Spinel-type spherical, black iron oxide particles and process for the producing the same

Spinel-type spherical black iron oxide particles having the formula:

• Zn.sub.x Fe.sub.1-x O.Fe.sub.2 O.sub.3
• wherein x is a number in the range of 0.05 to 0.62, said particles containing 1.5 to 17.0 wt % of Zn, and having a specific surface area of 5 to 15 m.sup.2 /g, a magnetization of not less than 70 emu/g in an external magnetic field of 1 kOe and a coercive force (Hc) of not more than the value obtained from the following expression:
• Hc(Oe)=15+3.times.(specific surface area of the particles)
• said particles produced by preparing a mixed suspension which contains ferrous hydroxide and zinc hydroxide and which has a pH of 6 to 9 in a non-oxidizing atmosphere by using an aqueous ferrous salt solution, a zinc compound and an aqueous alkaline solution;
• and blowing an oxygen-containing gas into the mixed suspension so as to oxidize ferrous hydroxide until the pH reaches a value in the range of 4 and 6 and the oxidation degree is such that Fe.sup.3+ in the mixed suspensions is 60 to 70 mol % based on the total amount of iron and zinc in the oxidization step.

A process for producing spinel-type spherical black iron oxide particles having the formula:

• Zn.sub.x Fe.sub.1-x O.Fe.sub.2 O.sub.3
• wherein x is a number in the range of 0.05 to 0.62, said particles containing 1.5 to 17.0 wt % of Zn, and having a specific surface area of 5 to 15 m.sup.2 /g, a magnetization of not less than 70 emu/g in an external magnetic field of 1 kOe and a coercive force (Hc) of not more than the value obtained from the following expression:
• Hc(Oe)=15+3.times.(specific surface area of the particles),
• said process comprising the steps of:
• (a) preparing a mixed suspension which contains ferrous hydroxide and sufficient zinc hydroxide to produce black iron oxide particles containing 1.5 to 17.0 wt % zinc by weight based upon the product and which has a pH of 6 to 9 in a non-oxidizing atmosphere by using an aqueous ferrous salt solution, a zinc compound and an aqueous alkaline solution;
• and
• (b) oxidizing the ferrous hydroxide by blowing an oxygen-containing gas into the mixed suspension so as to oxidize ferrous hydroxide until the pH reaches a value in the range of 4 to 6 and the oxidation degree is such that Fe.sup.3+ in the mixed suspension is 60 to 70 mol % based on the total amount of iron and zinc in the oxidation step.