Method for stabilization treatment of ferromagnetic metal powder

A method of treating ferromagnetic metal powder to achieve a uniform stable oxide coating, comprising:

• (a) molding ferromagnetic metal powder into granules having a minimum diameter of 0.25 to 10 mm;
• (b) introducing said granules of ferromagnetic metal powder into an apparatus which comprises
• (1) a horizontal, cylindrical, rotating type reactor body provided with end walls substantially perpendicular to a cylindrical side wall which forms an inner wall of said reactor body;
• (2) a jacket for a heat transfer medium;
• (3) a port including a valve for feeding said ferromagnetic metal powder into said reactor body;
• (4) a port for withdrawing said ferromagnetic metal powder from said reactor body, which port may be used in common with said feeding port;
• (5) a passageway for blowing an oxidizing gas into said reactor body comprising an oxidizing gas inlet in communication with a pipe arranged within said reactor body coaxially with respect to the axis of rotation of said reactor body, said passageway arranged to be in substantial non-contacting relationship with ferromagnetic metal powder and said pipe, having an outlet end, extends longitudinally through said reactor body, said outlet end positioned proximate the side wall remote from said oxidizing gas inlet;
• (6) a passageway for withdrawing said oxidizing gas from said reactor body comprising an oxidizing gas outlet in communication with a duct connected to said reactor body and arranged coaxially with respect to the axis of rotation of said reactor body, said passageway arranged to be in substantial non-contacting relationship with ferromagnetic metal powder;
• and
• (7) a plurality of scraping up plates for said ferromagnetic metal powder arranged on said inner wall of said reactor body parallel to the axis of rotation of said reactor body and extending longitudinally from one end of the reactor body to the other end thereof;
• (c) mixing said ferromagnetic metal powder by rotating the reactor body;
• (d) surface oxidizing said ferromagnetic metal powder in said reactor body by blowing oxidizing gas through the oxidizing gas blowing passageway toward said ferromagnetic metal powder during said mixing thereof;
• and
• (e) withdrawing oxidizing gas through the oxidizing gas withdrawing necessary.