Abstract: The process for producing granular hematite particles of the present invention, comprises passing an oxygen-containing gas through an aqueous ferrous nitrate solution in the presence of iron materials and hematite seed crystals at a temperature of 80 to 90° C., thereby obtaining reddish brown precipitates containing granular hematite particles as a main component; subjecting the reddish brown precipitates to filtration, water-washing and then drying; and heat-treating the reddish brown precipitates in air at a temperature of 450 to 900° C. In accordance with the process of the present invention, the granular hematite particles exhibiting a high purity, a small equilibrium water content and a reduced oil absorption can be produced using inexpensive iron raw materials without use of a specific device such as autoclave in an industrially and economically useful manner.

Abstract: The process for producing granular hematite particles of the present invention, comprises passing an oxygen-containing gas through an aqueous ferrous nitrate solution in the presence of iron materials and hematite seed crystals at a temperature of 80 to 90° C., thereby obtaining reddish brown precipitates containing granular hematite particles as a main component; subjecting the reddish brown precipitates to filtration, water-washing and then drying; and heat-treating the reddish brown precipitates in air at a temperature of 450 to 900° C. In accordance with the process of the present invention, the granular hematite particles exhibiting a high purity, a small equilibrium water content and a reduced oil absorption can be produced using inexpensive iron raw materials without use of a specific device such as autoclave in an industrially and economically useful manner.

Abstract: A fine yellow composite iron oxide hydroxide pigment of the present invention has an average major axial diameter of not less than 0.005 .mu.m and less than 0.1 .mu.m, which comprises:fine goethite particles as core particles, containing aluminum inside thereof in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said core particles; anda composite oxide hydroxide of Fe and Al, coated on the surface of each core particle. Such fine yellow composite iron oxide hydroxide pigment are excellent in transparency and heat resistance.

Abstract: In accordance with the present invention, there are provided non-magnetic black particles composed primarily of Fe and having a hematite structure, which shows an excellent blackness, heat resistance, and dispersibility in a vehicle, and which particles contain manganese in an amount of 5 to 40% by weight based on the weight of the non-magnetic black particles, a soluble sodium salt in an amount of not more than 500 ppm (calculated as Na), and a soluble sulfate in an amount of not more than 200 ppm (calculated as SO.sub.4), and have an average particle size of 0.1 to 10.0 .mu.m and a pH of not less than 5.

Abstract: In accordance with the present invention, there are provided non-magnetic black particles composed primarily of Fe and having a hematite structure, which shows an excellent blackness, heat resistance, and dispersibility in a vehicle, and which particles contain manganese in an amount of 5 to 40% by weight based on the weight of the non-magnetic black particles, a soluble sodium salt in an amount of not more than 500 ppm (calculated as Na), and a soluble sulfate in an amount of not more than 200 ppm (calculated as SO.sub.4), and have an average particle size of 0.1 to 10.0 .mu.m and a pH of not less than 5.5.

Abstract: Yellow iron oxide hydroxide particles comprising:iron oxide hydroxide particle as a core, andcomposite oxide hydroxide of Fe and Al deposited on surface of said iron oxide hydroxide particle as a core wherein the amount of Al in said composite oxide hydroxide is 0.1 to 10% by weight, calculated as Al, based on the weight of said iron oxide hydroxide as a core, the amount of Fe in said composite oxide hydroxide is 0.1 to 50% by weight, calculated as Fe, based on the weight of said iron oxide hydroxide particle as a core, and the ratio of Al to Fe in said composite oxide hydroxide is 0.05:1 to 2:1, calculated as Al and Fe atoms,said yellow iron oxide hydroxide particles having an average major axis diameter of 0.1 to 1.0 .mu.m, an average minor axis diameter of 0.02 to 0.10 .mu.m, an aspect ratio (major axis diameter/minor axis diameter) of 2 to 20, a BET specific surface area of 10 to 180 m.sup.2 /g and a heat-resistance temperature of not less than 255.degree. C.

Abstract: Disclosed herein are magnetic composite iron oxide particles comprising spinel-type Fe.sub.3 O.sub.4 particles as core particles and spinel-type CoFe.sub.2 O.sub.4 outerlayer of the composite iron oxide particles, a process for producing the same, and a magnetic recording medium comprising a substrate and a magnetic recording layer containing the said magnetic iron oxide particles.

Abstract: Disclosed herein are magnetic iron oxide particles comprising spindle-like magnetite or maghemite particles having an aspect ratio(major:minor) of less than 4:1 and containing 0.1 to 20 atomic % of Si to Fe, and a process for producing the same.

Abstract: Disclosed herein is spindle ferromagnetic alloy particles having the aspect ratio (major:minor) of less than 3:1 and the coercive force of 500 to 100 Oe, and containing 0.1 to 13 atomic % of Si to Fe, and a process for producing the same.

Abstract: In the production of acicular ferromagnetic iron oxide of highly uniform particle size by oxidizing an iron (II) hydroxide suspension at a pH of at least 11 with an oxygen-containing gas to form acicular iron (III) oxide hydroxide, reducing and optionally further reoxidizing this into ferromagnetic iron oxide, the improvement which comprises providing the iron (II) hydroxide suspension with a water-soluble silicate in an amount of less than 1.7% by atom based on the amount of the iron (II) hydroxide in the suspension and calculated as the ratio of Si to Fe (II) while prior to the oxidation of the suspension with the oxygen-containing gas.

Abstract: A process for treating acicular magnetite containing Co to stabilize the magnetic properties thereof comprises the steps of heating acicular magnetite containing 0.1 to 2.5% by atom of Co with respect to Fe as starting material at a temperature in the range of 300 - 1000.degree. C in an inert gas atmosphere in which the partial pressure of oxygen is controlled at a value in the range of 0.001 - 1% by weight and for the time sufficient to homogenize the crystal structure of the acicular magnetite by thermal-diffusion of oxygen thereinto, and thereafter saturating the resultant acicular magnetite with oxygen by heating it at a temperature in the range of 40 - 100.degree. C in an oxygen rich atmosphere such as air.