Patent GB-965803-A (GB)
Title: Magnetic particles and method of making same
IPC: H01F1/06
Filed: 1961-11-06
Abstract: A microscopic acicular particle of improved magnetic properties comprises an acicular core of a material capable of receiving a colloidal charge in aqueous medium and of retaining a cicular shape when heated to at least 300 DEG  C. which is an inorganic silicate or a non-magnetic oxide, having a length of 500<\>rA-5m , a transverse dimension of 30<\>rA-0.1m , a ratio of length: transverse :: 15-150 : 1, a surface area of 100-350 m2/g. and having a surface film of metallic iron 50-150<\>rA thick.  The core material may be clays, e.g. beneficiated hectorite, activated attapulgus, palygorskite, floridin, sepiolite or garnierite, or synthetic silicates or glass fibres, e.g. of diameter not more than 0.1m , or the oxide may be Al2O3.  The particles may be produced by forming an aqueous dispersion of negatively charged core particles, e.g. 1-10%, precipitating on the particles a surface film of ferric oxide, e.g. 5-20 g./g. core, positive sol particles, separating the particles from the aqueous medium, e.g. by filtration or centrifuging; the particles may then be washed with water and air dried at not above 105 DEG  C. or using e.g. acetone or methanol, heating the particles to e.g. not above 300 DEG  C. in a reducing gas, e.g. H2, CO, water-gas, which may be preheated and passed through a bed of particles, or the H2 may be generated by mixing an alkali or alkaline earth metal, e.g. Na, K or CA, hydride with the particles, at e.g. not above 100 mm. Hg. using an inert gas, to convert the oxide to iron; the particles may be cooled in a vacuum under N2. The particles may be used to make permanent magnets with or without the use of a binding agent, e.g. epoxy or phenolic resin and a filler, e.g. sand. The attapulgite may be activated by heating to 120-150 DEG  C. or by extrusion.  The hectorite may be beneficiated by grinding to a powder which is slurried with heated deionized water to form a gel from which impurities are e.g. centrifuged.  The core particles may be negatively charged by including in the aqueous medium an inorganic or organic surfactant, e.g. sodium chloride, sulphate or silicate; alkali metal, e.g. Na, phosphates or polyphosphates; alkali metal, e.g. Na, salts of carboxylated polyelectrolytes or of condensed sulphonic acids.  The iron oxide particles may first be formed, dispersed in aqueous medium and combined with the core particles dispersion, and/or the oxide particles may be formed in the core dispersion.  The ferric oxide particles may be made by the method of Specification 665,554 or by bubbling air through a ferrous salt solution, e.g. sulphate or chloride, or iron carbonyl.ALSO:A microscopic acicular particle of improve magnetic properties comprises an acicular core of a material capable of receiving a colloidal charge in aqueous medium and of retaining acicular shape when heated to at least 300  DEG C. which is an inorganic silicate or a non-magnetic oxide, having a length of 500<\>rA-5m , a transverse dimension of 30<\>rA-0.1m , a ratio of length:transverse: : 15-150:1, a surface area of 100-350 m.2/g. and having a surface film of metallic iron 50-150<\>rA thick.  The core material may be clays, e.g. beneficiated hectorite, activated attapulgus, palygorskite, floridin, sepiolite or garnierite, or synthetic silicates or glass fibres, e.g. of diameter not more than 0.1m , or the oxide may be Al2O3.  The particles may be produced by forming an aqueous dispersion of negatively charged core particles, e.g. 1-10%, precipitating on the particles a surface film of ferric oxide, e.g. 5-20% g./g. core, positive sol particles, separating the particles from the aqueous medium, e.g. by filtration or cemtrifuging; the particles may then be washed with water and air dried at not above 105  DEG C. or using e.g. acetone or methanol, heating the particles to e.g. not above 300  DEG C. in a reducing gas, e.g. H2, CO, water-gas, which may be preheated and passed through a bed of particles, or the H2 may be generated by mixing an alkali or alkaline earth metal, e.g. Na, K or Ca, hydride with the particles, at e.g. not above 100 mm./Hg., using an inert gas, to convert the oxide to iron; the particles may be cooled in a vacuum under N2. Specification 665,554 is referred to.