The invention relates to a method of manufacturing acicular goethite by forming a suspension of a poorly water-soluble iron (II) compound in a liquid consisting essentially of water oxidizing the iron to form goethite crystals, and filtering off, washing and drying the geothite crystals.
Of the four known crystalline iron (III) oxide hydrate modifications (.alpha.-, .beta.-, .gamma.-, .delta.-) of the chemical formula FeOOH, goethite (.alpha.-FeOOH) is the most stable and least soluble phase (solubility product [Fe.sup.+++ ].[OH.sup.- ].sup.3 =10.sup.-44 mol.sup.4 1.sup.-4). For structural reasons the crystallite formation of the goethite is acicular in the direction of the c-axis. .alpha.-FeOOH serves as a starting material for the manufacture of magnetic powders. Such magnetic powders are used in the manufacture of carriers for magnetic recording and reproducing of data, audio and video signals. The magnetic powders may consist of acicular maghemite, .gamma.-Fe.sub.2 O.sub.3, or of acicular iron. They are manufactured by reducing acicular geothite pseudomorphously (that is without the particles loosing their acicularity) to magnetite, Fe.sub.3 O.sub.4, and then either oxidizing the magnetite to acicular .gamma.-Fe.sub.2 O.sub.3 or further reducing it into acicular iron.
U.S. Pat. No. 3,288,563 (corresponding to German Offenlegungsschrift No. 1447134) describes a method of manufacturing a powder for magnetic recording. The powder consists of magnetic iron oxide, .gamma.-Fe.sub.2 O.sub.3. In the method, an iron (II) compound having a low water-solubility is suspended in an alkaline liquid consisting essentially of water with an (OH).sup.- ion concentration of more than 0.2 normal. The iron is oxidized, by introducing air or another oxidizing gas mixture, to form crystalline iron (III) oxide hydrate with a chemical composition of the formula Fe.sub.2 O.sub.3.n H.sub.2 O, in which 1.ltoreq.n.ltoreq.2. The iron (III) oxide hydrate produced is converted into magnetic iron oxide, .gamma.-Fe.sub.2 O.sub.3, by reduction and subsequent oxidation.
In the manufacture of acicular .alpha.-FeOOH as a starting material for magnetic recording materials, the starting materials generally are (strongly) alkaline solutions. For example, air or oxygen is blown into a suspension of Fe(OH).sub.2 gel in 1 to 2.5 molar NaOH, for example, at temperatures between 0.degree. and 60.degree. C. to produce the desired acicular .alpha.-FeOOH.
In the manufacture of goethite on a production scale, it is difficult to control particle growth and spontaneous nucleation. This is due to the very poor solubility of the .alpha.-FeOOH (one liter comprises in equilibrium at p.sub.H =14 only 5.10.sup.-11 g Fe.sup.3+ besides solid .alpha.-FeOOH). Accordingly, in the manufacture of goethite on a production scale an irreproducible, rather wide spectrum of particles sizes is found.
In order to produce magnetic powders having a coercive field strength as large as possible, the goethite particles should have a uniform structure and a pronounced acicularity. Moreover, in order to achieve an acceptable signal-to-noise ratio the particle sizes should fall within a narrow distribution. Particle assemblies manufactured to have optimum characteristics with respect to these criteria, are said to be thermodynamically "more monoenergetic" than particle assemblies manufactured without taking these criteria into account.