JPS5942441B2 - Manufacturing method of magnetic iron oxide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing magnetic iron oxide containing cobalt (hereinafter abbreviated as Co), which is suitable for use as a magnetic recording medium by coating it on a support together with bisodar. The present invention relates to a method for manufacturing a Co-containing iron oxide fine powder for magnetic recording media that has excellent properties by efficiently incorporating Co into the iron oxide fine powder.

Conventionally, in a magnetic recording material in which an undercoat layer is optionally provided on a support and a magnetic layer dispersed using a binder is applied, the magnetic material has a length of 0.3 to 1.5 μm) and a length to width ratio of 3. :1 to 10:1, and has a coercive force (hereinafter H
(abbreviated as c) is an acicular magnetic iron oxide (γ
-Fe2O3, Fe3O4), or 0.1 to 1.0 μm
Corner cubic magnetic iron oxide (γ-Fe2O3, Fe304)
etc. have been used.

However, among such magnetic iron oxides, the upper limit of Hc of industrially mass-produced iron oxides is currently about 430 Oe, which is too low for some uses.

For example, in home video tapes, H
c is required to be around 5000e, and for magnetic cards such as cash dispensers, approximately 65
An Hc of 0 Oe is required. In addition, with master tape for magnetic contact transfer, in order to perform high-efficiency transfer without damaging the master signal, it is necessary to
3.0 times more Hc is required, therefore 800-14
A high Hc of about 000e is required. One way to meet the above requirements is to use Co-doped iron oxide ((
Co)γ-Fe2O3, (Co)Fe3O4) have been conventionally used, and many manufacturing methods are known. For example, Japanese Patent Publication No. 41-6538, US Pat. No. 3,117,933, West German Publication No. ALS (ALS) No.
No. 43375, No. 3075191, No. 3278440
No. 3671435, No. 3725126, West German Patent Application (OLS) No. 2036612, Special Publication No. 1971-61
No. 13, JP-A-48-46899, JP-A-48-23
No. 753, Special Publication No. 48-10994, etc. These methods can be roughly divided into the following two types. One method is to add an alkali to an aqueous solution of Fe and CO salts and gradually oxidize the solution to precipitate and grow iron oxide containing CO. However, in this method, even in an acidic region, if a certain value determined by the reaction temperature, concentration, etc. is exceeded, precipitation of magnetite, which is an undesirable substance, will occur, so the reaction is generally carried out below that value. Therefore, due to the low pH, CO ions are not completely precipitated and some to half remain in the solution, and a portion of the CO salt, which is much more expensive than Fe salt, is wasted. I was getting used to it. In addition, iron oxide magnetic materials containing CO generally obtain high Hc by utilizing the crystal anisotropy caused by containing CO, and therefore the Hc of the product is controlled by the CO content. Although it is possible to do so, it is difficult to do so if there is a residual amount as described above. Furthermore, when the solid and liquid are separated by filtration and water washing after the reaction is completed, residual CO ions may be discharged from the filtrate or supernatant to the outside, which is not good for purifying wastewater. Another method is to deposit or mix a CO salt on iron oxide that does not contain CO and then heat treat it, but this is done only on the surface of the iron oxide particles and decomposes on heating, but the It is said to be difficult to spread CO uniformly throughout the iron particles.

In addition, in order to uniformly distribute CO to the iron oxide particles,
It is necessary to uniformly disperse the iron oxide particles, and it is also necessary to separately prepare an aqueous solution of CO salt, an alkali salt, a dispersion device, etc., which causes problems such as the process becoming complicated. Accordingly, the first object of the present invention is to provide a method for efficiently incorporating CO in a method for manufacturing fine particle iron oxide containing CO for a magnetic recording medium.

A second object of the present invention is to provide a method for producing CO-containing particulate iron oxide having excellent magnetic properties. In order to achieve the above objective, the present inventors have developed Fe()
Fe(
NI) In the reaction of oxidizing by blowing oxidizing gas into a solution containing ions, raising the pH of the solution to a higher level than during the reaction before and after the end of the reaction eliminates the risk of magnetite generation and also reduces CO. The present invention was completed based on the discovery of the novel fact that it is effective for efficiently depositing on particles.

In the reaction in which acicular crystals that will become the nucleus crystals are created in advance by blowing oxidizing gas into the Fe() salt aqueous solution, an alkali is added to adjust the pH of the solution to the acidic range, to avoid the risk of magnetite precipitation. are doing. In addition, Fe(
In the reaction of blowing oxidizing gas into an aqueous solution containing H) and CO(H) ions, it is necessary to avoid the risk of magnetite precipitation mentioned above and to incorporate CO into particles as efficiently as possible. is added to set the pH of the solution to be slightly acidic. (However, this level of P
As mentioned above, with H, it is impossible to expect the entire amount of CO used to be deposited on the particles. ) In this wet reaction, in addition to the pH, the concentration of the solution, the oxidizing gas blowing conditions,
The granularity of the generated particles can be changed by changing conditions such as stirring or by coexisting zinc ions, copper ions, cerium (l) ions, etc.

(Japanese Patent Publication No. 41-6538) The oxidizing gas used in the present invention is a gas having oxidizing ability, and includes, for example, air, oxygen gas, and ozone. The pH increase of the reaction solution, which is a feature of the present invention, is carried out before and after the completion of the wet reaction.

That is, before and after the completion of the reaction in which oxidizing gas is blown into a solution containing Fe() and CO() ions while stirring in a weakly acidic region, and also when Fe containing needle-shaped crystals generated by prior oxidation as nucleus crystals. A reaction in which an aqueous salt solution is added to a solution containing Fe() and CO() ions, and this mixed solution is oxidized in a weakly acidic region while stirring an oxidizing gas to form an iron oxide layer containing CO on the core crystal. This is done before and after the end. More specifically, during the course of the reaction, after the liquid becomes sufficiently yellow and there is little risk of producing magnetite even if alkali is added, or after the reaction is completed, filtration, decantation, etc. are carried out for solid-liquid separation. Before carrying out the water washing treatment, or immediately before or after the completion of the reaction, an operation of adding an alkali is performed.

Further, the final value of pH which is raised stepwise by adding alkali at the above-mentioned period is higher than that during the reaction, but there is no particular limitation on the value, and the higher the pH, the better the result.

The alkali used in the present invention is a water-soluble alkali metal,
Use oxides or hydroxides of alkaline earth metals, or those that show basicity in aqueous solution, such as alkali metal carbonates or ammonia.

For example, NaOH, KOH, Ba(0H)2, Li20
, Na20, Ba0, Na2C03, K2C03, NH
40H etc. By the above-described method, the entire amount of CO used can be deposited on goethite particles (α-FeOOH). Thereafter, this slurry is washed with water and dried as necessary, and further processed through dehydration and reduction to produce CO-containing magnetite ((CO)Fe3O4), and in some cases, is re-oxidized to CO-containing maghemite (CO).
γ-Fe2O3). As the final product of the present invention, either one of these two types of CO-containing magnetic iron oxides is used, or
07, No. 48-39639, the above two types of compounds ((CO)Fe3O4, (CO)γ
-Fe2O3) or an intermediate CO-containing Bertrade compound ((CO)FeOx, where 1.
33 x x 1.50) is used. The CO-containing magnetic iron oxide produced by the method of the invention can be dispersed in a binder and coated onto a support, optionally with an undercoat layer, to form a magnetic recording medium.

At this time, other ferromagnetic materials (such as CrO2, etc.)
It may also be applied together. As the binder used in the magnetic recording medium using magnetic iron oxide produced by the method of the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof are used.

The thermoplastic resin has a softening temperature of 150°C or less, an average molecular weight of 10,000 to 200,000, and a degree of polymerization of about 20.
0 to 500, such as vinyl chloride vinyl acetate copolymer, vinyl chloride vinylidene chloride copolymer, vinyl chloride acrylonitrile copolymer, acrylic acid ester acrylonitrile copolymer, allyl acid ester vinylidene chloride copolymer, acrylic Acid ester ethylene copolymer, methacrylic acid ester acrylonitrile copolymer, methacrylic acid ester vinylidene chloride copolymer, methacrylic acid ester styrene copolymer, urethane elastomer, nylon-silicon resin, nitrocellulose-polyamide resin, polyfluoride Vinyl, vinylidene chloride acrylonitrile copolymer, butadiene acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), styrene butadiene copolymer Polyester resins, chlorovinyl ether acrylate copolymers, amino resins, various synthetic rubber-based thermoplastic resins, and mixtures thereof are used.

Examples of these resins are given in Japanese Patent Publication No. 37-6877, 39-
No. 12528, No. 39-19282, No. 405349,
40-20907, 41-9463, 41-140
No. 59, No. 41-16985, No. 42-6428, 42
-11621, 43-4623, 43-15206
No., 44-2889, 44-17947, 44-1
No. 8232, No. 45-14020, No. 45-14500, No. 47-18573, No. 47-22063, No. 47-2
It is described in publications such as No. 2064, No. 47-22068, No. 47-22069, No. 47-22070, and No. 47-27886.

The thermosetting resin or reactive resin has a molecular weight of 200,000 or less in the state of a coating liquid, and when heated after coating and drying, the molecular weight becomes infinite due to reactions such as condensation and addition.

Also, among these resins, those that do not soften or melt before the resin is thermally decomposed are preferred. Specifically, examples include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, silicone resins, acrylic reaction resins, epoxy-polyamide resins, nitrocellulose melamine resins, high molecular weight polyester resins, and isocyanate resins. mixtures of polymers, mixtures of methacrylate copolymers and diisocyanate prepolymers, mixtures of polyester polyols and polyisocyanates, urea formaldehyde resins, mixtures of low molecular weight glycols/high molecular weight diols/triphenylmethane triisocyanates, polyamine resins and A mixture of these, etc. Examples of these resins are given in Japanese Patent Publication No. 39-81
No. 03, No. 40-9779, No. 41-7192, 41-
No. 8016, No. 41-14275, No. 42-18179, No. 43-12081, No. 44-28023, No. 45-1
No. 4501, No. 45-24902, No. 46-13103, No. 47-22065, No. 47-22066, 47-2
No. 2067, No. 47-22072, No. 47-22073, No. 47-28045, No. 47-28048, No. 47-2
It is described in publications such as No. 8922.

These binders may be used alone or in combination;
Other additives may be added.

The mixing ratio of the CO-containing magnetic iron oxide powder and the binder is 10 to 200 parts by weight of the binder per 100 parts by weight of the CO-containing magnetic iron oxide powder. Additives include dispersants, lubricants, abrasives, antistatic agents, and the like.

Dispersants include chifurylic acid, chipurinic acid, lauric acid,
Fatty acids with 12 to 18 carbon atoms such as myristic acid, palmitic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, stearolic acid
, Rl is an alkyl group having 11 to 17 carbon atoms): a metal soap consisting of an alkali metal (Li, Na, K, etc.) or an alkaline earth metal (Mg, Ca, Ba, etc.) of the above fatty acid:
Lecithin etc. are used.

In addition, higher alcohols having 12 or more carbon atoms and sulfuric esters thereof can also be used. These dispersants are added in an amount of 1 to 20 parts by weight per 100 parts by weight of the binder. Lubricants include silicone oil, graphite, molybdenum disulfide, tungsten disulfide, fatty acid esters consisting of monobasic fatty acids with 12 to 16 carbon atoms and monohydric alcohols with 3 to 12 carbon atoms, and 17 carbon atoms.
Fatty acid esters made of a monobasic fatty acid and a monohydric alcohol having 21 to 23 carbon atoms in total can be used. These lubricants are added in an amount of 0.2 to 20 parts by weight per 100 parts by weight of the binder. Regarding these, Japanese Patent Publication No. 43-23889, Japanese Patent Application No. 42-28647, Japanese Patent Application No. 43-815
It is stated in the specifications of No. 43, etc. Commonly used abrasive materials include fused alumina, silicon carbide chromium oxide, corundum, artificial corundum, diamond,
Synthetic diamonds, garnet, emery (main ingredients: corundum and magnetite), etc. are used.

These abrasives have an average particle diameter of 0.05 to 5 .mu.m, particularly preferably 0.1 to 2 .mu.m. These abrasives are used in an amount of 7 to 2 parts by weight per 100 parts by weight of the binder.
It is added in an amount of 0 parts by weight. Regarding these special requests
8-26749. Natural surfactants such as saponin as antistatic agents; nonionic surfactants such as alkylene oxide, glycerin, and glycidol; higher alkylamines, quaternary ammonium salts, pyridine and other heterocycles, phosphoniums or sulfoniums. Cationic surfactants such as: carboxylic acid, sulfonic acid, phosphoric acid, sulfate ester group, anionic surfactant containing acidic groups such as sulfate ester group: amino acids,
Ampholytic activators such as aminosulfonic acids, sulfuric acid or phosphoric acid esters of amino alcohols, and the like are used.

Some examples of surfactant compounds that can be used as antistatic agents are disclosed in U.S. Pat.
No. 2288226, No. 2676122, No. 26
No. 76924, No. 2676975, No. 2691566
No. 2727860, No. 2730498, No. 27
No. 42379, No. 2739891, No. 3068101
No. 3158484, No. 3201253, No. 32
No. 10191, No. 3294540, No. 3415649
No. 3441413, No. 3442654, No. 34
No. 75174, No. 3545974, West German Patent Application (0LS) No. 1942665, British Patent No. 1077317
No. 1198450, etc., "Synthesis of Surfactants and Their Applications" by Ryohei Oda et al. (Maki Shoten 1964 edition):
A. W. Perry, "Surf S Active Agents" (Interscience Publications, Inc. 1958 edition): T. P. ``Encyclopedia of Surf S Active Agents Volume 2'' (Chemical Publishing Company, 1964 edition) by Sisli: ``Surfactant Handbook'' 6th edition (Sangyo Tosho Co., Ltd., December 20, 1964) ) and other books.

These surfactants may be added alone or in combination. Although these are used as antistatic agents, they are sometimes applied for other purposes, such as dispersion, improving magnetic properties, improving lubricity, and as coating aids. To form the magnetic recording layer, the above-mentioned composition is dissolved in an organic solvent and applied as a coating solution onto a support.

The thickness of the support is about 5 to 50μ, preferably 10 to 40μ.
Materials used include polyesters such as polyethylene terephthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and polycarbonate. Methods for applying the magnetic recording layer onto the support include air doctor coating, blade coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, scratch coating, cast coating, and spray coating. etc., and other methods are also possible, and detailed explanations of these can be found in "Coating Engineering" 253 published by Asakura Shoten.
It is described in detail on pages 277 to 277 (published on March 20, 1972). Organic solvents used during coating include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohols such as methanol, ethanol, propanol, and butanol; methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, Ester types such as glycol acetate and monoethyl ether: Glycol ether types such as ether, glycol dimethyl ether, glycol monoethyl ether, and dioxane:
Tar-based (aromatic hydrocarbons) such as benzene, toluene, xylene, etc.: methylene chloride, ethylene chloride,
Carbon tetrachloride, chloroform, ethylene chlorohydrin,
Chlorinated hydrocarbons such as dichlorobenzene are used. By such a method, the magnetic layer coated on the support is optionally treated to orient the magnetic powder in the layer, and then the formed magnetic layer is dried.

Furthermore, by subjecting the material to surface smoothing or cutting it into a desired shape if necessary, it is possible to produce a magnetic recording material using the CO-containing magnetic iron oxide obtained by the method of the present invention as a medium. In this case, the orienting magnetic field is approximately 50
It is about 0 to 2000 Gauss, and the drying temperature is about 50 to 1
The temperature is about 00°C and the drying time is about 3 to 10 minutes.

Using the method described in the invention and the CO-containing magnetic iron oxide obtained thereby gives the following advantageous facts.

That is, PH7. The remaining CO(H) ions, which have not fully precipitated and crystallized in the wet reaction below O, are deposited on the particles by gradually increasing the pH to higher than that while blowing in oxidizing gas, and then deposited on the particles in the high-temperature gas phase. In order to incorporate CO into particles during dehydration, reduction, reoxidation, etc.
Almost all of the CO added as a raw material can be contained in the particles. Furthermore, compared to the conventional method in which CO salt is precipitated on the surface of iron oxide particles that do not contain CO and is heated to decompose and contain it, in the method of the present invention, CO is already contained inside the particles. Particles containing CO uniformly throughout the interior can be obtained, and high coercive force can be achieved. Furthermore, this coercive force can be controlled by changing the concentration of CO() ions and the pH value at the end of the wet reaction. (For example, Hc5OO
When it is desired to obtain a coercive force having a certain value between ~10,000e, the pH is changed between 5.0 and 14.0 in relation to the CO() ion concentration, etc. ) In addition, wet reaction slurry tends to cause agglomeration between particles due to changes in temperature and pH, cessation of stirring, and the passage of time.
Although it has been difficult to obtain uniform CO salt deposition on individual particles, the method of the present invention can eliminate this problem. In addition, since most of the CO() ions that may be emitted during solid-liquid separation are eliminated, this method is advantageous in terms of purifying the waste liquid compared to conventional methods, and the operation is a feature of the present invention. No special equipment is required for this process, the reaction equipment, stirring equipment, alkali, etc. used in the wet reaction can be continued to be used, and the process is relatively simple.

Furthermore, a magnetic recording material obtained by dispersing the CO-containing magnetic iron oxide fine powder produced by the method of the present invention in a binder and coating it on a support with an undercoat layer if necessary can be produced by a conventional method. It provides a better squareness ratio (Br/Bm) than a magnetic recording medium using CO-containing magnetic iron oxide fine powder as a medium. This is a novel fact that could not have been predicted using conventional techniques. Next, the present invention will be explained in more detail with reference to Examples.

Examples 1 and 2 also include a conventional method for comparison. Furthermore, it goes without saying that the present invention should not be limited to this embodiment. Add 31 dissolved in water to 21 dissolved in water, and stir while blowing air while keeping the temperature at 35°C and the pH at 5.5 with a 2N solution of sodium hydroxide. When the reaction was continued, acicular goethite (with a long axis of 0.8 μm and a short axis of 0.1 μm) was formed.
A yellow slurry containing α-FeOOH) was obtained.

When passed through this slurry, the filtrate showed the pick color of residual CO ions. 50% of this slurry
After removing 0CC, washing with water, and drying, it was dehydrated in air at 400°C, reduced in hydrogen gas at 400°C, and reoxidized in air at 300°C to form CO-containing maghemite ((CO)γ-Fe2
O3) was obtained. CO-containing maghemite ((CO)γ-Fe2O3) obtained by this conventional method was dispersed in a polymeric binder, coated on a support, dried, and measured for B-H characteristics. The results shown in A-1 were obtained. On the other hand, 500 cc of the above yellow slurry was taken, and while vigorously stirring and blowing air, 1N sodium hydroxide solution was added dropwise to give various pH values as shown in Table 1.
After that, stirring was continued for 30 minutes while blowing air, and the same treatment as above was carried out, and the B-H characteristics of each were measured, and the values shown in Table 1 B14 were obtained. The same table also shows the amount of CO contained in the filtrate obtained by filtering the slurry after each PH treatment, as the intensity ratio (relative value) given by the fluorescent X-rays. An aqueous solution of iron salt and cobalt salt is neutralized with alkali, and acicular goethite (α-Fe) is gradually oxidized.
In the conventional method of producing OOH), CO() ions were not sufficiently incorporated into the crystal, and some remained.

The reason for this is that while the pH of the reaction solution is 6.5 or less,
One of the reasons is that the pH could not be increased because harmful magnetite would be generated if it exceeded a certain value determined by temperature, concentration, etc. However, according to the method of the present invention, near the end of the reaction, there is no risk of magnetite generation even if the pH is increased, and CO is taken in almost to the set value, which is reflected in Hc. Furthermore, according to the method of the present invention, it is possible to almost eliminate residual CO() ions in the waste liquid, and especially when the pH is set to 9.0 or higher, CO() ions in the waste liquid can be almost eliminated.
( ) ions are not detected.

This is a very advantageous fact in terms of wastewater purification measures. The squareness ratio (Br/Bm) of a magnetic recording medium is a characteristic of electromagnetic conversion, and the higher the value, the more useful it is, but according to the method of the present invention, the squareness ratio can be increased by increasing the pH of the solution before and after the end of the wet reaction. has been improved.

This is a completely new fact that could not have been predicted using conventional techniques. Between PH5.5 and 10.0, Hc increases as the pH increases, and PHLO. Even if the temperature exceeded 0, Hc did not decrease.

5 Example 2 The same reagent-grade ferrous sulfate 1009 used in Example 1 was dissolved in 400 cc of water, and this was added to a solution in which the same reagent-grade sodium hydroxide 159 used in Example 1 was dissolved in 800 cc of water. The reaction was continued at a liquid temperature of 35° C. while stirring while blowing air.

After the liquid turned yellow, the precipitate was filtered and dispersed again in 11 water. At this time, the particle has a long axis of 0.5 μm and a short axis of 0.05 μm.
It was a needle-like crystal of m. Next, while adjusting the reaction temperature to 50 ° C. and adjusting the pH to 6.0 with 2N sodium hydroxide solution,
4 mol% of the same reagent grade cobalt sulfate used in Example 1
A 1/2 molar solution of ferrous sulfate containing 11 is added at a constant rate of 8 hours.

When the crystal growth is finished, the major axis is 1.1 μm and the short axis is 0.2 μm.
The particles were shaped like acicular particles. That is, α-goethite was obtained which had a layer containing CO on acicular fine particles that did not contain CO as a core crystal. The CO content of this goethite is 2.9
%, which was lower than the calculated value of 3.12%. A portion of the goethite-containing slurry thus obtained was collected at the end of the reaction, and to the remaining slurry, aqueous ammonia was added dropwise to the remaining slurry while stirring and blowing air so that the pH gradually increased until the pH reached 8. 5 and continued stirring for an additional 30 minutes. Both slurries were washed with water, dried, dehydrated at 300°C, and reduced in city gas at 400°C to produce CO.
containing magnetite ((CO)Fe3O4), and further oxidize a portion of each in air at 250°C to reduce carbon
O-containing maghemite ((CO)γ-Fe2O3) was used. These were dispersed in a binder and applied to a support, and the B-H characteristics and CO content of each were measured.

The results are shown in Table 2. However, in the column A-2 of the same table, the measured values based on the conventional method are also shown.
The column shows the measured values according to the present invention. Therefore, it can be seen that the method according to the present invention can be applied not only to CO-containing maghemite ((CO)γ-Fe2O3) but also to CO-containing magnetite ((CO)Fe3O4) in the same manner as the final product.

Example 3: A solution of 3001 in water was added to a solution of 2001 in water, and the reaction temperature was adjusted to 35%.
pH 6. with normal sodium hydroxide solution at 2°C. The reaction was carried out by continuously blowing air while stirring and maintaining the temperature at O.

At 8 hours from the start of the reaction, when the reaction solution has sufficiently turned yellow and is just before the end point of the reaction, 2N sodium hydroxide solution is added dropwise with continued stirring and air blowing to gradually raise the pH to 9. O and after 30 minutes,
A yellow slurry containing acicular goethite with a long axis of 0.8 μm and a short axis of 0.1 μm was obtained. After filtering, washing with water and drying, the same treatment as in Example 2 was carried out to obtain CO-containing maghemite ((CO)γ-Fe2O3). In addition, some of it is reduced and becomes CO-containing magnetite ((CO)Fe3O4
). Both were dispersed in a binder, coated on a support, and dried in the same manner as in Example 1 to obtain a magnetic recording medium.

When these B-H characteristics were measured, the results shown in Table 3 were obtained. Further, FIG. 1 shows changes over time in Fe() and CO() (ions) contained in the filtrate of the reaction solution during the reaction. As shown in Figure 1, Fe() and CO(H) ions in the solution decreased as the reaction time progressed, but at 8 hours from the start of the reaction just before the end of the reaction, F
Although there are almost no e(...) ions remaining, CO()
Approximately 20% of the ions still remained in solution.

However, by adding alkali according to the method of the present invention, the remaining amount of CO() rapidly decreased, and almost all of the CO() ions disappeared from the solution. This indicates that almost the entire amount of CO() was deposited on the particles.

[Brief explanation of the drawing]

FIG. 1 shows the residual amounts of Fe and CO in the reaction solution as the reaction progresses in Example 3.

Claims (1)

[Claims] 1 In a reaction in which an oxidizing gas is blown into a solution containing Fe(II) and Co(II) ions, or a solution containing FeIII ions which has been oxidized by blowing an oxidizing gas into this solution is oxidized. The pH of the solution is raised stepwise before and after the end of the reaction to complete the deposition of Co onto the particles, thereby creating α-goethite, followed by dehydration, reduction, and oxidation if necessary. A method for producing Co-containing magnetic iron oxide.