Spindle-shaped magnetic metal particles containing iron as main component and processes for producing the same

A process for producing spindle-shaped magnetic metal particles containing iron as a main component, comprising:

• a first step comprising:
• aging a water suspension containing a ferrous-containing precipitate produced by reacting a mixed aqueous alkali solution comprising an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution with an aqueous ferrous salt solution, in a non-oxidative atmosphere;
• conducting an oxidation reaction of the water suspension by passing an oxygen-containing gas therethrough, thereby producing spindle-shaped goethite seed crystal particles;
• and passing again an oxygen-containing gas through the water suspension containing both the ferrous-containing precipitate and the spindle-shaped goethite seed crystal particles so as to conduct an oxidation reaction thereof, thereby forming a goethite layer on the surface of the seed crystal particles to obtain spindle-shaped goethite particles,
• upon the production of said seed crystal particles, a Co compound being added in an amount of from 0.5 to less than 5 atm %, calculated as Co, based on whole Fe, to the water suspension containing the ferrous-containing precipitate during the aging thereof but prior to the elapse of 20% of the whole aging time before initiation of the oxidation reaction;
• the oxidation reaction being conducted while passing the oxygen-containing gas through the water suspension at a superficial velocity of 2.3 to 3.5 cm/s such that 30 to 50% of whole Fe2+ is oxidized;
• an Al compound being added in an amount of 5 to 10 atm %, calculated as Al, based on whole Fe;
• and the oxidation reaction being successively conducted to obtain spindle-shaped goethite particles;
• a second step comprising treating the spindle-shaped goethite particles obtained in the above first step, with an anti-sintering agent containing a rare earth compound in an amount of 1.5 to 5 atm %, calculated as a rare earth element, based on whole Fe;
• and then heat-treating the thus treated spindle-shaped goethite particles at a temperature of 650 to 800° C. in a non-reducing atmosphere, thereby obtaining spindle-shaped hematite particles;
• and
• a third step comprising charging the spindle-shaped hematite particles obtained in the above second step into a reducing apparatus to form a fixed-bed having a height of 3 to 15 cm;
• heating the fixed-bed of the spindle-shaped hematite particles to a temperature of 400 to 700° C. at a temperature-rising rate of 10 to 80° C./minute in an atmosphere of a reducing gas passed though the reducing apparatus at a superficial velocity of 40 to 150 cm/s, thereby reducing the spindle-shaped hematite particles;
• and then forming an oxide coating film on the surface of the thus reduced particles, thereby obtaining the magnetic metal particles containing iron as a main component.

A magnetic recording medium comprising:

• a non-magnetic substrate;
• and
• a magnetic recording layer formed on the non-magnetic substrate comprising a binder resin and spindle-shaped magnetic metal particles containing iron as a main component, which have an average major axis diameter (L) of 0.05 to 0.15 μm;
• a coercive force of 111.4 to 143.2 kA/m;
• a Co content of from 0.5 to less than 5 atm % based on whole Fe;
• a crystallite size of 150 to 170 Å;
• a ratio of Al to Co from 1.0:1 to less than 2.0:1;
• a specific surface area (S) represented by the formula:
• S≦−160×L+65;
• an oxidation stability (Δσs) of saturation magnetization of not more than 5.0%;
• and an ignition temperature of not less than 140° C.

A magnetic recording medium comprising:

• a non-magnetic base film;
• a non-magnetic undercoat layer formed on said non-magnetic base film;
• and
• a magnetic recording layer formed on the non-magnetic substrate comprising a binder resin and spindle-shaped magnetic metal particles containing iron as a main component, which have an average major axis diameter (L) of 0.05 to 0.15 μm;
• a coercive force of 111.4 to 143.2 kA/m;
• a Co content of from 0.5 to less than 5 atm % based on whole Fe;
• a crystallite size of 150 to 170 Å;
• a ratio of Al to Co from 1.0:1 to less than 2.0:1;
• a specific surface area (S) represented by the formula:
• S≦−160×L+65;
• an oxidation stability (Δσs) of saturation magnetization of not more than 5.0%;
• and an ignition temperature of not less than 140° C.

Spindle-shaped magnetic metal particles containing iron as a main component, having an average major axis diameter (L) of 0.05 to 0.15 μm;

• a size distribution (standard deviation/average major axis diameter) of not more than 0.30;
• an aspect ratio of 4:1 to 8:1;
• a coercive force of 111.4 to 143.2 kA/m;
• a Co content of from 0.5 to less than 5 atm % based on whole Fe;
• an Al content of from 5 to 10 atm % based on whole Fe;
• a rare earth element content of from 1.5 to 5 atm % based on whole Fe;
• a crystallite size of from 150 to less than 170 Å;
• a ratio of Al to Co from 1.0:1 to less than 2.0:1;
• a specific surface area (S) represented by the formula:
• S≦−160×L+65;
• an oxidation stability (Δσs) of saturation magnetization of not more than 5.0%;
• and an ignition temperature of not less than 140° C.