Helium, a colorless and odorless noble gas, is a technologically important element with the lowest boiling point of any known substance. Commercial helium is not mined like an ore; instead, it is extracted as a byproduct from specific natural gas streams collected underground. The process involves a complex series of steps, beginning with a unique geological formation and culminating in a highly specialized separation technique.
The Geological Origin of Terrestrial Helium
The helium found deep within the Earth’s crust is continuously generated through the slow radioactive decay of heavy elements, primarily uranium and thorium, present in crustal rocks. These unstable elements spontaneously emit alpha particles. Once emitted, these particles capture two electrons from the surrounding material, becoming neutral helium gas (\(\text{He-4}\)). Over millions of years, this radiogenic helium migrates through the Earth’s layers and accumulates in porous rock formations, trapped beneath an impermeable caprock, often mixing with natural gas reserves like methane and nitrogen.
Locating and Tapping Helium-Rich Natural Gas Reserves
Commercial extraction of helium is only economically viable in rare geological structures where the gas has accumulated in sufficient concentration. For a natural gas field to be considered a viable source, the helium content must typically exceed \(0.3\%\) to \(0.5\%\) by volume. Exploration teams use specialized seismic surveys and test drilling to identify these high-concentration reservoirs. Once located, drilling taps into the rock layer, allowing the mixed, raw gas stream—predominantly methane and nitrogen—to flow to the surface for subsequent separation.
The Cryogenic Separation Process
The most widely employed technique for isolating helium is cryogenic fractional distillation, which exploits the significant difference in the boiling points of the gas components. The mixed gas is first treated to remove impurities like water vapor and carbon dioxide, preventing equipment damage at low temperatures. The purified stream is progressively cooled, causing methane and heavier hydrocarbons to condense into a liquid, followed by nitrogen. Because helium has the lowest boiling point of all elements, it remains a gas until extremely low temperatures (\(\text{-269}^\circ\text{C}\)). The resulting crude helium stream, sometimes containing up to \(50\%\) helium, is drawn off and routed to a final purification stage, often involving a cryogenic pressure swing adsorption (PSA) unit, to remove traces of nitrogen and neon.
Refining, Liquefaction, and Strategic Storage
The crude helium stream must undergo significant refining to achieve the purity required for sensitive industrial and scientific applications. Commercial “Grade-A” helium is typically \(99.995\%\) pure, though specialty applications require ultra-high purity levels exceeding \(99.999\%\). Final purification uses activated charcoal beds at liquid nitrogen temperatures to adsorb trace contaminants. The purified helium is then cooled and compressed into its liquid state at approximately \(\text{-269}^\circ\text{C}\). Liquefaction is necessary because it drastically reduces the volume, making long-distance transportation economically feasible, as liquid helium dewars hold significantly more volume than gaseous tube trailers.
In the United States, significant volumes of crude helium have historically been stored in the National Helium Reserve, a strategic stockpile located near Amarillo, Texas. This facility uses the Bush Dome Reservoir, a natural geological formation, to store the crude gas. This strategic infrastructure ensures a stable supply of the gas for government use and acts as a central hub for the global supply chain.