1. Field of the Invention
The present invention relates to a method and a system for recovering an extra fluorescent material (phosphor) produced in a step for coating phosphor screens of color image receiving tubes, and purifying the fluorescent material for recycle and reuse thereof, and more particularly to a method and a system for easily and simply effecting the above described purification at the manufacturing location of such color image receiving tubes.
2. Description of the Related Art
Color image receiving tube (color picture tube, color television picture tube) is manufactured through steps of preparing fluorescent material (phosphor) slurries of red, green, and blue colors being three primary colors, respectively, and applying these slurries to predetermined dot-shaped or stripe-shaped positions on phosphor screen, thereby forming phosphor film. In this case, a large amount of extra phosphor slurries are produced, and when such extra phosphor slurries are dumped without any consideration, it is not only wasteful of resources and manufacturing cost, but also it is affected adversely in view of safeguard for environment. Typically, rare earth fluorescent materials used as red fluorescent material, for example, a fluorescent material represented by Y.sub.2 O.sub.2 S:Eu is expensive, so that reuse of the material after recovering the same is important for reducing the manufacturing cost of color image receiving tube.
However, these phosphor slurries contain additives such as carbon (dag), pigments, polyvinyl alcohol, chromium compounds such as bichromates and dispersants in addition to fluorescent materials. Furthermore, there is such a case where inclusion of fluorescent materials having different colors deteriorate color-developing characteristics of the fluorescent material to be reused in case of recovery thereof. In order to remove these additives and the fluorescent materials having different colors, very complicated steps are required in recovery and reuse of fluorescent material. In this respect, any simple and efficient regenerating or recycling method by which a fluorescent material can be purified up to a satisfactory degree could have not yet been found.
For instance, Japanese Patent Application Laid-Open No. 51-131486 discloses a regenerating or recycling method for fluorescent materials (phosphors) wherein a recovered fluorescent material containing a photosensitive binder prepared by dispersing a bichromate into polyvinyl alcohol is treated with an oxidizing agent, and examples of the oxidizing agent include hydrogen peroxide, alkali peroxide, periodic acid and the salts thereof, perchloric acid and the salts thereof, hypochlorous acid and the salts thereof, peroxosulfuric acid and the salts thereof, bichromates and the like.
In Japanese Patent Application Laid-Open No. 51-149186, disclosed is a regenerating or recycling method for fluorescent materials wherein tannic acid is added to a recovered fluorescent material containing graphite and chromium compounds, the resulting mixture is heat-treated at a temperature of 400 to 600.degree. C., whereby the graphite is oxidized to vaporize away the same as carbon dioxide, and the remaining oxide is washed with water to remove the same.
Among the impurities contained in the above described recovered fluorescent materials, particularly, carbon exhibits black color so that it absorbs easily light rays, and therefore emission luminance of fluorescent material is remarkably reduced in the case where such carbon remains in the regenerated fluorescent material. Furthermore, since carbon is chemically stable, it is difficult to dissolve or decolor the recovered fluorescent material by reacting carbon with any material in accordance with any chemical treatment, and accordingly, it is required to separate carbon from the fluorescent material to remove the same in regenerating process.
Other than a method for washing simply a recovered fluorescent material with water for separating and removing carbon from the fluorescent material, disclosed are a method wherein a recovered fluorescent material is treated with an oxidizing material such as periodic acid and the salts thereof, and sodium hypochlorite, and then the material so treated is washed with water (Japanese Patent Preliminary Publication Nos. 2-504160 and 2-504162), a method wherein a recovered fluorescent material is subjected to reduction treatment, and then the material so treated is washed with water (Japanese Patent Application Laid-Open No. 63-154784) and the like methods. Any of these methods is efficient for separating and removing carbon at a certain degree.
In recent years, however, with increase in strict demand for emission luminance of color image receiving tube, improvements in luminance of phosphor which decides the luminance of color image receiving tube are required, while regenerated and recycled fluorescent material has been used as a part of raw fluorescent material. For this reason, improvements in characteristic properties of the regenerated fluorescent material are increasingly required. For such recent requirement of improvements in characteristic properties of regenerated fluorescent material, the above described conventional methods are insufficient for the one for removing carbon from the regenerated fluorescent material.
Japanese Patent Application Laid-Open No. 53-51190 discloses a method wherein for removing included sulfide fluorescent materials from recovered rare earth fluorescent materials, a specified amount of silver ion is reacted with the sulfide fluorescent materials.
Japanese Patent Application Laid-Open No. 53-52052 discloses a method for regenerating fluorescent materials characterized in that recovered Y.sub.2 O.sub.2 S fluorescent materials are subjected to acid washing and water washing, and then the resulting slurry is treated with a solution containing polyvinyl alcohol and ammonium bichromate, or with a solution prepared by adding further acrylic emulsion thereto without drying the slurry. Moreover, as an improved method of the above described one, Japanese Patent Application Laid-Open No. 53-53588 discloses a method for treating a recovered fluorescent material with a solution prepared by adding further an anionic surfactant to the above described solution to prevent inclusion of other fluorescent materials.
In Japanese Patent Preliminary Publication No. 2-504160, a method wherein periodate is used for removing an organic polymer such as polyvinyl alcohol stuck to a recovered fluorescent material as well as a method wherein chromium compounds existing in the aforesaid fluorescent material are permitted to dissolve into water by the use of an aqueous solution of an oxidizing material such as sodium hypochlorite, ammonium peroxodisulfate, and hydrogen peroxide, whereby the resulting solution is removed are disclosed. Furthermore, Japanese Patent Preliminary Publication No. 2-504162 discloses a method for recovering organic contaminants from a recovered phosphor slurry without requiring a calcining or sintering step by employing the above described method.
However, an individual method of the above described methods is insufficient for recovering an extra phosphor slurry produced in a step for coating a phosphor screen and removing a variety of substances contained in the slurry to regenerate the phosphor, while a mere combination of these methods only brings about complicated steps. Particularly, organic materials such as polyvinyl alcohol and carbon cannot be completely removed, so that a fluorescent material having reusable characteristics cannot efficiently be obtained. For this reason, luminance and grade of coating film in a phosphor screen manufactured by employing the regenerated fluorescent material are adversely affected.
Furthermore, there arises a necessity for regenerating or recycling a recovered phosphor slurry in a image receiving tube manufacturing plant without spending labor and cost for long distance transportation of such slurry instead of conducting concentratively regenerating treatment of the recovered phosphor slurry. In this respect, a simple regenerating method of fluorescent material for satisfying the above described necessity is demanded, but the purpose therefor could have not been attained by the combination of conventional methods as mentioned above.
On the other hand, the regenerating treatment as described above is carried out usually by such a manner that a recovered phosphor slurry is successively agitated, admixed, and washed with each of aqueous solutions of the above described chemicals in a variety of agitation tanks. Although it is also possible to treat such recovered phosphor slurry at normal temperatures, the treatment is mostly conducted with the use of hot aqueous solutions for elevating washing effect. For this reason, a mild steel material on the surface of which had been coated with polyvinyl chloride, polypropylene, or had been applied glass lining has been heretofore used for a tank body for the agitation tank and a baffle (baffle plate) disposed in the tank for elevating agitating efficiency, and such baffles have been fixed to the tank body by means of welding or bolting.
Such conventional agitation tank as described above has involved the following problems.
(1) Baffles have been prepared separately from tank bodies and the former has been mounted on the latter, and in this connection, the working and mounting therefor are troublesome.
(2) Mounted portions of baffles are liable to be loaded, and such portions are easily damaged.
(3) Fluorescent material is easily stuck to and deposited on the mounted portions of baffles.
(4) Protecting tubes for protecting a drain tube, a level sensor and the like to be inserted into a tank from agitation flow are necessary, and mounting for such protecting tubes are troublesome as same as in the case of mounting for baffles.
(5) Agitating effect for liquid into which has been dispersed a fluorescent material is not sufficient.
In addition to the above described problems, polyvinyl chloride used for a surface coating of mild steel has a poor heat resistance, so that the polyvinyl chloride surface coating is corroded with hot aqueous solution. On one hand, polypropylene exhibits poor workability. Further, glass lining is easily attacked by aqueous alkaline solution in addition to exhibiting poor impact resistance. Other problems than those described above are such that sections to which such surface coating as described above is applied hardly are corroded with aqueous chemical solutions to be used, and the like problems.