A method for treating materials or material mixtures such as materials containing mineral substance, fiber mass, recycle fiber, other rejects, waste and biomass in an apparatus which comprises a treatment apparatus operating on the principle of the double-action impact mill which comprises - a housing with a feed opening, and at least one discharge opening in association with the outermost ring of the housing, - a first rotor fitted inside the housing, equipped with blades (18, 18', 18") having impact surfaces (a, a', a") whose blades form one or several, typically at least two coaxial rings (16, 16', 16") with the said rotor, and - a stator, which is coaxial with the first rotor, or a second rotor, coaxial with the first rotor, fitted inside the housing, which stator or the second rotor are equipped with blades (24, 24') having impact surfaces (b, b') and which form one or several, typically at least two, coaxial rings (22, 22') with the said stator or the second rotor, and which rings are staggered with the ring or rings of the first rotor, and a discharge screw, and in which method - material to be treated is fed through the feed opening, into the hub of the treatment apparatus, formed of ring blades, from where the material to be treated is transported by the effect of the rotor or rotors (12, 14), through the rings formed by the outer blades, to the outermost ring, and is discharged from the treatment apparatus through a discharge opening located in the housing, and - the material discharged from the housing is further transported using the discharge screw, wherein from the material exiting from the outermost ring through the discharge opening, a material layer is maintained between the discharge opening and the discharge screw, which prevents a free discharge of the material to be treated through the discharge opening from the handling apparatus, characterized in that the discharge of the material from the treatment apparatus through the discharge opening is is controlled using the discharge screw, by adjusting the material flow from the material layer to the discharge screw. - a housing with a feed opening, and at least one discharge opening in association with the outermost ring of the housing, - a first rotor fitted inside the housing, equipped with blades (18, 18', 18") having impact surfaces (a, a', a") whose blades form one or several, typically at least two coaxial rings (16, 16', 16") with the said rotor, and - a stator, which is coaxial with the first rotor, or a second rotor, coaxial with the first rotor, fitted inside the housing, which stator or the second rotor are equipped with blades (24, 24') having impact surfaces (b, b') and which form one or several, typically at least two, coaxial rings (22, 22') with the said stator or the second rotor, and which rings are staggered with the ring or rings of the first rotor, and a discharge screw, and in which method - material to be treated is fed through the feed opening, into the hub of the treatment apparatus, formed of ring blades, from where the material to be treated is transported by the effect of the rotor or rotors (12, 14), through the rings formed by the outer blades, to the outermost ring, and is discharged from the treatment apparatus through a discharge opening located in the housing, and - the material discharged from the housing is further transported using the discharge screw, wherein from the material exiting from the outermost ring through the discharge opening, a material layer is maintained between the discharge opening and the discharge screw, which prevents a free discharge of the material to be treated through the discharge opening from the handling apparatus, characterized in that the discharge of the material from the treatment apparatus through the discharge opening is is controlled using the discharge screw, by adjusting the material flow from the material layer to the discharge screw.