Method for the selective concentration and / or separation of metal compounds

Abstract

The present invention relates to a method for the selective concentration and / or separation of metal compounds from particulate material using magnetic nanoparticles modified with at least one organic acid in an aqueous solution, preferably with a pH value in the range of pH 3.5 to pH 11, and the use for the recycling of rare-earth compounds, preferably for the recycling of rare-earth oxides from fluorescent lamps.

Description

[0001] Method for the selective concentration and / or separation of metal compounds

[0002] The present invention relates a method for the selective concentration and / or separation of metal compounds from particulate material using magnetic nanoparticles and the use for recycling of metal compounds, preferably for the recycling of rare-earth oxides from fluorescent lamps.

[0003] Due to the diverse application of rare-earth elements (REE) in the field of high technology but also in the pharmaceutical industry, an increasing demand for these strategic raw materials is to be expected. Rare-earth elements, also rare-earth metals, are a group of metals including the elements of the third subgroup of the periodic table (with the exception of actinium) and the lanthanoids, in particular rare-earth elements include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).

[0004] Fluorescent lamps are one of the main applications of REE and their waste represents one of the highest values of secondary REE resources inside the European Union (Grohol and Veeh 2023). Due to European legal framework that regulates the release of hazardous substances to the environment, mercury-containing fluorescent lamps are efficiently collected as a separate stream of hazardous waste (Binnemans and Jones 2014). The estimated amount of REE waste from fluorescent lamps amounts to 25,000 tons in 2020. The collected lamps are crushed and the relatively unvaluable parts such as the glass tubes and the bulk metals are recycled. However, the very fine fluorescent material comprising REE cannot feasibly be recycled with current technologies and are stored in large containers instead.

[0005] One possible way to recycle the fluorescent material is direct chemical recycling by hydro- and / or pyrometallurgical treatment, which involves 1) dissolving the REE out of the fluorescent particles, 2) separating and purifying the individual REE and 3) manufacturing new particles from the purified REE (Binnemans et al. 2013).

[0006] The direct chemical recycling of REE from end-of-life fluorescent lamps is very energy and chemical intensive, because 1) the REE have to be dissolved out of the particles, 2) the individual REE have to be separated and purified and 3) new particles have to be synthesized from the pure REE. Furthermore, undesired reactions can occur during the direct chemical treatment of the waste powder mixture, such as the transformation of glass particles to soluble silicates. Currently, a direct chemical recycling of REE from end-of-life fluorescent lamps is not economically feasible. Another way to recycle the REE fluorescent material from end-of-life fluorescent lamps is the physically separation of the particles from the powder mixture obtained after crushing of the lamps. Various physical separation processes of REE materials from end-of-life fluorescent lamps have been reported in literature, including: pneumatic separation (Takahashi et al. 2001), flotation (Hirajima et al. 2005a), dense medium centrifugation (Hirajima et al. 2005b) and magnetic separation (Boelens et al. 2021). The particles very fine sizes, broad size distributions and similarity in terms of size, volumetric density and magnetic susceptibility, limit the applicability of these separation technologies as industrial solutions for the recycling sector.

[0007] The use of carrier magnetic separation with composite magnetic beads faces problems of feasibility, upscalability and process control, whereas the use of carrier magnetic separation with unstabilized iron oxide nanoparticles faces difficulties related to nanoparticle agglomeration and aggregation.

[0008] In case of magnetic separation, the magnetic velocity of a particle in an inhomogeneous magnetic field is linearly proportional to the particles' magnetic susceptibility ( ) and quadratically proportional to the particles' diameter (d): vmagnetic■ a ■ x ■ d2. For REE materials from end-of-life fluorescent lamps, due to their broad size distributions and similarity in terms of magnetic susceptibility, this implies that the magnetic velocity can be equal for two particles A and B if A > XB and dA < ds. Hence, the particles cannot simply be separated by the magnetic force because they have an equal response to the magnetic field.

[0009] Wada et al. have solved this problem by using the magneto-Archimedes method, in which the phosphor with highest T is levitated in a magnetic medium (Wada et a. 2014). However, this method requires the use of 7.5 wt% of MnCh aq. and a superconducting magnet and has a low processing speed, making it unviable as an industrial application. JP 5120963 B2 describes an approach using vertical magnetic field gradients in a high gradient separator and compensating for the broad particle size distribution by utilizing the gravitational force in addition to the magnetic force. However, this method requires a very carefully chosen configuration for each type of particle and the processing speed remains limited by the intrinsic properties of the weakly magnetic particles.

[0010] Boelens et al. have recently shown that composite magnetic beads can be used to readily separate REE compounds through selective electrostatic surface interactions between composite magnetic beads and target particles (Boelens et al. 2022). The use of magnetic carriers with a very high magnetic susceptibility allows separating the REE compounds with a very high velocity, even in a weak magnetic field with low magnetic field gradient and is more or less independent of the particles' intrinsic properties (e.g. particle size distribution, volumetric density and magnetic susceptibility). However, the composite magnetic beads are costly materials produced for diagnostic purposes and are not available in sufficient amounts for an industrial separation process. Furthermore, due to the large particle size of these beads, their interaction with the target material is difficult to control.

[0011] An alternative would be the use of iron oxide magnetic nanoparticles synthesized by a feasible coprecipitation method as magnetic carriers. However, these magnetic nanoparticles tend to agglomerate and aggregate, leading to a loss of free surface area and challenging conditions of process control. Carlos et al. describes the use of magnetite nanomaterials with a surface functionalisation, in particular with carboxyl, amine or thiol groups; for the removal of heavy metal ions from water, e.g. copper, chromium, mercury and arsenic (Carlos et al. 2013).

[0012] US 2015 / 368126 A1 describes a method and device for the selective removal of heavy ions, cations, anions or other elements from liquids using magnetic nanoparticles, in particular for the recovery of nanoparticles in wastewater treatment, wherein the nanoparticles can be modified, in particular with dextran (a natural polysaccharide) or fatty acids.

[0013] US 2017 / 128952 A1 discloses a method for separating a magnetic substance and a non-magnetic substance from a mixture using a high gradient magnetic separator, in particular a liquid, preferably a slurry containing solid materials containing the magnetically attractable substance, such as recycling samples, fluorescent substances, samples from fluorescent lamps, samples obtained from mines and packaging samples for food and pharmaceuticals, obtained by dispersing the solid materials in suspension liquids.

[0014] US 2020 / 384480 A1 describes polymer-functionalized particles comprising a magnetic core for use in the isolation and extraction of dissolved substances (“solutes”) such as rare earth metals, lithium and the like. The polymer-functionalized particles have a high resistance to agglomeration and degradation, even at high ionic strength and / or temperature. In addition to a method for producing the polymer-functionalized particles, a magnetic separation device for use and regeneration of the polymer-functionalized particles in the extraction of dissolved substances is disclosed. US 2020 / 384480 A1 further describes a possible modification of nanoparticles with a wide range of possible ligands for the chelating the solutes or stabilizing the chelate: “alkyl amines (e.g., diethylene-triamine (or DETA)), amoidoxime, phosphonic acid, sulfonic acid, carboxylic acids, resorcinol, formaldehyde, macrocycles (e.g., crown ethers like 14-crown-4-ethers; calixarenes; porphyrins; cyclodextrins; and the like).”

[0015] US 2023 / 219919 A1 discloses a "molecular recognition technology" (MRT) for the selective separation of lithium from natural and synthetic brines, leachates or other chemical mixtures, in particular electronic waste, using liquid-liquid extraction systems with extraction agents (e.g. small molecules or polymeric crown ethers) as the functionalization and chemical bonding of these extraction agents to solid sorbents for the sequestration of lithium.

[0016] DE 102017219800 A1 describes special biofunctionalized nanoparticles for the purpose of the separation of REE compounds and a method for selective separation of submicro- and microparticles using the biofunctionalized nanoparticles.

[0017] The object is therefore to provide a method for the selective concentration and / or separation of metal compounds from particulate material and / or a method for the selective concentration and / or separation of metal compounds with increased yield and / or purity of the targeted metal compound compared to the state of the art.

[0018] According to the invention, the object is solved by the method according to the independent claim. Advantageous embodiments of the invention are indicated in the dependent claims.

[0019] A first aspect of the invention is a method for the selective concentration and / or separation of metal compounds from particulate material comprising the following steps: a. Providing magnetic nanoparticles modified with at least one organic acid, b. Contacting the modified magnetic nanoparticles with at least one particulate material, preferably with a particle diameter in the range of 1 pm to 100 pm, comprising at least one metal compound in an aqueous solution, preferably with a pH value in the range of pH 3.5 to pH 11 , wherein the modified magnetic nanoparticles interact in the aqueous solution with the at least one metal compound by electrostatic interactions, c. Magnetic concentration and / or separation of the modified magnetic nanoparticles and the metal compound from the particulate material, and d. Removal of the modified magnetic nanoparticles from the at least one metal compound. Suitably, the method is carried out with a sequence of steps a), b), c) and d).

[0020] Advantageously, with the method according to the invention metal compounds can be separated from particulate material, preferably with a particle diameter in the range of 1 pm to 100 pm, for example from suspensions. Thus, the method for separating particles does not require any pretreatment of the particle mixture to be separated, during which the target material is dissolved. This results in a simplified method or a method that can be used for insoluble particle mixtures. Suitably, the metal compound exhibits at least partially a positive surface charge in an aqueous solution, preferably with a pH value in the range of pH 3.5 to pH 11. Based on this at least partially positive surface charge the metal compound can be separated from the particulate material mainly through electrostatic interactions with the modified magnetic nanoparticles and hence, an attachment to the modified magnetic nanoparticles. Suitably, the magnetic nanoparticles attach due to their lower size to the surface of the target material, the particulate material comprising the metal compound and preferably form a monolayer on the surface of the metal compound.

[0021] Advantageously, the method according to the invention has lower production costs, lower material requirements, higher process controllability and higher material reusability of the magnetic nanoparticles compared to the use of magnetic beads. The higher process controllability means that by the variation of the amount of magnetic nanoparticles (MNP), the formation of a MNP monolayer at the surface of the target metal compound is adjustable. Hence, a targeted magnetization of the target metal compound particle is possible. The less MNP are added, the less MNP interact with the surface of the target metal compound material, which results in a lower magnetization of the target metal compound particle. Compared to that, magnetic beads are much bigger and interact about one bead per one target particle. Using a high concentration of magnetic beads can result in contamination with other particles, which reduces the purity of the separation. Using a too low concentration of magnetic beads, not all the target metal compound particles are attached, which reduces yield of the separation.

[0022] As used herein, the term “metal” refers to a chemical element that is located in the periodic table of elements to the left of and below a dividing line from boron to astatine. Suitably, metals according to the invention also comprise metalloids and metal alloys.

[0023] As used herein, the term “metal compound” refers to a compound, in particular an oxide, comprising at least one metal. In embodiments, the compound comprises the at least one metal, preferably at least one rare-earth element, with a mass ratio of at least 10%, preferably at least 50%, more preferably at least 75%, in relation to the total mass of the compound. In embodiments, the metal compound is a metal oxide.

[0024] In embodiments, the metal compound comprises an element selected from the group comprising Li, Be, Al, Sc, Ti, V, Cr, Mn, Co, Ni, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ta, W, Re, Os, Ir, Pt and Au.

[0025] In embodiments, the metal compound is a compound of a heavy metal. As used herein, the term “heavy metal” refers to metals with relatively high densities, atomic weights, or atomic numbers, in particular with a density above 5 g / cm3. In embodiments, the metal compound comprises a heavy metal selected from the group comprising V, Cr, Mn, Co, Ni, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ta, W, Re, Os, Ir, Pt and Au.

[0026] In embodiments, the metal compound comprises a rare-earth-element selected from the group comprising Al, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

[0027] In embodiments, the metal compound is a compound of a rare-earth element. In embodiments, the metal compound comprises a rare-earth-element selected from the group comprising Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

[0028] In embodiments, the metal compound is a rare-earth oxide.

[0029] In embodiments, the metal compound is Y2O3:Eu3+or BaMgAlioOi?:Eu2+. In embodiments, the metal compound is Y2O3:Eu3+. In embodiments, the metal compound is AI2O3.

[0030] As used herein, the term “nanoparticle” refers to particles with a diameter in the range of 1 nm to 100 nm.

[0031] Advantageously, the synthesized and / or modified magnetic nanoparticles can be stored for years without any changes.

[0032] In embodiments, the magnetic nanoparticles comprise iron (Fe) or cobalt (Co). In embodiments, the magnetic nanoparticles are modified by coating a layer of graphene onto their surface.

[0033] Advantageously, Fe nanoparticles are biodegradable and therefore do not cause any problems with disposal. Disposal through targeted biodegradation with microbial organisms, organic acids are created, which can be used either for metal leaching or for the functionalization of magnetic nanoparticles. In embodiments, the magnetic nanoparticles are iron oxide nanoparticles. In embodiments, the magnetic nanoparticles comprise FeaO4 or Fe2Oa, in particular y-Fe2O3.

[0034] In embodiments, the magnetic nanoparticles are modified by coating a layer of silica onto their surface. The silica shell can be easily modified with various surface functional groups via covalent bonds between organo-silane molecules and silica shell.

[0035] In embodiments, the magnetic nanoparticles have a hydrodynamic diameter in the range of 5 nm to 100 nm, preferably 10 nm to 50 nm, more preferably 10 nm to 30 nm. Suitably, the hydrodynamic diameter is measured using Dynamic Light Scattering (DLS).

[0036] Advantageously, the modification with an organic acid enables the selective interaction with the target material (metal compound). Further advantageously, the modification with an organic acid prevents agglomeration of the magnetic nanoparticles, so that the magnetic nanoparticles can form a monolayer on the surface of the metal compound.

[0037] In embodiments, the at least one organic acid is selected from the group comprising citric acid, gluconic acid and fatty acids.

[0038] In embodiments, the magnetic nanoparticles modified with at least one organic acid have a hydrodynamic diameter below 200 nm.

[0039] In embodiments, the particulate material comprises the metal compound and has a particle diameter in the range of 1 pm to 100 pm, preferably in the range of 1 pm to 50 pm.

[0040] In embodiments, the particulate material is selected from particulate, preferably fine particulate, material of waste of electronic and electrical equipment (WEEE), in particular pulverized fluorescent lamps, pulverized solar cells, pulverized permanent magnets and particulate residual materials, preferably fine particulate residual materials, of catalysts or batteries. As used herein, the term “fine particulate” refers to material with a particle diameter in the range of 1 pm to 100 pm.

[0041] In embodiments, the pulverized solar cells are CIGS solar cells. As used herein, the term “CIGS solar cell” refers to a type of solar cell whose absorber is made of copper-indium-gallium- diselenide (CulnxGa(i-X)Se2, CIGS). In embodiments, the concentration of the at least one metal compound in the aqueous solution is in the range of 0.1 g / l to 1 ,000 g / l, preferably in the range of 5 g / l to 500 g / l, more preferably in the range of 10 g / l to 200 g / l.

[0042] In embodiments, the aqueous solution is water, in particular tap water or deionized water, or a buffered aqueous solution, preferably an aqueous solution of ammonium chloride and / or a phosphate buffered saline (PBS). In embodiments, the buffered aqueous solution has a salt concentration in the range of 5 mM to 150 mM.

[0043] In embodiments, the pH value of the aqueous solution is in the range of pH 3.5 to pH 11 . Suitably, the pH value is adapted according to the isoelectric point of the metal compound or the range, wherein the compound has a positive zeta potential. As used herein, the term “Zeta potential” refers to the electrokinetic potential of a colloidal dispersions, in particular an aqueous solution of the metal compound. The Zeta potential can be determined by measuring the electrophoretic mobility or the electrokinetic sonic amplitude. Suitably, the pH value is adapted according to the pkaof the organic acid used for the modification of the magnetic nanoparticles or the range, wherein the organic acid is at least partially dissociated.

[0044] In embodiments, the pH value of the aqueous solution is in the range of pH 3.5 to pH 11 , preferably in the range of pH 5 to 8. Suitably, the step b) is carried out with a pH value in the range of pH 3.5 to pH 11 , preferably in the range of pH 5 to 8, when AI2O3 is concentrated and / or separated with the method according to the invention.

[0045] In embodiments, the pH value of the aqueous solution is in the range of pH 3.5 to pH 8. Advantageously, rare-earth oxides have a positive surface charge in a pH range of 3.5 to 8. Suitably, the step b) is carried out with a pH value in the range of pH 3.5 to pH 8, when a rare- earth oxide is concentrated and / or separated with the method according to the invention.

[0046] In embodiments, the pH value of the aqueous solution is in the range of pH 3.5 to pH 8, preferably in the range of pH 5 to 8. Suitably, the step b) is carried out with a pH value in the range of pH 3.5 to pH 8, in the range of pH 5 to 8, when Y2C>3:Eu3+is concentrated and / or separated with the method according to the invention.

[0047] In embodiments, the pH value of the aqueous solution is in the range of pH 3.5 to pH 6, preferably in the range of pH 5 to 6. Suitably, the step b) is carried out with a pH value in the range of pH 3.5 to pH 6, in the range of pH 5 to 6, when BaMgAlioOi?:Eu2+is concentrated and / or separated with the method according to the invention.

[0048] In embodiments, the modified magnetic nanoparticles are contacted with the at least one particulate material in a mass ratio in the range of 0.02% to 200%, preferably 0.04% to 40%, more preferably 0.1 % to 10%.

[0049] The mass ratio of the magnetic nanoparticles (MNP) to the target material or particle (metal compound) - - required for the formation of a monolayer of MNP on the surface of the target particle target particle is calculated the following: with nMNPthe number of MNP bound to a single target particle, mMNPthe mass of a single MNP and mtarget particlethe mass of a single target particle.

[0050] For equation (1), the number of MNP bound to a single target particle: with Atarget particiethe surface area of a single target particle, Aprojected>MNPthe projected surface area of a single MNP.

[0051] For equation (2), the surface area of a single target particle: with dtarget particle^ diameter of the target particle.

[0052] For equation (2), the projected surface area of a single MNP: with dMNPthe diameter of the MNP.

[0053] For equation (1), the mass of a single MNP: with pMNPthe density of the MNP.

[0054] For equation (1), the mass of a target particle: with Ptarget particle^ density of the target particle.

[0055] Hence:

[0056] In embodiments, the modified magnetic nanoparticles are used in step b) with a concentration in the aqueous solution in the range of 1 mg / L to 6,000 mg / L. Suitably, the concentration of the modified magnetic nanoparticles is adjusted according to the target material (metal compound), in particular the concentration and / or particle size of the target material.

[0057] In embodiments, the step b) is carried out under stirring. In embodiments, the step b) is carried out under stirring with a stirring speed in the range of 300 rpm to 1 ,000 rpm, preferably 500 rpm to 800 rpm.

[0058] In embodiments, step b) and / or c) are carried out with a temperature in the range of 0°C to 100°C, preferably in the range of 5°C to 60°C, more preferably in the range of 10°C to 40°C.

[0059] In embodiments, the magnetic concentration and / or separation is carried out using a magnetic field with a strength in the range of 1 mT to 1 T. The strength of the magnetic field is given by the magnetic flux density, whose SI unit is Tesla (T). The strength of the magnetic field can be measured with a gaussmeter. The magnitude of the magnetic force on a particle in a magnetic field is calculated with equation (7): where Fmrepresents the magnitude of the magnetic force, V represents the volume of the particle, B

[0060] M = — represents the magnetization of the particle, B represents the magnetic field, represents Ho the magnetic susceptibility of the particle, p0represents the magnetic permeability of vacuum, and VB represents the gradient of the magnetic field.

[0061] In embodiments, the magnetic concentration and / or separation is carried out using a heterogeneous magnetic field, preferably a high-gradient magnetic field. In embodiments, the magnetic concentration and / or separation is carried out using a magnetic field with a magnetic field gradient in the range of 1 T / m to 5,000 T / m. As used herein, the term “high-gradient magnetic field” refers to a magnetic field with a magnetic field gradient in the range of 100 T / m to 5,000 T / m. In embodiments, the high-gradient magnetic field is produced by a non-ferromagnetic object, such as steel wires, steel balls, steel plates or steel wool, placed in a fixed magnetic system. As used herein, the term “non-ferromagnetic object” refers to an object comprising a metal, which cannot generate an own magnetic field, but are magnetized inside the magnetic field of an external magnetic system, such as a permanent magnet or electromagnet.

[0062] Hoffmann et al. describe a high-gradient magnetic separator (HGMS, Hoffmann et al. 2002).

[0063] A non-homogeneous magnetic field, preferably high-gradient magnetic field, is produced by a fixed magnetic system, in particular a permanent magnet, and preferably a non-ferromagnetic object. The magnetic nanoparticles are attracted by the magnetic force and are transported to the separation zone (separation cell). The non-magnetic material is not attracted and passes through the separating zone.

[0064] In embodiments, the aqueous solution comprising the particulate material comprising a metal compound is pumped through a separation cell comprising a non-ferromagnetic object in the magnetic field off mode to fill the separation cell. In embodiments, after filling the separation cell the device is switched to the magnetic field on mode.

[0065] In embodiments, the magnetic concentration and / or separation is carried out using a magnetic field with a strength in the range of 1 mT to 1 T and a magnetic field gradient in the range of 100 T / m to 5,000 T / m.

[0066] The person skilled in the art knows different methods for the magnetic concentration and / or separation of magnetic nanoparticles from non-magnetic particles and to generate a magnetic field.

[0067] In embodiments, the magnetic field is generated by an electromagnet. As used herein, the term “electromagnet” refers to a magnetic field generating unit in which a magnetic field is only generated by applying an electric current. Various electromagnets, their structure and properties are known to those skilled in the art. Advantageously, the magnetic field strength of the electromagnet can be adjusted, and the electromagnet can be switched on and off. In embodiments, the electromagnet consists of a coil and an iron core through which the magnetic field is amplified and directed. In embodiments, the magnetic field is generated by a permanent magnet. As used herein, the term “permanent magnet” refers to a magnet that exhibits static magnetic behavior. The person skilled in the art is familiar with the various permanent magnets, such as NdFeB magnets or alloys made of iron, cobalt, nickel, ferrites, and their properties. As used herein, the term “NdFeB magnets” refers to permanent magnets consisting of an alloy of neodymium, iron and boron. Advantageously, NdFeB magnets are known to be the strongest permanent magnets according to the current state of the art. In embodiments, the permanent magnet is arranged to be removable from a flow line.

[0068] In embodiments, the magnetic concentration and / or separation of the modified magnetic nanoparticles and the metal compound from the particulate material is carried out using a column comprising magnetic beads.

[0069] In embodiments, the magnetic concentration and / or separation of the modified magnetic nanoparticles and the metal compound from the particulate material is carried out using a high- gradient magnetic separator.

[0070] In embodiments, the high-gradient magnetic separator is a rotary permanent magnet separator. In embodiments, the high-gradient magnetic separator is of the type Steinert HGF-10 or HGF-50 (Steinert Elektromagnetbau GmbH, Kbln, Germany). In embodiments, the high-gradient magnetic separator comprises a stainless-steel wire matrix with a wire, which is introduced into the separation cell to serve as the magnetizable matrix for the high-gradient magnetic separator. In embodiments, the separation cell contains at least one opening at the bottom and at least one opening at the top, which are connected to a tube system. In embodiments, the separation cell contains two openings at the bottom and two at the top, which are connected to a tube system.

[0071] In embodiments, at least one wire mesh, preferably at least one woven stainless steel wire mesh is positioned within the separation cell. In embodiments, the wire diameter is in the range of 0.1 pm to 10,000 pm, preferably of 5 pm to 5,000 pm, more preferably 50 pm to 1 ,000 pm.

[0072] In embodiments, the removal of the modified magnetic nanoparticles from the non-ferromagnetic object is carried out by rinsing the separation cell and / or removing the magnet for further treatment.

[0073] In embodiments, rinsing the separation cell is carried out through pumping an aqueous solution and / or a multiphase flow, preferably air-water flow. As used herein, the term “multiphase flow” is the simultaneous flow of materials with two or more thermodynamic phases, preferably a liquid and a gaseous phase.

[0074] In embodiments, the removal of the modified magnetic nanoparticles from the metal compound (target material) is carried out chemically by changing the temperature, the pH value and / or the salt concentration of the aqueous solution, and / or mechanically by using ultrasonication. The removal of the modified magnetic nanoparticles comprises the detachment or desorption of the magnetic nanoparticles from the target material (metal compound) after the magnetic concentration and / or separation and removal of the modified magnetic nanoparticles from the sample.

[0075] In embodiments, the removal of the modified magnetic nanoparticles is carried out by changing the temperature of the aqueous solution to a temperature in the range of 50 °C to 100 °C, preferably 50 °C to 70°C.

[0076] In embodiments, the removal of the modified magnetic nanoparticles is carried out by changing the pH value of the aqueous solution, in particular by increasing the pH value of the aqueous solution. Suitably, the pH value of the aqueous solution is increased above the isoelectric point of the metal compound, in particular to the range, wherein the compound has a negative zeta potential.

[0077] In embodiments, the removal of the modified magnetic nanoparticles is carried out by changing the pH value of the aqueous solution to a pH in the range of 8.5 to 14, preferably 9.5 to 12.

[0078] In embodiments, the removal of the modified magnetic nanoparticles is carried out by changing the salt concentration of the aqueous solution, in particular by increasing the salt concentration.

[0079] In embodiments, the removal of the modified magnetic nanoparticles is carried out with a pH value about 10 and with an aqueous NH4CI solution with a concentration of 10 mM.

[0080] In embodiments, the removal of the modified magnetic nanoparticles is carried out using ultrasonication with a sonotrode or an ultrasonic bath.

[0081] In embodiments, the removal of the modified magnetic nanoparticles is carried out by changing the pH value of the aqueous solution and by using ultrasonication. In embodiments, the removal of the modified magnetic nanoparticles from the metal compound is further carried out by size separation, preferably by centrifugation or filtration, more preferably membrane separation; or by magnetic separation.

[0082] In embodiments, the method is carried out repeating at least steps b) to d).

[0083] In embodiments, different metal compounds are concentrated and / or separated with the method according to the invention, in particular by repeating steps b) to d), wherein step b) is carried out with different pH values in the range of pH 3.5 to pH 11. Suitably, the pH value is adapted according to the isoelectric point of the metal compound, in particular the range, wherein the compound has a positive zeta potential.

[0084] Advantageously, the pH value can be varied, so that different positively charged metal compounds can be gradually isolated.

[0085] In embodiments, different metal compounds, in particular Y2C>3:Eu3+and / or BaMgAlioOi?:Eu2+, are concentrated and / or separated from a particulate material with the method according to the invention, in particular by repeating steps b) to d) for each metal compound, wherein step b) is carried out with different pH values in the range of pH 3.5 to pH 11 , in particular wherein step b) is carried out with a pH value in the range of pH 7 to pH 8 to concentrate and / or separate Y2C>3:Eu3+from the particulate material, and / or wherein step b) is carried out with a pH value in the range of pH 5 to pH 6 to concentrate and / or separate BaMgAlioOi?:Eu2+from the particulate material.

[0086] Suitably, the concentrate and / or separate of Y20s:Eu3+and / or BaMgAlioOi?:Eu2+from one particulate material is carried out with the method according to the invention, in particular by repeating steps b) to d) for each metal compound in the following order: Y20s:Eu3+and BaMgAlioOi?:Eu2+.

[0087] In embodiments, the method is combined with a method for the selective concentration and / or separation of metal compounds from particulate material using magnetic nanoparticles modified with peptides specific for at least one metal compound, in particular metal oxides, preferably rare- earth oxides. Advantageously specific binding peptides bind selectively, and therefore, can be used in very complex mixtures, preferably of very fine particles. The person skilled in the art knows surface-binding peptides or peptides specific for at least one metal compound, and how to generate and select suitable peptides, in particular with phage display. Phage display or biopanning is a biotechnological process in which peptides, protein parts or complete proteins from recombinant libraries are functionally presented on the surface of bacteriophages in order to isolate and identify suitable ligands for specific target molecules. Starting from the so-called phage libraries, some of which are commercially available and the number of phage variants extends over several orders of magnitude (typically in the order of 109), the phages which have a good binding to the target molecule are selected in a multi-stage process. DE 10 2017 219 800 B4 discloses a method for the selective separation of materials by the use of selective binding peptides.

[0088] In embodiments, the magnetic nanoparticles removed in step d) are reused in step a) of the method according to the invention.

[0089] In embodiments, the metal compound concentrated and / or separated from the particulate material after step d) is recycled.

[0090] Another aspect of the invention is the use of the method according to the invention for recycling of metal compounds, preferably rare-earth compounds, more preferably for the recycling of rare- earth oxides, from particulate, preferably fine particulate, material of waste of electronic and electrical equipment, in particular pulverized fluorescent lamps, pulverized solar cells, pulverized permanent magnets and particulate residual materials, preferably fine particulate residual materials, of catalysts or batteries. Advantageously, the method according to the invention allows for a controllable, effective, cost-efficient separation and / or recycling of rare-earth compounds, in particular from end-of-life fluorescent lamps, requiring only small amounts of consumables - in particular the magnetic nanoparticles can be reused. In embodiments, rare-earth compounds from fluorescent lamps are powder mixtures rich in rare-earths compounds.

[0091] In embodiments, the method according to the invention is used for recycling of metal compounds, preferably metal oxides, more preferably rare-earth oxides, from permanent magnets, lamp phosphors, catalysts, or batteries.

[0092] Another aspect of the invention is the use of a high-gradient magnetic separator in the method according to the invention. Another aspect of the invention is the use of a high-gradient magnetic separator and magnetic nanoparticles modified with at least one organic acid in a method for the selective concentration and / or separation of metal compounds from particulate material.

[0093] The invention is not limited to the embodiments shown and described, but also includes all embodiments having the same effect within the meaning of the invention. Furthermore, the invention is also not limited to the specifically described combinations of features but may also be defined by any other combination of specific features of all the individual features disclosed as a whole, provided that the individual features are not mutually exclusive, or a specific combination of individual features is not explicitly excluded.

[0094] In the following, the invention will be explained in more detail by means of an example. The example is intended to describe the invention without limiting it. Implementations of the invention will be described, by way of example only, with reference to accompanying figures in which:

[0095] Fig. 1 shows the interaction of the magnetic nanoparticles (MNP) with Y2C>3:Eu3+(YOX), LaPO4:Ce3+,Tb3+(LAP), BaMgAl Oi7:Eu2+(BAM) and CeMgAlnOi9:Tb3+(CAT): A) the fraction of attached magnetic nanoparticles on each metal compound. B) Time dependent attachment of the MNP to YOX with an exponential fit. C) Concentration of the MNP in the sample (system) and attached to YOX, and comparison with the expected concentration that is required to form a monolayer.

[0096] Fig. 2 shows fractions of magnetic nanoparticles attached to the metal compound (Y2O3:Eu3+, YOX) in different media: A) deionized water (DIW), an aqueous 10 mM NH4CI solution, a phosphate buffered saline (PBS), PBS with 10 mM NH4CI, B) deionized water (DIW) or an aqueous 10 mM NH4CI solution with a pH value of 5.6, 7.4, 8.0, 9.0, 9.5 and 10.5, C) Zeta potential of YOX at pH 4 to 12.

[0097] Fig. 3 shows the removal of magnetic nanoparticles from the metal compound (Y2O3:Eu3+, YOX) by changing the pH value of the medium (to pH 7.4 or 10.5) and shaking in comparison to changing the pH value of the medium (to pH 7.4 or 10.5) and sonotrode, determined by the extinction of the supernatant.

[0098] Fig. 4 shows fractions of magnetic nanoparticles (MNP) removed from the metal compound (Y2O3: EU3+, YOX) with two different methods and after reuse, in particular by changing the medium and ultrasonicating with a sonotrode, and separation from YOX by 1 .) filtration with a 0.20 pm membrane and 2.) a 0.45 pm membrane. 3.) The recovered MNP were successfully attached to YOX a second time. Fig. 5 shows the relative concentration of the compounds Y2C>3:Eu3+(YOX), LaPC>4:Ce3+,Tb3+(LAP), CeMgAlnOi9:Tb3+(CAT) and BaMgAlioOi?:Eu2+(BAM) in a high-gradient magnetic separator after contacting to the magnetic nanoparticles.

[0099] Synthesis of magnetic nanoparticles

[0100] Maghemite magnetic nanoparticles (y-Fe2C>3, with a mean diameter of 12.3 nm) were prepared following the procedure described by Massart et al. (Massart et al. 1981). They were subsequently size sorted in order to have a narrower size distribution and then coated with citrate anions (Fauconnier et al. 1996). The solution (121.2 mL) of citrated maghemite magnetic nanoparticles obtained an iron concentration of 1.9 mol / L.

[0101] Separation of rare-earth compounds

[0102] The following rare-earth compounds were used: red Y2C>3:Eu3+(YOX), green LaPO4:Ce3+,Tb3+(LAP), green CeMgAlnOi9:Tb3+(CAT) and blue BaMgAlioOi?:Eu2+(BAM). The magnetic nanoparticles are mixed with the fluorescent lamp waste comprising the rare-earth compounds. Then, the interaction is used to selectively separate YOX in an inhomogeneous magnetic field. Of the four compounds, only YOX has a positive surface charge in an aqueous solution in the range of pH 5 to pH 8, in particular at a pH value of 7.4 and interacts electrostatically with the negatively charged magnetic nanoparticles modified with citrate. Finally, YOX attached to the magnetic nanoparticles is separated from the reusable magnetic nanoparticles, e.g. by means of centrifugation, filtration, magnetic separation or settling.

[0103] Figure 1 shows the characteristics of the selective magnetic nanoparticle (MNP) interaction with Y2O3: EU3+(YOX). Adjustment of the MNP amount during mixing and shaking with the compounds overnight resulted in almost complete attachments of YOX to MNP (Fig. 1A). Fig. 1 B shows the attachment rate of the MNP to YOX. The attachment followed an exponential fit under shaking of the sample and almost all of the MNP were attached to YOX after 2 hours. Fig. 1C shows the concentration dependent attachment of MNP to YOX. It can be seen that below a certain concentration of MNP, close to all the MNP attach to YOX. When a concentration of MNP is reached, the attachable MNP saturate on the surface of YOX and the additional MNP remain in suspension. Based on the specific surface area and density of YOX, and the projected surface area and density of the MNP, the theoretical amount of MNP that is required to form a monolayer on the surface of YOX was calculated. The figure shows that the theoretically required amount corresponds very well to the observed amount at saturation. Figure 2 shows the separation of the rare-earth element YOX in different media at pH 7.4: deionized water, a buffered solution (NH4CI and / or PBS). The attachment of the magnetic nanoparticles at YOX is successful in all media at pH 7.4 Furthermore, the influence of pH value was investigated in DIW and an aqueous NH4CI solution (10 mM). Fig. 2B shows that at pH 5.6 to 8.0 MNPs are attached to the YOX surface in both DIW and 10 mM NH4CI solution. In both solutions, when the pH is further elevated to 9.0, 9.5 and 10.5, there is no attachment due to electrostatic interactions. This pH region concurs very well with the phosphor’s I EP. The zeta potential measurements in Fig. 2C show that the isoelectric point (IEP) of YOX is about 9.5 and that YOX has a positive surface charge in the pH range of 4 to 9.

[0104] Figure 3 shows the removal of magnetic nanoparticles from YOX by changing the pH value of the medium (to pH 7.4 or 10.5) and shaking in comparison to changing the pH value of the medium (to pH 7.4 or 10.5) and using sonotrode in one-step. The attachment was carried out under the conditions described above (in deionized water with a pH value of pH 7.4), the sample is centrifuged and the supernatant is replaced. All tested removal conditions (medium change and shaking and medium change and sonotrode) showed an increase in the extinction of the supernatant after 1 .5 h of shaking in 10 mM NH4CI buffered solution and hence, a removal of the magnetic nanoparticles from YOX. The combination of sonication with pH increase above the IEP of YOX resulted in a complete detachment of the MNP from YOX. After just 10 s of sonication in a solution with pH 10.5, the MNPs are completely detached from YOX and the MNP concentration in the supernatant after the 2nd centrifugation is about 100% of the initial MNP concentration according to the measured extinction.

[0105] After detachment, MNPs and YOX were separated by either size separation, e.g. centrifugation or membrane separation; or magnetic separation.

[0106] The separation and reusability of the MNP was tested by first attaching MNP to YOX in PBS at pH 7.4. The MNP were removed by changing the medium to an aqueous NH4CI solution (10 mM) with pH 10.5 and ultrasonicating with a sonotrode. Then the detached MNP were separated from YOX by filtration with a 0.20 pm membrane or a 0.45 pm membrane. Then the recovered MNP were attached to YOX a second time in an aqueous NH4CI solution (10 mM) with pH 7.4.

[0107] Figure 4 shows the successful recovery of MNP by membrane separation after detachment and the successful reusability of recovered MNP. Figure 5 shows an upscaled method for a magnetic separation process. The removal dynamics of the different compounds Y2C>3:Eu3+(YOX), LaPC>4:Ce3+,Tb3+(LAP), CeMgAlnOi9:Tb3+(CAT) and BaMgAlioOi?:Eu2+(BAM) are shown in a high-gradient magnetic separator. The attachment of MNP to YOX strongly increases its removal rate and allows a separation from the other compounds of the waste material. MNPs were recovered by sonotrode and hence, could be reused upon after step d).

[0108] Cited non-patent literature

[0109] Binnemans K, Jones P (2014) Perspectives for the recovery of rare earths from end-of-life fluorescent lamps. Journal of Rare Earths 32 (3), 195-200, https: / / doi.org / 10.1016 / S1002- 0721(14)60051-X.

[0110] Boelens P, Lei Z, Drobot B, Rudolph M, Li Z, Franzreb M, Eckert K, Lederer F (2021) High- Gradient Magnetic Separation of Compact Fluorescent Lamp Phosphors: Elucidation of the Removal Dynamics in a Rotary Permanent Magnet Separator. Minerals 11 , doi:10.3390 / min11101116.

[0111] Boelens P, Bobeth C, Hinman N, Weiss S, Zhou S, Vogel M, Drobot B, Azzam SSA, Pollmann K, Lederer F (2022) Peptide functionalized Dynabeads for the magnetic carrier separation of rare- earth fluorescent lamp phosphors. J. Magn. Magn. Mater. 563,169956, https: / / doi.Org / 10.1016 / j.jmmm.2022.169956.

[0112] Carlos L, Garcia Einschlag FS, Gonzalez MC, Martire DO (2013) Applications of Magnetite Nanoparticles for Heavy Metal Removal from Wastewater. InTech Chapter 3, 63-77, http: / / dx.doi.org / 10.5772 / 54608

[0113] Grohol M, Veeh C (2023) Study on the critical raw materials for the EU 2023. Final Report. Publications Office of the European Union, European Commission DG Grow; doi: 10.2873 / 725585.

[0114] Fauconnier N, Bee A, Roger J, Pons JN (1996) Adsorption of gluconic and citric acids on maghemite particles in aqueous medium. Progress in Colloid & Polymer Science 100, 212-216. Hirajima T, Abel B, Sasaki K, Nakayama K, Hirai H, Tsunekawa M (2005a) Floatability of rare earth phosphors from waste fluorescent lamps. Int. J. Miner. Process 77,187-198, doi: 10.1016 / j.minpro.2005.05.002.

[0115] Hirajima T, Sasaki K, Abel B, Hirai H, Hamada M, Tsunekawa M (2005b) Feasibility of an efficient recovery of rare earth-activated phosphors from waste fluorescent lamps through dense-medium centrifugation. Sep. Purif. Technol. 44,197-204, doi:10.1016 / j.seppur.2004.12.014.

[0116] Hoffmann C, Franzreb M, Holl WH (2002) A Novel High-Gradient Magnetic Separator (HGMS) Design for Biotech Applications. IEEE Transactions on Applied Superconductivity 12 (1), 963- 966. Massart R (1981) Preparation of Aqueous Magnetic Liquids in Alkaline and Acidic Media. IEEE Transactions on Magnetics 17 (2) 1247 - 1248.

[0117] Takahashi T, Takano A, Saitoh T, Nagano N, Hirai S, Shimakage K (2001) Separation and Recovery of Rare Earth Elements from Phosphor Sludge in Processing Plant of Waste Fluorescent Lamp by Pneumatic Classification and Sulfuric Acidic Leaching. Shigen-to-sozai 177,579-585, doi: 10.2473 / shigentosozai.117.579.

[0118] Wada K, Mishima F, Akiyama Y, Nishijima S (2014) The Development of the Separation Apparatus of Phosphor by Controlling the Magnetic Force. Physics Procedia 58, doi:10.1016 / j.phpro.2014.09.068.

Claims

Claims1. A method for the selective concentration and / or separation of metal compounds from particulate material comprising the following steps: a. Providing magnetic nanoparticles modified with at least one organic acid, b. Contacting the modified magnetic nanoparticles with at least one particulate material comprising at least one metal compound in an aqueous solution, wherein the modified magnetic nanoparticles interact in the aqueous solution with the at least one metal compound by electrostatic interactions, c. Magnetic concentration and / or separation of the modified magnetic nanoparticles and the metal compound from the particulate material, and d. Removal of the modified magnetic nanoparticles from the at least one metal compound.

2. The method according to claim 1 , wherein the metal compound is a metal oxide.

3. The method according to claim 1 or 2, wherein the metal compound is a rare-earth oxide.

4. The method according to any one of claims 1 or 2, wherein the metal compound is selected from the group comprising Y2C>3:Eu3+, BaMgAlioOi?:Eu2+and AI2O3.

5. The method according to any one of claims 1 to 4, wherein the magnetic nanoparticles are iron oxide nanoparticles.

6. The method according to any one of claims 1 to 5, wherein the magnetic nanoparticles have a hydrodynamic diameter in the range of 5 nm to 100 nm.

7. The method according to any one of claims 1 to 6, wherein the at least one organic acid is selected from the group comprising citric acid, gluconic acid and fatty acids.

8. The method according to any one of claims 1 to 7, wherein the particulate material is selected from particulate material of waste of electronic and electrical equipment, preferably pulverized fluorescent lamps, pulverized solar cells, pulverized permanent magnets, particulate residual materials of catalysts or batteries.

9. The method according to any one of claims 1 to 8, wherein the modified magnetic nanoparticles are contacted with the at least one particulate material in a mass ratio in the range of 0.02% to 200%.

10. The method according to any one of claims 1 to 9, wherein the magnetic concentration and / or separation is carried out using a magnetic field with a strength in the range of 1 mT to 1 T.11 . The method according to any one of claims 1 to 10, wherein the magnetic concentration and / or separation is carried out using a magnetic field with a magnetic field gradient in the range of 1 T / m to 5,000 T / m.

12. The method according to any one of claims 1 to 11 , wherein the removal of the modified magnetic nanoparticles is carried out by changing the pH value and / or the salt concentration of the aqueous solution, and / or by using ultrasonication.

13. The method according to any one of claims 1 to 12, wherein metal oxides are recycled from particulate materials of waste of electronic and electrical equipment, preferably pulverized fluorescent lamps, pulverized solar cells, pulverized permanent magnets, particulate residual materials of catalysts or batteries.

14. The method according to any one of claims 1 to 13, wherein the magnetic concentration and / or separation is carried out using a high-gradient magnetic separator.

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