JPH11140440A - Fluorescent substance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a fluorescent substance that comprises fluorescent particles uniformly and firmly coated with pigment particles and can realize electronic tubes having excellent contrast characteristics with reduced peeling and removal of the pigment particles, when they are used as a fluorescent layer in an electronic tube. SOLUTION: Fluoresent particles 2 comprising a rare earth trisulfide as a parent substance and containing at least one selected from europium(Eu), terbium(Te), and samarium(Sm) as an activator are coated with pigment particles 3a. In this case, the pigment particles 3a are made of iron oxide converted from iron hydroxide and coated on the surfaces of the fluorescent particles 2 and simultaneously the pigment particles are adhered via a crosslinking membrane formed by an acetalization reaction to the surfaces of the fluorescent particles.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phosphor and a method for producing the same, and more particularly to a phosphor in which pigment particles are uniformly and firmly adhered to the surface of the phosphor particles. The present invention relates to a phosphor capable of realizing an electron tube excellent in contrast characteristics with less detachment and detachment of pigment particles, and an effective manufacturing method thereof.

[0002]

2. Description of the Related Art Generally, blue (B), green (G), and red (R) light emitted by electron beam excitation are provided in a picture tube (CRT) of a color television receiver or a cathode ray tube (CRT) for a computer display. Light emitting phosphors are used.

[0003] For example, as a blue light emitting phosphor for color television, silver and chlorine activated zinc sulfide phosphor (Zn) is used.
S: Ag, Cl), silver and aluminum activated zinc sulfide phosphor (ZnS: Ag, Al), silver, gold, aluminum activated zinc sulfide phosphor (ZnS: Ag, Au, Al), etc. are used, Examples of the green light emitting phosphor include copper and aluminum activated zinc sulfide phosphors (ZnS: Cu, Al),
Copper, gold, aluminum activated zinc sulfide phosphor (ZnS: C
u, Au, Al). As the red light emitting phosphor, europium-activated yttrium sulfide phosphor (Y ₂ O ₂ : Eu) or the like is generally widely used.

In particular, in the above-described red light-emitting phosphor for an electron tube, the surface of the phosphor particles is generally covered with colored pigment particles such as iron or the like made of ferric oxide (α-Fe ₂ O ₃ ), and the filter effect is obtained. Thus, the visible light is partially cut, and the reflected light is reduced by the effect of absorbing external light, thereby improving the contrast of the electron tube.

For example, according to Japanese Patent Publication No. 53-31831, as a method of coating the pigment particles, latex colloid particles are used and coagulated together with the pigment particles in an aqueous medium, whereby the pigment particles are converted into a phosphor. A method for attaching to a particle surface is disclosed.

On the other hand, according to JP-A-55-164283, pigment particles are coated on the surface of phosphor particles by using a solution of polyacrylic acid, polymethacrylic acid and gelatin and lowering the pH of the solution to cause aggregation. A method is described. JP-A-55-164282 discloses that a non-functional monomer is copolymerized with a compound having an epoxy group or a polyfunctional monomer molecule having a plurality of polymerizable double bonds, and a pigment particle is formed by a crosslinking structure. A method for attaching the phosphor particles to the surface of the phosphor particles is described.

[0007]

However, in the conventional phosphors to which the pigment particles are adhered by the above-described method, the adhesion of the pigment particles to the phosphor particles is weak, and a high-viscosity phosphor slurry is used. There was a problem that the pigment particles peeled off due to prolonged stirring in the inside, and the filter effect by the pigment particles was not sufficiently exhibited.

That is, in the electron tube manufacturing process, the phosphor for an electron tube is dispersed in a highly viscous liquid containing polyvinyl alcohol (PVA), ammonium bichromate (ADC), and several kinds of surfactants and dispersants. It becomes a slurry. This phosphor slurry is applied to an electron tube panel such as a cathode ray tube after a long stirring operation for 20 to 60 hours. However, the above-described long-time stirring makes the peeling and falling off of the pigment particles remarkable, and the improvement of the contrast characteristics by the filter effect of the pigment particles is insufficient.

On the other hand, in the conventional method of attaching pigment particles using gelatin, it is difficult to continuously obtain natural gelatin having a stable quality.
There is a problem unfavorable in manufacturing control. On the other hand, in the method using polyacrylic acid or polymethacrylic acid, these compounds are unstable in an aqueous solution due to the presence of a carboxyl group, and are likely to deteriorate such as a change in the degree of polymerization, which affects the coating with pigment particles. Accordingly, there is a problem that the pigment particles are easily peeled off in the phosphor slurry.

Furthermore, in the above three methods, all are made of iron oxide (α-Fe ₂ O ₃ ) as a pigment, and the adhered state is as shown in FIG. The pigment particles 3 were non-uniformly adhered to the surface, the filter effect of the pigment particles 3 was not sufficiently exhibited, and it was difficult to improve the contrast characteristics of the electron tube.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and is a phosphor in which pigment particles are uniformly and firmly adhered to the surface of the phosphor particles. An object of the present invention is to provide a phosphor capable of realizing an electron tube excellent in contrast characteristics with less detachment and detachment of pigment particles and an effective method for producing the same.

[0012]

Means for Solving the Problems In order to achieve the above object, the present inventors have made various pigment particles adhere to the surface of the phosphor particles, and the manufacturing method, type, and adhesion method of the pigment particles are adjusted to the phosphor characteristics. The effects were compared by experiments. As a result, the pigment particles are generated by the precipitation reaction, and when the generated pigment particles are adhered to the surface of the phosphor particles through the cross-linked film formed by the acetal reaction, the pigment particles have a high adhesive force on the surface of the phosphor particles. It has been found that the phosphor can be uniformly adhered, and a phosphor excellent in a filter effect with little peeling off of pigment particles can be effectively obtained. The present invention has been completed based on the above findings.

That is, the phosphor according to the present invention has a rare earth oxysulfide as a host and contains at least one selected from europium (Eu), terbium (Te) and samarium (Sm) as an activator. In the phosphor comprising particles and pigment particles attached to the surface of the phosphor particles, the pigment particles are made of iron oxide converted from iron hydroxide on the surface of the phosphor particles, and the pigment particles are The phosphor particles are attached to the surface of the phosphor particles via a crosslinked film formed by an acetal reaction. Further, the iron hydroxide is characterized in that it is iron hydroxide formed by a precipitation reaction between at least one iron salt selected from iron nitrate, iron sulfate and iron chloride and aqueous ammonia. Further, the iron oxide is characterized by being iron oxide generated by baking treatment of iron hydroxide.

The method for producing a phosphor according to the present invention comprises a rare earth oxysulfide as a host and at least one selected from europium (Eu), terbium (Te) and samarium (Sm) as an activator. After dispersing the phosphor particles in an aqueous iron salt solution, aqueous ammonia is added to the dispersion to adjust the pH to a range of 5.0 or more and 7.0 or less. Depositing and adhering, and baking the iron hydroxide precipitated and adhering to the surface of the phosphor particles in a temperature range of 600 to 800 ° C. to convert the iron hydroxide to iron oxide to obtain pigment particles, and baking treatment Dispersing the phosphor particles thus obtained in pure water and dissolving the aldehyde and alcohol in the obtained dispersion; and adjusting the pH of the dispersion in which the aldehyde and alcohol are dissolved to 1. Adjusted to the range of 3.0, yielding a crosslinked film with alcohol acetal reaction, characterized by comprising the step of attaching to the surfaces of phosphor particles and pigment particles through the crosslinked membrane.

Here, in the above manufacturing method, first, Y ₂
Phosphor particles containing a rare earth oxide such as O ₃ S as a base material and containing at least one selected from europium (Eu), terbium (Tb) and samarium (Sm) as an activator are prepared. . Next, iron nitrate (Fe
_{(NO 3) 3 · 9H 2} O), iron sulfate (FeSO ₄ · 7H
₂ O) and an aqueous solution of at least one iron salt selected from ferric chloride (FeCl ₂ · 4H ₂ O) were dispersed the phosphor particles to prepare a dispersion. Next, ammonia water was added to this dispersion to adjust the pH to a range of 5.0 to 7.0, so that iron hydroxide (III
) Is deposited.

Next, the phosphor particles on which the iron hydroxide has been deposited are baked (baked) at a temperature in the range of 600 to 800 ° C., whereby the deposited iron hydroxide (III) is deposited.
) To iron oxide (III). Here, when the baking treatment temperature is as low as less than 600 ° C., the produced iron oxide (III) is γ-type iron oxide, which is not preferable. on the other hand,
When the baking temperature exceeds 800 ° C., the luminance of the phosphor is deteriorated, which is not preferable. Therefore, the baking temperature is set in the range of 600 to 800 ° C.

Next, the phosphor particles having undergone the above-mentioned baking treatment step are dispersed in pure water, and polyvinyl alcohol (PVA) is dissolved in the dispersion, and then the aldehyde is dissolved. Here Although aldehydes are generally compounds given by the chemical formula of C _n H _{2n + 1} CHO, in the method of the present invention, in order to utilize the reaction in particular in aqueous solution, as formaldehyde or acetaldehyde with a hydrophilic, It is preferable to use a lower aldehyde having a straight-chain carbon number (n) of 3 or less. If the number of carbon atoms (n) exceeds 3, the hydrophobicity becomes high, and the acetal reaction described later hardly proceeds, which is not preferable.

An acid such as hydrochloric acid is added dropwise to the dispersion in which the alcohol and aldehyde such as PVA are dissolved to adjust the pH of the dispersion to a range of 1.5 to 3.0. The acetal reaction proceeds. This acetal reaction proceeds rapidly at room temperature only by adjusting the pH of the dispersion. As the acid used for the pH adjustment, it is preferable to use an acid that does not damage the rare earth oxysulfide as the phosphor base material, for example, hydrochloric acid. When the pH value is less than 1.5, the phosphor particles and the pigment particles are liable to be damaged. On the other hand, when the pH value exceeds 3.0, the acetal reaction proceeds insufficiently, and the crosslinked film Is small and the pigment particles are easily peeled off from the phosphor particle surfaces.

Pigment particles made of iron oxide (III) are produced by baking treatment of iron (III) hydroxide produced by the above-mentioned precipitation reaction, and further, by a crosslinked film produced by an acetal reaction between an alcohol such as PVA and an aldehyde, A phosphor (a phosphor with a pigment) in which pigment particles are uniformly and firmly attached to the surface of the phosphor particles is obtained. In order to further enhance the dispersibility of the phosphor, it is also effective to perform a bead milling treatment or an ultrasonic dispersion treatment in water after the above reaction. The processing time for the above-mentioned bead milling or the like is 60 minutes or more.

According to the phosphor and the method for producing the same according to the above-described constitution, the pigment particles are made of iron oxide converted from iron hydroxide on the surface of the phosphor particles, and the phosphor particles pass through the crosslinked film formed by the acetal reaction. Since the pigment particles are adhered to the particle surface, the pigment particles are uniformly adhered to the phosphor particle surface with high adhesion strength. Therefore, a phosphor with less peel-off of pigment particles can be obtained. When this phosphor is used as a phosphor layer of an electron tube, a filter effect by the pigment particles can be sufficiently exerted, and an electron tube with excellent contrast characteristics can be obtained.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be specifically described based on the following examples.

Example 1 Eu ₂ O ₃ powder was converted to Eu with respect to Y ₂ O ₃ powder.
1050 g of sodium carbonate, 750 g of sulfur, and 120 g of calcium phosphate were mixed with 1.5 kg of the coprecipitate to which 7% by weight was added, and the mixture was filled in an alumina crucible and heated at a temperature of 1150 ° C. in a sulfur atmosphere. It was baked for 4 hours. The obtained fired product was washed several times with diluted nitric acid to produce Y ₂ O ₂ S: Eu phosphor particles.

On the other hand, 1.4 g of iron nitrate (Fe (NO ₃ )
₃ · 9H ₂ O) into After sufficiently dissolved in pure water of 1000 ml, aqueous solution was prepared by heating to a temperature 90 ° C.. 100 g of the above-mentioned Y ₂ O ₂ S: Eu phosphor was suspended in this aqueous solution and stirred for 10 minutes to obtain a dispersion.

Next, a dilute aqueous ammonia solution was added to the dispersion to adjust the pH of the dispersion to 6.5, followed by stirring for 60 minutes to deposit and precipitate iron hydroxide on the surface of the phosphor particles. During this time, the temperature of the dispersion was kept at 90 ° C. Further, the dispersion liquid after the precipitation reaction was washed with pure water and dried, and then baked at a temperature of 600 ° C. to convert iron (III) hydroxide on the surface of the phosphor particles into iron (III) oxide.

Next, the phosphor particles after the baking treatment were suspended in 300 ml of pure water and stirred for 10 minutes. Further, 2.55 g of PVA powder was added to the obtained suspension and stirred for 10 minutes. Further, after adding 15 ml of formalin to the suspension, hydrochloric acid was added to adjust the pH to 1.5.
Then, the mixture was stirred for 60 minutes to cause an acetal reaction. PH of the dispersion after this acetal reaction
Was adjusted to 6.0 with a dilute aqueous ammonia solution, and after the reaction product was allowed to settle, the supernatant was removed and washed three times with pure water.

Thereafter, the washed sediment was washed with 500
in 25 ml of pure water and 0.2 ml of a 25% strength water glass and a 0.4 mol / l ZnSO ₄ solution.
After the surface treatment was performed by stirring for 30 minutes, the supernatant was removed, and the precipitate was washed three times with pure water. Further, the washed sediment was filtered, and the residue cake was filtered.
After drying at 00 ° C. for 20 hours, the phosphor according to Example 1 was prepared by sieving.

50 g of the phosphor according to Example 1 thus prepared was dispersed in 65 ml of pure water, and 60 g of PVA, 3 ml of ADC, 0.5 g of sodium dodecyl sulfate and 10 ml of tamol were added. The phosphor slurry was settled after forcibly stirring for 3 hours, and the state of adhesion of the pigment particles on the surface of the phosphor particles was observed. As a result, the pigment particles uniformly adhered to the entire surface of the phosphor particles. And no peeling-off was observed. The phosphor slurry is dried to form a phosphor layer and has a wavelength of 450 nm.
When the body color reflectance of the phosphor layer was measured by irradiating ultraviolet light of nm, it was 53.0%, and it was found that the peeling of the pigment particles was small. Further, even when the phosphor slurry was applied to a screen of an electron tube to form a phosphor layer, an electron tube having good contrast characteristics was obtained.

[0028] Example 2 1.4 g of iron sulfate _{(FeSO 4 · 7H 2 O)} 1000
After sufficiently dissolved in ml of pure water, the solution was heated to a temperature of 90 ° C. to prepare an aqueous solution. 100 g of the Y ₂ O ₂ S: Eu phosphor particles prepared in Example 1 were suspended in this aqueous solution and stirred for 10 minutes to form a dispersion.

Next, an aqueous solution of dilute ammonia was added to the dispersion to adjust the pH of the dispersion to 6.9, and the mixture was stirred for 60 minutes to precipitate iron hydroxide on the surface of the phosphor particles. During this time, the temperature of the dispersion was kept at 90 ° C. Further, the dispersion liquid after the precipitation reaction was washed with pure water and dried, and then baked at a temperature of 600 ° C. to convert iron (III) hydroxide on the surface of the phosphor particles into iron (III) oxide.

Next, the phosphor particles after the baking treatment were suspended in 300 ml of pure water and stirred for 10 minutes. Further, 2.8 g of PVA powder was added to the obtained suspension and stirred for 10 minutes. Further, after adding 15 ml of formalin to the suspension, hydrochloric acid was added to adjust the pH to 2.5, and the mixture was stirred for 60 minutes to cause an acetal reaction. The pH of the dispersion after the acetal reaction was adjusted to 6.0 with a dilute aqueous ammonia solution, and the reaction product was settled. Thereafter, the supernatant was removed and the resultant was washed three times with pure water.

Thereafter, the washed precipitate was subjected to the same surface treatment, washing, filtration, drying and sieving as in Example 1 to prepare a phosphor according to Example 2.

Using the phosphor according to Example 2 thus prepared, a phosphor slurry similar to that of Example 1 was prepared, and the phosphor slurry was forcibly stirred for 3 hours and then settled to obtain a phosphor particle surface. When the state of adhesion of the pigment particles was observed, the pigment particles were uniformly attached to the entire surfaces of the phosphor particles, and no peeling-off was observed. The phosphor slurry is dried to form a phosphor layer and has a wavelength of 450 nm.
When the body color reflectance of the phosphor layer was measured by irradiating ultraviolet rays of nm, it was 55.0%, and it was found that the peeling of the pigment particles was small. Further, even when the phosphor slurry was applied to a screen of an electron tube to form a phosphor layer, an electron tube having good contrast characteristics was obtained. Example 3 1.4 g of iron chloride (FeCl ₂ .4H ₂ O)
After sufficiently dissolved in ml of pure water, the solution was heated to a temperature of 90 ° C. to prepare an aqueous solution. 100 g of the Y ₂ O ₂ S: Eu phosphor particles prepared in Example 1 were suspended in this aqueous solution and stirred for 10 minutes to form a dispersion.

Next, a dilute aqueous ammonia solution was added to the dispersion to adjust the pH of the dispersion to 5.5, followed by stirring for 60 minutes to deposit and precipitate iron hydroxide on the surface of the phosphor particles. During this time, the temperature of the dispersion was kept at 90 ° C. Further, the dispersion after the precipitation reaction was washed with pure water and dried, and then baked at a temperature of 650 ° C. to convert iron hydroxide (III) on the surface of the phosphor particles into iron oxide (III).

Next, the phosphor particles after the baking treatment were suspended in 300 ml of pure water and stirred for 10 minutes. Further, 2.8 g of PVA powder was added to the obtained suspension and stirred for 10 minutes. Further, after adding 15 ml of formalin to the suspension, hydrochloric acid was added to adjust the pH to 2.2, and the mixture was stirred for 60 minutes to cause an acetal reaction. The pH of the dispersion after the acetal reaction was adjusted to 6.8 with a dilute aqueous ammonia solution, and the reaction product was settled. Thereafter, the supernatant was removed, and the resultant was washed three times with pure water.

Thereafter, the washed precipitate was filtered, and 100 g of the obtained residue cake, 200 g of beads, 100 ml of pure water, and 0.5 ml of tamol as a dispersant were placed in a 1-liter pot, and the beads were added for 60 minutes. Milling was performed. Thereafter, the same surface treatment, washing, filtration, drying and sieving as in Example 1 were carried out to obtain Example 3
Was prepared.

Using the phosphor according to Example 3 thus prepared, a phosphor slurry similar to that of Example 1 was prepared, and the phosphor slurry was forcibly stirred for 3 hours and then settled to obtain a phosphor particle surface. When the state of adhesion of the pigment particles was observed, the pigment particles were uniformly attached to the entire surfaces of the phosphor particles, and no peeling-off was observed. The phosphor slurry is dried to form a phosphor layer and has a wavelength of 450 nm.
When the body color reflectance of the phosphor layer was measured by irradiating ultraviolet light of nm, it was 54.6%, and it was found that the peeling of the pigment particles was small. Further, even when the phosphor slurry was applied to a screen of an electron tube to form a phosphor layer, an electron tube having good contrast characteristics was obtained.

Comparative Example 1 kg of the Y ₂ O ₂ S: Eu phosphor particles prepared in Example 1 were suspended in 4 liters of pure water. Next, 12.5 g of a 7.5% by weight of a dispersion of bran was added to the suspension.
The mixture was further stirred for 10 minutes. Further, after adding 0.5 ml of an acrylic emulsion solution having a concentration of 45% by weight, the pH was adjusted to 1.9 with dilute sulfuric acid, followed by stirring for 60 minutes. Further, the dispersion was allowed to stand, the supernatant was removed, and the precipitate was washed three times with pure water. Thereafter, the precipitate was subjected to the same surface treatment as in Example 1 to prepare a phosphor according to a comparative example by a conventional production method.

After preparing the same phosphor slurry as in Example 1 using the phosphor according to the comparative example thus prepared, the same forced stirring test as in Example 1 was carried out, and the precipitated phosphor surface was observed. As a result, it was found that many pigment particles were peeled off from the phosphor particle surfaces. Further, this phosphor slurry is dried to form a phosphor layer and has a wavelength of 450 nm.
When the body color reflectance of the phosphor layer was measured by irradiating ultraviolet rays of nm, it was 67.8%, and it was found that the pigment particles were largely peeled.

The body color reflectance at a wavelength of 450 nm of each of the phosphors according to Examples 1 to 3 and Comparative Example is shown in Table 1 below.

[0040]

[Table 1]

FIG. 1 shows a phosphor 1a according to Examples 1 to 3.
FIG. 3 is a cross-sectional view illustrating a particle structure of the present invention. In the phosphor 1a according to each of the examples, the pigment particles 3a are uniformly arranged on the surface of the phosphor particles 2, and the pigment particles 3a are separated via the crosslinked film 4 generated by the acetal reaction. Firmly adhered to the surface of

FIG. 2 is a sectional view showing the particle structure of a phosphor 1 according to a comparative example prepared by a conventional manufacturing method. The phosphor 1 according to this comparative example has a structure in which the pigment particles 3 are non-uniformly adhered to the surface of the phosphor particles 2, and unlike the case of each embodiment, a crosslinked film is not formed. The bond strength is reduced.

As is clear from the results shown in Table 1 and FIGS. 1 and 2, the phosphor according to each of the examples has a lower reflectance than the phosphor according to the comparative example. It was confirmed that there was little falling off and that it was effectively attached.

[0044]

As described above, according to the phosphor of the present invention and the method for producing the same, the pigment particles are made of iron oxide converted from iron hydroxide on the surface of the phosphor particles, and are formed by the acetal reaction. Since the pigment particles are adhered to the surface of the phosphor particles via the crosslinked film, the pigment particles are uniformly adhered to the surface of the phosphor particles with high adhesion strength. Therefore, a phosphor is obtained in which the pigment particles are hardly peeled off. When this phosphor is used as a phosphor layer of an electron tube, a filter effect by the pigment particles can be sufficiently exerted, and an electron tube having excellent contrast characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view showing one embodiment of a phosphor according to the present invention.

FIG. 2 is a cross-sectional view showing a phosphor manufactured by a conventional manufacturing method.

[Explanation of symbols]

 1,1a phosphor 2 phosphor particles 3,3a pigment particles 4 crosslinked film

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Kinno Funayama, No. 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Inside the Toshiba Yokohama office

Claims (5)

[Claims] A rare earth oxysulfide is used as a base material, and europium (Eu), terbium (Te) and samarium (S
m) a phosphor comprising phosphor particles containing at least one selected from the group consisting of an activator and pigment particles attached to the surface of the phosphor particles, wherein the pigment particles are on the surface of the phosphor particles. A phosphor comprising iron oxide converted from iron hydroxide, and wherein the pigment particles are attached to the surface of the phosphor particles via a crosslinked film formed by an acetal reaction. 2. The iron hydroxide according to claim 1, wherein the iron hydroxide is iron hydroxide formed by a precipitation reaction between at least one iron salt selected from iron nitrate, iron sulfate and iron chloride and ammonia water. 2. The phosphor according to 1. 3. The phosphor according to claim 1, wherein the iron oxide is iron oxide generated by baking iron hydroxide. 4. A rare earth oxysulfide as a base material, comprising europium (Eu), terbium (Te) and samarium (S).
m) phosphor particles containing at least one selected from activators as an activator are dispersed in an aqueous iron salt solution, and ammonia water is added to the dispersion to adjust the pH to 5.0 or more and 7.0 or less. By adjusting to a range, the step of depositing and depositing iron hydroxide on the surface of the phosphor particles and the step of baking and treating the iron hydroxide deposited on the surface of the phosphor particles in a temperature range of 600 to 800 ° C. Converting iron into iron oxide into pigment particles, dispersing the baked phosphor particles in pure water, dissolving the aldehyde and alcohol in the resulting dispersion, and dissolving the aldehyde and alcohol The pH of the resulting dispersion was adjusted to a range of 1.5 to 3.0 to form a crosslinked film by the acetal reaction of alcohol, and the pigment particles were adhered to the phosphor particle surfaces via the crosslinked film. The method for producing a phosphor, characterized in that it comprises a step. 5. The method according to claim 4, wherein at least one iron salt selected from iron nitrate, iron sulfate and iron chloride is used as the iron salt.