KR830000289B1 - Method for producing magnetic iron oxide particles - Google Patents

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Description

Method for producing magnetic iron oxide particles

The present invention relates to a method for producing a composition mainly composed of acicular non-stoichiometric magnetic iron oxide particles having a unique chemical and physical magnetism. In particular, the ratio of ferrous iron to ferric iron is 0.05 to 0.25, and the oxide is based on the total content of iron. A non-stoichiometric magnetic iron oxide particle containing 0.4-5.0 atomic% zinc and 0-10.0 atomic% cobalt is disclosed.

Needle-shaped ferric oxide or magnetic iron oxide particles containing zinc and cobalt are known and described, for example, in US Pat. No. 2,047,429 US Pat. Nos. 3,047, 505 and US Pat. No. 3,573,980. Needle-shaped iron oxides containing both zinc and cobalt and containing 4-22% by weight of ferric iron are described in Belgian Kingdom Patent No. 804,556 and have higher saturation magnetization than cobalt-modified magnetic iron oxide, and also storage safety. This is known to be good. However, the magnetic oxides of the Belgian Kingdom Patent No. 804,556 are not uniform in shape, and they do not provide modern industrial conditions because they are not provided with a magnetic film of a conventional competitive material having good surface smoothness, low noise and durability against stress deformation or good signal response. It is not satisfied.

Techniques for modifying the surface of the starting material goethite (alpha-FeOOH) with gamma ferric oxide by treating with hydrolyzable phosphorus oxyacids or salts thereof are described in US Pat. No. 3,652,334. 0.1-5% content of oxides, hydroxides or phosphates of polyvalent metals such as zinc or iron on the gamma Fe ₂ O ₃ particles which exhibit higher coercivity and better self-alignment than gamma Fe ₂ O ₃ of untreated goethite When the film is formed by using the cobalt-treated gamma Fe ₂ O ₃ particles, the residual magnetic loss is reduced and the coercive force decreases in tempering, compared with the case where no oxide or phosphate film is formed. It is described that acidity is improved.

U.S. Patent No. 3,931,025 describes that post-treatment with phosphate or silica can improve dimensional stability for acicular particles upon conversion to magnetic oxides. The patent also contains 0.1-4% zinc ions and 0.1-2% zinc ions and 0.1-2% phosphate ions when forming alpha-FeOOH seed particles. Containing -4% of zinc ions and 0.1-2% of phosphate ions improves the needle bed and produces dendritic tissues as compared with the case of producing seed particles without zinc and only one of zinc or phosphorus. V) It is described that formation is prevented and a more uniform particle size distribution is obtained. The three patents, ie, phosphate-containing gamma ferric oxide, are limited in their use because they do not provide the magnetic distortion stability, signal output or saturation moment required for audio and video tapes.

In addition, US Pat. No. 4,053,325 discloses that hydrated ferric oxide or magnetic iron oxide particles are coated with 1-20% of an insoluble metametal salt such as iron or zinc metaphosphate, thereby improving thermal stability, and furthermore, dehydration or oxidation reactions. The color of the conventional yellow and blue iron oxide pigments, which tend to change in color, is improved. Therefore, it can be inferred that the characteristics of magnetic iron oxide are improved by using iron oxide coated as a gamma Fe ₂ O ₃ precursor. Gamma ferric oxide particles in the above patents are limited in use because they do not meet the high magnetism required for video use, such as the ferric oxide particles of US Pat. No. 3,652,334.

According to the present invention, acicular magnetic iron oxide particles containing a specific amount of both phosphorus and zinc and ferrous iron ions at a specific ratio have better powder magnetism, dispersibility, and tissue generation than conventional coating compositions. It has been found that magnetic tapes exhibiting high stability to temperature and physical stress, good wear resistance and stress strain provide good signals for low noise ratios. Therefore, in the present invention, the atomic ratio of ferrous iron to ferric iron is 0.05: 1 to 0.25: 1, and the oxide content is 0.4-5.0 atomic%, 2.0-10.0 atomic% zinc and 0-10.0 atomic% based on the total content of iron. The present invention relates to the preparation of cobalt-containing non-stoichiometric magnetic iron oxide needle particles by the following method: (a) Needle particles composed of nonmagnetic ferric oxide or its hydrate form a dispersion solution, and (b) Treated with a water-soluble compound containing 0.4-5.0 atomic% phosphorus based on the total content of iron, (c) the particles treated with a zinc compound of 2.0-10.0 atomic% or higher based on the total content of iron Contact with and absorb onto the surface of the particles, (d) recover the particles produced from the aqueous medium, and (e) heat the recovered particles under reducing conditions to form a magnetite (m) Oxidation or oxidation and partial reduction To the ore for ferrous ferric atomic ratio of 0.05: changes in the magnetic iron oxide 1: 1 to 0.25. The composition is useful for use in magnetic parts of magnetic recording media.

The term non-stoichiometric magnetic iron oxide means that iron oxide is inconsistent with a certain proportion of the law. The oxides are called Bertolide compounds, and magnetite Fe ₂ O ₄ (FeOx, where X = 1.33), maghemite, gamma-Fe ₂ O ₃ (Fe Ox, where X = 1.50) Has an oxidation degree of. The atomic ratio of ferrous iron to all iron in nonstoichiometric magnetic iron oxide is less than 1: 3.

As described above, the non-stoichiometric magnetic iron oxide particles of the present invention contain 0.4-5.0 atomic% phosphorus and 2.0-10.0 atomic% zinc based on the total content of iron, and the atomic ratio of ferrous iron to ferric iron is 0.05: 1. To 0.25: 1. It is preferable that atomic% of phosphorus is 0.25-0.25, and atomic% of zinc is 2.0-7.0 atomic%. When the cobalt is contained, 0.1-10.0 atomic% is included based on the total iron content, and cobalt-to-zinc atom of 0.6 or more is preferable. In particular, the atomic ratio of ferrous iron to ferric iron in magnetic iron oxide is preferably 0.05-0.22, most preferably 0.05-0.20.

The non-stoichiometric magnetic iron oxide of the present invention exhibits excellent magnetic properties and is excellent in use in digital, audio, and helical video, and does not require a newer hardware method, especially in wear, and also in temperature. And the film is completely flawless since there is no harm in magnetic and electronic stability against physical stress.

The needle-shaped magnetic iron oxide of the present invention comprises a method consisting of the following series of steps: (a) preparing a dispersed aqueous solution of acicular interest consisting of nonmagnetic ferrous tetrachloride or a hydrate thereof; (b) treating the particles with a water-soluble compound containing 0.4-0.5 atomic percent phosphorus based on the total content of iron; (c) adsorbing a zinc compound of 2.0-10.0 atomic% on the basis of the total amount of iron to particles treated with pH 8.0 or higher to be absorbed on the surface of the particles; (d) recovering the particles produced from the aqueous medium; (e) heating the recovered particles under reducing conditions to produce magnetite; And (f) a process of changing the magnetite to non-stoichiometric magnetic iron oxide having an atomic ratio of ferrous iron to ferrous iron by 0.05: 1 to 0.25: 1 by partial oxidation or oxidation and partial reduction method.

The ferric oxide, hydrate, or ferric oxide particles used as starting materials in the process of the invention are nonmagnetic and needle-shaped. The length of the particles is preferably 0.2-1.0 micron, the ratio of length to width is preferably at least 5: 1, and most preferably from 8: 1 to 20: 1. It is particularly preferred that the starting materials are goethite (alpha-FeOOH) hematite (alpha-Fe ₂ O ₃ ) and lepidocr oite (gamma-FeOOH). Needle-shaped particles of these oxides are known and commercially available, and are also conveniently prepared from ferrous salt solutions using more than this stoichiometric equivalent to less than one-third of the stoichiometric equivalent of alkali hydroxides, It is possible to produce particles having a desired size of hydroxide precipitated in excess ferrous solution) or base medium (excess alkaline solution).

In a preferred embodiment of the present invention, the acicular particles are alpha-FeOOH particles, which are washed to remove soluble salts and then kneaded again in water to obtain a dispersion solution. The concentration of the particles in the dispersion is not a very important problem, and what is actually needed is to contain a sufficient amount of water to obtain proper fluidity and to be easily stirred.

The treatment step with the phosphorus-containing compound is preferably carried out so that an aqueous solution of phosphoric acid or a water-soluble salt thereof is gradually added to the dispersion while stirring, and stirring is continued to uniformly distribute and absorb the iron oxide particles. Preferred compounds containing phosphorus include inorganic acids or acids such as phosphoric acid, mono-phosphate, di- or tri-alkali metal salts and especially dihydrogen phosphate, disodium orthophosphate, trisodium phosphate, sodium pyrophosphate, sodium metaphosphate, etc. to be. Phosphorus-containing compounds are usually added in the form of dilute aqueous solution, and the temperature range of the dispersion is about 30 ° C to about 100 ° C.

The amount of the compound containing phosphorus should be used to provide a sufficient amount of phosphorus from about 0.4 to about 5.0 atomic%, preferably from 0.5 to about 2.5 atomic%, based on the total iron content.

The particles treated as described above are brought into contact with the zinc compound at a pH of 8.0 or higher. Usually, it is preferable to add zinc compound in aqueous solution or dispersion state to the dispersion, and the pH should be adjusted as necessary using a value of about 8.5 to about 10.0. Any zinc compound can be used as long as it is water-soluble or dispersible in water, such as zinc sulfate, zinc oxide, zinc chloride or zinc acetate, and the zinc content is about 2.0 to about 10.0, preferably about 2.0, based on the total iron content. It should be used in an amount sufficient to provide about 7.0 atomic% of zinc precipitates. The exact way to deposit zinc onto the particles is not fully understood. However, it has been found that the treatment method is important for obtaining product uniformity and the superiority of the present invention.

After the particles were treated with zinc, the dispersion was compressed, passed through a screen or the like, or separated from the aqueous medium by a conventional method such as centrifugation, and the recovered particles were washed and dried. All lumps of about 1 cm are usually comminuted and broken up.

The process of converting the treated particles into magnetic iron oxide is carried out by conventional techniques. The particles are injected into the furnace, heated to remove all hydrates, and continue heating in a reducing atmosphere to convert dehydrated particles to magnetite. Subsequent oxidation of the particles by heating in the atmosphere yields the desired atomic ratio of ferrous to ferric iron.

It is also desirable to magnetize the magnetite particles completely to ferric state and then partially reduce to the desired atomic ratio of ferrous iron to ferric iron. Particles are usually introduced into a cold furnace to raise the temperature from about 250 ° C to about 600 ° C, introduce a reducing gas such as hydrogen and / or carbon monoxide, and continue heating at 300 ° C-500 ° C. Alpha-Fe ₂ O ₃ particles dehydrated for about 1-4 hours in a reducing atmosphere are converted to magnetite. The reduction to magnetite is carried out carefully, and after the reduction is almost finished, almost no abnormality is performed. The amount usually required varies depending on the gas flow, temperature, reducing agent, inert gas and water vapor content, in addition to the oxide inlet and specific surface. Appropriate conditions and the like for a given system can be easily determined by performing a series of experiments by those skilled in the art.

After the particles are reduced to magnetite, the atmosphere of the furnace is changed to an oxidized state. After the supply of the reducing gas is completed, the exhaust gas is degassed with an inert gas and air or oxygen is introduced to heat the particles from 225 to 375 ° C. It is preferable to make it. The particles are placed in an oxidizing atmosphere until the desired atomic ratio of ferrous to ferrous iron is reached, and the particles are completely oxidized to ferric state and then partially reduced to the desired atomic ratio of ferrous to ferrous iron. The resulting particles are recovered by conventional methods such as cooling in an inert atmosphere and then pulverizing in a bottom as shown in US Pat. No. 2,954,303.

An aqueous solution or dispersion of the cobalt compound is added to the dispersion of particles of phosphorus-treated particles or a dispersion of particles of nonmagnetic ferric oxide or a salt thereof and introduced into the dispersion of phosphorus-treated particles before the introduction of a zinc compound. The particles can be modified with cobalt, preferably by manipulation of the pH from about 8.2 to about 10, to obtain these oxides with higher coercivity. In some cases, it is usually desirable to add the total amount of phosphorus needed during the addition of the cobalt compound to the particles treated with phosphorus, two processes, namely before and after the addition of cobalt. Any compound may be used as long as it is water-soluble or easily dispersible in water such as cobalt sulfate or cobalt hydrate. In the case of the preferred embodiment of the present invention, the content of cobalt is 0.1-10.0 atomic% and 0.5-7.0 atomic% based on the total amount of iron. The ratio of zinc to cobalt is preferably at least 0.6, most preferably 0.8 to 5.

The nonstoichiometric magnetic iron oxides described in the present invention are particularly useful for the production of magnetic recording tapes and can be easily dispersed in conventional solvent and binder heating, so that they are coated on a carrier and dried as necessary. It can be hardened and used as a magnetic recording device such as a tape, a disk or a sheet. The magnetic tape containing iron oxide of the present invention has superior characteristics compared to currently available tapes, and has excellent surface smoothness and abrasion resistance, and excellent stability when subjected to temperature and physical stress.

The present invention will be described in more detail according to the following examples. All percentages are by weight unless otherwise indicated in the examples.

Example 1

1.89 kilometers of aqueous slurry containing 34.5 g / liter of washed needle-like alpha FeOOH particles with an average length of about 0.6 microns and a ratio of length to width of about 12 microns in a tank equipped with a stirrer, heater and thermometer I was. The injected material was heated to 70 ° C. while stirring. The gas containing oxygen was passed through the stirred slurry at a rate of 1.9 liters per second. An aqueous 17% sodium hydroxide solution was added to the slurry to adjust to 5.4, and the slurry material was stirred at 70 ° C. for 1 hour while slowly adding 1.5 liters of 21.2% acid aqueous solution. Next, 6.67 kg of zinc sulfate monohydrate (ZnSO ₄ , H ₂ O) was added to the slurry material which is 30% zinc sulfate aqueous solution, and the pH of the slurry material was adjusted to 9.0 and stirred at 70 ° C. for 2 hours. The compressed cake obtained by compressing the slurry material to an oven was washed to remove soluble salts, and then the washed compressed cake was dried at 110 ° C. for 24 hours, and the dried cake was press-crushed into chunks. Next, the pressed cake was transferred to a calciner, heated and dehydrated at 365 ° C, and reduced at 365 ° C. It was then oxidized at 327 ° C until 33.9% of the iron became iron.

It was then reduced at 343 ° C. until the weight percentage of FeO in the product became 0.7. It was then heated to 415 ° C. for 1 hour in an inert atmosphere until 16.1% of the iron became ferrous and then cooled to room temperature in an inert atmosphere. The resulting product was then pulverized into powder and then obtained as non-stoichiometric needle-shaped iron oxide oxide particles having the same shape as the starting material alpha FeOOH particles and having an atomic ratio of 0.16 of ferrous iron. The obtained product contained 0.75 atomic% phosphorus and 5.0 atomic% zinc with respect to the iron weight analysis of 65.5% and the iron content, and exhibited the following magnetic properties when measured with a B-H meter having a magnetic field of 2,000 erste.

Coercive force (Hc) -385 Hersted.

Residual Magnetic (Br) -2620 Gauss

Maximum Inductance (Bm)-4800 Gauss.

Magnetic iron oxide was used to make magnetic tape by the following method. An injection consisting of a 34.8 g iron oxide, 61% xylene 26% methyl isobutyl ketone, a colorant containing 7% dioctyl and 7% terephthalate, and a 6% lecithin-type wetting agent was carried out by boil-milling for 40 minutes to contain magnetic iron oxide. After preparing the dispersion coating composition, 45 milliliters of a vinyl chloride and vinyl acetate (90:10) 15% copolymer solution in toluene / methylethylketone (2.3 / 1) was added to the infusion and then continued for 20 minutes. Milling was performed. The resulting dispersion liquid (# 4 spins were subjected to a buke feed viscosity of 30 ° C. and 45 poise at 20 RPM), and was then 25 microns in length using a knife extruder with an orientation magnet. The resulting film was dried in air by applying a biaxially oriented (terephthalic acid) poly (ethylene) film at a film speed of about 18 meters per minute, and then a tape of about 7 centimeters wide was cut from the film.

The microscopic examination of the film surface (film thickness of 5 microns) at 100 × magnification and low angle showed excellent gloss, texture, smoothness, and dispersion quality. The magnetic properties of this tape were measured with a B-H meter having an intensity of 1,000 Hersted magnetic field, and the following characteristics were obtained.

Coercive force (Hc) -354 Hersted.

Residual Magnetic (Br) -1,290 Gauss

Maximum Inductance (Bm) -1518 Gauss.

Squareness (Br / Bm) -0.85

Switching Field Distribution (H △) -105 Hersted

Normal switchboard distribution (H △ / Hc) -0.30

Example 2

Injecting 1.89 km of an aqueous slurry solution containing 31.8 g of needle-shaped alpha-FeOOH particles into a tank; Using 1.86 liters of an aqueous 21.2% phosphoric acid solution; 20% cobalt sulfate heptahydrate (CoSO4 7H2O) 5.35 kilogram was slowly added to the slurry material containing the particles treated with phosphoric acid to adjust the pH to 8.5 and stir the slurry material for 1 hour before injecting zinc sulfate. Thing; Stirring the slurry material for 1 hour before injecting zinc sulfate; Using 6.23 kilograms of zinc sulfate monohydrate; Dehydrating at 371 ° C; Example 1 except that the first reduction operation was carried out at 371 ° C. until 34.2% of the iron became the first iron state, and the reduction of the chitin species was carried out until 17.1% of the iron became the first iron state. The operation was repeated. The resulting product was repeated in Example 1 except for performing the final reduction operation. The resulting products were non-stoichiometric needle-shaped iron oxide oxide particles with an atomic ratio of 0.17 to the first iron and ferric iron, which were almost identical in shape to the starting material FeOOH particles. Analysis of the product revealed that it contained 0.80 atomic percent phosphorus, 2.8 atomic percent cobalt and 5.1 atomic percent zinc on a Fe 62.5 percent iron basis. The magnetic properties of this product were measured with a B-H meter having a 2,000 erste magnetic field, and the following characteristics were obtained.

Coercive force (Hc) -656

Residual Magnetic (Br) -2380 Gauss

Maximum Inductance (Bm)-4350 Gauss.

When the tape was made according to the operation of Example 1 using magnetic iron oxide, the tape was very smooth and glossy, and the magnetic properties of the tape were measured by measuring the magnetic properties of the tape with a strength of 2,000 Hersted field. .

Coercive force (Hc) -689 Hersted.

Residual Magnetic (Br) -1420 Gauss

Maximum Inductance (Bm)-1775 Gauss.

SQUARE (Br / Bm) -0.80

Switch length long distribution (H △)-224 Hersteds.

Normal switchover distribution (H △ / Hc) -0.34.

A regulator was prepared using the above operation except that the pH of the slurry material was adjusted to 8.3 before introducing zinc sulfate without adding phosphoric acid to the slurry material. Analysis of the resulting product contained 62.8% Fe, 2.8 atomic% cobalt and 5.1 atomic% zinc on the basis of iron, and showed the following magnetic properties (magnetic field of 2,000 Hersted).

Coercive force (Hc) -510 Hersted.

Residual Magnetic (Br) -2580 Gauss

Maximum Inductance (Bm)-4810 Gauss.

This standard oxide was used to make a tape according to the operation of Example 1, which showed the following magnetic values.

Coercive force (Hc) -628 elstede.

Residual Magnetic (Br) -1275 Gauss

Maximum Inductance (Bm)-1725 Gauss.

SQUARE (Br / Bm) 0.74

Switching length distribution (H △)-268 esthet.

Normal Switching Distribution (H △ / Hc) -0.43.

The gloss, smoothness, structure, and dispersibility of the standard control tape on the magnetic surface were inferior to the tape coated with the magnetic oxide of the present embodiment.

The X-ray diffraction patterns for the product of this example and the standard control were compared using Guinier technology and chromium target tubes. The shapes were similar but not identical, and the shapes for the standard control were not constant, the two were not the same, and there were lines that did not exist in the shape of the present example product. The cell constant of the product of this example was 8.381, while the cell constant of the standard control was 8.378.

Example 3

In a tank equipped with a stirrer, a heater and a thermometer, 1.89 kiloliters of an aqueous slurry solution containing 30.8 grams of needle-like alpha FeOOH particles in 1 liter was injected. While stirring, the injection was heated to 70 ° C. and an oxygen containing gas was passed through the stirred slurry material at a rate of 1.9 liters per second. A dispersion aqueous solution containing 17% cobalt hydrate was separately prepared and this dispersion was slowly added over 9.31 kilograms of stirred tank slurry material over 30 minutes to bring the final pH of the slurry material to 8.5. Then 2.1 liters of 21.2% phosphoric acid aqueous solution was added and the slurry material was stirred for 1 hour. A dispersion aqueous solution containing 16% zinc oxide was separately prepared, and 17.5 kg of this dispersion aqueous solution was slowly added to the stirred slurry material, and the stirring was continued for 1 hour at a pH of 9.0. The slurry material was filtered and pressed, the pressed cake was washed to remove soluble salts, and then the washed pressed cake was dried at 110 ° C. for 24 hours to be crushed into chunks. Next, the crimped cake is transferred to a calciner, heated and dehydrated to 371 ° C., and reduced at 371 ° C. until 33.8 iron is in the first iron state. It is then reduced at 349 ° C until the FeO weight percentage of the product is 0.3. It was then heated to 399 ° C. for 1 hour in an inert atmosphere until 16.8% of the iron became ferrous and then cooled to room temperature in an inert atmosphere. Next, the product formed by grinding powder into powder was non-stoichiometric needle-like iron oxide particles having an atomic ratio of 0.16 in ferrous iron, almost the same shape as the starting material alpha-FeOOH particles. This product contains Fe 2.8%, it contains 2.5 atomic% cobalt, 1.0 atomic% phosphorus and 5.0 atomic% zinc on the basis of iron. The measurement of the product showed the following characteristics of magnetic properties.

Coercive force (Hc) -702 Hersted.

Residual Magnetic (Br) -2300 Gauss

Maximum Inductance (Bm)-4250 Gauss.

This magnetic iron oxide was used to make a tape by the operation of Example 1, which had good gloss and good surface smoothness, and exhibited the following magnetic value at the intensity of the magnetic field of 2,000 ersted.

Coercive force (Hc) -742 Hersted.

Residual Magnetic (Br) -1820 Gauss

Maximum Inductance (Bm)-2330 Gauss.

Squareness (Br / Bm) -0.78

Switch length long distribution (H △) -236 esthet.

Normal Switching Distribution (H △ / Hc) -0.32

Example 4

Injecting a slurry aqueous solution containing 34.8 g / l of acicular alpha-FeOOH into a 1.89 g tank; 1.52 liters of aqueous 21.2% phosphate solution; Using 7.81 kg of cobalt hydrate dispersion aqueous solution instead of 5.35 kg of cobalt sulfate and heptahydrate of Example 3, and continuing stirring for 2 hours; 6.19 kg of zinc oxide dispersed aqueous solution instead of 6.23 kg of zinc sulfate of Example 3, and adjusting pH to 9.3; Dehydration at 366; Carrying out a first reduction operation at 366 ° C. until 33.6% of iron is in the state of ferrous iron; Continuing the oxidation operation until the weight percentage of FeO in the product becomes 0.4; The operation of the example was repeated except that the final reduction operation was carried out at 349 ° C. and the product was heated in an inert atmosphere for 1 hour before cooling to 16.3 iron. The next milled product was non-stoichiometric needle-shaped iron oxide oxide particles with an atomic ratio of 0.15 in ferrous iron and ferric iron, and nearly identical in shape to the starting alpha-FeOOH particles. This product contains Fe63.6%, and it contains 0.7 atomic% phosphorus, 1.9 atomic% cobalt and 5 atomic% zinc on the basis of iron. Indicated.

Coercivity (Hc) -535 Hersted.

Residual Magnetic (Br) -1350 Gauss

Maximum Inductance (Bm)-1730 Gauss.

Squareness (Br / Bm) -0.78

Switching box distribution (H △) -164 Hersteds.

Rated switching field distribution (H △ / Hc) -0.30.

The spiral video tape made of the magnetic iron oxide of the present embodiment has excellent smoothness, video clarity, temperature stability, and noise ratio for video signals, and no chroma noise, and long-term scanning testing, compared to commercially available spiral video tapes. It was extremely durable against.

Example 5

Using a slurry containing 29.5 g / l of acicular alpha-FeOOH particles as a starting material: using 19.6 kg of cobalt dispersion, 21.2% aqueous phosphate solution 3.9 meters, and 19.9 kg zinc oxide dispersion; Performing a first reduction operation until 33.5% of iron is in the ferric state; The operation of Example 3 was repeated except that the oxidation operation was continued until the weight% of FeO became 0.4 and the final reduction operation was performed until 17.0% of iron was in the first iron state. The bowl milled products were nonstoichiometric needles having an atomic ratio of 0.16 in ferrous iron and ferrite, and almost identical in shape and size to the starting alpha-FeOOH particles. This product contains 60.6% of iron, and this product contains 2.25 atomic% phosphorus, 5.53 atomic% cobalt and 5.0 atomic% zinc on the basis of iron. The characteristics of

Coercive force (Hc) -955 Hersted.

Residual Magnetic (Br) -2220 Gauss

Maximum Inductance (Bm)-3970 Gauss.

The tape was made by the operation of Example 1 using this magnetic iron oxide. When the tape was measured at an intensity of 3,000 Hertz magnetic field, the following magnetic properties were exhibited.

Coercive force (Hc) -1078

Residual Magnetic (Br) -1670 Gauss

Maximum Inductance (Bm)-2250 Gauss.

Squareness (Br / Bm) -0.74

Switching box distribution (H △) -390 esthet.

Normal Switching Distribution (H △ / Hc) -0.36

Example 6

Using 7.1 kg of 8% cobalt hydrate dispersion aqueous solution and 18.1 kg of 7% zinc oxide dispersion aqueous solution; Dehydrating at 404 ° C .; Performing a first reduction operation at 404 ° C. until 35.3% of iron is in the first iron state; Performing oxidation operation at 354 ° C until the FeO weight percentage in the product becomes 0.2; The operation of Example 4 was repeated except that the final reduction operation was carried out until 7.4% of iron became the first iron state, and the product was heated to 427 ° C. for 1 hour in an inert atmosphere before cooling the product.

The next product obtained by ball milling was non-stoichiometric needles with an atomic ratio of 0.07 in ferrous iron and ferric iron, which were almost identical in shape and size to the starting alpha-FeOOH particles.

This product contains Fe64.7%, and this product contains 0.75 atomic% phosphorus, 0.75 atomic% cobalt and 2.19 atomic% zinc on the iron basis, Magnetic properties are shown.

The magnetic iron oxide was used to make a tape by the operation of Example 1. The tape exhibited high surface smoothness and gloss, and was measured at an intensity of 1,000 Hersted field, and exhibited the following magnetic properties.

Claims (1)

Needle-shaped particles of nonmagnetic ferrous oxide or its hydrate dispersed in an aqueous medium are treated with a water-soluble phosphorus compound and a zinc compound, the treated particles are recovered from an aqueous medium, and the particles are converted into non-stoichiometric magnetic iron oxide. In the method for producing stoichiometric needle-shaped iron oxide particles, the particles are treated with a water-soluble phosphorus-containing compound in an aqueous dispersion to provide 0.4-5.0 atomic percent of phosphorus on the basis of iron absorbed on the particle surface, and then the particles on an iron basis. The magnetite produced by contacting the treated particles with a zinc compound in an aqueous acid solution of pH 8.0 or higher to provide 2.0-10.0 atomic% of zinc absorbed to the surface, and then heating the recovered particles to the ferrous / ferric iron atomic ratio of 0.05 to 0.25. Method for producing magnetic iron oxide particles, characterized in that the conversion to / 1 non-stoichiometric magnetic iron oxide.