Abstract: An apparatus for comminuting a substance which comprises a rotatable disc having a disintegrating surface and a base surface proximate to and facing the disintegrating surface. The apparatus has a means for rotating the disc and for feeding particles of the substance between the disintegrating surface and base surface. At least one of the disintegrating surface and base surface comprise an abrasive and at least one of the base surface and disintegrating surface comprise breaker bars. The invention also includes a novel disc for use in a disc refiner having an abrasive disintegrating surface and a novel comminuting plate for use as part of such a disc. The apparatus also desirably comprises a means for elevating the temperature of the substance being comminuted and desirably comprises a means for pressurizing the environment surrounding the substance being comminuted especially when the substance is wet wood chips and the desired product is fiberized wood pulp.