1. Field of the Invention
The present invention relates to an apparatus for manufacturing a plated steel sheet, and more particularly, to an apparatus for continuously dissolving metal powder for use in plating to supply a plating solution continuously to an electrolytic tank used for electroplating a steel sheet, and a method of dissolving metal powder using the apparatus.
2. Description of the Related Art
Conventionally, there have been two methods for replenishing metal ions consumed in a plating bath when various types of metals are electroplated on the surface of a metal sheet such as a steel sheet. One is a method in which an anode comprised of a plating metal is immersed into a plating bath, which is an electrolytic tank to dissolve the anode and to replenish the metal ions. The other is a method in which an insoluble anode is immersed into the plating bath, and the metal ions are fed to the plating bath from a dissolving device which is otherwise provided separately.
In the former method, however, the anode is consumed by the passage of electric current through the electrolytic tank and the operation of replenishing the metal ions is often interrupted due to an anode exchange operation, thereby causing a number of disadvantages from an operation viewpoint. For this reason, the latter method using the insoluble anode has been a mainstream recently.
Thus, research and development with respect to a method of or an apparatus for dissolving the metal ions is being increasingly carried out. Since a technique for separating/recovering metal powder (such as zinc powder, nickel powder, ion powder or the like) in the plating bath is a key factor in the above method and apparatus, a number of techniques using a press filter, a cyclone, a centrifugal separator, a leaf filter and the combination thereof have been disclosed.
In Japanese Unexamined Patent Publication No. 6-41796, there is disclosed an apparatus for continuously dissolving a plating metal (a metal to be plated) which comprises a dissolving tank of the plating metal, a cyclone for separating undissolved residue of the plating metal and a plating solution, a pump and a pipeline, and a technique for increasing a yield of using the plating metal by connecting the dissolving tank and the cyclone with a circuit to circulate the plating solution containing the undissolved residue. In Japanese Unexamined Patent Publication No. 5-33199, there is disclosed a technique for passing the plating solution containing the undissolved residue of the metal powder through a leaf filter having a pre-coated layer formed on the filter fabric thereof by mixing a filter aid into the plating solution so as to improve filtering efficiency. Furthermore, various suggestions concerning a dissolving tank or the like employing a press filter or a centrifugal separator as a filter have been made.
However, a technique using a press filter or a leaf filter as the filter has the following problems. A dissolving operation should be paused to discharge the undissolved metal powder deposited on a filter fabric after filtering out of the system. When the undissolved metal powder is disposed of, unit requirement (yield) of the metal powder is deteriorated. In order to reuse the undissolved metal powder, an additional operation is required for recovering and transferring the metal powder to the dissolving tank. On the other hand, a technique using a cyclone or a centrifugal separator as the filter has the following problem. According to this technique, the particle diameter of the metal powder to be collected 100% is 30 .mu.m or more. Thus, when the metal powder is dissolved to have the particle diameter of 30 .mu.m or less, or when the metal powder having the particle diameter is used for improving the dissolving ability, the undissolved metal powder is mixed into the plating solution to bring about deterioration of quality of the plating unless a secondary filter (leaf filter or the like) is used in combination with the primary filter to separate the plating solution and the undissolved metal powder. Furthermore, when nickel is used as metal powder particularly in zinc-nickel plating, there is a problem that the dissolving rate of the nickel is not necessarily sufficient from a viewpoint of practical use.