Magnetic particles comprising magnetite core and process for producing the same

Magnetic particles having an area-average diameter of 0.05 to 1.0 .mu.m, a compaction degree of not more than 60%, an oil absorption of not more than 20 ml/100 g and a charging stability of not more than 1.0 .mu.C/g, which magnetic particles comprise:

• magnetite core particles containing a silicon compound in the inside thereof, and
• a hydrous coprecipitate of silica and alumina deposited or adhered on the surfaces of the magnetite core particles,
• wherein the elemental silicon content of the silicon compound in the magnetite core particles is 0.10 to 1.00 wt % based on the weight of the magnetic particles, the elemental silicon content in the hydrous coprecipitate composed of silica and alumina is 0.01 to 0.5 wt % based on the weight of the magnetic particles, and the elemental aluminum content in the hydrous coprecipitate composed of silica and alumina is 0.01 to 2.0 wt % based on the weight of the magnetic particles, wherein said particles are produced by the process off
• (a) reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe.sup.2+ in said ferrous salt solution, and adding a water-soluble silicate in an amount of 0.4 to 4.0 mol %, calculated as Si, based on Fe, to (1) said aqueous alkali hydroxide solution prior to reaction with the aqueous ferrous salt solution or (2) the produced ferrous salt reaction solution containing colloidal ferrous hydroxide;
• (b) oxidizing the ferrous salt reaction solution of step (a) by passing an oxygen-containing gas through said ferrous salt reaction solution containing colloidal ferrous hydroxide while heating at 85.degree. to 100.degree. C.;
• (c) adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent to Fe.sup.2+ remaining in suspension after completion of the oxidation reaction;
• (d) oxidizing the product of step (c) while heating at 85.degree. to 100.degree. C. to produce the spherical magnetite particles containing a silicon compound in an alkaline suspension containing residual silicate;
• then
• (e) adding a water-soluble aluminum salt to the alkaline suspension containing residual silicate;
• and
• (f) adjusting the pH of the suspension to 5 to 9, thereby causing said residual silicate and the water-soluble aluminum salt added in step (e) to deposit or adhere on the surfaces of said spherical magnetite particles as a hydrous coprecipitate composed of silica and alumina.

Magnetic particles having after compaction treatment an average diameter of 0.05 to 1.0 .mu.m, a compaction degree of not more than 40%, an oil absorption of not more than 18 ml/100 g, a charging stability of not more than 0.8 .mu.C/g and an initial dispersibility in resin of not less than 1.4, and the magnetic particles prior to compaction treatment having an area-average diameter of 0.05 to 1.0 .mu.m, a compaction degree of not more than 60%, an oil absorption of not more than 20 ml/100 g and a charging stability of not more than 1.0 .mu.C/g,

• which magnetic particles comprise magnetite core particles containing a silicon compound in the inside thereof, and a hydrous coprecipitate of silica and alumina deposited or adhered on the surfaces of the magnetite core particles, wherein the elemental silicon content of the silicon compound in the magnetite core particles is 0.10 to 1.00 wt % based on the weight of the magnetic particles, the elemental silicon content in the hydrous coprecipitate composed of silica and alumina is 0.01 to 0.5 wt % based on the weight of the magnetic particles, and the elemental aluminum content in the hydrous coprecipitate composed of silica and alumina is 0.01 to 2.0 wt % based on the weight of the magnetic particles and
• adhered on said magnetic particle surfaces, non-magnetic oxides particles, non-magnetic hydrous oxides particles or mixed particles thereof, which contain at least one element selected from the group consisting of Fe, Ti, Zr, Si and Al, wherein the amount of non-magnetic oxides particles, non-magnetic hydrous oxides particles or mixed fine particles is 0.1 to 10 wt % based on the magnetic particles prior to compaction.

Magnetic particles having after compaction treatment an average diameter of 0.05 to 1.0 .mu.m, a compaction degree of not more than 40%, an oil absorption of not more than 18 ml/100 g, a charging stability of not more than 0.8 .mu.C/g and an initial dispersibility in resin of not less than 1.4, and the magnetic particles prior to compaction treatment having an area-average diameter of 0.05 to 1.0 .mu.m, a compaction degree of not more than 60%, an oil absorption of not more than 20 ml/100 g and a charging stability of not more than 1.0 .mu.C/g,

• which magnetic particles comprise magnetite core particles containing a silicon compound in the inside thereof, and a hydrous coprecipitate of silica and alumina deposited or adhered on the surfaces of the magnetite core particles, wherein the elemental silicon content of the silicon compound in the magnetite core particles is 0.10 to 1.00 wt % based on the weight of the magnetic particles, the elemental silicon content in the hydrous coprecipitate composed of silica and alumina is 0.01 to 0.5 wt % based on the weight of the magnetic particles, and the elemental aluminum content in the hydrous coprecipitate composed of silica and alumina is 0.01 to 2.0 wt % based on the weight of the magnetic particles
• wherein said particles prior to compaction are produced by:
• (a) adding non-magnetic oxides particles, non-magnetic hydrous oxides particles or mixed particles thereof containing at least one element selected from the group consisting of Fe, Ti, Zr, Si and Al in an amount of 0.1 to 25 parts by weight to 100 parts by weight of said magnetic particles;
• and
• (b) subjecting the product of step (a) to compaction, shearing end spatula-stroking.