Process for recovering raw materials from paper industry mechanical waste sludge

The method of recovering raw material for the manufacture of paper, pulp board or cardboard from the residual waste water slurry of a mechanical clarification or separation apparatus by separating said slurry into a fiber-rich portion and a filler-rich portion, which comprises the sequential steps of

• a) adjusting the solids content of said waste water slurry to a value of 1 to 5% by weight by adding mechanically clarified waste water to form a homogeneous suspension,
• b) separating the coarse contaminant component of said suspension by passing it through a 1 to 2 mm mesh screen,
• c) separating the black particle components of said coarse contaminant component having a particle size greater than 10 micrometers by centrifugation in a hydrocyclone or in a plurality of hydrocyclones connected in series or in parallel, said hydrocyclones having a nominal diameter of 10 to 100 mm, and accept nozzle diameter of 7 to 14 mm and a reject nozzle diameter of 2 to 8 mm, the input pressure on the inlet side of said hydrocyclone or hydrocyclones being between 0.5 and 6 bar,
• d) fractionally separating the accept component exiting from the hydrocyclone or hydrocyclones into a fiber component, an agglomerate component and a filler and pigment component, by passing the accept component through an 80 to 300 um mesh screen for separation of the fiber component and through a screen with a mesh size greater than 45 um for separation of the agglomerate component, and returning the fiber component and the filler and pigment component into the raw material processing step of the paper, pulp board or cardboard manufacturing process.