DE1031771B - Process for the production of iron powders consisting essentially of metallic iron particles, which are suitable for the production of permanent magnets - Google Patents

Description

GERMAN

It is known that magnetizable iron powder are processed by pressing into pressed bodies, which after magnetization are permanent magnets deliver good properties. The coercive force of such magnets depends to a large extent on the Crystallite size of the powder particles used, namely the better, the smaller the crystallites are. It is therefore desirable to start from iron powders with the smallest possible crystallite size.

There are two fundamentally different ways of producing finely divided iron powders: firstly direct reduction of technically accessible iron compounds, especially oxidic iron compounds, and secondly, production of special preparations using easily decomposable iron salts of organic acids (oxalates, Acetates, formates, etc.). In the first-mentioned way iron powder are obtained which are used for the production of Permanent magnets are poorly or not at all suitable; by decomposition of the iron salts more organic Acids with a possibly connected reduction, on the other hand, can be used to produce usable iron powder.

A further improvement of the second method results when one according to the German patent 939 085 iron salts are used, which at the same time contain lower amounts of calcium, magnesium, aluminum or Contain cadmium compounds. After decomposition, possible reduction, agglomeration through Pressures and magnetization are obtained compacts, which are particularly good magnetic Properties are excellent. Although it is not clearly known why the addition of calcium, Magnesium, aluminum or cadmium compounds with otherwise identical processing to better compact magnets leads than when using pure iron salts, according to the current state of knowledge however, it must be assumed that by adding these light metal compounds in the first place the decomposition and possible reduction of the iron salt is influenced, namely to the extent that at the formation of the iron primary crystallites, the crystal growth is disturbed. This creates proportionally small iron crystallites.

The present invention is aimed at the same goal, namely the production of iron powders Iron crystallite size (<C 5000 A) directed, it reached this goal, however, in a simpler way. According to the method of the invention, it is possible to make whole starting from considerably cheaper raw materials, namely from oxidic iron compounds such as iron oxides, Iron hydroxides or iron oxide hydrates. In the oxidic iron compound there is also one Crystallization-inhibiting light metal compound, in the present case an alkaline earth compound, fine ver process for extraction

of essentially metallic

Iron powder consisting of iron particles,

those used to make permanent

Magnets are suitable

Applicant:

S. A. Societe d'Electro-Chimie,

d'Electro-Metallurgie et des Acieries

electriques d'Ugine, Paris

Representative: Dipl.-Ing. H. Marsch, patent attorney,
Schwelm (Westphalia), Drosselstr. 31

Claimed priority:
Great Britain 22 May 1952

Nigel Conrad Tombs, London,
has been named as the inventor

divides, but without the production of mixed crystals would be required.

In principle it is of course possible to use a natural or synthetic, oxidic iron compound to mix intimately with an alkaline earth compound and reduce the mixture. That way it gets However, as with alkaline earth-free oxide powders, none for the production of permanent magnets suitable iron powder, probably because a uniform and extremely fine, If possible, molecular distribution of the alkaline earth compound in the oxide powder through mechanical mixing processes cannot be reached. The anti-crystallization effect of the alkaline earth compound in the Reduction can therefore not come into play.

On the other hand, if one tries to achieve an approximately molecular distribution through the temperature of the mixture achieve, so you get even when using extremely finely divided iron oxides due to the required proportion highly crystalline at high temperatures

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ized substances. During the reduction, these also do not lead to sufficiently small iron crystallites and are therefore also unsuitable for making permanent compact magnets.

A way had to be found, the alkaline earth compound at relatively deeper To distribute the temperature evenly in the iron oxide and at the same time to obtain an iron oxide, which was not exposed to high temperatures during its manufacture and is therefore still very fine crystalline.

According to the invention, this has now been achieved by the distribution of the alkaline earth compound does not make directly by sintering an iron oxide together with an alkaline earth compound, but in addition, sufficient amounts of an oxidic alkali compound are added to the starting mixture. then Alkali ferrites are formed when heated. One achieves in this way - already at temperatures between 500 and 800 ° C, depending on the type of added Alkaline compound, a completely uniform molecular distribution of the alkaline earth compound in the matrix and excludes the possibility of simultaneous sintering or crystal enlargement of the iron oxide completely off. You have no iron oxide at all in the during the distribution process Mixture, but an alkali ferrite. In order to regenerate the iron oxide, the heating product, that is, from a homogeneous mixture of Alkali ferrite, alkaline earth compound and possibly an excess of alkali salt, treated with water. Here the alkali ferrites decompose, the alkali part goes into solution, the iron oxide falls into extremely fine distribution. The alkaline earth compound remains completely uniform and of course approximately molecularly distributed in the total mass.

It is clear that the crystallite size of the precipitated iron oxide is largely independent in this process on the crystallite size of the oxidic starting material, since the crystal lattice of the starting material Destroyed during ferrite formation and the crystal lattice of the precipitate is completely reformed will. For this reason, there is no need for the starting material in the process of the invention extreme quality requirements, commercial, cheap substances can be used will.

As a result of the decomposition of the ferrite at low temperatures, an extremely fine particle is formed during hydrolysis Iron oxide or oxide hydrate, just as if a hydroxide is precipitated from an iron salt solution. This iron oxide, which is formed at low temperatures, can under mild conditions, i. H. at temperatures between 250 and 500 ° C, can be reduced with hydrogen. This enables the representation of a Iron powder with a crystallite size of less than 5000 A.

In summary, the method according to the invention is characterized by the following main features: A mixture of finely ground iron oxide or oxide hydrate, an alkali oxide, hydroxide or carbonate and an alkaline earth oxide or hydroxide or an oxide-forming alkaline earth compound with certain proportions of the components is prepared and added to a Temperature between 500 and 800 ° C heated; the reaction product is cooled down to ^T W ater decomposed, drained and then filtered; the residue obtained is then reduced in a manner known per se at temperatures below 500.degree.

Any iron oxide or iron oxide hydrate that is not too large in grain size can be used as the oxidic iron compound, for example a — Fe ₂ O ₃ .

The main alkaline earth components are the oxides, hydroxides and carbonates of calcium, barium and Suitable for strontium. Other compounds which decompose to oxides under the process conditions, such as the formates and oxalates, etc., are also usable. The alkaline earth compound is in such ίο Amount introduced into the mixture that the ratio of iron to alkaline earth in the mixture is equal to or greater 9: 1 and the end product contains between 0.1 and 10 percent by weight of alkaline earth metal. Probably lies this in the form of the oxide.

The hydroxides or carbonates of potassium or sodium are preferred as alkali compounds used. The amount must be sufficient to remove the oxidic iron compound during the heating of the Mixture completely converted into the alkali ferrite. An excess over the stoichiometric amount is allowed.

Those to be observed during the heating for ferrite formation and molecular distribution of the alkaline earth compound Temperatures depend on the type of alkali compound used. Using of alkali hydroxides they are 500 to 800 ° C, the mixture comes here as a result of melting Hydroxyds in a semi-liquid to liquid state, depending on the amount of the hydroxide. When using Alkali carbonates, temperatures between 700 and 800 ° C are required, there is no melt flow, but a strong sintering. The heating itself can take place in air or in a non-oxidizing atmosphere be made, e.g. B. under nitrogen.

The last-mentioned way of working is slightly more expensive, but on the other hand leads to even more uniformity and better powders. In general, the coercive force is also that from heated under nitrogen Powder-made magnets are slightly larger.

The further processing of the heating products - cooling, leaching, filtering and reducing - takes place essentially according to known working methods. The reduction is at temperatures carried out below 500 ° C and preferably between 250 and 500 ° C. Such mild reduction conditions are possible because of the process According to the invention, extremely finely divided iron oxides or oxide hydrates are reduced by decomposition of a ferrite was created at normal temperature and was never exposed to elevated temperatures have been. Although the reduction itself does not constitute a feature of the invention, the application is milder However, reducing conditions (250 to 500 ° C) is required and supported by the presence the molecularly distributed alkaline earth compound, the forced formation of iron particles is lower Crystallite size. The reduction is expedient at space velocities of at least 300 l / hour.

Hydrogen carried out on 10 to 15 g iron powder, even higher space velocities (e.g. 600 1 / hour. 10 to 15 g each) and shorter reduction times are even more favorable because of the magnetic properties the end products (coercive force, remanent induction, etc.) can be further improved as a result.

How one proceeds in detail, in each case, when adhering to the invention Conditions obtained iron powder, which are characterized by crystallite sizes below 5000 Å. Of course contain the iron powder in addition to the basic

Substance ferrous metal smaller amounts of alkaline earth and - if the reduction is not quantitative up to At the end of the process - minor remnants of iron oxide. Those that are effective in the end product and far more in the powder The predominant components, however, are the iron particles with crystallite sizes below 5000 Å. It is therefore it is justified to speak simply of "iron powder" in the description and in the claims. Of course, this includes the aforementioned, consisting essentially of metallic iron particles Understand product. The alkali component usually becomes quantitative by the water leaching removed. If traces of alkali remain in the end product due to incomplete leaching, so do not disturb them.

It has already been indicated that the alkaline earth compound added to the starting mixture and almost molecularly distributed by the heating process with alkali ferrite formation probably acts as a crystallization inhibitor in the reduction of the oxide powder. The processes involved have not yet been clarified in detail. However, it has clearly been shown that _a5 manentmagnete are obtained from the present invention prepared with the addition of alkaline earth iron powders by pressing and magnetizing per- whose coercive force is considerably greater than that of the magnets, but made in otherwise identical manner in the absence of alkaline earth compound have been. Particularly good results are achieved when using calcium compounds.

For further explanation, some exemplary embodiments of the method of the invention are given below described.

Examples

35

1. An intimate mixture of 1520 g of small particle size O _{-Fe 2 O 3} and 74 g of powdered calcium hydroxide was heated to 750 ° C. for 15 minutes after addition of an amount of potassium hydroxide sufficient to achieve a semi-liquid consistency. After cooling, leaching with water and filtration, the residue was washed with boiling water, dried and comminuted to a particle size of about 1 to 3 mm. The granulate was then 4 hours at 350 ° C in a dry, oxygen-free hydrogen stream of about 300 l / h. and 15 g of granules reduced.

The powder obtained consisted mainly of metallic iron; it contained small amounts Iron oxide and 3.6 percent by weight calcium. This was evenly distributed in the iron powder, probably in the form of a solid solution of calcium oxide in iron oxide. The powder was in a container collected with dry, oxygen-free hydrogen. To the powder even during the later To protect against atmospheric oxidation, it was treated with a solution of a phenol-formaldehyde resin impregnated in acetone and then evaporated the acetone. The amount of used Resin was so large that the dry powder contained approximately 5 weight percent resin, which encased the individual metal particles.

To make permanent magnets, the resin-coated iron powder in a metal mold was subjected to a pressure of about 12,500 kg / cm ² . A compact obtained in this way had a density of 3.92. After magnetization in a magnetic field of 10,000 örsted, the coercive force was 530 örsted and the remanent induction was 3200 Gauss.

For comparison, a second press magnet was manufactured using the same method, which -from an in iron powder produced in the same way, with the only difference that the initial mixture contained no calcium hydroxide. This Magnet had a density of 3.88, a coercive force of 330 örsted and a remanent induction of 2400 Gauss.

2. There was an intimate powder mixture of 1520 g of orferrioxide according to Example 1, 74 g of pulverized Calcium hydroxide and 1311 g of potassium carbonate, by crushing in methyl alcohol for 50 hours manufactured. This mixture was dried and sintered at a temperature of 800 ° C. for 1 hour. After cooling, the product was treated with water, filtered, granulated and in the same way reduced as in example 1.

The powder obtained was coated with phenol-formaldehyde resin and pressed by applying pressure and magnetized. The obtained magnet had a density of 3.8, a coercive force of approximately 500 örsted and a remanent induction of 2200 Gauss.

3. The iron powder was prepared as in Example 1, but the initial heating was carried out in made and on an oxygen-free nitrogen atmosphere at a temperature of 800 ° C 1 hour extended. The reduction and further treatment of the powder to produce the magnets was made in the same manner as in Example 1.

The magnet obtained had a density of 3.6, a coercive force of 590 örsted and a remanent Induction of 2000 Gauss.

4. The iron powder was produced in the manner described in Example 3, but the Reduction at 350 ° C for 2 hours in a hydrogen stream of 600 1 / hour. made for 15 g of powder.

A from this powder in the same W ^T else as prepared in Example 1 magnet had a density of 3.6, a coercive force of 620 oersteds and a residual induction of 2700 Gauss. For comparison, a second magnet was produced in exactly the same way, but the starting mixture, which was otherwise prepared from the same substances, did not contain any calcium hydroxide. This magnet had a density of 3.8, a coercive force of 360 örsted and a remanent induction of 2000 Gauss.

Claims (7)

Patent claims: 1. Process for the production of consisting essentially of metallic iron particles Iron powders suitable for making permanent magnets with crystallite sizes of less than 5000 Å, characterized in that a mixture of finely ground Iron oxide or oxide hydrate, an alkali oxide, hydroxide or carbonate in at least the for Ferrite formation stoichiometric amount and an alkaline earth oxide or hydroxide or an oxide-forming Alkaline earth compound (iron to alkaline earth equal to or greater than 9: 1) at 500 to 800 ° C heated, the reaction product filtered after leaching with water and the residue in itself is heated in a known manner at temperatures below 500 ° C reducing to the metal stage. 7 8 2. The method according to claim 1, characterized 4. The method according to any one of claims 1 to 3, distinguishes that when using an alkali oxide, characterized in that the heating of the or -hydroxvds heats the mixture to 500 to 800 ° C starting mixture in an oxygen-free sticker will. Substance atmosphere takes place. 3. The method according to claim 1, characterized in 5 indicates that when using an alkali car, the following documents are considered: bonats the mixture heated to 700 to 800 ° C Abeggs "Handbook of Inorganic Chemistry", will. Vol. 4, 3rd section, second part B, 1935, pp. 290 and 292. © SOS 530/371 6, 58