Helium (He) is a colorless, odorless, non-toxic noble gas, the second-lightest element, and possesses the lowest boiling point of any substance. These unique properties make it indispensable for applications requiring low temperatures or an inert atmosphere, such as in Magnetic Resonance Imaging (MRI) machines and specialized welding. Helium is not chemically manufactured; rather, it is extracted and purified from natural sources, primarily certain underground gas reservoirs.
The Origin of Terrestrial Helium
The helium recovered from underground reserves is a byproduct of a natural geological process occurring deep within the Earth’s crust. This noble gas is continuously generated by the radioactive decay of heavy elements, primarily uranium and thorium, present in crustal rocks. As these unstable elements decay, they emit alpha particles, which are helium nuclei. Over millions of years, these particles capture electrons to become neutral helium atoms that migrate upward. Helium only becomes economically viable when trapped beneath thick, non-porous rock formations, accumulating alongside natural gas.
Initial Extraction from Natural Gas Reserves
Commercial recovery begins by drilling into natural gas wells containing a sufficiently high concentration of helium. Extraction is typically only economical when the concentration reaches 0.3% or more by volume. The raw gas stream, composed mainly of methane, is first pretreated to remove impurities like water vapor, hydrogen sulfide, and carbon dioxide, which could freeze equipment. The gas is then cooled and compressed to separate the bulk of the hydrocarbons. This liquefaction leaves a concentrated gaseous mixture, referred to as “crude helium,” typically containing 50% to 70% helium, with the remainder being mostly nitrogen.
Cryogenic Purification Methods
The crude helium stream must undergo cryogenic distillation to achieve commercial-grade purity. This energy-intensive process relies on the elements’ drastically different boiling points, requiring the mixture to be progressively cooled to extremely low temperatures to remove contaminants. Nitrogen, the primary remaining impurity, is removed by cooling the mixture to approximately -193°C (-315°F), causing it to liquefy. Because helium has the lowest boiling point of all elements (-269°C or -452°F), it remains a gas while nearly every other substance turns to liquid or solid. Specialized methods, such as pressure-swing adsorption units, are then used to produce helium with a purity of 99.999% or higher.
Why Helium is a Finite Resource
Despite its abundance in the universe, terrestrial helium is considered a finite resource due to Earth’s relatively weak gravitational field. Once extracted and used, helium released into the atmosphere is lost forever. Being the second-lightest element, helium atoms quickly rise to the top of the atmosphere, where they exceed the escape velocity and are lost to space. The rate at which new helium is generated by radioactive decay deep within the crust is too slow to replenish the quantities currently consumed. Therefore, the current supply is limited to the reserves trapped underground, making conservation efforts necessary.