Method for performing the purifying separation of crystals out their suspension in a mother liquor

1. A process for purifying removal of crystals from their suspension in mother liquor, in which

the suspension is fed to a washing column which has a wall which encloses a process chamber, and which wall on its side facing away from the process chamber contacts a first contact area and on its side facing towards the process chamber contacts a second contact area,

mother liquor from the suspension fed into the process chamber is passed out of the process chamber while holding back the crystals and forming a crystal bed in the process chamber,

the crystal bed is conveyed within the process chamber, at least one force other than gravity acts in the process chamber in the conveying direction of the crystal bed and conveys the crystal bed within the process chamber,

pure melt comprising molten crystals which have been purifyingly removed by the process claimed is conducted in the process chamber in countercurrent to the crystal bed in such a manner that a washing front forms in the crystal bed which divides the crystal bed into a mother liquor zone and a pure melt zone,

wherein

the wall is a metal wall which, on its side facing away from the process chamber, is heated at least along the length of the pure melt zone, such that the temperature of the first contact area of said length of the metal wall on its side facing away from the process chamber is higher than the temperature of the second contact area of said length of the metal wall on its side facing toward the process chamber, so that a heat flux flows from the outside of the process chamber through the wall to the inside of the process chamber, and wherein realization of heating comprises encasing the washing column and maintaining a gas in the casing at a temperature above the pure melt temperature.

2. A process as claimed in claim 1 , wherein the crystals to be purifyingly removed are crystals of acrylic acid, xylene, methacrylic acid or N-vinylpyrrolidone.

3. A process as claimed in claim 2 , wherein the metal wall is manufactured from stainless steel, carbon steel or a nickel-based alloy.

4. A process as claimed in claim 1 , wherein the metal wall is manufactured from stainless steel, carbon steel or a nickel-based alloy.

5. A process as claimed in claim 1 , wherein the washing column is a hydraulic washing column.

6. A process as claimed in claim 1 , wherein the heating conducts a specific heat flux of from 10 to 100 W/m 2 into the pure melt zone.