DE2221264A1 - COBALT AND SILICON OXIDE CONTAINING NEEDLE-SHAPED FERRIMAGNETIC IRON OXIDES - Google Patents

Description

AYER AG

LEVERKUSEN-Bayerwerk Central area 2 April 8th «72 Patents, Maiken and Licenses

GB / Schu

Acicular ferrimagnetic ones containing cobalt and silica Iron oxides

The present invention relates to acicular ferrimagnetic ones Iron oxides containing cobalt and silicon. These acicular iron oxides containing cobalt and silica show after they have been heated to 150 ° C for 30 minutes, one reduced remanence loss.

The iron oxides, Fe ^ Or and y-FepCU with or without doping with Co and other elements are suitable for the production of magnetogram carriers. The literature usually speaks of ferromagnetic iron oxides. However, since the work of L. Neel at the latest, it has been known that the underlying phenomenon leading to magnetic behavior is
differs from that in ferromagnetic materials and as
ferr! is to be referred to as magnetic. The analogy of ferrimagnetism to ferromagnetism remains on the macroscopic
Limited behavior in the external magnetic field.

In ferromagnetism, all spin moments of the individual atoms are coupled and in individual areas, the so-called Weiss'

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see districts or domains strictly aligned in parallel. This creates a resulting total moment _{t <} for each domain, which corresponds to the sum of all magnetic atomic moments.

Ferrimagnetism, on the other hand, is a special case of antiferromagnetism, in which the atomic spin moments are equally large and aligned antiparallel. Act with ferrimagnetism different strong atomic moments magnetically against each other and thus lead to a total moment of the elementary areas, which, as in ferromagnetism, is different from zero. The atoms with oppositely directed spin occupy different crystal lattice sites and form so-called sublattices. The resulting magnetic moment then no longer corresponds to the sum of all atomic moments, but to the difference between the resulting magnetic moments of the individual sublattices.

In the case of the magnetic iron oscides that crystallize in the spinel lattice the sublattices are formed by the tetrahedral and octahedral gaps of the cubic closest oxygen packing. The spin moments of all ions on tetrahedral sites couple parallel to one another and are in turn antiparallel to the Spin moments of the ions on octahedral sites, which in turn are also aligned parallel to one another.

The resulting total moment of the domains results from the difference in the magnetic moment of all ions on octahedral sites minus the magnetic moment, all ions on tetrahedral sites. This means that there is ferrimagnetism in the case of magnetic iron oxides. As a result, in the present application, the magnetic iron oxide becomes no longer ferromagnetic but rather ferromagnetic marked.

The invention now relates to acicular ferrimagnetic iron oxides with a cobalt content of about 0.5-10 atomic percent and reduced remanence loss, characterized in that they additionally contain 0.05-3 percent by weight SiO ₂ Le A 14 282-2 -

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The invention also relates to a method for production of these ferromagnetic iron oxides.

The ferrimagnetic iron oxides produced by the present process have a somewhat lower remanence compared to the SiC ^ -free material, but the loss of remanence is usually less than 10 % under thermal stress, while the remanence in SiC ^ -free material same treatment decreases by more than 23% depending on the cobalt content in the iron oxide. They become magnetic. Pigments with a sufficient for most areas of application and; above all, largely constant remanence is obtained. The ferrimagnetic cobalt-containing iron oxides, which have been stabilized with SiOp, are suitable for magnetic recording and reproduction in and on all materials, such as tapes, plates, foils, papers in printing inks, encryption substances, etc. ■ ■

Iron oxides containing cobalt are suitable because of their high coercive force particularly good for recording high-frequency acoustic and optical signals or digital information.

It is known from the literature that magnetic tapes with cobalt-containing cube-shaped iron oxides show a drop in the original signal when they frequently pass a recording and playback head of a tape machine. For example, JR Morrison and DE Speliotis, IEEE Transactions on Electronic Computers, Volume EC 15, No. 5, page 782 (1966) found an output signal drop of 55 % after 6000 passes on a recording head at a speed of 76 cm / sec. This effect is known as mechanical instability. An effect of a similar order of magnitude is also obtained if the tape is used with cobalt-containing iron oxide as the recording

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material heated to temperatures around 150 ° C. Here too will a drop in previously recorded signals by a percentage similar to that when the tape passes the tape head frequently observed. This effect is known as the thermal instability of cobalt-containing iron oxides. Both appearances are based on an instability of the remanent magnetization iron oxides containing cobalt. It is also known that acicular cobalt-containing iron oxides - albeit to a lesser extent - thermal and mechanical instability of the remanence occurs.

In DOS 1 907 236 a magnetic cobalt-containing acicular iron oxide is described, which on the aligned magnetic tape a remanence loss of at most 20% parallel to the alignment of the acicular particles and perpendicular to the alignment at least 1.5 times the remanence loss after heating the magnetic tape for 30 minutes 150 ° C. The best stabilization of the remanence of the cobalt-containing iron oxides is achieved with an FeO content of 5 to 17% by weight FeO.

The FeO content necessary to stabilize the remanence cannot be achieved in technology without great technical effort to adjust. Even with the slightest fluctuations in the reduction conditions, changes in the FeO content will occur, see above that the production of a product with a constant loss of remanence becomes very difficult. On the other hand, it is known that with decreasing particle size of FeO-containing iron oxides the risk of oxidation increases during storage and processing in air. With certain particle sizes, those stabilized with FeO Iron oxides containing cobalt can even oxidize so quickly that they burn off.

The acicular iron oxide or iron oxide hydrate containing cobalt is produced by methods known per se.

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After DAS 1,226,997, a mixture of an iron (2) - and cobalt (2) salt solution at temperatures from 0 to 30 ⁰ C by the addition of a basic precipitant to achieve a pH of about 4.5 to 6.5 offset. The reaction mixture is then treated with oxidizing agents and the Oi (Fe, Co) OOH particles grow in the pH range from 4.5 to 6.5 at temperatures from 30 to 65 ^° C. with simultaneous addition of iron / cobalt - Saline solution carried out. The conversion of the cobalt-containing iron (3) oxide hydroxide produced in this way into cobalt-containing / Fe ₂ O takes place according to known processes.

In a strongly alkaline medium (pH value> 13), DAS 1 204 644 a cobalt-containing iron hydroxide or after a Hitherto unpublished proposal for a cobalt-containing iron hydroxide or carbonate suspension in excess alkali hydroxide solution oxidized to the needle-shaped <* - FeO0H.

According to German Offenlegungsschrift 2 022 013, acicular oC -FeOOH, Fe ₂ O ₅ or Fe, O ^ is mixed with a cobalt-containing liquid, the sludge is dried and the mass is converted _{into 2 "-Fe 2 O by known processes} which uses nitrate, acetate, hydroxide or formate.

The SiO ₂ on cobalt-is preferred acicular FeOOH ⁰ ^ with a length to width ratio of 5: deposited 1: 1 to 50th This process can be carried out directly during or immediately after the production of the Ct-FeOOH without any major additional work.

Surprisingly, it was found that acicular ferrimagnetic iron oxides containing cobalt, with constant chemical composition and containing 0.05 to 3% by weight of SiO ₂ , have a remanence loss of less than 20% after heating the powder at 150 ° C. for 30 minutes. Magnet-

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Ribbons which are produced with the iron oxide containing cobalt and silicon oxide described according to the invention show a improved thermal stability of the remanence and, consequently, also an improved ability to transmit high-frequency information without excessive output signal drop at higher temperatures and / or mechanical stress, so that an improved reproduction of signals is possible.

The novel, cobalt and silicon oxide-containing acicular ferromagnetic iron oxides are produced by depositing 0.05 to 3% by weight SiOp, preferably 0.1 to 1 %, on finished acicular cobalt-containing iron oxide or iron oxide hydroxide, or the SiOp is also precipitated during the production process of the pigment .

The amount and concentration of the SiOp-containing solutions depends on the size of the batch and the desired SiOp content of the finished pigment. Generally will SiOp solutions with a content of 0.5 to 10% by weight SiOp are used.

The SiOp is made from solutions of SiOp (silica sols), alkali silicates, for example. Sodium or potassium water glass, or guanidine silicate deposited on the cobalt-containing acicular iron oxide hydroxide. The solution, which contains the SiOp in dissolved form, can be added to the suspension either during the last 20 % of the pigment formation time in the case of the <* -FeOOH preparation, during and after the pigment formation or immediately after the oxidation to the oil FeOOH has ended, see above that SiO ₂ is deposited on the cobalt-containing d-FeOOH needles. If the cobalt-containing oil-FeOOH is produced in an acidic medium according to the method, e.g. DAS 1 226 997 at pH 4.0 to 6.5, a sodium silicate solution can be added during the last 10 to 20 % pigment formation time or immediately after pigment formation has ended. In this pH range, SiO _{2 is} deposited directly on the needles. Another possibility of SiO ₂ -ab-

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The distinction is that the cobalt-containing ct-FeOOH suspension adjusts to a pH value> 12, adds the water glass solution and then slowly adjusts the pH value of the suspension lowered to pH = 7 with hydrolysis of the water glass solution.

Another way of depositing the SiOp on cobalt-containing t-FeOOH is to treat the suspension of freshly prepared acicular cobalt-containing oc-FeOOH with a waterglass solution at a pH of> 12. It is found that even under these conditions SiOp is deposited on the oi - FeOOH. For this reason, the cobalt-containing acicular oil- FeOOH formed in a strongly alkaline medium (pH value 13), e.g. according to DAS 1 204 644, can be used during the last 20 % of the oxidation time, during and / or after complete oxidation to OC- ( Fe, Co) OOH, treat with a _{solution containing SiO 2} at pH> 13. Here too, SiO ₂ is deposited.

According to another proposal that has not yet been published four times After dehydration, the pigments containing cobalt oxide and silicon dioxide can initially be used at temperatures of approx 650 to 800 ° C are tempered before they are subjected to the reduction and reoxidation. Depending on the applied Temperature, a period of about 0.25 to about 2 hours is sufficient for the tempering. There is time and Temperature in inverse proportion. In any case, the tempering must be carried out in such a way that the pigment grains coarsen if possible does not occur.

The cobalt and silicon oxide-containing one produced according to the invention needle-shaped cx-FeOOH is dehydrated to oC-FepO ^ and then in a known manner by reduction and reoxidation

converted into magnetic iron oxide containing cobalt and silicon oxide. So a product is obtained that has a reduced Loss of remanence after heating at 150 ° C for 30 minutes.

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However, in another variant of the process, the SiOp can also be deposited _{on cobalt-containing ^ -Fe 2 O.} The procedure can be carried out, for example, as follows:

Acicular cobalt-containing OC-Fe ₂ O ^, produced by dehydrating cobalt-containing C ^ -FeOOH according to the method described above, is suspended in water and the pH is adjusted to 12. Then the SiO ₂ -containing solution is added and the pH value is gradually lowered to pH = 7 with acid, so that the SiO _{2 is} deposited on the acicular cobalt-containing ° C-Fe ₂ O. Another possibility is that the dry cobalt-containing cL-Fe ^ O-? before the conversion into the ^ -Fe ₂ O, the SiO ₂ -containing solution is sprayed onto the material.

The great advantage of the cobalt according to the invention described and siliceous acicular ferromagnetic iron oxide is that it has been possible to produce a product with a constant composition in a simple manner that a Remanenzverlust of less than 20% after a 30 minute heating at 150 ⁰ on the powder C and in which there is no change in remanence due to changes in chemical composition, e.g. due to oxidation in air, even during storage and processing.

Table 1 shows the loss of remanence on the powder and on the magnetic tape parallel and perpendicular to the alignment and the mean value calculated therefrom for some cobalt-containing acicular iron oxides obtained according to the following examples. The magnetic tapes were produced according to the method of British patent specification (1 080 614) by grinding for 3.5 hours in a pearl mill and subsequent application of 15 g / m 2 of the lacquer with a layer thickness of 12 μ on a 23 μ film. Of the

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Loss of remanence was observed after each 30 minutes of heating

150 ° C certainly. Loss of remanence (%)
perpendicular to the orientation
B. middle
value
(A + B): 2 - Tabel 1 9.0 5.2 sample
No. parallel to
Alignment
A. 9.3. 6.2 powder 1 1.3 16.1 11.2 3.1 2 3.1 21.8 13.7 5.35 3 6.2 26.6 19.7 8.85 4th 5.6 35.5 27.6 11.5 5 12.7 38.5 32.3 17.0 6th 19.7 19.0 7th 26.0 32.2

It can be seen from Table 1 that the mean value of the loss of remanence from column A and B and the loss of remanence of the powder have a parallel course. For example, products with a high loss of remanence on the powder also show unfavorable results Values on the magnetic tape, while low values on the powder indicate a lower remanence drop of the same size on the tape let it close.

It is thus possible to measure the decrease in remanence by means of a relatively simple measurement cobalt-containing iron oxides on the powder receive a statement about the decrease in remanence on the magnetic tape. This method is therefore used for assessment in the following examples, which are intended to serve to further explain the present invention of the properties of cobalt-containing iron oxides.

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example 1

a) Production of the cobalt-containing ° £ -FeOOH

According to DAS 1 204 644, in a 25 1 glass vessel to a receiver of 2.8 1 sodium hydroxide solution with 1218 g NaOH, with vigorous stirring and simultaneous introduction of 250 1 air / hour at 50 ⁰ C 9.4 1 of a solution are added 1910 g FeSO ^ .7 H ₂ O and 50.4 g CoCl ₂ .6 H ₂ O contains. This is how the iron (II) hydroxide containing cobalt is precipitated. It is oxidized for 30 minutes while maintaining the introduction of air and stirring at 50 ° C., then the mixture is heated to 70 ° C. and the formation of d- (Fe, Co) OOH is terminated with 500 liters of air / hour. If 75% ⁰ C- (Fe, Co) OOH is present in the suspension in 5 minutes, 200 ml of a dilute water glass solution containing 2.22 g of SiO ₂ are added dropwise and the suspension after complete oxidation for a further 2.5 hours at this Temperature stirred. The SiO ₂ deposition takes place during and after the complete oxidation of the ^ -FeOOH in a strongly alkaline medium at pH> 13. A total of 6 hours after the precipitation is filtered off, washed alkali-free and dried at 130.degree.

The ° t- (Fe, Co) OOH has an X-ray determined at the (OII) -reflex Needle width of 115 Å.

b) Conversion to cobalt and silicon oxide T-Fe ^ O ^

The conversion of the acicular iron oxide hydroxide into magnetic cobalt-containing acicular Fe ₂ O takes place as follows: dewatering of the "i- (Fe, Co) OOH, if necessary tempering at 68O ⁰ C to 750 ⁰ C, reduction at 400 ⁰ C to cobalt-containing Fe, 0 ^. Then allowed to cool in a stream of nitrogen and oxidized at 260 ⁰ C with air to the cobalt-containing

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^ · 222Ί264

The loss of remanence of the cobalt-containing P ^ Fe ^^ samples is determined after heating the powder at 150 ° C. for 30 minutes

Table 2 :

The OC- (Fe, Co) OOH comparison _{samples are prepared as described in Example 1a, but without addition of SiO 2} , and converted into cobalt-containing ^ -Fe _{P 0 by the usual method.}

Sample no. a) ¹ b) • 400 a) ² D) Amount of CoCI ₂ .6 H ₂ O 56.0 67.2 per oi — FeOOH approach Atom% Co in 0 ^ Fe ₂ O ₃ 3.2 3.9 Wt% SiO ₂ in ^ -Fe ₂ O ₃ without SiO ₂ 0.37 without SiO ₂ 0.37 Coercive Force I _Hc 583 - 511 691 531 (Oe) Remanence, Br / j 533 460 568 423 (Gcm ³ g ¹ ) Loss of remanence after 29 % 13 % 39 % 15.6 % 30 minutes 150 ^° C Remanence, Br / j 378 346 ' . 357

(Gem g) after heating
at 150 ^° C

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It can be seen from Table 2 that in the cobalt and silicon-containing samples the percentage loss of remanence after heating at 150 ^° C. for 30 minutes is always less than the decrease in remanence of the corresponding pure cobalt-containing comparison samples. The final value of the remanence Br / j> , which is essential for magnetic recording, is considerably higher for the samples containing SiOp after the thermal treatment than for the products without deposited SiOp

Example 2

From the acicular cobalt and silicon oxide-containing ^ Fe ₂ O * with 2.6 Aton #> Co produced in Example 1, magnetic tapes directed in a magnetic field are cast by a known method (GB 1 080 614). U thick layer having 15 g / m applied; After 3.5 hours, grinding in a Perl Miil u-foil 12 is a to a 23 _X. The strips are heated for 30 minutes at 15O ⁰ C and then the Remanenzverlust is determined in parallel and perpendicular to the alignment.

Table 3

Example 1, sample 1 Loss of remanence A. B. 00 middle
value
(A + B): 2 Pul
ver 2.6 atom% Co parallel to
Alignment perpendicular
to the off
direction 27.6 19.0 19.7 35.5 19.7 17.0 a) without SiO ₂ 12.7 26.6 b) with 0.36 % SiO ₀

It can be seen that the SiO ₂ -containing product in the strip direction (A) has a greatly reduced remanence loss of only 12.7% compared to 19.7 for the comparison product.

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Example 3

a) In a 30 1 boilers are to 4.2 1 sodium hydroxide solution 1770 g NaOH while stirring and introducing nitrogen of 100 1 / hour at 50 ⁰ C 3 minutes 14.2 1 Solution 2860 g FeSO ^ .7 H ₂ O and 74.3 g of CoCl ₂ .6 H ₂ O. After the precipitation has ended, 100 1 air / hour is passed in for 30 minutes while maintaining the temperature and continuing stirring. The suspension is then heated to X 70 ⁰ C and initiated 200 1 air until the end of the oxidation "After 80% are oxidized to the Ct (Fe, Co) 0OH, during the oxidation in 60 minutes, 200 ml of a dilute water glass solution with 1 , 65 g SiOp dripped into the alkaline suspension (pH> 13). 6 hours after the precipitation, the <= £ - (Fe, Co) OOH formation has ended. It has a radiographically determined needle width of 155 Å.

The conversion to cobalt and siliciumoxidhaltiges 9 "-Fe ₂ O, results in the powder after 30 minutes of heat treatment of the d, - (Fe, Co) ₂ O, at 75O ⁰ C and reducing at 400 ⁰ C.

a coercive force, l _Hc = 540 Oe

a remanence, Br // = 407

a decrease in remanence of 9.1 % to 370 Gcnrg after 30

Minutes 150 ⁰ C.

The cobalt-containing T-Pe ₃ O ₃ contains 0.10 90

b) The production of the ^ - (Fe, Co) OOH takes place as described in Example 3a, but a water glass solution with 3.3 g of SiO _{2 is} added dropwise.

The conversion into cobalt and silicon oxide-containing 0 ^ ₂ results in the powder after tempering at 700 ^° C. for 30 minutes

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Reduction at 400 ° C

a coercive force, I ₁ - = 556 Oe

3-1 a remanence, BrA = 417 according to g

a Remanenzverlust from 8.4% to 383 Gcnrg after 30 minutes 150 ⁰ C.

The cobalt-containing T-Fe ₂ O- contains 0.18 % SiO ₂ Example 4

Under the conditions described in Example 3b, with the addition of 33 g of CoCl ₂ .6 H ₂ O, a cobalt-containing ° f-FeOOH with a needle width of 250 Å determined by X-ray is produced in 5 hours. The water glass is added as described above. The conversion to cobalt and siliciumoxidhaltiges 2 ^ Fe ₂ O-, yields after 30 minutes of tempering at 700 ⁰ C and 400 ⁰ C reduction in

a coercive force, I _u " = 437 Oe

3-1 a remanence, Br / p = 445 according to g

3-1

a Remanenzverlust from 3.1% to 431 Gem g after 30 minutes at 150 ⁰ C.

The cobalt-containing ^ -Fe ₂ O "contains 0.22 % SiO ₂ according to the analysis.

A magnetic tape is produced from this material according to the information in Example 2 and the loss of remanence after 30 minutes Heating of the tape to 150 ° C measured.

^A B mean value powder

parallel to alignment perpendicular to alignment (A + B): 2

direction

1.3 5.1 3.2 3.1

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Example 5

To 8.2 1 of NaOH with 1770 g of NaOH, 10 1 of a solution with 1860 g of FeSO ^ .7 H ₂ O and 74.3 CoCl ₂ .6 H ₂ O were added under the conditions described in Example 3 and the subsequent oxidation, as described in Example 3 carried out. After 90 % of the suspension has been oxidized to od- (Fe, Co) OOH, a solution containing 3.3 g of SiO _{2 is} added dropwise over the course of 60 minutes. The oxidation is complete after a total of 6 hours. The conversion into cobalt and silicon oxide containing ^ Fe ₂ O with 3 atom% Co after 30 minutes of tempering at 700 ^° C. and reduction at 400 ^° C. results

a coercive force ·, I- = 536 Oe

■ tlC '-ζ _ A

a remanence, Br Iy = 429 Gem g ~

a loss of remanence of 8.8 % to 391 Gcnrg ~ after 30 minutes

O
th 150 C.

Example 6

From a Yorlage of 14.1 1 solution 2860 g FeSO ^ .7 H ₂ O and 74.3 g of CoCl ₂ .6H ₂ O, with 4.2 1 sodium hydroxide solution (1770 g of NaOH) at 50 ⁰ C. Stirring and simultaneous introduction of 100 liters of nitrogen / hour, the cobalt-containing iron (II) hydroxide is precipitated. ^{100 1 of air / S + ande are then passed in at 50 °} C. for 30 minutes with stirring, then 200 1 of air / hour until the end of the oxidation (7 hours). After 90 % to the cobalt-containing

-FeOOH are oxidized, a dilute waterglass solution with 0.8 g SiO _{2 is added} to the suspension in 3 minutes so that SiO _{2 is} deposited in the strongly alkaline medium.

The öt- (Fe, Co) OOH has a needle width determined radiographically from 150 Ä.

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The conversion into cobalt- and silicon oxide-containing W-Fe after 30 minutes of tempering at 700 ^° C. and reduction at 400 ° C. yields

a coercive force, I _u "= 524 Oe

nc 3-1
a remanence, Br / f = 446 according to g

3 - 1

a loss of remanence of 12.3% to 391 gem g after 30 minutes 150 ^° C.

The corresponding comparison product without Si0 _Q has a significant

3-1 gave a higher loss of remanence of 24 % from 479 to 364 Gem g

Example 7

a) nucleation

According to DAS 1 226 997, 3040 g FeSO ^ and 346 g CoSO ^ are dissolved in 20 l of water and 915 g of technical NaOH in 2.5 l of water are added while stirring at 20-25 ° C. A Schlitzflügelrühres at 1500 rpm are introduced 100 liters of air / hour into the hydroxide suspension with the aid, wherein the temperature in the course of 4 hours at 55 ⁰ C increases. During this time the pH drops to 5.0 and the color of the suspension changes from dark green to dark blue and green to orange-brown. The resulting iron oxide hydroxide suspension (germ) now contains 50.2 g / l FeSO. and 38.2 g / l FeOOH. The degree of precipitation was thus 56.6 %.

b) pigment formation

20 1 Germ suspension are heated to 60 to 70 ⁰ C, while stirring in the course of 8 hours with 4.6 1 of a 12.65% strength NaOH. Simultaneously with the addition of NaOH, 20 liters of air per hour are introduced. According to this process, the germs grow at pH 4.5 to 6.5 by oxidation of the II-valent me-

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metal ions to the III-valent stage and are then precipitated by subsequent hydrolysis. After the (Fe, Co) SO ^ concentration has been reduced, a solution of Fe and cosulphate is introduced at the same time as the sodium hydroxide solution. After adding 14.4 mol of FeSO ^ and 1.6 mol of CoSO ^ dissolved in 15.4 l of water and 15.4 l of 8.3% sodium hydroxide solution with an air supply of 30 l / hour, pigment formation is complete after 25 hours. The pigment is filtered off, washed and divided into 4 portions. The cobalt-containing egg-FeOOH has a needle shape with a needle width of about 200 S and a length: width ratio of 5: 1 to 40: 1. 350 g of this Ot- (Fe, Co) OOH are resuspended before drying and heated as a 6% suspension to 80 ° C with stirring, the pH is adjusted to 5.5 with dilute sulfuric acid and over 80 minutes with 6.5 g of sodium water glass, accordingly 1.75 g SiO ₂ , added. The pH rises to 5.8. After stirring for a further 10 minutes, the pigment is filtered off with suction and dried at 120.degree. A tempering of the <* .- (Fe, Co) ₂ O ^ at 680 ° C for one hour reduction at 400 ° C and reoxidation at 260 ⁰ C yields

a coercive force, I _x - = 570 Oe
a remanence, Br / p = 465 Gcnrg

a loss of remanence of 10.5 % to 417 gem g after 30 minutes at 150 ° C.

The acicular ferromagnetic iron oxide containing cobalt contains 6.5 atom% of Co and 0.46% by weight of SiO ₂ .

Example 8

175 g of cobalt-containing iron oxide hydroxide from Example 7 are dispersed, ^{heated to 80 °} C., the pH is adjusted to 12.2 with NaOH, 3.5 g of SiO _{2 are} added as water glass and then the pH is adjusted with H in the course of 140 minutes ₂ SUN. lowered to pH = 7, filtered off and dried. The SiO ₂ deposition leads

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to a more difficult growth of the pigment during the conversion. 't- (Fe, Co) ₂ O ^ ₅ , which after one hour of tempering at 730 C before

_25th

the reduction at 46O ⁰ C and reoxidation at 290 ° C is obtained, shows a remanence loss of less than 10 %,

a coercive force ,! "= 433 Oe

3-1 a remanence, Br / V = 332 according to g

3–1 a loss of remanence of 8 % to 306 Gern g after 30 minutes

150 ⁰ C.

The cobalt-containing ^ Fe ₂ O ₅ contains 2.1% by weight of Si Example 9

The encapsulation of the oC-dTe, Co) OOH is carried out according to Example 7, but the pH value is kept at pH = 12.2 throughout the entire time. The ÖC- (Fe, Co) OOH obtained contains 0.19 % SiO ₂ after drying. The conversion to cobalt and siliciumoxidhaltiges / ¹ Fe ₂ O gave after annealing of one hour at 730 ⁰ C, reduction at 430 ⁰ C, reoxidation at 26O ⁰ C

a coercive force, I _u = 456 Oe

3 -1
a remanence, Br /? = 396 according to g

a Br ^ loss of 10% per 356 Gerng ~ of 30 minutes

150 ° C.

Example 10
a) nucleation

In a 30 l reactor with a slotted vane stirrer, a 14.7% solution containing 21.375 mol FeSO ₄ and 1.125 mol CoSO ^ is converted into basic (Fe, Co) II with 915 g technical NaOH in 2.5 l water at 35 C -Sulfate precipitated and oxidized with 100 l air / hour with stirring at 1500 rpm. The pH-

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Value over 4 hours to 4.2 while the temperature
increases from 35 to 66 ° C. The nucleation is finished when
the initially blue-green suspension has turned brown-yellow, they
then contains 58 g / l FeSO ^ and 38.3 g / l FeOOH accordingly
an initial degree of precipitation of 53%.

b) Pigment formation without SiO ₂

22 1 of seed suspension from Example 10 a are oxidized at 60 C to 70 ⁰ C by bubbling 20 I.Luft / hour and stirring at 1500 rpm, and the resulting trivalent ions by the addition of 126.5 g NaOH in one liter of water in the course from 8
Hydrolyzed for hours, the pH being kept between 4.2 and 5.2. The resulting cobalt-containing iron oxide has a needle width of approximately 0.023 _/ um and a length:
Width ratio from 5: 1 to 40: 1.

The pigment is washed and divided. Part will
converted directly into ^ (Fe, Co) ₂ O, another portion redispersed and with 0.5 wt .-% SiO ₂ according to Example 8
post-treated.

The conversion of both samples into ^ - (Fe, Co) ₀ O, is with and without

2-3

Annealing performed reduction conditions 400 C to 460 C, reoxidation at 290 ⁰ C.

The cobalt-containing P ^ Fe ₂ O, contained 3.5 atom% Co.
The magnetic values are summarized in Table 4.

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ZO

Table 4

Tempering 1 h ( ⁰ C)

Coercive force

^L Hc

(Oe)

^{/ S} 3 1
(GcnTg)

Br / j

Rema-

loss after 30 min. at 150 ⁰ C

without SiO,

630

495 513

487
483

397 416

18.5 14.0

with 0.55 %
SiO ₂

730

477 415

462
449

402 422

13.0 6.0

It can be seen that both with and without tempering according to the invention described cobalt and silicon oxide containing ^ -FepO, has a lower loss of remanence.

Example 11

Sufficient water is added to 100 g of cobalt-containing, SiO ₂ - "free, needle-shaped CU-Fe ₂ O, with 2.2 atoms of 6 Co, that a 10 J & Lge suspension is formed, then the pH is adjusted to 12.2. A water glass solution containing 0.4 g of SiO _{2 is} added to this suspension and, as in Example 8, the SiO _{2 is} deposited on the O6- (Fe, Co) ₂ O.

The. Conversion into cobalt and silicon oxide containing ^ Fe ₂ O, resulted after 30 minutes of tempering at 700 ^° C. and reduction at 400 ^° C.

a coercive force, I _x , = 519 Oe a remanence, Br / p

= 446 Gem g ~

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■ ζ _ί

a Remanenzverlust from 12.1% to 392 Gcnrg after 30 minutes 150 ⁰ C.

The corresponding SiOp-free comparison product had a remanence loss of 24 %.

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Claims (5)

Claims ίΐ) J acicular ferrimagnetic iron oxides with a cobalt content of about 0.5 to 10 atomic percent and reduced remanence loss, characterized in that they additionally contain 0.05 to 3 percent by weight SiO ₂ . 2) Acicular ferrimagnetic iron oxides according to claim 1 with an additional SiOp content of 0.1 to 1 percent by weight. 3) A process for the production of acicular ferrimagnetic iron oxides containing cobalt and silicon oxide with reduced remanence loss according to claim 1 or 2, characterized in that SiO _{2 is} deposited on acicular cobalt-containing iron oxide hydroxide during and / or after its production, the product obtained is dehydrated and then by reduction and possibly converting reoxidation into ferrimagnetic iron oxide. 4) Process for the production of acicular ferromagnetic iron oxide containing cobalt and silicon oxide with reduced remanence loss according to claim 1 or 2, characterized in that the SiO ₂ is deposited on acicular cobalt-containing c * .- Fe ^ O, which is then reduced by reduction and optionally reoxidation is converted into ferrimagnetic iron oxide. 5) Method according to claim 3 or 4, characterized in that the cobalt and silicon oxide containing iron oxide or hydrated oxide is annealed after the dewatering prior to the reduction at temperatures from 650 to 800 ⁰ C. '' Use of acicular ferrimagnetic iron oxides • according to claim 1 or 2 for the production of magnetic recording media and magnetic printing inks. Le A 14 282 - 22 - 309846/1008