JOM 0610: Cobalt-Its Recovery, Recycling, and Application

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Overview: Cobalt: Winning, Recycling, and Applications

Vol. 58, No.10, pp. 47-50

Cobalt—Its Recovery,
Recycling, and Application

Shijie Wang

OCTOBER 2006 ISSUE

ALSO APPEARING IN PRINT
The print and/or PDF versions of the article can be acquired via the TMS Document Center.

FIGURE A(a).
A block flow diagram of a cobalt plant.

FIGURE A(b).
A block flow diagram of a cobalt plant.

FIGURE A(c).
A block flow diagram of a cobalt plant.

FIGURE A(d).
A block flow diagram of a cobalt plant.

FIGURE A(e).
A block flow diagram of a cobalt plant.

TABLE A.

REACTIONS

(1)	Co → Co²⁺ + 2e^– E⁰ = –0.28 V
(2)	6FeSO₄ + NaClO₃ + 3H₂SO₄ → 3Fe₂(SO₄)₃ + NaCl + 3H₂O
(3)	2Fe(OH)₃ + 3H₂SO₄ → Fe2(SO₄)₃ + 6H₂O
(4)	2H₃AsO₄ + Fe₂(SO₄)₃ → 2FeAsO₄ + 3H₂SO₄
(5)	2H₃AsO₄ + 8Fe(OH)₃ → (Fe₂O₃)₄·As₂O₅↓ + 15H₂O
(6)	MnSO₄ + NaClO + H₂O → MnO₂↓ + NaCl + H₂SO₄
(7)	MnSO₄ + NaClO + Na₂CO₃ → MnO₂↓ + NaCl + Na₂SO₄ + CO₂
(8)	2CoSO₄ + NaClO + 5H₂O → 2Co(OH)₃↓ + NaCl + 2H₂SO₄
(9)	2NiSO₄ + NaClO + 2Na₂CO₃ + 3H₂O → 2Ni(OH)₃↓ + 2Na₂SO₄ + NaCl + CO₂
(10)	CoSO₄ + Ni(OH)₃↓ → Co(OH)₃↓ + NiSO4
(11)	Extraction: nRH + Men+ → RnMe + nH⁺
(12)	Stripping: RnMe + nH⁺→ nRH + Meⁿ⁺
(13)	Cathode: Co²⁺ + 2e^– → Co
(14)	Anode: 2Cl^– → Cl₂ + 2e^– (chloride medium)
(15)	H2O → 1/2O₂ + 2H⁺ +2e^– (sulfate medium)
(16)	2Fe(BF₄)₃ + M → 2Fe(BF₄)₂ + M(BF₄)₂

RELATED WEB SITES
KENNECOTT UTAH COPPER GULF CHEMICAL AND METALLURGICAL CORP UNIVERSITY OF CALIFORNIA–BERKELEY

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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.

INTRODUCTION

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 processes^4–8 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 recovery process.^1–3

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
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 MRT^10,11 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 metallic powders.

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, and residues.

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 hydrometallurgy.^14–16

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 cobalt refineries.

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 purchase specification.

Recovering from Cobalt-Bearing Solutions
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/ Residue
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 system.

COBALT APPLICATION

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 as follows.^23–26

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, and corrosion.

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 oxidation.

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 unprecedented levels.

CONCLUSION

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.

REFERENCES

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23. L. Perron, “Cobalt,” Canadian Minerals Yearbook, 1995, (Ottawa, Canada: Natural Resources Canada, 1995), pp. 23.1– 23.16
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26. Cobalt Facts (Guildford, U.K.: Cobalt Development Institute, 2002), www.thecdi.com/cobaltfacts.php.

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 wangs@kennecott.com.