Although cobalt is one of the least
abundant elements compared to copper
and nickel, it is an important part of the
composition of nearly all alloys developed
since the 19th century and has
been of considerable interest in recent
years. In this paper, cobalt processes
that were developed for mixed ore are
summarized. New cobalt purification
and electrodeposition developments
are described, and the most important
aspects of cobalt recycling and application
are also presented.
For years, cobalt was thought by some
to be radioactive. This misconception
may be attributable to the Planet of
the Apes movies, which centered on a
“divine bomb”—a nuclear missile with
a cobalt casing. Another possible source
of the cobalt misconception is that the
radioactive isotope cobalt-60 is widely
used in industrial applications and in
hospital radiotherapy. Cobalt is, in fact,
a metal that is stable, nonradioactive, and
found in nature, while cobalt-60 (Co-60)
is unstable, radioactive, and manmade.
COBALT PRODUCTION PROCESS AND PRINCIPLES
Cobalt’s extractive metallurgy from sulfide ores is most frequently linked with that of copper and nickel. The cobalt mineralization and typical examples are summarized in Table A. Owing to its great similarity to nickel in chemical properties, cobalt tends to follow nickel throughout the various concentrating operations of milling, smelting, and metal separation. Further, since cobalt minerals are commonly associated with ores of nickel, iron, silver, bismuth, copper, manganese, antimony, and zinc, the process includes intensive separation and purification techniques. The primary cobalt production process is based on the following principles:
- Treating cobalt minerals within copper or nickel production processing
- Concentrating cobalt-bearing material through copper or nickel pyrometallurgical and hydrometallurgical operations
- Purifying cobalt solution/electrolyte by employing separation and purification techniques (e.g., selective precipitation, solvent extraction, and ion exchange)
- Producing cobalt metal, cobalt powder, or cobalt chemicals in a selected independent process via electrorefining, electrowinning, or reduction precipitation
A generalized block flow diagram illustrating main cobalt commercial processes4–8 is given in Figure A. As shown in Figure Aa, the cobalt recovery process is based on the continuous selective oxidation and hydrolysis of cobalt from relatively impure nickel electrolyte. Cobalt-bearing slime, containing 8–10% Co, 28–34% Ni, 0.4–0.8% Cu, and 5–6% Zn, is treated by selective reduction with sulfur dioxide to solubilize the cobalt and nickel to thus separate them from the iron and some of the copper. The leach solution contains 14–16 g/L Co, 45–65 g/L Ni, 0.4–0.8 g/L Cu, and 3–5 g/L Zn. Following further solution purification, the cobalt is separated from the nickel by hypochlorite precipitation to produce cobaltic hydroxide. The purified cobaltic hydroxide is then calcined to oxide, which is melted for making soluble anodes (>95% Co, <0.45% Ni, <0.05% Cu, and <1% Zn), and electro-cobalt (99.98% Co) is finally electrolytically refined in the tankhouse.
During electrorefining, cobalt is electrochemically dissolved from the anode into the electrolyte producing cobalt cations plus electrons (see Reaction 1; all reactions are shown in the Reactions table).
Manganese (E0 = –2.37 V), zinc (E0 = –1.66 V), and iron (E0 = –0.36/–0.44 V) are less noble than cobalt and they dissolve almost completely in the electrolyte. Elements such as nickel (E0 = –0.25 V) and lead (E0 = –0.25 V), which have similar activity with cobalt, also dissolve in the electrolyte. Although copper (E0 = +0.337 V) is more noble than cobalt, it dissolves from the anode when the copper content is less than 10%. The dissolved copper, however, reports to the slimes because copper is cemented by cobalt: Cu2+ + Co → Cu + Co2+.
The name “cobalt” was derived from
the German “Kobold,” meaning evil
sprite. The interest in cobalt’s uses goes
back to 2000 B.C. when cobalt was used
by Egyptian artisans as a coloring agent.
Cobalt salts have been used for centuries
to produce brilliant and permanent blue
colors in porcelain, glass, pottery, tiles,
and enamels. After mining techniques
became widely known in the 16th century through the works of Georgius
Agricola, a German mineralogist, cobalt
was supplied as raffre, a cobalt arsenide
of sulfide ore that was roasted to yield a
cobalt oxide. In 1735, a Swedish scientist,
Georg Brandt, demonstrated that a
blue color common in colored glass was
caused by a new element, cobalt. Metallic
cobalt was later established as an element
by Torbern Bergman in 1780.
In 1926, and again in 1929, a process
based on the use of nickel peroxide for
selective precipitation of cobalt from
nickel electrolyte/sulfate-boric acid
was studied, but was found to be unattractive.
In 1942, the use of a sulfate-chloride
electrolyte for nickel refining
was introduced, thus paving the way for
the successful development of a cobalt
COBALT ELECTROLYSIS AND WINNING
Although the electro-cobalt recovered
from the process described in the sidebar
and shown in Figure Aa shows a very high
purity of cobalt cathode (99.98% cobalt)
commercially produced, processes for
cobalt recovery are significantly changed
by following hydrometallurgical lines.
Solvent extraction-electrowinning (SXEW)
technology is widely embraced in
these changes (Figure Aa, b, c, d, and
e) because of commercial needs and
environmental reasons (i.e., eliminating
fire refining). Since cobalt electrolyte is
very sensitive to metallic impurities, in
particular, iron, nickel, copper, magnesium,
and manganese, solution purification
must be effectively conducted prior
to cobalt winning to meet an increasing
demand for high-purity cobalt.
Solution purification, in term of separating
impurities from cobalt to produce
a pure product, may be achieved by such
means as chemical precipitation, solvent
extraction, ion exchange, and electrowinning.
Chemical precipitation removes
iron and arsenic by using oxidation neutralization
and hydrolysis. The chemical
reactions are expressed as Reactions
2–5. Removing manganese by oxidative
hydrolysis produces Reactions 6 and 7.
Separating nickel from cobalt produces
Reactions 8 and 9. With an excess of
Co2+, Reaction 10 is produced.
Solvent extraction using selective
P507 (similar to PC-88 and SME-418)
and Cyanex 272 separates cobalt and
nickel from both sulfate and chloride
media, as shown in Reactions 11 and
12. Using selective D2EHPA, P204
(HR2PO4), Cyanex 302/301, Chelex 20,
and Purolite S-930 at pH 2–5, SX strips
zinc, iron, copper, and aluminum with
sulfuric acid. LIX 841 (ketoxime reagent)
can be used to selectively recover the
nickel to a purified nickel electrolyte.
With the ion exchange method,
Lewatit TP 207 can effectively remove
copper and zinc from nickel and cobalt
solution. This type of cation exchange
resin can also separate nickel from
cobalt solution or vice versa. The newest
Lewatit MonoPlus TP 207 is used in
separating magnesium and calcium from
nickel and cobalt solution as well as in
nickel recovery.9 MRT10,11 SuperLig® 241
can be used to extract and polish nickel
from a cobalt feedstock solution, which
is very effective in separating nickel from
cobalt when the Co/Ni ratio ranges up to
500–1,000:1. Also, SuperLig® 176 can be
used for the combined removal of copper
and Fe(III) together with nickel from the
cobalt electrowinning electrolyte.
For cobalt recovery in a copper sulfide
ore process, copper is removed via electrowinning
and iron, zinc, and more copper are precipitated by increasing the
pH of the solution.
Cobalt Winning Operation
After a series of steps to remove
impurities, cobalt is produced by electrowinning
from cobalt electrolyte either in
chloride or sulfate media using insoluble
anodes. The main reactions occurring at
the electrodes in the electrowinning are
shown in Reactions 13–15.
Due to the stresses inherent in cobalt
metal, electrodeposited cobalt tends to
peel from the cathode blank in electrowinning.
To avoid this and maintain an
adherent deposit in practice, cobalt is
electrowon onto 6 mm thick stainless-steel
blanks and allowed to grow around
the edges of the blank.12 The cobalt metal
produced can either be broken cathode or
it can be cut in small pieces. Cobalt is also
electrowon as “round” discs 2.53 cm in
diameter, deposited onto stainless-steel
mandrel sheets, or electrown directly as
Dimensionally stable anodes (DSA)
are used exclusively in cobalt chloride
electrolytes. The anodes are bagged
to collect the chlorine gas generated.
Alloyed lead anodes are most widely
used in sulfate medium. Electrowinning
cobalt metal from sulfate electrolyte
makes the winning operation possible in
undivided cells and enables significant
process changes to adapt the SX-EW
technology from sulfides to laterites.
During electrowinning, hydrogen
evolution is a possible competitive side
reaction that reduces metal deposition
current efficiency. However, some laboratory
findings13 indicated that “plating
and dissolving simultaneously” within
an oxidation system (i.e., presenting of
residual chlorine or hydrogen peroxide
in electrolyte) may be a root cause of the
low current efficiency.
COBALT RECYCLING AND RECOVERY
In 1997, the U.S. annual consumption
of cobalt was 9,200 tonnes and 22% of
the cobalt consumed was recycled and
recovered from alloy scrap, cemented
carbide scrap, and spent catalysts. Cobalt
resources can be maximized with its
recovery from leaching solution, electrowinning
bleed, smelting slag, sludge,
Recycling of Spent Catalysts
Catalysts are essential in the petroleum
refining and petrochemical industries for
production of gasoline, diesel fuels, jet
fuels, heavy oil hydrocarbons, etc.
Assessments of the economical recycling
of spent catalysts have focused on those
containing binary oxide/sulfide mixtures
of cobalt, nickel, molybdenum, and
tungsten. The recycling technology
identified is based in pyrometallurgy and
Gulf Chemical and Metallurgical
Corporation processed Ni/Mo, Co/Mo,
and Ni/Co/Mo/V catalysts in a 14 MW
electric arc furnace (EAF). Alumina
concentrate, produced after roasting and
leaching the spent catalysts, is mixed
with a reductant and melted in the EAF
to produce a mixed metallic alloy containing
37–43% nickel and 12–17%
cobalt, which is finally sold to nickel and
Recycling of Alloy Scrap
Pure metallic cobalt has solid-state
transition at approximately 417°C. When
its face-centered-cubic (fcc) phase is
stabilized by adding certain elements
such as nickel, manganese, or titanium,
the energy of crystallographic stacking
faults is high and thus results in increased
hardness and strength. The high microstructural
stability of cobalt-based alloys
makes them resistant to failure at the
elevated service temperatures.
To recover strategic and critical
metals from superalloy scrap (SAS), the
former U.S. Bureau of Mines developed
a process that utilized novel double membrane
electrolytic cells (DMEC) to
electrorefine SAS into high-purity cobalt
and nickel cathodes.17 Integrated with
copper cementation, iron SX, carbon
adsorption, and cobalt SX operations,
the DMEC cobalt cathode produced had
a purity of >99.95% cobalt, above the
Recovering from Cobalt-Bearing
Significant values are present in the
leaching liquors produced by the leaching
of low-grade ores or electrolyte bleed
from a copper tankhouse.18 The chelating
ion-exchange resin XFS-4195,19 after a
big screen selection, was used as the most
promising cobalt sorbent, and D2EHPA
in kerosene was the extractant for zinc,
aluminum, and iron removal. Cyanex
272 (20%) in kerosene was then used for
removal of the remaining iron. Although
successful, one of the most significant
practical problems with liquid-liquid
extraction systems is the loss of the
expensive organic extractants by entrainment
in the aqueous phase. One method
to overcome this, and to also avoid using
large volumes of the organic phase, is the
use of supported liquid membranes.20
Recovering from Sludge/
An effective scheme for treatment of
stockpile sludges/residue from one of the
largest nickel refineries was reported.21
The feed material contained 41.8%
copper, 5.29% nickel, and 1.48% cobalt.
After leaching, copper was recovered
as 99.9% copper cathode, and the spent
electrolyte was then precipitated as
carbonates that contained 0.49% copper, 31% nickel, and 13.6% cobalt.
Fluoborate technology,22 whose leaching
process is expressed in Equation
16, was also identified as an effective
lixiviate in treating the sludge at both
room and elevated temperatures, where
the oxidation reaction happened to solubilize
the base metals in the fluoborate
At temperatures below 417°C, cobalt
exhibits a hexagonal-close-packed
structure. Between 417°C and its melting
point of 1,493°C, cobalt has an fcc
structure. Cobalt is ferromagnetic and
retains this property to 1,100°C.
Cobalt has played an important part
in the composition of nearly all the new
alloys developed since the 19th century
for cutting tools and wear resistance.
The special properties of cobalt have
been utilized in such applications as
catalysts, paint dryers, polymerization
promoters, and rechargeable battery
chemicals. Some of them are described
Superalloys are major applications
where cobalt improves the strength, wear,
and corrosion-resistant characteristics
of alloys at elevated temperatures. The
major uses of cobalt-based superalloys
(45% cobalt) are in turbine blades for
aircraft jet engines and in gas turbines
for pipeline compressors.
Hardfacing alloys are specialized
applications where the cobalt-based
alloy is used for machining very hard
materials. In such applications the most
important group of cobalt-based alloys is
the stellite group, which contains cobalt,
tungsten, chromium, and molybdenum
as principal constituents. The hardfacing
tools with cobalt alloy, known as stellite
alloys (Co-Cr), invented by Haynes in
1907 and applied first in knives, provides
greater resistance to wear, heat, impact,
Magnets are another important application
for metallic cobalt where it is used
in the manufacture of various magnetic
materials. The remarkable and unexpected
magnetic properties of the alloy
Fe2Co were announced by Weiss in 1912.
Perhaps the most important of these
properties is the permeability in medium
fields; that is, for a magnetizing force of
50–600 gilberts per centimeter.
Hard materials-carbides are an important
application for cobalt metal powder
which is used as a binder in the production
of cemented tungsten carbides for
heavy-duty and high-speed cutting tools.
In cemented carbides (3–25% cobalt),
cobalt provides a ductile bonding matrix
for tungsten-carbide particles.
The application of cobalt in catalysts
is based on its property of multivalence
(Co2+ and Co3+). A cobalt-molybdenumalumina
compound is used as a catalyst
in hydrogenation and for petroleum
desulfurization. Cobalt metal is used in
electroplating because of its attractive
appearance, hardness, and resistance to
Radioactive Cobalt-60 was discovered
by Glenn T. Seaborg and John Livingood
at the University of California–Berkeley
in the late 1930s. Cobalt-60, a radioactive
isotope of cobalt, is an important source
of gamma rays and is used to treat some
forms of cancer and as medical tracer.
Cobalt-based batteries are another
extraordinary application where the use
of cobalt in rechargeable batteries grew
enormously between 1995 and 2000.
Batteries are electrochemical devices that
convert chemical energy into electrical
energy with an anode and a cathode.
The addition of cobalt to the electrodes
substantially enhanced the cell’s life,
increased hydride thermodynamic stability,
and inhibited corrosion.
The increasing use of cobalt in
rechargeable batteries for electric vehicle
applications is expected to increase the
use of cobalt even further. A major shift
to hybrid-electric vehicles in automotive
technology will dramatically increase
the demand for cobalt-based batteries.
Newly emerging demand combined with
increases in cobalt use for other types
of turbine engines and gas-to-liquid
catalysts is underpinning the recent
growth in cobalt demand and is expected
to drive future cobalt consumption to
Based on its unique properties, cobalt
is useful in applications that utilize its
magnetic properties, corrosion resistance,
wear resistance, and its strength at
elevated temperatures. The blossoming
of a wide variety of new applications,
in addition to the old ones, will assure
cobalt’s place in metallurgy for many
years to come.
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Shijie Wang is senior engineer at Kennecott Utah
Copper Corp. in Magna, Utah.
For more information, contact Shijie Wang,
Kennecott Utah Copper Corp., 11600 W 2100 South,
P.O. Box 6001, Magna, UT 84044; (801) 569-6747;
fax (801) 569-6753; e-mail email@example.com.