U.S. patent application number 10/865406 was filed with the patent office on 2004-11-18 for magnetite-iron based composite powder, magnetite-iron based powder mixture, method for producing the same, method for remedying polluted soil, water or gases and electromagnetic wave absorber.
This patent application is currently assigned to JFE Steel Corporation, a corporation of Japan. Invention is credited to Nakamaru, Hiroki, Nakamura, Yukiko, Ozaki, Yukiko, Saito, Shingo, Takajo, Sawae, Takajo, Shigeaki, Uenosono, Satoshi, Unami, Shigeru.
Application Number | 20040226404 10/865406 |
Document ID | / |
Family ID | 19176406 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040226404 |
Kind Code |
A1 |
Ozaki, Yukiko ; et
al. |
November 18, 2004 |
Magnetite-iron based composite powder, magnetite-iron based powder
mixture, method for producing the same, method for remedying
polluted soil, water or gases and electromagnetic wave absorber
Abstract
A magnetite-iron based composite powder includes magnetite with
a ratio of X-ray diffraction intensity to that of .alpha.-Fe of
about 0.001 to about 50 and has an average primary particle size of
about 0.1 to about 10 .mu.m. The composite powder can highly
dehalogenate organic halogen compounds and exhibits satisfactory
absorption power of high frequency electromagnetic waves after
molding. An ultrafine nonferrous inorganic compound powder may
adhere to the surface of the composite powder, or at least the
composite powder may adhere to the surfaces of small particles of a
nonferrous inorganic compound to thereby yield a composite powder
composition. The composite powder can be produced by partial
reduction of a material powder containing a hematite based powder
or by complete reduction and subsequent partial oxidation of the
material powder.
Inventors: |
Ozaki, Yukiko; (Chiba,
JP) ; Uenosono, Satoshi; (Chiba, JP) ;
Nakamaru, Hiroki; (Chiba, JP) ; Nakamura, Yukiko;
(Chiba, JP) ; Takajo, Shigeaki; (Chiba, JP)
; Takajo, Sawae; (Chiba, JP) ; Unami, Shigeru;
(Chiba, JP) ; Saito, Shingo; (Chiba, JP) |
Correspondence
Address: |
IP DEPARTMENT OF PIPER RUDNICK LLP
ONE LIBERTY PLACE, SUITE 4900
1650 MARKET ST
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE Steel Corporation, a
corporation of Japan
Tokyo
JP
|
Family ID: |
19176406 |
Appl. No.: |
10/865406 |
Filed: |
June 10, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10865406 |
Jun 10, 2004 |
|
|
|
10157604 |
May 29, 2002 |
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Current U.S.
Class: |
75/252 ;
75/348 |
Current CPC
Class: |
C01P 2002/52 20130101;
C01P 2002/54 20130101; C01P 2004/64 20130101; C01P 2004/61
20130101; C01G 49/06 20130101; C01P 2006/10 20130101; C01G 51/006
20130101; C01P 2006/80 20130101; C01P 2002/74 20130101; C01P
2004/45 20130101; C01G 49/08 20130101; B82Y 30/00 20130101; C01P
2006/42 20130101; C01P 2006/60 20130101; C01G 53/006 20130101; C01G
49/009 20130101; C01G 53/00 20130101; C01G 23/003 20130101; C01P
2004/62 20130101 |
Class at
Publication: |
075/252 ;
075/348 |
International
Class: |
C22C 001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2001 |
JP |
2001-366521 |
Claims
1-9. (cancelled)
10. (original) a method for producing a magnetite-iron based
composite powder comprising: reducing by heating a hematite based
powder having an average primary particle size of from about 0.01
to about 10 .mu.m in a reducing gas; and stopping reduction at
about midstream of reduction to yield a partially reduced powder
which is a composite powder comprising magnetite and iron.
11. A method for producing a magnetite-iron based composite powder
mixture comprising: reducing by heating a hematite based powder
having an average primary particle size of from about 0.01 to about
10 .mu.m in a reducing gas and in the presence of a nonferrous
inorganic compound powder; and stopping reduction at about
midstream of reduction to yield a partially reduced powder which is
a composite powder comprising magnetite and iron.
12. A method for producing a magnetite-iron based composite powder
comprising: heating a hematite based powder having an average
primary particle size of from about 0.01 to about 10 .mu.m in a
reducing gas to reduce the powder substantially completely; and
oxidizing a surface of resulting substantially completely reduced
powder with an oxygen-containing gas to thereby yield a composite
powder comprising magnetite and iron.
13. A method for producing a magnetite-iron based composite powder
mixture comprising: heating a hematite based powder having an
average primary particle size of from about 0.01 to about 10 .mu.m
in a reducing gas in the presence of a nonferrous inorganic
compound powder to reduce the powder substantially completely; and
oxidizing a surface of resulting substantially completely reduced
powder with an oxygen-containing gas to thereby yield a composite
powder comprising magnetite and iron.
14. A method for producing a magnetite-iron based composite powder
comprising: heating a hematite based powder having an average
primary particle size of from about 0.01 to about 10 .mu.m in a
reducing gas to thereby partially reduce the powder; stopping
reduction at about midstream of reduction to yield a partially
reduced powder; and oxidizing a surface of the partially reduced
powder with an oxygen-containing gas to thereby yield a composite
powder comprising magnetite and iron.
15. A method for producing a magnetite-iron based composite powder
mixture comprising: reducing by heating a hematite based powder
having an average primary particle size of from about 0.01 to about
10 .mu.m in a reducing gas in the presence of a nonferrous
inorganic compound powder; stopping reduction at about midstream of
reduction to yield a partially reduced powder; and oxidizing a
surface of the partially reduced powder with an oxygen-containing
gas to thereby yield a composite powder comprising magnetite and
iron.
16. A method for remedying polluted media comprising: bringing a
magnetite-iron based composite powder comprising magnetite and iron
and having an average primary particle size of from about 0.01 to
about 10 .mu.m into contact with a media contaminated with an
organic halogen compound; and causing the organic halogen compound
to decompose.
17. A method for remedying polluted media comprising: bringing a
magnetite-iron based composite powder mixture comprising magnetite
and iron and having an average primary particle size of from about
0.01 to about 10 .mu.m and a nonferrous inorganic compound powder
into contact with a media contaminated with an organic halogen
compound; and causing the organic halogen compound to
decompose.
18. An electromagnetic wave absorber comprising a compacted mixture
of a magnetite-iron based composite powder comprising magnetite and
iron and having an average primary particle size of from about 0.01
to about 10 .mu.m and a rubber, a resin or a mixture thereof.
19. An electromagnetic wave absorber comprising a compacted mixture
of a magnetite-iron based composite powder mixture comprising
magnetite and iron and having an average primary particle size of
from about 0.01 to about 10 .mu.m and a nonferrous inorganic
compound powder and a rubber, a resin or a mixture thereof.
20. The method according to claim 16, wherein the ratio of maximum
diffraction intensity of magnetite to that of .alpha.-Fe in X-ray
diffraction is from about 0.001 to about 50.
21. The method according to claim 16, wherein the powder further
comprises at least one component selected from the group consisting
of nickel, cobalt, chromium, manganese and copper.
22. The method according to claim 16, wherein the ratio of maximum
diffraction intensity of magnetite to that of .alpha.-Fe in X-ray
diffraction is from about 0.001 to about 50, and the powder further
comprises at least one component selected from the group consisting
of nickel, cobalt, chromium, manganese and copper.
23. The method according to claim 16, wherein the powder further
comprises nickel.
24. The method according to claim 17, wherein an average primary
particle size of the nonferrous inorganic compound powder is less
than or equal to about 0.1 .mu.m and is less than that of the
magnetite-iron based composite powder, and wherein the nonferrous
inorganic compound powder adheres to a surface of the
magnetite-iron based composite powder.
25. The method according to claim 17, wherein an average primary
particle size of the nonferrous inorganic compound powder is
greater than or equal to about 1 .mu.m and less than or equal to
about 100 .mu.m and is greater than that of the magnetite-iron
based composite powder, and wherein the magnetite-iron based
composite powder adheres to a surface of the nonferrous inorganic
compound powder.
26. The method according to claim 17, wherein the nonferrous
inorganic compound powder comprises a first nonferrous inorganic
compound powder and a second nonferrous inorganic compound powder,
wherein an average primary particle size of the first nonferrous
inorganic compound powder is less than or equal to about 0.1 .mu.m,
wherein the average primary particle size of the second nonferrous
inorganic compound powder is greater than or equal to about 1 .mu.m
and less than or equal to about 100 .mu.m and is greater than that
of the magnetite-iron based composite powder, and wherein the
magnetite-iron based composite powder and the first nonferrous
inorganic compound powder adhere to a surface of the second
nonferrous inorganic compound powder.
27. The composition according to claim 18, wherein the ratio of
maximum diffraction intensity of magnetite to that of .alpha.-Fe in
X-ray diffraction is from about 0.001 to about 50.
28. The composition according to claim 18, further comprising at
least one component selected from the group consisting of nickel,
cobalt, chromium, manganese and copper.
29. The composition according to claim 18, further comprising at
least one component selected from the group consisting of nickel,
cobalt, chromium, manganese and copper.
30. The composition according to claim 18, further comprising
nickel.
31. The composition according to claim 19, wherein an average
primary particle size of the nonferrous inorganic compound powder
is less than or equal to about 0.1 .mu.m and is less than that of
the magnetite-iron based composite powder, and wherein the
nonferrous inorganic compound powder adheres to a surface of the
magnetite-iron based composite powder.
32. The composition according to claim 19, wherein an average
primary particle size of the nonferrous inorganic compound powder
is greater than or equal to about 1 .mu.m and less than or equal to
about 100 .mu.m and is greater than that of the magnetite-iron
based composite powder, and wherein the magnetite-iron based
composite powder adheres to a surface of the nonferrous inorganic
compound powder.
33. The composition according to claim 19, wherein the nonferrous
inorganic compound powder comprises a first nonferrous inorganic
compound powder and a second nonferrous inorganic compound powder,
wherein an average primary particle size of the first nonferrous
inorganic compound powder is less than or equal to about 0.1 .mu.m,
wherein the average primary particle size of the second nonferrous
inorganic compound powder is greater than or equal to about 1 .mu.m
and less than or equal to about 100 .mu.m and is greater than that
of the magnetite-iron based composite powder, and wherein the
magnetite-iron based composite powder and the first nonferrous
inorganic compound powder adhere to a surface of the second
nonferrous inorganic compound powder.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] This invention relates to a magnetite-iron based composite
powder, a magnetite-iron based composite powder mixture and a
method for producing the magnetite-iron based composite powder
mixture. The invention also relates to a method for remedying
polluted soil, water or gases with the aid of the reducing activity
of the magnetite-iron based composite powder and to an
electromagnetic wave absorber and other magnetic material using the
magnetism of the magnetite-iron based composite powder.
[0003] 2. Description of the Related Art
[0004] Iron powders are used as materials for powder metallurgy for
fabricating mechanical parts and magnetic parts such as magnetic
powder cores and electromagnetic wave absorbers. In addition, iron
powder is used as a powder in catalysts, food additives,
antioxidants and carriers for copier toner and for remedying soil
and other media. Conventional techniques and problems thereof will
be illustrated below specifically regarding methods for remedying
soil and other media using a fine iron powder as a reducing agent
and electromagnetic wave absorbers using the magnetic properties of
the iron powder.
[0005] Remediation of Soil and Other Media Using Iron Powder
[0006] Methods for remedying soil or groundwater polluted by an
organic halogen compound can roughly be classified as (1) a method
in which polluted soil or groundwater is decomposed in situ (an in
situ decomposition method), (2) a method for treating gases in the
polluted soil or polluted groundwater after pumping from the ground
(treatment after in situ extraction), and (3) a method for treating
the polluted soil after excavation (excavation treatment).
[0007] Methods using an iron powder as a reducing agent for
decontamination of harmful organic halogen compounds by
dehalogenation have been proposed. For example, PCT Japanese
Translation Patent Publication No. 5-50120 and Japanese Unexamined
Patent Application Publications No. 10-263522 each propose a method
for remedying soil and soil moisture by forming a dispersed iron
powder layer in the soil followed by bringing groundwater into
contact with the layer to thereby decompose organic halogen
compounds. Japanese Unexamined Patent Application Publication No.
11-235577 also proposes a method for remedying soil by adding iron
powder to and mixing with the soil (excavated or not) to thereby
decompose organic chlorine compounds.
[0008] The iron powder used in the former method (PCT Japanese
Translation Patent Publication No. 5-501520 and Japanese Unexamined
Patent Application Publications No. 10-263522) is, for example,
scrap iron produced in the cutting process of iron. It is hard to
control the composition and structure of the iron powder to be
suitable as a reducing agent for organic halogen compounds. As a
result, the iron powder exhibits insufficient effects. In addition,
the former two publications mention that iron oxides are formed on
the surfaces of iron particles by reaction with oxygen in the soil
to thereby deteriorate reduction power of the iron powder. As a
countermeasure to this problem, the publications also propose
deoxygenation of soil in the vicinity of the iron powder by
allowing a reducing substance to disperse in the soil. This means
that the iron powder used in this method does not have sufficient
durability in its reduction power.
[0009] The latter method (Japanese Unexamined Patent Application
Publication No. 11-235577) proposes an iron powder conitaining
equal to or more than 0.1% by weight of carbon and having a
specific surface area of equal to or more than 500 cm.sup.2/g. This
iron powder comprises sponge like particles having a pearlite
texture as a structure with a particle size distribution that
allows equal to or more than 50% by weight of the total powder to
pass through a 150 .mu.m sieve. However, even this configuration
may not effectively dehalogenate such organic halogen
compounds.
[0010] Japanese Unexamined Patent Application Publication No.
12-80401 proposes an iron powder containing 0.020 to 0.5% by weight
of phosphorus, sulfur or boron as iron powder that can effectively
remove phosphor compounds in drainage. The iron powder contains
specific trace elements and the objective effect thereof is to
accelerate decontamination of phosphor in the drainage by
increasing the dissolving speed of the iron into the drainage.
Specifically, according to the mechanism of the aforementioned iron
powder, a compound which hardly dissolves and has a low solubility
product constant, such as iron phosphate, is formed between the
dissolved iron and phosphor in the drainage to remove phosphor from
the drainage by precipitation. This technology is fundamentally
different from the technology for reductive decomposition of
harmful substances on the surface of iron according to this
invention.
[0011] Japanese Unexamined Patent Application Publication No.
2000-5740 proposes an iron powder containing 0.1 to 10% by weight
of copper as an iron powder that can efficiently remove organic
chlorine compounds from soil and/or groundwater. However, copper
itself is a harmful metal with a danger of causing secondary
pollution.
[0012] In addition, all of the aforementioned iron powders mainly
contain iron having a valency of zero (Fe.sup.0) and thereby
exhibit insufficient decomposition power for organic halogen
compounds.
[0013] An iron powder having an average primary particle size of
less than 1 .mu.m has not been readily available, and those having
a large average primary particle size of about 80 .mu.m have been
used for the above application. However, such iron powders having a
large particle size cannot sufficiently be dispersed into the soil
or groundwater, have a small specific surface area and thereby
cannot sufficiently decompose the organic halogen compounds with
efficiency. Even if such fine iron powder particles can be
obtained, their reduction power may be rapidly consumed.
[0014] Application of Iron Powder to Magnetic Materials
[0015] A carbonyl ion powder obtained by reduction of carbonyl
iron, and an acicular iron powder obtained by reduction of goethite
iron (acicular iron oxide) are widely used as magnetic materials
for use in electronic equipment and communication equipment.
However, a demand has been made on magnetic materials that can
exhibit their functions in the high frequency regions as a result
of recent advances in electronic and communication equipment.
[0016] The carbonyl iron powder comprises relatively large
particles with a particle size of about several micrometers and its
permeability decreases with increasing frequency. The carbonyl iron
powder cannot, therefore, be used in a noise filter or an
electromagnetic wave absorber in the high frequency regions with a
frequency on the order of gigahertz (Ghz).
[0017] The acicular iron powder comprises relatively small
particles with a particle size of about 0.1 .mu.m, but such
constitutive small particles easily aggregate to thereby form an
aggregate having a relatively large particle size. Accordingly, the
acicular iron powder is also limited in its application as in the
carbonyl iron powder.
[0018] In addition, the material carbonyl iron and goethite iron
cannot stably be manufactured in a high volume and are expensive,
thereby causing increased manufacturing costs.
[0019] As such an electromagnetic wave absorber for use in the high
frequency regions on the order of gigahertz, a sheet prepared by
molding a mixture of a flat powder and a resin is used (e.g., Yasuo
Hashimoto: "Ceramics" vol. 35, No. 10 (2000), p. 857-862). The flat
powder used herein is prepared by processing a Fe--Si alloy powder,
sendust powder or stainless steel powder into a flat powder.
However, such flat powders require expensive material powder and
expensive treatments for pressing the material powders, thereby
causing increased manufacturing costs of the product
electromagnetic wave absorber.
[0020] Japanese Unexamined Patent Application Publication No.
1-136910 proposes a method of manufacturing a reduced iron
including fewer impurities and oxide films by reducing an iron
oxide obtained from a pickling device for hoop steel. However, fine
particles of the resulting pure iron with a particle size of from
0.1 to 3.0 .mu.m are immediately oxidized in the air and thereby
undergo self-combustion due to oxidation heat.
SUMMARY OF THE INVENTION
[0021] Remediation of Soil and Other Media Using Iron Powder
[0022] Polluted groundwater may bring about far more crucial damage
over surface drainage, since identification of pollution sources is
usually difficult in polluted groundwater as compared to polluted
surface drainage. Accordingly, prompt decontamination of polluted
groundwater is urgently needed. Durability of the activity of the
iron powder as a reducing agent is also strongly required for using
the iron powder because the iron powder cannot frequently be
replaced. The organic halogen compounds may also be present as a
gas in the polluted soil and air different from the organic halogen
compounds in the drainage and groundwater. Therefore, it is
advantageous to establish a method for efficiently decontaminating
organic halogen compounds in the gas for remediation of the
polluted soil and air. Accordingly, it would be advantageous to
provide a method for rapidly decomposing the organic halogen
compounds, a fine composite iron powder suitable for decomposition,
and a method for producing the composite iron powder.
[0023] Application of Iron Powder to Magnetic Materials
[0024] It would also be advantageous to provide a composite
magnetic iron powder that is a low-cost magnetic material, can
exhibit its functions in the high frequency regions and is not
oxidized to generate heat even in air, a method for producing the
same, and an electromagnetic wave absorber using the composite
magnetic iron powder.
[0025] We have discovered that reduction of an iron oxide can yield
a composite iron powder including a different component and a
different texture from conventional fine iron powders and that the
resulting composite iron powder has advantageous properties.
Specifically, this invention provides, in a first aspect, a
magnetite-iron based composite powder including magnetite and iron
and having an average primary particle size of from about 0.01 to
about 10 .mu.m.
[0026] Preferably, the ratio of the maximum diffraction intensity
of the magnetite to that of .alpha.-Fe in X-ray diffraction is from
about 0.001 to about 50. The magnetite-iron based composite powder
preferably contains at least one component selected from the group
consisting of nickel, cobalt, chromium, manganese and copper, of
which nickel is typically preferred. The magnetite-iron based
composite powder according to the first aspect of the invention is
also briefly referred to as "composite powder".
[0027] In a second aspect, the invention provides a magnetite-iron
based composite powder mixture including the magnetite-iron based
composite powder and a nonferrous inorganic compound powder. The
term "nonferrous inorganic compound" as used herein also includes
nonferrous pure elements, but does not exclude inclusion of
elemental iron in the nonferrous inorganic compound in an amount as
much as that of impurities (less than or equal to about 1%).
[0028] Preferably, the nonferrous inorganic compound powder has
average primary particle size of less than or equal to about 0.1
.mu.m and adheres to a surface of the magnetite-iron based
composite powder. In this configuration, the average primary
particle size of the nonferrous inorganic compound powder should be
smaller than that of the magnetite-iron based composite powder.
[0029] Alternatively, the magnetite-iron based composite powder
preferably adheres to the surface of the nonferrous inorganic
compound powder having an average primary particle size of equal to
or more than about 1 .mu.m and less than or equal to about 100
.mu.m. In this configuration, the average primary particle size of
the nonferrous inorganic compound powder should be greater than
that of the magnetite-iron based composite powder.
[0030] Further alternatively, the magnetite-iron based composite
powder and a nonferrous inorganic compound powder having an average
primary particle size of less than or equal to about 0.1 .mu.m (a
first nonferrous inorganic compound powder) adhere to a surface of
a nonferrous inorganic compound powder having an average primary
particle size of equal to or more than about 1 .mu.m and less than
or equal to about 100 .mu.m (a second nonferrous inorganic compound
powder). In this configuration, the average primary particle sizes
of the first nonferrous inorganic compound powder and the
magnetite-iron based composite powder are smaller than that of the
second nonferrous inorganic compound powder. The magnetite-iron
based composite powder mixture according to the second aspect of
the invention is also briefly referred to as "composite powder
mixture".
[0031] Preferred magnetite-iron based composite powders for use in
the magnetite-iron based composite powder mixture are the same as
in the magnetite-iron based composite powder according to the first
aspect of the invention.
[0032] Preferably, the nonferrous inorganic compound powder is a
silicate and/or an inorganic compound including carbon (inclusive
of allotropes of carbon). More preferably, the nonferrous inorganic
compound powder having an average primary particle size of less
than or equal to about 0.1 .mu.m is a silicate and/or an inorganic
compound including carbon and having an average primary particle
size of less than or equal to about 0.1 .mu.m, and the nonferrous
inorganic compound powder having an average primary particle size
of equal to or more than about 1 .mu.m and less than or equal to
about 100 .mu.m is a silicate and/or graphite having an average
primary particle size of equal to or more than about 1 .mu.m and
less than or equal to about 100 .mu.m.
[0033] The nonferrous inorganic compound powder having an average
primary particle size of less than or equal to about 0.1 .mu.m may
be referred to as an "ultrafine powder", and the nonferrous
inorganic compound powder having an average primary particle size
of equal to or more than about 1 .mu.m and less than or equal to
about 100 .mu.m may be referred to as "small particles".
[0034] When special emphasis is placed on the magnetic properties
of the composite powder mixture, the nonferrous inorganic compound
powder (inclusive of the ultrafine powder and small particles) is a
dielectric powder having a relative dielectric constant of more
than about 2.0.
[0035] The dielectric powder preferably has a standard Gibbs free
energy of formation less than that of hematite.
[0036] Preferably, the dielectric powder is at least one material
selected from the group consisting of a titanium oxide powder, a
silicon oxide powder and an aluminium oxide powder.
[0037] In a third aspect, the invention provides a method for
producing a magnetite-iron based composite powder. The method
includes the steps of heating and thereby reducing a hematite based
powder having an average primary particle size of from about 0.01
to about 10 .mu.m in a reducing gas, and stopping reduction of the
powder at about midstream of reduction to thereby yield a partially
reduced powder as a composite powder comprising magnetite and
iron.
[0038] In a fourth aspect, the invention provides another method
for producing a magnetite-iron composite powder mixture. This
method includes the steps of heating and thereby reducing a
hematite based powder having an average primary particle size of
from about 0.01 to about 10 .mu.m in a reducing gas in the presence
of a nonferrous inorganic compound powder, and stopping reduction
of the powder at about midstream of reduction to thereby yield a
partially reduced powder as a composite powder comprising magnetite
and iron.
[0039] The phrase "stopping reduction of the powder at about
midstream of reduction" means that the reduction operation of the
hematite based powder is stopped during formation of water formed
as a result of reduction.
[0040] The invention further provides, in a fifth aspect, another
method for producing a magnetite-iron composite powder. The method
includes the steps of heating a hematite based powder having an
average primary particle size of from about 0.01 to about 10 .mu.m
in a reducing gas to reduce the powder substantially completely,
and oxidizing a surface of the substantially completely reduced
powder with an oxygen-containing gas to thereby yield a composite
powder comprising magnetite and iron.
[0041] The invention provides, in a sixth aspect, another method
for producing a magnetite-iron composite powder mixture. The method
includes the steps of heating a hematite based powder having an
average primary particle size of from about 0.01 to about 10 .mu.m
in a reducing gas in the presence of a nonferrous inorganic
compound powder to reduce the powder substantially completely, and
oxidizing a surface of the substantially completely reduced powder
with an oxygen-containing gas to thereby yield a composite powder
comprising magnetite and iron.
[0042] In a seventh aspect, the invention provides yet another
method for producing a magnetite-iron based composite powder. The
method includes the steps of heating and thereby reducing a
hematite based powder having an average primary particle size of
from about 0.01 to about 10 .mu.m in a reducing gas, stopping
reduction of the powder at about midstream of reduction to yield a
partially reduced powder, and oxidizing a surface of the partially
reduced powder with an oxygen-containing gas to thereby yield a
composite powder comprising magnetite and iron.
[0043] In addition, the invention provides, in an eighth aspect,
yet another method for producing a magnetite-iron composite powder
mixture. The method includes the steps of heating and thereby
reducing a hematite based powder having an average primary particle
size of from about 0.01 to about 10 .mu.m in a reducing gas in the
presence of a nonferrous inorganic compound powder, stopping
reduction of the powder at about midstream of reduction to yield a
partially reduced powder, and oxidizing a surface of the partially
reduced powder with an oxygen-containing gas to thereby yield a
composite powder comprising magnetite and iron.
[0044] According to the third through eighth aspects of the
invention, the ratio of the maximum diffraction intensity of the
magnetite to that of .alpha.-Fe in X-ray diffraction is preferably
from about 0.001 to about 50 as in the first and second aspects of
the invention. The reducing gas is preferably hydrogen gas, carbon
monoxide gas or a gaseous mixture thereof. Further, the reducing
gas may comprise a gas of hydrocarbon such as methane or
ethane.
[0045] The magnetite-iron based composite powder mentioned in the
third to sixth aspects of the invention preferably contains at
least one component selected from the group consisting of nickel,
cobalt, chromium, manganese and copper, of which nickel is
typically preferred, as in the first and second aspects of the
invention.
[0046] In the above methods according to the fourth, sixth and
eighth aspects of the invention, the nonferrous inorganic compound
powder preferably includes a silicate and/or an inorganic compound
including carbon.
[0047] In addition, the nonferrous inorganic compound powder is
preferably a nonferrous inorganic compound powder having an average
primary particle size of less than or equal to about 0.1 .mu.m
and/or a nonferrous inorganic compound powder having an average
primary particle size of equal to or more than about 1 .mu.m and
less than or equal to about 100 .mu.m. More preferably, the
nonferrous inorganic compound powder having an average primary
particle size of less than or equal to about 0.1 .mu.m includes a
silicate and/or an inorganic compound including carbon, and the
nonferrous inorganic compound powder having an average primary
particle size of equal to or more than about 1 .mu.m and less than
or equal to about 100 .mu.m includes a silicate and/or an inorganic
compound including carbon.
[0048] The nonferrous inorganic compound powder is preferably a
dielectric powder having a relative dielectric constant of more
than about 2.0.
[0049] The dielectric mentioned above preferably has a standard
Gibbs free energy of formation less than that of the iron
oxide.
[0050] At least one of a titanium oxide powder, a silicon oxide
powder and an aluminium oxide powder is preferably used as the
dielectric powder.
[0051] The material hematite powder preferably contains at least
one component selected from the group consisting of nickel, cobalt,
chromium, manganese and copper, of which nickel is typically
preferred.
[0052] The reducing gas is preferably hydrogen gas or carbon
monoxide gas.
[0053] The invention provides, in a ninth aspect, a method for
remedying polluted soil, water or gases. The method includes the
steps of bringing the magnetite-iron based composite powder
according to the first aspect or the magnetite-iron based composite
powder mixture according to the second aspect of the invention into
contact with at least one of soil, water or a gas polluted with an
organic halogen compound, and thereby decomposing the organic
halogen compound.
[0054] In addition and advantageously, the invention provides, in a
tenth aspect, an electromagnetic wave absorber including a molded
mixture of the magnetite-iron based composite powder according to
the first aspect or the magnetite-iron based composite powder
mixture according to the second aspect with a rubber and/or a
resin.
[0055] The resin is preferably a thermosetting resin or a
thermoplastic resin.
[0056] The composite powder (mixture) of the invention has a larger
specific surface area and more active sites than conventional iron
powders for use in dehalogenation of organic halogen compounds.
Accordingly, the composite powder (mixture) can rapidly
dehalogenate the organic halogen compounds, keep its activity over
a long time and is, therefore, suitable for remediation of polluted
soil, polluted groundwater or polluted air. The composite powder
(mixture) can also be used as a magnetic material for use in high
frequency regions, and the resulting magnetic material obtained by
molding the composite powder (mixture) can keep its satisfactory
permeability and absorption capability of electromagnetic waves in
the high frequency regions.
BRIEF DESCRIPTION OF THE DRAWING
[0057] The single FIGURE (FIG. 1) is a schematic perspective view
showing a cylindrical vessel filled with hematite and a mixture of
a coke powder and a calcium carbonate powder according to an aspect
of the production method according to the invention.
DETAILED DESCRIPTION
[0058] We discovered that iron powder particles comprising both
iron and a magnetite phase can accelerate dehalogenation
(reduction) of organic halogen compounds and thereby accelerate
decontamination of the organic halogen compounds. Accordingly, the
magnetite-iron based composite powder of the invention for use in
remedying soil and other media is a magnetite-iron based composite
powder (hereinafter also briefly referred to as "composite powder")
carrying magnetite exposed in part or the whole of the surface
thereof and having an average primary particle size of from about
0.01 to about 10 .mu.m. The mechanism of the effective activity of
the composite powder is not completely understood. However, without
being limited to any particular theory, we believe that when the
magnetite phase is exposed to the surface of the iron powder in
coexistence with iron with a contact grain boundary therebetween,
the surface of the exposed magnetite phase acts as a local cathode
and thereby accelerates a local cell reaction. The term "primary
particle size" as used herein means the particle size of a single
particle or the particle size of each particle constituting an
aggregate particle. The particle size is determined by observation
on scanning electron microscope (SEM).
[0059] Specifically, anodes and cathodes are formed in the vicinity
of the surface of the composite powder, and oxidation of iron takes
place at the anodes while reduction of the organic halogen
compounds proceeds at the cathodes (a local cell reaction). In a
local cell reaction, electrons are transferred between the anodes
and cathodes. A phase serving as the local cathode must conduct
electricity. The reduction yields a dehalogenated organic compound.
Thus, the polluted soil, groundwater (water), and gas (air) can be
decontaminated or remedied.
[0060] Separately, the magnetite-iron based composite powder of the
invention is a magnetite-iron based composite powder having an
average primary particle size of from about 0.01 to about 10 .mu.m.
The use of such a magnetite-iron system can yield an
electromagnetic wave absorber from a fine iron powder as a material
without the problem of self-combustion.
[0061] The fine iron powder according to the invention is fine and,
therefore, exhibits less decrease in permeability due to an induced
current generated in an alternating magnetic field and can serve as
a magnetic material in the high frequency regions up to several
tens megahertzs. Accordingly, it is useful and effective as an
electromagnetic wave absorber utilizing magnetic loss in the high
frequency regions ranging from several megahertz to several tens of
megahertz. In the regions with frequencies on the order of
gigahertz, the fine iron powder acts less as a magnetic substance
and more as a dielectric due to its decreased permeability.
However, the permeability in this case is as low as less than or
equal to about 10. By incorporating a highly dielectric material,
the fine iron powder can be converted into a composite having a
dielectric constant optimal to electromagnetic wave absorption.
[0062] An iron phase (excluding magnetite and other oxides) in the
composite powder of the invention is preferably a pure ferrite
phase, but may comprise about 50% by mass or less of an austenite
phase. The iron phase preferably comprises nickel to improve
corrosion resistance and magnetic properties of the composite
powder. The content of nickel is preferably less than or equal to
about 50% by mass and more preferably from about 5% to about 10% by
mass, based on the total amount of metallic components in the
composite powder. The presence of excessive nickel increases
production costs of the composite powder and may deteriorate the
magnetic properties thereof.
[0063] By the same token, the iron phase may comprise at least one
component selected from the group consisting of cobalt (Co),
chromium (Cr), manganese (Mn) and copper (Cu). The content of these
elements is less than or equal to about 50% by mass based on the
total amount of metallic components in the composite powder. Each
of these elements can be added alone or in combination with nickel.
The content of secondary impurities such as carbon and silicon
other than iron and iron oxides in the composite powder is
preferably less than or equal to about 1% by mass based on the
total amount of the composite powder.
[0064] The composite powder of the invention has an average primary
particle size of less than or equal to about 10 .mu.m. This
characteristic is important to increase the specific surface area
of the composite powder. This increases its reduction power for the
organic halogen compounds. If the average primary particle size
exceeds about 10 .mu.m, the specific surface area of the composite
powder decreases and the reduction power thereof decreases. In
contrast, if the average primary particle size is less than about
0.01 .mu.m, the constitutive particles aggregate with each other
with increasing adhesion between particles, and the resulting
particles are not sufficiently dispersed into the water or soil in
which the organic halogen compound is decomposed. The average
primary particle size of the composite powder is preferably from
about 0.1 to about 110 .mu.m and more preferably from about 0.2 to
about 0.8 .mu.m.
[0065] When the magnetic properties of the composite powder of the
invention are utilized, the composite powder may comprise
aggregated particles. Such aggregated particles specifically in the
form of a chain and having a thin shape exhibit increased
anisotropy in shape to thereby reduce a demagnetizing field and
increase permeability in the high frequency regions.
[0066] When the composite powder is used as a magnetic material,
its average primary particle size should also range from about 0.01
to about 10 .mu.m and is preferably from about 0.05 to about 3
.mu.m and more preferably from about 0.1 to about 1 .mu.m. If the
average primary particle size exceeds about 10 .mu.m, the resulting
composite powder having a large particle size decreases in
permeability and cannot be used as a magnetic material in the high
frequency regions. If the average primary particle size is less
than about 0.01 .mu.m, the composite powder cannot sufficiently be
dispersed into a rubber or resin and cannot satisfactorily be
molded into an electromagnetic wave absorber.
[0067] The magnetite phase in the composite powder of the invention
may be present either inside the powder or on the surface of the
powder to thereby cover the powder. When the composite powder is
used for reduction of the organic halogen compounds, the magnetite
phase serving as a local cathode is preferably exposed to the
surface of the composite powder. The content of the magnetite phase
in the composite powder is preferably such that the ratio of
diffraction intensity of the magnetite to that of .alpha.-Fe ranges
from about 0.001 to about 50. The magnetite phase may not
effectively accelerate reduction or may not effectively inhibit
self-combustion if the ratio of diffraction intensity is
excessively low. The proportion of iron exposed to the surface is
decreased to thereby decrease the reduction power if the ratio of
diffraction intensity is excessively high. When the composite
powder is used as a magnetic material for absorption of
electromagnetic waves, the absorption power decreased with a
decreasing ratio of iron. The diffraction intensity ratio is more
preferably from about 0.01 to about 50 and typically preferably
from about 0.01 to about 5. The diffraction intensity ratio of
about 0.001 corresponds to the amount of magnetite of about about
0.1% by volume, and the diffraction intensity ratio of about 5
corresponds to the amount of magnetite of about 83% by volume.
[0068] The ratio of X-ray diffraction intensity does not correspond
to magnetite present in the surface alone. However, there is very
little possibility that a magnetite-iron powder carries no
magnetite on its surface.
[0069] In the X-ray diffraction analysis, Cu or Co is used as a
X-ray source, and the obtained diffraction spectrum of the
composite powder is separated into individual spectra of individual
phases constituting the composite powder. In this procedure, the
ratio of diffraction intensity is defined as the ratio of the
maximum diffraction intensity of magnetite to that of iron, i.e.,
the ratio of the diffraction peak derived from the (311) planes of
magnetite to the diffraction peak derived from the (110) planes of
iron.
[0070] The composite powder of the invention may further comprise
wustite (FeO) as an iron oxide component in addition to magnetite
(Fe.sub.3O.sub.4). Parts of elements in the nonferrous inorganic
compound of the ultrafine powder or small particles mentioned below
may comprise an iron oxide in which part of iron in magnetite is
replaced with another element.
[0071] The total amount of such iron oxides and iron-based complex
oxides other than magnetite is preferably less than or equal to
about 5% by mass based on the total amount of the composite
powder.
[0072] The hematite (Fe.sub.2O.sub.3) based iron oxide powder
(referred to as hematite based powder) for use as a raw material
for the composite powder of the invention can be prepared by, for
example, atomizing and roasting of an aqueous solution of an iron
salt such as an iron chloride, iron sulfate or iron nitrate.
Roasting is performed by atomizing the aqueous solution of the iron
salt as droplets into the air and heating the atomized solution at
temperatures ranging from about 600.degree. C. to about 700.degree.
C. using a gas burner. An iron oxide powder having an average
primary particle size of from about 0.01 to about 10 .mu.m,
preferably from about 0.1 to about 1 .mu.m, can be prepared by
controlling roasting conditions such as the concentration of the
solution, roasting temperature and particle size of the droplets.
If an iron oxide powder having an average primary particle size
exceeding about 10 .mu.m is used as the raw material of the
composite powder, the resulting composite powder has a large
particle size and exhibits decreased permeability and cannot be
used as a magnetic material in the high frequency regions. If an
iron oxide powder having an average primary particle size less than
about 0.01 .mu.m is used as the raw material, the resulting
composite powder cannot sufficiently be dispersed into a rubber or
a resin and cannot satisfactorily be molded into an electromagnetic
wave absorber. In remediation of media, the iron oxide powder
preferably has an average primary particle size of from about 0.01
to about 10 .mu.m to ensure sufficient reduction power.
[0073] The content of nickel in hematite is preferably less than or
equal to about 50% by mass based on the total amount of metal
elements, since a composite powder obtained by reduction of
hematite containing nickel in an amount exceeding about 50% by mass
in terms of nickel cannot be used as a magnetic material in the
high frequency regions. To ensure sufficient reduction power, the
nickel content is also preferably less than or equal to about 50%
by mass.
[0074] By the same token, the content of Co, Cr, Mn and Cu in
hematite is preferably less than or equal to about 10% by mass
based on the total amount of metal elements.
[0075] The material iron oxide (hematite based) powder may contain
other iron based oxides less than or equal to about 40% by mass.
Even if the iron oxide powder is a pure ion oxide (hematite based)
powder, the resulting composite powder can keep its satisfactorily
stable permeability and dielectric constant even in the high
frequency regions with frequencies on the order of gigahertz and
can show satisfactory reduction power.
[0076] The content of impurities such as C and Si other than iron
oxides in the raw material hematite powder is preferably less than
or equal to about 5% by mass based on total amount of material
powders.
[0077] The composite powder of the invention tends to undergo
sintering during heating and reduction. Accordingly, it is
preferred that an ultrafine powder of a nonferrous inorganic
compound having an average primary particle size smaller than that
of the composite powder adheres to a surface of the composite
powder to thereby prevent the composite powder from sintering
during heating operation in its manufacture and to prevent an
active surface from decreasing. The proportion of the ultrafine
powder adhering to or mixed with the composite powder is preferably
less than or equal to about 10% by mass and more preferably form
about 1% to about 5% by mass based on total amount of material
powders. If the proportion exceeds about 10% by mass, the active
surface of the composite powder may be decreased.
[0078] Such ultrafine nonferrous inorganic compound powders for use
herein are not specifically limited as long as they can prevent
sintering of the composite powder and are preferably silicates
and/or inorganic compounds including carbon. Preferred silicates
are colloidal silical and fumed silica, of which colloidal silica
is typically preferred in a view of adhesion of ultrafine powders.
Preferred inorganic compounds including carbon are graphite and
carbon black. Carbon black may be amorphous carbon black or
graphitized carbon black. Each of these substances can be used
alone or in combination.
[0079] If an ultrafine powder having an average primary particle
size exceeding about 0.1 .mu.m is used, constitutive particles of
the ultrafine powder cannot homogeneously be dispersed into gaps
among the composite powder particles, and the resulting composite
powder becomes susceptible to sintering during the heating
procedure in the manufacturing process. The average primary
particle size of the ultrafine powder is preferably from about 0.05
to about 0.1 .mu.m.
[0080] It is also preferred to allow the composite powder to adhere
to, or be mixed with, surfaces of small particles of a nonferrous
inorganic compound having an average primary particle size greater
than that of the composite powder to prevent sintering and
aggregation of the composite powder of the invention and to prevent
the active surface from decreasing. The average primary particle
size of the small particles is substantially equal to or more than
about 1 .mu.m and less than or equal to about 100 .mu.m, preferably
from about 1 to about 80 .mu.m and typically preferably from about
3 to about 50 .mu.m. If the average primary particle size of the
small particles is less than about 1 .mu.m, the resulting small
particles aggregate with each other, and the composite powder
cannot significantly adhere to or be mixed with the small
particles. If it exceeds about 100 .mu.m, the resulting composite
powder mixture is hardly handled or treated in dispersion into soil
or other media when the composite powder mixture is used for
reduction of organic halogen compounds.
[0081] The proportion of the small particles adhered to or mixed
with the composite powder is preferably from about 10% to about 80%
by mass and more preferably from about 30% to about 60% by mass
based on total amount of material powders. If the proportion of the
small particles is less than about 10% by mass, the composite
powder may aggregate. If it exceeds about 80% by mass, the
proportion of the composite powder that substantially contributes
to reduction of the organic halogen compound is relatively
decreased to thereby decrease the reduction power for the organic
halogen compound.
[0082] Such small particles of nonferrous inorganic compounds are
not specifically limited as long as they can prevent aggregation
and sintering and are preferably silicates and/or inorganic
compounds containing carbon. Preferred silicates for use herein are
silicon oxides, zeolite and pulverized powders of by-products in
steel manufacture, such as fly ash powder and slag powder. Graphite
is preferred as the inorganic compound including carbon as an
allotrope of carbon. The graphite may be whichever of naturally
occurring graphite and artificial graphite. The graphite serves to
activate an interface with iron into an active site. Each of these
substances can be used alone or in combination.
[0083] According to the invention, both the composite powder and
ultrafine powder may adhere to the surfaces of the small
particles.
[0084] It is preferred that the composite powder adheres to the
ultrafine powder or to the small particles by means of bonding with
solid state diffusion of atoms during the heating and reduction
procedure mentioned below. The resulting composite powder mixture
produced by the solid state diffusion bonding is resistant to be
separated by delamination at the interface even if it is charged
into a vessel or is placed underground.
[0085] Alternatively, these components may be bonded with the use
of a binder.
[0086] The composite powder, ultrafine powder and small particles
for use in the invention may each comprise plural particles
aggregated with each other to some extent. However, the particle
size of each powder should be indicated in the primary particle
size. In addition, it is preferred that aggregation is limited so
that about 3 to about 100 particles aggregate in the powder, since
excessive aggregation is not preferred.
[0087] To enable the composite powder to maintain a stable
permeability in the high frequency regions with frequencies on the
order of gigahertz, a dielectric powder having a relative
dielectric constant exceeding about 2.0 and preferably from about
5.0 to about 15 is preferably used as the nonferrous inorganic
compound constituting the ultrafine powder and/or small particles.
If the dielectric has an excessively low relative dielectric
constant, the resulting composite powder mixture may exhibit
deteriorated dielectric properties.
[0088] In the above case, a dielectric powder having an average
primary particle size of less than or equal to about 0.1 preferably
adheres to the surface of the composite powder. Alternatively, it
is preferred that the composite powder is mixed with or adheres to
the surface of a dielectric powder having an average primary
particle size of equal to or more than about 0.1 .mu.m and less
than or equal to about 100 .mu.m.
[0089] If the dielectric has a standard Gibbs free energy of
formation equal to or more than that of hematite (-763.6 kJ/mol),
it is reduced ahead of hematite and other iron oxides. Accordingly,
when the hematite powder and the dielectric powder are subjected to
reduction, the dielectric may be reduced into an electric conductor
such as an elementary metal or an alloy to thereby lose its
physical properties as a dielectric before hematite is reduced to a
desired extent. In this case, the resulting composite powder
mixture may not be used as a dielectric composite material in the
high frequency regions. The dielectric therefore preferably has a
standard Gibbs free energy of formation less than that of
hematite.
[0090] The dielectric powder is preferably an oxide powder such as
an aluminium oxide powder, an anatase-type titanium oxide powder
and a silicon oxide powder (prepared from colloidal silica or fumed
silica). Each of these powders can be used alone or in
combination.
[0091] Methods for Producing Composite Powder and Composite Powder
Mixture
[0092] The composite powder of the invention can be produced by
heating and reducing an iron oxide mainly containing hematite
(Fe.sub.2O.sub.3) and having an average primary particle size of
from about 0.01 to about 10 .mu.m in, for example, a
hydrogen-containing gas at temperatures preferably from about
200.degree. C. to about 700.degree. C. for about 1 minute to about
3 hours.
[0093] The composite powder obtainedbyreduction has an average
primary particle size of from about 0.01 to about 10 .mu.m and
comprises a magnetite phase exposed to part or overall surfaces of
constitutive particles with the balance (excluding incidental
impurities) of iron having a valency of zero (Fe.sup.0). If the
iron oxide is reduced at high temperatures of equal to or more than
about 570.degree. C., wustite (FeO) may be formed in addition to
magnetite (Fe.sub.3O.sub.4), and part of elements in the small
particles of the nonferrous inorganic compound may diffuse into the
magnetite phase on the surface of the composite powder where the
composite powder is in contact with the small particles to thereby
yield an iron oxide in which part of iron is substituted with
another element. An example of such an iron oxide in which part of
iron is substituted with another element is fayalite
(Fe.sub.2SiO.sub.4) which is formed during heating and reduction of
the iron oxide powder when a silicate is used as the small particle
nonferrous inorganic compound.
[0094] The rate of the reduction reaction is decreased if the
reduction temperature is excessively low. In contrast, if it is
excessively high, the relative ratio of magnetite decreases with an
increasing amount of wustite, and the resulting composite powder
undergoes sintering. The reduction temperature is preferably from
about 200.degree. C. to about 700.degree. C. as mentioned above,
and is more preferably from about 300.degree. C. to about
570.degree. C. at which reduction proceeds with a sufficient
reaction rate and the two phases, .alpha.-Fe and magnetite phases,
are in coexistence in equilibrium. The iron oxide powder is not
sufficiently reduced and the ratio of .alpha.-Fe is low if the
reduction time is excessively short. In contrast, if it is
excessively long, the iron oxide powder is excessively reduced, the
iron oxide in the composite powder is decreased in such an amount
below the detection limit in X-ray diffraction analysis, and the
resulting composite powder undergoes sintering. The reduction time
is preferably from about 1 minute to about 3 hours as mentioned
above and is more preferably from about 5 minutes to about 1
hour.
[0095] When carbon monoxide gas is used as the reducing gas, the
reduction temperature is preferably from about 30.sup.0.degree. C.
to about 900.degree. C. If the reduction temperature is lower than
about 300.degree. C., the hematite powder may be reduced at a
decreased rate to thereby deteriorate the productivity of the
composite powder. In contrast, if it exceeds about 900.degree. C.,
the reduced fine iron powder may undergo sintering to have a large
particle size. When the carbon monoxide gas is used as the reducing
gas, it may be supplied to a reduction reactor containing the
hematite powder and the dielectric powder. Alternatively, coke and
calcium carbonate are put into the reduction reactor containing the
hematite powder and the dielectric powder, and carbon monoxide gas
formed as a result of a reaction between coke and calcium carbonate
can be used. The carbon monoxide gas may further comprise a gas of
a reducing hydrocarbon such as methane or ethane.
[0096] The content of the magnetite phase in the composite powder
can be controlled by appropriately setting the reduction
temperature and reduction time in heating and reduction procedure.
Specifically, the content of the magnetite phase can be increased
by decreasing the reduction temperature and/or shortening the
reduction time. In contrast, by increasing the reduction
temperature and/or prolonging the reduction time, reduction is
accelerated, and the content of the magnetite phase is decreased.
As a result, a pure iron phase can be obtained in extreme cases.
When the composite powder is used for reducing the organic halogen
compound, the content of the magnetite phase in the composite pow
der is such that the ratio of the X-ray diffraction intensity of
magnetite to that of .alpha.-Fe is preferably from about 0.001 to
about 50 and more preferably from about 0.01 to 50 and typically
preferably from about 0.5 to about 1.5. The content of the
magnetite phase is not limited to the above ranges in some
applications.
[0097] To achieve the above diffraction intensity ratio, it is
preferred that reduction of hematite is stopped before hematite is
completely reduced and yields a pure iron phase. The degree of
proceeding of reduction can be determined by previously determining
the dew point Td of the material hydrogen at the reduction
temperature and determining the dew point Td of exhausted hydrogen
during reduction of hematite. Specifically, Td is higher than
Td.sup.i due to water formed as a result of reduction until
hematite is substantially completely reduced by heating (i.e.,
until hematite is reduced via the magnetite phase or another phase
into the pure iron phase). Accordingly, the reaction may be stopped
after a predetermined time period to sufficiently proceed with
reduction of hematite into magnetite and before Td becomes less
than or equal to Td.sup.i. This method can yield a magnetite-iron
based composite powder carrying magnetite at least part of which is
present on the surface of the composite powder. Magnetite herein is
formed as a result of reduction of hematite.
[0098] Alternatively, the above diffraction intensity ratio can be
achieved by a method in which reduction is completed when Td
becomes less than or equal to Td.sup.i to yield the pure iron phase
as a result of complete reduction of hematite, the hydrogen gas
feed is then stopped and replaced with an inert gas, the iron
particle is oxidized with an oxygen-containing gas at temperatures
less than or equal to about 570.degree. C. at which the iron and
magnetite phases can be coexistent in equilibrium to thereby
increase the proportion of the magnetite phase. This method can
also yield a composite powder carrying magnetite at least part of
which is present on the surface of the composite powder.
[0099] The composite powder of the invention can also be obtained
by partially reducing hematite (preferably reducing to a state
where magnetite and iron can be coexistent) and subjecting the
partially reduced hematite to the above partial re-oxidation
procedure. However, this method invites somewhat higher costs.
[0100] When hematite is reduced in the presence of the ultrafine
nonferrous inorganic compound powder, a composite powder mixture
comprising the composite powder and the ultrafine powder adhering
to the surface of the composite powder as a result of solid phase
diffusion bonding of atoms can be obtained. Likewise, a composite
powder mixture comprising the small particles and the composite
powder adhering to the surfaces of the small particles as a result
of solid phase diffusion bonding of atoms can be obtained when
hematite is reduced in the presence of small particles of the
nonferrous inorganic compound.
[0101] A composite powder mixture comprising the small particles
and the composite powder and the ultrafine powder adhering to the
surfaces of the small particles can be obtained when hematite is
reduced in the presence of the ultrafine nonferrous inorganic
compound powder and small particles of the nonferrous inorganic
compound. Naturally, the ultrafine powder and the small particles
can be added one by one and be allowed to adhere sequentially.
[0102] To avoid heat generation or ignition caused by rapid
oxidation, it is preferred to slightly oxidize the surface of the
composite powder obtained as a result of reduction of hematite in
an atmosphere of a weakly oxidizing gas having a low oxygen
content.
[0103] Method for Remedying Soil and Other Media Using Iron
Powder
[0104] The organic halogen compounds to which the composite powder
or composite powder mixture of the invention can be applied are
those containing halogen such as chlorine bound to the molecule.
Such organic halogen compounds include, but are not limited to,
volatile organic halogen compounds such as trichloroethylene
(hereinafter may be abbreviated as TCE), tetrachloroethylene,
1,1,1-trichloroethane, 1,1,2-trichloroethane, dichloroethylenes,
dichloroethanes, dichloromethane and carbon tetrachloride. In
addition, PCB and dioxin, for example, can also be subjected to the
method of the invention.
[0105] Such organic halogen compounds typically leak from tanks and
drainage, permeate into the soil and reside there. Part of the
organic halogen compounds are dissolved in the moisture in the soil
and groundwater by slow degrees, while part of the remaining
organic halogen compounds are gasified in the soil or air.
[0106] The organic halogen compounds are reduced with the composite
powder and converted into harmless compounds such as non-halogen
compounds and hydrogen halides. For example, TCE receives electrons
(is reduced) on the surface of the composite powder to form
unstable intermediate compounds such as chloroacetylene by
beta-elimination. The intermediate compounds are ultimately
decomposed to acetylene and other compounds containing no chlorine
atoms. Although reduction may proceed further, harmful compounds
are converted into harmless compounds in any case by initiation by
reception of electrons (reduction) on the surface of the composite
powder.
[0107] The composite powder of the invention is fine and carries
magnetite exposed in part or over all of its surface and,
therefore, it does not excessively aggregate in spite of its
fineness, has a large specific surface area and exhibits high
reduction power for the organic halogen compounds. The amount of
the composite powder for use in remediation of polluted soil and
other media can therefore be decreased.
[0108] The composite powder mixture comprising the composite powder
and the ultrafine powder adhering to the composite powder more
effectively prevents the composite powder particles from sintering
and can, therefore, more effectively exhibit the above
advantages.
[0109] The composite powder mixture comprising the small particles
and the composite powder adhering to the small particles can more
effectively prevent the composite powder particles from sintering
and aggregation, has a significantly large specific surface area
and can further effectively exhibit the above advantages. The
composition has a relatively large particle size and, therefore,
exhibits good workability.
[0110] The composite powder mixture comprising the small particle
with the composite powder and the ultrafine powder adhering to the
small particles can exhibit the advantages of the above two
configurations additively.
[0111] The composite powder and the composite powder mixture of the
invention can be applied to any contaminated media, including but
not limited to, polluted soil or waste materials such as municipal,
refinery or chemical sludges or particulates, waterway and lagoon
sediments and the like, groundwater, drainage, wastewater, run-off
or the like, or air according to conventional procedures. For
example, the composite powder or composite powder mixture is
brought into contact with the organic halogen compounds by spraying
or mixing, or by injection of the composite powder or composite
powder mixture or a slurry thereof, for remediation of polluted
soil and/or polluted groundwater. The moisture content of the soil
is preferably equal to or more than about 40% by mass. A reduction
accelerating agent may be used together with the composite powder
or composite powder mixture.
[0112] When the composite powder or composite powder mixture is
applied to excavated polluted soil, the composite powder or
composite powder mixture may also be brought into contact with the
organic halogen compounds by spraying or mixing, or by injection of
the composite powder or composite powder mixture or a slurry
thereof, considering the moisture content, soil quality and soil
pressure. The soil is preferably previously crushed to have a small
particle size to bring the soil into contact with the composite
powder or composite powder mixture, when the excavated soil is
viscous and has a large particle size. Groundwater may be allowed
to pass through a permeable layer in the ground in which the
composite powder or composite powder mixture has been added.
[0113] The amount of the composite powder or composite powder
mixture used relative to the amount of the soil and groundwater is
appropriately determined depending on the type of decontamination
or the degree of contamination of the polluted soil or groundwater.
The amount of the composite powder or composite powder mixture is
generally from about 0.1% to about 10% by mass and preferably from
about 0.5% to about 5% by mass relative to the object to be
remedied, (1) when the polluted water or groundwater is treated in
situ, (2) when the polluted groundwater is pumped (extracted) for
remediation and/or (3) when the polluted soil is treated by
excavation.
[0114] When the composite powder or composite powder mixture of the
invention is applied to polluted air, the air may be allowed to
flow through a vessel filled with the composite powder or composite
powder mixture to bring the air into contact with the composite
powder or composite powder mixture. While the surface of the
composite powder should be wet, adsorbed water is sufficient. One
or more layers of water molecule layers are preferably formed on
the surface of the composite powder. Relative humidity of the air
is preferably equal to or more than about 50%. Fillers and a
reduction accelerating agent may be filled in the vessel in
addition to the composite powder or composite powder mixture.
[0115] Method for Manufacturing Electromagnetic Wave Absorber
[0116] To manufacture magnetic materials having different shapes
(e.g., an electromagnetic wave absorber), a rubber and/or a resin
is added to the composite powder or composite powder mixture and
the resulting mixture is molded. Molding can be performed according
to a conventional procedures such as pressure molding, injection
molding, sheet forming or the like. When the magnetic material is
manufactured by pressure molding, a molding pressure is preferably
from about 5 to about 50 MPa, and a molding temperature is
preferably from room temperature to a temperature about 100.degree.
C. higher than the softening temperature of the rubber and/or resin
for thermoplastic resins.
[0117] Preferred resins for use herein are polyethylenes,
polypropylenes, nylons, ethylene-vinyl acetate resins, and other
thermoplastic resins; and epoxy resins, phenol resins, and other
thermosetting resins. Preferred rubbers are urethane rubber and
silicone rubber. The rubbers also include acrylic elastomers and
styrene-butadiene elastomers.
[0118] The total amount of the rubber and resin is preferably from
about 10 to about 80 parts by weight relative to about 100 parts by
weight of the composite powder or composite powder mixture. An
appropriate amount of the rubber and resin is selected within the
above range depending on the dielectric constants of the rubber and
resin.
[0119] By mixing with an organic solvent, the composite powder or
composite powder mixture can be used as a coating or paint for the
application onto inner or outer walls of buildings, vessels and
cases, for example. The resulting magnetic material can yield a
stable permeability and absorption capability of electromagnetic
waves in the high frequency regions with frequencies on the order
of several gigahertz.
[0120] The composite powder or composite powder mixture of the
invention can also be applied to uses other than those mentioned
above. For example, it can effectively be used as a reducing agent
for reduction of nitrogen in the form of nitric acid.
EXAMPLES
[0121] The invention will be illustrated in further detail with
reference to several examples and comparative examples below, which
are not intended to limit the scope of the invention.
Examples 1 to 23, 32 to 43 and Comparative Examples 1 To 3
[0122] Production of Composite Powder
[0123] A series of material hematite based powders (hereinafter
briefly referred to as "material hematite") was prepared by mixing
hematite based powders, ultrafine powders and small particles. The
composition and average primary particle size of each of the
hematite based powders, and the average primary particle size of
each of the ultrafine powders and small particles used herein are
shown in Table 1. Each of the material hematite was subjected to
reduction with hydrogen, or subjected to reduction with hydrogen
followed by oxidation with oxygen, if desired, (Production Methods
1 to 4), or subjected to reduction with carbon monoxide and to
reduction with hydrogen to thereby remove remaining carbon
substances, if any, followed by oxidation with oxygen (Production
Method 5) and thereby yielded a composite powder or a composite
powder mixture. The resulting composite powder mixtures contained
the composite powder and the ultrafine powder adhering to the
composite powder, contained the small particles and the composite
powder adhering to the small particle, or contained the small
particles with the ultrafine powder and composite powder adhering
to the small particles. The conditions (reduction temperature,
reduction time and dew point) in reduction with hydrogen,
conditions (temperature, oxygen partial pressure and oxidation
time) in oxidation with oxygen and those in reduction with carbon
monoxide are shown below.
[0124] The dew point in the exhausted hydrogen at the time when
reduction was stopped and the X-ray diffraction intensity ratio of
the composite powder after oxidation are shown in Table 1.
[0125] Production Method 1: The material hematite (50 g) was
reduced in a batch furnace at 450.degree. C. in an atmosphere of
hydrogen gas. The dew point of the hydrogen gas used which had been
determined at 450.degree. C. was -30.degree. C. A dew-point
hygrometer was arranged in an exhaust pipe, and reduction was
stopped before the time when the dew point decreased to -30.degree.
C. as shown in Table 1, the supply of hydrogen was stopped, and the
inner atmosphere of the furnace was replaced with an inert gas. The
resulting powder was cooled to room temperature, exposed to
nitrogen gas containing 5% by volume of oxygen for equal to or less
than 2 hours and then taken out from the furnace (partial reduction
and re-oxidation).
[0126] Production Method 2: The material hematite (50 g) was
reduced in a batch furnace at 550.degree. C. in an atmosphere of
hydrogen gas. The dew point of the hydrogen gas used which had been
determined at 550.degree. C. was -30.degree. C. A dew-point
hygrometer was arranged in an exhaust pipe, and reduction was
stopped after the time when the dew point reached -30.degree. C.,
the supply of hydrogen was stopped, and the inner atmosphere of the
furnace was replaced with an inert gas. The resulting powder was
then cooled to room temperature, allowed to stand in nitrogen gas
containing 10% by volume of oxygen for 2 to 24 hours and then taken
out from the furnace (complete reduction and re-oxidation).
[0127] Production Method 3: The material hematite (50 g) was
reduced in a batch furnace at 550.degree. C. in an atmosphere of
hydrogen gas. The dew point of the hydrogen gas used which had been
determined at 550.degree. C. was +10.degree. C. A dew-point
hygrometer was arranged in an exhaust pipe, and reduction was
stopped before the time when the dew point decreased to +10.degree.
C. as shown in Table 1, the supply of hydrogen was stopped, and the
inner atmosphere of the furnace was replaced with an inert gas. The
resulting powder was cooled to room temperature, gradually exposed
to air and then taken out from the furnace (partial reduction).
[0128] Production Method 4: The material hematite (50 g) was
reduced in a batch furnace at 550.degree. C. in an atmosphere of
hydrogen gas. The dew point of the hydrogen gas used which had been
determined at 550.degree. C. was +30.degree. C. A dew-point
hygrometer was arranged in an exhaust pipe, and reduction was
stopped after the time when the dew point reached -30.degree. C.,
the supply of hydrogen was stopped, and the inner atmosphere of the
furnace was replaced with an inert gas. The resulting powder was
cooled to room temperature, exposed to nitrogen gas containing 5%
by volume of oxygen at 200.degree. C. for 5 minutes to 2 hours and
then taken out from the furnace (complete reduction and
re-oxidation).
[0129] Production Method 5: As shown in FIG. 1, the material
hematite (100 g) 3 and a mixture of a coke powder and calcium
carbonate (5:1 by mass) 2 were concentrically filled in a
cylindrical vessel 1. While monitoring formation of carbon monoxide
gas, the vessel was heated at 850.degree. C. in a batch furnace,
and the material hematite was reduced until the formation of carbon
monoxide gas completed. The reduced hematite powder was further
reduced in hydrogen gas at 550.degree. C., for 30 minutes, which
hydrogen gas had a dew point of -30.degree. C. as previously
determined. While removing carbon substances, reduction was stopped
after the time when the dew point of the exhausted hydrogen gas
reached -30.degree. C., the supply of hydrogen was stopped, and the
inner atmosphere of the furnace was replaced with an inert gas. The
resulting powder was cooled to room temperature, allowed to stand
in nitrogen gas containing 5% by volume of oxygen at room
temperature and then taken out from the furnace (complete reduction
and re-oxidation).
[0130] Structure of Composite Powder Mixture
[0131] Each of the composite powders obtained by reduction or
reduction and re-oxidation was subjected to X-ray analysis to
thereby identify constitutional phases. The diffraction intensity
ratios of maximum peaks of .gamma.-Fe and iron oxides to that of
.alpha.-Fe were determined.
[0132] The composite powder mixtures according to the examples of
the invention were subjected to observation on scanning electron
microscope (SEM) and it was found that the ultrafine powder and/or
small particles satisfactorily adhered to the composite powder. The
average primary particle size of each of the powders was determined
according to the following procedure. Twenty or more exposures of
fields of view on SEM were made, and based on the resulting SEM
photographs, an average outer diameter of each target primary
particle was defined as the particle size of the primary particle,
and the average primary particle size was determined as an
arithmetic mean of primary particle sizes of twenty or more
particles. The term "primary particle" as used herein means a
particle constituting an aggregated particle. When the particle is
not an aggregated particle, it is considered as one primary
particle.
[0133] Performance of Composite Powder as Reducing Agent
[0134] (1) In a 100-ml glass vial were placed 50 ml of an aqueous
solution containing 40 mg/L (liter) of calcium carbonate, 80 mg/L
of sodium sulfite and 5 mg/L of TCE, followed by addition of 5 g of
the composite powder (composite powder mixture). The vial was then
sealed with butyl rubber with a fluorinated resin seal and an
aluminium cap. The sealed sample was shaken in the vertical axis
direction of the vial at a rotational speed of 180 rpm in a
constant temperature chamber controlled at 23+2.degree. C. The
concentration of the TCE gas stored in the head space of the vial
was analyzed with a gas detector tube at a predetermined time
interval after initiation of shaking to determine the concentration
of TCE in water. The vial once opened was not used for analysis
thereafter.
[0135] The TCE concentration in water was measured, and the shaking
time (reaction time) and the TCE concentration were plotted along
the horizontal and vertical axes, respectively. The reduction power
of the sample composite powder was determined as the time (hr) at
which the TCE concentration reached half the initial concentration.
The results are shown in Table 1.
[0136] (2) An aqueous solution of TCE was added to 40 g of a loam
layer soil and thereby yielded soil polluted with 100 mg/kg of TCE.
A sample composite powder was then mixed with the polluted soil in
an amount of 1% by mass. The resulting soil was sealed in a 120-ml
glass vial. The vial was then stored in a constant temperature
chamber controlled at 23.+-.2.degree. C. The concentration of the
TCE gas stored in the head space of the vial was analyzed with gas
chromatography/mass spectrometer at a predetermined time interval
after initiation of storage. In this procedure, the ratio of the
TCE concentration of the soil added with the composite powder to
that of soil without the composite powder was defined as the TCE
residual ratio, and the residual ratio 3 days into the storage
(reaction) was defined as the reduction power of the sample
composite powder.
1 TABLE 1-1 TCE Material powder Ultrafine powder Small particles
Dew Oxidation decomposition Average Compo- Average Compo- Average
Production point at time Half- primary sitional primary sitional
primary Compo- method termination of time in particle ratio
particle ratio particle sitional of of composite X-ray diffraction
intensity aqueous Residual size (% by size (% by size ratio
composite reduction powder ratio of composite powder* solution
ratio Type** (.mu.m) mass) Type (.mu.m) mass) Type (.mu.m) (% by
mass) powder (.degree. C.) (hr) .alpha.-Fe .gamma.-Fe Magnetite
Fayalite Wustite (hr) (%) Comparative atomized 75 100 -- -- -- --
-- -- -- -- -- 1.0 0 0 0 0 70 50 Example 1 iron powder Comparative
magnetite 0.3 100 -- -- -- -- -- -- -- -- -- 0 0 All*** 0 0 >500
hr 100 Example 2 for toner Comparative hematite 0.3 100 -- -- -- --
-- -- 2 -30 0 1.0 0 0 0 0 >100 hr 80 Example 3 Example 1
hematite 0.6 100 -- -- -- -- 2 -30 12 1.0 0 0.02 0 0 20 5 Example 2
hematite 0.05 100 -- -- -- -- -- -- 1 20 0 1.0 1.1 0 0 6 5 Example
3 hematite 0.6 100 -- -- -- -- -- -- 1 60 0 1.0 0 9.0 0 0 36 5
Example 4 hematite 0.3 95 colloidal 0.02 5 -- -- -- 2 -30 6 1.0 0
0.02 0.01 0 32 3 silica Example 5 hematite 0.3 95 colloidal 0.02 5
-- -- -- 1 0 0 1.0 0 0.7 0.01 0 8 2 silica Example 6 hematite 0.3
98 carbon 0.01 2 -- -- -- 2 -30 12 1.0 0 2.6 0 0 24 6 black Example
7 hematite 6.3 98 carbon 0.01 2 -- -- -- 1 60 0 1.0 0 8.1 0 0 38 5
black Example 8 hematite 0.3 98 graphitized 0.02 2 -- -- -- 2 -30 2
1.0 0 0.06 0 0 26 5 carbon black Example 9 hematite 0.3 98
graphitized 0.02 2 -- -- -- 1 60 2 1.0 0 0.55 0 0 16 2 carbon black
Example 10 hematite 0.3 94 colloidal 0.02 5 -- -- -- 2 -30 24 1.0 0
1.10 0.01 0 11 5 silica carbon 0.01 1 black Example 11 hematite 0.3
94 colloidal 0.02 5 -- -- -- 1 30 1 1.0 0 9.0 0.01 0 34 5 silica
carbon 0.01 1 black Example 12 hematite 0.4 99 -- -- -- silica 25 1
4 -30 1 1.0 0 2.1 0.01 0 16 5 Example 13 hematite 0.4 99 -- -- --
silica 25 1 1 25 2 1.0 0 5.3 0.01 0 23 2 Example 14 hematite 0.3 90
-- -- -- natural 5 10 2 -30 12 1.0 0 1.8 0 0 29 2 graphite Example
15 hematite 2.1 90 -- -- -- natural 5 10 1 25 2 1.0 0 6.3 0 0 37 3
graphite Example 16 hematite 0.3 70 colloidal 0.02 5 natural 30 25
4 -30 5 min 1.0 0 0.95 0 0 21 2 silica graphite Example 17 hematite
0.3 70 colloidal 0.02 5 natural 30 25 3 15 0 1.0 0 2.0 1 0 16 2
silica graphite Example 18 hematite 0.3 19 colloidal 0.02 5 zeolite
80 80 4 -30 0.5 1.0 0 0.90 0.03 0 25 5 silica graphitized 0.02 0.5
carbon black Example 19 hematite 0.3 19 colloidal 0.02 0.5 zeolite
80 80 3 21 0 1.0 0 0.50 0.01 0 39 5 silica graphitized 0.02 0.5
carbon black Example 20 hematite 0.3 35 colloidal 0.02 4.5 zeolite
80 30 5 -30 2.0 1.0 0 3.10 0.02 0 45 5 silica natural 50 60
graphitized 0.02 0.5 graphite carbon black Example 21 hematite 8.3
35 colloidal 0.02 4.5 zeolite 80 30 3 40 0 1.0 0 8.0 0.01 0 50 6
silica natural 50 60 graphitized 0.02 0.5 graphite carbon black
Example 22 15% Ni- 0.6 95 colloidal 0.02 5 -- -- -- 1 40 2 1.0 1.0
21.0 0.01 0 22 5 hematite silica Example 23 hematite 0.5 40 -- --
zeolite 80 60 3 20 0 1.0 0 1.6 0.05 0 31 5 *X-ray diffraction
intensity ratio is the ratio to X-ray diffraction intensity of
.alpha.-Fe **hematite = Fe.sub.2O.sub.3 ***pure magnetite
[0137]
2 TABLE 1-2 Material powder Ultrafine powder Small particles
Average Average Average primary Compositional primary Compositional
primary Compo- particle ratio particle ratio particle sitional size
(% by size (% by size ratio (% Type (.mu.m) mass) Type (.mu.m)
mass) Type (.mu.m) by mass) Example
(Ni.sub.0.05Fe.sub.0.95).sub.2O.sub.3 0.4 100 32 Example
(Ni.sub.0.1Fe.sub.0.9).sub.2O.sub.3 0.4 100 33 Example
(Ni.sub.0.05Fe.sub.0.95).sub.2O.sub.3 0.4 99 silica 25 1 34 Example
(Ni.sub.0.07Fe.sub.0.93).sub.2O.sub.- 3 0.4 80 colloidal 0.02 15
natural 5 5 35 silica graphite Example
(Co.sub.0.3Fe.sub.0.7).sub.2O.sub.3 0.4 100 36 Example
(Cr.sub.0.03Fe.sub.0.97).sub.2O.sub.3 0.4 95 carbon 0.01 5 37 black
Example (Mn.sub.0.02Fe.sub.0.98).sub.2O.sub.3 0.4 95 natural 18 25
38 graphite Example (Cu.sub.0.01Fe.sub.0.99).sub.2O.sub.3 0.4 70
carbon 0.01 5 silica 25 1 39 black Example
(Mn.sub.0.02Ni.sub.0.07Fe.sub.0.91).sub.2O- .sub.3 0.5 85 colloidal
0.02 15 40 silica Example
(Co.sub.0.3Cr.sub.0.01Fe.sub.0.69).sub.2O.sub.3 0.5 94 carbon 0.01
5 silica 20 1 41 black Example (Mn.sub.0.01Cu.sub.0.01Fe.s-
ub.0.98).sub.2O.sub.3 0.5 75 natural 18 25 42 graphite Example
(Ni.sub.0.1Co.sub.0.3Cr.sub.0.01 0.5 100 43
Fe.sub.0.6).sub.2O.sub.3 Dew Oxidation TCE decomposition Production
point at time Half- method termination of time in of of composite
X-ray diffraction intensity ratio of aqueous Residual composite
reduction powder composite powder* solution ratio powder (.degree.
C.) (hr) .alpha.-Fe .gamma.-Fe Magnetite Fayalite Wustite (hr) (%)
Example 4 -10 2 1 0.00 0.01 0.00 0 25 5 32 Example 2 -30 6 1 0.15
0.02 0.00 0 30 7 33 Example 1 10 2 1 0.00 0.03 0.03 0 28 6 34
Example 1 10 2 1 0.05 0.02 0.00 0 28 8 35 Example 2 -30 6 1 0.00
0.02 0.04 0 29 5 36 Example 1 20 2 1 0.00 0.04 0.00 0 31 10 37
Example 1 -10 2 1 0.00 0.02 0.00 0 33 9 38 Example 4 -20 0.5 1 0.00
0.02 0.03 0 32 9 39 Example 1 -20 2 1 0.32 0.02 0.03 0 29 7 40
Example 3 15 0 1 0.00 0.03 0.03 0 18 6 41 Example 2 -30 12 1 0.00
0.02 0.00 0 31 9 42 Example 1 -10 2 1 0.12 0.03 0.00 0 32 10 43
*X-ray diffraction intensity ratio is the ratio to X-ray
diffraction intensity of .alpha.-Fe
Example 24 and Comparative Example 4
[0138] Production of Composite Powder
[0139] Hematite having an average primary particle size of 0.3
.mu.m was reduced in a tube furnace at 550.degree. C. in an
atmosphere of hydrogen gas having a dew point of -40.degree. C.
During reduction, the dew point in the tube furnace was determined
with a dew point hygrometer to thereby determine changes in the
amount of water formed. The dew point once elevated due to the
water formed as a result of reduction of hematite, but converged on
a temperature of -40.degree. C. upon the completion of the
reaction. Accordingly, the time when the dew point converged on a
temperature of -40.degree. C. was considered as the completion of
the reaction. After completion of the reaction, the furnace was
cooled to room temperature, the inner atmosphere was replaced with
nitrogen gas, and again replaced with nitrogen gas containing 5% by
volume of oxygen. Thus, the surface of the obtained metal powder
was slightly oxidized to form magnetite and thereby yielded a
composite powder.
[0140] Performance of Composite Powder as Electromagnetic Wave
Absorber
[0141] The above-prepared composite powder had an average primary
particle size of 0.55 .mu.m as determined by air-permeametry. The
composite powder was subjected to X-ray diffraction pattern
analysis to verify that the composite powder contained 0.2% by
volume of magnetite with the balance of a pure .alpha.-Fe phase
where the ratio of the intensity of magnetite to that of .alpha.-Fe
was 0.002. The magnetization of the composite powder was determined
at 800 kA/m with a vibrating sample magnetometer.
[0142] To the composite powder were added 1.25% by mass of an epoxy
resin and 0.25% by mass of zinc stearate, and the resulting mixture
was molded at room temperature at a pressure of 686 MPa into a ring
having an outer diameter of 12 mm, an inner diameter of 8 mm and a
thickness of 2 mm. The constitutive resin was then cured by heating
at 180.degree. C. and thereby yielded a magnetic powder core
(Example 24). The ratio of permeability to initial permeability of
the magnetic powder core was determined as a complex impedance
measured with an impedance analyzer at frequencies from 10 kHz to 1
GHz. The ratio of permeability to initial permeability at 10 kHz
(.mu..sub.ri/.mu..sub.o: 10 k) was measured, and the frequency at
which this ratio was decreased to eight tenth (critical frequency
f.sub.cr) was determined. The results are shown in Table 2.
[0143] As a comparison, a commercially available carbonyl iron
powder having an average particle size of 3.00 .mu.m was molded in
the same manner as above to yield a magnetic powder core
(Comparative Example 4) and the properties thereof were determined.
The results are also shown in Table 2.
[0144] The magnetization of each of the powders was near to that of
pure iron
[0145] Wb/m.sup.2) to verify that the powders were magnetically
pure iron. When the powders were molded into magnetic powder cores,
the magnetic powder core according to Example 24 comprising the
composite powder of the invention showed a slightly low ratio of
permeability to initial permeability, but a higher critical
frequency as determined at the same density than the magnetic
powder core according to Comparative Example 4 comprising the
carbonyl iron powder. This verified that the magnetic powder core
of Example 24 can stably keep the ratio of permeability to initial
permeability even in the high frequency regions.
3 TABLE 2 Properties of Characteristics of composite powder
magnetic powder core Average Ratio of primary permeability particle
.sigma.800k relative to initial size (Wb/ Density permeability
f.sub.cr (.mu.m) m.sup.2) (Mg/m.sup.3) .mu..sub.n/.mu..sub.o:10 k
(MHz) Example 24 0.55 2.13 6.54 35 55 Comparative 3.00 2.14 6.56 40
32 Example 4
Examples 25 to 31 and 44 to 48
[0146] Production of Composite Powder (Mixture)
[0147] A series of powder mixtures was obtained by mixing hematite
powders with nonferrous inorganic compound (dielectric) powders in
compositional ratios indicated in Table 3. The compositions and
average primary particle sizes of the hematite powders and the
nonferrous inorganic compound powders are shown in Table 3. The
powder mixtures were reduced with hydrogen gas in a box furnace
under conditions shown in Table 3, cooled to room temperature and
then oxidized in nitrogen gas with 5% by volume of oxygen gas for
equal to or less than 12 hours.
[0148] The obtained powders were subjected to X-ray diffraction
analysis, and the volume ratios of individual phases were
determined based on the ratios of the maximum peaks of the
individual phases. The results are shown in Table 3. Any of the
composite powders mainly contained .alpha.-Fe, magnetite and
nonferrous oxides, and a very small part of the nonferrous oxides
were converted into iron compounds. This verifies that, by the
reduction procedure, almost none of the nonferrous oxides were
reduced, but hematite was selectively reduced. Next, while
distinguishing iron powder particles from nonferrous powder
particles by scanning electron microscopy-energy dispersive X-ray
spectroscopy (SEM-EDX), the average primary particle size of each
powder was determined. The results are shown in Table 3.
[0149] Performance of Composite Powder (Mixture) as Electromagnetic
Wave Absorber
[0150] Each of the above-prepared composite powders was mixed with
a resin or rubber such as ethylene-vinyl acetate resin in a ratio
shown in Table 3, and the mixture was molded into a sheet of 3 mm
thick. The electromagnetic wave absorption of the sheet was
determined at frequencies from 1.8 to 18 GHz.
[0151] Table 3 shows that the sheets according to Examples 25 to 31
and 44 to 48 could satisfactorily absorb the electromagnetic
waves.
4 TABLE 3-1 Hematite (Fe.sub.2O.sub.3) Nonferrous inorganic
compound Average Average Oxidation primary Compositional primary
Compositional time particle ratio particle ratio Reduction
Reduction of composite size (% by size (% by time temperature
powder (.mu.m) mass) Type (.mu.m) mass) (hr) (.degree. C.) (hr)
Example 25 0.5 100 -- -- 0 2.0 550 12 Example 26 1.1 70 titanium
0.2 30 2.0 550 0 oxide** Example 27 1.1 50 titanium 0.2 50 2.0 550
0 oxide** Example 28 1.1 10 titanium 0.2 90 1.5 550 6 oxide**
Example 29 0.6 50 titanium 0.2 50 1.5 550 0 oxide** Example 30 2.3
50 aluminium 0.1 50 1.5 550 12 oxide*** Example 31 0.8 50 silicon
0.15 50 1.5 550 0 oxide**** Resin/rubber Amount relative to Content
in composite powder as determined by X-ray composite diffraction (%
by volume) powder By- Nonferrous (% by Electromagnetic produced
inorganic weight wave .alpha.-Fe Magnetite* Wustite oxide oxide
Type in sheet) absorption***** Example 25 99.8 0.2 0.1 -- --
ethylene- 40 good vinyl acetate resin Example 26 65.3 0.2 0 --
titanium ethylene- 20 good oxide** 35.5 vinyl acetate resin Example
27 45.5 0.3 0 Fe.sub.2TiO.sub.3 0.2 titanium ethylene- 30 good
oxide** 54 vinyl acetate resin Example 28 45.5 0.3 0
Fe.sub.2TiO.sub.3 0.2 titanium ethylene- 50 good oxide** 73.3 vinyl
acetate resin Example 29 43.7 0.7 0 Fe.sub.2TiO.sub.3 0.5 titanium
ethylene- 90 good oxide** 55.1 vinyl acetate resin Example 30 46.1
0.8 0 Fe.sub.2AlO.sub.3 0.1 aluminium ethylene- 80 good oxide*** 53
vinyl acetate resin Example 31 45.3 0.9 0 Fe.sub.2SiO.sub.3 0.1
silicon ethylene- 80 good oxide**** vinyl acetate 53.7 resin *X-ray
diffraction intensity ratio of magnetite = (volume percentage of
magnetite)/(volume percentage of .alpha.-Fe)(within the range shown
in Table 3) **Titanium oxide: anatase-type,: relative dielectric
constant: 48.0, standard Gibbs free energy of formation of oxide
(.DELTA.G.degree. f.): -887.6 kJ/mol ***Aluminium oxide: relative
dielectric constant: 9.34 to 11.54 (varying depending on direction
of electric field to the crystal axis), standard Gibbs free energy
of formation of oxide (.DELTA.G.degree. f.): -1581.9 kJ/mol ****
Silicon oxide: relative dielectric constant: 4.27 to 4.68 (varying
depending on direction of electric field to the crystal axis),
standard Gibbs free energy of formation of oxide (.DELTA.G.degree.
f.): -856.5 kJ/mol Standard Gibbs free energy of formation of
hematite (Fe.sub.2O.sub.3) (.DELTA.G.degree. f.): -763.6 kJ/mol
.DELTA.G.degree. f.: as determined under standard condition at 298
K *****good: reflection ratio is equal to or less than -5 dB
[0152]
5 TABLE 3-2 Hematite (Fe.sub.2O.sub.3) Nonferrous component
Nonferrous inorganic in metal compound Oxidation (ratio Average
Average time based on primary Compositional primary Compositional
of total amount particle ratio particle ratio Reduction Reduction
composite of metal size (% by size (% by time temperature powder
component) (.mu.m) mass) Type (.mu.m) mass) (hr) (.degree. C.) (hr)
Example 5% Ni 0.5 10 titanium 0.2 90 1 500 1 44 oxide** Example 25%
Co 0.6 20 titanium 0.2 80 1 500 1 45 oxide** Example 6% Mn 0.6 45
silicon 0.1 55 1.5 550 1 46 oxide**** Example 2% Cr- 0.6 55
aluminium 0.1 45 2 550 1 47 1% Cu oxide** Example 1% Ni- 0.5 50
aluminium 0.1 50 2 550 1 48 30% Co- oxide*** 1% Mn Resin/rubber
Amount relative to composite Content in composite powder as
determined powder by X-ray diffraction (% by volume) (% by
Non-ferrous weight Electromagnetic By-produced inorganic in wave
.alpha.-Fe .gamma.-Fe Magnetite* Wustite oxide oxide Type sheet)
absorption***** Example 7.1 0.6 0.2 0.1 Fe.sub.2TiO.sub.3 3.5
titanium urethane 70 good 44 oxide rubber 88.5 Example 18.2 0.3 0.1
Fe.sub.2TiO.sub.3 2.1 titanium urethane 80 good 45 oxide rubber
78.3 Example 43.4 0.5 0.1 F.sub.2SiO.sub.3 3.0 silicon urethane 70
good 46 oxide rubber 53.0 Example 53.9 0.3 0.1 Fe.sub.2Alo.sub.3
2.6 aluminium urethane 50 good 47 oxide rubber 43 Example 48.8 0.3
0.1 Fe.sub.2AlO.sub.3 2.3 aluminium urethane 40 good 48 oxide
rubber 48 *X-ray diffraction intensity ratio of magnetite = (volume
percentage of magnetite)/(volume percentage of .alpha.-Fe)(within
the range shown in Table 3) **Titanium oxide: anatase-type,
relative dielectric constant: 48.0, standard Gibbs free energy of
formation of oxide (.DELTA.G.degree. f.): -887.6 kJ/mol
***Aluminium oxide: relative dielectric constant: 9.34 to 11.54
(varying depending on direction of electric field to the crystal
axis), standard Gibbs free energy of formation of oxide
(.DELTA.G.degree. f.): -1581.9 kJ/mol ****Silicon oxide: relative
dielectric constant: -4.27 to 4.68 (varying depending on direction
of electric field to the crystal axis), standard Gibbs free energy
of formation of oxide (.DELTA.G.degree. f.): -856.5 kJ/mol Standard
Gibbs free energy of formation of hematite (Fe.sub.2O.sub.3)
(.DELTA.G.degree. f.): -763.6 kJ/mol .DELTA.G.degree. f.: as
determined under standard condition at 298 K *****good: reflection
ratio is equal to or less than -5 dB
[0153] Other embodiments and variations will be apparent to those
skilled in the art, and this invention is not to be limited to the
specific matters stated above.
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