An impeller for use in association with an extraction plate adapted for defibering liquid slurry stock and causing defibered material to pass through the extraction plate. The impeller includes a cylindrical central body portion and a plurality of vanes extending outwardly in a generally tangential direction from the central body portion. The vane leading face height from an extraction side to an air foil surface is substantially constant from the vane outer end to where the vane is attached to the central body portion at the radially outward edge of the central body portion, and then thereafter gradually decreasing in height from the radially outward edge of the body central portion to the airfoil surface at the vane inner end.