Method for preparing a precoat on the surface of the filter medium of a polishing filter, polishing filter and use of a polishing filter

1. A method comprising forming a precoat on the surface of a filter medium of a polishing filter, where the precoat is at least partially formed of the underflow of at least one of a thickener and a clarifier, where the precoat serves as a filter aid medium in polishing filtration of the overflow of said at least one of a thickener and a clarifier, where particles of the filter aid medium created by the precoat formed of the underflow serve as seed crystals, where particles contained in a supersaturated solution of the overflow are crystallized, and where the precoat is formed of essentially the same material as that to be filtered from the supersaturated solution of the overflow.

2. A method according to claim 1 , where the precoat is a filter aid medium in pressurized polishing filtration, on the surface and/or in the structure of which there are separated nanoparticle-size solids from an overflow that has a smaller particle size than the underflow.

3. A method according to claim 1 , where the underflow has a larger particle size and serves as a filter aid medium forming a porous precoat in which nanoparticle-size solids that are fed at a slow filtering rate and have a smaller particle size are at least one of filtered, precipitated, adsorbed and crystallized.

4. A method according to claim 1 , where the size of the particles contained in the underflow and in the particles contained in the overflow is generally less than 1,000 nm, by average 40-400 nm.

5. A method according to claim 1 , where the precoat is formed of essentially the same substance as the substance to be filtered.

6. A method according to claim 1 , where the underflow and the overflow comprise those of a CaSO4 clarifier.

7. A method according claim 1 , where the precoat is conducted back to polishing filtration, and reused as the filter aid medium creating the precoat.

8. A polishing filter having a filter medium comprising a precoat, where the precoat is at least partially formed of the underflow of at least one of a thickener and a clarifier, and where the precoat serves as a filter aid medium in polishing filtration of an overflow of said at least one of a thickener and clarifier, where particles of the filter aid medium created by the precoat formed of the underflow serve as seed crystals, where particles contained in a supersaturated solution of the overflow are crystallized, and where the precoat is formed of essentially the same material as that to be filtered from the supersaturated solution of the overflow.

9. A polishing filter according to claim 8 , where the precoat is a filter aid medium of a pressurized polishing filter, on the surface and/or in the structure of which there are separated nanoparticle-size solids from an overflow that has a smaller particle size than the underflow.

10. A polishing filter according to claim 9 , where the solids from the underflow that has a larger particle size than the overflow are arranged as a filter aid medium comprising the precoat in connection with the filter medium of polishing filtration, and boost at least one of the filtering, precipitation, adsorption and crystallization of nanoparticle-size solids contained in the overflow that have a smaller particle size than the underflow.

11. A polishing filter according claim 8 , where the filter aid medium arranged in the polishing filter enables the filtering of such nanoparticle-size solids from the process solution where the particle size is generally less than 1,000 nm, by average 40-400 nm.

12. A polishing filter according claim 8 , including elements for conducting used precoat back to polishing filtration, to be reused to form the precoat serving as a filter aid medium.