Patent Application
20230357885
The present invention relates to ultrafast flash Joule heating synthesis methods and systems, and more particularly, ultrafast synthesis methods to recover precious metals and other metals from electronic waste (e-waste).
Highly efficient and low-cost synthesis of nanomaterials is the prerequisite for their commercial applications.
Nanosized transition metal carbides (TMCs) have been widely used as the precursors for ultra-hard and ultra-strong ceramics [Zou 2013; Zhang 2019; Reddy 2012], high-performance electrochemical catalysts because of their platinum-like electronic structures [Li 2018; Zhong 2016; Gao 2019; Gong 2016; Han 2018], and catalyst supports due to the strong metal-substrate interactions [Lin 2017; Yao 2017]. Traditional methods for bulk carbide syntheses include carburization of metal precursors with gaseous carbon precursors or sintering of metal precursors with graphitic carbon at high temperature. [Rosa 1983] These procedures can be problematic since they result in coked carbide surfaces due to the excessive supply of carbon sources, and large particle sizes with low surface areas that are detrimental to catalytic performance. [Chen 2013; Zeng 2015].
Much effort has been devoted to synthesizing carbides with fine particle sizes, including temperature-programmed reduction [Oyama 2992], carbothermic 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 well-optimized reaction windows. [Claridge 2000]. The carbothermic reduction of metal precursors in a furnace is universal in synthesis of TMCs [Wu 2020]; however, extended high-temperature conditions are essential to compensate the slow solid-solid reaction kinetics, which inevitably result in sintering or agglomeration [Wang K 2019].
To avoid severe agglomeration, a microwave combustion method was developed for rapid synthesis of Mo2C and WC nanodots within 2 min. [Wan 2019]. The pyrolysis of metal complexes involves the use of costly 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].
The type of carbide is also limited by the availability of volatile metal compounds. The solution-based precipitation and carburization requires long annealing times for full conversion. For example, annealing at 850° C. for 12 to 24 h is needed for the synthesis of MoC using ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) as the precursor [Wan 2014].
Recently, several non-conventional electrical thermal processes have been developed towards energy-efficient high temperature synthesis. [Wang 2020; Giorgi 2018; Yan 2018]. The thermal shock (CTS) process used short current pulses for synthesis of high-entropy alloy nanoparticles on carbon supports at ˜2000 K. [Yan 2018]. The ultrahigh temperature sintering (UHS) based on current-induced heating was proposed for sintering and screening of ceramics within 10 s. [Wang 2020]. The spark flash sintering (SPS) applied an electric current for the reactive carbothermic synthesis of zirconium carbide (ZrC) in 10 min. [Giorgi 2018]. However, these approaches are targeting the sintering of bulk ceramics and lack the ability in synthesis of fine nanocrystals.
Furthermore, phases and crystal surface structure play significant roles in the behavior of carbides, such as in their hydrogen adsorption/desorption energy. [Gong 2016; Politi 2013] However, there are very few procedures to selectively engineer the phases and crystal surfaces of carbides for maximal performance. [Gong 2016; Wan 2014].
Electrocatalytic hydrogen evolution (HER) reactions depend on the availability of low-cost electrocatalysts. TMC are highly promising in HER due to their platinum-like electronic structures. [Gao 2019]. However, state-of-art methods to synthesize metal carbides nanoparticles have the limitation of high cost and low productivity. [Gong 2016]. Critically, most methods are too specific and lack generality, and are also hard for the phase control. [Wan 2014].
High-surface-area corundum nanoparticles (α-Al2O3NPs) have widespread applications. For examples, corundum is widely used in ceramics for prosthetic implants [De Aza 2002] and high-speed cutting tools [Kumar 2003]. α-Al2O3NPs precursors provide access to nanometer-grained alumina ceramics with significantly improved fracture toughness [Ighodaro 2008], wear resistance [Krell 1996], and high density under reduced sintering temperature [Guo 2016]. Even though γ-Al2O3NPs are primarily used as catalyst supports due to their high surface area [Peterson 2014], the α-Al2O3NPs are also used as catalyst supports and they have higher mechanical stability in auto-exhaust Pt—Mo—Co catalytic converters [Frank 1998], and enhanced Ru catalyst activity for ammonia synthesis. [Lin 2019].
Much effort has been made toward improving the synthesis of α-Al2O3, yet few of the processes afford high-surface-area NPs due to the intrinsic thermodynamic limits. [Guo 2016; McHale 1997; Amrute 2019]. Even though corundum is the thermodynamically stable phase of coarsely crystalized aluminum oxide (Al2O3), the synthesis of nanocrystalline Al2O3 usually leads to γ-Al2O3 because of its lower surface energy when surface areas are greater than 125 m2 g−1. [McHale 1997].
Another reason is the high activation energy barrier of ˜485 kJ mol−1 for the phase transformation from the cubic close-packed structure of the γ-phase to the hexagonal close-packed structure of the α-phase. [Steiner 1971]. As a result, the thermal processes usually require temperatures >1470 K with prolonged annealing times of 10 to 20 h to facilitate the transformation. [Steiner 1971; Levin 1998]. The high-energy input and extended high-temperature annealing leads to surface area <10 m2 g−1 because of the substantial mass transfer. [Amrute 2019]. Moreover, the polymorphism of Al2O3 during the phase transformation further increases the complexity and could lead to the mixed transition (t)-alumina with undesired δ- and θ-Al2O3. [Steiner 1971; Chang 2001; Laine 2006]. Representative methods for corundum nanoparticles are rather time- and energy-consuming, such as, for example, annealing of γ-Al2O3 at 1473-1673K for 10-20 h [Lodziana 2004], and hydrothermal reaction of γ-AlOOH at 723K and 1200 bar for 35 days [McHale 1997; Loffler 2003].
Accordingly, the production of α-Al2O3 by phase transformation from the cubic close-packed gamma phase (γ-Al2O3) is usually hampered by the high activation energy barrier (˜485 kJ mol−1), which requires extended high-temperature thermal annealing (˜1500 K, 10 to 20 h) and suffers from severe agglomeration. Hence, developing a method that is ultrafast and energy-saving is critical for the broad applications of α-Al2O3 nanoparticles.
Recovery of valuable metals from waste is significant for the circular economy and is also critical for solving environmental issues. Specifically, electronic wastes (e-waste) that contain rich valuable elements.
The e-wastes comes from discarded electrical or electronic devices. Precious metal recovery from electronic waste, termed “urban mining,” is important for a circular economy.
Present methods for urban mining, mainly smelting and leaching, suffer from lengthy purification processes and negative environmental impacts.
More than 40 million tons of electronic waste (e-waste) are produced globally each year [Zhang 2012; Zeng 2018], which is the fastest-growing component of solid wastes due to the rapid upgrade of personal electrical and electronic equipment [Ogunseitan 2009; Wang 2016]. Most e-waste is landfilled with only ˜20% being recycled [Ghosh 2015], which could lead to negative environmental impact due to the broad use of heavy metals in electronics [Leung 2008; Julander 2014; Awasthi 2019].
E-waste could become a sustainable resource because it contains abundant valuable metals. [Kaya 2016]. The concentrations of some precious metals in e-waste are higher than those in ores. [Zhang 2012]. Precious metals recovery from e-waste, i.e., urban mining, is becoming more cost-effective than virgin mining [Zeng 2018] and important for a circular economy [Awasthi 2019].
Similarly, due to the broad use of heavy metals in electronics, including Cd, Co, Cu, Ni, Pb, and Zn, e-waste could lead to significant health risks and negative environmental impacts. [Leung 2008; Julander 2014; Awasthi 2019]. The heavy metal leakage due to improper landfill disposal leads to environmental disruption. [Zhang 2012; Awashthi 2019].
The release of hazardous components during the recycling processes in the form of dust or smoke [Leung 2008] deteriorates the health of recycling workers and local residents. For example, a significantly higher concentration of Pb has been found in the blood of e-waste workers. [Julander 2014; Popoola 2019].
The lack of high-yielding and environmentally friendly recovery processes are main obstacles to urban mining. [Kaya 2016]. The traditional method for e-waste recycling is based on a pyrometallurgy process [Hall 2007], where metals are melted by heating at high temperature. 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]. The hydrometallurgical process is more selective and done by leaching the metals using acid, base, or cyanide. [Sun Z 2017]. The leaching kinetics are usually slow. The use of highly concentrated leaching agents renders the hydrometallurgical process difficult for large-scale applications, and large amounts of liquid waste and sludge are produced that could result in secondary pollution. [Jajhav 2015]. Biometallurgy could be highly selective and environmentally sustainable, yet it is still in its infancy. [Zhuang 2015]. The separation of valuable metals from various materials matrices, including plastics, glass, and ceramics, are based upon their difference in physical or chemical properties. For example, the gravity separation technique relies on differing specific densities. [Sarvar 2015]. Magnetic separation is used to separate magnetic metals from nonferrous waste. [Yamane 2011]. Hydrometallurgical separation is based upon the chemical reactivity of metals with leaching agents. [Sethurajan 2019].
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 result in a number of waterborne or even airborne diseases. At the same time, the circuit boards contains many precious metals, like gold (Au), silver (Ag), and platinum (Pt), as well as rare earth elemental metals that are hard to mine and considered critical elements for electronics manufacture and electric motors, including neodymium (Nd) and dysprosium (Dy). Mining or processing of these latter rare earth elements are controlled by foreign governments, raising concerns for the US essential element security for its manufacturing needs. However, less than 20% of e-waste is recycled, with 80% being landfill. One way for e-waste recycling is by melting circuit boards and leaching the valuable metals. [Sthiannopkao 2013]. The traditional recycling method that is usually handled in developing countries exposes workers to hazardous and carcinogenic substances. Hence, an ultraclean and highly efficient way to recycle the valuable metals from e-waste is highly needed.
Similar situations pertains to ores, fly ash, and red mud (red mud is more recently referred to as bauxite residue), again because rare earth elements (REE) are strategic resources in modern electronics, clean energy, and automotive industries. [Cheisson 2019]. Concentrated aqueous acid leaching of the REE minerals followed by biphasic solvent extraction has been the dominant scheme for REE mass production. [Cheisson 2019]. However, the resource- and pollution-intensive production has a large environmental footprint, where the degrative environmental cost reached $14.8 billion in 2015, warranting a search for a sustainable solution. [Lee 2018]. As the easily accessible REE minerals diminish, the extraction of REE from industrial wastes has gained much attention. [Jyothi 2020]. The applicable secondary wastes include coal fly ash (CFA) [Taggart 2016; Smith 2019; Zhang 2020; Liu 2019; Sahoo 2016; Middleton 2020], bauxite residue (BR, which is also called red mud) [Deady 2016; Rivera 2018; Reid 2017], which results from bauxite processing for aluminum production, and, electronic waste (e-waste) [Maroufi 2018; Deshmane 2020; Peelman 2018] from consumer electronics and electric vehicles. Annual production of alumina in 2018 was approximately 160 million tons. Red mud is a highly alkaline waste composed of mainly oxides including Fe2O3, Al2O3, TiO2, CaO, SiO2, and Na2O. Moreover, red mud also contains valuable rare earth elements, including La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y. [Deady 2016]. Accordingly, similar situations as discussed above with respect to needs to recovery metals from e-waste likewise pertain to ores, fly ash, and bauxite residue (red mud).
The reuse of these wastes in turn reduces the environmental burden of their disposal. [Sahoo 2016]. However, the REE contents in these secondary wastes are usually less than that in REE minerals, and the recycling yields are still extremely low, which exacerbate the quest to establish a circular economic program. [Taggart 2016].
Taking CFA as an example, it is the by-product of coal combustion with an annual production rate of ˜750 million tons worldwide. [Sahoo 2016]. CFA has an average total REE content of ˜500 ppm, which is variable based upon the geological origin of the feed coals. [Taggart 2016; Middleton 2020]. The acid extractable REE content, however, is usually much smaller and highly dependent on the CFA feeds. For example, Taggart 2016 reported the HNO3 extractability of REE ranging from 1.6% to 93.2% with a median value of ˜30% from major U.S. power plants, or 7.4 ppm to 372 ppm with a median value of ˜127 ppm. REE extractability in CFA depends on the REE species, such as oxides, phosphates (churchite, xenotime, monazite, etc.), apatite, zircon, and glass phases. [Liu 2019]. The low REE extractabilities in most CFA resources are attributed to the large ratios of hard-to-dissolve REE species such as REE phosphates, zircon, and glass phases. [Liu 2019].
Optimizing acid leaching processes could, to some extent, improve the extractability by using highly concentrated mineral acids, such as 15 M HNO3 at 85-90° C. for an extractability of 70% [Taggart 2016], and 12 M HCl at 85° C. for an extractability of 35-100%, depending on the feeds [King 2018]. The use of concentrated acid, however, inevitably increases the cost of extraction and the disposal burden. Chemical or thermal pretreatments of the CFA prior to acid leaching contribute to achieving high REE recovery. [Wang Z 2019; Taggart 2018]. For examples, a total REE recovery of 88% is achieved by the NaOH hydrothermal treatment followed by acid leaching. [Wang Z 2019]. Alkali roasting using NaOH leads to a recovery yield >90%. [Taggart 2018]. However, those pretreatment processes are usually lengthy and energy-intensive, which greatly reduce the profit margin and incentive.
Moreover, there are environmental hazards in the discharge of these materials. Discharge of red mud is very environmentally hazardous because of its alkalinity. In October 2010, about one million cubic meters of red mud was accidently released into the countryside in Hungary, killing ten people and polluting the surrounding areas. Indeed, developed methods to separate and recover rare earth elements, such as, for instance, leaching and cation-exchange chromatography [Ochsenkuhn-Petropulu 1995] can result in secondary pollution in view of the large amounts of acid used.
Thus, present methods for REE recovery suffer from lengthy purifications, low extractability, and high wastewater streams. Hence, there remains a need for a rapid and energy-efficient pretreatment for the REE recovery from ores, fly ash, and bauxite residue (red mud). There further remains a need to develop a “dry” method to directly recovery the rare earth elements in ores, fly ash, and bauxite residue (red mud).
The present invention relates to ultrafast flash Joule heating synthesis methods, and more particularly, embodiments of the present invention include ultrafast synthesis methods to recover precious metals and other metals from electronic waste (e-waste).
Such solvent-free processes based on flash Joule heating can provide for a solvent-free and sustainable process to recover precious metals and remove hazardous heavy metals in electronic waste within one second. The sample temperature can ramp to ˜3400 K in milliseconds by the ultrafast electrical thermal process. Such a high temperature enables the evaporative separation of precious metals from the supporting matrices, with the recovery yields greater than 80% for Rh, Pd, Ag, Ir, Ru, and Pt, and greater than 60% for Au. The heavy metals in electronic waste, some of which are highly toxic including Cr, As, Cd, Hg, and Pb, are also removed, leaving a final waste with minimal metal content, acceptable even for agriculture soil levels. Urban mining by FJH would be 80 times to 500 times less energy consumptive than using traditional smelting furnaces for metal-component recovery and more environmentally friendly.
In general, in one embodiment, the invention features a method of recovering metal. The method includes mixing a material with a conductive additive to form a mixture. The material is prepared from e-waste. The method further includes applying a voltage across the mixture to recover metal from the material. The voltage is applied in one or more voltage pulses. Duration of each of the one or more voltage pulses is for a duration period. The method further includes collecting the recovered metal.
Implementations of the invention can include one or more of the following features:
The conductive additive can be a carbon source.
The e-waste can be a printed circuit board.
The e-waste can include a plastic.
The e-waste can be a waste material from a device selected from a group consisting of computers, smartphones, electronic devices, and displays.
The material can be prepared by performing a mechanical process to transform the material into a fine powder.
The mechanical process can be selected from a group consisting of cutting the material into small pieces, crushing the material, grinding the material, milling the material, and combinations thereof.
The fine powder can be a microscale fine powder.
The conductive additive can be selected from a group consisting of elemental carbon, carbon black, graphene, flash graphene, coal, anthracite, coke, metallurgical coke, calcined coke, activated charcoal, biochar, natural gas carbon that had been stripped of its hydrogen atoms, activated charcoal, shungite, plastic waste, plastic waste-derived carbon char, food waste, food waste-derived carbon char, biomass, biomass-derived carbon char, hydrocarbon gas, and mixtures therefrom.
The conductive additive can be carbon black.
The conductive additive can be predominately elemental carbon.
The conductive additive can be selected from the group consisting of metals, metal salts, metal oxides, metalloids, metal complexes, conductive posphorus, and non-metal conductive materials.
The conductive additive can be selected from the group consisting of metals, metal salts, metal oxides, metalloids, and metal complexes.
The conductive additive can be a metalloid.
The metalloid can be selected from the group consisting of B, Si, As, Te, and At.
The material and the conductive additive can be mixed at a weight ratio in a range of 1:2 and 4:1.
The voltage applied can be in a range of 15 V and 300 V.
The mass of the mixture to which the voltage is applied can be more than 1 kg. The voltage applied can be between 100 V and 100,000 V.
The mass of the mixture to which the voltage is applied can be more than 100 kg.
The mass of the mixture to which the voltage is applied can be more than 1 kg. The current applied can be between 1,000 amps and 30,000 amps.
The mass of the mixture to which the voltage is applied can be more than 100 kg.
The mixture can have a resistance in the range of 0.1 ohms and 25 ohms when the voltage is applied.
The duration period for the duration of each of the one or more voltage pulses can be between 1 microsecond and 25 seconds.
The duration period for the duratrion of each of the one or more voltage pulses can be between 1 microsecond and 10 seconds.
The duration period for the duration of each of the one or more voltage pulses can be between 1 microsecond and 1 second.
The duration period for the duration of each of the one or voltage pulses can be between 100 microseconds and 500 microseconds.
The one or more voltage pulses can be between 2 voltage pulses and 100 voltage pulses.
The voltage pulse can be performed using direct current (DC).
The method can be performed utilizing a pulsed direct current (PDC) Joule heating process.
The voltage pulse can be performed using alternating current (AC).
The voltage pulse can be performed by using both direct current (DC) and alternating current (AC).
The method can switch back and forth between the use of direct current (DC) and alternating current (AC).
The method can concurrently use direct current (DC) and alternating current (AC).
The one or more voltage pulses can increase the temperature of the mixture to at least 3000 K.
The metal can include a rare earth element.
The metal can include precious metal.
The metal can include a toxic heavy metal.
The materials can include a metal oxide. The step of applying a voltage across the mixture can result in a carbothermic reaction of the metal oxide to recover the metal.
The applying of the voltage across the mixture to recover the metal from the material can be performed at a pressure between 1 and 25 atmospheres.
The pressure can be below 0.5 atmospheres.
The pressure can be below 0.001 atmospheres.
The pressure can be around 1 atmosphere.
The pressure can be at least 2 atmospheres.
The pressure can be at least 10 atmospheres.
The pressure can be at least 20 atmospheres.
The method can be performed using a pressurized cell.
The applying of the voltage across the mixture to recover the metal from the material can result in a majority of the metal remaining with graphene created by the method.
The collecting of the recovered metal can include separating the metal from the graphene.
The separating of the metal from the graphene can include oxidizing the graphene away chemically.
The graphene can be oxidized with an oxidant.
The oxidant can be HNO3 or H2O2.
The oxidant can be HNO3 or H2O2 with H2SO4.
The separating of the metal from graphene can include calcinating the graphene away to leave a metal species selected from a group consisting of metal, metal oxide, metal carbide, metal salt, and combinations thereof.
The mixture of the material and the conductive additive can further include a halogen containing compound.
The halogen containing compound can be selected from a group consisting of NaCl, NaF, KCl, NaI, halogentated polymers, halogenated organics, halogenated inorganics, halogenated salts, and combinations thereof.
The halogen containing compound can include a halogenated polymer selected from a group consisting of PTFE, PVDF, PVC, and CPVC.
The step of collecting can include collecting a gas stream comprising volatized products produced by the application of the voltage across the mixture.
The volatized products can include a metal halide.
The step of collecting can further include cooling the gas stream.
The step of applying a voltage across the mixture can heat and evaporate metals from the mixture forming a metal vapor. The step of collecting the recovered materials can include transporting the metal vapors under low pressure. The step of collecting the recovered materials can include utilizing a condenser or cold trap to condense the metal vapor for collection.
The metal vapor comprises metal halides.
The transporting of the metal vapors can be under a vacuum.
The step of collecting further can include performing a leaching process after applying the voltage across the mixture.
The leachability of metals in the mixture after applying a voltage across the mixture can be more than two times the leachability content of the metals in the mixture before applying the voltage across the mixture, when conducted using the same pH and same volume of aqueous treatment.
The leaching process can be performed using diluted acid.
The diluted acid can be at least 1 M of the acid.
The applying of the voltage across the mixture to recover the metal from the material can be performed at a pressure above 1 atmosphere such that volatile components of the e-waste are trapped in residual solids of the material after the application of the voltage.
The method can be performed in a continuous process or automated process.
In general, in another embodiment, the invention features a system for performing the method of recovering metal utilizing at least one of the above described methods. The system includes a source of the mixture comprising the material and conductive additive. The system further includes a cell operably connected to the source such that the mixture can be flowed into the cell and held under compression. The system further includes electrodes operatively connected to the pressure cell. The system further includes a flash power supply for applying a voltage across the mixture to recover the metal from the material.
Implementations of the invention can include one or more of the following features:
The cell can be a pressurized cell. The system can further include a gas supply for pressurizing the pressure cell.
The system can further include an adjustable relief value.
The system can further include a particle collector.
The system can further include a gas collector.
The system can be operable to perform a continuous process or automated process.
The present invention relates to ultrafast flash Joule heating synthesis methods, and more particularly, embodiments of the present invention include ultrafast synthesis methods to form carbides, ultrafast synthesis methods to form corundum nanoparticles, ultrafast synthesis methods to recover precious metals recovery from electronic waste (e-waste), and ultrafast synthesis methods to recover metal from ores, fly ash, and bauxite residue (red mud).
Synthesis Process
The present invention includes flashing Joule heating [see Luong 2020; Stanford 2020; Tour PCT '000 Application] for ultrafast processes to synthesize metal carbide nanoparticles. Metal carbides were synthesized within seconds, which is hundreds of time faster than previous methods [Gong 2016; Wan 2014; Ma 2015]. Accordingly, in some embodiments, the present invention provides for phase controlled synthesis of transition metal carbide nanocrystals by ultra flash Joule heating.
Such solvent-free process based on flash Joule heating, can provide for the ultrafast synthesis of coke-free carbide nanocrystals within 1 s. A milliseconds current pulse can pass through the precursors, which brings the sample to ultrahigh temperature (>3000 K) and then it is rapidly cooled to room temperature (>104 K s−1). Thirteen element carbides 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 for excellent generality. Moreover, by controlling the FJH pulse voltage, phase-pure molybdenum carbides including β-Mo2C, and metastable α-MoC1-x and η-MoC1-x can be selectively synthesized, showing the phase engineering ability of the synergistic electrical-thermal process. The phase-dependent HER performance of molybdenum carbides was also discovered; the β-Mo2C exhibited the best HER performance (with an overpotential of −220 mV, Tafel slope of 68 mV dec−1, and good durability).
Methods for ultrafast synthesizing of carbides can include the following.
Select a reaction precursor (or precursors) and mix with a conducting carbon additive, such as carbon black. Alternatively, the conducting additive can be other carbon sources (in addition or in the alternative of carbon black, since these temperatures will convert any carbon source to almost all carbon at these temperatures. The carbon black could be substituted by graphene, flash graphene, coal, anthracite, coke, metallurgical coke, calcined coke, activated charcoal, biochar, natural gas carbon that had been stripped of its hydrogen atoms, activated charcoal, shungite, plastic waste, plastic waste-derived carbon char, food waste, food waste-derived carbon char, biomass, biomass-derived carbon char, hydrocarbon gas, and mixtures therefrom. The use of carbon black as described herein is representative of the conducting additives that can be utilized in the present invention. In certain embodiments of the present invention, the ratio of precursor to conductive additive is in the range between 1:2 to 15:1 by weight, and in further certain embodiments, the ratio of precursor to conductive additive is in the range between 1:2 to 2:1 by weight.
As shown in
Flash Joule heating the mixture of carbon black (or other conductive additive) and metal precursors. As shown in
In an embodiment, a mixture of metal precursors and commercial carbon black was slightly compressed inside a quartz tube between two graphite electrodes (
A rapid light emission was observed during the FJH process (see photos 110-112 in
The cooling rate is ultrafast and on the order of 104 K s−1. The temperature distribution of the sample is simulated by using a finite element method (FEM), which further provides insight into the effects of FJH parameters on the reachable temperature. It was found that higher temperature values could be obtained by applying a larger FJH voltage and suitable sample electrical conductivity. In contrast, the higher thermal conductivity of the sample results in lower temperature due to faster thermal dissipation. A temperature map showed that the temperature distribution is uniform throughout the whole sample, showing the homogeneous heating feature of the FJH process.
FJH of the sample to such a high temperature (˜3000 K) volatilized most of the non-carbon components. According to the temperature-vapor pressure relationships (
Phase Controlled Synthesis of Molybdenum Carbide Nanocrystals
Molybdenum carbides 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 stability [Hugosson 1999]. Different phases have distinct geometric and electronic structures [Politi 2103; Baek 2019], and the catalytically relevant phases are hexagonal β-Mo2C [Wan 2014; Ma 2015; Fan 2017], cubic α-MoC1-x [Yao 2017; Baek 2019; Song 2019], and hexagonal η-MoC1-X34 [Song 2019].
MoCl3 was chosen as the precursor because of its high vapor pressure (
The phase transformation from hexagonal β-Mo2C to cubic α-MoC1-x and then to hexagonal η-MoC1-x is a newly found topotactic transition pathway, which is distinct from the previous report [Wan 2019], where the α-MoC1-x was transformed to β-Mo2C after a ˜24 h annealing at 850° C., and η-MoC1-x was only stabilized by using a NiI2 additive at an higher temperature.
To investigate the electronic structures, X-ray photoelectron spectroscopy (XPS) spectra of the Mo 3d core level was collected (
Morphology characterization by scanning electron microscopy (SEM) shows the fine powder feature of all three carbide phases. The energy dispersive spectroscopy (EDS) mapping images showed a uniform distribution of Mo and C.
Transmission electron microscopy (TEM) and XRD were used to characterize the size and crystallinity of the molybdenum carbides. The particle sizes of the molybdenum carbide phases were determined by the FJH voltages. The β-Mo2C synthesized at the lowest voltage has the largest average size of ˜26.4 nm, followed by α-MoC1-x (˜21.2 nm) and η-MoC1-x (size of ˜20.1 nm). The smaller particle size obtained under higher voltage could be attributed to the faster nucleation kinetics at higher temperature [Jang 1995].
The particle size values measured by TEM match well with the crystal size determined by XRD using the Halder-Wagner method (see TABLE I), indicating that the single-crystal feature of the synthesized carbide particles.
1:1.25
The typical bright-field TEM (BF-TEM) image of a β-Mo2C nanocrystal showed the regular hexagonal nanoplate (depicted by hexagon 201) with a lateral size of ˜20 nm supported on carbon (
The high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image and EDS elemental maps under STEM mode reveal the uniform spatial distribution of Mo, C, and O (
Phase Transformation Process of Molybdenum Carbides
To explain the voltage-dependent phase formation, the current passing through the samples and the temperature under different FJH voltages were firstly recorded. A higher voltage leads to higher temperatures and energy inputs. The maximum temperatures at FJH voltages of 30 V, 60 V, and 120 V were measured to be 839 K, 1468 K, and 3242 K, respectively.
The formation energies of β-Mo2C, α-MoC1-x, and η-MoC1-x varied with carbon content were calculated by first principles density functional theory (DFT) (
In contrast, the α-MoC1-x and η-MoC1-x were metastable phases [Hugosson 1999] and were formed and stabilized at a higher temperature according to the Mo—C phase diagram. The α-MoC1-x (x=½) structure has a slightly higher formation energy and the same stoichiometric composition with β-Mo2C (
The η-MoC1-x phase becomes the relatively stable phase near x=⅜ (
The FJH process with broadly tunable energy input permits the access of the metastable phases with higher formation energy than the thermodynamically stable phase; then, the ultrafast cooling rate of the FJH process (>104 K s−1) helps to kinetically retain the metastable phases, including α-MoC1-x and η-MoC1-x phases, to room temperature. As a control, at the same temperature when metastable α-MoC1-x phase was produced by FJH, the synthesis using a conventional tube furnace with its slow cooling rate of ˜10 K min−1 only produced the thermodynamically stable β-Mo2C phase. This explicitly showed the role of the ultrafast cooling rate of the FJH process in kinetically accessing the metastable phases.
Phase Dependent HER Performance of Molybdenum Carbides
The side-by-side electrochemical comparison of the three phases of molybdenum carbide reveal the effect of the phase control on their individual intrinsic characteristics and catalytic behaviors. To demonstrate their catalytic properties, the HER performances of the three molybdenum carbide phases were measured in 0.5 M H2SO4 using a standard three-electrode configuration. Linear scan voltammogram (LSV) curves of the different electrocatalysts as well as the Pt/C benchmark are shown in
The phase-dependent HER activity of molybdenum carbides was observed. The overpotential (q) vs a reversible hydrogen electrode (RHE) at geometric current densities of 10 mA cm−2 for J-Mo2C, α-MoC1-x, and η-MoC1-x were −220 mV, −310 mV, and −510 mV, respectively (
The fast electrode kinetics of β-Mo2C phase is reflected in the small charge transfer resistance of ˜60Ω at the potential of −0.5 V vs RHE according to the electrochemical impedance measurement. (See
The durability of the three molybdenum carbides phases was evaluated by sweeping the electrocatalysts for 1000 cycles using the cyclic voltammetry method. The LSV curves of the 1st and 1000th cycle (curves 431-432, respectively) for the three phases of molybdenum carbides are shown in
DFT calculations were conducted to elucidate the phase dependent HER performance. The Gibbs free energy of hydrogen adsorption (ΔGH) has been a descriptor in the selection of HER electrocatalysts [Mavrikakis 2006], and optimal catalysts have ΔGH near 0 eV according to the Sabatier principle [Greenley 2006]. The ΔGH of β-Mo2C(001), α-MoC1-x(110), and η-MoC1-x(001) were calculated to be 0.48 eV, 0.71 eV, and 1.09 eV, respectively (
Generalized Strategy for Carbide Nanocrystals Synthesis
Because of the ultrahigh available temperature by the FJH process, various TMCs are readily synthesized regardless of the availability of metal precursors with high vapor pressure. A series of carbide nanocrystals from transition groups IVB, VB, and VIB were successfully synthesized (
According to the Ellingham diagram, the reduction temperatures of the metal oxides were calculated, which serve as reference values to evaluate carbide formation since the reaction of metal with carbon is exothermic (
Group IVB carbides only have the stable rock salt crystal structure, including TiC, ZrC, and HfC, which were readily synthesized (
System and Synthesis Process
Accordingly, for the synthesis of metal carbides, the present invention provides, among other things, (i) an ultrafast synthesis that is thousands of times faster than previous reported methods; (ii) the phase control ability, which is hard to realize by other methods; (iii) the generality, as demonstrated by the synthesis of up to 13 carbides, which is impossible by any other methods.
The metal carbides resulting from the present invention, especially molybdenum carbides and tungsten carbides, can be utilized as electrocatalysts, such as for hydrogen evolution, which is critical for the application of fuel cells in clean energy. Moreover, the nanoscale carbides are important precursors for the fabrication of high-performance carbide ceramics.
An exemplary system and process used included the electrical circuit diagram and setup of the FJH system are shown in
The capacitor bank was charged by a direct current (DC) supply that can reach voltages up to 400 V. A relay with programmable ms-level delay time was used to control the discharge time. The charging, flash Joule heating, and discharging were automatically controlled by using the National Instruments Multifunction I/O (NI USB-6009) combined with a customized LabView program. After the FJH reaction, the apparatus rapidly cooled on its own to room temperature. Before removing the sample, make sure that the capacitor bank is fully discharged. The detailed conditions for the synthesis of various carbides are listed in TABLE I.
Features and Applications
In embodiments, the as-synthesized carbide nanocrystals were supported on flash graphene. The necessity of separation of graphene and carbides depends on the further application. For the application of nanocrystalline carbides in electrocatalysts, the graphene support is beneficial for improving the performance by providing conduction and preventing particle aggregation. For another major application of nanocrystalline carbides as precursors for ultra-strong ceramics, the removal of excess carbon is necessary.
It was realized that the efficient purification of the carbides by post-synthesis processes, including the simple calcination in air for SiC; the Ca metal etching [Dyjak 2013] for TiC, ZrC, HfC, VC, NbC, TaC, Cr3C2, J-Mo2C, and W2C; and the density-in-liquid purification procedure for metastable molybdenum carbides, α-MoC1-x and η-MoC1-x. In addition, the greatly improved purity of B4C was shown by using controlled feeding during the synthesis.
Due to the ultrafast heating/cooling rate, the direct sampling heating feature, and the short reaction duration within 1 s, the FJH process for carbide synthesis is highly energy efficient compared to traditional furnace heating where large amounts of energy are used to maintain the temperature of the chamber. The carbide nanocrystals were synthesized at only 2.2 to 8.6 kJ g−1 in electrical energy. The FJH synthesis possesses excellent scalability, that a constant temperature value and uniformity on different mass scales could be obtained by adjusting the discharging voltage and/or the capacitance.
The synthesis of carbide nanocrystals up to gram scale was demonstrated by increasing the FJH voltage. The FJH process can be extended to the synthesis of carbide alloys [Sarker 2018], heteroatom-decorated carbides, [Song 2019], and phase engineering of metastable carbides, [Demetriou 2002], which provides a powerful technique for carbide production.
The controlled synthesis of metastable phases is challenging in the synthesis of inorganic materials [Chen 2020]. The FJH process provides broadly tunable energy input that can exceed 3000 K coupled with kinetically controlled ultrafast cooling rate (>104 K s−1). Hence, the FJH process provide access to many non-equilibrium phases and subsequently retain it at room temperature, thus serving as a potential tool for engineering the metastable phases of various materials, such as metal nanomaterials [Chen 2020], layered oxides [Bianchini 2020], metal nitrides [Sun W 2017], and two-dimensional materials.
The present invention further includes flashing Joule heating [see Luong 2020; Stanford 2020; Tour PCT '000 Application] for ultrafast processes to synthesize metal corundum nanoparticles, i.e., ultrafast phase transformation from γ-Al2O3 (as well as γ-AlOOH) to α-Al2O3 by a flash Joule heating method. Briefly, carbon black (or other carbon additive, such as discussed above) was mixed with γ-Al2O3 (or γ-AlOOH) nanoparticles, which then is subjected to flash Joule heating. The phase transformation is ultrafast within 1 s, thousands of times faster than other start-of-art methods.
Embodiments of the present invention thus include a Joule heating process based on pulsed direct current (PDC) to complete the phase transformation from γ- to α-Al2O3 at a significantly reduced average bulk temperature and reaction duration (˜573 K, <1 s). The rapid transformation can be enabled by the resistive hotspot-induced local heating in the PDC process when an appropriate volume fraction ratio of γ-Al2O3 precursors and carbon black conductive additives are used. The pulsed and local heating mitigates the agglomeration, leading to the synthesis of α-Al2O3NPs with average particle size of ˜23 nm and surface area ˜65 m2 g−1. Ab initio calculations reveal that the topotactic phase transformation process (from γ- to δ′- to α-Al2O3) is determined by the surface energy difference of the three phases. A particle size of ˜21 nm was achieved that is the thermodynamic limit for the synthesis of dehydrated α-Al2O3NPs with the δ′-Al2O3 as the intermediate phase by a thermal process.
Further, based on the Joule heating technique, an alternating current sintering (ACS) process has been developed that shows the ultrafast and pressureless sintering of these α-Al2O3NPs into alumina ceramics with nanoscale grain size and improved strength and hardness.
A calcination process was also developed to totally remove the carbon black or formed flash graphene, and pure phased α-Al2O3 was obtained. In embodiments, the synthesized α-Al2O3 was shown to have a surface area up to 65 m2/g, that means that these materials are useful in the applications of catalyst support and high-strength ceramics.
Phase Transformation Synthesis
Methods for ultrafast synthesizing of corundum nanoparticles (i.e., the transformation from γ-Al2O3 (as well as γ-AlOOH) to α-Al2O3 can include the following.
Since the γ-Al2O3NPs precursors are electrically insulative, commercial carbon black (CB) was used in embodiments as the conductive additive. For instance, the mixture of γ-Al2O3NPs and CB were compressed inside a quartz tube between two graphite electrodes. See
The CB also works as separators to avoid the agglomeration of Al2O3NPs during heating. The resistance was controlled by the compressive force on the two electrodes, which is shown in TABLE II.
The electrodes were connected to a capacitor bank with capacitance of C=0.624 F and charging voltage up to V0=500 V. The discharge circuit was a series resistor-inductor-capacitor circuit with the characteristic time of τ=0.1 ms, which permitted the PDC with frequency of f=1000 Hz.
Joule heating affects the entire electric conductor; for a homogeneous conductor, the current density is uniform so the Ohmic dissipation enables the homogeneous temperature distribution throughout the sample. [Johnson 2011]. However, when an electrical field is applied to an inhomogeneous medium, as in the composite of conductive CB and insulative Al2O3, the current and powder densities have strong spatial variation. [Soderberg 1987]. The power dissipation is substantially larger than the neighboring regions at some regions, which are termed resistive hotspots 702 (illustrated in
By using this effect, the phase transformation from γ-Al2O3 to α-Al2O3 accompanied by the intermediate t-phase of δ′-Al2O3 at an average bulk temperature of ˜573 K in <1 s was realized. See pulsed direct current method 814 shown in
The liquid-feed flame spray pyrolysis method 811 produced α-Al2O3 at temperatures near 1873 K; however, the kinetically controlled process may render it difficult to access the pure phase (80-85% purity of α-phase). [Laine 2006]. Traditional heating methods that supply heat through the sample boundary, such as furnace annealing method 812, require an extended period to permit uniform heating; hence 1473 K and 10 to 20 h was necessary to complete the phase conversion. [Steiner 1971]. Other room-temperature nonequilibrium processes, such as high-energy ball milling method 813, have been reported to form α-Al2O3. [Amrute 2019]. Nevertheless, the γ-Al2O3 could agglomerate, which leads to loss of surface area during the extended time and high-energy collisions. [Zielinski 1993; Chauruka 2015].
The detailed phase transformation process of γ-Al2O3 was investigated by the PDC approach. See
Unlike previous report [Luong 2020] on the synthesis of graphene by the high-voltage flash Joule heating at a high temperature of ˜3000 K, the 60 V PDC did not provide enough energy to graphitize the CB.
Raman spectra are sensitive to even a monolayer of carbon [Wang 2008]; intriguingly, no characteristic Raman bands of carbon were detected after calcination at 700° C. (
Characterization of the Corundum Nanoparticles
The α-Al2O3NPs derived by PDC followed by mild calcination were further characterized in detail. Bright field transmission electron microscopy (BF-TEM) images showed the well-dispersed particles. See
Brunauer-Emmett-Teller (BET) measurement showed that the surface area of the α-Al2O3NPs is ˜65 m2 g−1. See inset 1401 of
D=6/(ρS) Eq. (1)
where ρ is the density of α-Al2O3 (3.96 g cm−3) and S is the specific surface area [Karagdov 1999].
The pore size determined from the N2 adsorption-desorption isotherm using the density functional theory (DFT) model indicates the distribution with high probability at 3 to 10 nm. See
Unlike the starting γ-Al2O3NPs that had hydrated surfaces, the synthesized α-Al2O3NPs surfaces were highly dehydrated because of the thermal process.
The XPS fine spectra showed the dominate O2− peak at a binding energy of ˜531.2 eV and single Al3+ peak at a binding energy of ˜74.0 eV from the α-Al2O3NPs. See
Resistive Hotspot Effect
The composition of the inhomogeneous media can be important for local power dissipation during the PDC process. To quantitatively show the effect of the composition on the phase transformation, a series of precursors with different mass ratio of γ-Al2O3 and CB were treated by PDC under the same voltage and time.
The phase transformation degree was increased as the f(γ-Al2O3) increased from 0.41 to 0.73; the phase-pure α-Al2O3 was obtained at f(γ-Al2O3)˜0.73. Further increase in the f(γ-Al2O3) to >0.78 led to no phase transformation.
To explain the f(γ-Al2O3)-dependent phase transformation, the electrical conductivity and temperature were measured. The conductivities were determined based on the measured resistance (R) and the feature size of the samples. TABLE II;
where V is the voltage, and a is the conductivity of the sample.
Since the start voltages were fixed to V0=60 V, the power was proportional to the conductivity of the sample. Intriguingly, the phase pure α-Al2O3NPs were obtained at a low average bulk temperature of ˜573 K with f(γ-Al2O3)˜0.73.
Such a low temperature was not supposed to trigger the phase transformation from γ- to α-Al2O3 with a high activation energy of ˜485 kJ mol−1. [Steinr 1971]. Moreover, the higher phase transformation degree at a lower temperature is counterintuitive.
To explain the phenomenon, a numerical simulation was conducted based on the finite element method (FEM) on the current density distribution of the γ-Al2O3/CB composite during PDC process. As shown in
Q∝j
2
R Eq. (3)
The large thermal dissipation in the regions with high current densities leads to the hotspots near γ-Al2O3NPs with much higher temperature than the bulk regions, which triggers the phase transformation. A shown in
Topotactic Transition Pathway
To provide deeper insight into the topotactic transition pathway, thermodynamic analysis of the three Al2O3 phases were conducted based on DFT. The bulk energy and surface energy of the three Al2O3 phases were calculated.
The ultrafast, pulsed, and low-temperature PDC process to a large extent avoids mass transfer and grain coarsening during the phase transformation process.
To gain insight into the structural origin of the phase-dependent bulk and surface energy, the partial charge density contour at the highest bands (0.3 eV below the Fermi levels) of the surface states of the three Al2O3 phases were plotted.
Applications
Accordingly, for the synthesis of corundum nanoparticles, the present invention provides, among other things, an ultrafast synthesis, which is within 1 second, and is much faster than any reported methods, which requires at least several hours. The corundum (α-Al2O3) nanoparticles resulting from the present invention have small particles size and high-surface area, which can utilized in a number of applications, such as for stable catalysis support and in ceramics with high fracture strength and toughness.
For instance, one prominent application of α-Al2O3NPs is as a precursor for sintering nanometer-grained alumina ceramics (i.e., ultrafast ACS for nano-grained alumina ceramics). The typical alumina ceramics sintering processes occur under high-pressure and high-temperature conditions (HP-HT), such as hot isostatic pressing, [Mizuta 1992], spark plasma sintering [Balima 2019], and pulse electric current sintering [Zhou 2004]. The high pressure, usually several GPa, retains the grain growth and advances densification [Wang 2013], which can be a main factor for dense ceramic sintering using coarse grained precursors. However, the HPHT process is not suitable for complex structures. The nanocrystalline precursors could undergo the pressureless sintering yet it would suffer from an elevated sintering temperature and prolonged time (>10 h). [Guo 2016; Cao 2017; Li 2006]. Very recently, an ultrafast high-temperature sinter method [Wang 2020] based on direct current heating was reported for the rapid screening of ceramics.
Here, based on the Joule heating technique, an alternative current sintering (ACS) process has been created for ultrafast sintering of the alumina ceramics. The ACS system is capable of providing stable and high energy output with voltages up to 63 V and currents up to 100 A (
Two separated, highly graphitized carbon papers 1801a-1801b in
The XRD patterns confirm the pure α-phase of the alumina ceramics.
Accordingly, the ACS process can be utilized in the sintering of functional ceramics, porous ceramics, or for materials screening. [Wang 2020].
Effectiveness and Scalability
Being a highly efficient energy supplies technology, Joule heating has a coefficient of performance of 1.0. The localized heating by resistive hotspots in PDC makes the process more effective because most of the electrothermal energy was directly targeted to the phase transformation, making the synthesis possible with a low energy input of ˜4.77 kJ g−1 or 0.027 $ kg−1 in electrical energy cost. Moreover, the PDC process can be scaled by adjusting sample cross-sectional area and the PDC voltage. A synthesis of α-Al2O3NPs up to 1.4 g-scale has been performed. See
Recovery of Metal From E-Waste
The present invention includes flashing Joule heating [see Luong 2020; Stanford 2020; Tour PCT '000 Application] for ultrafast processes to recover metals (precious metals) from waste (such as e-waste). Waste can be mixed with carbon black, then subjected to ultrafast Joule heating flashing. According to the Ellingham diagram, multiple precious metals are reduced to elemental metal by the carbothermic reaction. The recycling process is ultrafast, within seconds. Of import, the process is a totally dry process without any solvents, and hence is extremely environmentally friendly.
Synthesis Processes
Methods for ultrafast synthesis to recover metal from waste can include the following.
The method can include preparation of the electronic wastes for flashing. For instance, a printed circuit board (PCB) from a used electronic printer was used as the starting materials. The PCB board was first cut into pieces and then crushed into small particles. Ball milling was used to grind it to a microscale fine powder, which was then available for flash Joule heating by adding carbon black (or other carbon materials as discussed above) and treated as described below in a flash Joule heating apparatus.
Evaporative Separation
It has been discovered that the different vapor pressure of metals—compared to that of substrate materials (carbon, ceramics, and glass)—enables the separation of metals from e-waste. This is termed “evaporative separation.” The high vapor pressure of precious metals is obtained by an ultrafast flash Joule heating (FJH) process under vacuum. A subsecond current pulse is passed through the precursors, which brings the sample to an ultrahigh temperature of ˜3400 K, enabling the evaporative separation of precious metals. Halide additives are used to improve the recovery yield greater than 80% for Rh, Pd, and Ag, and greater than 60% for Au that are abundant in the tested e-waste. Alternatively, compared with directly leaching e-waste raw materials, by leaching the residual solids after FJH, the recovery yield is significantly improved with tens of times increase for Ag and few times increase for Rh, Pd and Au. The toxic heavy metals, including Cd, Hg, As, Pd, and Cr, could also be removed and collected, minimizing the health risks and environmental impact of the recycling process.
The FJH process to recover precious metals from e-waste involves three stages. See
To establish baseline concentrations, the PCB was digested using dilute aqua regia [Hong 2020], and the concentration of precious metals was determined by inductively coupled plasma mass spectrometry (ICP-MS). Among the precious metals, Rh, Pd, Ag, and Au are abundant with concentration of several to tens of parts per million (ppm), as shown in
In a FJH process, the mixture of PCB powder and ˜30 wt % CB was slightly compressed inside a quartz tube between two sealed electrodes.
The high-voltage discharge of the capacitor bank brings the reactant to a high temperature. With the fixed sample resistance of ˜1Ω. the current passing through the sample was measured under different FJH voltages. See
Since the resistance of the sample is much larger than that of the graphite and porous Cu electrode, the voltage drop was mainly imposed on the sample. Hence, the high-temperature region was limited to the sample and the FJH setup has good durability even though it can achieve a high temperature of >3000 K. Such a high temperature (>3000 K) volatilizes most of the non-carbon components. According to the calculated vapor pressure-temperature relationships (
As a result, the metals are evaporated, and the major carbon-containing components such as plastics were carbonized. [Luong 2020; Algozeeb 2020] The evaporated metal vapors were captured by condensation in a cold trap (
The content of the precious metals in the condensed solid was measured and the recovery yield was calculated.
Halide Assisted Improvement Of Recovery Yield
The high recovery yield of the evaporative separation relies on the generation of more volatile components. To improve the recovery, halides were used as additives because of the much higher vapor pressure of metal halides compared with the elemental metals. [Lide 2005]. Fluorine-containing components were first used as the additive, including the sodium fluoride (NaF) and polytetrafluoroethylene (PTFE, Teflon). With the additives, the recovery yields of Rh and Pd were improved to >80% and 70%, respectively. See
Chlorine-containing compounds were tried because of their abundance and low cost. Both sodium chloride (NaCl) and potassium chloride (KCl) were used (
Even with the F and Cl additives, the recovery yield of Au is <10%. Interestingly, the recovery yields of all four precious metals were improved when sodium iodine (NaI) was used as the additive; the recovery yield of Au was improved to >60% (
A total composition analysis of the collected metals in the cold trap was conducted. In both cases with or without the chemical additives, in additions to the precious metals, the most abundant metals were Cu with mass ratio >60 wt %, followed by other prominent metals in e-waste including Al, Sn, Fe, and Zn. Further purification and refining could be done by selective precipitation, solvent extraction, and solid-phase extraction, which are commercially well-established practices and are known in the art. [Ueda 2016].
The morphology and chemical composition of the condensed solids were characterized using scanning transmission electron microscopy (STEM) and energy dispersion spectroscopy (EDS). The elemental maps showed the clustered alloy particles of Rh, Pd, Ag, and Au (FIG. 31G), which were formed by the ultrafast heating and rapid cooling of the FJH process. This is similar to the case of the carbothermic shock synthesis of high-entropy alloy nanoparticles, which could be potentially used in catalysts. [Yao 2018]. In other regions, the precious metals spreading over the entire product was also observed. Moreover, the XPS analysis of the collected volatiles showed that Ag and Au were mainly in the elemental state, while elemental state and higher oxidation state coexisted for Rh and Pd, presumably due to their different chemical reactivity.
Improved Leaching Efficiency Of Precious Metals
Apart from the condensation of the volatile composition, the other pathway to recover the precious metals was by leaching the residual solids obtained by FJH. See
Based on the pressure drop and the size of the FJH chamber, the gas diffusion was simulated under different pressures (Pout) (
The leaching of the residual solids after FJH (denoted as PCB-Flash) was started at 120 V and atmospheric pressure using dilute acids (1 M HCl, 1 M HNO3). The leachable content of Rh, Pd, and Ag in PCB-Flash was substantially higher than that in the PCB raw materials (
The thermogravimetric analysis (TGA) of the PCB-Flash showed that the carbon could be removed in air at −700° C. (
The XPS analysis showed the efficient removal of carbon by calcination (
The mechanism of the improved leaching efficiency by FJH is shown in
The effect of the FJH voltage and pressure on the recovery yield were evaluated. It was found that the modest FJH voltages between 30 to 50 V led to the best recovery yield (
Removal and Collection of Toxic Heavy Metals
Removal of toxic components is another major concern for 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, the heavy metals, including Cr, Pb, Cd, As, and Hg, have much higher vapor pressures and lower boiling points (
After one FJH, the heavy metal contents in the remaining solid (PCB-Flash) were greatly reduced (
The concentration of heavy metals in the residue solids could be further reduced by multiple FJH reactions. After one FJH reaction, the concentration of Hg was reduced to below the safe limit of Hg in soils for agriculture (0.05 ppm) (
Metal Separation
The above-described processes utilizing the evaporative separation scheme is discussed directed to the recovery of metals from e-waste. Nevertheless, such processes could exhibit the capability for the separation of metals. Calculation shows that large separation factors up to ˜105 could be realized for most metals with large vapor pressure differences. The chart of
The different recovery yields of precious metals (
The chemical additives (
The separation ability of the evaporative separation scheme could be further improved by progressively increasing the FJH temperature.
Carbothermic Reduction
The flash Joule heating process can also be used for carbothermic reduction of metal from oxide. Before recovery, various metal oxides were used that showed the availability to recovery metal by the flash Joule heating method. As shown in
In certain embodiment of the present invention, the process can include a mechanism used to trap the metals in waste. For example, the mechanism can use reduced pressure and have the volatilized metals, metal carbides, metal oxides, or other metal complexes volatilize out of the reaction chamber enter a cold trap upon flash Joule heating of the source. The cold trap can be, but need not be, liquid N2. Even at room temperature, these can be collected in the trap.
Further, for example, the mechanism can use atmospheric or higher pressure (such, as, for instance, 10 atmospheres or 20 atmospheres), and have the metals remain in with the newly formed graphene. The graphene can be calcined away (such as, for instance, at 700-800° C. in air), leaving the metals (or metal oxides, etc.) isolated. Or the graphene can be oxidized away chemically, like with HNO3. For this latter mechanism, a pressure release valve can be used at the end of the electrode-hole assembly for the flash Joule heating process. Some of the metals being recovered have very high boiling points and they will stay with the carbon, especially at the higher pressures utilized
Designs and Scalability
System 3800 is a pressurizable flash Joule heating cell that has a gas collector 3818 should gas overpressure ensue. In some embodiments, system 3800 utilizes electrodes having 5/16 inches or 8 mm diameter. Conduits can have ⅛ inch outer diameter.
In system 3800, the two brass electrodes with O-ring grooves are inserted into the quartz tube that is tightly wrapped with a compression spring to put the quartz under compression and resist the outward force of the pressure. One electrode is hollow, with a PTFE tube inserted to provide a smooth and continuous exit path. A reducing Swagelok fitting provides a pressure and vacuum tight seal to the PTFE tube, which exits the electrode without a joint. System 3800 is capable of withstanding tens of atmospheres of pressure. Generally, with respect to pressure, the limiting factor is the quartz tube, and how well a strong spring can prevent breakage. The twin-screw supporting frame also should be sufficiently robust to resist the thrust when the sample is pressurized or when pressure is created by the flash. The quartz tube can be replaced by any non-conductive tube, and crosslinked polyethylene has also been used since the temperature reach on the tube is generally below 250° C. and generally for less than 1 second. While not shown in
Rubber bushings between the nut and the support frame can be useful in absorbing the shock when short-duration flashes are used.
System 3800 can be sealed with O-rings. Silicon O-rings are heat resistant and even with overheating, do not melt but tend to harden, and should maintain a seal. Because no hot gases can typically flow past the O-rings, they do not overheat. While discoloration of the first O-ring has been observed, the double O-ring remained sealed.
System 3800 can be fully evacuated and would hold pressure following the flashing of the sample 3813 as the gases exited into a heavy wall glass pressure tube. In some embodiments, a right angle joint can be used so that the gas exhaust would not interfere with the end connections of the electrodes. However, if particulates or nanoparticles are ejected, a straight exit tube is generally preferred. System 3800 shows a straight and continuous conduit 3814 (PTFE exit tube), and the wires are connected with rings on threaded brass electrodes 3808a-3808b.
System 3800 uses of the twin-screw translation, which provides consistent alignment of the electrodes. It was found that for single-screw translators, when pressure or force is applied, the electrodes angle upward, which in turn had put strain on quartz tube 3809. The twin screws are connected by timing sprockets and a belt 3801 for simultaneous thrust, and can be driven either manually or with a stepper motor.
As for the vacuum and gas supply, the tubing that exits the end of the hollow electrode can be connected through valves to vacuum 3823, a gas supply 3824, and a pressure gauge 2925. The gas supply can be inert, or be used to infuse reagents into the sample like hydrogen, methane, or other reactive species like halocarbons, ammonia, boron compounds, etc. These can be added to the porous carbon/graphene in a subsequent flash.
The pressure relief can be preset for system 3800. Adjustable pressure relief valve 3817 determines the ultimate pressure on the sample 3813. The cell can be fully pressurized before the flash, or allow the flash to generate high pressure. The opening pressure is set by a spring and threaded cap on the valve, and when the pressure exceeds the set force of the spring, the valve opens and the gases enter the gas collector, which was evacuated previously. Subsequently, the gases can be analyzed, or just pumped away. Pressure gauge 3826 and the volume of gas collector 3818 provide information on the total yield of gases. Gas collector 3818 also has a pressure relief valve 3821 connected to a vent 3820 in case of excessive gas production.
As for the effect of wide range of pressures that can utilized by system 3800, with the sealed flash chamber and adjustable relief valve, the effect of a wide range of pressures on the yield of the flash has been evaluted. Because of the pressure, volatile additives can be incorporated in the sample and will not depart until the relief valve opens.
System 3800 can be utilized for a variety of particle/metal collection methods. For instance, when it is desirable to collect particulates, the PTFE tube can go straight (without bends) into 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 tube while the non-condensable gases can be pumped away. This can be used to collect volatile metals and metal compounds, which will aggregate as they cool and form nanoparticles that will adhere to particle collector 3816.
This design can be varied and modified as needed with materials changes and design changes depending upon the intended use.
The cost and benefit of the FJH processing were evaluated since economic incentives are the main driver for waste recycling. [Awasthi 2019]. FJH is a highly efficient heating process due to the ultrafast heating/cooling rate, the direct sample heating feature, and the short reaction duration, compared to traditional smelting furnaces where large amounts of energy are used to maintain the temperature of the whole chamber. [Khaliq 2014]. The FJH method has an energy consumption of ˜939 kWh ton−1, which is ˜ 1/500th of that for a lab-scale tubular furnace, [Balaji 2020], and ˜ 1/80th of that for a commercially used Kaldo furnace in industrial scale [Theo 1998]. Hence, the FJH process for e-waste processing have advantages over traditional pyrometallurgical processes.
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.
While the schemes of
Through the use of an automation system integrated with a FJH setup, a production rate of >10 kg day−1 has already been realized.
Accordingly, for the metal recovery from e-waste, the present invention provides, among other things, (i) the flash Joule heating is a dry process without usage of any solvent, which endows it as environmentally friendly; (ii) the flash Joule heating can recover most of metal elements in waste in one step, which is hard to realize by other methods; (iii) the flash Joule heating process also removes nearly all the harmful materials in waste, so it will not result in secondary pollution; and (iv) the flash Joule heating process uses far less electrical energy than a furnace since the heating durations are short and the is little energy that escapes the sample being flash Joule heated.
The precious metals recovered from e-waste, are very important raw materials for various industry. Indeed, mixtures of metals like this are quite valuable since many mining companies already implement automated systems that do the base metal separations.
Moreover, the recovery process removes the harmful materials such as heavy metal within the waste, which has import for solving the environmental issues raised by those waste.
Similar situations to e-waste likewise pertains to ores, fly ash, and red mud (red mud is more recently referred to as bauxite residue), again because rare earth elements (REE) are strategic resources in modern electronics, clean energy, and automotive industries. Thus, the above-described methods and systems can likewise be implemented for the recovery of metals from ores, fly ash, and bauxite residue (red mud).
Embodiments of the present invention include the ultrafast electrothermal process based on flash Joule heating (FJH) to activate the ores, fly ash, and red mud to improve the acid extractability of REE simply using a mild acid such as 0.1 M HCl. A pulsed voltage in seconds brings the raw materials to a temperature of ˜3000° C., leading to the thermal decomposition of the hard-to-dissolve REE phosphates in CFA into highly soluble REE oxides, and the carbothermic reduction of REE components to highly reactive REE metals. The activation process can enable the increase of REE recovery yields to ˜206% for class F-type CFA (CFA-F) and ˜187% for class C-type CFA (CFA-C) compared to directly leaching the raw materials with more concentrated acids. The activation strategy is feasible for various secondary wastes, as demonstrated by coal fly ash (CFA) and red mud (bauxite residue (BR)). The rapid FJH process is scalable and highly energy-efficient with a low electrical energy consumption of (such as 600 kWh ton−1 or $12 ton−1) enabling a profit percentage of greater than 10 times.
FJH System and Process
The FJH system that can be utilized is similar to those described and discussed above. For instance, an electrical diagram of the FJH system that can be utilized for fly ash is shown in
In a typical experiment, the secondary wastes (CFA, BR) were mixed with carbon black with the mass ratio (such as 2:1) by using the ball miller (MSEsupplies, PWV1-0.4L). The carbon black served as the conductive additive. 200-mg mixture (133 mg waste and 67 mg CB) was added into a quartz tube (inner diameter of 8 mm and outer diameter of 12 mm). The resistance was controlled by compressing the two electrodes. The samples were loaded into a jig (
Acid-Extractable REE Content In CFA
There are two types of CFA categorized by the chemical composition, CFA-F, with the total content of SiO2, Al2O3, and Fe2O3>70 wt %, and CFA-C, with a higher abundance of CaO. [Liu 2019]. In examples evaluated herein, CFA-F was collected from the Appalachian Basin (App), and CFA-C from the Powder River Basin (PRB) [Taggart 2016], both in the US.
CFA is composed of primary amorphous phases (60-90%) [Zhang 2020], and the remaining crystalline materials include mainly quartz and mullite, as shown by the X-ray diffraction patterns (XRD).
The total quantification of REEs in CFA was done by the HF:HNO3 digestion method. [Taggart 2016]. The total REE content, ctotal(CFA Raw), was 516±48 mg kg−1 for CFA-F, and 418±71 mg kg−1 for CFA-C.
The acid extractability of REE from CFA-C was higher than that from CFA-F. This is consistent with [Liu 2019], which attributes the higher extractability to the higher content of easy-to-dissolve REE species like REE oxides in CFA-C. The morphology image by scanning electron microscopy (SEM) of CFA-F is shown in
Improved Recovery Yield of REE from CFA by Electrothermal Activation
In the electrothermal activation process by FJH, CFA raw materials were first mixed with carbon black (CB), which serves as the conductive additive. The mixture of CFA and CB (˜30% CB) was loaded inside a quartz tube between two graphite electrodes.
In a typical discharging process with FJH voltage of 120 V, R of 1Ω, and discharging time (t) of 1 s, the current curve passing through the sample was recorded with the peak current at ˜120 Å followed by a current plateau at ˜7 Å.
A series of FJH voltage ranging from 50 V to 150 V were applied.
For CFA-C, under the optimized FJH condition, the acid leachability of REE from the activated CFA-C was measured to be Y ˜103% using the HCl leaching procedure (1 M HCl, 85° C.) (
Even using a dilute acid (pH 1, 0.1 M HCl), the recovery yield of REE from the activated CFA-C remains Y ˜94%, significantly higher than that of the CFA-C raw materials (Y0˜54%). This would render far more manageable wastewater streams.
For individual REE, with the FJH activation process, the acid leachability was improved ranging from 170% to 230% for CFA-F (
As 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. Hence, the use of carbon black does not induce significant error into these measurements. In practical applications, the carbon black could be substituted with anthracite coal or any other inexpensive sources of mildly conductive carbon, but the REE content in that source should be considered in yield calculations.
The Mechanism of the Improved REE Extractability
The mechanism of the improved REE leachability by the electrothermal activation process was investigated. The REE speciation and distribution in CFA determine the REE extractability. REE phosphate, including monazite and xenotime, is one of the primary counterions of REE in coal. [Liu 2019; Stuckman 2018]. REE phosphates are rather stable components, and no melting or thermal dissociation occur up to −2000° C. in air. [Ushakov 2001; Hikichi 1987]. The coal-fire combustion temperature typically ranges from 1300° C. to 1700° C. [Stuckman 2018]. As a result, the REE-bearing trace phases, including monazite and xenotime, persist in CFA. [Kolker 2017; Smolka-Danielowska 2010]. The REE could also be partitioned and encapsulated into the glass fraction of CFA by diffusion into the melt (e.g., aluminosilicates) formed at the coal boiler temperature. [Dai 2014]. Those hard-to-dissolve REE phosphates and glass phases are detrimental for REE extraction [Liu 2019], while REE oxides and carbonates in CFA are relatively easier to extract by acid leaching.
The high temperature of −3000° C. generated by the FJH process, which is significantly higher than the coal boiler temperature, could thermally degrade the REE species. Lanthanum phosphate (LaPO4) and yttrium phosphate (YPO4) were used as representatives for REE phosphates. As shown in
To further provide insight on the solubility of REE phosphates and oxides, the dissolution curves as a function of pH were calculated.
In addition to the thermal decomposition of REE phosphates, the ultrahigh temperature could also trigger the thermal reduction of REE compounds. According to the Ellingham diagram (
Y2O3 and La2O3 were used as representatives to verify the carbothermic reduction of REE oxides by the FJH process. The fitting of the XPS fine spectrum of Y2O3 after FJH shows four peaks.
The XPS analysis proved the reduction of Y2O3 to Y metal by the FJH process, while the small ratio of Y2O3 might be from the surface oxidation. Similarly, the fitting of XPS fine spectra of La2O3 precursor and La2O3 after FJH (
This suggests that the required temperature for the thermal activation is >2000° C. for thermal decomposition of REE phosphates, and >2500° C. for carbothermic reduction of REE oxides, which also provides insight on the voltage dependent REE leachability.
In addition to speciation, the REE distribution also affects the extractability, where the REE encapsulated in or distributed throughout the glass phases are hard to dissolve. [Liu 2019]. The FJH permits an ultrafast heating and rapid cooling (>104 K s−1,
Generality of the Electrothermal Activation Process
The electrothermal activation process is applicable to other waste products for REE recovery, including BR [Deady 2016; Rivera 2018; Reid 2017] and e-waste (including as discussed above) [Maroufi 2018; Deshmane 2020; Peelman 2018].
BR (red mud) is the waste product of the Bayer process for alumina production. BR is one of the most abundant industrial wastes with 3 billion tons already stored in waste ponds and an additional 150 million tons produced each year, yet just 3% is currently recycled [Service 2020]. BR contains a significant amount of REE, for example, a total REE content of ˜1000 ppm is found in BR from MYTILINEOS “Aluminum of Greece.” [Deady 2016]. The BR is a dried powder with fine particle size, and has major components including Fe2O3, CaCO3, FeO(OH), and SiO2.
Similar to CFA, the REE extractability of the BR after the electrothermal activation process is also dependent on the FJH voltage.
This FJH strategy was also applied for for activating e-waste and it is shown here as a complement to the methods described that used no mild acid leaching. More than 40 million tons of e-waste are produced globally each year due to the rapid upgrade of personal electronics, with <20% being recycled. [Zeng 2018]. REE are widely used in electronics in permanent magnets [Deshmane], and capacitors [Alam 2012]. In turn, the recovery of REEs from high-grade e-waste has its economic feasibility compared to REE mining from ores.
The e-waste used in this FJH process was a printed circuit board (PCB) from a discarded computer.
Different from CFA or BR, the REE species in e-waste are usually in the form of easy-to-dissolve REE metals or oxides. [Alam 2012]. However, the REEs are usually embedded into the matrix materials due to the laminated configuration of the electronics, which could hinder the REE extraction by the hydrometallurgical process. The FJH process could expose the metals by cracking the matrices, accelerating the leaching rate and extent of metal extraction.
Scalability and Utility
The FJH process for REE recovery is scalable. To maintain a constant temperature when scaling up the sample mass per batch, the FJH voltage or the total capacitance of the capacitor bankcan be increased. A production rate of >10 kg day−1 by the batch-by-batch process has already been realized. The FJH process can be integrated into the continuous production manner for further automation, such as by using the schemes shown in
The economics since the profit margin is often the sustainer of recycling. Due to the direct sample heating feature, short duration, and rapid heating/cooling rate, embodiments of the FJH process are highly energy-efficient with a low electrical energy consumption of 600 kWh ton−1 or $12 ton−1, enabling a profit percentage of >10× compared to directly leaching the raw materials.
For further refining, the removal of dissolved impurities, including mainly Al, Si, Fe, Ca, and Mg, in the REE-containing leachate and subsequent separation are needed. It was observed that the content ratio of REE and impurity (c(REE)/c(Impurity)) in the leachate was improved with the FJH process in most cases, indicating that the FJH process would also be beneficial for the subsequent REE separation.
Ores
Since monazite, (Ce, La, Y, Th)PO4, and xenotime, YPO4, are the main commercial sources for REE production [Cheisson 2019], embodiments can also be used for REE mining to improve the leachability from REE ores. Commercially, alkaline digestion (70% NaOH, 140-150° C.) is the main leaching technology for monazite [Peelman 2016], or acid baking (concentrated H2SO4, 200° C.) for monazite and xenotime [Kim 2016]. This FJH process could be faster and less dependent on the use of concentrated bases and acids. Existing individual elemental separation technologies, such as solvent extraction and ion exchange [Xie 2014] can utilized to work with the REE mixtures obtained by FJH since these are often less contaminated than those generated through traditional mining methods.
While embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described and the examples provided herein are exemplary only, and are 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 description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.
The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference in their entirety, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.
Amounts and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of approximately 1 to approximately 4.5 should be interpreted to include not only the explicitly recited limits of 1 to approximately 4.5, but also to include individual numerals such as 2, 3, 4, and sub-ranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as “less than approximately 4.5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the presently disclosed subject matter belongs. Although any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the presently disclosed subject matter, representative methods, devices, and materials are now described.
Following long-standing patent law convention, the terms “a” and “an” mean “one or more” when used in this application, including the claims.
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 indicated to the contrary, the numerical parameters set forth in this specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently disclosed subject matter.
As used herein, the term “about” and “substantially” when referring to a value or to an amount of mass, weight, time, volume, concentration or percentage is meant to encompass variations of in some embodiments ±20%, in some embodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1% from the specified amount, as such variations are appropriate to perform the disclosed method.
As used herein, the term “substantially perpendicular” and “substantially parallel” is meant to encompass variations of in some embodiments within ±10° of the perpendicular and parallel directions, respectively, in some embodiments within ±5° of the perpendicular and parallel directions, respectively, in some embodiments within ±1° of the perpendicular and parallel directions, respectively, and in some embodiments within ±0.5° of the perpendicular and parallel directions, respectively.
As used herein, the term “and/or” when used in the context of a listing of entities, refers to the entities being present singly or in combination. Thus, for example, the phrase “A, B, C, and/or D” includes A, B, C, and D individually, but also includes any and all combinations and subcombinations of A, B, C, and D.
This application claims priority to U.S. Patent Appl. Ser. No. 63/082,592, filed Sep. 24, 2020, entitled “Ultrafast Flash Joule Heating Synthesis Methods And Systems For Performing Same,” which patent application is commonly owned by the owner of the present invention. This patent application is incorporated herein in its entirety.
This invention was made with government support under Grant No DE-FE0031794, awarded by the United States Department of Energy and under Grant No. FA9550-19-1-0296, awarded by the United States Department of Defense/Air Force Office of Scientific Research. The United States government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/052043 | 9/24/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63082592 | Sep 2020 | US |
