EP0105110B1 - Process for producing acicular ferromagnetic metal particles essentially consisting of iron - Google Patents

Description

The invention relates to a process for the production of acicular ferromagnetic metal particles consisting essentially of iron by reducing acicular iron (III) oxide hydroxide provided with a shape-stabilizing surface coating or the iron (III) oxide obtained therefrom by dewatering by means of decomposable organic compounds and hydrogen.

Acicular ferromagnetic metal particles are of particular interest for the production of magnetic recording media because of their high saturation remanence and high coercive force.

It is known to remove iron particles by reducing finely divided acicular iron compounds, such as. B. the oxides with hydrogen or another gaseous reducing agent. In order for the reduction to take place at a practical speed, it must be carried out at temperatures above 300 ° C. However, this brings with it the difficulty that the metal particles formed sinter. As a result, the particle shape no longer corresponds to that required to achieve the specified magnetic properties.

To reduce the reduction temperature, it has already been proposed to catalyze the reduction by applying silver or silver compounds to the surface of finely divided iron oxide (DE-A-20 14 500). The treatment of iron oxide with tin (II) chloride has also been described (DE-A-1907691).

Attempts could also be made to provide the iron oxides to be reduced with a surface coating layer in order to prevent the sintering of the individual particles which occurs due to the required reduction temperature, such as e.g. B. in DE-A-24 34 058, 24 34 096, 26 46 348, 27 14 588, EP-A-24 694 and FR-A-24 04 289, do not fully satisfy.

The object of the invention was therefore to provide a method for producing acicular ferromagnetic metal particles consisting essentially of iron, with which it is possible in a simple manner to obtain pronounced shape-anisotropic particles which are distinguished by a high degree of fineness, a narrow particle size distribution and a high coercive field strength.

It has now been found that acicular ferromagnetic metal particles consisting essentially of iron are formed by reducing acicular iron (III) oxide hydroxide provided with a shape-stabilizing surface coating or the iron (III) oxide obtained therefrom by dewatering, to which additionally decomposable organic compounds are applied can be produced according to the task if the pretreated iron (III) oxide hydroxide or iron (III) oxide in a first stage in an inert gas atmosphere at a temperature between 270 and 650 ° C to FeO _x with values for x from 1.33 to 1.44 and in a second stage by means of hydrogen at a temperature between 270 and 450 ° to the metal.

The starting material for the process according to the invention is iron (III) oxide hydroxide in the form of a-FeOOH and γ-Fe00H in a mixture of 80 to 100% a-FeOOH and 0 to 20% γ-FeOOH or 70 to 100% γ-FeOOH and 0 to 30% a-Fe00H suitable. The corresponding iron (III) oxide hydroxides expediently have a BET surface area of at least 20 and up to 120 m ² / g, the average particle length is between 0.10 and 1.5 μm with a length-to-thickness ratio of at least 5, expediently 8 to 40. In the same way, the iron (III) oxides obtained from the iron (III) oxide hydroxides mentioned by dewatering at more than 250 ° C. can also be used. Correspondingly modified iron oxides are used in a known manner as starting materials for producing metal particles which contain further alloy constituents, such as cobalt, nickel and / or chromium, in addition to iron.

These iron (III) oxide hydroxides or iron (III) oxides are now provided in a known manner with a shape-stabilizing surface coating, which helps to maintain the outer shape during the further processing steps. Suitable for this is e.g. B. the treatment of iron (III) oxide hydroxides or oxides with an alkaline earth metalation and a carboxylic acid or another organic compound which has at least two groups capable of chelating with the alkaline earth metalation. These processes are described in DE-A-24 34 058 and 2434096.

Also known and described in DE-A-26 46 348 is the shape-stabilizing treatment of the iron (III) oxide hydroxides or oxides on their surface with hydrolysis-resistant oxygen acids of phosphorus, their. Salts or esters and aliphatic mono- or polybasic carboxylic acids. Possible hydrolysis-resistant substances are phosphoric acid, soluble mono-, di- or triphosphates such as potassium, ammonium dihydrogen phosphate, disodium or dilithium orthophosphate, trisodium phosphate, sodium pyrophosphate and metaphosphates such as sodium metaphosphate. The compounds can be used alone or as a mixture with one another. Advantageously, the esters of phosphoric acid with aliphatic monoalcohols with 1 to 6 carbon atoms, such as. B. use tert-butyl ester of phosphoric acid. Carboxylic acids in the process are saturated or unsaturated aliphatic carboxylic acids with up to 6 carbon atoms and up to 3 acid groups, it being possible for one or more hydrogen atoms in the aliphatic chain to be substituted by hydroxyl or amino radicals. Oxidic and oxitricarboxylic acids such as oxalic acid, tartaric acid and citric acid are particularly suitable. Other shape-stabilizing agents suitable within the scope of the method according to the invention Equipment is the known surface coatings with tin compounds (DE-C-19 07 691) or with silicates or Si0 ₂ (JP-A-121 799/77 and JP-A-153 198/77).

Decomposable organic compounds are now applied to the iron (III) oxide hydroxides or oxides thus equipped.

Suitable organic compounds are all organic substances which are decomposable in the temperature range between 270 and 650 ° C. in the presence of the iron oxide hydroxides or iron oxides. So are u. a. longer-chain carboxylic acids and their salts, amides of long-chain carboxylic acids, long-chain alcohols, starch, oils, polyalcohols, waxes, paraffins and polymeric substances such. B. polyethylene. A high boiling point or sublimation point is advantageous in order to avoid losses of organic matter before the reduction effect begins.

For finishing with the organic reducing agent, the iron (III) oxide hydroxides or oxides are mechanically mixed with the solid or liquid organic substances or coated with them in a suitable solution or suspension of the substance. Shape stabilization and application of the organic substance can also simultaneously or immediately one after the other, for. B. in an aqueous suspension of the particles. Likewise, the organic compounds can also be present during or before the crystal growth of the iron (III) oxide hydroxides. For this purpose, the organic substance is already at the beginning of the Fe00H synthesis, e.g. B. added before the precipitation of Fe (OH) ₂ . The addition after nucleation has ended or during or after the growth stage is also suitable. In these cases, the shape-stabilizing surface coating is subsequently formed in the aqueous suspension of the particles or after the filter cake has been freed from inorganic salts in water. In general, carbon contents of 0.5 to 20% by weight, based on Fe00H or Fe ₂ 0 _{3, are} sufficient.

When carrying out the process according to the invention, in a first stage the iron (III) oxide hydroxide or oxide provided with a surface coating is converted to FeO _x with values for x by means of the organic decomposable compound under inert gas, usually nitrogen, at temperatures between 270 and 650 ° C. reduced from 1.33 to 1.44. In a second stage immediately following, the FeO _x is then reduced to hydrogen at 270 to 450 ° C. to the metal.

The reductions and, if appropriate, the dewatering of FeOOH to Fe ₂ O ₃ before and at the start of the reduction can be carried out batchwise or continuously, for. B. each in its own reactor. Depending on the number and type of reactors available, e.g. B. rotary tube or fluid bed technology, the type of product used, for. B: Fe00H or Fe ₂ 0 ₃ , and the reduction process can use cocurrent or countercurrent operation of solids and gas or steam streams. Furthermore, in some organic reducing agents the organic reduction to FeO _x can be carried out simultaneously with the dewatering of FeOOH and at the same point in the reactor or, in the case of a continuous procedure, the dewatering to Fe ₂ 0 ₃ and the organic reduction to FeO _x in a reactor by addition of the organic matter take place locally at the appropriate location of the reactor.

The acicular ferromagnetic metal particles obtainable by the process according to the invention still largely have the shape stemming from the starting materials and, despite the preceding conversion reaction, they are uniform and particularly fine-particle according to the starting material. As a result, they are characterized by high values with regard to their magnetic properties, such as the coercive force and above all the remanence. The high squareness of the hysteresis loop indicates a narrow switching field thickness distribution due to the uniform shape.

Such metal particles are outstandingly suitable as magnetic materials for the production of magnetic recording media. However, these substances are expediently passivated before further processing. This means encasing the metal particles with an oxide layer by controlled oxidation in order to eliminate the pyrophoricity caused by the large free surface area of the small particles. For example, this is achieved by passing an air / nitrogen mixture over the metal powder. The passivation can also be carried out by wetting the pigments with organic solvents in the presence of oxygen or with other known oxidation and / or coating processes.

Used for the production of magnetic recording media, the metal particles obtainable by the process according to the invention can be oriented magnetically particularly easily. In addition, important electroacoustic values, such as depth and height controllability, and especially the noise due to the fine nature of the material have been improved.

The present invention is explained by way of example using the following experiments. The magnetic values of the samples were measured with a vibration magnetometer at a magnetic field of 160 kA / m or after premagnetization in a pulse magnetizer in a vibration magnetometer. The values of the coercive field strength, H _e , measured in kA / m, were related to a stuffing density of 0 = 1.6 g / cm ³ in the powder measurements. Specific remanence (M _r / δ) and saturation (M _m / δ) are given in nTm ³ / g.

example 1

In a rotary flask, 4 ml of olive oil were added to 20 g of γ-FeOOH (0.87% by weight PO ₄ ³ aus, 0.08% by weight C from oxalic acid) equipped with oxalic acid / phosphoric acid and the mixture was brought to 370 for 15 minutes in a stream of nitrogen ° C heated. The resulting material had the composition FeO _1.34 . Now 10 g of FeO _{1.34 were} reduced to metal at 350 ° C in a hydrogen stream within 8 hours. The coercive field strength of the pyrophoric material was 73.7 kA / m.

Comparative experiment 1

A γ-FeOOH which had been treated with oxalic acid / phosphoric acid in accordance with Example 1 was reduced directly with hydrogen as indicated in Example 1. The coercive field strength of the pyrophoric material was 63.1 kA / m.

Comparative experiment 2

A γ-FeOOH with a specific surface area of 30 m ² / g was provided with a tin oxide coating in accordance with the information in DE-AS 19 07 691 by neutralizing the acidic SnCl ₂ -containing aqueous suspension of the particles. The amount of tin was 1% by weight based on FeOOH. Subsequently, a coating of 3% by weight of olive oil was produced in the same dispersion by adding olive oil. The Fe00H equipped in this way was reduced to the metal in a hydrogen stream (30 NI / h) at 370 ° C. for 7 hours. The measurement results of the pyrophoric material (py) and of a material (pa) passivated with acetone in the presence of air are given in Table 1.

Example 2

The procedure is as described in Comparative Experiment 2, but the material treated with tin oxide / olive oil is first reduced to passivium in a nitrogen stream at 520 ° C. within 30 minutes to FeO _1.33 and only then as in Comparative Experiment 2 with hydrogen to the metal. The measurement results are given in Table 1.

Comparative experiment 3

A mixture of α-FeOOH with a proportion of 13% by weight γ-FeOOH and a surface S _N2 of 77.3 m ² / g, produced according to DE-OS 15 92 398, was dispersed in water. Then 1.5% by weight of H ₃ PO ₄ and 4% by weight of olive oil were added to the suspension with further intensive stirring. After the addition is complete, the mixture is dispersed for a further 20 min, filtered and the filter cake is dried at 80 ° C. in a vacuum drying cabinet. This material was then reduced to metal in a stream of hydrogen at 350 ° C. for 8 hours. The measurement results are shown in Table 2.

Example 3

The procedure is as described in Comparative Experiment 3, but the material finished with phosphoric acid / olive oil is first reduced to FeO _{1.33 in a stream of} nitrogen at 470 ° C. in 30 minutes and then to metal as indicated. The measurement results are shown in Table 2.

Example 4

5 kg of y-FeOOH (S _N2 = 31 m ² / g) were suspended in 40 l of water in a 60 l can. 100 g H ₃ P0 ₄ and 150 g of olive oil were stirred in 4 l of water and added to the suspension with vigorous stirring. The suspension was then pumped through an intensive mill at a throughput of 80 kg / h. The suspension thus obtained was filtered off and dried at 130 ° C (γ-FeOOH with 1.8% P0 ₄ and 1.3% C). The FeOOH thus equipped was then reduced to FeO _1.35 in a nitrogen stream at 475 ° C. and then reduced to metal in a fluidized bed using hydrogen at 340 ° C. The specific surface area of the metal particles is 26.6 m ² / g. The magnetic values of the sample passivated with acetone / air are at 160 kA / m: H _c = 69.2; M _r = 62; M _m = 112 and in the pulse magnetometer: H _{c =} 77.0; M _{r =} 79.

Example 5

40 kg of α-FeOOH (S _N2 = 50 m ² / g) were mixed in a 1 m ³ kettle with 700 l of water and stirred intensively for 3 hours. A mixture of 50 liters of water, 612 g of 85% phosphoric acid and 1.2 kg of olive oil was slowly added. The mixture was then stirred for a further 5 hours and then filtered off and air-dried at 120 ° C. 4 kg of α-FeOOH treated in this way was then reduced in an N ₂ stream in a discontinuous rotary kiln at 475 ° C to FeO _1.33 (0.36% P0 ₄ , 0.86% C, S _N2 = 38.7 m ² /G). This FeO _1.33 was then reduced to metal in a stirred fixed bed with 8.25 Nm ³ / h H ₂ at 340 ° C. and then stabilized at 40 ° C. with an N ₂ / air mixture. The measurement results are shown in Table 3.

Example 6

The procedure was as described in Example 5, but instead of adding phosphoric acid / olive oil, a mixture of 761 g of SnCl _{2 was used} . 2 H ₂ 0 and 1.2 kg of olive oil are added to the suspension and air is passed through after the addition for 2 hours. The first stage of the reduction led to FeO _1.34 , (1.2% Sn, 0.13% C) and the reduction to the metal was carried out at 310 ° C in a fluidized bed furnace. The measurement results on the sample stabilized with a nitrogen / air mixture at 40 ° C. are given in Table 3.

Example 7

250 g of γ-FeOOH (S _N2 = 50.2 m ² / g) were dispersed in 5 l of water for 10 minutes, then the solution of 23 g of 15% water glass in 500 ml of water was added and the mixture was dispersed for a further 30 minutes. The solid portion was filtered off and dried for 40 hours at 80 ° C / 25 Torr. 35 g of which were mixed with 1.1 g of polyethylene (molecular weight 250,000) and heated to 550 ° C. in a 250 ml rotary flask in 37 minutes. After cooling to 370 ° C., the sample was reduced in a stream of hydrogen for 32 hours and, after cooling to room temperature, superficially oxidized in a stream of 99% N ₂ and 1% O _{2 for} 8 hours.

The powder obtained showed an H _c of 87.3 [kA / m] and an M _r / δ of 61 [nTm3 _/ g _{] in the} pulse magnetometer.

Example 8

In the same way as described in Example 7, 250 g of α-FeOOH (S _N2 = 51.5 m ² / g) were reacted. Values obtained: H _c = 93.9 [kA / m): M _r / δ = 80 [nTm ³ / g].

Example 9

3 kg of α-FeOOH (S _N2 = 52 m ² / g) were predispersed in 60 l of water in a container. After 15 minutes, 42 ml of an 85% H ₃ PO ₄ and 30 g oxalic acid (H ₂ C ₂ O ₄ .2 H ₂ O) - both dissolved in a total of 400 ml H ₂ O - were introduced within 5 minutes with further stirring. The mixture was then dispersed for a further 15 minutes, then filtered and the filter cake was dried at 130.degree. The finished a-FeOOH had the following properties (sample A): S _N2 = 51.7 m ² / g; PO4 ^3― = 1.1% by weight; C = 0.05% by weight.

80 g of sample A were dewatered in air at different temperatures (sample B). The conditions and results are summarized in the table.

40 g of the dewatered products B 1, B 2 and B 3 were mixed with 3% by weight of stearic acid, then kept in an oven at 1000 ° C. for 1 hour. The samples were then reduced to FeO _1.35 at 360 ° C. in a nitrogen stream of 10 NI / h within 30 minutes and then, without isolating the FeO _1.35 , directly reduced to iron at 360 ° C. The results are shown in Table 4.

Example 10

In the 1.8 l grinding pot of a laboratory agitator ball mill, 1,800 g of steel balls with a diameter of 4 mm, 100 parts of the metal particles according to Example 4, 3 parts of lecithin, 9 parts of a silicate-based filler, 110 parts of a solvent mixture of equal parts of THF and Dioxane and 127 parts of a 13.5% lacquer, produced by dissolving 13.75 parts of an elastomeric polyester urethane from adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane and 3.4 parts of a polyphenoxy resin with a molecular weight of 30,000 in 109.85 parts of a mixture of equal parts of tetrahydrofuran and dioxane, and then finely ground at 1500 rpm for 14 hours. After the dispersion had ended, 6.3 parts of a 75% strength solution of a triisocyanate, prepared from 3 mols of tolylene diisocyanate and 1 mol of 1,1,1-trimethylolpropane in ethyl acetate, were added and the mixture was stirred for a further 15 minutes. After the dispersion had been filtered, it was applied in layers to a 12 μm thick polyethylene terephthalate film with simultaneous alignment of the magnetic particles by means of a permanent magnet. After drying, the magnetic layer was compacted and smoothed by the action of warm steel rollers. The resulting magnetic layer has a layer thickness of 4 µm. The magnetic foils produced in this way were cut into 3.81 mm wide magnetic tapes and then tested. The measurement of the magnetic properties was carried out in a measuring field of 160 kA / m, the coercive field strength H _e in [kA / m], remanent magnetization M, and saturation magnetization M _m in [mT] as well as the guideline factor Rf, the remanence longitudinal / transverse, specified. In the recording properties, the noise-to-noise ratio RG _A against the reference band IEC IV and the copy loss K _{o were} determined. The results are shown in Table 5.

Example 11

The procedure was as described in Example 10, but the metal particles obtained in Example 5 were used. The results are shown in Table 5.

Example 12

The procedure was as described in Example 10, but the metal particles obtained in Example 6 were used. The results are shown in Table 5.

Example 13

The procedure was as described in Example 10, but the metal particles obtained according to Example 7 'were used. The results are shown in Table 5.
(See table 5 page 7 f.)

Claims (1)

1. A process for the preparation of acicular ferromagnetic metal particles consisting essentially of iron by reducing acicular iron(III) oxide hydroxide provided with a shape-stabilizing surface coating, or the iron(III) oxide obtained therefrom by dehydration, a decomposable organic compound having been additionally applied to said hydroxide or oxide, wherein, in a first stage, the so-pretreated iron(III) oxide hydroxide or iron(III) oxide is reduced in an inert gas atmosphere at from 270 to 650 °C to FeO_x, where x is from 1.33 to 1.44, and, in a second stage, this product is reduced with hydrogen at from 270 to 450 °C to the metal.