [Song 2019], and phase design of metastable carbides [Demetriou 2002], which provides a powerful technique for carbide production.

[0187] Controlled synthesis of metastable phases is challenging in the synthesis of inorganic materials [Chen 2020]. The FJH process provides a widely adjustable energy input that can exceed 3000 K, combined with a kinetically controlled ultrafast cooling rate (> 10 4 K s -1 ). Therefore, the FJH process can access many non-equilibrium phases and subsequently retain them at room temperature, thereby serving as a potential tool for designing metastable phases of various materials such as metal nanomaterials [Chen 2020], layered oxides [Bianchini 2020], metal nitrides [Sun W 2017], and two-dimensional materials.

[0188] Ultrafast synthesis of corundum nanoparticles

[0189] The present invention further includes a flash Joule heating [see Luong 2020; Stanford 2020; Tour PCT’000 application] of the ultrafast process for the synthesis of corundum nanoparticles, i.e., the ultrafast phase transformation from γ-Α1203(and γ-AIOOH) to α-Α1203by flash Joule heating. In brief, carbon black (or other carbon additives, as described above) is mixed with γ-Α1203(or γ-AIOOH) nanoparticles, which are then subjected to flash Joule heating. The phase transformation is ultrafast, within 1 second, thousands of times faster than other prior art methods.

[0190] Embodiments of the present invention thus include a pulsed direct current (PDC)-based Joule heating method to accomplish the phase transformation from γ- to α-Α1203at significantly reduced average bulk temperature and reaction duration (~573 K, < 1 s). When using a suitable volume fraction ratio of γ-Α1203precursor and carbon black conductive additive, the local heating induced by resistive hot spots in the PDC process can enable the fast transformation. The pulsed and local heating mitigates agglomeration, resulting in the synthesis of α-Α1203NPs with an average particle size of ~23 nm and a surface area of ~65 m 2 g -1 Ab initio calculations reveal that the topological phase transformation process (from γ- to δ ′ - to α-Α1203) is determined by the surface energy difference between the three phases. A particle size of ~21 nm is achieved, which is the thermodynamic limit for the synthesis of dehydrated α-Α1203NPs with δ ′ -Α1203as the intermediate phase by thermal processes.

[0191] In addition, based on the Joule heating technique, an alternating current sintering (ACS) process has been developed, which shows the ultrafast and pressureless sintering of these α-Α1203NPs into alumina ceramics with nanoscale grain size and improved strength and hardness.

[0192] A calcination process has also been developed to completely remove the carbon black or formed flash graphene and obtain pure phase α-Α1203. In embodiments, the synthesized α-Α1203shows a surface area of up to 65 m 2 / g, which means that these materials can be used for catalyst support and high-strength ceramic applications.

[0193] Phase transformation synthesis

[0194] The method for the ultrafast synthesis of corundum nanoparticles (i.e., the transformation from γ-Α1203(and γ-AIOOH) to α-Α1203) can include the following.

[0195] Since the γ-ΑΙ203NP precursor is electrically insulating, commercially available carbon black (CB) was used as a conductive additive in embodiments. For example, a mixture of γ-ΑΙ203NPs and CB was compressed inside a quartz tube between two graphite electrodes. See Figure 7 (illustrating a PDC device 701 and resistive hot spots 702 around and in the insulating γ-ΑΙ203NP gap, with arrows depicting current lines) and Figure 13A (an aluminum electrolytic capacitor (450 V, 13 mF) with a total capacitance of 0.624 F was used for charging),

[0196] The CB also served as a spacer to avoid agglomeration of the Al203NPs during heating. The resistance was controlled by the compression force on the two electrodes, which is shown in Table II.

[0197] Table II

[0198] Parameters of PDC Joule heating

[0199]

[0200] Note: V0: initial voltage, Vi: voltage after Joule heating.

[0201] The electrodes were connected to a capacitor bank with a capacitance C = 0.624 F and a charging voltage up to V0= 500 V. The discharge circuit was a series resistance-inductance-capacitance circuit with a characteristic time τ = 0.1 ms, which allowed a PDC with a frequency f = 1000 Hz. Figure 13B The pulse voltage generation that can be used in this system to generate a PDC is shown, with a frequency of 1000 Hz and the ON state set to 20%, which gives a voltage pulse of 0.2 ms.

[0202] Joule heating affects the entire electrical conductor; for a homogeneous conductor, the current density is uniform, so Ohmic dissipation allows a uniform temperature distribution throughout the sample to be achieved [Johnson 2011]. However, when an electric field is applied to a heterogeneous medium, as in the composite of conductive CB and insulating Al203, the current and powder density have a strong spatial variation [Soderberg 1987]. In some regions, the power dissipation is significantly greater than in adjacent regions, which are referred to as resistive hot spots 702 (shown in Figure 7 Even if the average bulk temperature is low, the hot spots allow local heating and trigger the transition that occurs at much higher temperatures.

[0203] By exploiting this effect, a transition of the δ ′ -Al203 intermediate t-phase to α-Al203 was achieved at an average bulk temperature of ~573 K in <1 s. See Figure 8The pulsed DC method 814 is shown in the figure. (As shown in the figure...) Figure 8 As shown, the pulsed DC method 814 is compared with representative phase change methods reported in the literature, namely flame spray pyrolysis 811 [Laine 2006], furnace annealing 812 [Steiner 1971] and high-energy ball milling 813 [Amrute 2019].

[0204] Liquid-feed flame spray pyrolysis 811 produces α-Al₂O₃ at temperatures close to 1873 K; however, the kinetically controlled process can make it difficult to obtain a pure phase (80-85% pure α-phase) [Laine 2006]. Conventional heating methods that supply heat through the sample boundary, such as furnace annealing 812, require extended times to allow for uniform heating; therefore, 1473 K and 10 to 20 hours are needed to complete the phase transformation [Steiner 1971]. Other room-temperature non-equilibrium methods, such as high-energy ball milling 813, have been reported to form α-Al₂O₃ [Amrute 2019]. However, γ-Al₂O₃ can aggregate, leading to a loss of surface area during prolonged high-energy collisions [Zielinski 1993; Chauruka 2015].

[0205] The detailed phase transition process of γ-Al₂O₃ was investigated using the PDC method. See [link to PDC study]. Figures 9-11 (exist Figure 9 In the text, the symbols represent γ-Al2O3(■) and δ-Al2O3(■). ′ -Al₂O₃(▲), α-Al₂O₃(●), and γ-AlOOH(○); the precursor is γ-Al₂O₃ (crystal system: monoclinic; space group: P21 / n; PDF No. 07-0324) with a small amount of γ-AlOOH phase; and the sample was calcined for 0.8 seconds. Particle sizes of ~10 nm and surface areas of ~156 m² were used. 2 g -1 Commercially available γ-Al₂O₃ NP was used as a precursor. A small proportion of the γ-AlOOH phase appeared in the precursor. Figure 9 (0 seconds). A sample resistance of ~8 Ω was obtained with a γ-Al₂O₃ NP to CB mass ratio of 4:1 (Table II). A discharge voltage of 60 V was applied, and different discharge times were controlled by a relay. X-ray diffraction (XRD) patterns of products with different PDC on-state times are shown in... Figure 9 In the middle stage, as the discharge time increases, γ-AlOOH disappears first at 0.3 seconds; subsequently, γ-Al2O3 transforms into δ-Al2O3 between 0.4 and 0.5 seconds. ′ - and α-Al2O3 phase; finally, after 0.8 seconds of discharge, the intermediate δ ′ -Al2O3 phase is completely transformed into α-Al2O3 phase. Figure 11In this case, curves 1121-1123 are γ-Al2O3, δ ′ -Al2O3 and a-Al2O3, respectively. The orthorhombic δ ′ -Al2O3 as a single intermediate phase Figure 10 , which is different from other thermal processes where δ- and θ-Al2O3 usually appear before the final a-Al2O3 phase Figure 8 [Steiner 1998; Levin 1998; Lamouri 2017].

[0206] Unlike previous reports on the synthesis of graphene by high-voltage flash Joule heating at high temperatures of ~3000 K [Luong 2020], the 60 V PDC could not provide enough energy to graphitize the CB. Figure 13C (No 2D peaks of the product were observed after Joule heating at 60 V). As a result, the CB could be easily removed by heating in air, according to thermogravimetric analysis (TGA). Here, the mixture of synthesized a-Al2O3 NPs and CB was calcined in air at 700 °C for 1 h to purify the product. The X-ray photoelectron emission spectroscopy (XPS) of the a-Al2O3 product after calcination showed a very small carbon signal, which could be caused by the adsorption of carbon in air.

[0207] Raman spectroscopy is even sensitive to carbon monolayers [Wang 2008]; interestingly, no characteristic Raman bands of carbon were detected after calcination at 700 °C Figure 12 , curves 1224-1226 are 700 °C calcined, 650 °C calcined and CB / Al2O3, respectively), confirming the efficient removal of carbon. As a control, it was determined that the calcination process itself does not trigger phase transformations and has negligible effect on the coarsening or agglomeration of the γ-Al2O3 phase.

[0208] Characterization of corundum nanoparticles

[0209] The a-Al2O3 NPs obtained by PDC and subsequent mild calcination were further characterized in detail. Bright-field transmission electron microscopy (BF-TEM) images show well dispersed particles. See Fig. 6. Figure 14A High-resolution TEM (HRTEM) shows the high crystallinity of the a-Al2O3 NPs. See Fig. 7. Figure 14B and ​d(104) and d(113) of a- Al203, respectively. Some a- Al203 NPs were observed to have surface roughness features of a few nanometers, similar to the particle size of the g- Al203 precursor. This indicates that the fast PDC process triggered the phase transformation while no significant agglomeration of the NPs occurred. TEM images show particle sizes of 14 to 36 nm, with an average particle size of 25.4 nm and a standard deviation (σ) of 5.8 nm. See Figure 14C .

[0210] Brunauer-Emmett-Teller (BET) measurements show the surface area of the a- Al203 NPs to be ~65 m 2 g -1 . See Figure 14D Insert 1401 (which shows the N2adsorption-desorption isotherm of a- Al203 NPs at 77 K). The average particle size (D) was estimated to be ~23 nm by Equation (1):

[0211] D = 6 / (pS) Equation (1)

[0212] where p is the density of a- Al203 (3.96 g cm -3 ), and S is the specific surface area [Karagdov 1999].

[0213] The pore size distribution determined from the N2adsorption-desorption isotherm using a density functional theory (DFT) model indicates a high probability at 3-10 nm. See Figure 14D . The observed surface area is attributed to the nanoscale grain size, as well as the pores and surface roughness features in the NPs. The crystal size of the a- Al203 NPs was estimated to be ~22 nm based on the Halder-Wagner method. The crystal size (~22 nm) is in very good agreement with the particle sizes measured by TEM statistics (~25 nm) and BET estimation (~23 nm), demonstrating the single-crystal character of the NPs.

[0214] Unlike the starting g- Al203 NPs with a hydrated surface, the synthesized a- Al203 NPs surface is highly dehydrated due to the thermal process. Figure 14E Curves 1411-1412 show the a- Al203 product and the g- Al203 precursor, respectively (black arrow 1413 points to the hydroxyl absorbance).

[0215] XPS fine spectra show O 2- peaks at a binding energy of ~531.2 eV and Al 3+ monopeaks from the a- Al203 NPs at a binding energy of ~74.0 eV. See Figure 14FThis indicates that the ultrafast PDC process did not lead to significant oxygen defects or carbothermic reduction of Al2O3 even in the presence of CB, presumably due to the high reduction potential of Al 3+ No other peaks were detected in the XPS survey, indicating the high purity synthesis capability of the electrothermal process. This makes it superior to solvent-based methods, including ball milling [Amrute 2019] or co-precipitation [Guo 2016], which suffer from lengthy purification processes and chemical contaminants.

[0216] Resistive hot spot effect

[0217] The composition of the heterogeneous medium can be important for the local power dissipation during the PDC process. To show quantitatively the influence of the composition on the phase transition, a series of precursors with different mass ratios of γ- Al2O3 to CB were treated by PDC under the same voltage and time. Figure 15A (With labels: γ-Al2O3 (■), δ'-Al2O3 (▲), and α-Al2O3 (·), numbers are mass ratios of γ-Al2O3 to CB); Table II. From the densities of γ-Al2O3 and CB, the volume fraction of γ-Al2O3 (f) was obtained (shown in Table III) and the phase mass ratio as a function of f(γ-Al2O3) after the PDC process was calculated. Figure 15B (α-Al2O3 and δ ′ -Al2O3, respectively, curves 1501-1502).

[0218] Table III

[0219] Volume fraction of γ-Al2O3

[0220]

[0221] The extent of phase transition increased as f(γ-Al2O3) increased from 0.41 to 0.73; pure phase α-Al2O3 was obtained at f(γ-Al2O3) ~ 0.73. Further increase of f(γ-Al2O3) to > 0.78 resulted in no phase transition.

[0222] To explain the f(γ-Al2O3)-dependent phase transition, the conductivity and temperature were measured. The conductivity was determined based on the measured resistance (R) and the characteristic dimensions of the sample. Table II; Figure 15C (curves 1503 and 1504 for conductivity and temperature vs f(γ-Al2O3), respectively). The conductivity is inversely proportional to f(γ-Al2O3) Figure 15C (curve 1504 in Figure 1), which is reasonable since γ-Al2O3 is electrically insulating. The real-time temperature was measured using an infrared (IR) thermometer. The average bulk temperature decreased as f(γ-Al2O3) increased Figure 15C(Curve 1503 in the figure). This can be explained by equation (2) from the power (P) equation for Joule heating:

[0223]

[0224] Where V is the voltage and σ is the conductivity of the sample.

[0225] Since the initial voltage is fixed at V0 = 60V, the power is proportional to the conductivity of the sample. Interestingly, pure-phase α-Al2O3 NPs were obtained using f(γ-Al2O3) of ~0.73 at a low average bulk temperature of ~573K. Figure 15C .

[0226] Such low temperatures should not trigger a reaction with ~485kJ / mol. -1 The phase transition from γ- to α-Al₂O₃ with high activation energy [Steinr 1971]. Furthermore, the higher degree of phase transition at lower temperatures is counterintuitive. Figure 15C .

[0227] To explain this phenomenon, the current density distribution of the γ-Al₂O₃ / CB composite material during the PDC process was numerically simulated using the finite element method (FEM). For example... Figures 15D-15F As shown, the current density is non-uniform in the γ-Al₂O₃ and CB composite; the current density is greater in the vertical gap region between γ-Al₂O₃NPs than in the bulk region (in...). Figures 15D-15F In the diagram, the sphere is γ-Al₂O₃, the continuous phase is CB, and the vertical side pillars represent the current density values. The gap narrows as f(γ-Al₂O₃) increases, resulting in significantly larger current densities in those regions. Considering that the resistivity (R) of the conductive CB phase is constant, according to equation (3), the heat (Q) generated per volume by the PDC is proportional to the square of the current density (j):

[0228] Q ∝j 2 Equation (3)

[0229] Large heat dissipation in regions with high current density leads to hot spots near γ-Al₂O₃ NPs, which have much higher temperatures than the bulk region, triggering a phase transition. For example... Figure 16 As shown in the figure, quantitative analysis of current density indicates that as f(γ-Al2O3) increases, the bulk temperature decreases (curve 1601), but the hotspot temperature increases (curve 1602), which is consistent with... Figure 15C The temperature measurement results shown are very consistent.

[0230] Topological transformation pathways

[0231] To gain deeper insight into the topotactic transformation pathway, thermodynamic analysis of the three Al203phases was performed based on DFT. Bulk energies and surface energies of the three Al203phases were calculated. Figure 17A The bulk energy of a-Al203is the lowest, followed by d ′ -Al203, and then g-Al203, indicating that a-Al203is the most stable phase as a dense bulk crystal. In contrast, the surface energies are reversed: g-Al203(100) has the lowest surface energy, followed by d ′ -Al203(100), a-Al203(110), and (001). As the surface area increases, the surface energy difference determines the thermodynamic stability of the three Al203phases. Figure 17B , a-Al203, d ′ -Al203, and g-Al203, respectively. When the surface area is less than ~79 m 2 / g or the particle size is larger than ~21 nm, the a-Al203phase becomes more stable than the d ′ -phase. Therefore, a particle size of ~21 nm is suggested as the thermodynamic limit for synthesizing dehydrated a-Al203by a thermal process involving an intermediate d ′ -phase. The particle size of a-Al203synthesized by PDC (~23 nm) is close to the thermodynamic limit and smaller than the particle sizes obtained by most other thermal processes (Table IV).

[0232] Table IV

[0233] Synthesis of a-Al203by thermal processes

[0234]

[0235] This ultrafast, pulsed, and low-temperature PDC process largely avoids mass transport and grain coarsening during phase transformation.

[0236] To gain insight into the structural origin of phase-dependent bulk and surface energies, the contour plots of partial charge densities at the highest energy bands (0.3 eV below the Fermi level) of the surface states of the three Al203phases were plotted. Figures 17C-17D All surface atoms on a-Al203(001) are active, while d ′ -Al203(100) and g-Al203(100) surfaces are relatively active at sites with missing Al atoms. Figure 17C ) Closer analysis shows that the active states are deep in the bulk for d ′ -Al203(100) and g-Al203(100) but not for a-Al203(001). Figure 17D). This explains the bulk and surface energy sequence of the three Al2O3 phases and identifies Al vacancies in the γ- and δ ′ - phases (relative to the α-phase) as the structural source of their thermodynamic stability / instability.

[0237] Applications

[0238] Thus, for the synthesis of corundum nanoparticles, the present invention provides, inter alia, ultrafast synthesis, within 1 second, and much faster than any reported method - which takes at least several hours. The corundum (a- Al2O3) nanoparticles obtained by the present invention have small particle size and high surface area, which can be used for many applications, such as for stable catalytic supports and for ceramics with high fracture strength and toughness.

[0239] For example, one prominent application of a- Al2O3 NPs is as a precursor for sintering nanocrystalline grain alumina ceramics (i.e., ultrafast ACS for nanocrystalline alumina ceramics). Typical alumina ceramic sintering processes are carried out under high pressure and high temperature conditions (HP-HT), such as hot isostatic pressing [Mizuta 1992], spark plasma sintering [Balima 2019] and pulse current sintering [Zhou 2004]. High pressure (typically several GPa) retards grain growth and promotes densification [Wang 2013], which can be a major factor for the sintering of dense ceramics using coarse-grained precursors. However, HPHT processes are not suitable for complex structures. Nanocrystalline precursors can undergo pressureless sintering, but they will suffer from elevated sintering temperatures and prolonged times (>10 hours) [Guo 2016; Cao 2017; Li 2006]. Recently, an ultrafast high-temperature sintering method based on direct current heating was reported [Wang 2020] for rapid screening of ceramics.

[0240] Here, an alternative alternating current sintering (ACS) process for ultrafast sintering of alumina ceramics has been developed, based on Joule heating technology. The ACS system is capable of providing stable and high energy output of up to 63 V of voltage and up to 100 A of current Figure 18A ), making it suitable for sintering structural ceramics. For the ACS system, the total capacitance is 1.5 F, and the maximum available voltage is 63 V. The capacitor is simultaneously charged by the AC power supply and provides energy output to the sample by discharging. The energy output is continuous and enables prolonged sintering of several seconds with high energy output.

[0241] Two separate, highly graphitized carbon papers 1801a-1801b in the Figure 18C are used as heating elements. See Figure 18B(where carbon papers 1801a-1801b are attached to a glass slide and adhered by copper tabs). a- AI2O3 NPs [Taktak 2011] mixed with polyethylene glycol (PEG) binder were pressed into pellets 1802 at 500 MPa. Commercial a- AI2O3 nanopowder (-300 nm) was used as a control. After binder removal (5 °C min -1 to 500 °C, holding for 2 hours; in air), pellets 1802 were placed between carbon papers and under ACS at -15 V.

[0242] Figure 19A Rapid heating 1901, stable sintering 1902, and rapid cooling 1903 are shown. Temperature was recorded by fitting a blackbody radiation. The temperature was ramped up to -2250 K at a heating rate of -10 3 K s -1 . After stable sintering for 5 seconds, the sample was also cooled down at a rapid cooling rate of -10 3 K s -1 . See Figure 19B . Figure 19C Sintered ceramic pellets 1911-1912 loaded on carbon papers 1913 are shown.

[0243] This XRD pattern confirms the pure a-phase of alumina ceramic. Figure 19D Microstructure measured by scanning electron microscopy (SEM) shows grains of the same size and tightly bonded grain boundaries with a polyhedral morphology Figure 19E , indicating good sintering. The average grain size of the alumina ceramic is -270 nm Figure 19F . In contrast, alumina ceramic sintered from commercial a- AI2O3 powder exhibits high residual porosity and a grain size of -1200 nm, indicating sintering in its initial stage. This result shows that the fine grain size of a- AI2O3 NPs contributes to ultrafast sintering, presumably assisted by grain growth at high temperatures [Guo 2016]. The mechanical properties of the ceramic were measured. See Figures 19G-19H The ceramic sintered from a- AI2O3 NP precursors exhibits a Young’s modulus of -11.7 GPa, significantly higher than the Young’s modulus from commercial a- AI2O3 powder (-1.5 GPa). The mechanical properties of alumina ceramic from a- AI2O3 NPs can be improved by using conventional high-pressure-based sintering processes [Mizuta 1992; Balima 2019; Zhou 2004] or by extending the sintering time [Guo 2016; Laine 2006].

[0244] Therefore, the ACS method can be used for sintering of functional ceramics, porous ceramics, or for material screening [Wang 2020].

[0245] Effectiveness and scalability

[0246] As a high energy supply technique, Joule heating has a coefficient of performance of 1.0. Localized heating by resistive hotspots in PDC makes the process more efficient as most of the electrothermal energy is directly targeted at phase change, enabling synthesis at ~4.77 kJ g -1 at low energy input or 0.027 $kg -1 of electrical energy cost. Furthermore, the PDC process can be scaled up by adjusting the sample cross-sectional area and PDC voltage. Synthesis of a- Al2O3 NPs up to 1.4 gram scale has been performed. See Figures 20A-20B and 21A-21B (in Figure 20A and 21A , black powder is the synthesized mixture of CB and a- Al2O3, white powder is a- Al2O3 after calcination). The PDC process, in combination with resistive hotspot effect, greatly reduces the temperature required for reactions that would otherwise be triggered at high energy input, serving as a cost-effective alternative technique for synthesis.

[0247] Recycling metals from electronic waste

[0248] The invention includes a flash Joule heating of ultrafast method for recycling metals (precious metals) from waste (such as electronic waste) [see Luong 2020; Stanford 2020; Tour PCT’000 application]. The waste can be mixed with carbon black, followed by ultrafast Joule heating flash. According to the Ellingham diagram, a variety of precious metals are reduced to elemental metals by carbothermal reaction. The recycling process is ultrafast, within seconds. Importantly, the method is a completely dry method without any solvent, thus is extremely environmentally friendly.

[0249] Synthesis method

[0250] The method for ultrafast synthesis of recycling metals from waste can include the following.

[0251] The method can include preparing electronic waste for flash. For example, printed circuit boards (PCB) from used electronic printers are used as starting materials. The PCB boards are first cut into small pieces, then crushed into small particles. Using ball milling, they are ground into micron-sized fine powder, then made available for flash Joule heating by adding carbon black (or other carbon materials as described above), and processed in a flash Joule heating apparatus as described below.

[0252] Evaporative separation

[0253] It has been found that the different vapor pressures of metals compared to base materials (carbon, ceramic, and glass) enable the separation of metals from electronic scrap. This is referred to as “vaporization separation”. High vapor pressures of noble metals are obtained by the ultrafast flash Joule heating (FJH) process under vacuum. Subsecond current pulses pass through the precursor, which brings the sample to ultra-high temperatures of ~3400 K, enabling the vaporization separation of noble metals. Halide additives are used to improve the recovery of Rh, Pd, and Ag, which are rich in the electronic scrap under test, to greater than 80%, and the recovery of Au to greater than 60%. Alternatively, by leaching the residual solids after FJH, the recovery is significantly improved compared to direct leaching of electronic scrap raw materials, with the recovery of Ag improved by several orders of magnitude, and the recovery of Rh, Pd, and Au improved by several times. Toxic heavy metals, including Cd, Hg, As, Pd, and Cr, can also be removed and collected, minimizing the health risks and environmental impact of the recovery process.

[0254] The FJH process for the recovery of noble metals from electronic scrap involves three stages. See Figure 22 , which shows a schematic of the system 2200. In the metal vaporization stage 2201 (which includes a FJH device with a capacitor bank 2205 and a porous Cu electrode 2206), the metals in the electronic scrap are heated and vaporized by ultra-high temperature FJH. Subsequently, in the mass transfer stage 2202, the metal vapor is transported under vacuum (using a vacuum system with a pump 2207), and in the condensation stage 2203, the metal vapor is collected by condensation (using a cold trap 2208). Printed circuit boards (PCBs) from discarded computers, a representative electronic scrap, are used as the starting material. See Figure 23 . The PCBs are ground into fine powder and mixed with carbon black (CB) that acts as a conductive additive. Figure 23 , which shows an inset 2310.

[0255] To determine the baseline concentrations, the PCBs were digested using dilute aqua regia [Hon 2020] and the concentrations of noble metals were determined by inductively coupled plasma mass spectrometry (ICP-MS). As shown in Figure 24 , the Rh, Pd, Ag, and Au contents are rich in noble metals, ranging from parts per million (ppm) to tens of ppm.

[0256] In the FJH process, the mixture of PCB powder with ~30 wt% CB is slightly compressed inside a quartz tube between two sealed electrodes. Figure 22 . Figure 29A A photograph of the system is shown, which includes a flash table 2901, a power supply 2902, a pump 2903, and a cold trap 2904 (liquid nitrogen, Dewar). One electrode is a porous Cu electrode to facilitate gas diffusion, and the other electrode is a graphite rod. Figure 30The resistance of this sample is tunable by adjusting the compressive force on the two electrodes. The two electrodes are connected to a capacitor bank with a total capacitance of 60 mF. Detailed separation conditions are shown in Table V.

[0257] Table V

[0258] Parameters for FJH under vacuum

[0259]

[0260] The high voltage discharge of the capacitor bank brings the reactants to high temperatures. The current through the sample is measured at different FJH voltages at a fixed sample resistance of ~1 Ω. See Figure 25 , which shows curves 2521-2523 for 150 V, 120 V, and 100 V, respectively. The real-time temperature of the sample is estimated by fitting the blackbody radiation in the 600-1100 nm emission. This temperature varies with the FJH voltage and reaches ~3400 K at 150 V within <50 ms. See Figure 26 , which shows curves 2631-2633 for 150 V, 120 V, and 100 V, respectively.

[0261] Since the resistance of the sample is much larger than that of the graphite and porous Cu electrodes, the voltage drop is mainly applied on the sample. Therefore, the high-temperature region is confined to the sample, and the FJH device has good durability even though it can achieve high temperatures >3000 K. Such high temperatures (>3000 K) volatilize most of the non-carbon components. According to the calculated vapor pressure-temperature relationship Figure 27 ), noble metals have a higher vapor pressure than carbon, which does not sublimate before ~3900 K [Abrahamson 1974].

[0262] As a result, metals are evaporated, and the main carbon-containing components such as plastics are carbonized [Luong 2020; Algozeeb 2020]. The evaporated metal vapor is captured by condensing in a cold trap Figure 22 and 29A ). Some of the vapor remains gaseous even at liquid N2 temperature (77 K); these gases are assumed to be H2 and CO [Algozeeb 2020].

[0263] The noble metal content in the condensed solid is determined and the recovery rate is calculated. Figure 28The recovery of Ag was ~40%, while Rh, Pd and Au had relatively lower recoveries of ~3%. This is because Ag has a high vapour pressure and a relatively low boiling point. The concentration of noble metals in the starting commercial CB was 1-2% of that in the PCB, so their presence in the CB did not introduce significant errors. Furthermore, due to the extremely low C solubility of noble metals, stable carbide phases tend not to form even at high temperatures [Okamoto 2016]. Therefore, the use of CB as the conductive additive did not affect the evaporation behaviour of the noble metals.

[0264] Halide-assisted improvement of recovery

[0265] The high recovery of evaporation separation relies on the formation of components with higher volatility. To improve the recovery, halides were used as additives, as the vapour pressure of metal halides is much higher compared to elemental metals [Lide 2005]. Fluorine-containing components were first used as additives, including sodium fluoride (NaF) and polytetrafluoroethylene (PTFE, Teflon). With this additive, the recovery of Rh and Pd improved to >80% and 70%, respectively. See Figures 31A-31B , showing an improvement of ~20 times compared to the experiment without additives. The concentration of noble metals in the additives was <2% of that in the PCB, so this ruled out the additives introducing significant errors in the recovery of noble metals.

[0266] Chlorine-containing compounds were tried as they are abundant and low-cost. Sodium chloride (NaCl) and potassium chloride (KCl) Figure 31C were used. For both NaCl and KCl additives, the recovery of Rh, Pd and Ag was improved. Furthermore, polyvinyl chloride (PVC) and chlorinated PVC (CPVC) plastics Figure 31D were used. The recovery of all four noble metals was improved, especially for Ag, with a recovery of >80%. The plastic additives were post-consumer samples that had extremely low or negative value, so they would not introduce significant material costs in e-waste recycling processes.

[0267] Even with F and Cl additives, the recovery of Au was <10%. Interestingly, when sodium iodide (Nal) was used as an additive, the recovery of all four noble metals was improved; the recovery of Au improved to >60% Figure 31E . Among the halides used for Au recovery, the I additive had the best performance. According to the hard-soft acid-base (HSAB) theory, Au + is a soft Lewis acid, I - is a soft Lewis base, while F - and Cl - are harder than I -Hard [Pearson 1963], favorable for AuI. With the use of additive mixtures of NaF, NaCl, and NaI, good recovery rates were achieved for all noble metals: Rh > 60%, Pd > 60%, Ag > 80%, and Au > 40%. Figure 31F Compositional analysis of the raw materials and residual solids after FJH by X-ray photoelectron spectroscopy (XPS) showed that 10-40% of the halide additives evaporated during the FJH process, and could be recovered and reused through water washing and precipitation processes.

[0268] Overall composition analysis was performed on the metals collected in the cold trap. In both cases, with and without chemical additives, the most abundant metal, excluding precious metals, was Cu at a mass ratio >60 wt%, followed by other major metals found in e-waste, including Al, Sn, Fe, and Zn. Further purification and refining can be performed by selective precipitation, solvent extraction, and solid-phase extraction, which are commercially established practices and known in the art [Ueda 2016].

[0269] The morphology and chemical composition of the condensed solids were characterized using scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS). The elemental diagram shows clustered alloy particles of Rh, Pd, Ag, and Au. Figure 31G This process, known as FJH, involves ultrafast heating and rapid cooling to form high-entropy alloy nanoparticles. This is similar to the carbon thermal shock synthesis of high-entropy alloy nanoparticles, which could potentially be used as catalysts [Yao 2018]. Noble metals were also observed throughout the product in other regions. Furthermore, XPS analysis of the collected volatiles showed that Ag and Au were primarily in elemental form, while Rh and Pd coexisted in both elemental and higher oxidation states, likely due to their different chemical reactivity.

[0270] Improved precious metal leaching efficiency

[0271] Besides condensing volatile compositions, another route to recover precious metals is by leaching the residual solids obtained from FJH. See also Figure 32A Unlike evaporation separation schemes that use vacuum to promote metal volatilization (…), this method… Figure 22 ), and establish a pressurization device to capture metal in the reactor ( Figure 33A An inert gas (N2) cylinder is connected to the FJH reactor, where the pressure is monitored by a pressure gauge. Based on the amount of gas collected, the internal pressure (P0) in the FJH process is estimated to be ~5 atmospheres.

[0272] Based on the pressure drop and size of the FJH chamber, at different pressures (P out Simulated gas diffusion () Figure 33B When using vacuum (P) out=0 atmospheres), as in evaporation separation ( Figure 22 The gas velocity reached as high as 800 m / s. -1 Such high gas velocities facilitate the rapid diffusion of volatile components into the cold trap and prevent condensation losses at the tube sidewalls. In contrast, gas velocities decrease significantly with increasing pressure. Figure 33B As a result, more of the initially volatile components are retained in the residual solids within the reactor. Detailed reaction conditions for pressurized FJH are shown in Table VI.

[0273] Table VI

[0274] Parameters of FJH under pressure

[0275]

[0276] The leaching of residual solids after initiating FJH (referred to as PCB-flash evaporation) using dilute acids (1M HCl, 1M HNO3) at 120V and atmospheric pressure. The leaching content of Rh, Pd, and Ag in PCB-flash evaporation is significantly higher than that in PCB raw materials. Figure 33C The recovery rates of PCB leaching-flash evaporation (Y) and leaching of PCB raw materials (Y0) were calculated. FJH, combined with leaching, was significantly more effective than leaching alone. The recoveries of Rh, Pd, and Ag increased by 4.17±0.48, 2.90±0.31, and 56.0±18.1 times, respectively. Figure 33C )(exist Figure 33C In the figure, Y0 and Y represent the recovery rates by leaching printed circuit boards (PCBs) and PCB-flash evaporation, respectively. The dashed line indicates that Y / Y0 = 1. The error bars represent the standard deviation (where n = 3). This deviation may be due to the uneven distribution of precious metals in e-waste. Interestingly, the Au recovery rate decreases after the FJH process. This is likely because Au forms covalent bonds with carbon [Olavarria-Contreras 2016], which significantly increases the difficulty of acid leaching.

[0277] Thermogravimetric analysis (TGA) of PCB flash evaporation showed that carbon could be removed in air at ~700°C. Figure 32B ). ( Figure 32B The TGA curves show that PCB flash evaporation begins to lose weight at ~400°C and remains stable at ~800°C. Therefore, the PCB flash evaporation solid was calcined at 700°C for 1 hour (denoted as PCB flash evaporation-calcination). Inset 3201 shows photographs of PCB flash evaporation and PCB flash evaporation-calcination. PCB raw material was also calcined as a control (denoted as PCB calcination). Figure 32C ).

[0278] XPS analysis showed that calcination effectively removed carbon ( Figure 32D). (In Figure 32D , the XPS of PCB mainly shows C and some inorganic signals, the XPS of PCB- flash shows mainly C signal, indicating that O was removed by FJH and no inorganic element peak was detected, probably because inorganic materials were covered by carbon during FJH process. The XPS of PCB-flash-calcination shows rich elemental signals, demonstrating the removal and exposure of inorganic materials). With FJH and calcination processes, the recovery of Rh, Pd, Ag and Au was increased by 3.11 ± 0.37, 2.64 ± 0.39, 28.5 ± 9.8, 7.24 ± 2.22 times, Figure 33D ). (In Figure 33D , Y0and Y represent the recovery by leaching of PCB and PCB-flash-calcination, respectively. The dotted line indicates Y / Y0= 1. Error bars represent standard deviation, where n = 3). The values are greater than those obtained by the process of calcination only Figures 32E-32F ).

[0279] The mechanism of improving leaching efficiency by FJH is shown in Figures 34A-34E . Modern electronic devices are manufactured and packaged by planar processes and have laminated structures, in which useful metals are embedded in polymer or ceramic matrix Figure 34A [Sun Z 2017]. Even after comminution, the particle size is as large as ~5 pm Figure 34B . The laminated structure hinders the extraction of metals in typical hydrochemical processes, leading to prolonged leaching time and low leaching efficiency [Sun Z 2017]. During the FJH process, the matrix is made into ultrafine powder Figures 34C-34D at ultra-high temperature and the metals are exposed Figure 34E , which greatly accelerates the leaching rate and the degree of metal extraction.

[0280] The effect of FJH voltage and pressure on recovery was evaluated. It was found that a moderate FJH voltage of 30 to 50 V provides the best recovery Figure 33E , curves 3301-3304 show for Rh, Pd, Ag and Au, respectively, Figure 33E the shaded area of Figure 33F is the approximate optimal voltage for recovery of all metals). Too low a voltage cannot provide enough energy to thermally decompose the matrix, while too high a voltage can lead to evaporation loss. It was found that higher ambient pressure is beneficial Figure 33B , curves 3311-3314 show for Rh, Pd, Ag and Au, respectively). This is because volatile components are trapped in the residual solid, as predicted by gas flow simulation). Compared to other hydrometallurgical processes that use high concentrations of inorganic acids such as aqua regia [Sun Z 2017; Park 2009] or toxic cyanide [Sethurajan 2019; Quinet 2005] as extractants to achieve high recovery rates, the mild acid leaching conditions used in the present process (1 M HC1, 1 M HNO3) are more cost-effective and environmentally friendly.

[0281] Removal and collection of toxic heavy metals

[0282] Removal of toxic components is another major concern in e-waste processing [Ogunseitan 2009; Leung 2008; Julander 2014; Sun 2020]. The heavy metal removal capability of the FJH process was evaluated. Compared to precious metals, heavy metals (including Cr, Pb, Cd, As, and Hg) have much higher vapor pressures and much lower boiling points ( Figure 35A ). Especially for the most toxic Cd, As, and Hg, based on theoretical analysis, the separation factor between them and precious metals can reach ~10 5 . The levels of heavy metals in PCB waste are 0.1-20 ppm Figure 35B . These values are higher than the safety limits of heavy metals in agricultural soils recommended by the World Health Organization (WHO). [Kinuthia 2020].

[0283] After one FJH, the heavy metal content in the remaining solid (PCB-flash) is greatly reduced Figure 35C . The removal efficiency of Hg and Cd is calculated to be >80%, followed by Pb and As (>50%) and Cr (>35%) Figure 35D . These efficiencies are consistent with their vapor pressure values Figure 35A . The heavy metals are collected by condensation in the cold trap, as done for the evaporation separation, and the collection yield is calculated Figure 35D . The collection yield matches well with the removal efficiency, indicating that most of the evaporated heavy metals are trapped by the cold trap, minimizing the leakage of heavy metals to the environment during the recovery process.

[0284] The concentration of heavy metals in the residue solid can be further reduced by multiple FJH reactions. After one FJH reaction, the concentration of Hg is reduced to below the safety limit of Hg in agricultural soils (0.05 ppm) Figure 35E [Kimuthia 2020], which is the highest standard for waste disposal. For Cd, three consecutive FJH cycles reduce the concentration to below the safety limit (0.003 ppm) Figure 35F[Kimuthia 2020]. With increasing FJH reaction times, the concentrations of As, Pb, and Cr all decrease. Since each FJH reaction takes only 1 second, multiple flash evaporations are easily achievable.

[0285] Metal separation

[0286] The aforementioned method utilizing an evaporation separation scheme, relating to the recovery of metals from electronic waste, has been discussed. However, such methods can exhibit the ability to separate metals. Calculations show that for most metals with large vapor pressure differentials, separation rates as high as ~10 can be achieved. 5 A large separating factor. Figure 36 The charts provide theoretical separation factors for evaporative separation processes based on vapor pressure differentials. These factors represent the practical value of separating trace metals from abundant metals. To separate abundant metals, these values ​​should be corrected for their activity in the alloy melt.

[0287] Separation using the FJH process based on vapor pressure difference has demonstrated varying recoveries of precious metals. Figure 28 ).like Figure 28 As shown, without chemical additives, the recoveries of precious metals are Y(Rh) = 4.0%, Y(Pd) = 3.1%, Y(Ag) = 38.0%, and Y(Au) = 1.3%. These different recovery rates demonstrate the separation capability of the FJH process. See Table VII below.

[0288] Table VII

[0289] Separation factor of precious metals

[0290] Rh Pd Ag Au Rh 1 1.29 9.5 3.1 Pd 1.29 1 12.3 2.4 Ag 9.5 12.3 1 29.2 Au 3.1 2.4 29.2 1

[0291] Chemical additives ( Figures 31A-31F It also regulates the separation of precious metals, likely due to their different chemical reactivity. See Tables VIII-X below.

[0292] Table VIII

[0293] Separation factor of noble metals by using NaCl additive

[0294] Rh Pd Ag Au Rh 1 1.44 3.0 840 Pd 1.44 1 4.33 58.3 Ag 3.0 4.33 1 253 Au 840 58.3 253 1

[0295] Table IX

[0296] Separation factor of noble metals using NaF additive

[0297] Rh Pd Ag Au Rh 1 1.52 1.8 146 Pd 1.52 1 1.19 96 Ag 1.8 1.19 1 81 Au 146 96 81 1

[0298] Table X

[0299] Separation factor of noble metals by using NaI additive

[0300] Rh Pd Ag Au Rh 1 1.05 1.08 1.59 Pd 1.05 1 1.03 1.51 Ag 1.08 1.03 1 1.48 Au 1.59 1.51 1.48 1

[0301] The separation capability of the evaporation separation scheme can be further improved by gradually increasing the FJH temperature.

[0302] Carbothermic reduction

[0303] Flash Joule heating can also be used for the carbothermic reduction of metals from oxides. Various metal oxides have been used prior to recovery, demonstrating the applicability of metal recovery via flash Joule heating. For example... Figures 37A-37F As shown, it was found that Al can be recovered from Al₂O₃, Fe from Fe₂O₃, Cu from CuSO₄, Ni from NiSO₄, Mn from MnO₂, and Pb from PbNO₃. During the flash Joule heating process, carbon from the carbon black reduces metal oxides and metal salts to metals, while carbon is approximately oxidized to carbon dioxide and carbon monoxide.

[0304] In some embodiments of the invention, the process may include a mechanism for trapping metals in the waste. For example, this mechanism may utilize reduced pressure and, upon flash Joule heating of the source, cause volatile metals, metal carbides, metal oxides, or other metal complexes to evaporate from the reaction chamber and enter a cold trap. The cold trap may be, but is not necessarily, liquid N2. These can be collected in the trap even at room temperature.

[0305] Furthermore, for example, this mechanism can utilize atmospheric pressure or higher (e.g., 10 or 20 atmospheres) and retain the metal within the newly formed graphene. The graphene can then be burned off (e.g., in air at 700-800°C), leaving the separated metal (or metal oxides, etc.). Alternatively, the graphene can be chemically oxidized, such as with HNO3. For the latter mechanism, a pressure relief valve can be used at the end of the electrode-hole assembly used in the flash Joule heating process. Some of the recovered metals have very high boiling points and will remain with carbon, especially at the higher pressures used.

[0306] Design and scalability

[0307] Figure 38 A flash Joule heating pressure and gas collection system 3800, which can be used in embodiments of the present invention, is shown. System 3800 includes the following:

[0308] (a) Timing sprocket and belt 3801;

[0309] (b) Manual or motor driven 3802;

[0310] (c) Driver 3803 (e.g., twin screw driver);

[0311] (d) power supply 3804 (e.g., AC or DC from a flash power supply);

[0312] (e) sample compression 3805;

[0313] (f) nuts 3807a-3807b and soft spacers 3806a-3806b;

[0314] (g) electrode 3808a (e.g., threaded solid brass electrode) and electrode 3808b (e.g., threaded brass electrode with drilled hole);

[0315] (h) tube 3809 (e.g., quartz tube);

[0316] (i) copper wool 3810;

[0317] (j) torsion spring compression 3811;

[0318] (k) electrode 3812 (e.g., brass electrode with O-ring seal and axial hole);

[0319] (l) sample 3813;

[0320] (m) conduit 3814 (e.g., PTFE tube) inside electrode 3812;

[0321] (n) pressure seal 3815 (e.g., using Swagelok reducer);

[0322] (o) particle collector 3816;

[0323] (p) adjustable pressure relief valve 3817;

[0324] (q) gas collector 3818;

[0325] (r) flow to vacuum or gas analysis 3819;

[0326] (s) exhaust 3820;

[0327] (t) safety valve 3821;

[0328] (u) conduit 3822 (e.g., PFE tubing);

[0329] (v) flow to vacuum 3823;

[0330] (w) pressurized input from gas supply 3824; and

[0331] (x) pressure gauges 3825-3826.

[0332] System 3800 is a pressurizable flash Joule heating unit with a gas collector 3818 that should then develop a gas overpressure. In some embodiments, system 3800 utilizes electrodes with 5 / 16 inch or 8 mm diameter. The conduit can have an outer diameter of 1 / 8 inch.

[0333] In system 3800, two brass electrodes with O-ring grooves are inserted into a quartz tube that is tightly wound with a compression spring to keep the quartz under compression and resist the outward force of pressure. One electrode is hollow, with a PTFE tube inserted to provide a smooth and continuous exit path. A reducer Swagelok fitting provides pressure and vacuum sealing for the PTFE tube, which exits the electrode without a joint. System 3800 can withstand tens of atmospheres of pressure. Generally, the limiting factor with respect to pressure is the quartz tube, and how well the robust spring can prevent breakage. The dual screw support frame should also be robust enough to resist the push when the sample is pressurized or when pressure is generated by the flash. The quartz tube can be replaced with any non-conductive tube, and cross-linked polyethylene has also been used, as the temperature reached on the tube is generally below 250°C and typically less than 1 second. While not shown in Figure 38 A motor driver can be added and utilized. Also, because the system is fully sealed, there is no need for an external vacuum chamber around the flash assembly.

[0334] Rubber bushings between the nuts and the support frame can be used to absorb shock when short duration flashes are employed.

[0335] System 3800 can be sealed with O-rings. Silicon O-rings are heat resistant and do not melt, even in the case of overheating, but rather tend to harden and should maintain the seal. Because there is generally no hot gas that can flow through the O-rings, they do not overheat. While discoloration of the first O-ring has been observed, the double O-ring maintains the seal.

[0336] System 3800 can be fully evacuated, and will maintain pressure when the gas exits into the thick-walled glass pressure tube after the sample 3813 is flashed. In some embodiments, a right angle fitting can be used so that the exhaust does not interfere with the end connections of the electrodes. However, a straight exit tube is generally preferred if particles or nanoparticles are being ejected. System 3800 shows straight and continuous tubing 3814 (PTFE exit tube), and wires are connected to the loops on the threaded brass electrodes 3808a-3808b.

[0337] System 3800 uses dual screw translation, which provides consistent alignment of the electrodes. It has been found that with a single screw translator, the electrode angle goes up when pressure or force is applied, which in turn creates strain on the quartz tube 3809. Dual screws are connected by timing sprockets and belts 3801 to advance simultaneously, and can be driven manually or with stepper motors.

[0338] With respect to vacuum and gas supply, the tubing exiting the hollow electrode tip can be connected through valves to vacuum 3823, gas supply 3824, and pressure gauge 2925. The gas supply can be inert, or can be used to inject reagents into the sample, such as hydrogen, methane, or other active species such as halocarbons, ammonia, boron compounds, etc. These can be added to the porous carbon / graphene in a subsequent flash.

[0339] A pressure release can be preset for the system 3800. An adjustable pressure relief valve 3817 determines the limit pressure on the sample 3813. This unit can be pressurized sufficiently prior to flash, or the flash can be allowed to generate high pressure. The opening pressure is set by a spring and a threaded cap on the valve, which opens when the pressure exceeds the set force of the spring, and gas enters a previously evacuated gas collector. Subsequently, the gas can be analyzed, or simply pumped away. The pressure gauge 3826 and the volume of the gas collector 3818 provide information about the total yield of gas. The gas collector 3818 also has a pressure relief valve 3821 that connects to a vent 3820 in the event of excess gas generation.

[0340] With respect to the wide range of pressures available to the system 3800 with a sealed flash chamber and adjustable safety valve, a wide range of pressures have been evaluated for their effect on flash yield. Volatile additives can be mixed into the sample due to the pressure, and will not exit until the safety valve opens.

[0341] The system 3800 can be used for a variety of particle / metal collection methods. For example, when it is desirable to collect particles, the PTFE tube can be straight (without bends) into the particle collector 3816 (i.e., a test tube impactor). This would be inside a larger evacuated vessel (not shown), and the momentum of the particles can cause them to stick to the tube, while non-condensable gases can be pumped away. This can be used to collect volatile metals and metal compounds, which will agglomerate and form nanoparticles that will stick to the particle collector 3816 as they cool.

[0342] This design can be changed and modified as needed with changes in materials and design, depending on the intended use.

[0343] The cost and benefit of FJH processing were evaluated because economic incentives are the main driver for waste recycling [Awasthi 2019]. Compared to conventional smelting furnaces where a large amount of energy is used to maintain the temperature of the entire chamber, FJH is an efficient heating process due to the ultrafast heating / cooling rate, direct sample heating feature, and short reaction duration [Khaliq 2014]. The energy consumption of the FJH process is ~939 kWh / ton, which is ~1 / 500 of the energy consumption of a tubular furnace at the laboratory scale [Balaji 2020] and ~1 / 80 of the energy consumption of a commercial Kaldo furnace at the industrial scale [Theo 1998]. Therefore, the FJH process for electronic waste processing has advantages over traditional pyrometallurgical processes.

[0344] The FJH process is scalable. According to the analysis performed, the FJH voltage and / or the capacitance of the capacitor bank can be increased when scaling up the sample mass. Figures 39A-39D Scaling up of the flash Joule heating (FJH) process is shown. Figure 39A are photos of samples processed with conditions m0=0.2 g, V0=150 V, and C0=0.06 F (sample 3901), m1=2 g, V1=150 V, and C1=0.6 F (sample 3902), m2=4 g, V2=300 V, and C2=0.6 F (sample 3903). Figures 39B-39D are real-time temperature profiles of test samples 3901-3903, respectively.

[0345] Figure 40A is a scheme of a continuous flash Joule heating (FJH) reactor 4000 with continuous feed 4001 (such as electronic waste and carbon black), Cu electrodes 4002-4003 (Cu electrodes with holes), a porous electrode 4004, a graphite electrode 4005, an O-ring 4006, and a baffle 4007. Volatile components can enter a collection system 3108 to be collected with a cold trap, and non-volatile components can be collected in a collector 4009.

[0346] Figure 40B is a scheme of a continuous flash Joule heating (FJH) reactor 4020 with continuous feed of feedstock 4021 (such as electronic waste and carbon black) flowing from a bin 4022. In step 4031, the feedstock 4021 is loaded onto a chamber 4023 of a conveyor belt 4024. In step 4032, the feedstock 4021 in the chamber 4023 is compressed (using a compressor 4025) to a predetermined resistance. Then, in step 4033, the feedstock 4021 is subjected to a FJH reaction using a FJH system 4026 with Cu electrodes 4027 and graphite electrodes 4028. In step 4034, the product 4034 is subsequently unloaded in a collector 4029.

[0347] Although the scheme of flash Joule heating is described for e-waste and carbon black as feedstock, these can be used for other materials used in FJH reactions. Figures 40A-40B

[0348] By using an automated system integrated with the FJH apparatus, a production rate of >10 kg per day has been achieved.

[0349] Therefore, for the recovery of metals from e-waste, the present invention provides, inter alia: (i) flash Joule heating is a dry process that does not use any solvent, which makes it environmentally friendly; (ii) flash Joule heating can recover most of the metallic elements in the waste in one step, which is difficult to achieve by other methods; (iii) the flash Joule heating process will also remove almost all the hazardous substances in the waste, so it will not cause secondary pollution; and (iv) since the heating duration is short and almost no energy escapes from the sample of flash Joule heating, the flash Joule heating process uses much less electrical energy than a furnace.

[0350] Precious metals recovered from e-waste are very important feedstock for various industries. In fact, a mixture of metals like this is quite valuable, as many mining companies have implemented automated systems to separate base metals.

[0351] In addition, the recovery process removes hazardous substances such as heavy metals from the waste, which is important in addressing environmental problems caused by these wastes.

[0352] Ore, fly ash and bauxite residue (red mud)

[0353] Similar to the case of e-waste, the same also applies to ore, fly ash and red mud (red mud is recently referred to as bauxite residue), also because rare earth elements (REEs) are strategic resources in the modern electronics, clean energy and automotive industries. From this, the above-mentioned method and system can also be used to recover metals from ore, fly ash and bauxite residue (red mud).

[0354] ​Embodiments of the present invention include a flash-joule heating (FJH)-based ultrafast electrothermal process to activate ores, fly ash, and red mud to improve the acid extractability of REEs using only mild acid (e.g., 0.1 M HC1). The pulsed voltage in seconds brings the temperature of the feedstock to ~3000 °C, resulting in the thermal decomposition of the poorly soluble REE phosphates in CFA to highly soluble REE oxides and carbothermic reduction of the REE components to highly reactive REE metals. This activation method is able to improve the REE recovery to ~206% for CFA-F and ~187% for CFA-C compared to direct leaching of the feedstock with a more concentrated acid. This activation strategy is viable for various secondary wastes, as demonstrated by coal fly ash (CFA) and red mud (bauxite residue (BR)). The fast FJH process is scalable and energy efficient with low electrical energy consumption (e.g., 600 kWh / ton or $12 / ton), making the profit percentage greater than 10 times.

[0355] FJH system and process

[0356] FJH systems that can be used are similar to those described and discussed above. For example, Figure 41A An electrical diagram of a FJH system that can be used for fly ash is shown in FIG. Figure 6A , 13A and 30.

[0357] In a typical experiment, secondary waste (CFA, BR) was mixed with carbon black in a mass ratio (e.g., 2: 1) by using a ball mill (MSE supplies, PWV1-0.4L). The carbon black acts as a conductive additive. 200 mg of the mixture (133 mg of waste and 67 mg of CB) was added into a quartz tube (8 mm inner diameter, 12 mm outer diameter). The resistance was controlled by compressing two electrodes. The sample was loaded into a jig Figures 41B-41C ), and the electrodes were connected to a capacitor bank. In such embodiments, 10 aluminum electrolytic capacitors (450 V, 6 mF, Mouser #80-80-PEH200YX460BQU2) were used for charging, and a capacitor bank with a total capacitance of 60 mF was charged by a direct current (DC) power supply. A relay with programmable millisecond relay was used to control the discharge time. Table XI reflects the detailed parameters of some secondary wastes used. After FJH, the sample was rapidly cooled to room temperature.

[0358] Table XI

[0359] FJH parameters for activating secondary waste

[0360]

[0361] * As a comparison to the method for e-waste, results on printed circuit boards are shown.

[0362] Acid-extractable REE content in CFA

[0363] There are two types of CFA classified by chemical composition, CFA-F with total content of SiO2, Al2O3, and Fe2O3 > 70 wt.%, and CFA-C with higher CaO abundance [Liu 2019]. In the examples evaluated here, CFA-F were collected from the Appalachian Basin (App) and CFA-C from the Powder River Basin (PRB), both in the USA [Taggart 2016]. Figure 42 are photographs of CFA-C 4201 and CFA-F 4202 (scale bar, 4 cm).

[0364] CFA consists of a major amorphous phase (60-90%) [Zhang 2020] with the remaining crystalline material mainly comprising quartz and mullite, as shown by X-ray diffraction patterns (XRD). Figure 43A In addition to the enrichment of Ca in CFA-C, elemental analysis by X-ray photoelectron spectroscopy (XPS) Figure 43B ) and energy dispersive X-ray spectroscopy (EDS) also showed a high C content in CFA-F, which can be caused by incomplete combustion of the coal feedstock. The high C content in CFA-F is also evident by a large weight loss at ~700 °C by thermogravimetric analysis (TGA).

[0365] Total quantification of REE in CFA was performed by the HF:HNO3 digestion method [Taggart 2016]. The total REE content c 总计 (CFA Raw) was 516 ± 48 mg kg -1 for CFA-F and 418 ± 71 mg kg -1 . Figure 43C CFA from App had higher REE content than CFA from PRB, consistent with Taggart 2016. The acid-leachable REE content c0(CFA Raw) from the CFA feedstock was measured by using 1 M HC1 or 15 M HNO3 [Taggart 2016; Middleton 2020]. For CFA-F, the REE content extractable by HNO3 and HC1 was 144 ± 32 mg kg -1 and 160 ± 50 mg kg -1 ( Figure 43C ​), corresponding to ~ 28% and ~ 31% of the REE extractability (Yo), respectively. For CFA-C, the REE content extracted by HNO3and HC1 was 246 ± 71 mg kg -1 and 231 ± 81 mg kg -1 ( Figure 43C ), corresponding to ~ 59% and ~ 55% of the REE extractability, respectively. It is concluded that once the acid concentration is greater than 1 M, its effect on REE leachability is limited. Therefore, 1 M HC1 leaching was used in the standard protocol for subsequent evaluation.

[0366] The acid extractability of REE from CFA-C was higher than that of REE from CFA-F. This is consistent with [Liu 2019], which attributed the higher extractability to the higher content of easily soluble REE species, such as REE oxides in CFA-C. The morphological images of CFA-F by scanning electron microscopy (SEM) are shown in Figure 43D , and the high carbon content can hinder the accessibility of the aqueous acid to the REE-bearing species, resulting in a low extractability of 21% to 42% of the individual REE. Figure 43E In contrast, CFA-C is composed of fine, uncoated spherical particles Figure 43F , which benefit the acid leaching process, resulting in a relatively higher extractability of 33% to 67% of the individual REE Figure 43G .

[0367] Improving REE recovery from CFA by electrothermal activation

[0368] During the electrothermal activation process by FJH, the CFA feedstock was first mixed with carbon black (CB) acting as a conductive additive. The mixture of CFA and CB (~ 30% CB) was loaded into a quartz tube between two graphite electrodes. Figure 41A and 44A . The resistance (R) of the sample was adjustable by adjusting the compression force between the two electrodes, which were connected to a 60 mF capacitor bank. The sample reached high temperatures by high-voltage discharge of the capacitor. The detailed experimental parameters are shown in Table XI.

[0369] During a typical discharge process with FJH voltage of 120 V, R of 1 Ω, and discharge time (t) of 1 s, the current curve across the sample was recorded, with a peak current at ~ 120 A, followed by a current plateau at ~ 7 A. Figure 44B . The corresponding real-time temperature curve showed a peak temperature of up to ~ 3000 °C, followed by a steady heating at ~ 1150 °C. Figure 44C . The solid obtained after FJH was called activated CFA. Figure 45(Flow diagram showing REE recovery from CFA 4501 to (via FJH) synthesised CFA+CB 4502 to activated CFA 4503). The acid leachable REE content c(activated CFA) from activated CFA was measured by a 1 M HC1 leach procedure. The REE recovery from activated CFA (Y) was calculated and compared to the REE recovery from the CFA feedstock (Y0).

[0370] A range of FJH voltages from 50 V to 150 V were applied. Figure 44D At ~120 V, the total REE content from activated CFA-F that was HC1 leachable (1 M HC1, 85 °C) improved to 329 ± 14 mg kg -1 . Figure 44D This corresponds to a recovery of Y ~ 64% which represents an improvement of ~ 206% compared to the recovery from the CFA-F feedstock (Y0 ~ 31%). The pH dependent leaching kinetics of the REE from the CFA-F feedstock and activated CFA-F were investigated. Figure 44E (CFA-F feedstock and activated CFA-F are curves 4401-4402, respectively). In general, the yield decreases with increasing acid pH. Notably, at pH 2 (or 0.01 M HC1), the REE recovery from activated CFA-F remains Y ~ 45%, which is significantly higher than the REE recovery from the CFA feedstock under the same leaching conditions (Y0 ~ 9% at pH 2), and even at much higher acid concentrations (Y0 ~ 31% at pH 0).

[0371] For CFA-C, the acid leachability of the REE from activated CFA-C was measured to be Y ~ 103% ( Figure 44F , CFA-F feedstock and activated CFA-F are curves 4403-4404, respectively), which corresponds to a proportion of ~ 187% of the acid leachability from the CFA-C feedstock (Y0 ~ 55%).

[0372] Even with dilute acid (pH 1, 0.1 M HC1), the REE recovery from activated CFA-C remains Y ~ 94%, which is significantly higher than the recovery from the CFA-C feedstock (Y0 ~ 54%). This would make the wastewater stream more manageable.

[0373] For the individual REEs, with the FJH activation process, using the same leach procedure (1 M HC1, 85 °C), the acid leachability improved from 170% to 230% for CFA-F ( Figure 44G ) and from 130% to 170% for CFA-C ( Figure 44H) from 170% to 210%. Similar improvements were achieved using dilute acid leaching (0.1 M HC1, 85 °C). No significant bias was observed in REEs, indicating that the FJH activation process acted equally on all REEs.

[0374] As a control, the REE content in carbon black was measured using the same digestion method. The total REE content in carbon black was ~5 mg kg -1 , corresponding to ~1% of the REE content in CFA. Therefore, the use of carbon black does not introduce significant error in these measurements. In practical applications, carbon black can be replaced with anthracite or any other inexpensive source of moderately conductive carbon, but the REE content in that source should be considered in the yield calculations.

[0375] Mechanism of improved REE extractability

[0376] The mechanism of improved REE leachability by the electrothermal activation process was investigated. The morphology and distribution of REEs in CFA dictate REE extractability. REE phosphates, including monazite and xenotime, are one of the main counterions of REEs in coal [Liu 2019; Stuckman 2018]. REE phosphates are quite stable components and do not melt or thermally dissociate in air up to ~2000 °C [Ushakov 2001; Hikichi 1987]. Coal fire combustion temperatures are typically 1300 °C to 1700 °C [Stuckman 2018]. As a result, trace phases containing REEs, including monazite and xenotime, persist in CFA. [Kolker 2017; Smolka-Danielowska 2010]. REEs can also be partitioned and occluded into the glassy portion of CFA by diffusing into melts (e.g., aluminosilicates) formed at coal-fired boiler temperatures [Dai 2014]. Those insoluble REE phosphates and glass phases are detrimental to REE extraction [Liu 2019], while REE oxides and carbonates in CFA are relatively easier to extract by acid leaching.

[0377] The high temperature of ~3000 °C generated by the FJH process, which is significantly higher than coal-fired boiler temperatures, can thermally degrade REE species. Lanthanum phosphate (LaP04) and yttrium phosphate (YP04) were used as representatives of REE phosphates. As shown in FIG. 1 1 A, a La203phase was identified after FJH of the LaP04 precursor. Similarly, YP04 thermally decomposed to Y203after the FJH process. Figure 46A Figure 46B REE oxides have much higher solubility (log 10 K sp -27 to -24) than REE phosphates (log 10 K sp ​REE metal, oxide, and phosphate dissolution reactions at 25 °C. See Table XII.

[0378] Table XII

[0379] Gibbs free energy change and solubility product constant for REE metal, oxide, and phosphate dissolution reactions at 25 °C.

[0380]

[0381]

[0382] To further provide insight into the solubility of REE phosphates and oxides, dissolution curves as a function of pH were calculated. Figure 46C (La2O3, Y2O3, LaPO4, and YPO4 are curves 4601-4604, respectively). It was found that LaPO4 and YPO4 only show significant solubility at pH close to 0, while the oxide counterparts readily dissolve at low acidity at pH ~ 6. This partially explains the pH-dependent REE leaching kinetics, i.e. higher REE leachability is achieved for activated CFA than for raw material using dilute acid. Figures 46E-46F (For Figure 46E , the Si signal can come from the quartz tube in the FJH process).

[0383] In addition to the thermal decomposition of REE phosphates, ultra-high temperatures can also trigger the thermal reduction of REE compounds. According to the Ellingham diagram Figure 46D ), the carbothermal reduction temperature of REE oxides is estimated to be ~ 1900 °C (Eu2O3) to ~ 2500 °C (Dy2O3). The FJH at ~ 120 V produces temperatures up to ~ 3000 °C Figure 44C ), which is able to reduce REE oxides.

[0384] Y2O3 and La2O3 were used as representatives to verify the carbothermal reduction of REE oxides by the FJH process. The XPS fine spectral fitting of Y2O3 after FJH shows four peaks. Figure 46E and Table XIII. In Y2O3, the peaks at 157.5 and 159.6 eV were assigned to Y’s 3d 5 / 2 and 3d 3 / 2 [Barreca 2001], and in Y(0), the peaks at 156.4 and 158.5 eV were assigned to Y’s 3d 5 / 2 and 3d 3 / 2 [Cole 2020].

[0385] Table XIII

[0386] XPS peak fitting of La and Y

[0387]

[0388] XPS analysis confirmed that Y₂O₃ was reduced to Y metal via the FJH process, while a small proportion of Y₂O₃ likely originated from surface oxidation. Similarly, after FJH, the XPS fine-fit spectra of the La₂O₃ precursor and La₂O₃ were obtained (…). Figure 46F Table XIII) verifies the reduction of La₂O₃ to La metal [Deasha 1995; Li 2019]. Reduced REEs with low oxidation states are highly reactive materials that readily react with even pure water [Greenwood 1997]. The calculated Gibbs free energy change (ΔG) for the REE metal dissolution reaction is more negative than that for REE oxides. Figure 46G Table XII shows that REE metals have significantly greater thermodynamic solubility than their oxide counterparts.

[0389] This indicates that the required temperature for thermal activation is >2000℃ for the thermal decomposition of REE phosphates and >2500℃ for the carbothermic reduction of REE oxides, which also provides insight into the voltage-dependent REE leaching properties. Figure 44D To achieve temperatures >2000°C, an FJH voltage of ≥120V is required, while a voltage <100V has limited impact on REE leaching. However, excessively high FJH voltages of ≥150V can lead to prolonged high temperatures >3000°C, which can further result in REE evaporation loss during the FJH process.

[0390] Besides morphology, REE distribution also affects extractability, with REEs encapsulated in or distributed throughout the glass phase being difficult to dissolve [Liu 2019]. FJH allows for ultrafast heating and rapid cooling (>10). 4 K s -1 , Figure 44C This will cause thermal stress and cracking of the glass phase in the CFA, which helps to improve leachability.

[0391] The universality of electrothermal activation process

[0392] Regarding REE recycling, the electrothermal activation process is applicable to other wastes, including BR [Deady 2016; Rivera 2018; Reid 2017] and e-waste (including those mentioned above) [Maroufi 2018; Deshmane 2020; Peelman 2018].

[0393] BR (red mud) is a waste product of the Bayer process for the production of alumina. BR is one of the most abundant industrial wastes, with 3 billion tons already stored in waste ponds and 0.15 billion tons produced annually, but currently only 3% is recycled [Service 2020]. BR contains significant amounts of REEs, for example, a total REE content of ~1000 ppm was found in BR from MYTILINEOS "Aluminum of Greece" [Deady 2016]. BR is a dry powder with fine particle size and has major components including Fe2O3, CaCO3, FeO(OH) and SiO2. Figures 47A-47B REEs in BR were extracted by direct leaching using 0.5 M HNO3[Ochsenkuhn-Petropulu 1996]. The acid-extractable REE content from BR feedstock was 428 ± 9 mg kg -1 . Figure 47C and 48A -48B.

[0394] Similar to CFA, the REE extractability of BR after electrothermal activation also depends on the FJH voltage. Figure 48A At the optimized FJH voltage of 120 V, the extractable REE content increased to 757 ± 30 mg kg -1 ( Figure 48B ), corresponding to Y / Y0~177% ( Figure 47C ) from BR feedstock. The mechanism by which REE extractability from BR is improved by the FJH process is thought to be similar to that of CFA ( Figures 46A-46G ), as phosphate is one of the main counterions of BR [Boni 2013].

[0395] This FJH strategy is also applicable to the activation of electronic waste, and is shown here as a complement to the method without mild acid leaching. Due to the rapid upgrade of personal electronic devices, more than 40 million tons of electronic waste are generated globally each year, of which <20% is recycled [Zeng 2018]. REEs are widely used in electronic devices in permanent magnets [Deshmane] and capacitors [Alam 2012]. In turn, the economic viability of recovering REEs from high-grade electronic waste is greater compared to mining REEs from ores.

[0396] The electronic waste used in the FJH process is printed circuit boards (PCB) from discarded computers. Figure 49A (shown ground into a powder). As Figure 49BAs shown, the abundant metals in electronic waste include copper and aluminum, which are mainly used for interconnects. REEs in PCB waste were extracted via a 1M HCl leaching process at 85°C. The acid-leached REE content from the electronic waste raw material was 61 ± 4 mg / kg. -1 . Figures 50A-50B After optimizing the activation process under the voltage ( Figures 50A-50B The extractable REE content increased to 94.6 ± 0.2 mg / kg. -1 This corresponds to Y / Y0~156% from electronic waste raw materials. Figure 49C and 50A -50B.

[0397] Unlike CFA or BR, REEs in e-waste are typically in the form of readily soluble REE metals or oxides [Alam 2012]. However, due to the laminated structure of electronic devices, REEs are often embedded in the matrix material, which hinders REE extraction by hydrometallurgical processes. The FJH method can accelerate the leaching rate and extent of metal extraction by exposing the metal through matrix cracking.

[0398] Scalability and usability

[0399] The FJH process for REE recovery is scalable. To maintain a constant temperature while scaling up the sample quality of each batch, the FJH voltage or the total capacitance of the capacitor bank can be increased. A production rate of >10 kg / day has been achieved through the batch process. The FJH process can be integrated into continuous production for further automation, such as by using… Figures 40A-40B The scheme shown in the image. The continued commercial expansion of the FJH process to tons / day paves the way for the future recovery of REEs from large-scale waste products.

[0400] Economic viability, as profit margins are typically a strong supporter of recycling. Due to the direct sample heating characteristics, short duration, and rapid heating / cooling rate, the FJH process implementation is highly energy efficient, with low power consumption of 600 kWh / ton or $12 / ton, enabling a profit percentage >10× compared to direct leaching of feedstock.

[0401] For further refining, it is necessary to remove dissolved impurities (mainly Al, Si, Fe, Ca, and Mg) from the REE-containing leachate, as well as subsequent separation. It was observed that in most cases, the ratio of REE to impurities in the leachate (c(REE) / c(impurities)) improved with the FJH process, indicating that the FJH process will also benefit subsequent REE separation.

[0402] ore

[0403] Since monazite (Ce,La,Y,Th)PO4 and xenotime ore YPO4 are the main commercial sources of REE production [Cheisson 2019], the implementation scheme can also be used in REE mining to improve the leaching properties of REE ores. Commercially, alkali digestion (70% NaOH, 140-150°C) is the primary leaching technique for monazite [Peelman 2016], or acid roasting (concentrated H2SO4, 200°C) is the primary leaching technique for both monazite and xenotime ore [Kim 2016]. This FJH process is faster and less dependent on the use of concentrated alkali and acid. Existing single-element separation techniques, such as solvent extraction and ion exchange [Xie 2014], can be used to process the REE mixture obtained by FJH, as these generally produce less contamination than those generated by conventional mining methods.

[0404] While embodiments of the invention have been shown and described, modifications can be made to them by those skilled in the art without departing from the spirit and teachings of the invention. The embodiments and examples provided herein are merely exemplary and not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. The scope of protection is not limited by the foregoing description but only by the appended claims, which include all equivalents of the subject matter of the claims.

[0405] All patents, patent applications and publications cited herein are incorporated herein in their entirety, to the extent that they provide exemplary, procedural or other details that supplement those set forth herein.

[0406] Quantities and other numerical data may be presented in range format herein. It should be understood that such range format is used for convenience and brevity only and should be flexibly interpreted to include not only the numerical values ​​explicitly described as the boundaries of the range, but also all individual numerical values ​​or subranges encompassed within that range, as if each numerical value and subrange were explicitly described. For example, a numerical range of approximately 1 to approximately 4.5 should be interpreted to include not only the explicitly described boundary of 1 to approximately 4.5, but also individual numbers such as 2, 3, 4, and subranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges describing only a single numerical value, such as “less than approximately 4.5,” which should be interpreted to include all the aforementioned values ​​and ranges. Furthermore, this interpretation should apply regardless of the width of the range or the characteristics described.

[0407] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter of this disclosure pertains. While any methods, apparatuses, and materials similar to or equivalent to those described herein may be used in the practice or testing of the subject matter of this disclosure, representative methods, apparatuses, and materials are described hereafter.

[0408] In accordance with long-standing patent law practice, the terms "a" and "an" when used in the context of this application (including the claims) are to be construed to cover "one or more." Therefore, use of the terms "a" and "an" in this specification and the appended claims is not intended to be limiting.

[0409] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless otherwise indicated, the numerical parameters set forth in the specification and claims are approximations that can vary depending upon the desired properties sought to be obtained by the processes of the present disclosure.

[0410] The terms "about" and "substantially" as used herein when used in the context of a value or amount, weight, time, volume, concentration, or percentage, means that the value or amount can vary from the stated amount by plus or minus 20% in some embodiments, by plus or minus 10% in some embodiments, by plus or minus 5% in some embodiments, by plus or minus 1% in some embodiments, by plus or minus 0.5% in some embodiments, and by plus or minus 0.1% in some embodiments, such variations being suitable for the practice of the methods disclosed.

[0411] The terms "substantially perpendicular" and "substantially parallel" as used herein mean that the value or amount can vary from the stated amount by plus or minus 10° in some embodiments, by plus or minus 5° in some embodiments, by plus or minus 1° in some embodiments, and by plus or minus 0.5° in some embodiments, respectively, in the perpendicular and parallel directions.

[0412] The term "and / or" as used herein when used in a list of entities, means that at least one of the entities, but that the entities can be combined with others on the list. Thus, for example, the phrases "A, B, C, and / or D" individually include A, B, C, and D, but also includes any and all combinations and subcombinations of A, B, C, and D.

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Claims

1. A method of recovering metals, wherein the method comprises: (a) mixing a material with an electrically conductive additive to form a mixture, wherein: (i) the material is prepared from ore, fly ash, and / or bauxite residue; and (ii) the electrically conductive additive is a carbon source; (b) applying a voltage across the mixture to recover metals from the material, wherein (i) the voltage is applied in one or more voltage pulses, and (ii) the duration of each of the one or more voltage pulses is a duration period; and (c) collecting the recovered metals, wherein the recovery and collection of metals comprises a leaching process after the voltage is applied across the mixture.

2. The method of claim 1, wherein the material is prepared from ore.

3. The method of claim 1, wherein the material is prepared from fly ash.

4. The method of claim 1, wherein the material is prepared from bauxite residue.

5. The method of claim 1, wherein the material is prepared by subjecting the material to a mechanical process to convert the material into a fine powder.

6. The method of claim 5, wherein the mechanical process is selected from the group consisting of cutting the material into small pieces, pulverizing the material, grinding the material, milling the material, and combinations thereof.

7. The method of claim 5, wherein the fine powder is a micron-sized fine powder.

8. The method of claim 1, wherein the electrically conductive additive is selected from the group consisting of elemental carbon, carbon black, graphene, flash graphene, coal, anthracite, coke, metallurgical coke, calcined coke, activated carbon, biochar, natural gas carbon from which its hydrogen atoms have been removed, subgraphite, plastic waste, carbon char derived from plastic waste, food waste, carbon char derived from food waste, biomass, carbon char derived from biomass, hydrocarbon gas, and mixtures thereof.

9. The method of claim 1, wherein the electrically conductive additive is carbon black.

10. The method of claim 1, wherein the electrically conductive additive is predominantly elemental carbon.

11. The method of claim 1, wherein the material is mixed with the electrically conductive additive in a weight ratio ranging from 1:2 to 25:

1.

12. The method of claim 1, wherein the applied voltage ranges from 15 V to 300 V.

13. The method of claim 1, wherein (a) the mass of the mixture to which the voltage is applied exceeds 1 kilogram; and (b) the applied voltage is from 100 V to 100,000 V.

14. The method of claim 13, wherein the mass of the mixture to which the voltage is applied exceeds 100 kilograms.

15. The method of claim 1, wherein (a) the mass of the mixture to which the voltage is applied exceeds 1 kilogram; and (b) the applied current is from 1,000 amps to 30,000 amps.

16. The method of claim 15, wherein the mass of the mixture to which the voltage is applied exceeds 100 kilograms.

17. The method of claim 1, wherein the mixture has an electrical resistance of from 0.1 ohm to 25 ohms when the voltage is applied.

18. The method of claim 1, wherein the duration period of each of the one or more voltage pulses is from 1 microsecond to 25 seconds.

19. The method of claim 1, wherein each of the one or more voltage pulses has a duration of 1 microsecond to 10 seconds.

20. The method of claim 1, wherein each of the one or more voltage pulses has a duration of 1 microsecond to 1 second.

21. The method of claim 1, wherein each of the one or more voltage pulses has a duration of 100 microseconds to 500 microseconds.

22. The method of claim 1, wherein the one or more voltage pulses is from 2 voltage pulses to 100 voltage pulses.

23. The method of claim 1, wherein direct current (DC) is used to perform the voltage pulses.

24. The method of claim 1, wherein the method is performed using pulsed direct current (PDC) Joule heating.

25. The method of claim 1, wherein alternating current (AC) is used to perform the voltage pulses.

26. The method of claim 1, wherein the voltage pulses are performed using both direct current (DC) and alternating current.

27. The method of claim 26, wherein the method switches back and forth between using direct current (DC) and alternating current (AC).

28. The method of claim 26, wherein the method uses both direct current (DC) and alternating current (AC) simultaneously.

29. The method of claim 1, wherein the one or more voltage pulses increases the temperature of the mixture to at least 3000 K.

30. The method of claim 1, wherein the metal comprises a rare earth element.

31. The method of claim 1, wherein the metal comprises a noble metal.

32. The method of claim 1, wherein (a) the material comprises a metal oxide; and (b) the step of applying a voltage across the mixture causes a carbothermic reaction of the metal oxide to recover the metal.

33. The method of claim 1, wherein the voltage is applied across the mixture at a pressure of 0.001 to 25 atmospheres to recover the metal from the material.

34. The method of claim 33, wherein the pressure is 1 atmosphere.

35. The method of claim 33, wherein the pressure is at least 2 atmospheres.

36. The method of claim 33, wherein the pressure is at least 10 atmospheres.

37. The method of claim 33, wherein the pressure is at least 20 atmospheres.

38. The method of claim 33, wherein the method is performed using a pressurization unit.

39. The method of claim 38, wherein the applying a voltage across the mixture to recover the metal from the material causes a majority of the metal to remain with the graphene produced by the method.

40. The method of claim 39, wherein the collecting the recovered metal comprises separating the metal from the graphene.

41. The method of claim 1, wherein the collecting step comprises collecting a gas stream, the gas stream comprising volatilized products produced by the applying a voltage across the mixture.

42. The method of claim 41, wherein the collecting step further comprises cooling the gas stream.

43. The method of claim 1, wherein the leachability of the metals in the mixture after the voltage is applied across the mixture is more than twice the leachability content of the metals in the mixture before the voltage is applied across the mixture when using the same pH value and the same volume of water treatment.

44. The method of claim 1, wherein the leaching process is performed using a dilute acid.

45. The method of claim 44, wherein the dilute acid is at most 1 M acid.

46. The method of claim 44, wherein the dilute acid is at most 0.1 M acid.

47. The method of claim 44, wherein the dilute acid is at least 1 M acid.

48. The method of claim 1, wherein the method is performed in a continuous process or an automated process.

49. A system for performing a method of recovering metals, the system employing the method of any one of claims 1-48, wherein the system comprises: (a) a source comprising a mixture of the material and a conductive additive; (b) a cell operably connected to the source such that the mixture can flow into the cell and be held under compression, wherein (i) the cell is a pressure cell; (c) an electrode operably connected to the pressure cell; and (d) a flash power source for applying a voltage across the mixture to recover metals from the material; and (e) a gas supply for pressurizing the pressure cell.

50. The system of claim 49, wherein the system further comprises an adjustable safety valve.

51. The system of claim 49, wherein the system further comprises a particulate collector.

52. The system of claim 49, wherein the system further comprises a gas collector.

53. The system of claim 49, wherein the system is operable to perform a continuous process or an automated process.

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