Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Cobalt metal agglomerates, a method of producing them and their use
The present invention relates to cobalt metal agglomerates consisting of
peanut-
shaped primary particles, to a process for the production thereof and to the
use
thereof.
Finely divided cobalt metal is mainly used as a binder in the production of
hard
metal and cutting tools based on various hard materials, such as for example
WC,
diamond, SiC and CBN. The cobalt metals used, for example, in the production
of
diamond tools must fulfil specific requirements. These include, in the first
instance, that impurities such as Al, Ca, Mg, S and Si should be avoided as
these
elements readily form stabile oxides with the residual oxygen of the cobalt
metal
powder, so causing unwanted porosity in the segments.
It is also necessary, especially when producing segments with synthetic
diamonds,
to use only cobalt metal powders with very active sintering properties, as
minimum densities of 8.5 g/cm3 are required in this case. These densities
should
be achieved at a sintering temperature range of as low as < 900 C because the
diamond may be converted into graphite at higher temperatures. If the
sintering
activity of the cobalt metal is inadequate, sufficient hardness is not
achieved.
Under the extreme stresses to which annular drilling bits or cutting tools are
exposed, the abrasive action of stone dust leads to deep erosion and unwanted
detachment of the diamonds or other hard materials and consequently a loss of
cutting performance.
According to the prior art, cobalt metals are used, on the one hand, in the
form of
mixtures of atomised cobalt metal powders with hydrogen-reduced powders, as
are
disclosed in DE-A 4 343 594, on the other hand as ultra-fine and extra-fine
grade
cobalt metal powders.
Ultra-fine powders are differentiated by their FSSS value of < 1.0 gm from
extra-
fine powders which have FSSS values of between 1.2 and 1.4 gm.
The small particle size and the resultant large surface areas of the described
cobalt
metal powders promote the absorption of atmospheric oxygen and moisture, which
frequently leads to degradation of the flowability of the powders.
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The present invention provides a sintering active
cobalt metal which does not exhibit or at least mitigates
the stated disadvantages, but does allow the production of
segments with elevated density and hardness.
It has now proved possible to provide a cobalt
metal powder which exhibits these required properties.
These are cobalt metal agglomerates consisting of
peanut-shaped primary particles, characterised in that the
primary particles have average particle sizes in the range
from 0.1 to 0.7 m. These cobalt agglomerates are the
subject matter of this invention. They preferably have a
spherical secondary structure with average agglomerate
diameters of 3 to 50 m, preferably of 5 to 20 m. By
virtue of their spherical secondary structure they are
distinguished by good flow properties.
The irregularly elongated primary particles
preferably have an average particle length of 0.5 to 1 m
and, generally, a diameter of < 0.5 m.
Figure 1 shows the hardness values of a sintered
article produced from the cobalt metal powder agglomerate of
the invention in comparison with sintered articles produced
from commercially available ultra- and extra-fine cobalt
metal powders as a function of sintering temperatures.
Figure 2 shows the densities of a sintered article
produced from the cobalt metal powder agglomerate of the
invention produced according to example 3 in comparison with
sintered articles produced from commercially available
ultra- and extra-fine cobalt metal powders as a function of
sintering temperatures.
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Figures 3a and 3b show 5000 and 15000 times
magnification scanning electron micrographs of the cobalt
metal powder agglomerates of the invention produced
according to example 3.
Figures 4a and 4b show 500 and 5000 times
magnification scanning electron micrographs of cobalt metal
powders according to the invention.
The specific surface areas of the cobalt metal.
agglomerates according to the invention (determined using
the nitrogen single point method to DIN 66 131) are
preferably 2 to 6 m2/g. These surface areas and the small
particle sizes of the primary particles are responsible for
the elevated sintering activity of the cobalt metal
agglomerates according to the invention, from which sintered
articles having densities of 8.5 g/cm3 may be produced at
temperatures of as low at 700 C.
Figure 2 and Table 2 show the densities of a
sintered article produced from the cobalt metal powder
agglomerate of the invention produced according to example 3
in comparison with sintered articles produced from
commercially available ultra- and extra-fine cobalt metal
powders as a function of sintering temperatures.
Hardness values of 110 HRB may be achieved with
segments hot pressed at temperatures of only up to 620 C.
These hardness values are among the highest hitherto
achieved. With prior art cobalt metal powders, sintering
temperatures of
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approximately 780 C are required for HRRB values of 110. Figure 1 an d Table 1
show the hardness values of a sintered article produced from the cobalt metal
powder agglomerate of the invention in comparison with sintered articles
produced
from commercially available ultra- and extra-fine cobalt metal powders as a
function of sintering temperatures. It may clearly be seen -that elevated
hardness
values are obtained with the cobalt metal powder- according to the invention
at
temperatures of as low as 620 C, the hardness values moreover remaining
constant
over the entire temperature range up to 900 C. This affords the manufacturer
of
sintered composite hard materiai and drilling tools great production latitude
without any need to fear any quality fluctuations caused by differing hardness
values of the cobalt binder.
The present invention also provides a process for the production of the cobalt
metal agglomerates according to the invention.
The process is characterised in that in a first stage an aqueous cobalt(II)
salt
solution of the general formula CoX,, wherein X' = Cl', N03 and/or '/z S042"
is
reacted, preferably in a continuously operated tubular flow reactor with
vigorous
stirring, with an aqueous solution of alkali metal and/or ammonium, carbonates
and/or hydrogen carbonates. The temperature range for
the reaction is here between 40 and 100 C,
preferably between 60 and 90 C. In this process, in contrast with the
conventional
precipitation process, a rod-shaped crystallised cobalt carbonate is not
formed, but
instead a spherical basic cobalt carbonate. This is filtered and washed until
free of
neutral salt. The resultant basic cobalt carbonate is converted in a further
processing stage into spherical cobalt(II) hydroxide by adding alkali liquors,
so
achieving the secondary morphology, and is then oxidised with suitable
oxidising
agents to yield cobalt(III) oxidehydroxide, heterogenite, CoO(OH). Suitable
oxidising agents are, inter alia, hypochlorites, peroxydisulphates, peroxides.
It has
surprisingly now been found that oxidation of the cobalt(II) hydroxide to
yield
heterogenite is accompanied by a reduction in primary particle size, while
completely achieving the secondary morphology. This fine particle size of 0.3
to
1.0 m is retained when the heterogenite is subsequently reduced to cobalt
metal
over a wide range of temperatures from 300 to 800 C. Gaseous reducing agents,
such as hydrogen, methane, dinitrogen oxide and/or carbon monoxide, are
preferably used at furnace temperatures of 350 to 650 C.
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Unlike conventional known extra-fine and ultra-fine cobalt powders, the cobalt
metal agglomerates according to the invention have very good flow properties
by
virtue of their spherical secondary structure.
By virtue of the described properties, the cobalt metal powders according to
the
invention are particularly suitable as binders in the production of hard metal
and/or diamond tools. It should be noted that the cobalt metal powder agglomer-
ates may here advantageously be used both alone and combined with other binder
metals.
The present invention accordingly provides the use of the cobalt metal
agglomerates according to the invention for the production of sintered cobalt
articles and for the production of composite sintered articles based on cobalt
metal
and hard materials from the group comprising diamond, CBN, WC, SiC and
Al, 03.
By virtue of the good flow properties and the fine primary structure of the
cobalt
metal powder agglomerates according to the invention, they are also
particularly
suitable for incorporation into the positive electrode composition containing
nickel
hydroxide in rechargeable batteries based on nickel/cadmium or nickel/metal
hydride technologies.
During the so-called forming cycles, the cobalt metal is initially oxidised in
accordance with its potential to cobalt(II). In the alkaline electrolyte (30%
KOH
solution), this forms soluble cobaltates(II) and is thus uniformly distributed
within
the electrode composition. On further charging, it is ultimately deposited as
an
electrically conductive CoO(OH) layer on the nickel hydroxide particles, so
allowing the desired full utilisation to be made of the nickel hydroxide in
the
storage battery. The described anodic dissolution of the cobalt metal powder
naturally proceeds all the faster and more effectively, the finer is the
primary
structure or the greater is the surface area of the metal powder.
The present invention thus also provides the use of the cobalt metal
agglomerates
according to the invention as a component in the production of positive
electrodes
in alkaline secondary batteries based on nickel/cadmium or nickel/metal
hydride
technologies.
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The invention is illustrated in the following examples below, without this
constituting any limitation.
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Examples
Example 1
20 1 of water were introduced into a stirred flow reactor and heated to 80 C.
5 1/h
of a 1.7 molar CoC12 solution and 19 1/h of a 0.9 molar NaHCO3 solution were
continuously metered into the reactor with vigorous stirring. Once the steady
state
had been reached, the resultant product was discharged from the reactor
overflow,
filtered and washed with water until free of neutral salt. The product was
then
dried to constant weight at T = 80 C.
Chemical analysis of the basic cobalt carbonate 6btained in this manner
revealed a
Co content of 54.3% and carbonate content was determined at 32.3%.
Example 2
500 g of basic cobalt carbonate, produced according to example 1, were
suspended
in 2 1 of water. This suspension was combined with 200 g of NaOH dissolved in
1.5 1 of water, heated to 60 C and stirred for 1 hour. The product was
filtered and
washed with 3 1 of hot water. While still moist, the filter cake was
resuspended in
2 1 of water and oxidised within 1.5 hours with 700 ml of H702 (30%) at a
temperature of 45 C. On completion of addition, stirring was continued for a
further 0.5 hour, the product was then filtered, rewashed with 2 1 of hot
water and
dried to constant weight at 80 C. 420 g of spherically agglomerated
heterogenite
with ~an agglomerate D50 value of 10.5 gm were obtained. Cobalt content was
analysed at 63.9%.
Example 3
200 g of spherically agglomerated heterogenite produced according to example
2,
were weighed into a quartz boat and reduced in a stream of hydrogen for 3
hours
at T = 450 C. 131 g of spherically agglomerated cobalt metal were obtained.
Figure 3 shows 5000 and 15000 times magnification scanning electron
micrographs of this agglomerate. The D50 value of the cobalt metal powder was
10.5 m. The FSSS value was 0.62 gm.
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Examnle 4
Sintering test
The cobalt metal agglomerates obtained according to example 3 were subjected
to
hot pressing tests under the following conditions:
Apparatus used: DSP 25-ATV (from Dr. Fritsch GmbH)
Heating time to final temperature: 3 min
Holding time: 3 min
Final pressure: 350 N/mm'`
Final temperature: see tables I and 2
Dimensions: 40 x 4 x 10 mm
Table I and Figure 1 show the hardness values of a sintered article produced
from
the cobalt metal powder agglomerate of the invention from example 3 in
comparison with sintered articles produced from commercially available ultra-
and
extra-fine cobalt metal powders as a function of sintering temperatures. It
may
clearly be seen that elevated hardness values are obtained with the cobalt
metal
powder according to the invention at temperatures of as low as 620 C, the
hardness values moreover remaining constant over the entire temperature range
up
to 980 C.
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Table 1: (Hardness values)
Rockwell hardness values (HRB)
620 C 700 C 780 C 900 C 980 C
Co uFl) 91.5 109.8 109.7 107
Co eF2) 102.5 105.0 104.6 97.2
Cobalt metal powder 110.6 110.9 110.1 110.5
agglomerate from
example 3
1) Ultra-fine cobalt metal powder supplied by Eurotungstene Grenoble, France
'-) Extra-fine cobalt metal powder supplied by Hoboken Overpelt, Belgium
IO Table 2 and Figure 2 show the densities of a sintered article produced from
the
cobalt metal powder agglomerate of the invention from example 3 in comparison
with sintered articles produced from commercially available ultra- and extra-
fine
cobalt metal powders as a function of sintering temperatures.
Table 2: (Densities)
Densities [g/cm3]
620 C 700 C 800 C 900 C 980 C
Co,uF') 7.72 8.58 8.60 8.59
Co eF') 8.42 8.62 8.67 8.61
Cobalt metal powder 8.47 8.49 8.53 8.50
agglomerate from
example 3
Table 3 compares the particle sizes and BET specific surface areas of the
cobalt
metal agglomerates (determined using the nitrogen single point method to DIN
66 13 1) from example 3 with those of commercially available ultra- and extra-
fine
cobalt powders.
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Table 3: (Particle size and specific surface areas)
FSSS [ m] BET [cm2/g]
Co uFl) < 1 1.4
Co eF2) 1.2-1.4 0.8-1.0
Cobalt metal agglomerate < 0.7 2.8-4.0
from example 3
