JPH0341963B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a manufacturing technology for acicular ferromagnetic iron powder used as a magnetic material for magnetic recording, and more specifically, hematite obtained by dehydrating goethite is sintered under specific conditions to obtain good acicular properties. Concerning technology for obtaining ferromagnetic iron powder. Conventionally, as a magnetic material for magnetism, acicular γ
- Iron oxides such as Fe ₂ O ₃ and Fe ₃ O ₄ or iron oxides doped with Co are used. However, in order to meet the recent demands for higher density magnetic recording, these iron oxide-based magnetic materials have insufficient performance, and ferromagnetic metal powders containing iron as a main component are required. The performance required for needle-shaped ferromagnetic iron powder is as follows:
Good gold shape, narrow particle size distribution,
It is required that the overall result of all properties, such as sufficient magnetic properties, good oxidation stability, and optimal specific surface area, be good. Various methods are known for producing acicular ferromagnetic iron powder, but the main method used industrially is to process acicular goethite with a reducing gas such as H ₂ or CO. This is a dry reduction method. Several methods have already been proposed for producing acicular ferromagnetic iron powder by dry reduction of goethite. For example, there are JP-A-56-39682, JP-A-56-23201, JP-A-56-20105, etc. However, with the techniques described in these, the magnetic properties,
When evaluated using various evaluation methods such as specific surface area, shape and particle size distribution observed in electron micrographs, oxidation stability, and tape properties when made into magnetic tape,
It cannot be said that these characteristics are necessarily sufficient. For example, firstly, the magnetic properties of iron powder are low. That is, as shown in Japanese Patent Publication No. 56-39682, the coercive force (Hc) and saturation magnetic flux density (σs) of metal powder are shown, both of which are lower than expected values for metal iron powder. Next, oxidation stability is insufficient. In other words, the metallic iron powder in JP-A-56-23201 is
When accelerated testing is performed under conditions of 60°C and 90% RH, the saturation magnetic flux density decreases significantly. Furthermore, magnetic tapes manufactured using metallic iron powder obtained by these prior art techniques have poor properties such as coercive force, residual magnetic flux density, and squareness ratio. For example, the characteristic values of a magnetic tape made using metallic iron powder obtained by the method of JP-A-56-20105 are extremely inadequate. The main cause of these problems is that the technology to maintain the acicular shape during the production of metallic iron powder is much more difficult than that for iron oxide, and because of this, the magnetic properties and oxidation stability This makes it difficult to obtain metallic iron powder that has good properties in all respects, such as , specific surface area, and properties when used as a magnetic tape. The production of iron oxide-based magnetic powder has been carried out for a long time, and the technology for maintaining the needle-like shape has been perfected, but applying this technology as is to the production of metallic iron powder will not work. As mentioned above, several new methods have been proposed for producing ferromagnetic metallic iron powder by the dry reduction method of goethite, but each still has some problems and It is still not satisfactory as a magnetic material for use. Even with these known methods, the difficulty of maintaining the acicular shape of metallic iron powder has not yet been sufficiently overcome, so that it is difficult to control the specific surface area when trying to obtain high magnetic properties, for example. If you try to improve the oxidation stability, you have to sacrifice the characteristics of the magnetic tape when it was made, so it is difficult to obtain high performance overall. do not have. For your reference, we will discuss here the reason behind the difficulty in maintaining the acicular shape of metallic iron powder compared to iron oxide. First, what is meant by maintaining the needle-like shape will be explained. The process of producing reduced iron from goethite is divided into a process of dehydration to form hematite and a process of further reduction. During the hematization process, goethite dehydrates, and the needle-shaped goethite particles, which were made up of one single crystal, develop many dehydration pores and turn into an aggregate of several hematite single crystals, that is, polycrystals. Convert. Then, by undergoing reduction, it becomes an aggregate of Fe single crystals. At this time,
It is desired that the external shape of the Fe single crystal aggregate retains well the acicular shape of the starting goethite particles. Disintegration or distortion of the acicular shape that prevents this may include breakage of the acicular particle into several pieces or sintering of two or more acicular particles during the hematization process and gas phase reduction process. It may become knotted and form a lump or become arched. Fragmented particles, lumpy particles, curved particles, etc. caused by the collapse or distortion of the acicular shape may cause an expansion of the particle size distribution, a decrease in magnetic properties, especially coercive force, squareness ratio, and poor orientation during tape production. cause deterioration of The difference in preparation between iron oxide powder and metallic iron powder is that, firstly, if we compare the reduction rate, of the total amount of oxygen contained in goethite, when producing magnetite, the amount of dehydration is Only 25% of the oxygen is removed by dehydration and 8.3% by reduction, but when producing metallic iron powder, 25% of the oxygen is removed by dehydration and 75% by reduction. Therefore, the reduction time becomes longer and the difficulty of maintaining the needle shape increases. Second, if we compare their crystal structures, hematite and magnetite or maghematite are very similar in that their crystals are constructed by bonding iron through oxygen, based on a close-packed structure of oxygen molecules. On the other hand, metal Fe differs greatly in that it does not contain oxygen molecules and its crystals are composed only of Fe atoms. Therefore, as the reduction progresses, the needle-like shape is likely to collapse from inside the particles. Thirdly, the density is 5.30 g/cm ³ for hematite and 5.19 g/cm ³ for magnetite, whereas it is 7.86 g/cm 3 for Fe.
It is significantly different from g/ ^cm3 . Therefore, needle-shaped distortion is likely to occur as the reduction progresses. With the aim of solving the above-mentioned technical problems, the present inventors made intensive efforts to overcome the difficulty of maintaining the acicular shape of reduced iron, and as a result, the acicular goethite The present invention was completed based on the discovery that it is extremely effective to subject the obtained hematite to a specific firing treatment. That is, in the present invention, acicular goethite is heated at 500°C.
The hematite obtained by dehydrating the hematite at a temperature of less than 500°C is
This is an improved method for producing ferromagnetic iron powder characterized by carrying out a firing treatment. As the acicular goethite referred to in the present invention, acicular goethite synthesized by a known technique may be used. For example, acicular goethite is obtained by mixing iron sulfate with an excess of caustic soda solution and then blowing in an oxygen-containing gas to obtain acicular goethite. It may be any so-called acid process goethite that obtains crystalline goethite. Also, when synthesizing game sites, in advance,
After adding various elements specified in claim 2, that is, components other than iron such as Cr, Zn, Ni, Si, Mn, and Co, to an aqueous ferrous salt solution or an aqueous alkaline solution, an oxygen-containing gas is added. Use of acicular crystal goethite containing subcomponents synthesized by blowing makes the effects of the present invention even more remarkable. Furthermore, the surface layer of goethite contains Si, Zn, Ni,
The effect of the present invention is also enhanced by depositing components other than iron such as Cr, Co, Mn, B, and Sn. Hematite that has undergone the dehydration process from goethite is extremely porous and has a low density, and the metal powder obtained by directly reducing it has low magnetic properties, probably due to its porous nature. Because it easily disintegrates, it is not very useful as a magnetic material for magnetic recording. Through many years of research, the present inventors have found that firing hematite obtained by dehydrating goethite under specific conditions is extremely effective in obtaining ferromagnetic iron powder for magnetic recording with excellent quality. They discovered something and completed the present invention. The moisture generated during the dehydration process is about 10v% by weight of the raw goethite, and the moisture concentration in the heating furnace easily reaches a high concentration of 5v% or more. Before proceeding to the firing process, it is necessary to purge moisture in the heating furnace. Methods for purging moisture in the heating furnace include venting dry air into the furnace, diffusing moisture from the opening of the furnace, and transferring the hematite to another heating furnace at the end of the dehydration process before firing. There are several ways to do this, and any of them may be used. When the conditions specified in the present invention are not met, that is, when the moisture concentration in the atmosphere is set to 5v% or more during the firing process, the magnetic properties of the obtained iron powder are not good.
The reason for this is thought to be that acicular hematite tends to sinter at high temperatures.
The reduced iron powder obtained by reducing sintered hematite is iron powder with a damaged acicular shape, so it lacks magnetic properties as a magnetic material for magnetic recording. The magnetic tape has poor orientation and, furthermore, the filling properties are not high, making it impossible to obtain a high-quality magnetic tape. As a heating method in carrying out the present invention, a horizontal rotary furnace is conveniently used, but any method such as a fluidized bed method or a fixed bed method may be used. Note that any known technique can be applied to the method of reducing hematite obtained by carrying out the present invention. The first advantage of the ferromagnetic iron powder obtained by implementing the present invention is that it has good magnetic properties. Metallic iron powder has 2 to 3 times the coercive force and 2 times the saturation magnetization force compared to conventional iron oxide magnetic materials.
The greatest advantage lies in the extremely large improvement, which is twice as large, and this advantage is fully demonstrated by implementing the present invention. Second, it has high oxidation stability. This means that the ferromagnetic iron powder obtained by the method of the present invention,
By conducting an accelerated deterioration test in air at 60°C and 90% RH, it is determined that the deterioration of magnetic properties is small. Thirdly, the tape manufactured using the ferromagnetic iron powder obtained by implementing the method of the present invention has dramatically improved coercive force, residual magnetic flux density, and squareness ratio compared to conventional standards. There is. The present invention will be specifically explained below using Examples. Example 1 After adding a caustic soda aqueous solution to a ferrous sulfate aqueous solution, air was blown into the mixture at 35° C. for 19 hours to obtain acicular goethite. After this goethite was dispersed in water, an aqueous solution of sodium silicate was added to coat it so that Si was 0.5% relative to Fe, and then it was dried. This dry powder is put into a horizontal rolling furnace, heated to 480℃, and then the moisture inside the furnace is purged by ventilating dry air.
Firing treatment was carried out for 75 hours while maintaining the temperature at 550°C.
The moisture content in the atmosphere during this firing process was 0.05% by volume. The obtained hematite was reduced by a conventional method to obtain iron powder. The magnetic properties of this iron powder are
Hc = 1250 Oe, σs = 175 emu/g, σr/σs = 0.51, the specific surface area of the iron powder was 30 m ² /g, and when observed using an electron microscope, a good needle-like shape was observed. Example 2 Adding a chromium sulfate aqueous solution to a ferrous sulfate aqueous solution
After adding a zinc sulfate aqueous solution of 0.2% in terms of Cr and 0.6% in terms of Ze relative to Fe, the mixture was mixed with an alkaline aqueous solution and air was blown in at 40°C for 12 hours to obtain goethite. After this goethite was dispersed in water, an aqueous solution of sodium silicate was added to coat it so that Si was 0.5% relative to Fe, and then it was dried. This dried powder was dehydrated, calcined, and reduced in the same manner as in Example 1, and
Obtained iron powder. The properties of this iron powder are shown in Table 1, and all had the necessary magnetic properties, appropriate specific surface area of the iron powder, and good acicular shape. Example 3 After adding a caustic soda aqueous solution to a ferrous sulfate aqueous solution, air was blown into the mixture at 35° C. for 19 hours to obtain acicular goethite. After this goethite was dispersed in water, an aqueous solution of sodium silicate was added to coat it so that Si was 0.5% relative to Fe, and then it was dried. This dry powder was placed in a fluidized bed and heated to 480° C. under N ₂ aeration. This hematite was then transferred to a horizontal rolling furnace and fired at 550°C for 7.5 hours. The moisture in the atmosphere during this firing process is
It was 4.0% by volume. The obtained hematite was reduced by a conventional method to obtain iron powder. The properties of this iron powder are shown in Table 1, and all had the necessary magnetic properties, appropriate specific surface area of the iron powder, and good acicular shape. Example 4 After adding a caustic soda aqueous solution to a ferrous sulfate aqueous solution, air was blown into the mixture at 35° C. for 5 hours to obtain acicular goethite. After this goethite was dispersed in water, an aqueous solution of sodium silicate was added to coat it so that Si was 0.5% relative to Fe, and then it was dried. After this dry powder was put into a horizontal rolling furnace and heated to 480℃, the moisture inside the furnace was purged by ventilating dry air, and then heated to 550℃.
The calcination treatment was carried out for 7.5 hours while maintaining the temperature at ℃.
The moisture content in the atmosphere during this firing process was 0.05% by volume. The obtained hematite was reduced by a conventional method to obtain iron powder. The properties of this iron powder are shown in Table 1, and all had the necessary magnetic properties, appropriate specific surface area of the iron powder, and good acicular shape. Comparative Example 1 After adding a caustic soda aqueous solution to a ferrous sulfate aqueous solution, air was blown at 35° C. for 19 hours to obtain acicular goethite. After this goethite was dispersed in water, an aqueous solution of sodium silicate was added to coat it so that Si was 0.5% relative to Fe, and then it was dried. This dry powder was placed in a horizontal rotary furnace and heated to 480°C, and then fired for 7.5 hours while maintaining the temperature at 550°C without purging the moisture in the furnace. The moisture content in the atmosphere during this firing process was 20 to 15% by volume. Table 1 shows the properties of iron powder obtained by reducing the obtained hematite using a conventional method. The magnetic properties were not good, the specific surface area of the metal powder was outside the appropriate range, and the needle shape was poor. Examples 5 to 8 Ferromagnetic iron powders obtained in Examples 1 to 3 (P1 to P3)
After stirring and dispersing a mixture of 23 parts by weight, 4 parts by weight of polyurethane resin, and 16 parts by weight of toluene in a ball mill for 15 hours, 11 parts by weight of the polyurethane resin and 46 parts by weight of toluene were further added to the ball mill, and 1 A magnetic paint was prepared by stirring and dispersing for a period of time. The obtained magnetic paint was applied to a 21 μm thick polyester film to a dry thickness of 5 μm.
The ferromagnetic iron powder was oriented in a magnetic field and then dried.The surface of the magnetic layer was mirror-finished by calendering, and then cut into a predetermined width to obtain a magnetic tape. Table 2 shows the measured values of coercive force (Hc), residual magnetic flux density, and squareness ratio of the obtained magnetic tape. All of them show good characteristic values. Comparative Example 2 A magnetic tape was obtained from the ferromagnetic iron powder (P-4) obtained in Comparative Example 1 in exactly the same manner as in Examples 5 to 8, and then the coercive force (Hc), residual magnetic flux density, and square shape were obtained. Table 2 shows the measured ratios. Both have insufficient characteristic values as magnetic tapes. Examples 9 to 12 Ferromagnetic iron powders obtained in Examples 1 to 4 (P-1 to
Table 3 shows the magnetic properties of P-4) after it was left in the air at 60° C. and 90% RH for 24 hours. In either case, the deterioration of magnetic properties is small because the properties are less deteriorated and the oxidation stability is high. Comparative Example 3 The ferromagnetic iron powder (P-4) obtained in Comparative Example 1 was
Table 3 shows the magnetic properties after being left under exactly the same conditions as Examples 9 to 12. Both have low oxidation stability, resulting in significant deterioration of magnetic properties.

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Claims (1)

[Claims] 1. Hematite obtained by dehydrating acicular goethite at a temperature below 500°C is subjected to a firing treatment at a temperature of 500°C or higher and a water vapor concentration in the atmosphere of 5% by volume or less. An improved method for producing ferromagnetic iron powder. 2 Acicular crystal goethite is a group consisting of metals belonging to group B, group, and group of the periodic table, tin, manganese, titanium, silicon, boron, bismuth, lead, phosphorus, antimony, chromium, molybdenum, and tungsten. 2. The method according to claim 1, wherein the goethite contains one or more selected elements.