PROCESS FOR PRODUCING ACICULAR IRON ALPHA-OXYHYDROXIDE PARTICLES

Abstract: A process for producing acicular α-FeOOH particles having a narrow particle size distribution and recitified shape and being highly dispersed, which process comprises mixing with stirring an aqueous solution of a Fe(II) salt with an aqueous solution of an alkali in an equivalent ratio of alkali to Fe(II) salt of 1.5 or more, oxidizing the mixture with a O₂-containing gas, heat treating the resulting suspension of iron α-FeOOH particles to 60° to 100°C, further adding an aqueous solution of a Fe(II) salt in a proportion of Fe(II) to α-FeOOH of 0.5 to 10% and again oxidizing the mixture with a O₂-containing gas at 35° to 55°C.

Description: This invention relates to a process for producing an acicular iron α-oxyhydroxide used as a starting raw material for acicular iron oxide and metal iron powder for magnetic recording. For the purposes of magnetic recording, magnetic powder materials having a higher level of magnetic characteristics and dispersibility have come to be required. Such improved magnetic powders give higher performance and more compactness. In order to satisfy such requirements, it has been found to be desirable to improve certain characteristics of the particulate iron α-oxyhydroxide powder, i.e. its dispersibility, particle shape and particle size distribution. Derwent Abstract 85-161541 discusses JP 60-090830 in which preparation of needle-like iron oxyhydroxide is discussed. An aqueous ferrous salt solution is partially oxidised by air and subsequently heated to 50 o C under an inert gas atmosphere. Further air is blown into the suspension to complete the oxidation reaction. Derwent Abstract 81-29791D discusses JP 56-022638 and discloses the preparation of α-iron oxyhydroxide from ferrous hydroxide. The ferrous hydroxide is obtained by adding a base to an aqueous ferrous solution and an oxygen containing gas is blown through the resulting suspension. α -iron oxyhydroxide is formed in two steps: below 40 o C and in the range 40 to 60 o C. Derwent Abstract 81-29790D discusses JP 56-022637 and discloses the manufacture of α-iron oxyhydride by deposition of ferrous hydroxide by addition of a basic reagent to aqueous ferrous salt solution. An oxygen containing gas is blown through the solution at 40 to 60 o C. In another system α-iron oxyhydroxide is prepared at less than 40 o C. Conventional iron α-oxyhydroxide is unsatisfactory as regards the dispersibility of each particle, i.e. preparation of single particle, dendritic particles are observed and a large amount of fine particles is present in the product so that the particle size distribution of the product is relatively broad. Thus, since the magnetic tape prepared using such magnetic powder particles is inferior in the dispersibility in magnetic paints and has dendritic particles in admixture, it is inferior in the degree of orientation, low in the remanent induction and broad in the particle size distribution so that its erosion characteristics and transfer characteristics have been inferior. An object of the present invention is to provide a process for producing an iron α-oxyhydroxide particle powder in which the presence of dendritic particles and fine particles has been reduced as much as possible, which has a narrow particle size distribution and which exhibits good dispersibility. Accordingly, the present invention provides a process for producing an iron α-oxyhydroxide particle powder, which process comprises mixing with stirring an aqueous solution of a ferrous salt with an aqueous solution of an alkali in an equivalent ratio of the alkali to the ferrous salt of 1.5 or more, followed by subjecting the mixture to a first oxidation reaction with an oxygen-containing gas, heat-treating the resulting suspension of iron α-oxyhydroxide particle powder as it is to a temperature of 60° to 100°C, further adding to the resulting suspension an aqueous solution of a ferrous salt in a proportion of atom to molecule of Fe(II)/α-FeOOH of 0.5 to 10% and subjecting the mixture to a second oxidation reaction with an oxygen-containing gas at 35° to 55°C. The ferrous salt in the aqueous solution thereof may be a ferrous salt such as ferrous sulfate, ferrous chloride, etc. As the alkali in the aqueous solution thereof, sodium hydroxide, potassium hydroxide, ammonia, etc. may be used. The oxygen-containing gas may be represented by air, but air diluted with an inert gas may also be used. In the process of the present invention, the aqueous solution of an alkali is mixed with the aqueous solution of a ferrous salt so that the equivalent ratio of an alkali to a ferrous salt, that is the ratio of the alkali gram equivalent/Fe²⁺ gram equivalent may be 1.5 or higher, preferably 2.0 or more. The oxidation temperature is preferably in the range of 10° to 35°C. If the alkali equivalent ratio is less than 1.5, magnetite is liable to be present in admixture and iron α-oxyhydroxide in the form of dendritic particles is liable to be formed; hence such lower ratios are undesirable. Further, if the oxidation temperature is too low, the oxidation reaction requires a long time, while if it is too high, the crystal growth is too much promoted so that the resulting particles are too large to correspond to the demand of preparing fine particles as a whole of particles. Next, the suspension of the iron α-oxyhydroxide obtained in the above process is heat-treated at a high temperature of 60° to 100°C. In this heat treatment of the suspension, the higher the temperature, preferably at 80°C or higher, and the higher the alkali concentration, the higher the solubility of iron α-oxyhydroxide particles; hence the effectiveness of the treatment is more evident. Namely, the amount of fine particles decreases, the minute branches of dendritic particles are not found to be present and main particles are rectified (that is, the pointed parts at the tip ends of particles are not present and the size of particles becomes uniform), and bundled particles are broken up so that particles having a considerably improved particle dispersibility, particle shape and particle size distribution can be obtained. In addition, the time for heat treatment is preferably in the range of 6 to 12 hours, although it varies depending on the treating temperature, alkali concentration, particle size, etc. Further, in order to enhance the present process, an alkali compound may be added in advance of the heat treatment. However, even when such a treatment is carried out, it cannot yet be said that particles which satisfy the desired properties can be obtained. Thus, a certain amount of fine particles remains, and singularization and rectification of the main particles are incomplete. Thus, as a further process step, to the suspension is added an aqueous solution of a ferrous salt in an atomic/molecular proportion of Fe(II) to α-FeOOH of 0.5 to 10%, preferably 1 to 5%, followed by subjecting the mixture to oxidation reaction with an oxygen-containing gas at a temperature of 35° to 55°C, preferably 40° to 50°C. If the oxidation reaction temperature is lower than 35°C, independent seed crystals of iron α-oxyhydroxide are formed, whereby fine particles of iron α-oxyhydroxide increase to the contrary. If the oxidation reaction temperature is higher than 55°C, magnetite is liable to be formed; hence temperatures outside the above range are undesirable. As to the aqueous solution of a ferrous salt, if the atom/molecule proportion of Fe(II)/α-FeOOH is less than 0.5%, the effectiveness of dispersing particles, rectifying the shape thereof and narrowing the particle size distribution is insufficient, while if the proportion exceeds 10%, there is the problem that seed crystals and dendritic particles are formed; hence the proportions outside the above range are undesirable. By carrying out the above-mentioned additional reaction under the above conditions, main particles are accompanied with residual fine particles by adsorption and the resulting main particles are doped with pure iron α-oxyhydroxide on the surface thereof, whereby it is possible to obtain an iron α-oxyhydroxide particle powder which has more satisfactory particle dispersibility, particle shape and particle size distribution. As described above, the object of the present invention has been achieved by a combination of a heat treatment in the range 60 to 100 o c with an additional oxidation reaction; if either one of these steps is omitted, the object cannot be achieved. The present invention will be described in more detail, by way of example only, but it should not be construed to be limited thereto. Example 1 (1) A 5% aqueous solution of ferrous sulfate (20 Kg) was mixed with stirring with a 30% aqueous solution of NaOH in an equivalent ratio of the alkali to ferrous sulfate of 5 (8.8 Kg), followed by blowing air at a rate of 20 ℓ/min. into the mixed solution at 20°C for 2 hours to obtain a slurry of iron α-oxyhydroxide. A photograph under a transmission electron microscope, of iron α-oxyhydroxide particles at this step is shown in Fig. 3. (2) Next, the slurry was heat-treated at 80°C for 10 hours. A photograph under a transmission electron microscope, of iron α-oxyhydroxide particles at this step is shown in Fig. 2. (3) The temperature of the resulting slurry was decreased to 45°C, followed by adding thereto an aqueous solution of ferrous sulfate in an atom/molecule proportion of Fe(II)/α-FeOOH of 1% and then blowing air at a rate of 0.5 ℓ/min. for 20 minutes. A photograph under a transmission electron microscope, of iron α-oxyhydroxide at this step is shown in Fig. 1. The resulting iron α-oxyhydroxide particles were measured in the sizes of 100 particles thereof under a transmission electron microscope. The average length of the major axis was 0.45 µm, the standard deviation therefrom was 0.083 µm and the average length of the minor axis was 0.042 µm. The specific surface area was 70.2 m²/g. These results are shown in Table 1. Example 2 The steps (1),(2) and (3) of Example 1 were repeated except that the atom/molecule proportion of Fe(II)/α-FeOOH in the aqueous solution of ferrous sulfate added at the step (3) of Example 1 was changed to 3%, to obtain an iron α-oxyhydroxide. The results are shown in Table 1. Example 3 The steps (1), (2) and (3) of Example 1 were repeated except that the atom/molecule proportion of Fe(II)/α-FeOOH in the aqueous solution of ferrous sulfate added at the step (3) of Example 1 was changed to 8% to obtain an iron α-oxyhydroxide. The results are shown in Table 1. Comparative example 1 Only the step (1) of Example 1 was carried out except that in the step (1), the amount of air blown was changed to 3 ℓ/min. to obtain an iron α-oxyhydroxide. The results are shown in Table 1. Comparative example 2 Only the steps (1) and (2) of Example 1 were carried out to obtain an iron α-oxyhydroxide. The results are shown in Table 1. Comparative example 3 Only the steps (1) and (3) of Example 1 were carried out to obtain an iron α-oxyhydroxide, except that in the step (3), the atom/molecule proportion of Fe(II)/α-FeOOH in the aqueous solution of ferrous sulfate was changed to 6%. The results are shown in Table 1. It will be apparent from Table 1 and Figs. 1 to 3, that the product obtained by the process of the present inventon is almost free of dendritic particles and fine particles. The resultant acicular iron α-oxyhydroxide particle powder product is one having a narrow particle size distribution, a rectified shape and a high dispersion.

Claims: A process for producing particles of iron α-oxyhydroxide, which process comprises mixing with stirring an aqueous solution of a ferrous salt with an aqueous solution of an alkali in an equivalent ratio of said alkali to said ferrous salt of 1.5 or more, followed by subjecting the mixture to a first oxidation reaction with an oxygen-containing gas, heat-treating the resulting suspension of iron α-oxyhydroxide particle powder to a temperature of 60° to 100°C, further adding to the resulting suspension an aqueous solution of a ferrous salt in an amount such that the atom to molecule ratio of Fe(II) to α-FeOOH is 0.5 to 10%, and subjecting the mixture to a second oxidation reaction with an oxygen-containing gas at 35° to 55°C. A process according to claim 1, wherein the said ratio of alkali to ferrous salt is 2.0 or more. A process according to claim 1 or 2, wherein the first oxidation reaction is carried out at the temperature in the range of 10° to 35°C. A process according to claim 1, 2 or 3, wherein the heat-treating step is carried out at the temperature in the range of 80° to 100°C. A process according to any one of the preceding claims, wherein the second oxidation reaction is carried out at the temperature in the range of 40° to 50°C. A process according to any one of the preceding claims, wherein the atom to molecule ratio of Fe(II) to α-FeOOH is in the range of 1 to 5%.

Inventors: I, JIRO, SAITO, MASAYOSHI
Applicants: CHISSO CORPORATION
Application year: 1987
Main classification: C01G49
IPC codes: C01G49, G11B5