Molybdenum is a metallic, silvery-white element with one of the highest melting points of all elements, reaching 2,623°C. This extreme thermal resistance makes it exceptionally valuable as an alloying agent in modern industry. It is most commonly found in the mineral molybdenite, a molybdenum disulfide (\(\text{MoS}_2\)). This raw ore must undergo a complex series of physical and chemical transformations, including extraction, concentration, and detailed chemical refining, before it can be used commercially.
Geological Sources and Initial Extraction
The primary source of molybdenum is the sulfide mineral molybdenite (\(\text{MoS}_2\)), found in large hydrothermal porphyry deposits. Mining operations are categorized into two main types based on the deposit’s composition. Molybdenum is mined as the primary product in deposits like the Climax-type, where the concentration is high enough to justify the operation alone.
A significant portion of the world’s supply is recovered as a byproduct from massive porphyry copper deposits. In these secondary mines, the economic driver is copper recovery, with molybdenum providing additional value. Extraction typically employs either open-pit or underground methods.
Open-pit mining is the most common method for large, near-surface, low-grade deposits, involving the removal of large volumes of overburden to access the ore body. For deeper or higher-grade deposits, underground methods, such as block caving, may be used. The goal is to remove the raw ore for subsequent processing.
Ore Concentration Through Flotation
After the ore is removed, it enters the mill for concentration. The ore is first subjected to crushing and grinding, reducing the massive rocks to a fine powder, often referred to as a slurry when mixed with water. This size reduction, known as comminution, is necessary to liberate the fine molybdenite particles from the surrounding worthless rock, or gangue.
The liberated molybdenite is separated from the slurry using froth flotation, which takes advantage of the natural hydrophobicity of molybdenite. The finely ground slurry is mixed with chemical reagents, including collectors that attach to the particles, and frothers that create a stable layer of bubbles on the water’s surface.
Air is introduced into the mixture, causing the collector-coated molybdenite particles to adhere to the bubbles and rise in a froth. The waste material and other minerals remain submerged. The froth is continuously skimmed off, capturing the molybdenite concentrate. To achieve the required purity, the rough concentrate is subjected to multiple cleaning and re-grinding stages.
Chemical Refining of Molybdenum Concentrate
The concentrated product is molybdenite (\(\text{MoS}_2\)), a sulfide unsuitable for most industrial applications. The next stage is chemical transformation, typically high-temperature roasting in a multi-hearth furnace. The \(\text{MoS}_2\) concentrate is heated in the presence of air between 500°C and 650°C, converting the molybdenum sulfide into technical molybdenum oxide (\(\text{MoO}_3\)) and releasing sulfur dioxide gas as a byproduct.
The resulting technical molybdenum oxide, or “tech oxide,” contains approximately 57% molybdenum and is the primary commercial product, used directly in steelmaking. For applications requiring higher purity, the tech oxide must undergo further refining. This can involve sublimation, which separates the volatile \(\text{MoO}_3\) from non-volatile impurities at high temperatures. Alternatively, a chemical wet process dissolves the oxide in an alkaline solution, such as ammonia, to create ammonium molybdate, which is then purified through crystallization.
The purified ammonium molybdate or molybdenum oxide is the precursor for other high-value molybdenum forms. Ferromolybdenum (\(\text{FeMo}\)), an alloy used extensively in the steel industry, is produced by mixing technical \(\text{MoO}_3\) with iron oxide and a reducing agent, then igniting the mixture in a thermite-like reaction. To produce pure molybdenum metal powder, the purified \(\text{MoO}_3\) is subjected to a two-stage reduction process using hydrogen gas at temperatures up to 1,100°C.
Primary Industrial Applications
Molybdenum is channeled into industries requiring exceptional strength and heat resistance. The largest application is in the production of alloy steels and superalloys, where it enhances hardenability, toughness, and corrosion resistance. Adding molybdenum allows for the creation of high-strength, low-alloy (HSLA) steels used in construction and pipelines, and stainless steels for chemical processing equipment.
Technical molybdenum oxide and ferromolybdenum are the forms introduced directly into molten steel. Beyond metallurgy, purified molybdenum compounds serve as catalysts in the petroleum industry for the desulfurization of fuels. The metal’s high melting point also makes it valuable in specialized electrical applications and high-temperature furnace parts.