DE2735315A1 - Acicular gamma ferric oxide for magnetic recording tape - made from lepidocrocite contg. copper and/or lead ions - Google Patents

Abstract

A ferrous salt soln. is treated with 0.05-1% (w.r.t. the wt. of Fe2+ ions) of cuprous or cupric ions and/or 0.05-1% Pb4+ ions; NH4OH or alkali(ne earth) hydroxides; oxidising gases; more Fe2+ cpds.; and more alkali to obtain a pH of 4.8-6, so that lepidocrocite is pptd. The lepidocrocite is reduced to magnetite and then oxidised to acicular gamma-Fe2O3 (I). The lepidocrocite is pref. dewatered at 300-620 degrees C prior to redn. The process is used for mfr. of (I) with high coercive force, good crystallinity, and good mechanical and magnetic stability for magnetic recording tapes.

Description

Process for the production of acicular gamma iron (III) oxide

The invention relates to a method for producing needle-shaped ferrimagnetic gamma iron (III) oxide by reducing needle-shaped lepidocrocite to magnetite and subsequent oxidation.

Acicular ferrimagnetic gamma iron (III) oxide has been used for a long time widely used as a magnetizable material in the manufacture of magnetic Recording media used.

A large number of methods have already become known for production. GB-PS 675 260 already describes a process for the production of Gwnma iron (ITI) oxide, at which needle-shaped # -iron oxide hydroxide (goethite) dehydrates to # -iron (III) oxide, converted to magnetite in a reducing atmosphere at more than 3000c and in air oxidized to acicular Gamna iron (III) at temperatures below 450 ° C In the same way, the gamma iron (II1) oxide can be obtained from @om @ iron oxide hydroxide (Lepidocrocite) obtained (DT-PS 1 061 760). The lepidocrocite that can be used for this is, for example, by precipitating iron (I1) hydroxide from an iron (II) chloride solution with ammonia at pil = 7 and a temperature of 20 to 50 ° C and then through Oxidation of the precipitated hydroxide with air while maintaining the pH (Schwertr, lann, Zeitschrift f. Anorg. Chemie 298 (1959), 337-3 # 8). From the DT-PS 1 223 352 a further process for the production of the lepidocrocite is known, in which the nucleation by precipitation of iron (II) salt solutions with alkali or Alkaline earth bases and oxidation of iron (II) hydroxide or carbonate with oxygen, Air, organic nitro compounds or other oxidizing agents takes place and optionally a nucleation of # -FeOOH in iron (II) salt solutions either in the presence of metallic iron or in iron (II) salt solutions with simultaneous addition of equivalent Amounts of iron (II) ions and alkali or alkaline earth solutions or suspensions or with simultaneous addition of equivalent amounts of iron (III) ions and alkali or Alkaline earth bases, solutions or suspensions in both cases using the Oxidizing agent is carried out and wherein before, during or after the nucleation or small amounts of arsenate ions or phosphate ions during the germ growth process, as well as small amounts of trivalent ions of the elements B, Al, Ga, Cr, Mn and / or Fe are added to the reaction medium.

Similarly, the lepidocrocite can also be obtained, though in a first stage a slurry of colloidal lepidocrocite germs is produced is by combining ferrous chloride with aqueous alkali, the concentration of ferrous chloride is about 29.95 to 59.9 g per liter, and the resulting mixture stir vigorously while introducing an oxygen containing gas until pH of the mixture is between 2.9 and 4.1 and then in a second stage the slurry with vigorous stirring at a temperature of 26.7 to 60 ° C. and a pli value maintains between 2.9 and 4.1 in the presence of an excess of ferrous chloride, while simultaneously and continuously alkali and an oxygen-containing gas is introduced, up to 1.2 to 5 parts by weight of the total product per part by weight of none have been formed.

In the course of efforts to improve the crystalline and magnetic Properties of gamma iron (III) oxide was not only the starting material in Investigated form of iron (III) oxide hydroxide, but also the subsequent one Conversion process varied several times in its individual stages.

Already with the DT-PS 801 351 it was disclosed that by treatment of non-magnetic iron oxides with the salts of short-chain carboxylic acids, followed by By heating, suitable magnetic iron oxides are available in the form of magnetite.

The magnetite obtained in this way is then oxidized at 200 to 400 ° C convertible into Gwina iron (III) oxide. The suitability of all organic compounds, which decompose at temperatures below 540 C with little tar and ash formation are, for the reduction of the non-magnetic iron oxides to magnetite is in the US-PS 2 900 236. For this purpose the iron oxide with the gaseous, solid or liquid organic substance brought into contact and brought to a temperature of 540 heated to 65000. While US Pat. No. 2,900,236 all corresponding organic substances with particular mention of wax, starch and oil as usable for this purpose, In DT-AS 12 03 656, salts of soluble soaps precipitated on the iron oxide, in DT-0S 20 64 804 both higher hydrocarbons, higher alcohols and amines, higher fatty acids and their salts, as well as oils, fats and waxes, in DL-PS 91 017 also long-chain carboxylic acids or their salts, in DT-AS 17 71 327 aliphatic monocarboxylic acids having 8 to 24 carbon atoms, optionally in a mixture with morpholine and in JA-OS 80 499/1975 organic compounds such as ethanol, containing inert gases as reducing agents for the extraction of magnetite from non-magnetic Iron oxides listed. In some of the previously mentioned processes, under Exclusion of air heated, whereby the reaction stops at the level of magnetite, or in the presence of air, which immediately turns the magnetite into gamma iron (III) oxide is oxidized.

The stated multiple efforts to improve for the Production of magnetic iron oxides suitable for magnetic recording media make the endeavor evident in this way both the rising technical To meet demands on the information carrier, as well as the disadvantages of others compensate for magnetic materials that can also be used.

The invention was based on the object, acicular gamma iron (III) oxide provide that is characterized by high values the coercive field strength, characterized by good crystallinity as well as mechanical and magnetic stability.

It has now been found that acicular gamma iron (III) oxide by reducing one in the implementation of aqueous iron (II) salt solutions with a Base from the group of ammonia, alkali or alkaline earth metal hydroxides and initiation oxidizing gases with the addition of further iron (II) compounds and bases a pH of 4.8 to 6.0 obtained lepidocrocite to magnetite and then Can produce oxidation with the desired properties if the iron (II) salt solutions in the production of the lepidocrocite mono- or divalent copper ions and / or bivalent or tetravalent lead ions in an amount of 0.05 to 1.0 percent by weight each, based on the amount of iron (II) ions are added.

When producing the lepidocrocite according to the invention, it is more advantageous proceeded so that aqueous iron (II) chloride solutions with a content of 40 to 60, in particular 30 to 35 g of iron (II) ions per liter with 0.05 to 1.0% by weight of copper (I) - or copper (II) ions and / or 0.05 to 1.0 percent by weight of lead (II) or lead (IV) ions be moved. This solution then becomes 35 to 70, in particular 38 to 45 fi the iron (II) ions by means of ammonia, alkali or alkaline earth bases under strong Stirring at temperatures from 15 to 30, in particular from 20 to 26 ° C as iron (II) hydroxide precipitated and then with the introduction of oxidizing gases, in particular air, for as long oxidized to iron (III) oxide hydroxide until the pH reaches a value of 4.5 to 5.0. After this first reaction stage, in a second stage to 38 to 45, in particular 40 to 1120c heated and by adding further amounts of alkali at the complete On oxidation, a pH of 4.8 to 6.0, in particular 5.2 to 5.8, is maintained. With the help of the amount of oxidizing gas passed through, as well as the agitation intensity the oxidation time in the second stage of the process is advantageously increased limited to a period of about one to a maximum of three hours.

The lepidocrocite produced according to the invention is characterized by a particularly uniform needle shape, the needles having a length of about 0.6 to 0.9 μm and grow almost without any dendrite formation. Opposite to the production of undoped lepidocrocite is that of the process according to the invention increased rate of oxidation and the absence of what otherwise occurs simultaneously highlight the cube-shaped magnetite.

The lepidocrocite produced according to the invention is known per se In a reducing atmosphere, e.g. by means of hydrogen or carbon monoxide, reduced to magnetite at 280 to 11200C and then at 250 to 3500C by means of Air or an air-nitrogen mixture is oxidized to gamma iron (III) oxide.

Also the transformation of the Lepidokrokits with is largely ash-and tar-free decomposable organic substances at temperatures between 300 and 6500C possible. For this purpose, the Lepidokrokit is made with the solid or liquid organic substance mechanically mixed or in a suitable solution or suspension the substance covered with it and then under inert gas to temperatures of Heated to 300 to 6500C. In the same way, the process is by means of gaseous organic Substances that are metered into the inert gas can be carried out. As usable organic Substances can be all compounds indicated as suitable according to the prior art use if they can be decomposed at temperatures between 300 and 6500c. The use of higher fatty acids or theirs has proven to be particularly expedient Salts, as well as lower polyhydric alcohols or their carboxylic acid esters, as well as highlighted saturated or unsaturated aldehydes. The organic Substances in an amount of 0.8 to 3.5 percent by weight, based on the iron oxide, used. Depending on the organic substance used and selected accordingly The reaction temperature is the conversion to acicular magnetite after about 1 to Finished 120 minutes.

The acicular magnetite obtained after this reduction reaction is expediently by passing air over it at temperatures of 200 to 4000c oxidized to gamma iron (III) oxide.

However, if the specified conversion reaction is not carried out under inert gas, but already carried out in the presence of air and temperatures below 11000C, this is how gamma iron (III) oxide is formed immediately.

One obtained from the lepidocrocite produced according to the invention Qamma iron (III) oxide can also be improved in its magnetic properties, if the Lepidokrokit before the specified further processing at temperatures between 300 and 6200C is drained.

The gamma iron (III) oxide materials> which according to the invention Processes available differ significantly from the known gamma iron (III) oxides due to their improved crystallinity and the higher coercive field strength, as well as by the narrow distribution of the grain sizes with an L / D ratio between about 8: 1 to 12: 1. Here, the pre-drainage takes place at 300 to 6200C, in particular at 450 to 5400C before the reworking leads to a further increase in the coercive field strength by 4 to 7 kA / m with also improved remanence and saturation values of 4 up to 6 nTm3 / g (measured at 100 kA / m). This pre-drainage lifts the mechanical Stability in such a way that almost no drop in the coercive field strength during the Dispersion of the crystallite needles occurs. When used to manufacture Magnetic recording media produced particularly advantageous products, which are particularly characterized by their high packing density of the magnetizable material in the layer, through an increased directivity factor, as well as through improved electroacoustic Characteristics distinguish.

Gamma iron (III) oxide is used to produce magnetic layers dispersed in polymeric binders. Suitable binders for this purpose are known compounds, such as homopolymers and copolymers of polyvinyl derivatives, polyurethanes, Polyesters and the like. The binders are in solutions in suitable organic Solvents used, which can optionally contain further additives. the Magnetic layers are on rigid or flexible substrates such as plates, foils and cards applied.

The subject matter of the invention is illustrated by the following examples.

The powder magnetic values are determined by measuring one on a plug weight oxide sample of D = 1.2 g / cm3 with a conventional oscillating magnetometer determined at 100 kA / m measuring field strength. The coercive field strength (Hc) is in [kA / m], the specific remanence (Mr / 8) and the specific magnetization (Mm /) become given in [nT # cm3 / g].

Example 1 In one with a plate stirrer and a nozzle ring for In a container provided with an air line, 1450 g FeCl2r4 H20 are dissolved in 9 l of water. 2.2 g of CuC12r2 H20 are then added to this solution. At one temperature from 20 to 230C are added to the iron chloride solution while stirring and passing through 290 g of sodium hydroxide in 4.5 1 of water were added dropwise of nitrogen within 10 minutes. Then air is introduced, whereby the iron (II) ions are oxidized. The pH falls from originally 7.0 to 5.6 within 20 minutes. To a reaction time of a further 40 minutes, the pH drops to 2.8. Now will the suspension is heated to 110.degree. C. while stirring and passing air through 290 g of NaOH in 8 l of water were added over the course of 50 minutes, the rate of addition the sodium hydroxide solution is regulated so that the pH value to about 5.5 increases and remains within the range of 5.2 to 5.8. By another Addition of sodium hydroxide solution, the pH is then adjusted to 6.0 to 6.5 and still Stirred for 30 minutes. After filtering off the resulting lepidocrocite, it becomes Washed free of chlorine and dried at 1050C.

The washed and dried lepidocrocite is divided into 2 parts. The first half is applied directly, the second half after 30 minutes of drainage 11300C mixed with 2% stearic acid. Both samples are separated into one each rotating quartz glass ball charged with N shielding gas quickly to 5400C brought and reduced to Fe3O11 within 25 minutes. After cooling to 3200 the nitrogen atmosphere is initially 10% air for 30 minutes, then 50% Air added for a further 70 minutes. The resulting gamma ferric oxide samples are measured magnetically. The results are shown in Table 1.

Example 2 In one with a plate stirrer and nozzle ring for introducing air provided container are 50 1 techn. FeCl2 solution (31.4% FeCl2 content), 178 1 Drinking water, 68.6 g CuC12 ~ 2 H20, 73.8 g CoCl2 6 6 H20, 103.0 g gelatin, dissolved in 2 1 H20. Thereafter, 22.8 1 techn. Sodium hydroxide solution (25%), diluted with 97.2 l of drinking water, stirred in within 15 minutes. At one temperature between 19 and 220C, 2 m3 of air are generated at a stirring speed of 100 rpm sprayed per hour. The oxidation of the precipitated Fe (OH) 2 to 7-FeOOH is 2 hours. The pH value initially drops from 6.8 to 5.8, leaving one and a half Hours in the range from 5.8 to 5.5, then rapidly decreasing to 4.5 to 11> 34.3. The mixture is then heated to 40 ° to 420 ° C., with stirring and the introduction of air, and then again 22.8 1 25% NaOH, mixed with 228 1 drinking water added in such a way that the pH is im Range from 5.2 to 5.8 remains. The amount of air here is 8m3 per hour increased. After about 2 hours, 40 minutes the pH is 6.2, followed by another 25 minutes is subsequently stirred.

After filtering off, the lepidocrocite obtained becomes chloride-free washed and dried at 105 to 12,000.

A 4 kg sample of the dried and ground filter cake is made dehydrated in a rotary kiln at 55,000 within 20 minutes. A second one too Comparative sample weighing 4 kg remains undewatered.

Both samples are separated with 1.5% stearic acid and 1% stearic acid amide equipped and each in the meanwhile pressurized with nitrogen as a protective gas Rotary tube filled at 55000. The reduction is complete after 15 minutes. The lower part of the rotary tube near the outlet, which was only heated to 28,000, served for oxidation; by adding 20% air to the nitrogen. After 40 minutes the oxidation is finished. The magnetic powder values are shown in Table 1.

Example 3 As described in Example 2, 50 1 techn. FeCl2 solution with a content of 29.1% Fell2, 180 liters of drinking water and 22.8 g of PbCl2 in a stirred tank given. Then in 2 stages with the addition of the dilute technical. Caustic soda initially 3 hours, 45 minutes at 230C with 2 m3 of air per hour at a stirring speed of 100 rpm, then in the second stage at 400C with 8 m3 of air per hour and unchanged Stirring speed oxidized for 3 hours. The second stage is the pH held at 5.3 to 5.6.

There are two samples of 1 kg each of the dried and finely ground Product manufactured. The first sample is used unchanged, the second sample beforehand in a continuous furnace on an endless belt at 52000 within 20 minutes drains. Both samples are then separated with 1.5% glycerine and 1.5% Mixed stearic acid. One after the other, the samples are placed on the endless belt in a Layer thickness of approx. 4 to 5 cm applied and through the electrically heated conveyor oven led without protective gas. The oven is only open in the first half of the inlet 52000 heated. The second half of the furnace section is used for cooling by from Cold air is introduced at the end of the furnace. Reduction and oxidation thus take place one after the other in the conveyor oven. The oxidation finds its conclusion only after The approx. 3000 hot powder runs out of the kiln section before being ejected from the belt into a collecting container. The time from entry into the furnace to discharge is 22 minutes.

The magnetic powder values are shown in Table 1.

Table 1 Example No. Hc Mr / # Mm / # 1 25.4 39 74 not dehydrated 1 28.3 44 79 drains 2 ~ 23.1 42 81 not dehydrated 2 26.0 44 82 drains 3 25.8 38 74 not dehydrated 3 27.1 41 79 drains Example 4 Starting from chem. pure FeCl2 ~ 4 H20 while observing the working conditions as described in example 1, the influence of the foreign metal doping indicated in table 2 on the surface of the lepidocrocite is investigated as a measure of the quality of the crystallization. The undoped lepidocrocite with the measure of 100 was used for the surface.

The smaller the measure of the spiked samples, the higher the quality of the gamma iron (III) oxides produced from it.

The formation of needles, their size distribution and the absence of dendrites is also visually classified into grades based on electron microscope images from 1 to 6, with quality class 1 for pronounced dendrite-free needles of uniform size and after the conversion to gamma iron (III) oxide very supplies well-suited products and grade 6 crystallized completely inadequately Products labeled with correspondingly unusable end products. The following Table 2 shows the test results.

Table 2 Experiment no. Doping in% surface needle formation; moved on to the grade Fe (II) content 1 ~ 100 4 2 V 0.4 105 6 Al 0.4 128 5 4 Mn 0.2 67 6 5 Mn 0.4 / Al 0.2 92 4 6 Cu 0.2 51 1 7 Pb 0.2 59 2 8 Co 0.2 78 4 9 Zn 0.1 120 6 10 Sb 0.2 82 6 11 Cr 0.2 75 4 12 Ni 0.2 71 4 13 Sn 0.2 77 5 14 Cu O, 1 / Pb 0.1 55 1 15 Cu O.1 / Co 0.2 58 1 16 Cu O.1 / Ni 0.2 56 1 17 Pb 0.1 / Co 0.2 60 2 18 Pb O.1 / Ni 0.2 60 2 Example 5 In accordance with the procedure described in Example 1, tests are carried out with various Cu-Pb doping quantities and with other doping metals. The lepidocrocite samples are then dehydrated without water (-) or by heating to 450 to 5200C (+). The resulting samples are then reduced by admixing 3% stearic acid amide at 5200 ° C. in 20 minutes and, after cooling to 280 to 320 ° C., oxidized by passing a nitrogen / air mixture over them. Table 3 shows the respective dopings and any dehydration of the lepidocrocite before further processing, as well as the magnetic measured values of the gamma iron (III) oxide samples obtained therefrom.

Table 3 Try doping in%, drainage HMM number based on Fe (II) content #c 1,2 # c came 1 - - 22.3 34 74 2 0.3 Pb - 23.1 36 76 3a 0.2 Pb - 24.5 37 75 3b 0.2 Pb + 27.1 41 79 4a 0.2 Cu - 23.8 36 75 4b 0.2 Cu + 26.3 41 81 5 1 Pb - 23.9 35 73 6a 0.2 Pb / 0.5 Co - 25.8 35 73 6b 0.2 Pb / 0.5 Co + 27.9 39 74 7 1 Cu + 27.3 40 78 8a 0.1 Cu / 0.1 Co / 0.1 Ni - 25.1 36 74 8b 0.1 Cu / 0.1 Co / 0.1 Ni + 27.5 41 81 9a 0.1 Pb / 0.1 Co / 0.1 Mi i 24.9 39 77 9b 0.1 Pb / 0.1 Co / 0.1 Ni + 26.0 43 83 10 0.3 Cu / 0.1 Co / 0.1 Ni + 27.0 43 80 11 0.5 Cu / 0.3 Co + 26.1 44 84 12 0.3 Cu / 0.2 Co + 28.2 44 79 13 0.05 Cu / 0.05 Co - 23.6 36 71 0.05 Pb / 0.05 Ni 14 0.1 Cu / 0.2 Co 0.1 Pb / 0.1 Mn + 28.3 44 81 Example 6 The procedure described in Example 1 is followed. 0.3% Cu and 0.2% Co are used for doping. In addition, at the beginning of the oxidation reaction, 12.5 g of stearic acid and 6.0 g of lauryl alcohol, dissolved in water containing NH3, are added.

Because of the strong foaming. in the manner of a floating foam preparation the main part of the hydrophobized lepidocrocite discharged. The needles of the resulting Lepidocrocits are particularly well formed and of very uniform length, i. approx. 80% of the product have a length between 0.7 and 0.8 μm.

The conversion of this lepidocrocite takes place as described in Example 1. The magnetic values of the resulting gamma ferric oxide are: Hc (1,2) = 25.9 kA / m Mr /! : 38 nTm3 / g: = 73 nTm3 / g Example 7 With the according to Example 5, experiment 1 to 14, gamma iron (III) oxide samples prepared are magnetic tapes in the following manner manufactured.

900 parts of the respective corresponding gamma iron (III) oxide with 300 parts of a 20% solution of a vinyl chloride copolymer, consisting from 80% vinyl chloride, 10% dimethyl maleate and 10% diethyl maleate, in equal measure Parts of tetrahydrofuran and dioxane, 22.5 parts of soy lecithin and a further 600 parts consisting of equal parts of tetrahydrofuran and dioxane Solvent in a 6,000 volume agitator ball mill together with 8,000 parts Introduced 6 mm steel balls and dispersed for about two hours. After that, the Approach 140 parts of a thermoplastic polyurethane from adipic acid, 1,4-butanediol and 4,4'-diisocyanate-diphenylmethane as a 20% solution in equal parts of tetrahydrofuran and dioxane, further 970 parts of said mixed solvent and 0.9 part a commercial silicone oil added. After an additional three hours Dispersing the dispersion is filtered through a cellulose / asbestos fiber layer and using a conventional coating machine on a 12 # µm thick polyethylene terephthalate film applied. After passing through a magnetic directional field, at 80 to 1000C dried for 2 to 5 minutes. The magnetic layer is heated by pulling over it and polished rollers smoothed and compacted at temperatures of 60 to 800. the finished magnetic layer is about 5 µm thick. The coated film is made into tapes 3.81 mm wide.

The magnetic properties of the ribbons are measured in a vibrating magnetometer measured at a field of 100 kA / m, namely the coercive field strength Hc in kA / m, as well as the remanence Mr and the saturation Mnl in mT. The preferred magnetic direction The RF of the particles is determined from the remanence along the remanence across the magnetic Preferred direction.

The electroacoustic properties of the tape samples are against the Standard reference tape T 308 S measured. The sensitivity given is 333 Hz (ET), as well as at 10 kHz (EH) and the tape flow for a recording at 333 Hz with a cubic distortion of 5% (AT) and the maximum tape flow with a Recording at 10 kHz (AH).

The results of the individual experiments in connection with the Gamma iron <III) oxide samples from Example 5 are listed in Table 4.

Table 4 Verse no. Ex. 5 Hc Mr Mm RF thickness ET EH AT AH Verse no. /around 1 1 21.2 150 185 2.2 4.6 + o ,! -6.2 -0.6 -6.4 2 2 23.6 124 148 | 2.8 5.0 -1, o -0.6 -1.7 -0.3 3 3a 24.0 144 168 2.6 4.4 -0.4 -0.7 -1.4 -0.1 4 3b 25.3 147 168 3.1 4s5 -0.7 +0.1 -0.9 +0.5 5 4a 23.5 145 170 2.9 4.2 -0.6 -1.0 -1.3 -0.7 6 4b 26.8 145 168 3.0 4.4 -0.2 +0.7 -0.2 +0.9 7 5 22.1 138 165 2.6 4.7 -0.1 -0.7 -0.5 -1.0 8 6a 24.6 146 172 2.8 4.3 -0.7 -0.4 -1.3 -0.1 9 6b 28.2 153 173 3.0 4.3 -0.6 +1.0 -0.6 +2.0 10 7 25.6 138 168 2.6 4.7 -1.1 -1.3 -2.4 +0.1 11 8a 24.8 140 169 2.5 4.8 -0.6 -0.4 -1.4 +0.1 12 8b 25.0 146 179 2.0 4.6 -0.5 -1.3 -1.2 -0.3 13 9a 24.7 139 171 2.1 4.8 -0.5 -1.6 -1.0 -0.6 14 9b 24.5 137 167 2.0 5.1 -0.5 -0.5 -1.4 +0.2 15 10 27.8 149 169 3.1 4.2 -1.1 +0.2 -1.9 +1.1 16 11 25.9 150 175 2.6 4.3 -0.2 +0.6 -0.8 +1.2 17 12 25.8 143 169 2.7 3.9 -o, l -0.2 -1.4 +0.3 18 13 22.1 139 170 2.4 4.5 -0.6 -1.6 -1.2 -0.9 19 14 25.3 145 174 2.6 4.2 -0.6 ~ r, -1.7 -0.1

Claims (2)

Godfather: in particular 1. Process for the production of acicular Gamzna iron (III) oxide by reducing one in the implementation of aqueous iron (TT) salt solutions with a Base from the group of ammonia, alkali or alkaline earth metal hydroxides and initiation oxidizing gases with the addition of further iron (II) compounds and bases a pIl of 4.8 to 6.0 obtained lepidocrocite to the magnetite and then Oxidation, characterized in that the iron (II) salt solution, s in the production of the lepidocrocite mono- or bivalent copper ions and / or bivalent or tetravalent Lead ions in an amount of 0.05 to 1.0 percent by weight, based on the amount of iron (II) ions, are added. 2. The method according to claim 1, characterized in that the Lep.idokrokit dehydrated before reduction to magnetite at temperatures between 300 and 62000 will.