Spindle-shaped goethite particles, spindle-shaped hematite particles and spindle-shaped magnetic iron based alloy particles

Spindle-shaped hematite particles comprising:

• a hematite seed containing 0.5 to 25 atm % of Co based on the total Fe in the spindle-shaped hematite particles,
• a hematite surface layer containing 0.5 to 15 atm % of Al based on the total Fe in the spindle-shaped hematite particles, and
• an upper surface layer comprising a rare earth element of 0.5 to 15 atm %, calculated as the rare earth element, based on the total Fe in the spindle-shaped hematite particles;
• and
• having an average major axial diameter of 0.05 to 1.0 μm, an aspect ratio (major axial diameter/minor axial diameter) of 10 to 15 and a ratio D104/D110 of an X-ray crystallite size of 0.20 to 0.65.

Spindle-shaped hematite particles comprising:

• a hematite seed containing 0.5 to 25 atm % of Co based on the total Fe in the spindle-shaped hematite particles, and
• a hematite surface layer containing 0.5 to 10 atm % of Al based on the total Fe in the spindle-shaped hematite particles and 0.5 to 15 atm % of a rare earth element based on the total Fe in the spindle-shaped hematite particles;
• and
• having an average major axial diameter of 0.05 to 1.0 μm, an aspect ratio (major axial diameter/minor axial diameter) of 10 to 15 and a ratio D104/D110 of an X-ray crystallite size of 0.20 to 0.65.

A process for producing spindle-shaped hematite particles having an average major axial diameter of 0.05 to 1.0 μm and an aspect ratio (major axial diameter/minor axial diameter) of 10 to 15 said particles comprising a hematite seed containing 0.5 to 25 atm % of Co based on the total Fe in the spindle-shaped hematite particles, and a hematite surface layer containing 0.5 to 15 atm % of Al based on the total Fe in the spindle-shaped hematite particles, said process comprising the steps of:

• aging an aqueous suspension containing a precipitate comprising ferrous obtained by reacting a mixed aqueous alkali solution of an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution with a ferrous salt solution, in a non-oxidizing atmosphere;
• introducing an oxygen-containing gas into said aqueous suspension to produce spindle-shaped goethite seed particles;
• adding a ferrous salt solution, and a mixed aqueous alkali solution of an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution to said aqueous suspension containing said seed particles;
• introducing an oxygen-containing gas into said aqueous suspension to grow goethite on the particle surfaces of said spindle-shaped goethite seed particles;
• adding a compound of a rare earth element in which the content of the rare earth element is 0.5 to 15 atm %, calculated as said rare earth element, based on the total Fe spindle-shaped hematite particles as an anti-sintering for treating the surfaces of said spindle-shaped goethite particles;
• and
• dehydrating said spindle-shaped goethite particles at a temperature of 400 to 850° C. to obtain spindle-shaped hematite particles;
• wherein in the process of producing said seed particles, a cobalt compound in which the Co content is 0.5 to 25 atm %, calculated as Co, based on the total Fe in the spindle-shaped hematite particles is added to one selected from the group consisting of said ferrous salt solution, said aqueous suspension containing said precipitate comprising ferrous, said aqueous suspension containing said precipitate comprising ferrous in the course of aging and before the beginning of the oxidization reaction, and said aqueous suspension in the course of producing said goethite seed particles;
• and
• in the process of growing said goethite, an aluminum compound in which the Al content is 0.5 to 15 atm %, calculated as Al, based on the total Fe in the spindle-shaped hematite particles is added to one of the alkali solutions selected from the group consisting of said ferrous salt solution, said aqueous suspension containing said spindle-shaped seed particles and said precipitate comprising ferrous, before the beginning of said oxidization reaction, and said aqueous suspension in the course of growing said goethite.