Cobalt, a hard, lustrous, bluish-gray transition metal with the symbol Co and atomic number 27, has become an increasingly important element in the modern technological landscape. The metal’s unique properties, particularly its thermal stability and high energy density, make it a necessary component for the cathodes of rechargeable lithium-ion batteries. This demand is driven primarily by the rapid growth of the electric vehicle market, where cobalt plays a role in battery performance and safety. However, the process of obtaining this metal is complex because cobalt is almost never found in its pure, elemental form in the Earth’s crust.
Primary Sources and Ores
Cobalt is typically recovered as a co-product or byproduct, meaning its supply is tied to the mining of other metals. Most of the world’s cobalt is obtained during the extraction of copper, particularly in the Democratic Republic of Congo (DRC). Significant quantities are also produced from nickel deposits, especially the magmatic sulfide and laterite ores found in countries like Russia and Australia.
The composition of the ore dictates the subsequent extraction process, which is tailored to the host metal. Sulfide ores, where cobalt is chemically bound with sulfur, are processed differently than oxide or laterite ores. Cobalt is not processed in a single, universal way but through specialized circuits attached to larger copper or nickel operations.
Mining Methods and Initial Concentration
The physical journey of cobalt begins with mining the host ore, using large-scale industrial methods like open-pit and underground mining. Open-pit operations are utilized when the ore body is shallow and disseminated, while underground techniques are necessary for deeper, more concentrated deposits. Once the ore is extracted, it undergoes mechanical preparation to physically separate the cobalt-bearing minerals from the non-valuable rock, known as gangue.
Initial concentration involves crushing the massive ore blocks into smaller pieces using jaw and cone crushers. The crushed material is then subjected to fine grinding in ball or rod mills to achieve a particle size small enough to liberate the cobalt minerals from the host rock. The most common technique for pre-concentration is froth flotation, where ground particles are mixed in a water slurry with chemical reagents. These reagents selectively attach to the cobalt-containing minerals, allowing them to cling to air bubbles and float to the surface, forming a mineral-rich concentrate that feeds the subsequent chemical separation processes.
Hydrometallurgy and Pyrometallurgy
The challenge of cobalt extraction lies in chemically separating it from the concentrated copper and nickel, a process accomplished through two primary industrial approaches: pyrometallurgy and hydrometallurgy. Pyrometallurgy uses high temperatures, often with sulfide ores, in a process called smelting. Smelting involves heating the concentrate in a furnace to 1,200°C to 1,500°C, which melts the material and separates it into a metal-rich matte and a waste slag.
Cobalt is initially captured within the matte, which is primarily a mixture of copper, nickel, and iron sulfides, and requires further processing. Pyrometallurgy is energy-intensive but highly effective at reducing the volume of material that needs to be handled. The resulting matte is often the starting point for a subsequent hydrometallurgical circuit, demonstrating how the two methods are frequently used in sequence.
Hydrometallurgy, in contrast, uses aqueous solutions and chemical reactions to dissolve and separate the metals. This method begins with leaching, where the cobalt concentrate is submerged in an acidic solution, typically sulfuric acid, which dissolves the cobalt and other metals into the liquid phase. The resulting solution, known as a pregnant leach solution, contains a complex mix of dissolved metals that must be separated.
Purification is achieved through techniques like solvent extraction (SX) or precipitation. Solvent extraction involves mixing the pregnant leach solution with an organic solvent that selectively binds to and removes unwanted impurities like iron and copper. This selective removal is repeated in multiple stages until the solution contains a high concentration of purified cobalt ions, ready for the final recovery step.
Final Purification and Product Forms
The final step is converting the purified solution into a marketable product. For the production of high-purity cobalt metal, the purified cobalt sulfate solution is typically fed into an electrowinning circuit. This process uses an electrical current to reduce the dissolved cobalt ions, plating the elemental cobalt onto a cathode as a solid metal sheet.
A significant portion of cobalt is sold as chemical compounds for the battery industry, not as metal. The most common form is cobalt sulfate, created by precipitating cobalt from the purified solution as an intermediate like cobalt hydroxide, which is then re-dissolved and crystallized. Cobalt sulfate and cobalt oxide are the chemical precursors required by cathode manufacturers, making them the most prevalent final products from many cobalt refineries. Achieving the necessary purity for battery applications requires control over the final precipitation and crystallization stages.