JPH0825731B2 - Method for producing rare earth phosphate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention mainly relates to a method for producing a rare earth phosphate useful as a raw material for a green light emitting phosphor and a rare earth phosphate for a green light emitting phosphor, and spray drying. By introducing a process into spherical particles of rare earth phosphate or phosphor fine particles, the powder characteristics of the rare earth phosphate or the emission characteristics and powder characteristics of the phosphor are improved. This is a manufacturing method that can simplify the manufacturing process.

(Prior Art) terbium (Tb) in activated with orthophosphoric acid cerium _{[(Ce x Tb y) PO} 4] is emitting green fluorescence by ultraviolet (253 nm) stimulation, mainly 545nm its spectrum by Tb Wavelength is a well-known fact (G.Blass
and A. Bril J. chem, physics 3252 51 (1969)). This phosphor is a rare earth phosphate [(Re which has improved emission characteristics by optimizing the composition, improving the manufacturing method, adding an activator, etc.
_1-xy Ce _x Tb _y ) PO ₄ (However, Re is at least one of La, Y, and Ga, X> 0, Y> 0, 0 <X + Y <1) Have been proposed as a green light emitting phosphor for a fluorescent lamp (Japanese Patent Application Laid-Open No. 60-10065, Japanese Patent Application Laid-Open No. 59-59165).
No. 179578, Japanese Patent Publication No. 41673/1989), and as its synthesis method, a synthesis method in which various phosphates are added to an aqueous solution of a rare earth chloride or a nitrate is known.

(Problems to be Solved by the Invention) However, the rare earth phosphates obtained by these synthetic methods convert rare earth chlorides or nitrates into a rare earth carbonate, an oxalate, an oxide or the like, and further Rare earth phosphates are manufactured by the dry melting method or the wet precipitation method by adding phosphate, but the manufacturing process is complicated and many raw materials are required.Furthermore, since the precipitation of phosphate is fine particles, it is washed with water and filtered. It takes a long time, and further, the cake has a drawback that it solidifies when dried and becomes difficult to pulverize. Therefore, industrial mass production of the green-emitting phosphor by these synthetic methods was extremely difficult. In addition, there is an increasing demand for the production of fine spherical phosphors having a uniform particle diameter for the purpose of improving the fluorescent characteristics and powder characteristics. For this purpose, for example, in Japanese Patent Publication No. 45-37296 and Japanese Patent Publication No. 52-37581, a phosphor slurry is jetted into a combustion flame using a nozzle for granulation.
-201989 discloses a manufacturing method in which a phosphor material is melt-cooled by high-frequency plasma for granulation, and JP-A-1-108294 discloses a manufacturing method in which a phosphor material is melt-cooled by a laser beam and granulated. It was However, the slurry of the phosphor raw material produced by these methods is highly viscous, and after filtering and washing with a filter press or the like according to the conventional production method and drying the cake, it solidifies after drying, which makes it difficult to pulverize. It was When the slurry is dried as it is in a vacuum dryer, a drum dryer, a freeze dryer, etc.,
Although it is considerably pulverized, it contains a large number of agglomerated particles and requires a pulverization step, so that these methods are still difficult to put into practical use industrially and economically.

An object of the present invention is to provide a method for producing a rare earth phosphate and a phosphor, which are excellent in fluorescence characteristics and powder characteristics, which solve the above disadvantages and drawbacks.

(Means for Solving the Problems) The inventors of the present invention have conducted extensive studies in order to solve such problems, and spray-dry a slurry containing a rare earth phosphate, which has been conventionally considered to be difficult, to obtain spherical particles. Successful,
The present invention has been reached by studying the spray drying conditions in detail.

The gist of the present invention is to dissolve a rare earth oxide in an inorganic acid to form a rare earth mixed acidic aqueous solution, add a solution containing phosphoric acid and phosphate ions to this, and obtain a rare earth hydrous phosphate obtained by reacting at a pH of 5 or less. A method for producing a rare earth phosphate characterized by spheroidizing a salt slurry by a spray drying method, and a green color characterized by spheroidizing a slurry containing a rare earth phosphate to which an activator is added by a spray drying method. A method for producing a rare earth phosphate for a light emitting phosphor.

 Hereinafter, the present invention will be described in detail.

The applicable range of the present invention includes Y as the rare earth element Re.
La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu are generic names of lanthanoid elements and are selected from these 1
It is a species or a mixture of two or more elements. The green light-emitting phosphor includes Ce and Tb as essential elements, at least one of La, Gd, and Y as a rare earth element, and the basic salt is a rare earth orthophosphate RePO ₄ and a rare earth orthophosphate. RePO ₄ acid salt as a main component with some activators partially substituted by several wt% SiO ₂ , Al ₂ O ₃ , B ₂ O _3, etc., several tens of ppm
Halogen elements (F, Cl, Br, I) and 1A group elements (Li, N)
Examples thereof include those in which a, K, Rb, Cs) and the like are activated to greatly improve the fluorescence characteristics. Specifically, (La _0.5 Ce _0.3 Tb
_0.2 ) PO ₄ , (La _0.5 Ce _0.3 Tb _0.2 ) PO ₄・ 0.2SiO ₂ , (La _0.5
Ce _0.3 Tb _0.2 ) PO ₄ .0.3Al ₂ O ₃ .0.2SiO _{2 and the} like.

 Next, a method for producing a rare earth phosphate will be described.

First, as a raw material, various rare earth oxides and carbonates corresponding to desired rare earth elements are dissolved with an inorganic acid such as hydrochloric acid, nitric acid or sulfuric acid to prepare a rare earth element mixed acidic salt aqueous solution. Nitric acid is particularly preferable for the dissolution of the raw material, because it is easy to remove in the washing process and the firing process which are the subsequent processes.
The concentration of these aqueous solutions may be any concentration from a dilute solution to a saturated solution, but is usually preferably 0.01 to 1 mol / about.

Then, these aqueous solutions are reacted with solutions containing various phosphates and phosphate ions to obtain a precipitate of a rare earth hydrous phosphate [RePO ₄ .nH ₂ O, 0 ≦ n ≦ 2]. Where C
Since e and Tb are easily oxidized by air in the alkaline region above PH7, it is difficult to precipitate them as pure trivalent rare-earth hydrous phosphate. For this reason, they are reacted in the acidic region below PH5. Is desirable. Further, in order to control the particle size of these precipitates and to remove impurities in the phosphor, phosphoric acid H ₃ PO ₄ solution or phosphoric acid 2
A powder or aqueous solution of ammonium hydrogen (NH ₄ ) H ₂ PO ₄ is desirable. The slurry of the rare earth hydrous phosphate obtained here has a solid content concentration of 0.01 to 30% by weight, a crystal grain size of 0.1 to 40 μm, an average grain size of 5 μm, and a shape of needle, sphere, or irregular shape.

Next, the slurry of the hydrous phosphate containing rare earth may be spray-dried as it is, but usually, it goes into a washing step, an activator mixing step, and a reaction step. The washing process employs a decanting method (a method of repeating sedimentation concentration and discarding the supernatant several times), a method of filtering with a filter press or the like, and a method of dispersing after washing in an organic solvent with a ball mill. Industrially, it is desirable to wash the slurry while concentrating the slurry with a continuous thickener or a rotary filter to remove unnecessary components from the solution. In addition, hot water is preferable to water for washing,
It is also effective to use a solvent such as ethanol or acetone.
By using these solvents, it is possible to produce a rare earth phosphate having a particularly low cohesiveness.

The rare earth phosphate is then separated and dried from these slurries, and the most important feature of the present invention is that the slurries are spray-dried here. By spray-drying, the step of separating the rare earth phosphate becomes unnecessary, and the step can be simplified. It's even easier to get a uniform particle size distribution, close to a true sphere,
Spherical particulate rare earth phosphates can be produced.
The particle size distribution of this rare earth phosphate changes depending on conditions such as slurry concentration and hot air temperature, but the average particle size is 0.5-100 μm.
It can be easily controlled between m and therefore does not require a grinding step.

The spray dryer may be a rotary disk type, a two-fluid nozzle type, or the like as a sprayer, but is not particularly limited. As for the dryer, there is also a so-called spray dryer in which spray particles are dried by a swirling hot air flow in a chamber or a method of instantaneously freezing spray particles and drying the frozen particles in a vacuum dryer.
The spray drying conditions are arbitrarily set in consideration of the desired particle size of the granulated powder, the spray drying model and its characteristics, the type and amount of the activator, and the like. Further, according to the spray drying method, in order to conventionally activate the rare earth orthophosphate with the activator, the powder of the salt containing the activator and the rare earth hydroxide are sufficiently pulverized and mixed in a ball mill or the like, A method of firing was proposed,
In the present invention, the activator can be activated very easily by mixing the activator in the form of powder or aqueous solution into the slurry of rare earth phosphate.

The final step is 1,200 rare earth phosphate in a weak reducing atmosphere
When fired at ℃ × 2Hr, a bright green light-emitting phosphor whose main component is the rare earth phosphate RePO ₄ can be obtained. Although the particle size of this phosphor is about 20% smaller than that of the rare earth phosphate, the particle shape is fine spherical and the flowability is extremely good, and the aggregation due to firing is small, and thus the pulverization step is unnecessary. Further, by performing flux treatment and mixed firing of the activator in this firing step, it is possible to manufacture a phosphor having higher brightness and higher characteristics.

As described above, the present invention is highly practical as a method for producing a phosphor raw material and has versatility for various phosphors containing a rare earth phosphate as a main component.

Hereinafter, the embodiments of the present invention will be specifically described with reference to Examples, but the present invention is not limited thereto.

(Example 1) Lanthanum oxide (La ₂ O ₃ ) 81.5 g, cerium carbonate [Ce ₂ (C
O ₃ ) ₃・ nH ₂ O, Ce 40% content] 105.0 g, terbium oxide (Tb ₄ O ₇ ) 37.4 g, nitric acid 3 mol, hydrogen peroxide solution (H ₂ O ₂ ).
It was dissolved in an aqueous solution of 70 ° C. containing 0.1 mol. In this rare earth acidic solution, ammonium dihydrogen phosphate [(NH ₄ ) H
₂ PO ₄ ] powder was added and mixed well, and after aging for 1 hour,
Ammonia solution was added to adjust to PH4, and the mixture was aged for 2 hours. In this way, a precipitate of a rare earth hydrous phosphate [RePO ₄ .nH ₂ O, 0 ≦ n ≦ 2] having a good settling property and a large particle size was obtained. After leaving the rare earth hydrous phosphate for several hours,
The supernatant liquid was discarded, water at 90 ° C was added, and decanting (precipitation concentration, washing method) was repeated to sufficiently wash the slurry.
The slurry of the rare earth hydrous phosphate thus obtained had a rare earth concentration of 1 mol / PH 6, a solid content concentration of 20% and a viscosity of 1000 cps. Next, this slurry was spray-dried with a rotary disk type spray dryer. The drying conditions were as follows: atomizer rotation speed 10,000 rpm, hot air temperature 200 ° C., slurry supply rate 0.5 / Hr. The rare earth hydrous phosphate powder thus obtained already emitted green fluorescence. This rare earth hydrated phosphate is
When fired at 200 ℃ × 2Hr, bright green rare earth phosphate
230g was obtained. Its composition is (La _0.5 Ce _0.3 Tb _0.2 ) PO ₄ , and the particle size is 15-50 μm measured by Coulter counter.
The average particle size was 26 μm, and the particle shape was a fine spherical shape with very good fluidity.

(Example 2) The same raw material as in Example 1 except that the rare earth hydrate salt slurry was spray-dried with a two-fluid nozzle spray dryer,
A rare earth hydrate salt slurry was obtained under the manufacturing conditions. The spray drying conditions were an air pressure of 1.8 kg / cm ² , a hot air temperature of 180 ° C., and a slurry feed rate of 2 / Hr. This rare earth hydrous phosphate powder was calcined in a weak reducing atmosphere at 1,200 ° C. for 2 hours to obtain 230 g of a bright green rare earth phosphate rare earth phosphate. The composition was (La _0.5 Ce _0.3 Tb _0.2 ) PO ₄ , the particle size was distributed in the range of 2 to 20 μm, the average particle size was 6 μm, and the particle shape was fine spherical and had good fluidity.

Example 3 8.15 g of lanthanum oxide (La ₂ O ₃ ) and cerium carbonate [{Ce ₂
(CO ₃ ) ₃ } ₃ · nH ₂ O, Ce 40% content product] 10.50 g, terbium oxide (Tb ₄ O ₇ ) 3.74 g, 70 ° C., 1 aqueous solution containing 0.3 mol of nitric acid and 0.01 mol of hydrogen peroxide solution Dissolved in.
Ammonium dihydrogen phosphate (NH
₄ ) 12 g of H ₂ PO ₄ powder was added and mixed well, and after aging for 1 hour, an ammonia solution was added to adjust to PH 4 and aging was performed for 2 hours. In this way, a precipitate of rare earth hydrous phosphate [RePO ₄ .nH ₂ O, 0 ≦ n ≦ 2] having a large particle size was obtained. After the rare earth hydrated phosphate was left standing for several hours, the supernatant was discarded, warm water at 90 ° C. was added, and decanting was repeated to sufficiently wash the slurry. Water was added to the slurry of the rare earth hydrous phosphate thus obtained, and diluted 10 times to obtain a rare earth concentration of 0.1 mol /, PH6, a solid content concentration of 2% and a viscosity of 50 cps.
And The slurry was then sprayed onto the surface of liquid nitrogen using a two-fluid nozzle to freeze and then the frozen powder was quickly placed in a freeze dryer to dry. The drying conditions were such that the degree of vacuum was 10 ⁻³ mmHg and the cold trap was liquid nitrogen cooling. The rare earth hydrous phosphate powder thus obtained already emitted green fluorescence. When this rare earth hydrated phosphate is calcined at 1,200 ℃ × 2Hr in a weak reducing atmosphere, 20g of bright green rare earth phosphate rare earth phosphate is produced.
I was able to manufacture. Its composition is (La _0.5 Ce _0.3 Tb _0.2 ) PO ₄ , and the particle size is _{0.5 to 5} μm on a coal tar counter.
The average particle size was 1.5 μm, and the particle shape was a slightly distorted fine spherical shape.

(Comparative Example 1) 81.5 g of lanthanum oxide (La ₂ O ₃ ) and cerium carbonate [Ce ₂ (C
_{O 3) 3 · nH 2 O} , Ce40% product containing] 105.0 g, terbium oxide (Tb ₄ O ₇₎ 37.4g, nitric acid 3 mol, 70 ° C. containing hydrogen peroxide 0.1 mol, dissolved in 20 solution of Then, an aqueous solution of 200 g of oxalic acid (H ₂ C ₂ O ₄ .2H ₂ O) dissolved in 2 was added dropwise to the rare earth acidic solution for 10 minutes for reaction. The generated rare earth oxalate was filtered, washed with water, and then calcined in air at 1,000 ° C for 2 hours to obtain 160 g of rare earth oxide. To this rare earth oxide, 230 g of ammonium dihydrogen phosphate [(NH ₄ ) H ₂ PO ₄ ] powder was added, mixed well, and fired at 1,000 ° C. for 2 hours in air to synthesize a rare earth phosphate. Since this baked product aggregated in the form of pumice, it was crushed well in a mortar and washed well with warm water at 90 ° C. This is 1,200 ℃ × 2 in a weak reducing atmosphere
Hr calcination produced 230 g of bright green rare earth phosphate rare earth phosphate. Its composition is (La _0.5 Ce _0.3 T
b _0.2 ) PO ₄ , the particle size was distributed in the range of 1 to 40 μm, and the average particle size was 5 μm. The particles had a short columnar shape and contained aggregated particles, and the fluidity was extremely poor.

(Effect of the Invention) The method for producing a rare earth phosphate and a rare earth phosphate for a green light emitting phosphor of the present invention is characterized in that a slurry containing the rare earth phosphate is spray-dried to obtain spherical particles. According to the method, compared with the conventional manufacturing method, the crushing step is not necessary, the raw material cost and the manufacturing cost are low, and it is economical. Moreover, since the rare earth phosphate obtained by this production method is spherical and has a sharp particle size distribution and improved powder characteristics, the fluorescent emission characteristics are also improved, and the coating work efficiency for fluorescent lamps, CRTs, etc. is also improved. Its industrial utility value is extremely high.

Claims (2)

[Claims] 1. A rare earth hydrated phosphate obtained by dissolving a rare earth oxide in an inorganic acid to prepare a rare earth mixed acidic aqueous solution, adding a solution containing phosphoric acid and a phosphate ion, and reacting the solution at pH 5 or less. A method for producing a rare earth phosphate, which comprises spheroidizing a slurry by a spray drying method. 2. A method for producing a rare earth phosphate for a green light emitting phosphor, which comprises spheroidizing a slurry containing a rare earth phosphate to which an activator is added by a spray drying method.