Spindle-shaped goethite particles, spindle-shaped hematite particles and magnetic spindle-shaped metal particles containing iron as main component

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

• aging a water suspension containing an Fe2+-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 the oxidation reaction by passing an oxygen-containing gas through the water suspension, thereby producing spindle-shaped goethite seed crystal particles;
• passing again an oxygen-containing gas through the resultant water suspension containing both said Fe2+-containing precipitate and said spindle-shaped goethite seed crystal particles to conduct the oxidation reaction of the water suspension, thereby growing a goethite layer on a surface of each spindle-shaped goethite seed crystal particle,
• wherein upon the production of said spindle-shaped goethite seed crystal particles, a Co compound is added in an amount of 8 to 45 atm %, calculated as Co, based on whole Fe, to said water suspension containing the Fe2+-containing precipitate during the aging treatment before initiation of the oxidation reaction, thereby oxidizing 30 to 80% of whole Fe2+, and
• wherein upon the growth of said goethite layer, a linear velocity of said oxygen-containing gas passing through said water suspension containing both the Fe2+-containing precipitate and the spindle-shaped goethite seed crystal particles, is adjusted to not less than two times that of the oxygen-containing gas passing through the water suspension containing the Fe2+-containing precipitate upon the production of the goethite seed crystal particles, and an Al compound being added in an amount of 5 to 20 atm %, calculated as Al, based on whole Fe,
• treating the obtained spindle-shaped goethite particles with an anti-sintering agent comprising a rare earth element-containing compound;
• and
• then heat-reducing said spindle-shaped goethite particles at 400 to 700° C. in a reducing atmosphere.

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

• aging a water suspension containing an Fe2+-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 the oxidation reaction by passing an oxygen-containing gas through the water suspension, thereby producing spindle-shaped goethite seed crystal particles;
• passing again an oxygen-containing gas through the resultant water suspension containing both said Fe2+-containing precipitate and said spindle-shaped goethite seed crystal particles to conduct the oxidation reaction of the water suspension, thereby growing a goethite layer on a surface of each spindle-shaped goethite seed crystal particle,
• wherein upon the production of said spindle-shaped goethite seed crystal particles, a Co compound is added in an amount of 8 to 45 atm %, calculated as Co, based on whole Fe, to said water suspension containing the Fe2+-containing precipitate during the aging treatment before initiation of the oxidation reaction, thereby oxidizing 30 to 80% of whole Fe2+, and
• wherein upon the growth of said goethite layer, a linear velocity of said oxygen-containing gas passing through said water suspension containing both the Fe2+-containing precipitate and the spindle-shaped goethite seed crystal particles, is adjusted to not less than two times that of the oxygen-containing gas passing through the water suspension containing the Fe2+-containing precipitate upon the production of the goethite seed crystal particles, and an Al compound being added in an amount of 5 to 20 atm %, calculated as Al, based on whole Fe;
• treating the obtained spindle-shaped goethite particles with an anti-sintering agent comprising a rare earth element-containing compound;
• heat-treating said spindle-shaped goethite particles at 400 to 850° C. in a non-reducing atmosphere;
• and
• then heat-reducing said treated particles at 400 to 700° C. in a reducing atmosphere.

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

• charging spindle-shaped goethite particles containing cobalt of 20 to 45 atm %, calculated as Co, based on whole Fe and having a major axial diameter of 0.05 to 0.15 μm, or spindle-shaped hematite particles obtained by heat-dehydrating said goethite particles, as starting particles, into a fixed-bed reducing reactor to form a fixed-bed having a height of not more than 30 cm;
• elevating the temperature of said starting particles to 400 to 700° C. in an inert gas atmosphere;
• replacing the inert gas atmosphere with a reducing gas atmosphere;
• and
• reducing said spindle-shaped goethite particles or spindle-shaped hematite particles with a reducing gas fed at a linear velocity of 40 to 150 cm/s, at temperature of 400 to 700° C.