In recent years, the trend of magnetic recording media to incorporate smaller-sized magnetic particles at higher densities has increased the necessity of improving the dispersion of such particles, such as magnetic iron oxide, magnetic metal, or magnetic barium ferrite, to a high degree in a binding agent and enabling the resultant magnetic particles to be oriented to a high degree in the produced magnetic recording medium. Such fine magnetic powder actually used as a raw material typically has a BET specific surface area of not less than 55 m.sup.2 /g. Magnetic raw material having a BET value of this order contains particles having small particle diameters such as, for example, from about 0.1 micron to about 0.02 microns, and this magnetic raw material exhibits a strong cohesive force compared with a magnetic raw material having a smaller BET value.
Japanese Unexamined Patent Publication No. JP02-178363 proposes a method for the production of a magnetic coating material that comprises blending a magnetic powder with a binding agent, an organic solvent, etc. in a ratio calculated to yield a solids concentration in the range between 65 and 95% by weight by using one twin-shaft type continuous kneading device provided with a pair of vaned shafts and a barrel for rotatably accommodating the shafts therein. This method is purportedly capable of producing a magnetic coating material having a magnetic powder packed and dispersed to a high degree therein. Since this method utilizes an ordinary continuous process kneader, the kneading of the magnetic raw material necessarily occurs in the presence of a binding agent.
When this method is executed with a granular magnetic metal raw material having a BET value of about 55 m.sup.2 /g, the kneading force is insufficient to effectively disperse the magnetic metal raw material. The resultant magnetic coatings show inferior magnetic properties. Further, kneading the magnetic metal raw material in the presence of the binding agent also may generate sufficient heat to alter, e.g., plasticize, the binding agent. When the magnetic coating material is diluted to a suitable degree of viscosity to allow kneading, since the dilution inevitably requires shearing force exertion, there is also the possibility that kneading the diluted solution in the presence of the binding agent may also cause alteration of the binding agent.
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