U.S. patent application number 10/515952 was filed with the patent office on 2005-10-06 for method and device for producing metal powder.
Invention is credited to Kagohashi, Wataru, Miyagi, Takuya.
Application Number | 20050217428 10/515952 |
Document ID | / |
Family ID | 29561399 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050217428 |
Kind Code |
A1 |
Kagohashi, Wataru ; et
al. |
October 6, 2005 |
Method and device for producing metal powder
Abstract
Raw metal is intermittently and continuously charged into a
chlorination furnace, the raw metal and chlorine gas are reacted in
the chlorination furnace to continuously generate metal chloride
vapor, and the metal chloride vapor and hydrogen gas are reacted in
a reduction furnace to continuously obtain metallic powder.
Furthermore, the chlorination furnace in which the chlorination
reaction is proceeding is weighed, and the charged amount of the
raw metal to the chlorination furnace is controlled depending on
the weighing result.
Inventors: |
Kagohashi, Wataru;
(Chigasaki-shi, Kanagawa, JP) ; Miyagi, Takuya;
(Chigasaki-shi, Kanagawa, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
29561399 |
Appl. No.: |
10/515952 |
Filed: |
November 29, 2004 |
PCT Filed: |
May 19, 2003 |
PCT NO: |
PCT/JP03/06191 |
Current U.S.
Class: |
75/367 |
Current CPC
Class: |
B22F 9/28 20130101; C22B
34/1209 20130101; B22F 2999/00 20130101; B22F 9/28 20130101; C22B
34/1286 20130101; B22F 2201/013 20130101; B22F 2999/00
20130101 |
Class at
Publication: |
075/367 |
International
Class: |
B22F 009/22 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 2002 |
JP |
2002-155101 |
Claims
1. A method for production of metallic powder, the method
comprising: charging raw metal into a chlorination furnace
intermittently or continuously, reacting the raw metal and chlorine
gas in the chlorination furnace to generate metal chloride vapor
continuously; continuously reacting the metal chloride vapor and
hydrogen gas in a reduction furnace to obtain metallic powder;
weighing the weight of the chlorination furnace in which a
chlorination reaction is proceeding; and controlling an amount of
the raw metal charged into the chlorination furnace according to
the weighing result.
2. The method for production of the metallic powder according to
claim 1, wherein weight changing rate of the chlorination furnace
is observed, and charged amount of the raw metal to the
chlorination furnace is controlled depending on the changing
rate.
3. The method for production of the metallic powder according to
claim 2, wherein the charged amount of the raw metal is temporarily
and suddenly increased in the case in which the changing rate
suddenly decreases.
4. The method for production of the metallic powder according to
claim 2, wherein the changing rate is measured, the charged amount
of the raw metal into the chlorination furnace is automatically
controlled according to the measurement, and chlorination reaction
rate of the raw metal and the chlorine gas in the chlorination
furnace is controlled.
5. The method for production of the metallic powder according to
claim 1, wherein the weight of the chlorination furnace is weighed
by a load cell.
6. The method for production of the metallic powder according to
claim 1, wherein the entirety of a raw material hopper which
charges the raw metal into the chlorination furnace is weighed.
7. The method for production of the metallic powder according to
claim 1, wherein the metal is nickel.
8. The method for production of the metallic powder according to
claim 7, wherein the metallic powder is nickel powder having an
average particle diameter in a range from 0.01 to 1 .mu.m.
9. The method for production of the metallic powder according to
claim 8, wherein the metallic powder is for a conductive paste.
10. The method for production of the metallic powder according to
claim 8, wherein the metallic powder is for a multilayer ceramic
capacitor.
11. A production device for the metallic powder, comprising: a raw
material hopper to charge raw metal; a chlorination furnace to
chlorinate the raw metal charged from the raw material hopper; and
a reduction furnace to reduce metal chloride vapor generated in the
chlorination furnace; wherein the raw material hopper and the
chlorination furnace are communicated with a raw material charging
pipe which charges the raw metal via a carrying valve for
controlling charged amount thereof; the chlorination furnace and
the reduction furnace are communicated with a carrying pipe which
carries metal chloride vapor generated in the chlorination furnace
to the reduction furnace; the chlorination furnace is comprising a
chlorine gas charging pipe to charge chlorine gas inside; the
reduction furnace is comprising a nozzle to inject the metal
chloride vapor inside, a hydrogen gas charging pipe to charge
hydrogen gas inside, and a cooling gas charging pipe to charge
inert gas for cooling the reduced metallic powder inside; the
producing device of the metallic powder further comprises a
weighing method to weigh the entire weight of the chlorination
furnace and a controlling method to control charged amount of the
raw metal to the chlorination furnace depending on the weighing
result of the weighing method.
12. The production for device of metallic powder according to claim
11, wherein the controlling method monitors the changing rate of
the weight of chlorination furnace, and controls the charged amount
of the raw metal into the chlorination furnace depending on the
changing rate.
13. The production device for metallic powder according to claim
12, wherein the controlling method increases the charged amount of
the raw metal temporarily and immediately in the case in which the
changing rate is suddenly decreased.
14. The producing device for metallic powder according to claim 12,
wherein the controlling method measures the changing rate, controls
the charged amount of the raw metal into the chlorination furnace
automatically according to the measurement, and controls the
chlorination reaction rate of the raw metal and the chlorine gas in
the chlorination furnace.
15. The production device for metallic powder according to claim
11, wherein the device further comprises a weighing means for
weighing the weight of the entirety of the raw material hopper.
16. The production device for metallic powder according to claim
11, wherein the weighing means is a load cell.
17. The production device for metallic powder according to claim
11, wherein the metal is nickel.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for production of
metallic powder and to a production device for the metallic powder,
and in particular, relates to a method for production of a metallic
powder and to a production device for the metallic powder, such as
one of nickel, which is appropriate for various purposes such as
for conductive paste filler used in electronic parts such as
multilayer ceramic capacitors or the like, bonding materials for
titanium materials, or catalysts.
BACKGROUND ART
[0002] Metallic powders of nickel, copper, or the like are widely
used in various fields such as those of electronic materials and
catalysts. In recent years, metallic powders which are called
"ultrafine particles" having average diameters of not more than 1
.mu.m, have attracted attention as materials for forming internal
electrodes used in multilayer ceramic capacitors. Conventionally,
noble metal powders such as those of silver, palladium, platinum,
and gold, or base metal powders such as those of nickel, cobalt,
iron, molybdenum, and tungsten, are used in conductive pastes for
electronic materials, and in particular, in internal electrodes of
multilayered ceramic capacitors. Generally, a multilayer ceramic
capacitor is made by alternately laminating ceramic dielectric
layers and metallic layers which are used as internal electrodes,
and by forming external electrodes which are connected to the
metallic layer of internal electrodes on both faces of the ceramic
dielectric layers. Materials which contain a high dielectric
constant material as a main component, such as barium titanate,
strontium titanate, and yttrium oxide, are used in the dielectric
substances. On the other hand, the noble metal powders or base
metal powders mentioned above are used as metals which form the
internal electrodes. However, since recently more inexpensive
electrical materials are required, multilayer ceramic capacitors in
which the latter base metal powders are used have been greatly
developed. In particular, nickel powder has been mainly
developed.
[0003] A multilayer ceramic capacitor may be made by the following
method. First, a dielectric powder such as barium titanate is mixed
with an organic binder so as to form a suspension. Next, the
suspension is formed into a sheet by a doctor blade method to form
a dielectric green sheet. In addition, metallic powder for an
internal electrode is mixed with an organic compound such as an
organic solvent, plasticizer, or organic binder to form a metallic
powder paste, and this paste is coated on the green sheet mentioned
above by screen printing. After several hundreds of layers thereof
are laminated, they are sintered at not less than 1000 .degree. C.,
and then outer electrodes are baked on both sides of the dielectric
ceramic layer to obtain a multilayer ceramic capacitor.
[0004] In an organic component removing process in which organic
components are vaporized from metallic paste or a sintering process
after the removing process in the production of the multilayer
ceramic capacitor described above, volume change occurs due to
expansion or shrinkage of the metallic powder. On the other hand,
similarly, volume change of the dielectric material itself also
occurs by sintering. That is to say, since dielectric materials and
metallic powders, which are different materials, are sintered at
the same time, differences in sintering behavior originating due to
volume changes such as expansion or shrinkage of each material
cannot be avoided, and as a result, there have been problems such
as destruction of layered structure, and so called delamination
including, for example, cracking or separation.
[0005] Furthermore, as the size of a capacitor is reduced and
capacity of the capacitor increases, not only ultrafine particles
having diameters of not more than 1 .mu.m, but also ultrafine
particles having diameters of not more than 0.5 .mu.m are required
as metallic powders used in internal electrodes so as to increase
the number of laminations, by reducing the thicknesses of layers of
internal electrodes, and thereby reduce the resistance. At this
time, the existence of coarse particles having diameter of not less
than 1 .mu.m or 2 .mu.m blocks reduction in the thicknesses of the
layers of the internal electrodes. Furthermore, occurrence of
uneven parts on the surface of the electrodes causes short of
circuiting, and results in delamination.
[0006] A method for production of metallic powder in which few such
coarse particles are contained is disclosed in Japanese Unexamined
Patent Application Publication No. 98-219313. In this method, a
chlorination process in which chlorine gas is contacted to metal to
generate metal chloride vapor continuously and a reduction process
in which the metal chloride vapor generated in the chlorination
process is contacted with reducing gas to reduce the metal chloride
continuously are included.
[0007] The method for the production mentioned above is an
efficient method because nickel powders, particularly having
diameters of not more than 1 .mu.m, can be produced reliably and
efficiently. However, coarse particles having diameters of not less
than 1 .mu.m or 2 .mu.m are contained in the yielded metallic
powder. A method for production or a method for improvement of the
device which can reduce generation of coarse particles are
required.
DISCLOSURE OF THE INVENTION
[0008] Therefore, in a method in which raw metal and chlorine gas
are reacted to generate metal chloride vapor, and the metal
chloride vapor and hydrogen gas are reacted to obtain metallic
powers, an object of the present invention is to provide a method
for production and a production device which can produce metallic
powders particularly having reliable average diameters of not more
than 1 .mu.m appropriate for internal electrodes in multilayer
ceramic capacitors, without generating coarse particles.
[0009] The inventors have researched to achieve the object
mentioned above and have found that when chlorine gas which is not
reacted yet in a chlorination furnace is charged into a reduction
furnace, this causes the reaction temperature to increase and
generates coarse particles.
[0010] The method for production of metallic powder of the present
invention was performed based on the information described above. A
method for production of metallic powder comprises: charging raw
metal into a chlorination furnace intermittently or continuously;
reacting the raw metal and chlorine gas in the chlorination furnace
to generate metal chloride vapor continuously; and reacting the
metal chloride vapor and hydrogen gas in a reduction furnace to
obtain metallic powder continuously and is characterized in
weighing the weight of the chlorination furnace in which
chlorination reaction is proceeding, and controlling the charged
amount of the raw metal into the chlorination furnace depending on
the weighing result.
[0011] Furthermore, a production device for the metallic powder of
the present invention is characterized in that the device has a raw
material hopper to charge raw metal, a chlorination furnace to
chlorinate the raw metal charged from the raw material hopper, and
a reduction furnace to reduce metal chloride vapor generated in the
chlorination furnace, wherein the raw material hopper and the
chlorination furnace are communicated with a raw material charging
pipe which charges the raw metal via a carrying valve for
controlling charged amount thereof, the chlorination furnace and
the reduction furnace are communicated with a carrying pipe which
carries metal chloride vapor generated in the chlorination furnace
to the reduction furnace, the chlorination furnace has a chlorine
gas charging pipe to charge chlorine gas inside, the reduction
furnace has a nozzle to inject the metal chloride vapor inside; a
hydrogen gas charging pipe to charge hydrogen gas inside; and a
cooling gas charging pipe to charge inert gas for cooling the
reduced metallic powder inside, the production device of the
metallic powder further has a weighing means to weigh the entire
weight of the chlorination furnace and a controlling means to
control the amount of the raw metal charged into the chlorination
furnace depending on the weighing result of the weighing
method.
[0012] In the present invention, since the charged amount of the
raw metal is controlled depending on the measuring result of the
weight of the entirety of the chlorination furnace, appropriate
amount of raw metal can be always filled in the chlorination
furnace. Therefore, the raw metal and the chlorine gas can be
reacted uniformly, and it becomes possible to reduce the charged
amount of chlorine gas which is not reacted yet.
[0013] As a metallic powder of the present invention, one of a
metal which can be used in an internal electrode or a catalyst of
multilayer ceramic capacitors can be used. For example, a noble
metal such as silver, palladium, platinum, gold or the like, or a
base metal such as nickel, cobalt, iron, molybdenum, tungsten or
the like. Among these metals, base metals are desirable from the
viewpoint of cost, and in particular, nickel is more desirable.
[0014] Although characteristics of particles of the metallic powder
produced in the present invention are not particularly limited as
long as there is no difficulty in use of each, average diameter of
the metallic powder is desirably in a range from 0.01 to 1 .mu.m,
more desirably in a range from 0.1 to 1 .mu.m, and further
desirably in a range from 0.1 to 0.5 .mu.m, in the case in which
the particles are used as an internal electrode of multilayer
ceramic capacitors. Furthermore, it is desirable that the specific
surface area of the metallic powder by BET be in a range from 1 to
20 m.sup.2/g. Furthermore, it is desirable that the particle shape
of the metallic powder be spherical to improve sintering
characteristics and dispersibility.
[0015] The present invention is based on a method in which raw
metal and chlorine gas are reacted in a chlorination furnace to
generate metal chloride vapor continuously, and the metal chloride
vapor and hydrogen are reacted in a reduction furnace to obtain
metallic powder continuously (hereinafter this may be called a
"chlorination reduction method"). Generally, in a production method
of metallic powder by such a vapor phase reduction reaction, a
metal atom is generated at a moment when metal chloride vapor and
hydrogen gas are contacted, the metal atoms collide and condense to
generate ultrafine particles and the particles grow. The diameter
of the generated metal powder is determined depending on conditions
such as partial pressure of the metal chloride vapor or
temperature. In this chlorination reduction method, the amount of
the metal chloride vapor which is charged in the reduction furnace
can be controlled by controlling the charged amount of chlorine gas
since the metal chloride vapor is generated depending on the
charged amount of the chlorine gas. In addition, since the metal
chloride vapor is generated by a reaction of chlorine gas and
metal, amount of carrier gas used can be reduced compared to a
method in which metal chloride vapor is generated by heating and
evaporating a solid of metal chloride. Furthermore, it is possible
not to use carrier gas depending on production conditions.
Therefore, production cost can be reduced by reducing the amount of
carrier gas used and heating energy therefor.
[0016] In addition, the partial pressure of the metal chloride
vapor in the reduction furnace can be controlled by mixing inert
gas with the metal chloride vapor which is generated in the
chlorination reaction. In this way, by controlling the charged
amount of the chlorine gas or the partial pressure of the metal
chloride vapor charged in the reduction furnace, the particle
diameter of the metallic powder can be controlled, and not only can
the diameter of metallic powder be stabilized, but also the
diameter can be freely set.
[0017] As explained up to this point, the chlorination reduction
method is superior from the viewpoints that metallic powder having
consistent particle diameter can be obtained and that the metallic
powder can be produced efficiently and at low cost. However, in the
case in which the metallic powder is produced continuously by the
chlorination reduction method, the chlorination reaction rate may
be changed. If the chlorination reaction rate is changed, the
amount of the metal chloride vapor which is generated in the
chlorination furnace is changed and therefore the partial pressure
of the metal chloride in the reduction furnace is changed, and as a
result, particle sizes of the metal chloride vapor become
nonuniform. There is a case in which metallic powder having desired
particle diameter cannot be obtained. In particular in a production
process for nickel powder for internal electrodes of a multilayer
ceramic capacitor, coarse particles having diameters of not less
than 1 .mu.m or 2 .mu.m may be greatly generated if such changing
in chlorination reaction rate occurs.
[0018] For example, in a production process of nickel powder, raw
nickel in a pelleted state of several millimeters is filled in the
chlorination furnace, heated up to about 800.degree. C., and
chlorine gas and raw nickel are continuously charged to perform the
chlorination reaction. At this time, raw nickel is chlorinated to
generate nickel chloride vapor, and filled layer of raw nickel in
the chlorination furnace is decreased. In this decreasing process,
the chlorination reaction rate can be maintained constant if the
filled layer of raw nickel is constantly decreased along the cross
section of the chlorination furnace.
[0019] However, temperature distribution of the filled layer of raw
nickel in the chlorination furnace is not uniform, and the central
part or outer circumferential part of the filled layer of raw
nickel may be selectively chlorinated and decreased depending on
the position of chlorine gas charged in the chlorination furnace
and the position of raw nickel. If such a non-uniform decreasing of
filled layer of raw nickel is continued, gaps which penetrate the
filled layer occur, and a part of the charged chlorine gas may be
directly charged to the reduction furnace together with nickel
chloride vapor without contacting raw nickel. If chlorine gas which
is not reacted yet is charged directly in the reduction furnace,
the partial pressure of the nickel chloride vapor in the reduction
furnace is decreased and generating rate of nickel powder is
increased owing to the existence of chlorine gas, and as a result,
coarse particles are extremely increased.
[0020] The inventors have found that occurrence of the coarse
particles is mainly caused by an inflow of chlorine gas which is
not reacted yet in such chlorination reaction into the reduction
furnace. Originally, such an abnormal phenomenon can be detected by
determining composition of vapor and gas generated in the
chlorination furnace continuously. However, a mixture gas of
chlorine gas and metal chloride gas as in the present invention
makes it difficult to perform separation and detection.
[0021] Therefore, it is desirable that changing rate of
chlorination furnace weight be observed to perform feedback control
of the chlorination reaction rate, since the chlorination reaction
rate corresponds to the changing rate of chlorination furnace. As a
controlling method of chlorination reaction rate, the amount of
chlorine gas charged in the chlorination furnace can be reduced or
the charged amount of the metal chloride vapor generated in the
chlorination furnace to the reduction furnace can be reduced since
deterioration of reaction rate mainly resulted from outflow of
non-reacted chlorine gas through a penetrated gap which occurred in
the filled layer of the raw metal in the chlorination furnace as
described above. However, in these methods, overall productivity of
metallic powder is deteriorated, reaction in the reduction furnace
becomes uneven, and particle size of generated metallic powder may
become nonuniform. Therefore, it is desirable that raw metal be
charged in the chlorination furnace so as not to form gaps among
filled layers of raw metal in the chlorination furnace. Usually, in
a continuous operation, although raw metal is charged continuously
or intermittently in the chlorination furnace, also in this case,
it is desirable that the charged amount of the raw material be
increased corresponding with detected decrease of reaction
rate.
[0022] In addition, if the chlorine gas which is not contacted with
raw metal and not reacted yet flows into the reduction furnace, the
reaction rate of chlorination is radically decreased, particle size
of generated metallic powder may be nonuniform if the phenomenon is
allowed to remain as it is, and large amounts of coarse particles
may be generated.
[0023] Therefore, in the present invention, it is desirable that
the changing rate of the chlorination furnace weight be monitored
and that the charged amount of the raw material be temporarily
increased if an indication that the changing rate is deteriorated
is observed. For example as shown in FIG. 4, if a fall in changing
rate P is detected, the same or greater amount as an amount of the
raw metal which is charged intermittently or continuously over 30
minutes is charged once or several times. After that, standard or
lower amounts are charged intermittently or continuously. In this
way, the condition of excess chlorine gas can be solved
simultaneously. Therefore, the particle size of obtained metallic
powder can be stabilized by decreasing the non-reacted chlorine gas
which is charged in the reduction furnace, and in particular,
generation of coarse particles can be reduced.
[0024] Specifically, it is desirable to use a load cell as a
weighing method of chlorination furnace weight, and it is more
desirable to use one which can detect the weight changing over
time. In the present invention, changing in chlorination furnace
weight is detected, weight changing amount per unit time is
calculated, and this is controlled as the reaction rate. That is,
this reaction rate means weight of metal chloride vapor generated
per unit time. If this reaction rate is always maintained constant,
the chlorination reaction is stabilized. Therefore, as a result,
reaction in the reduction furnace is also stabilized and particle
size of metallic powder obtained is also stabilized.
[0025] Furthermore, in the case in which the raw metal is charged
in the chlorination furnace continuously or intermittently as
described above, a raw material hopper in which raw metal is stored
and charged is also weighed by the load cell. In this way, the
reaction rate of chlorination can be detected and controlled by
observing changing in weight of the raw material hopper and the
chlorination furnace.
[0026] Desirable conditions of the method for production in the
present invention are shown as follows.
[0027] (1) A raw metal such as nickel metal is charged from a raw
material hopper using a weighing method by a load cell, to a
chlorination furnace using weighing method by a load cell, to form
filled layer of the raw metal having a height to some extent.
[0028] (2) After that, the chlorination furnace is heated and
chlorine gas. is charged in the chlorination furnace to start
chlorination reaction.
[0029] (3) At the same time, the raw metal is charged continuously
or intermittently.
[0030] (4) Reaction rate of chlorination reaction is continuously
detected by a weight changing of the raw material hopper and the
chlorination furnace.
[0031] (5) Charging of the raw metal is increased to make the
reaction rate reach a specific rate if changes, in particular
decrease, of reaction rate is observed.
[0032] In the features described above, the production system of
metallic powder in which the weights of raw material hopper and
chlorination furnace are weighed, changes in chlorination reaction
rate are detected, charged amount of raw metal is automatically
controlled according to this, and reaction rate is controlled, is
more desirable.
[0033] In the device of the present invention, as described above,
since the chlorination furnace is arranged upstream of the
reduction furnace, and since the both furnaces are connected
directly, chlorination and reduction can be performed at the same
time and continuously to produce metallic powder efficiently.
Furthermore, since the metal chloride vapor is generated depending
on the amount of the chlorine gas charged in the chlorination
furnace and since the chlorination furnace and the reduction
furnace are connected directly, the amount of the metal chloride
vapor charged in the reduction furnace can be controlled by
controlling the charged amount of the chlorine gas.
[0034] Furthermore, by arranging an inert gas charging pipe in the
chlorination furnace, inert gas can be charged in the chlorination
furnace to control the partial pressure of the metal chloride vapor
in the reduction furnace. Therefore, the particle diameter of the
metallic powder can be controlled by controlling the charged amount
of the chlorine gas or the partial pressure of the metal chloride
vapor which is charged in the reduction furnace. Furthermore, since
a weighing method which can weigh the entirety of the chlorination
furnace is prepared, changing of reaction rate in the chlorination
reaction can be detected. By controlling this, it has become
possible to stabilize the particle size of metallic powder and in
particular to reduce generation of coarse particles. Furthermore,
since a weighing method is also used in the raw material hopper, it
becomes possible to control the reaction rate with greater
accuracy.
[0035] Next, the production device for metallic powder of an
embodiment of the present invention is explained further by way of
drawings. It is desirable that chlorination reaction be performed
in a chlorination furnace 5 shown in FIG. 1. The chlorination
furnace 5 is supported by a load cell 9. A raw material hopper 1
which stores and charges raw metal 3 is arranged on the upper part
of the chlorination furnace 5, and the raw material hopper 1 is
connected to the top part of the chlorination furnace 5 by a raw
metal charging pipe 21 in which raw metal charging valves 4a and 4b
are arranged on the medium part thereof The raw material hopper 1
is supported by a load cell 2, and the load cell 2 is connected to
the load cell 9 of the chlorination furnace 5.
[0036] A chlorine gas charging pipe 6 is connected on the upper
side part of the chlorination furnace 5, and an inert gas charging
pipe 8 is connected on the lower side part. A heater 7 is arranged
around the chlorination furnace 5, and a metal chloride vapor
carrying pipe 12 is connected to the lower side part of the
chlorination furnace 5. Both perpendicular type and horizontal type
can be applied as the chlorination furnace 5, it is desirable to
use perpendicular type so as to perform a solid-gas contact
reaction uniformly. Furthermore, structures of medium parts of the
raw material charging pipe 21, the chlorine gas charging pipe 6,
and the inert gas charging pipe 8 have elasticity and flexibility
such as bellows, and the weights of raw material hopper 1 and the
chlorination furnace 5 can be weighed accurately. It should be
noted that at the bottom part of the chlorination furnace 5, filler
11 is arranged to form a hearth. The filler 11 is composed of small
pieces of, for example, quartz glass, and metal chloride vapor and
inert gas can pass through the filler 11 while preventing falling
of the raw metal.
[0037] Chlorine gas is caused to flow continuously through the
chlorine gas charging pipe 6 and the flow amount thereof is
measured. It is desirable that the chlorination furnace 5 and other
members be composed of quartz glass. The metal chloride vapor
carrying pipe 12 is connected to a metal chloride vapor injection
nozzle 14 arranged on an upper end part of a reduction furnace 13,
which is explained below.
[0038] Although the shape of the raw metal 3 which is a starting
raw material is not limited in particular, from the viewpoints of
contacting efficiency and prevention of pressure loss increasing,
granular structures having diameters in a range of from 5 to 20 mm,
massive structures, or plate structure is desirable. It is
desirable that the purity be about 99.5% or more. Height of a
filled layer of the raw metal 10 in the chlorination furnace 5 can
be appropriately set in a sufficient range in which charged
chlorine gas is converted to metal chloride vapor by considering
such as chlorine charging rate, temperature of the chlorination
furnace, continuous operation time, and shapes of the raw metal 3.
The temperature in the chlorination furnace 5 can be set in a range
in which the raw metal can be chlorinated, in the case of nickel
metal, it is desirable in a range of from not less than 800.degree.
C. which is necessary for proceeding the reaction, to not more than
1483.degree. C. which is a melting point of nickel. Considering the
reaction rate and durability of the chlorinated furnace 5, it is
desirable that it be in a range of from 900 to 1100.degree. C. in
practice.
[0039] Chlorine gas is continuously charged from the chlorine gas
charging pipe 6 to the chlorination furnace 5, and at the same
time, the raw metal 3 is continuously or intermittently charged
from the raw material hopper 1 by opening and closing the raw
material charging valve 4. At this time, the charged amount of the
raw metal is weighed by the load cell 2.
[0040] Metal chloride vapor generated in the chlorination furnace 5
can be carried to the reduction furnace 13 through the metal
chloride vapor carrying pipe 12, or according to circumstances, 1
to 30 mol % to the metal chloride vapor of inert gas such as
nitrogen or argon is mixed through the inert gas charging pipe 8,
and this mixture gas is carried to the reduction furnace. This
charging of inert gas can be a factor in controlling the particle
diameter of the metallic powder. If too much inert gas is mixed,
not only is too much of the inert gas consumed, but this also
causes energy loss and is uneconomical. From these viewpoints,
desirable partial pressure of the metal chloride vapor in the
mixture gas which flows through the carrying pipe 12 is in a range
of from 0.5 to 1.0 in the case in which total pressure is defined
as 1.0. In particular, it is desirable that the partial pressure be
in a range of from about 0.6 to 0.9 to produce fine metallic
powders having diameters in a range from 0.15 to 0.5 .mu.m.
Furthermore, as described above, the generated amount of metal
chloride vapor can be freely controlled by the charged amount of
chlorine gas, and the partial pressure of the metal chloride vapor
can be freely controlled by the charged amount of inert gas.
[0041] The metal chloride vapor which is generated in the
chlorination furnace 5 is continuously carried to the reduction
furnace 13. On the upper end part of the reduction furnace 13,
metal chloride vapor injection nozzle 14 (hereinafter simply
referred to as nozzle 14) which is connected to the metal chloride
vapor carrying pipe 12 is projecting downward. Furthermore, on the
upper end surface of the reduction furnace 13, hydrogen gas
charging pipe 15 is connected, and cooling gas charging pipe 17 is
connected on the lower side part of the reduction furnace 13.
Furthermore, heater 16 is arranged around the reduction furnace 13.
As is described below, the nozzle 14 has a function in which the
metal chloride vapor (inert gas can be contained) is injected from
the chlorination furnace 5 to the reduction furnace 13 in a
desirable flowing rate.
[0042] In the process in which reduction of the metal chloride
vapor by hydrogen gas is proceeding, reaction flame 18 which is
like a burning flame of gas fuel such as LPG is formed extending
downward from the top part of the nozzle 14. Charged amount of
hydrogen gas to the reduction furnace 13 is chemical equivalent of
the metal chloride vapor, that is to say, from 1.0 to 3.0 times
chemical equivalent of chlorine gas which is charged to the
chlorination furnace 5, desirably from about 1.1 to 2.5 times. But
the charged amount of hydrogen gas is not particularly limited to
these ranges. However, enormous hydrogen flow occurs in the
reduction furnace 13 if too much hydrogen gas is charged. This
hydrogen flow disturbs metal chloride vapor injecting flow which is
injected from the nozzle 14, resulting in an uneven reduction
reaction. In addition, it is uneconomical because gas which is not
consumed may be emitted. Furthermore, although the required
temperature of the reduction reaction is higher than a temperature
which is sufficient to complete the reaction, a temperature less
than a melting point of nickel may be used in the case of
production of nickel powder because nickel powder in a solid state
is easier to handle. Considering reaction rate, durability of the
reduction furnace 13, and economic efficiency, temperature in a
range of from 900 to 1100.degree. C. is practical. However, the
temperature is not limited to this.
[0043] As described above, chlorine gas which is introduced into
chlorination furnace 5 is converted to metal chloride vapor of the
same mole amount, which is reduction raw material. The metallic
powder 19 having appropriate particle diameter can be obtained by
controlling liner velocity of flow of metal chloride vapor or a
mixture of metal chloride vapor and inert gas, which is injected
from the top of the nozzle 14. That is to say, if the diameter of
the nozzle is constant, particle diameter of generated metallic
powder 19 in the reduction furnace 13 can be controlled in a
desired range by the charged amount of chlorine gas and inert gas
sent to the chlorination furnace 5. Desirable linear velocity of
gas flow (sum (calculated value which is converted to the charged
amount of gas at the reduction temperature) of metal chloride vapor
and inert gas) at the top of the nozzle 14 is set in a range from
about 1 to 30 m/sec at a reduction temperature of from 900 to
1100.degree. C., set in a range from about 5 to 25 m/sec in the
case in which nickel powder of small particles having diameters of,
for example, from 0.1 to 0.3 .mu.m, is produced, and is set in a
range of 1 to 15 m/sec in the case in which nickel powders having
diameters from 0.4 to 1.0 .mu.m are produced. The linear velocity
in the axial direction of hydrogen gas in the reduction furnace 13
is desirably in a range from about 1/50 to 1/300 of the injection
velocity (linear velocity) of the metal chloride vapor, and is more
desirably in a range from 1/80 to 1/250. Therefore, substantially,
the metal chloride vapor is injected from the nozzle 14 into a
static hydrogen atmosphere. It is desirable that the exit of
hydrogen charging pipe 15 be not directed toward the flame
side.
[0044] In the method for production of the present invention, the
particle diameter of metallic powder 19 generated in the reduction
furnace 13 is decreased as the charged flowing amount of chlorine
gas to the chlorination furnace 5 is increased, and conversely, the
particle diameter is increased as the charged flowing amount of
chlorine gas is decreased. Furthermore, by mixing inert gas with
the metal chloride vapor near the exit of the chlorination furnace
5 to control the partial pressure of the metal chloride vapor,
specifically by mixing inert gas for 1 to 30 mol % of the metal
chloride vapor, for example, the particle diameter of the metallic
powder can be increased by increasing the partial pressure, and
conversely, the particle diameter of the metallic powder can be
decreased by decreasing the partial pressure of the metal chloride
vapor.
[0045] During the process in which the chlorination reaction is
performed continuously in the chlorination furnace 5 and generated
metal chloride vapor is reduced in the reduction furnace 13 to
produce metallic powder as described above, weight change is
detected continuously by weighing the weight of the chlorination
furnace 5 by the load cell 9. On the other hand, the weight change
of the raw material hopper 1 is continuously weighed by the load
cell 2 to detect the weight of the raw metal 3 which is charged to
the chlorination furnace 5. The reaction rate of the chlorination
reaction is detected by these weight changes over time. That is to
say, the sum of the weight change of the raw material hopper 1 per
unit time and the weight change of the chlorination furnace 5 per
unit time correspond to the weight of the metal chloride vapor
which is generated in the chlorination furnace 5 per unit time, and
further corresponds to the reaction rate (weight of generated metal
chloride vapor/time) of the chlorination reaction.
[0046] This reaction rate is continuously observed through the
production process of the metallic powder, and in the case in which
an indication in which the reaction rate is deteriorated is
observed, charging of the raw metal 3 from the raw material hopper
1 is immediately and temporarily increased to stabilize the
reaction rate. Since the upper surface of the filled layer of raw
metal 10 is uneven at this time, it is desirable that the raw metal
be charged while observing visually, to make this upper surface
even. Furthermore, it is desirable that the raw metal charging
valve 4 and the weight change which is detected by the load cell 2
and 9 be interrelated by using, for example, a distributed
controlling system, in the case in which indication of
deterioration of reaction rate is observed, the metal charging
valve 4 is opened and the raw metal 3 is charged so as to stabilize
the reaction rate.
[0047] Cooling process can be arranged in the process for
production of the metallic powder of the present invention. As
shown in FIG. 1, the cooling process can be arranged in a space at
the opposite side of the nozzle 14 in the reduction furnace 13, and
alternatively, another casing which is connected to the exit of the
reduction furnace 13 can be used. It should be noted that the
cooling of the present invention means an operation which is
performed to stop or to reduce growth of the metallic powder in gas
flow (hydrochloric acid is included) generated at the reduction
process, specifically an operation in which gas flow having
temperature of about 1000.degree. C. after finishing the reduction
reaction is quickly cooled to about 400 to 800.degree. C. The
cooling process can be performed to a lower temperature than this
range.
[0048] As a desirable example of performing cooling, inert gas can
be blown to a space downward of the top of the flame. Specifically,
gas flow can be cooled by blowing nitrogen gas from cooling gas
charging pipe 17. By blowing inert gas, the particle diameter of
the metallic powder 19 can be controlled while clumping of the
metallic powder is prevented. Cooling conditions can be freely
changed by arranging the cooling gas charging pipe(s) at a point or
plural points in the reduction furnace 13 along vertical direction.
In this way, controlling of the particle diameter can be performed
even more accurately.
[0049] The metallic powder 19 and the mixture gas of hydrochloric
acid gas and inert gas which have passed through the processes
described above are carried to a recovering process, and the
metallic powder is separated and recovered from the mixture gas.
Although it is desirable that combination of one kind or more
selected from a bag filter, capture separation method in water,
capture separation method in oil, and magnetic separation method be
used to separate and recover the metallic powder, the methods of
the present invention are not particularly limited. For example, in
the case in which the metallic powder 19 is recovered by a bag
filter, after the metallic powder 19 and the mixture gas of
hydrochloric acid gas and inert gas which are generated in the
cooling process are induced to the bag filter to recover only
metallic powder 19, the metallic powder 19 can be carried to a
washing process. In the case of the capture separation method in
oil, normal paraffin having a carbon number of 10 to 18, or light
oil, can be preferably used. In the case of the capture separation
method in water or oil, clumping or oxidation of metallic powder
particles can be effectively prevented by adding one or more kinds
selected from polyoxyalkyleneglycol, polyoxypropyreneglycol or
derivatives thereof (monoalkyl ether, mono ester), or surface
active agent such as sorbitane, sorbitane monoester, known
antioxidants such as phenol-based or amine-based metal deactivators
represented by benzotriazole or derivatives thereof in a range from
about 10 to 1000 ppm to a scavenger.
[0050] As explained above, in the method and device for production
of the metallic powder by a conventional chlorination reduction
method, uneven reaction occurring in the filled layer of the raw
metal in the chlorination furnace causes flowing of non-reacted
chlorine gas to the reduction furnace. Therefore, particle size of
generated metallic powder is not stabilized, and in particular,
coarse particles are generated. However, in the method and device
of the present invention, reaction rate of the chlorination
reaction can be controlled and stabilized by weighing the weight of
the chlorination furnace, flowing of non-reacted chlorine gas to
the reduction furnace can be prevented, and as result, the metallic
powder having stable particle size and having no coarse particles
can be produced. Furthermore, although reaction rate cannot be
increased in the conventional method and device since the
chlorination reaction is unstable being severely deteriorated as
described above, the reaction rate can be increased in the present
invention since the reaction rate is stabilized. As a result,
productivity of the metallic powder can be improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 is a longitudinal cross section showing a structure
of the production device for metallic powder of the present
invention.
[0052] FIG. 2 is a graph showing particle size distribution of
nickel powder produced in Example and Comparative Example.
[0053] FIG. 3 (A) is a SEM photograph showing nickel powder
produced in an Example, and (B) is a SEM photograph showing nickel
powder produced in a Comparative Example.
[0054] FIG. 4 is a graph showing the reaction rate (changing rate
of the chlorination furnace weight) in the chlorination
furnace.
EXPLANATION OF REFERENCE NUMERAL
[0055] 5 . . . Chlorination furnace, 9 . . . Load cell, 1 . . . Raw
material hopper, 2 . . . Load cell, 13 . . . Reduction furnace, 3 .
. . Raw metal
BEST MODE FOR CARRYING OUT THE INVENTION
[0056] The present invention is further explained in detail by way
of an Example.
EXAMPLE
[0057] 15 kg of raw nickel having average diameters of 5 mm was
filled from raw material hopper 1 to a chlorination furnace 5 of a
production device of metallic powder shown in FIG. 1, the
temperature of the atmosphere in the furnace was maintained at
1100.degree. C., and chlorine gas was induced by flow amount of 4
Nl/min to start chlorination reaction. After that, 0.5 kg of raw
nickel was charged from the raw material hopper 1 to the
chlorination furnace 5 intermittently every 5 minutes. Nickel metal
was chlorinated in this way to generate nickel chloride vapor.
[0058] 10 mol % of charged amount of chlorine gas of nitrogen gas
was mixed with this, and a gas mixture of nickel chloride vapor and
nitrogen was induced from a nozzle 14 into a reduction furnace 13
in which the atmosphere was heated to 1000.degree. C. at a flow
velocity of 2.3 m/sec (converted to in the case of 1000.degree.
C.). At the same time, hydrogen gas was charged from the top part
of the reduction furnace 13 at a flow velocity of 7 Nl/min to
reduce nickel chloride vapor.
[0059] The chlorination reaction and the reduction reaction were
concurrently and continuously conducted (for 30 hours), each weight
of raw material hopper 1 and chlorination furnace 5 were weighed by
a load cell 2 and 9, and the reaction rate of chlorination reaction
in the chlorination furnace was continuously detected by the weight
changing. Since an indication of deterioration in the reaction rate
was observed 25 hours after starting of the production, charging
amount of raw nickel was increased to 5 kg at one time to stabilize
and the reaction rate, the production was continued.
[0060] Nitrogen gas was mixed with generated gas which is generated
in the reduction reaction including nickel powder so as to cool in
the cooling process. Then, mixed gas including nitrogen gas,
hydrochloric acid vapor and nickel powder was introduced into pure
water, and the nickel powder was separated and recovered. The
obtained nickel powder was washed with pure water and dried to
obtain a nickel powder product. The particle size distribution of
the obtained nickel powder is shown in FIG. 2 and the SEM
photograph is shown in FIG. 3 (A). Average particle diameter using
a BET method was 0.40 .mu.m, and the average particle diameter in
the case in which the nickel powder was suspended in organic
solvent was 1.50 .mu.m. 0% of coarse particles having diameters not
less than 5 .mu.m was contained. The average particle diameters in
the case in which the nickel powder was suspended in organic
solvent and the particle diameter distribution was measured by the
following method using a laser diffraction particle size analyzer
(trade name: Coulter LS230, produced by Coulter). Optimum amount of
metallic powder was suspended in .alpha.-terpineol, ultrasonic
waves were applied to disperse for 3 minutes, the sample was
measured in 1.8 of sample refraction factor, and the particle size
distribution of statistic of volume was measured.
COMPARATIVE EXAMPLE
[0061] Nickel powder was produced by a method similar to that in
Example 1, except that the raw material hopper 1 and chlorination
furnace 5 were not weighed and that the reaction rate of
chlorination reaction in the chlorination furnace was not
controlled. Particle size distribution of the obtained nickel
powder is shown in FIG. 2 and a SEM photograph is shown in FIG.
3(B). Average particle diameter using the BET method was 0.45 .mu.m
and the average particle diameter in the case in which the nickel
powder was suspended in organic solvent was 1.45 .mu.m. 3.0% of
coarse particles having diameters not less than 5 .mu.m was
contained.
[0062] With respect to the particle size distribution of nickel
powder which was produced in the method of the embodiment of the
present invention, extremely low amounts of coarse particles are
contained compared to nickel powder which is produced in the
Comparative Example as shown in FIG. 2. Furthermore, it is obvious
that coarse particles having particle diameters not less than 1
.mu.m are contained in extremely small amount in the nickel powder
produced in the Example as shown in the SEM photograph of FIG. 3,
although coarse particles having particle diameters not less than 1
.mu.m are contained to some extent in the nickel powder produced in
the Comparative Example.
[0063] As explained above, metallic powder such as nickel powder
used in, for example, internal electrodes of multilayer ceramic
capacitors in which fine particle diameter not more than 1 .mu.m is
required can be produced efficiently, and furthermore, metallic
powder having uniform particle diameter and having no coarse
particles can be produced by controlling the reaction rate of the
chlorination reaction, in the method for production and production
device for the metallic powder in the present invention.
* * * * *