Phosphorus-containing iron powders

Details Phosphorus-containing iron powders Phosphorus-containing iron powders

1998-02-12

2001-01-30

Le, Hoa T. (Department: 1773)

Stock material or miscellaneous articles

Coated or structually defined flake, particle, cell, strand,...

Particulate matter

C423S138000, C423S322000

Reexamination Certificate

active

06180235

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to phosphorus-containing iron powders and a process for preparing them.
DESCRIPTION OF THE PRIOR ART
Certain applications, for example in powder metallurgy, require metal powders having defined mechanical properties. Suitable powders for such applications are, for example, powders of iron-phosphorus alloys whose mechanical properties such as hardness and brittleness can be set by means of the phosphorus content.
Gmelins Handbuch der Anorganischen Chemie, Volume Iron, Part A, Section II, 8th Edition 1934/39, pages 1784-85 describes classical methods of preparing iron-phosphorus alloys or iron phosphides (having an integral iron-phosphorus ratio). In these methods, iron-phosphorus alloys or iron phosphides are prepared directly from the elements, by reduction of phosphorus oxides in the presence of iron or by coreduction of phosphorus and iron compounds.
Thus, preparations having a phosphorus content of up to 30% by weight can be prepared by melting iron together with red phosphorus under a nitrogen atmosphere or by action of phosphorus vapor on red hot iron. Higher phosphides having a phosphorus content of over 50% by weight are formed on heating the lower phosphides in an atmosphere of saturated phosphorus vapor.
Iron-phosphorus alloys can also be prepared by melting a mixture of iron turnings and P
2
O
5
with powdered carbon or without addition of carbon. Iron-phosphorus alloys and iron phosphides are also formed in the reduction of Fe
3
PO
4
by hydrogen or carbon or in the reduction of a mixture of calcium phosphate and Fe
2
O
3
by carbon.
The processes mentioned generally require high temperatures. In order to react iron with phosphorus, the former has to be heated at least to red heat. Furthermore, the iron-phosphorus alloys obtained by reduction have a high content of secondary constituents.
The production of phosphorus by reduction of phosphate-containing iron ores in an electric furnace produces an alloy of iron and phosphorus, ferrophosphorus, containing from 20 to 27% by weight of phosphorus, as by-product. Secondary constituents present in ferrophosphorus are 1-9% of silicon and further metals such as titanium, vanadium, chromium and manganese.
The iron-phosphorus alloys produced by the abovementioned processes are unsuitable for applications in which high-purity iron powders having a defined phosphorus content and particle sizes of <50 &mgr;Am are required.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for preparing phosphorus-containing iron powder having a phosphorus content which can be varied within wide limits and having a proportion of secondary constituents which is as low as possible.
We have found that this object is achieved by starting out from known processes for preparing phosphorus-containing iron powder in which metallic iron is heated with elemental phosphorus and the product obtained is comminuted to give a powder and, according to the present invention, using metallic iron in the form of finely divided carbonyl iron.
For the purposes of the present invention, finely divided carbonyl iron is carbonyl iron powder and/or carbonyl iron whiskers.
DETAILED DESCRIPTION OF THE INVENTION
Carbonyl iron powder and carbonyl iron whiskers can be obtained according to known processes by thermal decomposition of iron pentacarbonyl in the gas phase, as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A 14, page 599 or in DE 3 428 121 or in DE 3 940 347, and consist of particularly pure metallic iron. The high purity of the powder or whiskers is the result of the high purity of the iron pentacarbonyl. Powder or whiskers are formed depending on the decomposition conditions (pressure, temperature).
Carbonyl iron powder is a gray, finely divided powder of metallic iron having a low content of secondary constituents and consisting essentially of spherical particles having a mean particle diameter up to 10 &mgr;m.
In the process of the present invention, it is possible to use mechanically hard, unreduced carbonyl iron powders or mechanically soft, reduced carbonyl iron powders.
The unreduced carbonyl iron powders which are preferably used in the process of the present invention have an iron content of >97% by weight, a carbon content of <1.0% by weight, a nitrogen content of <1.0% by weight and an oxygen content of <0.5% by weight. The mean diameter of the powder particles is preferably from 1 to 10 &mgr;m, particularly preferably from 1.5 to 5.0 &mgr;m, and their specific surface area (BET) is preferably from 0.2 to 2.5 m
2
/g.
The reduced carbonyl iron powders which are preferably used in the process of the present invention have an iron content of >99.5% by weight, a carbon content of <0.06% by weight, a nitrogen content of <0.1% by weight and an oxygen content of <0.4% by weight. The mean diameter of the powder particles is preferably 1-8 &mgr;m, particularly preferably 4.0-8.0 &mgr;m. The specific surface area of the powder particles is preferably 0.2-2.5 m
2
/g.
Carbonyl iron whiskers are very fine, polycrystalline iron threads. The carbonyl iron whiskers which are preferably used in the process of the present invention consist of thread-like arrangements of spheres having sphere diameters of 0.1-1 &mgr;m, with the threads being able to have different lengths and being able to form tangles or knots, and have an iron content of >83.0% by weight, a carbon content of <8.0% by weight, a nitrogen content of <4.0% by weight and an oxygen content of <7.0% by weight.
The carbonyl iron powders and whiskers which are preferably used in the process of the present invention have a very low content of extraneous metals which is usually below the detection limit of atomic absorption analysis and is the result of their preparation from the very pure starting compound iron pentacarbonyl. The carbonyl iron powders contain, inter alia, the following proportions of further extraneous elements: nickel <100 ppm, chromium <150 ppm, molybdenum <20 ppm, arsenic <2 ppm, lead <10 ppm, cadmium <1 ppm, copper <5 ppm, manganese <10 ppm, mercury <1 ppm, sulfur <10 ppm, silicon <10 ppm and zinc <10 ppm.
Preference is given to using carbonyl iron powder in the process of the present invention.
Elemental phosphorus can be used in all known modifications, ie. as white, red, black or violet phosphorus. In the process of the present invention, preference is given to using red phosphorus. The red phosphorus used in the process of the present invention can additionally contain, in particular, water as a secondary constituent.
The reaction is carried out at a temperature above room temperature. For example, the reaction vessel used can be a heatable tube made of a heat-resistant material such as quartz. Carbonyl iron powder or whiskers and elemental phosphorus are intensively mixed and the reaction mixture of carbonyl iron powder or whiskers and elemental phosphorus is heated in the reaction vessel until the exothermic reaction commences. After the reaction has commenced, the temperature can rise further as a result of the heat of reaction. The reaction is preferably carried out at above 300° C., particularly preferably at from 380° C. to 550° C.
The reaction is preferably carried out under substantial exclusion of atmospheric oxygen. This can be achieved, for example, by carrying out the reaction in an inert gas atmosphere. The reaction is preferably carried out in an inert gas atmosphere of nitrogen, and is preferably carried out at atmospheric pressure.
An advantage of the process of the present invention is that the iron/phosphorus ratio of the powder can be varied as desired by selection of the starting composition.
Carbonyl iron powder and phosphorus are preferably reacted in a mass ratio of from 99.9:0.1 to 30:70, particularly preferably from 99:1 to 70:30.
Depending on the starting composition selected, the phosphorus content of the phosphorus-containing iron powder obtained c

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