Cobalt (Co) is a rare metal element found in nature (0.001% of earth’s crust) . It belongs to the first-row transition series of elements in the periodic table and does not exist in its purest form naturally – it only exists in combination with other elements. When fully extracted and refined from its ore, cobalt appears as a lustrous, bluish-white metal. It is usually combined with iron to form different alloys in many applications.
Cobalt is a ferromagnetic material, hard yet brittle. It has a high melting point and is hard-wearing at elevated temperatures. It has high strength and possesses good conduction properties. Below are some key properties of cobalt [1,2]:
It has a hexagonal closed pack crystal structure at temperatures below 417 °C (783 °F) and has 12 radioactive isotopes. The most popular is cobalt-60 with a half-life of 5.3 years, which is widely used in medical applications.
Cobalt occurs typically in nature in cobaltite, smaltite, and erythrite minerals. It is mined along with nickel, silver, lead, copper, and iron ore and is often obtained as a by-product. The extraction of cobalt can be done using solvent extraction . The laterite ore undergoes a reduction process at 525°C with carbon monoxide. This reduces nickel and iron to ferronickel, and the cobalt and copper to a metallic state. The minerals are then leached with a 100 g/L ammonium hydroxide (NH4OH), 300 g/L (NH4)2SO4 solution in a controlled oxidation leach to solubilise nickel, copper, and cobalt.
Cobalt has been used in many industrial, commercial, and military applications . Below are some of its common applications:
Cobalt-based superalloys form high-temperature resistant parts for gas turbine aircraft engines, space vehicles, rocket motors, and other aerospace applications. Cobalt-based superalloys have a higher melting point than iron or nickel, and have excellent resistance to hot corrosion and thermal fatigue. The weldability of this superalloy is also better than nickel superalloys. Altogether, cobalt-based superalloys perform exceptionally in applications with low stress and elevated temperature environments .
Cobalt steel is a variation of high-speed steel with common grades M-35 and M-42. It is an ideal cutting tool for its high red hardness that in turn provides high heat resistance. The cobaltic high-speed steel is able to run and withstand high feed rates and faster speed .
Cobalt is used widely as one of the metals needed to create hard permanent magnets with high coercivity, such as the aluminium-nickel-cobalt (Al-Ni-Co) alloy series. Alnico magnets are used in motors, hard disk drives, and sensors. Magnetic resonance imaging is an example of an application for these magnetic alloys.
Cobalt oxide, hydroxide, and metals are used in many electrochemical devices that convert chemical energy to electrical energy, such as rechargeable batteries. Portable devices such as mobile phones, laptops, and other consumer electronic devices right up to electric vehicles, all utilise rechargeable batteries. Cobalt acts as a raw material in the cathode technology which is essential for recharging batteries .
Cobalt is used as a catalyst for many industrial applications such as removing sulfur moieties from petroleum and natural gas products. Desulphurisation of diesel, petrol, kerosene and other fuel oils contributes to reducing emissions. The element as a catalyst, not only reduces emissions, but also activates the energy required for industrial processes, such as recycling plastics. 
Image source: Wikimedia Commons
Cobalt pigments are used to decorate ceramics and can also be added to glass as a colourant or decolouriser to create specific tints. Glass, porcelain, paints and inks, and enamelware use it in order to achieve a vivid blue colour. Cobalt has been found irreplaceable as a colouring agent in these applications, due to its unique properties in solubility, stability, and colouring effect. 
Fig 1. Other than the famed cobalt blue, cobalt is also known to produce many other brightly coloured complexes depending on its electronic structure. Here are the original Werner complexes in Zurich, Switzerland.
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