Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Reexamination Certificate
2000-09-26
2003-07-08
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
C428S570000, C428S681000, C428S687000, C428S402000, C075S343000, C075S010630, C075S770000, C075S953000
Reexamination Certificate
active
06589667
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel iron powder and a method for the preparation thereof.
BACKGROUND OF THE INVENTION
Iron powders of various types are well known and widely used for different applications within the chemical field.
Thus, iron powders are often used as raw materials for preparing iron containing compounds, such as inorganic and organic salts, iron oxide pigments and chelates. Iron powders are also used as auxiliary materials in various chemical processes and operations, where the powders are used e.g. as reduction agents, catalysts and extraction media for so-called cementation processes.
The rusting reaction of iron powder has two important features which are utilised in two important applications, namely its exothermic (heat generating) nature in hot-bags applications and the fact that oxygen is consumed during the reaction in oxygen absorption applications.
Several demands must be met by the iron powder material, which is to be used in any of the above mentioned chemical applications. The iron powder should have a high purity, be highly reactive, have a good flowability in order to improve the handling of the powder, have a high and active surface area, a high reactivity and good permeability for gases and liquids.
Iron powders are also used in electrophotography as carriers for toner particles.
The carrier should fulfil the following criteria. The carrier should have an appropriate triboelectric property which enables it to electrostatically hold the toner particles and to transfer the held toner particles to an electrostatic latent image on a photosensitive plate when contacted. The carrier should have a sufficient mechanical strength to protect the carrier particles from breaking or cracking and a good fluidity. The carrier particles should be uniform in their electric and magnetic properties, stable with respect to changes in the environmental conditions, such as humidity and a sufficient durability to ensure an acceptable lifetime.
At present, there are very few, if any, iron powder materials available which are useful in all the above mentioned applications. The main purpose of the present invention is to provide such an iron powder.
SUMMARY OF THE INVENTION
In brief the present invention concerns a powder useful in the above mentioned applications as well as a method for the preparation of this powder.
The most characterising features of the new powder are that the particles have an essentially spherical form, a porous structure throughout and a high specific surface area. Furthermore, the powder is distinguished by a low apparent density, a high saturation magnetisation, a high reactivity and a good flowability.
According to the present invention this new powder may be prepared by subjecting a dry powder of essentially spherical iron-containing agglomerates to a heat treatment in a reducing atmosphere at a temperature and time sufficient for obtaining particles essentially consisting of metallic iron and having a porous structure throughout. The obtained particles may then be subjected to sintering at a time and temperature sufficient for obtaining the required strength.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention the dry powder of essentially spherical agglomerates is prepared from a slurry of a selected starting material in a liquid, preferably water.
The starting material is preferably ground magnetite and/or hematite or another iron-containing compound, such as hydrated ferric oxides, such as goethite or 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. Preferably the slurry is an aqueous slurry containing e.g. 0.01-2% by weight of a binding agent, such as polyvinylalcohol, methyl cellulose and/or carbowax. Irrespectively of the origin of the starting material, the majority of the particles of this material in the slurry should have an average particle size of less than 15 &mgr;m preferably less than 5 &mgr;m. During the spray-drying spherical agglomerates of the particles of the iron-containing starting material are formed. Larger particle sizes than 15 &mgr;m could potentially be used. This would however result in more irregularly shaped agglomerates, especially for smaller agglomerate sizes.
The size of spray-dried agglomerates can be controlled and is determined by the conditions during this drying step, such as the composition of the slurry, the flow, the pressure and the type of rotary or spray nozzle used, and is selected by a man skilled in the art in view of the intended use of the final powder. Normally, the particles of the spray-dried powder have an average diameter within the range of from about 10 to 250 &mgr;m. Factors useful for controlling the porosity of the final powder particles are the particle size of the starting material and the sintering conditions.
As mentioned before relatively irregular particles could be obtained with this process e.g. by choosing the spray drying parameters and raw materials in such a way that more irregular shaped particles are formed. The main purpose is however to select the conditions is such a way that essentially spherical particles are formed.
The reduction of the spray-dried powder is important and the atmosphere, temperature and time conditions should preferably be selected so that an essentially complete reduction is obtained in order to get particles of essentially pure metallic iron having a porous structure throughout and a high specific surface area. However, for certain applications it may be sufficient or even preferred not to perform a complete reduction to metallic iron. In these cases the final powder particles include a minor amount of the starting material, e.g. magnetite or hematite. The extent of the reduction in each specific case can thus be tailored by the man skilled in the art depending on the intended final use of the powder.
The reduction is preferably performed in a continuous furnace or batch furnace in an atmosphere of hydrogen or hydrogen-containing nitrogen, cracked ammonia or carbon monooxide or combinations thereof. The reduction may also be performed in the presence carbon containing material.
In a preferred embodiment of the present method, the spray-dried spherical powder is reduced in a hydrogen based atmosphere at a temperature within the range of from about 450 to 700° C., preferably from about 500 to 6000° C. during a period of about 1 to 4 hours. The time required for the reduction will heavily depend on the type of furnace used, the amount of material in the furnace and the flow of gas etc. For this reason both shorter and longer tie might be needed than the 1-4 hours time range indicated above. The reduction to metallic iron in essentially pure hydrogen gas is preferably performed at temperatures below 600° C., and most preferably between 530 and 570° C. e.g. at 550° C.
As an alternative, the spray-dried spherical powder agglomerates is subjected to reduction in a carbon monoxide based atmosphere at a temperature within the range of from about 700 to 1300° C.
In yet another embodiment of the reduction, the spherical spray-dried material is subjected to reduction in the presence of carbon-containing material such as graphite at a temperature of from about 1100 to 1300° C., preferably about 1200° C.
According to a preferred embodiment the reduction is performed during such conditions that spherical particles consisting of essentially pure iron is obtained.
If necessary and in order to obtain a sufficient mechanical strength the reduced powder is subjected to a sintering process, which may be performed in the same atmospheres as indicated above or modifications thereof at temperatures within the range of from about 800 to 1300° C., preferably from about 800 to 1100° C. The sintering step is of particular interest when the reduction is performed at low temperature.
In order to remove the binding agent a
Allroth Sven
Hultman Lars
Burns Doane Swecker & Mathis L.L.P.
Hëganäs AB
Le H. Thi
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