DE10044095A1 - Production of iron oxide seeds containing aluminum, used directly for red pigment production, involves adding aluminum component to ferrous chloride solution, heating, adding sub-stoichiometric amount of precipitant and oxidation - Google Patents

Abstract

Production of iron (Fe) oxide seeds containing aluminum (Al), with alpha -FeOOH crystal structure and aspect ratio 2100-3100, comprises stirring 6-20 mole% Al component (based on Fe) into Fe-II chloride solution containing 20-100 g/l Fe (0.1-10 mole% Fe-III); heating to 30-60 deg C; adding 20-80% of the stoichiometric amount of precipitant (2-10 equivalent active/l); oxidation at 2-50 mole-%/hour and direct conversion to Fe oxide red pigment. Production of iron (Fe) oxide seeds containing aluminum (Al), with alpha -FeOOH crystal structure and an aspect ratio of 2100-3100, using Fe-II chloride (FeCl2) comprises: (a) adding 6-20 mole% Al component with respect to total Fe content to a stirred FeCl2 solution containing 20-100, preferably 40-65 g/l Fe, with 0.1-10 mole% Fe-III; (b) heating this mixture to a precipitation temperature of 30-60, preferably 35-50 deg C; (c) adding a precipitant (I) with an active content of 2-10, preferably 4-8 equivalent/l, so that the molar ratio of (Fe+Al)/(I) is 20-80, preferably 30-60% of the stoichiometry; (d) oxidizing the precipitated suspension with an oxidant, so that the oxidation rate of the Fe to be oxidized is 2-50, preferably 10-35 mole%/hour; and (e) using the seeds for direct production of iron oxide red pigment via alpha -FeOOH as intermediate stage without further isolation, after testing the properties.

Description

The present invention relates to a method for producing an aluminum-containing iron oxide nucleus with an α-FeOOH crystal structure from FeCl ₂ . This seed is suitable as a starting material for the production of iron oxide red (α-Fe ₂ O ₃ ) via α-FeOOH as an intermediate.

Synthetic iron oxides are usually produced using the Laux process, the pen niman process, the precipitation process, the neutralization process or roasting process (Ullmann's Encyclopedia of Industrial Chemistry, 5th. Ed., 1992, Vol A20, p. 297 ff). The iron oxides thus obtained are generally called Pigments used.

Basically, two methods are known for producing fine-particle α-FeOOH (needle width between 5 and 30 nm):

• - The so-called acid process
• - The so-called alkaline process

The acidic process uses an iron (II) component, i.e. a solution in water Iron salt, and this is an alkaline with intensive mixing Component, usually an alkali or Er dissolved or suspended in water dalkali metal compound or ammonia solution, metered. The metered The amount of alkaline component is usually between 15% and 70% of the amount required stoichiometrically. The pH is after the addition of the alkaline Component in the weakly acidic range.

After the addition of the alkaline component is complete with an Oxidati onsmittel, usually atmospheric oxygen, oxidized. The reaction takes place at temperatures performed between 20 ° C and 50 ° C. At significantly higher temperatures the risk of undesirable magnetite formation. The end point of the reaction can  can be recognized by a sharp drop in pH and redox potential. After completion of the reaction, the properties of the product obtained (generally called germ) and this, if suitable, immediately to α-FeOOH pigment processed further.

The alkaline process differs from the acid process in the amount the added alkaline component. This is included in the alkaline process at least 120% of the stoichiometrically required amount, but usually clearly about that. The temperatures at which this reaction is carried out can be somewhat are higher than the temperatures used in the acid process because of the danger the magnetite formation is lower here.

In principle, the alkaline process gives relatively long-needle α-FeOOH Crystallites with a length to width ratio of 10: 1 to 30: 1. Because these crystals lite are also very low in dendrites, this procedure is especially for those Production of α-FeOOH suitable as a starting product for magnetic tapes.

For the production of α-FeOOH pigments for use in paints and varnishes the germs produced according to the alkaline process are not directly or only in limited use, because with this method all exist in the Fe component where color-giving metals are installed. These metals (especially Mn, Cr, Cu, Ni) significantly deteriorate the color properties and thus limit the ver using germs produced in this way as color pigments.

For the production of iron oxide yellow pigments, an α-FeOOH seed is preferably used and this is then coarsened (built up) in acid, which reduces the incorporation of the coloring metals. If you want to produce a particularly pure iron oxide red (α-Fe ₂ O ₃ ) from α-FeOOH by calcination, then the starting material α-FeOOH may only contain a small proportion of coloring metals. Furthermore, the structure may only be used at pH values of less than approx. 4, since the coloring metals are increasingly incorporated at higher pH values. Furthermore, the particle shape of the α-FeOOH has a significant influence on the color properties, the viscosity of the paint and the need for binding agents.

For a desired low viscosity in the paint and a low binder Short-needle α-FeOOH particles are required. these can can be produced by intensive grinding from long-needle α-FeOOH particles. A cheaper alternative is the direct production of short-needle α-FeOOH Particles.

In order to control the particle shape of the α-FeOOH nucleus and thus that of the pigment built up from it in the direction of low length to width ratio, modifying additives are required. The use of B, Al, Ga, Si, Ge, Sn or Pb as seed modifiers is known from US-A4 620 879. This patent describes an iron oxide yellow with a particularly low silking index, which is achieved by a corresponding procedure in pigment build-up and by adding the above-mentioned modifiers. However, it is not specified in this document how to provide an α-FeOOH seed for the production of a particularly good (colorless) α-Fe ₂ O ₃ pigment.

The object of the present invention was to provide a method for the simple and inexpensive production of a short-needle α-FeOOH seed by the precipitation method. In a further step, this α-FeOOH seed is built up to an α-FeOOH pigment. From this subsequently isolated FeOOH pigment, an α-Fe ₂ O ₃ red pigment is ultimately produced by annealing.

This object is achieved by the process according to the invention: If iron (II) chloride is used instead of iron (II) sulfate and 3 to 16 mol% of aluminum, based on Fe, is added, then very finely divided α-FeOOH is obtained with a Aspect ratio from 2100 to 3100. In the present context, the aspect ratio is the product of BET surface area and average crystallite size, which was determined by X-ray analysis from the 110 reflex of α-FeOOH. These very fine-particle α-FeOOH seeds can be produced using the following procedure:

• 1. To an iron (II) chloride solution with a total Fe content of 20-100 g / l, preferably 40-65 g / l and an Fe-III content of 0.1-10 mol% Fe-III (be referred to total Fe), an Al component in amounts of 3- 16 mol% based Total Fe given with stirring,
• 2. This mixture is to a fall temperature between 30 ° C and 60 ° C, be preferably heated to 35-50 degrees,
• 3. a precipitant with an active ingredient content of 2-10 is added to the mixture Equivalents per liter, preferably 4-8 equivalents per liter, and the molar ratio Fe + Al to precipitant is 20% -80% before increases 30% -60% of the stoichiometry,
• 4. the precipitated suspension is then mixed with an oxidizing agent oxidized at such a rate that the oxidation rate 2-50 mol% / h, is preferably 5-20 mol% / h of the iron to be oxidized and
• 5. The Al-containing α-FeOOH seed obtained after the oxidation is given if isolated.

The Al-containing α-FeOOH nucleus obtained after the oxidation can optionally be without further insulation, after testing the properties, for the production of iron oxi red pigments over α-FeOOH can be used as an intermediate.  

The preferred procedure is as follows:

starting chemicals

FeCl ₂

Solution with an Fe content of 55 g / l Fe, of which 1.5 mol% Fe-III
AlCl ₃

-Solution,
NaOH solution with an NaOH content of 300 g / l = 7.5 equivalents of NaOH / 1
Al / Fe ratio: 12-13
Fe + Al / precipitant ratio: 35-40%

reaction conditions

Temperature: 34 ° C
Oxidation rate: 30-35 mol% Fe-II / h

AlCl ₃ (as an aqueous solution) is preferably used as the Al component. The use of Si or Ti as a seed modifier, in the form of their chlorides, is also possible if possible, but requires a higher technical outlay in production.

As precipitants, NaOH, KOH, Na ₂ CO ₃ , K ₂ CO ₃ , Mg (OH) ₂ , MgO, MgCO ₃ , Ca (OH) ₂ , CaO, CaCO ₃ , NH ₃ or secondary or tertiary aliphatic amines in aqueous Solution or aqueous slurry can be used.

Atmospheric oxygen, oxygen, ozone, H ₂ O ₂ , chlorine, nitrates of the alkali or alkaline earth metals or NH ₄ NO ₃ are used as the oxidizing agent.

Contains the iron (II) chloride solution used in larger quantities at pH values of less than 4 precipitable coloring metals, these can be added by adding one alkaline component to the iron-II-chloride solution can be precipitated up to pH 4. The Solid can be formed by sedimentation, filtration or centrifugation  the clear, supernatant solution above. In addition to the un Desired coloring metals are also removed Fe-III, which is the reaction to the α-FeOOH germ significantly undesirably influenced (formation of black Magnetite).

The reaction is carried out in batch or continuous stirred tanks, in Stirred tank cascades, loop reactors or stirrerless reactors with two substances nozzles carried out as mixing organs.

After the α-FeOOH nuclei according to the invention have been prepared, they are converted into a pig ment converted what ge by known coarsening of the germ particles looks (pigment build-up). However, since the α-FeOOH nuclei according to the invention are not To be used as such, it is necessary to build up the pigment and the glow to describe an iron oxide red pigment.

The Al-containing germ produced by the process according to the invention is pumped to a solution of FeCl ₂ or FeSO ₄ or another Fe-II salt. 7-15 mol of Fe-II salt are added to one mol of FeOOH in the nucleus as a solution with an Fe content of 30-100 g / l Fe. This suspension is now heated to the reaction temperature, which is between 50 ° C and 90 ° C. After the fall temperature has been reached, oxidation and precipitation are started at the same time. As a rule, atmospheric oxygen is added via a suitable gassing device and the pH of the suspension is regulated with an alkaline precipitant. The pH value is regulated in a range from 2.4 to 4.8. The rate of oxidation should be between 0.5 and 8 mol% Fe-III / h.

After the reaction has ended (ie when all Fe-II is oxidized), the solid formed is separated off by filtration. It is washed salt-free and can then be dried. Since, according to the invention, an α-Fe ₂ O ₃ red pigment is to be produced from this α-FeOOH, it makes sense to feed the washed solid directly to a suitable annealing unit.

The α-FeOOH pigment is expediently annealed in a continuously operating apparatus. Rotary tube furnaces, continuous shaft furnaces, fluidized bed reactors, falling shaft furnaces and push-through furnaces are suitable for this. The required annealing temperatures (measured in the product) are only between 550 ° C and 800 ° C. The required average residence times are between 10 and 80 minutes. Preferably, the resulting α-Fe ₂ O ₃ pigment is then subjected to screening grinding in order to remove oversize particles and agglomerates.

The red pigment obtained in this way is characterized by high color purity, almost isometric particle shape, low oil number and high chemical purity. The sum of its properties makes it particularly suitable for:

• - Application in the field of paints and varnishes
• - Applications as raw material for catalysts
• - Food coloring applications
• - Applications in the area of paper coloring
• - Applications in the field of polymer coloring
• - Applications as a UV stabilizer
• - Applications in the field of high-quality building materials (plasters, etc.)
• - Applications in the field of emulsion paints

Due to the relatively low annealing temperatures, the inexpensive raw materials and the High production rate in the production of yellow pigments is a particular feature of this process economically. Because of the special reaction and the application of a precisely specified germ, it is possible to use particularly high quality red pigments te, the application technology compared to pigments produced by other processes niche advantages to manufacture safely. Environmentally relevant chemicals not used in the production of the red pigments according to the invention.  

In the preferred embodiment (use of FeCl ₂ , AlCl ₃ , NaOH and air as starting materials), an almost closed material cycle is possible by electrolysis of the NaCl obtained as a by-product. The sodium hydroxide solution obtained in this way can be used directly in the process again. The products H ₂ and Cl ₂ formed during chlor-alkali electrolysis can be converted to HCl, which then serves to pickle the steel sheets. This particularly environmentally friendly technology is currently not possible with FeSO ₄ , since the electrolysis of Na ₂ SO _{4 is} not satisfactory.

Description of the measurement methods used 1. Measurement of the BET surface

The BET surface area is determined according to the so-called 1-point method according to DIN 66131. 90% He and 10% N ₂ are used as the gas mixture, it is measured at 77.4 K. Before the measurement, the sample is baked at 140 ° C for 60 minutes.

2. X-ray measurement of the crystallite size

The crystallite size is determined on the Phillips powder diffractometer. The 110 reflex is used to determine the crystallite size.

α-iron oxide hydroxide (M (FeOOH) = 88.9 g / mol

2.1 Area of application

Determination of the crystallite size in goethite in the range from 5 to 100 nm.  

2.2 basis

The determination in goethite is based on X-ray diffraction radiation through reflection detection. The evaluation is done on silicon as an external standard.

2.3 reagent

Silicon standard for angle calibration (ICDD No. 27-1402), Philips PW 1062/20

2.4 devices

2.4.1 Diffractometer: Philips PW 1800 goniometer type: Theta-2 Theta
2.4.2 Sample feed: 21-fold sample changer
2.4.3 Detector: Xe proportional counter tube
2.4.4 Reflex evaluation: X-Pert Software Rev. 1.2 on HP Vectra VL
2.4.5 Agate grater and pestle
2.4.6 Sample holder: Philips PW 1811/00 and PW 1811/27

2.5 X-ray diffractometric conditions

2.5.1 X-ray tube: long fine focus, Cu anode, 60 kV, 2200 W
2.5.2 Radiation: CuKα ₁

, λ = 0.154056 nm
2.5.3 Generator: 40 kV, 40 mA
2.5.4 Scan parameters:
2.5.4.1 Scan Type: Step Scan
2.5.4.2 Step size: 0.020 ° 2 theta
2.5.4.3 Step measurement time: 2.00 s
2.5.5 Silicon standard:
2.5.5.1 Starting angle: 27.00 ° 2 theta
2.5.5.2 End angle: 30.00 ° 2 theta
2.5.6 Sample:
2.5.6.1 Starting angle: 18.50 ° 2 theta
2.5.6.2 End angle: 23.50 ° 2 theta

2.6 Execution

2.6.1 External standard:
2.6.1.1 Place the silicon standard (2.1) in the sample holder of the diffractometer and start the measurement program.
2.6.1.2 Determine the maximum and half width of the silicon reflex with the Miller indices hkl = 111 in the 2 theta angle range 27.00 ° to 30.00 °. Print out the peak parameters (Tab. 1) and, if necessary, the diffractogram.
2.6.2 Determination in the sample:
2.6.2.1 Rub about 2 g of sample in the agate drip tray (4.5).
2.6.2.2 Place about 1 g sample in the sample holder (4.6) of the diffractometer and start the measurement program.
2.6.2.3 Determine the maximum and the integral width of the goethite reflex with Miller indices hkl = 110 in the 2 theta angle range from 18.50 ° to 23.50 °. Print out the diffractogram and the peak parameters (Tab. 2) and, if necessary, the diffractogram.

2.7 calculations

2.7.1 In the crystallite size determination table displayed by the computer (X'Pert Software, Rev. 1.2, (Philips Analytical GmbH, Kassel, DE) Profile Widths) the integral width (Width of Broadened Profile), the maximum (peak position / ° 2 theta) of the goethite reflex as well as the width of the standard profile (FWHM) of the silicon standard. Create and print out the evaluation report (Tab. 2).
2.7.2 The crystallite size in the X'Pert program is determined according to the Scherrer equation:

D (crystallite size) crystallite size in nm
k Form factor of the crystallites = 0.9 (literature mean)
λ wavelength in nm
W _Size Integral width of the goethite reflex half-width of the silicon standard
cosθ maximum of the goethite reflex in ° 2 theta

• 1.

Table 1

Table 2

Table 3 Crystallite size determination via X'Pert program; Scherrer equation Menu item: Additional functions in the X'Pert program section: X'Pert Organizer

Anodes - Material: Cu (copper)
Radiation type: Cu Kα
Wavelength: (nm) 0.154184
K factor (form factor mean): 0.9000
Intensity ratio: Cu Kα ₁

/ Cu Kα ₂

0.5000

3. Measurement of the color values

The color values are measured as described in EP-A 0 911 370.  

Examples example 1 Aluminum-containing germ from FeCl ₂ and AlCl ₃

In a discontinuous stirred tank with a useful volume of 30 liters, equipped with a three-stage cross-bar stirrer and a gassing device (ring with holes below the stirrer), 14.095 l FeCl ₂ solution with 55.07 g / l Fe and an Fe-III content of 1.5 mol were placed -% (based on total Fe). 914 g of AlCl ₃ solution with 5.06% by weight of Al and 3.95% by weight of HCl were then added.

The FeCl ₂ amount corresponded to 13.9 mol Fe (Fe-II and Fe-III), the AlCl ₃ amount 1.71 mol, the HCl amount 0.989 mol. The ratio Al / Fe total accordingly corresponded to 12.3 mol% based on total Fe. This solution was heated to 34 ° C. with stirring and gassing with 300 Nl / h nitrogen. After this temperature had been reached, 1615 ml of sodium hydroxide solution with a content of 300 g NaOH / l (corresponding to 7.5 equivalents per liter) were pumped in 6 minutes with a gear pump. Accordingly, 33.8% of the metals Fe + Al were precipitated. Immediately after the precipitation had ended, the gassing was stopped with nitrogen, and 97 Nl / h of air were gassed for the purpose of oxidation. The reaction was complete 180 minutes after the start of the oxidation. The oxidation rate was accordingly 33.3 mol% Fe-II / h. (Only the 33.8% of the Fe precipitated by the NaOH were taken into account. The Fe III content was calculated as Fe II. It was further assumed that Fe and Al were precipitated uniformly).

The germ obtained had the following properties:
BET surface area: 173 m ² / g
Crystallite size: 17.5 nm
AV: 3027

Example 2 Aluminum-containing germ from FeCl ₂ and AlCl ₃

In a discontinuous stirred tank with a useful volume of 30 liters, equipped with a three-stage cross-bar stirrer and a gassing device (ring with holes below the stirrer), 11.58 l FeCl ₂ solution with 55.09 g / l Fe and an Fe III content of 1.0 mol% (based on total Fe). 818 g of AlCl ₃ solution with 6.00% by weight Al and 1.6% by weight HCl were then added.

The FeCl ₂ amount corresponded to 9.5 mol Fe (Fe-II and Fe-III), the AlCl ₃ amount 1.82 mol, the HCl amount 0.36 mol. The Al / Fe total ratio accordingly corresponded to 19.2 mol% based on total Fe. This solution was heated to 44 ° C. with stirring and gassing with 300 Nl / h nitrogen. After this temperature had been reached, 1061 ml of sodium hydroxide solution containing 300 g of NaOH / l (correspondingly 7.5 equivalents per liter) were pumped in in 10 minutes using a gear pump. Accordingly, 32.1% of the metals Fe + Al were precipitated by the NaOH. Immediately after the precipitation had ended, the gassing was stopped with nitrogen and gassed with 67 Nl / h of air. The reaction was complete 135 minutes after the start of the oxidation. The oxidation rate was therefore 44.5 mol% Fe-II / h. (Only the 32.1% of the Fe precipitated by the NaOH were taken into account. The Fe III content was calculated as Fe II. It was further assumed that Fe and Al are precipitated uniformly).

The germ had the following properties:
BET surface area: 186 m ² / g
Crystallite size: 15.5 nm
AV: 2883

Example 3 Aluminum-containing germ from FeCl ₂ and AlCl ₃

In a discontinuous stirred tank with a useful volume of 30 liters, equipped with a three-stage cross-bar stirrer and a gassing device (ring with holes below the stirrer), 14.095 l FeCl ₂ solution with 55.07 g / l Fe and an Fe-III content of 1.5 mol were placed % (based on total Fe). 914 g of AlCl ₃ solution with 5.06% by weight of Al and 3.95% by weight of HCl were then added.

The amount of FeCl ₂ corresponded to 13.9 mol Fe (Fe-II and Fe-III). the amount of AlCl ₃ 1.71 mol, the amount of HCl 0.989 mol. The Al / Fe total ratio accordingly corresponded to 12.3 mol% based on total Fe. This solution was heated to 44 ° C. with stirring and gassing with 300 Nl / h nitrogen. After this temperature had been reached, 1615 ml of sodium hydroxide solution containing 300 g of NaOH per liter (corresponding to 7.5 equivalents per liter in 6 minutes using a gear pump were pumped in. Accordingly, 33.8% of the metals Fe + Al were precipitated by the NaOH. Immediately after the precipitation had ended, the gassing was stopped with nitrogen and gassed with 97 Nl / h of air. 180 minutes after the start of the oxidation, the reaction was complete. The oxidation rate was therefore 33.3 mol% Fe-II / h 33.8% of the Fe precipitated by the NaOH was taken into account. The Fe III content was calculated as Fe II. It was further assumed that Fe and Al were precipitated uniformly).

The germ had the following properties:
BET surface area: 100 m ² / g
Crystallite size: 27.0 nm
AV: 2700

Example 4 Yellow pigment production

2 mol of yellow germ suspension from Example 1 (calculated as mol of α-FeOOH) and 20 mol of FeCl ₂ with an Fe content of 95.7 g / l Fe were placed in a discontinuous stirred kettle with a gassing ring, pH measurement, temperature control and a three-stage crossbar stirrer , This suspension was heated to 60 ° C. with constant stirring. After this temperature had been reached, a pH of 3.4 was maintained by continuously adding sodium hydroxide solution with 300 g / l NaOH. At the same time, air was gassed with 40 Nl / h. After 1453 minutes of gassing, all Fe-II was oxidized, which corresponds to an oxidation rate of 4.1 mol% Fe / h.

The yellow pigment obtained had the following properties:
BET surface area: 32.7 m ² / g
Crystallite size: 32 nm
Color strength (against Bayferrox® 915): 101%
there *: -0.9
db *: -2.1
dL * (against Bayferrox® 915): -2.4
there *: -1.0
db *: -4.0

Example 5 Yellow pigment production

Instead of the germ from Example 1, the germ from Example 2 was used, and the air volume was set to 76 Nl / h. Under otherwise identical reaction conditions, a yellow pigment with the following properties was obtained after 1618 minutes (oxidation rate 3.7 mol% / h):
BET surface area: 45.6 m ² / g
Crystallite size: 25 nm
Color strength (against Bayferrox® 915): 97%
there *: 0.1
db *: -2.6
dL * (against Bayferrox® 915): -3.4
there *: 1.1
db *: -4.0

Example 6 Yellow pigment production

Instead of the germ from example 1, the germ from example 3 was used. The following yellow pigment was obtained under the reaction conditions of Example 5:
BET surface area: 22.0 m ² / g
Crystallite size: 27 nm
Color strength (against Bayferrox® 915): 97%
there *: 0.2
db *: -1.2
dL * (against Bayferrox® 915): -0.7
there *: 0.7
db *: -1.2

Examples 7-12 Rotglühungen

All anneals were carried out in a laboratory chamber furnace at the specified tempera doors carried out. The amount of product was 50 g, the bed height 40 mm, the glow takes 30 minutes at the specified temperature.

Claims (6)

1. Process for the production of aluminum-containing iron oxide nuclei with an α-FeOOH crystal structure with an aspect ratio of 2100 to 3100 using FeCl2 _, characterized in that

• 1. a.) To an iron (II) chloride solution with a total Fe content of 20-100 g / l, preferably 40-65 g / l and an Fe III content of 0.1-10 mol% Fe- III (based on total Fe) an Al component is initially added in amounts of 3-16 mol% based on total Fe with stirring,
• 2. b.) This mixture is heated to a falling temperature between 30 ° C. and 60 ° C., before being added to 35-50 degrees,
• 3. c.) A precipitant with an active ingredient content of 2-10 equivalents per liter, preferably 4-8 equivalents per liter is added to the mixture, and the molar ratio Fe + Al to precipitant 20% -80%, preferably 30% -60% of the stoichiometry is
• 4. d.) The precipitated suspension is then oxidized with an oxidizing agent at such a rate that the oxidation rate is 2-50 mol% / h, preferably 5-20 mol% / h, of the iron to be oxidized and
• 5. e.) The aluminum-containing α-FeOOH seed obtained after the oxidation is used without further isolation after testing the properties, for the production of iron oxide red pigments via α-FeOOH as an intermediate stage.

2. The method according to claim 1, characterized in that as an Al component Aluminum chloride is used.   3. The method according to claims 1 and 2, characterized in that as a precipitating agent NaOH, KOH, Na ₂ CO ₃ , K ₂ CO ₃ , Mg (OH) ₂ , MgO, MgCO ₃ , Ca (OH) ₂ , CaO, CaCO ₃ , NH ₃ or secondary or tertiary aliphatic amines can be used in aqueous solution or aqueous slurry. 4. Process according to Claims 1 to 3, characterized in that atmospheric oxygen, oxygen, ozone, H ₂ O ₂ , chlorine, nitrates of the alkali or alkaline earth metals or NH ₄ NO ₃ are used as the oxidizing agent. 5. The method according to claims 1 to 4, characterized in that the reacti on in discontinuous or continuous stirred tanks, in stirred tank tanks caden, loop reactors or stirrerless reactors with two-substance nozzles as Mixing organs is carried out. 6. Use of the α-FeOOH nuclei produced according to one of claims 1-5 for the production of α-Fe ₂ O ₃ .