What is Cerium?
Cerium (Ce), element number 58 on the periodic table, is the most abundant of the rare earth elements and belongs to the lanthanide series. It is a soft, silvery-white, ductile metal that tarnishes when exposed to air. Like other rare earths, cerium exhibits distinct chemical properties that make it valuable in various modern technologies and everyday applications.
Natural Occurrence and Extraction
Cerium is not found as a free element in nature due to its reactivity. Instead, it is typically found in minerals alongside other rare earth elements. The primary ore minerals containing cerium include monazite (a phosphate mineral) and bastnäsite (a fluorocarbonate mineral).
Vast deposits of these rare earth minerals are found globally. The Bayan Obo Mining District in Inner Mongolia, China, is the world’s largest known rare earth deposit, accounting for a significant portion of global production. Other notable reserves and mining operations exist in the United States (Mountain Pass, California), Australia (Mount Weld), India, Brazil, and Russia.
The extraction process for cerium and other rare earths is complex and multi-staged:
Mining and Concentration
Ore is initially extracted through open-pit or underground mining methods. After mining, the ore undergoes beneficiation processes, which involve crushing, grinding, flotation, and magnetic separation to concentrate the rare earth minerals into a more manageable form.
Chemical Leaching
The concentrated minerals are then subjected to chemical leaching using strong acids, such as sulfuric or hydrochloric acid. This step dissolves the rare earth compounds, separating them from other gangue minerals.
Separation of Individual Elements
Due to their very similar chemical properties, separating cerium from other rare earth elements is challenging. The most common industrial method is solvent extraction, which involves hundreds of stages where different solvents are used to selectively extract individual rare earth ions based on subtle differences in their solubilities. This process is often highly energy-intensive.
Reduction to Metal
Once cerium is isolated as a purified compound, it can be reduced to its metallic form. This is typically achieved through the electrolysis of molten cerium chloride or fluorides, or by metallothermic reduction using calcium or lithium.
Everyday Uses of Cerium
Cerium’s unique chemical and physical properties lend themselves to a variety of applications essential to modern life.
1. Catalytic Converters
Cerium oxide is a critical component in automotive catalytic converters, which are found in virtually all modern vehicles globally, from compact cars in Japan to large trucks in North America. Its role is to act as an oxygen storage material, absorbing oxygen when the engine runs lean (excess oxygen) and releasing it when the engine runs rich (deficient oxygen). This helps the catalyst maintain its efficiency in converting harmful pollutants like carbon monoxide, nitrogen oxides, and unburnt hydrocarbons into less harmful substances.
2. Glass Polishing
Cerium oxide is an extremely effective abrasive used for polishing optical glass, such as camera lenses manufactured in Germany or eyeglasses produced in facilities across Asia. It is also employed for polishing flat-panel display screens for televisions and smartphones globally. Its effectiveness stems from a combination of mechanical abrasion and a mild chemical reaction with the glass surface, resulting in a superior finish compared to other polishing agents.
3. Lighter Flints
An alloy of cerium and iron, known as ferrocerium, is widely used as the “flint” in disposable lighters and fire starters around the world. When struck or scraped, ferrocerium produces sparks with temperatures reaching over 1,500°C, capable of igniting flammable gases or tinder. This application has been crucial for providing portable ignition sources for decades.
4. Phosphors in Display Screens
Cerium compounds, particularly cerium-doped yttrium aluminum garnet (YAG:Ce), are used as phosphors. These materials are crucial in the production of white light-emitting diodes (LEDs) that backlight LCD screens for devices like smartphones, tablets, and televisions manufactured in countries like South Korea and China. When excited by blue light from the LED, the cerium-doped phosphor emits yellow light, which combines with the remaining blue light to create white light.
5. UV-Absorbing Glass
Cerium dioxide is incorporated into glass formulations to absorb ultraviolet (UV) radiation. This application is particularly important for specialized eyewear, such as sunglasses, and certain types of window glass designed to protect interiors from UV damage. It is also used in some optical instruments to filter specific wavelengths of light.