PHOSPHOR RECOVERY METHOD BACKGROUND

[0001] The invention relates generally to phosphor recovery processes, and, in particular, to methods of electrochemical 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 separating includes dispersing the starting mixture in a liquid medium comprising a surfactant; and applying a direct current (DC) cell potential in a range from about 0.1 volt to about 4 volts in an electrochemical cell.

[0005] In one embodiment, a method is disclosed. The method includes separating a phosphor material from a starting mixture. The separation of the phosphor material from the starting material includes ultrasonically dispersing the starting mixture in a liquid medium that includes a dispersant and a surfactant. The surfactant used may be TTA, EDTA, oleic acid, or combinations thereof. A DC cell potential in a range from about 0.3 to about 1.2 volts in an electrochemical cell may be used to separate the phosphor material.

DETAILED DESCRIPTION

[0006] Embodiments of the present invention include the methods for recovering a phosphor material from starting mixture that includes retorted fluorescent phosphor.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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 (CRT), 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 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.

[0011] 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.

[0012] 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.

Table 1.

[0013] 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 LaPCV Ce3+, Tb3+, 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. In one embodiment, the separated phosphor material is in the same electronic charge state as that of the phosphor material in the RFP, and the final phosphor material that is used for a visible light producing application.

[0014] 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.

[0015] In one embodiment, the phosphor material separated from an RFP has less than or equal to 10% 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 5%. As used herein, the "quantum efficiency" is defined as the ratio of visible light emission of the phosphor material with respect to the absorbed ultraviolet energy.

[0016] In one embodiment, the phosphor material is separated from the RFP
by the application of a direct current (DC) potential to the RFP in a liquid medium (electrolyte) across two electrodes (anode and athode). In one embodiment, a suspended mixture of RFP is formed by suspending the RFP in a liquid medium. The liquid that is used for the suspension of RFP may be organic, or inorganic. In one embodiment, the liquid medium includes ionic liquids. In one embodiment, one or more organic liquids such as, for example, acetonitrile, methanol, dichloromethane, chloroform, toluene, hexane, heptane, chloroform, pentane, acetone, isopropyl alcohol, ethanol, or any combination of the foregoing are used as the liquid medium for suspension of the leached mixture. In one embodiment, RFP is suspended in water. One of the phosphors may be selectively deposited on an electrode by choosing suitable operation parameters such as, for example, DC cell voltage, temperature of the mixture at the time of deposition, solid loading, electrode, or electrolyte composition. Further, this method may be extended to selectively deposit two different phosphors on the two different electrodes.

[0017] Dispersion of RFP may be aided by use of dispersants, stirring, ultrasonication, or a combination of these techniques. 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.

[0018] In one embodiment, the dispersant that is used to disperse the suspended mixture includes sodium hexametaphosphate, ammonium salts of acrylic polymer, or combinations thereof.

[0019] In one embodiment, the liquid medium used for the suspension of the
RFP includes a surfactant material dispersed or dissolved in it. Typically surfactants are surface active substances that lower surface tension of a liquid, the interfacial tension between two liquids, or the interfacial tension between a liquid and a solid. Depending on the nature, the surfactants are classified as an-ionic, non-ionic, cat-ionic, or amphoteric. As used herein a "surfactant" is a surface active agent that specifically attaches to the phosphor material surfaces. As used herein, a surfactant may also act as a dispersant, but the dispersant does not act as a surfactant.

[0020] The surfactant that is used along with the phosphor blend of the RFP
herein may include 2-thenoyl trifluoro acetone (TTA), ethylenediamine tetra acetic acid (EDTA), sodium dodecyl sulfate (SDS), dodecyl ammonium acetate (DAA), cetrimonium bromide (CTAB), ammonium lauryl sulfate (ALS), dioctyl sulfosuccinate sodium salt (AOT), hexadecyl (2-hydroxyethyl) dimethyl ammonium dihydrogen phosphate, 3- (N,N-dimethyl octadecyl ammonio) propane sulfonate, sodium oleate, oleic acid, Bis(2-ethylhexyl) phosphate, tetraoctyl ammonium bromide (TOAB), or a combination thereof.

[0021] In one embodiment, the RFP is reacted with a surfactant in the liquid medium to form a phosphor material-surfactant complex. This complex may include the phosphor material and at least a part of at least one of the surfactant that is present in the liquid medium. In one embodiment, at least about 50 milli molar of surfactant in the liquid medium is used for the electrochemical separation of a phosphor material. In one embodiment, concentration of the surfactant in the liquid medium is in a range from about 75 milli molar to about 200 milli molar.

[0022] In one embodiment, an electrochemical process is employed to separate and recover high-purity lanthanum phosphate (LAP), barium magnesium aluminate (BAM), or yttrium europium oxide (YEO) phosphors from a spent phosphor blend that includes all three phosphors (green, blue, red). Electrochemical driving forces may be used in cases where the two phosphors in combination with appropriate surfactants acquire opposing surface charges and can be separated to positive and negative electrodes.

[0023] The phosphor material-surfactant complex may be in an
electrostatically charged state or electrostatically uncharged state. As used herein an

"electrostatically charged state" is a state of the complex wherein the complex has an overall positive or negative charge that makes the complex move towards an electrode during the application of a DC cell potential. As used herein an "electrostatically uncharged state" is a state of the complex wherein the complex is neutral in overall charge, and therefore does not substantially move towards a negative or positive electrode during the application of a DC cell potential.

[0024] In one embodiment, the DC cell potential is applied to the phosphor
material-surfactant complex in a liquid medium using positive and negative electrodes. In one embodiment, wherein the complex is in an electrostatically charged state, the separation of the complex may be carried out through the electrostatic separation process. In one embodiment, wherein the complex is in an electrostatically uncharged state, the applied DC cell potential may induce charge in the phosphor material-surfactant complex, and aid in the separation of the complex from the rest of the material through an electrodeposition process. In one embodiment, wherein the complex is in an electrostatically charged state, the applied DC cell potential may reinforce or change charge polarity of the phosphor material-surfactant complex, and aid in the separation of the complex from the rest of the material.

[0025] In one embodiment, the phosphor material may itself be in the charged state in the phosphor material-surfactant complex and may get separated as the phosphor material-surfactant complex based on the charge on the phosphor material. In one embodiment, the charged part of the phosphor material-surfactant complex is in the surfactant part, without any electrostatic charge variation in the phosphor material part of the phosphor material-surfactant complex. In this state, the charge on the surfactant in the phosphor material-surfactant complex aids in the separation of that phosphor material-surfactant complex from the other materials present in the RFP. The surfactant part of the phosphor material-surfactant complex may further be separated from the phosphor material through chemical or heat treatment steps after electrochemical separation of the complex from the RFP.

[0026] The separation of phosphor material by the application of DC cell potential may require a certain minimum strength of the DC potential field. A phosphor material-surfactant complex that has a comparatively high electrostatic charge may need only a relatively small strength of the applied DC potential field, while the complexes that are electrostatically weaker may require a higher strength DC potential field to be applied for the separation. In one embodiment, the absolute value of DC cell potential applied herein is lower compared to what is needed for an electrostatic separation or electrodeposition of a material without using a surfactant attachment to the material. In one embodiment, the DC cell potential (anode versus cathode) applied for the separation of a phosphor material from a dispersed and suspended phosphor material-surfactant complex is in a range from about 0.1 volt to about 4 volts. In one embodiment, the applied DC cell potential may be in the range of about 0.3 volt to about 2 volts.

[0027] In one embodiment, a phosphor material is reacted with a surfactant such as EDTA, TTA, or oleic acid, or any combination of these surfactants, to form a phosphor material-surfactant complex that is easily separable by applying a DC cell potential less than about 1.3 volt. Without being bound by any particular theory, the phosphor material-surfactant (of EDTA, TTA, oleic acid, or combinations thereof) complex separation takes place through electrodeposition by charging the surfactant part of the phosphor material-surfactant complex.

[0028] In one example, a phosphor material was separated from the starting mixture through an electrochemical separation method. The starting mixture was dispersed with the help of a dispersant and ultrasonication in a liquid medium, and the phosphor material was reacted with a surfactant selected from the group consisting of EDTA, TTA, and oleic acid. The starting mixture in the liquid medium was subjected to an electrochemical separation in an electrochemical cell using a DC cell potential in a range from about 0.3 to about 1.2 volts.

[0029] In one embodiment, the RFP considered for the separation of the phosphor material has fine sized materials. In one embodiment, a median particle size of the RFP is less than about 5 microns. In one embodiment, the median particle size of the RFP is less than or equal to about 3.5 microns. A smaller size of the RFP powder helps in suspension and electrostatic separation of the phosphor material.

[0030] In one embodiment, the phosphor material-surfactant complex is heat- treated to form a heat-treated mixture. Without being bound by any particular theory, the heat-treatment is considered to aid in the removal of some of the organic impurities that may still be present in the separated mixture. Depending on the surfactant and any other impurities that are incidentally present in the phosphor material-surfactant complex, the heat-treatment of the complex 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.

[0031 ] The RFP present in the starting mixture 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 from the phosphor blend. The separation of the impurities from the RFP may include, among other things, leaching with an acid solution, dissolving in suitable solvents, or dispersing the impurities and phosphor materials in different solvent mediums in a biphasic solution.

EXAMPLE

[0032] 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.

[0033] A process is disclosed herein to separate and recover LAP, BAM and YEO phosphors from a phosphor blend. Electrochemical driving forces were used in cases where at least one of the phosphors in combination with appropriate surfactants form a complex and acquire surface charge and may be separated to a positive or a negative electrodes.

[0034] In one example, a phosphor blend was milled to reduce the particle
size of the phosphor powders to approximately 3.5 microns. The phosphor blend was mixed with water with about 15-40 grams per liter solid loading. The phosphor blend was dispersed in the water using dispersants and ultrasonication. The dispersants used included Calgon®, Darvan® 821a, or Dispex® A40 at a concentration of about 2-40 grams per liter of water. In one example, a YEO specific surfactant (TTA) was added to form a YEO-TTA complex that was further dispersed in water using ultrasonication. A DC cell potential was applied such that the working electrode was maintained in the range from about -0.3 volts to about -0.5 volts (half-cell voltage) with respect to an Ag/AgCl reference electrode. During the application of DC cell potential to the mixture, the mixture was subjected to continuous ultrasonication. Under the influence of the DC cell potential, the YEO-TTA complex was separated and deposited on the working electrode. The separated complex was washed with hexane to remove TTA.

[0035] In another example, a LAP and BAM mixture blend with approximately 3.5 microns average particle size was taken for LAP separation. The LAP+BAM blend was mixed with water with about 15-40 grams per liter solid loading and dispersed in the water using dispersants and ultrasonication. The dispersants used included Calgon®, Darvan® 821a, or Dispex® A40 at a concentration of about 2-40 grams per liter of water. The dispersion in water was treated with a LAP specific surfactant such as EDTA to form a LAP-EDTA complex and dispersed further using ultrasonication. A DC cell potential was applied such that the working electrode was maintained in the range from about -0.3 volts to about -0.4 volts (half-cell voltage) with respect to an Ag/AgCl reference electrode. Under the influence of the DC cell potential, the LAP-EDTA complex was separated from the phosphor blend and washed with water to remove EDTA. The separated LAP and BAM phosphors were further heat-treated in air at a temperature range of about 350°C to about 500°C.

[0036] 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.

CLAIMS:

1. A method, comprising:

separating a phosphor material from a starting mixture, wherein the separating comprises

dispersing the starting mixture in a liquid medium comprising a surfactant; and

applying a direct current (DC) cell potential in a range from about 0.1 volt to about 4 volts in an electrochemical cell.

2. The method of claim 1, wherein the DC cell potential is in a range from about 0.3 volt to about 2 volts.

3. The method of claim 1, wherein the surfactant comprises 2-thenoyl tri fluoro acetone (TTA), sodium dodecyl sulfate (SDS), ethylene diamine tetra acetic acid (EDTA), dodecyl ammonium acetate (DAA), cetrimonium bromide (CTAB), ammonium lauryl sulfate (ALS), dioctyl sulfo succinate sodium salt (AOT), hexa decyl(2-hydroxyethyl) dimethyl ammonium dihydrogen phosphate, 3- (N,N-dimethyl octadecyl ammonio) propane sulfonate, sodium oleate, oleic acid, Bis(2-ethylhexyl) phosphate, tetra octylammonium bromide (TOAB), or a combinations thereof.

4. The method of claim 3, wherein the surfactant comprises TTA, EDTA, oleic acid, or a combinations thereof.

5. The method of claim 1, wherein a median particle size of the starting mixture is less than about 5 microns.

6. The method of claim 1, wherein the dispersing comprises ultrasonication.

7. The method of claim 1, wherein the liquid medium further comprises a dispersant.

8. The method of claim 1, wherein a concentration of the surfactant in the liquid medium is at least about 50 milli molar.

9. The method of claim 7, wherein the concentration of the surfactant in the liquid medium is in a range from about 75 milli molar to about 200 milli molar.

10. The method of claim 1, wherein the liquid medium comprises water.

11. The method of claim 1, wherein the liquid medium comprises an organic medium.

12. The method of claim 11, wherein the organic medium comprises hexane, heptane, chloroform, decanol, pentane, or a combinations thereof.

13. A method, comprising:

separating a phosphor material from a starting mixture, wherein the separating comprises
ultrasonically dispersing the starting mixture in a liquid medium, wherein the liquid medium comprises a dispersant and a surfactant, wherein the surfactant comprises TTA, EDTA, oleic acid, or a combinations thereof; and

applying a DC cell potential in a range from about 0.3 to about 1.2 volts in an electrochemical cell.