Note: Descriptions are shown in the official language in which they were submitted.
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14090-MB
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METHOD FOR PRODUCING ACICULAR OR EXUDE
IRON OR IRON ALLOY PARTICLES
While the invention is subject to a wide range of
applications, it especially relates to a relatively
inexpensive apparatus and method of producing substantially
equiaxed or acicular iron or iron alloy particles for magnetic
recording purposes.
Hitherto, various magnetic powder materials have
been proposed or use in preparing magnetic recording
media; for example, y-Fe2O3, Co doped Foe, Foe,
Co doped Foe, Fife' Cry'
preparation of these powders requires a rather lengthy
and expensive process. For example, acicular iron
particles may be manufactured by fluidized bed
reduction of y-Fe2O3. These iron particles are
extremely pyrophoric and require extensive processing
to passivity them.
A number of different processes have been proposed
for producing ferromagnetic metal alloy powder
materials, such as disclosed in US. Patent No.
4,274,865. Besides disclosing a process for preparing
a magnetic powder suitable for magnetic recording
consisting mainly of iron, this patent sets out other
techniques for producing ferromagnetic acicular
particles. However, there is no teaching of the unique
process of manufacturing iron or iron alloy equiaxed or
acicular particles as disclosed in the present
invention.
US. Patent No. 4,290,799 discloses, for example,
"a ferromagnetic metal pigment for magnetic recording
purposes which consists essentially of iron and which
is distinguished by well-developed acicular particles
and superior properties as a recording support, and a
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process for the production of this material." The
process of producing metal powders, as taught by this
patent, is quite different from the present invention.
US. Patent No. 3,556,962 to Prior et at.
describes a method for reclaiming copper scrap
containing iron and US. Patent No. 4,264,419 to Prior
describes a method for electrochemically detaining
copper base alloys. In both patents, there is no
disclosure or leaching ox providing a strip having
lo fine iron particles distributed throughout or
dissolving the strip to recover the particles.
Therefore, the present invention can be clearly
distinguished from those disclosures.
- It is a problem underlying the present invention
to provide a method for producing substantially
equiaxed or acicular ferromagnetic particles of the
desired magnetic or shape an isotropy which are suitable for
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incorporation into conventional magnetic recording
mediums.
It is an advantage of the present invention to
provide a method for producing substantially equiaxed
iron or iron alloy particles which obviates one or
more of the limitations and disadvantages of the
described prior arrangement.
It is a further advantage of the present invention
to provide a relatively inexpensive method of producing
fine iron or iron alloy particles.
Accordingly, there has been provided a method and apparatus
for producing substantially equiaxed iron or iron alloy particles.
A metal or metal alloy strip having fine equiaxed particles of iron
or iron alloy distributed throughout is provided. The petal strip
is selectively dissolved without substantial dissolving of the iron
or iron alloy particles in order to recover the particles The
method may include use of the apparatus which facilitates the
collection of the particles. The collected wrought
particles have a length in the range of about 0.05 to
about 0.5 microns and an aspect ratio of between about
4:1 to about 15:1. The collected particles may be substantially
equiaxed in a size range of about 0.05 to about 0.5 microns.
The invention and further developments of the
invention are now elucidated by means of preferred
embodiments shown in the drawings:
Figure 1 is a schematic diagram of an apparatus
for forming a strip with substantially equiaxed iron
particles in accordance with this invention;
Figure 2 is a graph of anodic dissolution of
copper and iron in sodium sulfate solution;
Figure 3 is a schematic diagram of an apparatus
for carrying out this inventl-n; and
Figure 4 is a schematic diagram of an electron
magnetic container in accordance with the present
invention.
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The present invention relates to a method and
apparatus of producing substantially equiaxed or
acicular ferromagnetic particles. The method requires
a metal or metal strip having distributed throughout
fine, substantially equiaxed particles of iron or iron
alloys. A copper base alloy strip containing ferry-
magnetic particles may be prepared by rapid solidify-
cation so that substantially equiaxed iron or iron alloy
particles sized between about 0.05 to about 0.5 microns
are distributed substantially homogeneously or
isotropic ally throughout the solidified base metal
matrix. The equiaxed particles may be either spherical
or cubical in morphology. The resulting copper alloy
strip has substantially equiaxed ferromagnetic particles
with the desired magnetic or shape an isotropy. If
desired, these substantially equiaxed fine particles may
be elongated by cold rolling to produce particles whose
aspect ratio is between about I and about 15:1 and
preferably between about 5:1 and about 7:1. The
resulting copper alloy strip has acicular ferromagnetic
particles with the desired magnetic or shape an isotropy.
More specifically, a base metal is melted by any
desired conventional technique. The base metal
preferably comprises copper, copper alloy, gold or gold
alloy. It is further within the scope of the present
invention to provide small additions of transition
metals as described hereinbelow. Iron is mixed into the
molten base metal to form a substantially homogeneous
single phase melt. Although the iron may comprise above
about 20% by weight of the entire mixture, the iron is
preferably about 20 to about 60% by weight of the
mixture. The iron is preferably substantially pure
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although it may contain some impurities or doping
elements.
Although this disclosure primarily describes the
metallic strip as being comprised of copper and iron,
it is within the scope of the present invention to add
some other desired components to the melt to modify the
composition of the ferromagnetic particles. Transition
metal additions that enhance the magnetic performance
of the resulting acicular iron alloy particles may be
incorporated by alloying the welt. Nickel, cobalt,
manganese and other transition elements in an effective
amount up to weight percents of about 10% maximum and
preferably between about 2 to about 7% are advantageous
for this purpose and are within the scope of
conventional alloying techniques.
The strip is preferably prepared by rapid
solidification in any desired manner such as melt
spinning. Other applicable techniques, such as
atomization, are set forth in an article entitled
"Rapid Solidification of Metallic Particulate" by
Grant in Journal of Metals, January 1983. Using
these other techniques, the non-particles may be
disposed within non-continuous splats or pieces of
copper alloy matrix. In general, the process of
separating the desired iron particles is carried out as
disclosed regarding the strip dissolutions.
Referring to Figure 1, there an exemplary apparatus
10 is illustrated for producing a continuous long thin
strip 12 of copper or copper alloy dispersed with iron
or iron alloy. The mixture of molten copper base metal
and iron 14 may be introduced into a heat resisting
tube 16 of a material such as quartz. The tube 15 may
be provided with a nozzle 18 having a diameter of about
0.3 to about 1.5 mm at one end. The molten material 14
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is preferably maintained at a temperature slightly
above the liquids point of the melt by any suitable
means such as a heat resistor 20. Although the
temperature may be not more than about 200C above the
liquids point, it is preferably not more than about
100C above the liquids point. Notwithstanding the
above temperature limitations, the molten material may
be maintained at any desired temperature. A cooling
substrate 22, such as a chill wheel, may be rotatable
arranged below the heat resisting tube 16. The chill
wheel may be of any desired diameter and may be rotated
at a peripheral speed of between approximately 1,050 to
8,400 feet per minute (fpm) and preferably between
about 2,100 to 4,200 fpm. However, it is within the
scope of the present invention to rotate the wheel at
any desired speed. The open end 18 of the nozzle is
preferably positioned less than about 5 mm and
preferably less than about 2 mm from a smooth surface
24 of the wheel 22. The molten mate~lal is ejected
from the tube 16 onto the rotating surface 24 under a
pressure of between about 5 to about 40 psi and
preferably between about 15 to about 25 psi applied to
the melt 14. As soon as the molten material contacts
the rotating surface 24, the melt quickly cools and
solidifies into a thin continuous strip 12 having the
iron particles distributed substantially homogeneously
or isotropic ally throughout the copper base metal
matrix.
The thickness and width of the obtained thin strip
12 can be determined by a number of factors. For
example, the surface tension between the molten
material art the surface 24 of the moving chill wheel
22 effects the shape of strip 12. As the surface
tension of the melt increases relative to the wheel,
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the strop tends to be thicker and narrower. An
increase in the rotational speed of the chill wheel
worms a thinner, wider strip. The ejection pressure
ox the met 14 also effects the shape of the strop.
As the pressure increases, the width of the strip
increases while its thickness decreases. The diameter
of the nozzle between about 0;3 to 1.5 mm and
preferably between about 0.8 to about 1.2 mm is a
factor, The smaller the diameter of the nozzle, the
thinner and more narrow the strip. Of course, the
ejection temperature and viscosity ox the melt are
also critical factors. The hotter and less viscous
thy melt, the thinner and wider the strip. The
viscosity is thought to be in the range of about 0.01
to about l poise.
The selection of the material forming the chill
wheel must take into account the wettabllity between
the molten thin strip and the surface 24. This
~ettabillty is mainly determined by surface tensions
ox toe melt and the substrate. It has been wound that
a chill wheel formed of copper can be successfully
used to manufacture strip of the materials set forth
hereinabove. However, it is also within the terms of
the present invention to use other materials such as
for example, copper alloy, aluminum, aluminum alloy,
steel, steel alloy or graphite.
The temperature of the molten material or melt is
preferably slightly above its likelihoods point. As
mentioned above, although the temperature may be not
more than about 200~C above the llquidus point, it is
proudly not more than about 100C above the liquids
point. It the temperature were below the l quads
point, the mixture Gould contain some solid particles
and would not form properly. Conversely, if the
temperature were too high above the melting point,
the melt might either spread o'er the cooling surface
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of the chill wheel so that the strip becomes too thin
or spray off the wheel before solidifying into a strip.
Accordingly, the preferred temperature is slightly
above the liquids point so that the chill wheel can
extract enough heat to immediately make the strip
slightly solid and give it some mechanical stability
or strength. Depending upon the particular composition
of the melt and other operating parameters of- the
process, the cooling rate might be about 102 to 108K per second
and preferably between about 102 to about 106K per second.
Although-a chill wheel is described as the
preferred apparatus for forming the strip, it is also
within the terms of the present invention to form the
strip by any desired conventional means.
The present invention requires the formation of a
continuous strip or pieces of metal alloy that are
useful as an intermediate material to produce
substantially equiaxed iron an iron alloy particles.
The majority of the ferromagnetic particles are homogeneously
or isotropic ally distributed throughout the strip,
substantially equiaxed in shape, and preferably sized
so that each particle is a single magnetic domain, i.e.
in the range of about 0.05 to about 0.5 microns.
During the solidification, there are two precipitation
modes of iron for a rapidly solidified copper-lron
melt. The primary solidification tends to be
relatively coarse and plate-like and the iron particles
are generally sized above about I The secondary
solidification occurs near the terminal stage of
solidification and produces the majority of particles
with a substantially equiaxed morphology having a size
in the range of about 0 02 to bout 0.5~. The
equiaxed particles may be of a cubical or spherical
shape.
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The particle size is determined by solid
solidification time which in turn is determined by
solid solidification time which in turn is determined
by factors such as the casting rate, thickness of the
casting and the thermal conductivity of the alloy.
Thicker casting sections produce larger particles while
thinner casting sections produce smaller particles.
Also, a faster cooling rate results in the formation of
smaller particles.
The final copper or copper alloy strip to be
produced preferably has primarily substantially equiaxed
iron or iron alloy particles isotropic ally or homogeneously
dispersed throughout the matrix. However, it may be desirable that
the final copper or copper alloy strip to be produced preferably has
acicular ferromagnetic particles isotropic ally or homogeneously
dispersed throughout the matrix. Until now, the process described
has formed homogeneously or isotropic ally spaced equiaxed particles.
me next step may then be directed to elongating the particles. To
accomplish this, the cast strip is preferably rolled to obtain the
desired aspect ratio. This rolling may be conducted cold or hot
depending on the strength of the ferrous particles If
the rolling is conducted hot, it should be conducted at
a temperature no higher than between about 300C to
about 900C. The aspect ratio, i.e. ratio of the length
to width, of the particles is preferably between about
5:1 and about 7:1 although it may be in the range of
about 4:1 to about 15:1. The strip now contains
wrought, acicular, iron or iron alloy particles created
by the step of rolling. It may be desirable to anneal
cold rolled strip and soften the particles as required.
To soften the iron particles, the annealing would
require temperatures n the range of about 400 to about
900C. Note that if the iron particle is acicular, it
will not change shape in the anneal.
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The process continues with the matrix of the strip
or pieces being dissolved without substantial
dissolving of the iron or iron alloy particles
contained therein. The particles can then be
recovered. The matrix is preferably dissolved by
electrolysis; however, it is within the scope of the
present invention to dissolve the matrix by any other
desired method.
According to the preferred method of manufacture
lo a metallic strip containing substantially equiaxed or
acicular iron or iron alloy particles, as specified
above, is immersed into an aqueous electrolyte. The
specific electrolyte is chosen to passivity the iron or
iron alloy particles while permitting aggressive
electrolytic anodic dissolution of the metal or metal
alloy matrix. The control of the electrical potential
at which the strip is maintained is of critical
importance and is further described hereinbelow.
Sodium sulfate in the neutral pi range is a preferred
electrolytic for this purpose.
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The concentration ox thy sodium sulfate is not ultra
critical although concentrations between about 0.05
normal to about 4.0 normal are preferred. Other
electrolytes suitable for this application include
alkaline metal sulfates.
After the exemplary continuous or non-continuous strip is
immersed in an electrolytic bath ox the type mentioned above, an
electric current is passed between an electrode as a counter
electrode or cathode and the strip as working electrode or
anode. The strip is preferably supported in an
electrically conductive container (also serving as a
worming electrode) to which the external current may be
applied, as described below. As the strip dissolves,
the iron or iron alloy particles are collected in the
platform container from which they can easily be
recovered. The external potential is maintained within
the passive potential range of the iron or iron alloy
particles of the strip. The result is anodic
dissolution of the copper matrix and recovery of
passivated iron particles.
For example, the strip is submersed in a sodium
sulfate electrolyte and maintained at a critical
potential of about 0-0 vltSsHE standard Hydrogen
d ) to about 1.5 volts SHE. Furthermore, the
preferred range of this electric potential it about
0.25 volts SHE to about 1 volt SHE. The maximum voltages
are specified so that a high anodic current on the
order of approximately 2 amp/cm2 is drawn from the
copper or copper alloy matrix. Figure 2, which
represents the anodic dissolution of copper end iron
in sodium sulfate solution, illustrates that a low
current on thy order of less than a few mlcroamp/cm2
is drown from the passive iron or iron alloy particles
when the potential is established as described
above.
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Apparatus for carrying out the process ox this
invention is illustrated in Plugger 3. Working
electrode 30 consists essentially of an electrically
conductive support surface 31 and strip 12 connected
via feed wire 32 to the positive terminal 34 of a
potentlostat 36. The surface 31 must be passive in
the electrolyte. The negative terminal 38 of the
potentiostat is connected through current meter 44 tub
counter electrode 46 via lead wires 42 and 40. A
reference electrode 48 is connected to terminal 49 of
the potentiostat by a lead wire 52. A potentiostat 54
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is connected across lead wires 32 and 52 via lead wires
56 and 58 to monitor the difference in voltage between
worming electrode 30 and reference electrode 48. The
electrolyte bath 50, as mentioned above, is held within
a tank 60. Toe current meter 44 monitors the current
while the potentiostat 54 enables the potentiostatic
control of the working electrode 30 with the
potentiostat 36.
Toe support surface may include a container 33 for
supportln~ the strip 12 as it it dissolved Dye the
electrochemlcal process occurring within tank 60. The
container 33 is preferably an open top, box-like
structure having side walls 62 and a bottom floor 64.
The container 33 ma haze feet 65 to support the
platform on the bottom of container 60. The container,
being the working electrode, is preferably formed of an
inert material which will not dissolve during the
electrochemlcal process. It is within the terms of
this invention to use inert materials such as nickel,
stainless steel, platinum or palladium.
As the copper matrix of strip 12 dissolves, it is
plated onto the counter electrode 46 and the majority
of iron particles are set free. Some of the iron
particles could still be difficult to recover for
several reasons. First, the process releases pieces of
copper which still contain particles of iron. These
pieces of copper with iron can float in the electrolyte
and thereby stay out of electrical contact with the
inner surface of the container 33 or with the strip 12.
This prevents the copper from dissolving and setting
the trapped iron particles free. Second, free iron
particles which have been collated within container
33 may spill over the sides of walls 62 either during
the process or while being collected from the platform
container.
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To maximize the yield of particles prom this
process, a magnet pa may be positioned under the
support surface 31. The magnet preferably attracts
the free iron particles against the floor 64 ox
container 61 during the process. Also, the magnet
attracts the pieces of copper still containing iron
articles into contact with the floor or the
undissolved strip so that the copper can be farther
dissolved to free the remaining iron particles.
Preferably, the magnet is located outside of the tank
60 so that it is not subject to corrosion from toe
electrolyte 50. The magnet may be either a permanent
or electromagnetic type. -It must create a magnetic
field capable of attracting the iron particles to hold
them on the floor 64 or walls of the container. It is
also within the terms of the present invention to
place the magnet between the bottom surface of the
tank 60 and the support surface or container 31.
Locating the magnet in the electrolyte may require
certain precautions, such as inert coatings or use of
inert ferromagnetic material, to prevent it from
corroding. It is also within the terms of the present
invention to place the magnet within the floor 64 or
side walls ox the container 33 and encapsulate it with
some noncorrosive material such as the metal of the
working electrode.
A further posslblllt~J, as illustrated in Figure 4,
is to form the working electrode of a ferromagnetic
alloy material such as, for example, a nickel-chrome-
iron, and wrap a coil of wire 80 around this electrode energizing the coil with a current, the electrode
becomes magnetic and Jill attract the iron particles
as required. with this embodiment, the electromagnetic
field can be applied as desired. For example, while
the container 33 is being removed from tank 60 to
collect the iron particles, the field can be applied
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to attract the particles to the container walls. Then,
the field can be shut off so that the particles can be
easily taken from the container. The golfs may be
positioned near or around the container in any desired
configuration in order that the field be applied at
any desired location on the container.
urlng the electrochemlcal treatment, the metallic
copper is dissolved prom the anodlc strip material and
may be easily recovered as an integral part Or the
processing. It is highly advantageous to use a copper
counter electrode or cathode 48. The whole cathode can
then be melted without contamination and reused as
required. However, other petal counter electrodes such
as platinum, }cad, iron, stalnles~ steel, etc. may be
used and the electrodeposlted copper may be subset
quaintly stripped mechanically. The electrode potential
should be lowered sufficiently at the copper counter
electrode so that the copper ions passing into solution
anodically deposit as metallic copper on the cathode.
In general, an operating temperature range of about 20
to about 60C I preferred, but the process will
operate economically between about O to about 100C.
Once an iron particle is separated from electrical
contact with the support surface 31, it will rapidly lose
the passivity occasioned by its anodic treatment. The
iron or iron alloy particles are somewhat protected by
a thin outside film thought to be iron oxide or iron
hydroxide However, care us be exercised to prevent
corrosion of the iron particles for maximum utility of
process. Protection against
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corrosion Or the separated iron particles can be
achieved in several assay. The electrolytic medium
ma be decorated by flushing with an inert gas, such
as for example, nitrogen. Toe decoration acts to
prevent corrosion of the free iron particles.
A corrosion inhibitor for the iron or iron alloy
phase may be incorporated into the electrolyte
provided that it does not substantially reduce the
anodlc current carried by the copper or copper alloy
matrix. These inhibitors include sodium molybdate
concentrations prom about 5 x lo 5 to about lo 3
normal and sodium tungstate in concentrations from
about 10 4 to about 10 3 normal. Other adsorption
lnhib~tors may be added which have no specific
influence on the anodic corrosion of the copper
matrix. For example, copper may be anodically corroded
at current densities in excess of about 10 ma/cm2 in
sodium sulfate solution containing about 0.005 N
concentrations of either sodium molybdate or sodium
tungstate. These corrosion inhibitors of iron do not
adversely affect the anodic current that can be drawn
from the dissolution of copper at potentials in excess
of about Owe She.
After electrochemical separation of the particles
and their inhibition against corrosion in the sodium
sulfate base solution, the particles are preferably
rapidly filtered and washed with water to which an
oxidizing corrosion inhibitor has been added. These
inhibitors are drawn from the class of sodium chromates
sodium nitrite, sodium tungstate and sodium molybdate
in concentrations of about 0.001 N to about Owl I. The
washing is hollowed by rapid drying and storage under
dry conditions so as to prevent corrosion. An important
advantage of the present invention is that the separated
iron particles are not pyrophoric and can easily be
handled or processed. or additional protection from corrosion,
the particles may be stored under an iniquity was sunk a nitrogen.
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Also, the iron particles may be separated by size
using any conventional technique such as passing
through a sieve or through a fluidized bed filter.
It is also within the scope of the present
invention to protect the final collected iron or iron
alloy particles by coating them with a metal such as
copper or cobalt. The thickness of the coating may
be in the range of about 100 to about 500 mieroinches
and preferably in the range of about 200 to about 300
mourns. The coating may be applied in any
desired manner such as by using conventional electron
less plating technology.
The resulting iron or iron alloy assailer
or equiaxed particles may be used in any of the
conventional methods for preparing magnetic recording
media such as magnetic tapes, disks, floppy disks,
magnetic cards or identification systems.
It is apparent that there has been provided
in accordance with this invention a method for produce-
in iron or iron alloy particles which satisfies the objects, means, and advantages set forth above.
While the invention has been described in combination
with the embodiments thereof, it is evident that many
alternatives, modifications, and variations will be
apparent to those skilled in the art in light of the
foregoing description. Accordingly, it is intended
to embrace all such alternatives, modifications,
and variations as fall within the spirit and broad
scope of the appended claims.
