A study on synthesis of fine size cobalt powder from fluoride compounds

Article abstract of DOI:10.1016/j.jallcom.2006.07.098

Research studies were carried out to prepare extra fine and high purity cobalt powder from synthetic cobalt chloride solutions without the use of hydrogen. The two major steps involved in the process were, first, precipitation of an appropriate ammonium cobalt fluoride salt and second, its thermal decomposition to cobalt powder. The optimum conditions for precipitation of NH₄CoF₃ were found to be, temperature of 30 °C, 100% excess NH₄F and a solution pH of 5. Cobalt metal powder was prepared from NH₄CoF₃ in a closed reactor and decomposition cum reduction temperature and time were optimized to 550 °C and 6 h, respectively. Detailed XRD studies identified all the intermediate and final products. The powder was further characterised by Chemical and SEM analysis. SEM analysis revealed the formation of 100-500 nm cobalt powder having spherical/round morphology.

Full text of DOI:10.1016/j.jallcom.2006.07.098

Journal of Alloys and Compounds 437 (2007) 231–237
A study on synthesis of fine size cobalt powder from fluoride compounds
I.G. Sharma, Pamela Alex∗, S. Majumdar, Jugal Kishor, A.K. Suri
Materials Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
Received 2 May 2006; received in revised form 21 July 2006; accepted 21 July 2006
Available online 30 August 2006
Abstract
Research studies were carried out to prepare extra fine and high purity cobalt powder from synthetic cobalt chloride solutions without the use
of hydrogen. The two major steps involved in the process were, first, precipitation of an appropriate ammonium cobalt fluoride salt and second,
its thermal decomposition to cobalt powder. The optimum conditions for precipitation of NH4CoF3 were found to be, temperature of 30 ◦C, 100%
excess NH4F and a solution pH of 5. Cobalt metal powder was prepared from NH4CoF3 in a closed reactor and decomposition cum reduction
temperature and time were optimized to 550 ◦C and 6 h, respectively. Detailed XRD studies identified all the intermediate and final products. The
powder was further characterised by Chemical and SEM analysis. SEM analysis revealed the formation of 100–500 nm cobalt powder having
spherical/round morphology.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Ammonium cobalt fluoride; Thermal decomposition; Non-pyrophoric
1. Introduction
of 400 nm cobalt powder from Co(OH)2 solution, by hydrogen
reduction in presence of palladium catalyst. Considerable work
has also been carried out on the preparation of fine size cobalt
powder by several chemical methods such as carbonyl pyrolysis
[6] and solution phase metal salt reduction [7–9]. For example,
Zhu et al. [10] reports aqueous reduction of cobaltous hydroxide
slurry under hydrogen reduction conditions using 2 hydroxy-4-
(1-methylheptyl) benzophenone oxime to prepare nano-crystals
of cobalt. Most of the reported work on submicron cobalt powder
is from very lean solutions using complex organic compounds.
Cobalt metal and its compounds have found a wide range
of applications. However, cobalt powder of particular size,
purity and morphology find its application mainly in cemented
carbide tools. Metallic materials produced from nano-size high
purity powders offer new physical and mechanical properties
and applications. In India, cobalt powder is needed for its fabri-
cation to pellets (1 mm × 1 mm) and slugs (6 mm × 25 mm), of
96% theoretical density for its ultimate conversion to gamma
radiation source for cancer treatment. Most of the requirements
of cobalt powder in India is met by imports. India imported
about 36 tonnes of cobalt in the form of powders and flakes
in the year 2002–2003 [11]. Although cobalt metal powder
has proven applications in cemented carbide tools, meaningful
data and technical details are not available in open literature to
prepare cobalt metal powder for the above-cited distinguished
applications.
Conventional production methods for cobalt and nickel pow-
ders commonly use a multistage process involving hydrogen
reduction of aqueous salt solution of oxide, chloride or sulphate
under pressure and high temperature [1,2]. This reduction pro-
cess yields a powder with a purity of 99.6% and a particle size
of around 5 ␮m. Studies are also reported on direct hydrogen
reduction of powders of oxides, chlorides and sulphates [3]. Of
these, oxide intermediates only resulted in cobalt powder, of
size in the range of 2–5 ␮m. Thermal decomposition of cobalt
oxalate under reducing atmosphere of hydrogen [4] produces a
cobalt powder of 99.8–99.9% purity with a crystal size of around
1–2 ␮m, but requires preconditioning due to highly pyrophoric
nature and also suffers from a poor shelf life. Metal powders of
particle size below 1 ␮m are mostly produced by many physi-
cal and chemical methods. Among the chemical methods, the
reduction of a metal salt or oxide by an appropriate reducing
agent is frequently employed. The size, shape and purity of the
product largely depend on the type, purity of metallic precursor,
reducing agent and conditions of, precipitation and reduction.
However, Recently, Dong Jin Kin [5] reports on the preparation
∗
Corresponding author. Tel.: +91 22 25592629; fax: +91 22 25505151.
E-mail address: alexpamela57@yahoo.co.in (P. Alex).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.07.098

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I.G. Sharma et al. / Journal of Alloys and Compounds 437 (2007) 231–237
Fig. 1. Experimental set-up for decomposition of ACF to cobalt powder.
This paper describes an alternate method of cobalt powder
ride from cobalt chloride by ammonium fluoride is based on the
following reaction [2]:
production from an ammonium fluoride salt of cobalt, using
chemically bound hydrogen as reductant. In comparison to most
of the conventional powder production processes, this environ-
mentally acceptable process, offers additional improvements
including improved purity, reduced grain size, non-oxidized and
non-inflammable powders.
In the absence of any valuable and meaningful data sighted
in the literature, on preparation of ammonium cobalt fluoride,
decomposition to cobalt fluoride and ultimately to fine metal
powder, the paper attempts to provide an elaborate insight into
the same. Characterization studies of powder by XRD, SEM,
chemical analysis have also been incorporated.
CoCl2 + 3NH4F → NH4COF3 + 2NH4Cl
(1)
It is assumed that the following sequence of chemical reac-
tions take place during the course of preparation of cobalt metal
powder [12,13]:
3NH4CoF3 → 3CoF2 + 3NH3 + 3HF
2NH3 → N2 + 3H2
(2)
(3)
(4)
(5)
NH3 + HF → NH4F
3CoF2 + 3H2 → 3Co + 6HF
2. Experimental description
Putsimplyitisaone-stagemanufactureofmetalpowderfrom
an intermediate cobalt fluoride salt. The decomposition had to be
carried out in a closed reactor maintained under pressure of the
order of 1.5 × 105 Pa, so as to prevent any escape of ammonia
from the system, generated during heating of NH4CoF3. Ther-
mal decomposition of CoF2 under the operating conditions of
the reactor is not favorable. However, H2 reduction of CoF2 is
thermodynamically feasible and produces cobalt metal powder.
Hydrogen required for the reduction is generated in situ in the
system by the decomposition of ammonia. It is only the chemical
reduction reaction shown in reaction (5) results in generation of
cobalt metal powder. The reaction products are recycled except
for nitrogen, which is vent out. The overall reaction can thus be
expressed as
In this research, cobalt powder of different sizes is prepared from synthetic
cobalt chloride solutions, by varying different parameters, of precipitation and
decomposition of the cobalt salt. Particle size and purity were taken to be the
most important characteristics of the product in conjunction with the yield of the
reactions involved. In a typical procedure for precipitation of ammonium cobalt
fluoride, 100 ml CoCl2 (1 M) was adjusted to the required pH and precipitated
with ammonium fluoride dissolved in minimum amount of water. The solution
was maintained under constant stirring for 30 min at the required temperature.
The light pink amorphous precipitate was filtered without washing, dried at
a temperature less than 100 ◦C, weighed and then used as feed material for
preparation of cobalt powder in the next stage.
The decomposition of the salt to Co powder was carried out in a resistance
heating furnace with horizontal retort, as shown in Fig. 1. The air dried ammo-
nium cobalt fluoride in batches of 10 g was taken in a closed molybdenum reactor
and placed inside the retort. Firstly, argon was passed for half an hour in order to
drive away any air inside the retort. Then the furnace was closed from both sides
and maintaining slight argon pressure inside, heated to an inside temperature of
550 ◦C. After heating for the required time, the side valve was opened to release
the excess fluoride to an ammonia collector. The retort was then cooled thor-
oughly and the product removed and weighed. The products were subsequently
characterised by X-ray diffraction, scanning electron microscopy and chemical
analysis.
3NH4CoF3 → 3Co + NH4F + 8HF + N2
(6)
The decomposition of ammonium cobalt fluoride to interme-
diate products (CoF2) as per reaction (2) has only been supported
by TG-DTA curves given in Fig. 2. The appearance of endotherm
at 160 ◦C indicate loss of water of crystallization of the salt,
followed by decomposition to cobalt fluoride at 320 ◦C. The
appearance of two small peaks at around 245 and 260 ◦C might
be on account of formation of successive lower ammonium
cobalt fluorides. This is because the reaction takes place in sev-
eral stages before the formation of CoF2. The intermediate CoF2
phase, is further confirmed by XRD (Fig. 3). On further heating
to 550 ◦C, chemically bound hydrogen from ammonia reduces
the cobalt fluoride to metallic cobalt.
3. Results and discussion
Chemical precipitation/reduction are a frequently used
method for the preparation of metal powders of submicron
sizes. However, the conditions for precipitation and subsequent
decomposition of complex ammonium cobalt fluoride is not
adequately described in literature [12]. The chemical reactions
involved in the formation of synthetic ammonium cobalt fluo-

I.G. Sharma et al. / Journal of Alloys and Compounds 437 (2007) 231–237
233
Table 1
Effect of ammonium fluoride
S. no.
NH4F (g)
ACF (g)
Co in filtrate (g) (unreacted)
1
2
3
4
5
6
Stoic
7.20
8.50
10.00
12.00
14.50
14.20
4.2
3.8
3.4
3.0
2.38
2.4
50% excess
70% excess
90% excess
100% excess
110% excess
Co = 1 M (100 ml), pH 5, temperature = 60 ◦C.
cobalt ammonium fluoride. This is because of the differences
in thermodynamics and kinetics of formation and also during
the subsequent metal conversion stage of the two compounds. A
series of experiments were conducted by varying pH, solution
concentration and temperature of precipitation and the findings
arediscussedin thefollowingsequel. In each experiment, 100 ml
of synthetic cobalt chloride solution was used for precipitating
ammonium cobalt fluoride with a known amount of ammonium
fluoride solution. The compound resulted was mainly identified
by XRD analysis. The residual cobalt in the filtrate is the indi-
cator of the reaction yield.
Fig. 2. TG-DTA graph for decomposition of ammonium cobalt fluoride to cobalt
fluoride.
The two major stages as indicated above involved in the
preparation of cobalt metal powder from cobalt chloride solu-
tions are:
3.1.1. Influence of strength of ammonium fluoride solution
Ammonium fluoride concentration was varied from stoichio-
metric requirement to 110% excess, at pH 5, and a minimum
temperature of 60 ◦C for every 100 ml of 1 M cobalt chloride
solution. The results are presented in Table 1. The weight of
the product formed was very much influenced by the quantity
of the precipitating agent. Almost 100% excess of ammonium
fluoride was found necessary for complete precipitation. The
experiments also indicated the product to be highly soluble
at the experimental temperature of 60 ◦C. The filtrate analysis
(included in the same table) confirms the result. All the products
were further substantiated by XRD (Table 2).
i. synthesis of anhydrous ammonium cobalt fluorides;
ii. thermal decomposition of ammonium cobalt fluoride.
The findings of the entire study are therefore discussed
accordingly.
3.1. Synthesis of ammonium cobalt fluoride
Due to lack of published literature and relevant technical
data on preparation of ammonium cobalt fluoride from cobalt
chloride, a thorough study was initiated to identify the opti-
mum conditions. The preliminary studies indicated the presence
of different fluoride salts of cobalt during precipitation. How-
ever, the studies had been focused on precipitation of one single
3.1.2. Influence of temperature
The next set of experiments were conducted, to study the
effect of reaction temperature on ammonium cobalt fluoride pre-
cipitation by varying the temperature of cobalt solution from 30
to 90 ◦C. Table 2 summarizes the results of the experiments on
precipitation at these different temperatures. As can be seen from
the table, with rise in temperature, a decrease in weight of the
precipitate and increase in Co value in the filtrate occurs. This
can be attributed to an increase in solubility of the precipitate
with increase in temperature and thereby reduces the weight of
Table 2
Effect of temperature on precipitation
S. no.
Temperature (◦C)
ACF (g)
Co in filtrate (g)
1
2
3
4
5
6
30
50
60
70
80
90
24.4
20.5
19.6
18.0
17.8
17.5
Nil
1.9
1.92
2.60
2.75
2.80
Fig. 3. X-ray diffraction analysis of the intermediate cobalt fluoride powder
produced at 320 ◦C.
Co = 1 M (100 ml), pH 5, NH4F = 100% excess.

234
I.G. Sharma et al. / Journal of Alloys and Compounds 437 (2007) 231–237
Table 4
Results of study on thermal decomposition temperature and time for ACF
S. no.
Temperature (◦C)
Time (h)
Remarks
1
2
3
4
5
6
7
8
400
450
550
650
550
550
550
550
6
6
6
6
4
5
6
7
Incomplete redn
Incomplete redn
Completely reduced to metal
Agglomerated powder
Incomplete redn
Incomplete redn
Complete redn
Agglomerated powder
chloride solution. The pH of the solution was varied from 2 to
6, at a constant temperature of 30 ◦C with stirring. The precipi-
tate formed depends very much on the initial pH of the solution.
The precipitation of ammonium cobalt fluoride does not occur
at once, on attaining the desired temperature, but later, after
an induction time of stirring for half an hour. The precipitate
obtained was filtered without washing and air dried for 6 h. The
product was weighed and filtrate was analysed for calculating
the yield. As seen from the table, condition of precipitation sta-
bilizes at around pH 5 with maximum precipitate and minimum
loss of cobalt in the filtrate. At acidic pH, Co shows a tendency of
solubility while higher pH warrants the formation of hydroxide.
Hence, an optimum pH of 5 was chosen for all further experi-
ments.
Fig. 4. X-ray diffraction analysis showing peaks for NH4CoF3.
3.2. Decomposition of ammonium cobalt fluoride to cobalt
metal
The dried precipitate of ammonium cobalt fluoride was
decomposed in a closed reactor, in argon atmosphere, under
a positive pressure of around (1–1.5) × 105 Pa. Optimum con-
dition of decomposition was studied in detail by varying the
temperature and time of the reaction and the findings are pre-
sented in Table 4. From the results it was ascertained that an
optimum temperature of 550 ◦C was essential. Temperatures
lesser than 550 ◦C resulted in incomplete reduction, with sizable
amount of the intermediate product, cobalt fluoride, along with
partially reduced cobalt metal. Higher temperatures resulted in
agglomeration of the product with an increased particle size.
However, the decomposition had to be coupled with an opti-
mum duration also. Hence, by applying the optimum temper-
ature (550 ◦C), time of heating was varied from 4 to 7 h. A
minimum period of 6 h was adequate for complete conversion of
cobalt ammonium fluoride to cobalt metal. Longer duration of
reduction results in a tendency towards sintering of the product.
All the experiments were backed by XRD analysis to identify
and characterise the product.
Fig. 5. XRD data depicting the peaks for (NH4)2CoF4.
the precipitate formed. To reduce solubility, an optimum tem-
perature of 30 ◦C was chosen for further experiments which
also resulted in maximum amount of ammonium cobalt fluo-
ride precipitate. XRD pattern of the precipitate formed at room
temperature and 50 ◦C is shown in Figs. 4 and 5. It is evident
from Fig. 4 that precipitation reaction at room temperature (i.e.
30 ◦C) results in the formation of only NH4CoF3 phase. Any fur-
ther increase in temperature leads to formation of (NH4)2CoF4
as seen in Fig. 5.
3.1.3. Influence of pH on precipitation
Table 3 summarises the results of studies conducted by vary-
ing pH of precipitation of cobalt. In this set of experiments, a
known amount of ammonium fluoride was added to 1 M cobalt
Table 3
Effect of pH on ammonium cobalt fluoride precipitation
S. no.
pH
ACF (g)
Co in filtrate (g)
3.3. XRD analysis
1
2
3
4
5
2
3
4
5
6
18.4
19.0
19.8
24.0
23.0
2.45
2.10
1.80
Nil
XRD investigations conducted to identify the products at
intermediate and final stages of the process scheme are presented
in Figs. 4–7. Figs. 4 and 5 indicate the formation of compounds
NH4CoF3 and (NH4)2CoF4 at different conditions of precipita-
tions. It is evident from these two figures that the temperature
0.20
Co Conc = 1 M (100 ml), NH4F = 100% excess, temperature = 30 ◦C.

I.G. Sharma et al. / Journal of Alloys and Compounds 437 (2007) 231–237
235
Table 5
Chemical analysis (wt.%) of cobalt powder
Co
Ni
99.95
0.04
Cu
Fe
0.003
0.01
Mg
0.003
the process used to produce it. It is because either the surface
per unit weight of powder will vary or chemical reactivity of
the material itself will differ [14]. As far as the pyrophorocity
of the powder is concerned, the activity of a powder refers to
its propensity to react with the environment; and it depends on
the rate of material transport by bulk and surface diffusion, the
rate of adsorption and desorption, and similar reactions with the
environment. The average particle size or diameter of a powder
is one of the several factors which determines its activity. Small
or finely divided particles are usually, but not always of greater
activity than larger particles. With decreasing particle size of a
powder, the behavior of the particle is governed increasingly by
surface phenomena. The variables such as, interparticle poros-
ity, oxide films, solid impurities, adsorbed gases or moisture,
etc. have a pronounced effect on pyrophoric tendencies [15].
In this case, the SEM analysis of the powder revealed it to be
of rounded/spherical shape, often loosely agglomerated powder
with a minimum interparticle porosity. The minimum poros-
ity decreases the chances of oxygen adsorption which in turn
reduces the inflammability of the particles.
Fig. 6. XRD analysis depicting incomplete reduction of (NH4)2CoF4.
play a decisive role in the formation of a particular ammonium
cobalt fluoride compound. The decomposition products of these
compounds were analysed from Figs. 6 and 7. As seen from the
XRDdata, the compound NH4CoF3 formedatroom temperature
precipitation and further decomposition at 550 ◦C resulted in
pure cobalt powder while (NH4)2CoF4 (formed at a higher tem-
perature) on decomposition at a temperature of 550 ◦C results in
incomplete decomposition and a mixture of the products (Fig. 7),
perhaps due to the following reaction [13]:
(NH4)2CoF4 → NH4CoF3 + HF
which on further heating results in CoF2.
(7)
It was observed that, the additional step in the reaction
requires either longer duration of heating or still higher tem-
peratures for complete conversion to metallic powder. However,
these experiments were not attempted in detail, since maximum
conversion to cobalt metal powder was obtained from NH4CoF3
precipitate obtained at room temperature. Hence, room tem-
perature precipitation of 1 M CoCl2 at a pH 5 and subsequent
decomposition at 550 ◦C in an air tight reactor, for a duration
of 6 h was identified as optimum experimental condition for the
process. It may be stressed here that, the process was devoid of
any external use of hydrogen.
The purity of the cobalt powder was determined by atomic
emission spectroscopy and induction coupled plasma techniques
and its result is indicated in Table 5. Since the cobalt powder was
prepared from analytical grade cobalt chloride salts, the ultimate
cobalt powder was 99.95% pure.
3.4. Scanning electron microscopy
Cobalt powder was charecterised with respect to size and
morphology by scanning electron microscope (SEM). Though
a detailed investigation of all the powder is beyond the scope
of this paper, a few images of the Co powder are depicted in
Fig. 8a–c, respectively. The effect of cobalt concentration on
the morphology of the powder can be demonstrated with these
SEM micrographs. As shown in figures, there is a significant
change in the size of cobalt powders formed by varying the con-
centration of cobalt chloride solutions. Powders formed from
very dilute solutions such as 0.25 M results in very fine cobalt
powders of particle size in the range of 100–250 nm (Fig. 8a).
While particle size increases from 500 nm to 1.1 ␮m by increas-
ing the strength of cobalt solutions from 1 to 3 M (Fig. 8b and
c). As mentioned earlier in Table 4 (result 4 and 8) the cobalt
metal powder formed at temperature of 650 ◦C and by heat-
ing for longer durations were agglomerated. The SEM analysis
of these powders recorded slightly larger sizes, in the range of
1.1–1.5 ␮m. It is clearly evident from all the micrographs that
powders produced by these process are spherical/round in mor-
phology and loosely bound which qualifies it to be suitable for
binder in cemented carbide tools.
The cobalt powder obtained was of high purity, non-
pyrophoricandofveryfinesize. Inmanycasesthepyrophorocity
orinflammabilityofthepowderofagivenmaterialwillvarywith
Fig. 7. X-ray diffraction analysis of cobalt powder resulted from decomposition
of NH4CoF3.

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I.G. Sharma et al. / Journal of Alloys and Compounds 437 (2007) 231–237
Fig. 8. (a) SEM of Co powder from 0.25 M CoCl2 (100–250 nm); magnification: 20K×; (b) SEM of Co powder from 1 M CoCl2 (300–500 nm), magnification:
10K×; (c) SEM of powder from 3 M CoCl2 (450–1100 nm), magnification: 10K×.
4. Conclusions
ply of hydrogen and neither got oxidized nor caught fire during
handlingandstorageinspiteoftheextrafinesize. TheSEManal-
ysis of the powder revealed it to be of rounded/spherical shape,
often loosely agglomerated powder with a minimum interparti-
cle porosity.
An alternate method to form extra fine cobalt powder from
chloride solution via ammonium cobalt fluoride intermediate
was test examined. Optimum parameters were established for
the precipitation of an appropriate ammonium cobalt fluoride.
The presence of an additional compound of ammonium cobalt
fluoride deteriorate the quality of powder and yield of the pro-
cess. Complete decomposition parameters of ammonium, cobalt
fluoride to cobalt powder were also standardized. Cobalt metal
powder prepared by this process does not involve external sup-
Acknowledgements
The authors wish to thank Dr. P.K. Sinnha and Shri.
Tarashankar Mahata of ECMS, Materials Group, for their co-
operation and extensive support in the XRD analysis of all the

I.G. Sharma et al. / Journal of Alloys and Compounds 437 (2007) 231–237
237
samples. The authors also wish to thank Dr. S.K. Gupta and Shri
Souvik Bhattacharya of TPPED, BARC, for SEM analysis.
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