BACKGROUND

[0001] The invention relates generally to phosphor recovery processes, and, in particular, to methods of chemical and magnetic separation of phosphors from a retorted fluorescence powder mixture.

[0002] A phosphor is a substance that exhibits the phenomenon of luminescence.

Phosphors often include various types of rare earth compounds. Resources of concentrated deposits of rare earth compounds are limited in the earth leading to scarcity and high cost for the compounds. Therefore, there is a need for recycling and reusing spent or rejected phosphor materials.

[0003] Current methods of separating and recycling the spent or rejected phosphor blends are selective to particular phosphors, involve elaborate chemical changes, or require expensive multiple processing steps that increase the processing costs of the phosphors. Therefore, there is a need for a simple, cost-effective method for recovering and recycling pure individual phosphors from spent or rejected phosphor materials.

BRIEF DESCRIPTION

[0004] Various embodiments of separating a phosphor material are disclosed herein.

In one embodiment, separating a phosphor material from a starting mixture is disclosed. The separation includes the steps of leaching with a first mineral acid solution to form a leached solution and a residual solid mixture; applying a magnetic field to the residual solid mixture to magnetically separate a desirable portion comprising the phosphor material; heat-treating the desirable portion to form a heat-treated mixture; and chemically separating the phosphor material from impurities in the heat-treated mixture; and drying.

[0005] In one embodiment, a starting mixture is initially leached with hydrochloric acid solution to form a leached solution and a residual solid mixture. The residual solid mixture is subjected to the application of a magnetic field of strength in a range from about

0.3 tesla to about 2.3 tesla to form a magnetically separated mixture. The magnetically separated mixture is heat-treated at a temperature in a range from about 400°C to about 700°C to form a heat-treated mixture. The heat-treated mixture is then reacted with a mixture of hydrogen peroxide, hydrochloric acid, and sodium hydroxide to form a water soluble impurity product. The phosphor material is filtered, washed, dried, and compared with the unused phosphor material of the same composition.

[0006] In one embodiment, a starting mixture is initially leached with hydrochloric acid solution to form a leached solution and a residual solid mixture. The residual solid mixture is subjected to the application of a magnetic field of strength in a range from about 0.3 tesla to about 2.3 tesla to form a magnetically separated mixture. The magnetically separated mixture is heat-treated at a temperature in a range from about 400°C to about 700°C to form a heat-treated mixture. In one embodiment, the heat-treated mixture is reacted with water, hexane, and tetraoctyl ammonium bromide to form a biphasic mixture, and the phosphor material is separated through controlled filtration and leaving out the impurities.

DETAILED DESCRIPTION

[0007] Embodiments of the present invention include the methods for recovering a phosphor material from starting mixture that includes retorted fluorescent phosphor.

[0008] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about," is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.

[0009] In the following specification and the claims that follow, the singular forms

"a", "an" and "the" include plural referents unless the context clearly dictates otherwise.

[0010] A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. In one embodiment, phosphors convert radiation, such as for example, ultraviolet radiation (UV) to visible light. Different combinations of phosphors provide different colored light emissions. A phosphor material may convert UV or blue radiation to a lower energy visible light. The color of the generated visible light is dependent on the particular components of the phosphor material.

[0011] The phosphor material described above may be used in many different applications. For example, the material may be used as a phosphor in lamp, in a cathode ray tube, in a plasma display device, or in a liquid crystal display. The material may also be used as a scintillator in an electromagnetic calorimeter, in a gamma ray camera, in a computed tomography scanner, or in a laser. These uses are meant to be merely exemplary and not exhaustive. The most common uses of phosphors are in cathode ray tube (CRT) displays and fluorescent lights. Different embodiments of the present invention are described using the example of fluorescent lamps. However, the various methods disclosed here are not limited to fluorescent lamps but may be adapted to different applications.

[0012] One embodiment of the invention relates to a method of separating a phosphor material from a starting mixture. The starting mixture, in one embodiment, includes retorted fluorescent powder (henceforward referred as "RFP"). RFP is a crushed powder made from used phosphor-containing equipment, such as used fluorescent lamps. The RFP may have phosphor material along with impurities associated with the recovery process. In one embodiment, the RFP includes a phosphor blend along with impurities such as metals, crushed glass powder, basing cement, crushed electrode, and alumina. A phosphor blend used in a fluorescent lamp normally includes one or more high-purity green, blue, and red phosphors. Table 1 shows typical examples and compositions of phosphors.

Table 1.

YVO YV0 ₄ : Eu ³

[0013] In some embodiments, a mixture or a blend of two or more types of phosphors are present in RFP. For example, a phosphor blend may contain red, blue and green phosphors. The phosphor material is typically a multi-element complex compound, which normally decomposes into a mixture of different oxides or carbonates on different heat or chemical treatment.

[0014] One embodiment of the invention relates to a method of separating a phosphor material from starting mixture that includes RFP. The "phosphor material" as used herein is in the context of a single phosphor compound having a particular composition. As used herein, the "composition" of the phosphor material is the chemical composition with particular identities, and relative numbers, of the constituent elements that make up the compound. Two phosphor materials having the same chemical composition may or may not have all the corresponding atoms in similar electronic charge states. For example, a first lanthanum phosphate (LAP) phosphor may have the composition LaP0 ₄ : Ce ³⁺ , Tb ³⁺ , with a definite number of lanthanum, phosphorous, oxygen, cerium, and terbium elements in a first lattice with a first lattice structure. As used herein, a second LAP phosphor is said to have the same composition as the first LAP phosphor, if the number of lanthanum, phosphorous, oxygen, cerium, and terbium elements present in a second lattice is same as in the first LAP phosphor, regardless of the charge of each element, and lattice structure of the second lattice being similar to the first LAP phosphor.

[0015] As used herein, "separating" a phosphor material means that the phosphor material during separation does not undergo decomposition from the initial phosphor material and after isolation is substantially the same as the initial phosphor material. In other words, the phosphor material present in an RFP is "separated" from the RFP into the substantially same phosphor material, without significant decomposition of the phosphor material. As used herein "substantially" is a qualifier term used to accommodate any small incidental variations in the chemical composition of the phosphor material that does not alter the visible light emission of the phosphor material more than 5%. As used herein "decomposition" is any kind of alteration of the phosphor material compound wherein the number of elements present in an unit cell of the phosphor material is reduced from the original phosphor material.

[0016] In one embodiment, the phosphor material separated from an RFP has less than or equal to 5% variation in its quantum efficiency from the phosphor material of the substantially same chemical composition present in the RFP before the separation, as measured on the powder in a spectrometer. In one embodiment, the quantum efficiency variation between the starting phosphor material and the separated phosphor material is less than about 3%. As used herein, the "quantum efficiency" is defined as the ratio of visible light emission of the phosphor material with respect to absorbed ultraviolet energy.

[0017] The RFP may include different metallic, non-metallic, and ceramic impurities along with a phosphor blend. These impurities, if present, may reduce the quantum efficiency of the recovered phosphor material. Therefore, it is desirable to remove these impurities from the RFP before or after the separation of individual phosphors.

[0018] In one embodiment, the method of separating the phosphor includes leaching the starting mixture with a solution to form a leached solution and a residual solid mixture. As used herein the "residual solid mixture" is the solid that remains in the starting mixture after leaching. The leaching may be carried out using different solvents depending on the nature of the phosphor to remove some of the metallic and non-metallic impurities as a part of the leached solution. In one embodiment, the RFP is leached with a mineral acid. The "mineral acid" as used herein is an acid derived from one or more inorganic compounds, without having a carbon-carbon bonding. The RFP may be subjected to the reaction with the mineral acid solution to separate or dissolve some of the metallic impurities, and at least some part of silicon, organic compounds, and basing cement. Acids that may be used for leaching may include common acids such as hydrochloric acid, sulfuric acid, hydrofluoric acid, hydrobromic acid, nitric acid, phosphoric acid, boric acid, perchloric acid, or any combinations of the foregoing. In one embodiment, the acid used for leaching is hydrochloric acid. After leaching out some of the impurities, the residual solid mixture may include the phosphor blends and still some more impurities.

[0019] In one embodiment, the residual solid mixture is subjected to a magnetic field to magnetically separate a desirable portion comprising the phosphor material. As used herein "magnetically separate a desirable portion" means that a desirable portion that has magnetic nature is physically separated using a magnetic field. The magnetic field that is used herein may be an AC, DC, or pulsed magnetic field or a magnetic field from a permanent magnet. A phosphor material that includes a magnetic element may be susceptible to the magnetic field. A magnetic element may be an element in the periodic table that possesses a diamagnetic, paramagnetic, ferromagnetic, or antiferromagnetic nature. In one embodiment, one or more magnetic elements present in the phosphor material include terbium, manganese, or combinations of terbium and manganese. In one embodiment, the magnetically susceptible phosphor material is lanthanum phosphate (LAP) including any dopants present in the lanthanum and / or phosphorous sites. In one embodiment, terbium is a magnetic element present in LAP. The terbium may be present in the phosphor material as a trivalent ion or a tetravalent ion, as both the trivalent terbium and the tetravalent terbium ions are known to be susceptible to magnetic field.

[0020] Applying magnetic field to the residual solid mixture has the advantage of avoiding the metallic impurities that may be attracted to the magnetic field before, or along- with the magnetically susceptible phosphor material. In one embodiment, leaching the RFP before magnetic separation of phosphor material is more desirable than performing the leaching after the magnetic separation. The phosphor material that is separated by leaching the starting mixture first and then applying the magnetic field to the residual solid mixture is found to have greater quantum efficiency compared to the phosphor material that is separated by applying the magnetic field to the starting mixture first and then leaching the magnetically separated desirable portion.

[0021] In one embodiment, the residual solid mixture that is subjected to a magnetic field in the solid form. In one embodiment, the residual solid mixture is in the form of a suspension in a liquid carrier when magnetic field is applied. The liquid that is used for the suspension of residual solid mixture may be organic or inorganic. In one embodiment, organic liquids such as acetone, isopropyl alcohol, or ethanol are used for the suspension of the residual solid mixture. In one embodiment, the residual solid mixture is suspended in water.

[0022] The dispersion of the residual solid mixture may be aided by dispersants, mechanical dispersion, or a combination of the foregoing. Mechanical dispersion may include stirring, shaking, ultrasonication, or any other way of mechanically dispersing the residual solid mixture in the liquid medium. A dispersant is a dispersing agent that may be a surface-active substance, or a non-surface active polymer added to a suspension, to improve the separation of particles in the suspension and to prevent settling or clumping. A dispersant may or may not attach to the surface of the suspended particles.

[0023] In one embodiment, the dispersant that may be used for dispersing the residual solid mixture includes sodium hexametaphosphate, ammonium salts of acrylic polymer, or combinations thereof. The magnetic field may be applied to the residual solid mixture through a gradient or non-gradient magnetic field source. For example, the mixture may be passed through a magnetic field gradient of appropriate strength or may be subjected to the magnetic field using a constant strength magnetic field.

[0024] The separation of phosphor material by the application of magnetic field requires some minimum strength of the magnetic field. A phosphor material that has high magnetic susceptibility may need a comparatively small strength of the applied magnetic field, while phosphor materials that are magnetically weaker may require a higher strength magnetic field to be applied for the separation. In one embodiment, the strength of the magnetic field applied to the separation of a phosphor material from a dispersed and suspended residual solid mixture is less than about 10 tesla. In one embodiment, the strength of the magnetic field applied to the residual solid mixture is in a range from about 0.3 tesla to about 2.5 tesla.

[0025] In one embodiment, the RFP considered for the separation of the phosphor material has very fine sized materials. In one embodiment, a median particle size of the RFP powders is less than about 5 microns. In one embodiment, the median particle size of the RFP powder is less than or equal to about 3.5 microns. A smaller size of the RFP powder helps in one or more of dispersion, suspension, or magnetic separation of the phosphor material.

[0026] In one embodiment, the magnetic field is applied in a gradient to the suspended residual solid mixture. The applied magnetic field may be gradually increased to attract the magnetically susceptible phosphor materials in the decreasing magnetic susceptibility order. In one particular arrangement, the residual solid mixture particles suspended in a solvent are passed through a gradient magnetic field and the phosphor material that includes terbium is selectively separated by the magnetic field. Further, upon increasing the field strengths systematically, or tailoring the magnetic flux gradient, phosphors having relatively lower susceptibilities are sequentially or simultaneously positioned at different regions, thereby making the process of separation of multiple phosphor materials feasible.

[0027] In one embodiment, the magnetically separated mixture (including the phosphor material) from the residual solid mixture is heat-treated to form a heat-treated mixture. Without being bound by any particular theory, the heat-treatment may aid in the removal of organic impurities that may still be present in the separated mixture. Depending on the impurities that are still present in the separated mixture, the heat-treatment of the mixture may be conducted in oxidizing, neutral, or reducing atmosphere. In one embodiment, the heat-treatment is conducted in an oxidizing atmosphere or neutral atmosphere. In one embodiment, the heat-treatment is conducted in air. In one embodiment, the temperature of the heat-treatment varies from about 200°C to about 1000°C. In one embodiment, the heat-treatment temperature is in a range from about 400°C to about 500°C.

[0028] In one embodiment, the heat-treated mixture is subjected to further purification based on the chemical separation from impurities based on solubility or dispersion of the impurities or the phosphor material that has to be separated. The impurities present in the heat-treated mixture may include silicon, or iron. In one embodiment, the chemical separation step includes forming a water soluble chemical compound of at least one of the impurities. In one such chemical separation method, the heat-treated mixture is reacted with a combination of hydroxides, hydrogen peroxide, and a second mineral acid. The second mineral acid as used herein may or may not be similar to the first mineral acid. In one embodiment, the heat-treated mixture is reacted with a combination of sodium hydroxide, hydrochloric acid, and hydrogen peroxide. The concentration of the combination may be varied to separate a pure phosphor.

[0029] In one embodiment of the chemical separation, at least one of the impurities is dispersed in a biphasic solution comprising organic and aqueous mediums. In one embodiment, the at least one impurity becomes dispersed in the organic medium and the phosphor material desired to be separated is dispersed in the aqueous medium. The resulted organic-aqueous triphasic mixture may be separated based on the insolubility of the mediums and the phosphor material may be separated and dried.

[0030] In one example, a starting mixture is leached with hydrochloric acid solution before magnetically separating the mixture including the phosphor material by the application of a magnetic field of strength in a range from about 0.3 tesla to about 2.3 tesla. The magnetically separated mixture is heat-treated at a temperature in a range from about 400°C to about 700°C to form a heat-treated mixture and then reacted with a mixture of hydrogen peroxide, hydrochloric acid, and sodium hydroxide to form a water soluble impurity product, and a bleached phosphor material. The phosphor material is filtered, washed, and dried. In one embodiment, the heat-treated mixture is reacted with water, hexane, and tetraoctyl ammonium bromide to form a biphasic mixture, and the phosphor material is separated through controlled filtration.

EXAMPLES

[0031] The following example illustrates methods, materials, and results, in accordance with specific embodiments, and as such should not be construed as imposing limitations upon the claims. All components are commercially available, unless otherwise indicated.

[0032] A phosphor blend including about 22 % of barium magnesium aluminate

(BAM) and about 78% of LAP was dispersed in acetone using an ultrasound bath and few drops of dispersant Darwan 821 ^® A. An NdFeB permanent magnet was dipped in to the dispersion. Out of the BAM and LAP, LAP was found to be deposited over the permanent magnet. The LAP phosphor separated was found to be almost 100% pure. Similarly, LAP was separated magnetically from a mixture of strontium europium chlorapatite (SECA) and LAP, and further, from a mixture of tricolor RGB phosphor blend.

[0033] In another example, about 10 grams of RFP was dispersed in water with the help of a dispersant and ultrasonication. The prepared dispersion was subjected to magnetic stirring for about 30 minutes to remove any ferromagnetic impurities. The ferromagnetic impurities such as fine particles of iron that were found to be attached to the magnetic stirrer were removed. The remaining dispersion was washed with about 6 normal hydrochloric acid (HC1) solution to remove any residual iron, and any other metallic, or non-metallic impurities. The magnetic LAP was separated from the other phosphors by applying a magnetic field using an NdFeB permanent magnet. The separated LAP product was subjected to a heat-treatment at a temperature of about 450°C in air to further remove any organic impurities. The heat-treated LAP was further washed with a 1 : 1 : 1 solution of water, HCl, and hydrogen peroxide (¾(¾). This washed powder was found to have about 90% of quantum efficiency when compared to an internal standard. The LAP powder was further treated with about 0.2 milli molar of sodium hydroxide ( aOH) solution. The dried LAP powder after this treatment was found to have quantum efficiency of about 96% in comparison with the internal standard.

[0034] While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.