Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2002-11-25
2004-03-09
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S349000, C075S363000
Reexamination Certificate
active
06702870
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a new iron-based powder material and a process for the preparation thereof. Particularly the invention concerns a process permitting the preparation of a new powder, which as such or further processed is useful within a wide variety of different fields and which has magnetic and electric properties.
BACKGROUND OF THE INVENTION
Iron-based powders having magnetic and electric properties are used for many different purposes and within a wide variety of industrial fields. An example of such a field is electrophotograpy wherein iron-based powders may be used as carrier cores or as magnetic pigments in toners. Another field is magnetic core components where iron-based powders are compacted to obtain a specific component useful in e.g. electric circuits. Very fine (small particle) iron-based powders made by e.g. the carbonyl process are also used in magnetic core components, electromagnetic interference/shielding, for parts made by metal injection moulding and in magnetic fluids.
The present invention concerns a new powder which as such or when further processed is potentially useful for all these materials as well as for other applications where specific magnetic and/or electric properties are required. The powder may also be used in such fields where magnetic and/or electric properties are not necessary, e.g. within the food or pharmaceutical industries
A method of preparing an iron-based powder suitable for use within these technical fields is disclosed in the U.S. Pat. No. 5,538,656. More particularly, this patent concerns a magnetic oxide, which is produced by mixing a magnetisation control material (Ca, Ti, Sn, Si, Mg) with hematite and/or magnetite and sintering the obtained mixture in an inert gas at 550° C. to 1500° C. The desired magnetic oxide, which according to this patent can be easily mass-produced, is single phase magnetite (Fe
3
O
4
). Furthermore, the patent discloses that if the reaction parameters are not accurately selected other magnetic oxides, such as wustite (FeO) and hematite (Fe
2
O
3
) may be obtained as undesired impurities. The magnetic oxide prepared according to this patent has saturation magnetisation values varying between about 90 (when no addition of magnetisation control agent is made) to about 10 emu/g (when about 80% by weight magnetisation control agent is added).
The use of silica in iron-based powder is also disclosed in the Japanese patent publication 63-184764. Also this publication discloses the use of inert atmosphere and the obtained product mainly consists of magnetite and a non magnetic oxide. The purpose of the invention according to this publication is to reduce the saturation magnetisation to values lower than that of pure magnetite. About the same values of saturation magnetisation as in the US patent are obtained.
It has now been found that these values can be remarkably increased by performing the process in a reducing atmosphere in stead of an inert atmosphere. For comparable additions of magnetisation control agent it has thus unexpectedly been found that materials having magnetic saturation values ranging from 220 to 30 emu/g can be prepared.
Another publication which discloses silicon containing iron based powders is the Japanese patent publication 61-155595, which concerns a technology for the production of ferroalloy particles containing 0.1-5.0 atom-% silica. This process involves a reduction step of a silicate containing magnetite material. The powder obtained after the reduction had a magnetic saturation between 90 and 184 emu/g and remanence between 7 and 24 and coercivity between 80 and 205 Oe. Average particle sizes of less than 0.25 micron were obtained. A main purpose of the invention according to this publication is to prepare spherical particles for use as magnetic toners and carriers for electrostatic copying. A necessary feature of the invention according to this invention is a comparatively complicated precipitation process for the preparation of the silicate containing magnetite material, i.e. the starting material which is subjected to the reduction.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a simple and economically attractive process which makes it possible to prepare an iron based powder having a very fine particle size.
A second object is to provide a process for the preparation of an iron-based powder material having magnetic and electric properties, which can be adapted to a wide variety of different needs.
A third object is to provide a process for the preparation of an iron-based powder having a saturation magnetisation, which may be varied within a broad interval with maintained good soft magnetic properties, such as low coercivity and remanence.
A forth object is to provide an iron-based powder the soft magnetic properties of which are uniquely good in relation to the particle size.
A fifth object is to provide a new material that is environmentally friendly.
SUMMARY OF THE INVENTION
The iron-based powder according to the invention includes at least 0.5% of iron containing silicate and at least 10% by weight of metallic iron and/or alloyed iron and is prepared by mixing an iron containing powder and a silicon containing powder and reducing the obtained mixture at a temperature above about 450° C.
The new material according to the present invention essentially consists of a compounded structure comprising mainly a mixture of iron, which is present in the form of metallic iron and/or alloyed iron, iron containing silicate and minor amounts of iron oxides
The critical features of the new material according to the present invention are the content of metallic and/or alloyed iron, which has an impact on the magnetic and electrical properties, and the silicate content, which has an influence on the apparent density. By varying the amount of silicate and metallic and/or alloyed iron the properties of the base material may be tailored in view of the required properties of the final product.
DETAILED DESCRIPTION OF THE INVENTION
The iron-based starting material is preferably ground magnetite and/or hematite or another iron-containing compound, such as hydrated ferric oxides, such as goethite ore lepidocrocite, or ferric salts. Especially preferred in this respect are highly purified ores such as benificated ores. Instead of ground magnetite and/or hematite ore it is of course also possible to use synthetically prepared magnetite or hematite.
The silica containing starting material is preferably a silicon containing oxide such as silica, but other forms such as intermetallic silica or mixed silicas (containig elements like Mg, Ca, Al, Na) such as Olivine ((Fe,Mg)
2
SiO
4
) or Alumino-silicates such as ((Na,Al,Si)
2
O
6
) could be used as well.
Optionally the starting materials to be mixed may also contain a minor amount of an additive known to improve a magnetic and/or electrical property such as Ni, Sn, P, Co, Al and Mo. Ni, Sn, Co, Al and Mo may be added in metallic or intermetallic form and P may be added as ferrophosphorus.
The starting materials may be subjected to conventional dry blending. According to a preferred embodiment of the invention the starting materials are agglomerated before the reduction.
Agglomeration
A long range of methods to agglomerate powders exist and depending on the requirements in the final application the most suitable method may be selected. An example of a very simple technology is adding a binder (such as PEG) to raw powders followed by mixing whereupon agglomerates may be formed. The mixing procedure and equipment may vary. Specifically the new base material may be prepared by dispersing selected starting materials, the iron-containing powder and the silica-containing material, in a liquid, preferably water, in order to obtain a slurry which is subsequently spray-dried to form agglomerates. Preferably the slurry is an aqueous slurry containing e.g. 0.01-2% by weight of a binding agent, such as polyvinyl alcohol, methyl cellulose and/or carbowax. Irrespectively of the origin of the starting mat
Benemark Britt
Grahn Peter
Grahn Svante
Hultman Lars
Jonsson Nils
Burns Doane Swecker & Mathis L.L.P.
Grahn Svante
Hëganäs AB
Mai Ngoclan
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