WO2014081547A1

WO2014081547A1 - Phosphor recovery method

Info

Publication number: WO2014081547A1
Application number: PCT/US2013/067958
Authority: WO
Inventors: Karthick Vilapakkam Gourishankar; Srinidhi RAMACHANDRA; Malathi Yarra; Alok Mani Srivastava; William Winder Beers; Gopi Chandran Ramachandran; Prasanth Kumar Nammalwar
Original assignee: General Electric Company
Priority date: 2012-11-22
Filing date: 2013-11-01
Publication date: 2014-05-30

Abstract

A method of 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 varying the pH to sediment a yttrium europium oxide (YEO) phosphor, lanthanum phosphate (LAP) phosphor, or both. Sedimenting the YEO phosphor includes adjusting the pH to less than about (3) and then increasing the pH to be in a range from about (5) to about (6). Sedimenting the LAP phosphor comprises adjusting the pH to greater than about (10) and then decreasing the pH to be in a range from about (8) to about (9).

Description

PHOSPHOR RECOVERY METHOD

BACKGROUND

[0001] The invention relates generally to phosphor recovery processes, and, in particular, to methods of chemical separation of phosphors from a retorted fluorescent 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 dispersing the starting mixture in a liquid medium; and varying the pH to sediment an yttrium europium oxide (YEO) phosphor, lanthanum phosphate (LAP) phosphor, or both. Sedimenting the YEO phosphor includes first adjusting the pH to be less than about 3 and then subsequently increasing the pH to be in a range from about 5 to about 6. Sedimenting the LAP phosphor comprises adjusting the pH to be greater than about 10 and then subsequently decreasing the pH to be in a range from about 8 to about 9.

[0005] In one embodiment, a method is disclosed. The method includes separating a phosphor material from a starting mixture. The starting mixture includes YEO phosphor and LAP phosphor. The separation of the phosphor material from the starting material includes dispersing the starting mixture in an aqueous medium, addition of an acid, and addition of an alkali. The aqueous medium includes a dispersant and a surfactant. The surfactant includes at least about 2 Wt.% of SDS, TOAB, DDA, or any combinations. Addition of the acid forms a dispersed starting mixture having a pH in a range from 2 to about 3, and addition of the alkali increases the pH to be in a range from about 5 to about 6 and sediments the YEO phosphor.

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.

[001 1] 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.

[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.

[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 LaP0₄: 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 "separation" of the phosphor material may include separating a complex of the phosphor material from rest of the material, and then decomposing the complex to isolate the phosphor material, without decomposing the phosphor material itself in any stages of separation. In one embodiment, the complex of the phosphor material is a surface activated or surfactant attached phosphor material.

[0016] In one embodiment, the phosphor material separated from an RFP has less than or equal to 10% variation in its quantum efficiency from the standard phosphor material of the substantially same chemical composition, as measured on the powder in a spectrometer. In one embodiment, the quantum efficiency variation between the standard 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.

[0017] In one embodiment, the phosphor material is separated from the RFP based on isoelectric point (IEP) of the phosphor material. IEP, as used herein is the pH at which a particular molecule or surface carries no net electrical charge. A net charge on a molecule may be affected by pH of its surrounding environment, such as for example, the pH of the liquid in which the solid is submerged. The molecule may become more positively or negatively charged due to the interaction with protons (H⁺) or hydroxyl ions (OFF) of the surrounding environment. The IEP may affect dispersibility of a material at a given pH in a given liquid medium. At a particular pH and in a particle size range, a material at its IEP having zero charge state may coagulate or aggregate and settle down faster compared to other materials having a net charge. [0018] Some materials have minimum dispersibility in water or salt solutions at a pH that corresponds to their IEP and may undergo sedimentation at the liquid medium.

[0019] In one embodiment, a chemical 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). The IEP based chemical separation may be used in cases where two or more phosphors have different IEP and reaction kinetics with the [H⁺] and [OH ] ions.

[0020] In one embodiment, a dispersed mixture of a starting mixture including the phosphor material is formed by dispersing the starting mixture in a liquid medium. The liquid that is used for the dispersion of starting mixture may be organic, or inorganic. 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 dispersion of the starting mixture. In one embodiment, starting mixture is suspended in water.

[0021] In one embodiment, an yttrium europium oxide (YEO) phosphor, lanthanum phosphate (LAP) phosphor, or both YEO and LAP phosphors were separated by preferential sedimentation of one of the phosphors at a time. As used herein, "sedimentation" (further "sedimenting" or "sediment"-used as a verb) is a process in which the dispersed powder gets settled out from the suspended state. "Sediment" (used as a noun) is the solid deposit formed from the dispersed mixture as it settles out of suspension. As used herein a "preferential sedimentation" of a particular phosphor material is a process in which the sedimentation of the particular phosphor material is favored when compared with the other materials present in the dispersion.

[0022] Typically, sedimentation of a dispersed material is attempted by adjusting the pH of the liquid medium in which the material is dispersed to a value near the IEP of the material. The IEP of YEO is generally known to be in the range of about 7-8. The IEP of LAP is generally known to be in the range of about 4-6, and a BAM phosphor material in the range from about 8-9. The IEP of a material may vary a little from the expected range based on changes in the exact composition (including doping) and further based on surfactant-interaction of the material, if a surfactant is present in the dispersion. Considering the above factors, an YEO phosphor is generally expected to sediment from a YEO dispersion in a liquid medium, when the pH of medium is adjusted around the range of about 7-8. Similarly, a LAP may be sedimented from a LAP dispersion when the pH of the liquid medium is adjusted to be around 4-6, and BAM is expected to sediment from a BAM dispersion, when the pH of the BAM dispersion medium is adjusted to be around 7-8.

[0023] Without being bound by any particular theory, the pH for the preferential sedimentation of a phosphor material is herein disclosed to be slightly varied in a dispersion of mixture of phosphors when compared to single-phosphor dispersion. Additionally, it is herein disclosed that the preferential sedimentation of a phosphor material is further related to the directionality of pH adjustment of the liquid medium in which the phosphors are dispersed.

[0024] In one embodiment, in a starting mixture comprising YEO and LAP, the YEO sediments when pH of the liquid medium is increased from an acidic state to be in a range from about 5 to about 6. Thus, the preferential sedimentation of YEO from a starting mixture occurs when the dispersed starting mixture is initially made strongly acidic and then the pH of the mixture is increased to be in a range from about 5 to about 6. Thus, increasing the pH (or decreasing the acidity) from less than 3 to be in a range of 5-6 results in the separation of YEO from the other phosphors compared to the instance where the pH is directly adjusted to be in a range of 5-6 from a starting neutral dispersion.

[0025] Without being bound by any particular theory, the underlying mechanism responsible for this difference in preferential sedimentation behavior related to the directionality of pH adjustment (from acidic to pH of 5-6 range) is stated as follows. On highly acidifying the suspended phosphor materials in a liquid medium, the phosphor materials are effectively de-agglomerated. This de- agglomeration facilitates the separation of YEO phosphor at a particular pH range. For example, if a neutral pH suspension having YEO and other phosphors is directly acidified to a pH in a range from about 5 to about 6, the agglomerated nature of the phosphor mixture prevents the preferential sedimentation of YEO. On the other hand, if the suspension having YEO and other phosphors is first taken to a pH less than 3, the higher concentration of [H⁺] allows for de-agglomeration of the suspended phosphor materials. At this condition, adjustment of the pH to the preferred pH range of YEO results in preferential sedimentation of YEO. Therefore, the directionality of pH change is helpful in achieving separation of individual phosphors from a starting mixture. In an instance of directly adjusting the pH to be in a range of 5-6 from a neutral dispersion, even conventional methods of de-agglomeration such as use of dispersants and ultrasonication does not seem to aid in preferential sedimentation of YEO from the suspended starting mixture.

[0026] In one embodiment, the pH of the starting mixture dispersed in the liquid medium is lowered to be less than 3 before further adjusting the pH for YEO sedimentation. In one embodiment, the pH is lowered to be in a range from about 2 to about 3 and then raised to the 5-6 range for YEO sedimentation. The lowering of pH may be aided by the addition of strongly acidic materials such as hydrochloric acid, sulfuric acid, nitric acid, for example. In one embodiment, a hydrochloric acid solution is added to the dispersed starting mixture to lower the pH to be in a range from about 2 to about 3. Further, a basic solution is added to increase the pH to be in a range from about 5 to about 6.

[0027] Similar to the path dependent difference in the YEO sedimentation, the preferential sedimentation of LAP phosphor from a suspended starting mixture including phosphors is observed to be path dependent. Very little, if any, sedimentation is observed when the pH of the dispersed starting mixture is directly adjusted to be in a range from 8 to about 9. Preferential sedimentation of LAP is observed when the dispersed starting mixture is initially made strongly basic and then the pH is lowered to be in a range from about 8 to about 9. A similar de- agglomeration mechanism as put forward for preferential YEO sedimentation may further explain the IEP -based preferential separation of LAP by first preparing a more alkaline suspension and then reducing the alkalinity of the suspension. As observed in the case of YEO, in an instance of directly adjusting the pH to be in a range of 8-9 from a neutral dispersion, even conventional methods of de-agglomeration such as use of dispersants and ultrasonication does not seem to aid in preferential sedimentation of YEO from the suspended starting mixture.

[0028] In one embodiment, the pH of the starting mixture dispersed in the liquid medium is increased to be greater than 10 before further adjusting the pH for LAP sedimentation. In one embodiment, the pH is increased to be in a range from about 10 to about 11 and then lowered to the 8-9 range for LAP sedimentation. The lowering of pH may be aided by the addition of strongly basic materials such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, for example. In one embodiment, a sodium hydroxide solution is added to the dispersed starting mixture to increase the pH to be in a range from about 10 to about 1 1. Further, an acidic solution is added to lower the pH to be in a range from about 8-9.

[0029] Dispersion of RFP for preferential sedimentation of YEO, LAP or

BAM phosphor material may further 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.

[0030] In one embodiment, the dispersant that is used to disperse the suspended mixture includes sodium hexametaphosphate, ammonium salts of acrylic polymer, or combinations thereof.

[0031] 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.

[0032] The surfactant that is used along with the phosphor blend of the RFP herein may include 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- (Ν,Ν-dimethyl octadecyl ammonio) propane sulfonate, sodium oleate, oleic acid, Bis(2-ethylhexyl) phosphate, tetraoctyl ammonium bromide (TOAB), or a combination thereof.

[0033] In one embodiment, the RFP is reacted with a surfactant in the liquid medium to form a phosphor material-surfactant complex before any pH adjustment. The surfactants may further help in the dispersion of the phosphors and improve reaction kinetics for the [H⁺], [OFF], or both [H⁺] and [OFF] ions with specific phosphor sites. 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 1 Wt% to about 5 Wt% of surfactant in the liquid medium is used. In one embodiment, concentration of the surfactant in the liquid medium is in a range from about 2 Wt% to about 4 Wt%.

[0034] 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 in a range from about 2 microns to 4 microns. A smaller size of the RFP powder helps in suspension and electrostatic separation of the phosphor material.

[0035] 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

[0036] 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.

[0037] A process is disclosed herein to separate and recover YEO from a starting mixture including a phosphor blend using an IEP based separation method. In one example, a phosphor blend including YEO, LAP, and BAM was milled to reduce the particle size of the phosphor powders to approximately 3.5 microns. The mixture was mixed with water with about 15-40 grams per liter solid loading. The phosphor blend was dispersed and suspended 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. About 2 Wt% of DDA surfactant was added and dispersed by ultrasonicating for about 30 minutes.

[0038] The pH of the dispersed mixture was adjusted to be about 2.5 by the addition of about 1 molar hydrochloric acid during magnetic stirring. Further, the pH was raised to about 5.5 by a slow addition of ammonium hydroxide during continuous stirring. After the addition of ammonium hydroxide solution, the mixture was allowed to settle down for about 30 minutes and the suspension containing the dispersed mixture was decanted to separate other (green/blue) phosphors (in filtrate) from the YEO phosphor (in sediment). The preferentially settled YEO phosphor was recovered by filtration, washing, drying, and an optional heat-treatment. The process of sedimenting YEO by taking the dispersed mixture to a highly acidic condition and adjusting the pH back to about 5.5 was repeated for about 4 times to substantially sediment out the YEO in the starting mixture. It was particularly observed that the direct adjustment of the pH to a value of 5.5 from the initial dispersed mixture, without strongly acidifying the dispersion first, did not result in the preferential sedimentation of YEO.

[0039] In another example, a phosphor blend including LAP, YEO, and BAM having approximately 3.5 microns average particle size was taken for LAP separation. The mixture was mixed with water with about 15-40 grams per liter solid loading. The phosphor blend was dispersed and suspended 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. About 2 Wt% of TOAB surfactant was added and dispersed by ultrasonicating for about 30 minutes.

[0040] The pH of the dispersed mixture was adjusted to be about 10.5 by the addition of ammonium hydroxide during magnetic stirring. Further, the pH was lowered to about 8.3 by a slow addition of 1M hydrochloric acid during continuous stirring. After the addition of hydrochloric acid, the mixture was allowed to settle down for about 30 minutes and the suspension containing the dispersed mixture was decanted to separate other (red/blue) phosphors (in filtrate) from the LAP phosphor (in sediment). The preferentially settled LAP phosphor was recovered by filtration, washing, drying, and an optional heat-treatment. The process of sedimenting LAP by taking the dispersed mixture to a highly basic condition and adjusting the pH back to about 8.3 was repeated for about 4 times to substantially sediment out the LAP in the starting mixture. Further, it was observed that the direct adjustment of the pH to a value of 8.3 from the initial dispersed mixture, without first increasing the pH to make a highly basic dispersion, did not result in the preferential sedimentation of LAP.

[0041] In one embodiment, wherein YEO, LAP, and BAM are present, all the three phosphors may be separated by following the above steps sequentially to remove YEO and LAP and to recover BAM. Further, by the pH adjustment, it is feasible to remove LAP first and, then separate YEO and BAM using the sedimentation methods as explained above. [0042] 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

CLAIMS:

1. A method, comprising: separating a phosphor material from a starting mixture by dispersing the starting mixture in a liquid medium and varying the pH to sediment a yttrium europium oxide (YEO) phosphor, lanthanum phosphate (LAP) phosphor, or both, wherein sedimenting the YEO phosphor comprises adjusting the pH to be less than about 3 and then increasing the pH to be in a range from about 5 to about 6; and wherein sedimenting the LAP phosphor comprises adjusting the pH to be greater than about 10 and then decreasing the pH to be in a range from about 8 to about 9.

2. The method of claim 1, wherein the liquid medium further comprises a surfactant.

3. The method of claim 2, wherein the surfactant comprises sodium dodecyl sulfate (SDS), cetrimonium bromide (CTAB), ammonium lauryl sulfate (ALS), dioctyl sulfosuccinate sodium salt (AOT), hexadecyl(2- hydroxyethyl)dimethylammonium dihydrogen phosphate, 3-(N,N- dimethyloctadecylammonio)propanesulfonate, sodium oleate, oleic acid, Bis(2-ethylhexyl) phosphate, tetraoctylammonium bromide (TOAB), or combinations thereof.

4. The method of claim 3, wherein the surfactant comprises SDS, TOAB, AOT, or a combinations thereof.

5. The method of claim 2, wherein an amount of surfactant in the liquid medium is in a range from about 1 Wt% to about 5 Wt%.

6. The method of claim 2, wherein a median particle size of the starting mixture is less than about 5 microns.

7. The method of claim 6, wherein the median particle size of the starting mixture is in a range from about 2 microns to about 4 microns.

8. The method of claim 1, wherein adjusting the pH to be in a range from about 2 to about 3 comprises addition of an acid.

9. The method of claim 1, wherein increasing the pH to be in a range from about 5 to about 6, comprises addition of an alkali.

10. The method of claim 1, wherein adjusting the pH to be in a range from about 10 to about 1 1, comprises addition of an alkali.

11. The method of claim 1 , wherein decreasing the pH to be in a range from about 8 to about 9, comprises addition of an acid.

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

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

14. A method, comprising: separating a phosphor material from a starting mixture comprising YEO phosphor,LAP phosphor, and BAM phosphor,wherein the separating comprises dispersing the starting mixture in an aqueous medium comprising a dispersant and a surfactant, wherein the surfactant comprises at least about 2 Wt% of SDS, TOAB, DDA, or a combinations thereof; adding an acid to form an acidified dispersed starting mixture having a pH in a range from 2 to about 3 ; and sedimenting theYEO phosphor by adding an alkali to increase the pH to be in a range from about 5 to about 6.

15. The method of claim 14, further adding alkali to increase the pH to a range from about 10 to about 11.

16. The method of claim 15, further sedimenting the LAP by adding an acid to decrease the pH to be in a range from about 8 to about 9.