PURPOSE: To obtain a magnetic fluid which is stable for a long period of time with uniformly distributing particle sizes, good dispersibility and high saturation magnetization by adding a basic aqueous solution to the organic phase resulting form extraction of a solution in which the sol of hydrated iron oxide is coagulated, with an organic solvent to form ultrafine particles of magnetite.
CONSTITUTION: A basic aqueous solution is added to an aqueous solution containing a soluble ferrous salt and a soluble ferric salt to adjust the pH to 1 to 4.5 to form a sol of hydrated iron oxide which is clear and positive and has less than 300 A average particle size. Then, the sol is aged and stabilized at higher than room temperature. Then, the resultant dispersion is combined with a surfactant to coagulate the gel, to which an organic dispersant is added to transfer the sol in the organic phase. The organic phase containing the gel dispersed is rinsed with water to remove salts. The product is separated and the organic phase is collected, combined with water, heated over 50°C and combined with a basic aqueous solution under stirring to adjust the pH to over 9 whereby the objective ultrafine particles of magnetite are obtained.
IZEKI SEIJI
MATSUI TOSHITAKA
KIDA YOSHIE
