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How are the rare gases in electric light sources produced?
The neon, fluorescent, and xenon lights we see in everyday life all rely on a special type of material—rare gases.
Their names, such as helium, neon, argon, krypton, and xenon, sound impressive, but they're actually hidden in the air we breathe every day. So how are these gases isolated and used in electric light sources?
1. Helium: A Rare Guest "Swimmed" from Natural Gas
Helium is present in very low concentrations in Earth's atmosphere, making it nearly impossible to extract directly from air. However, scientists have discovered that it comprises a relatively significant proportion in natural gas—some gas fields contain 1% to 2%, with some areas even approaching 8%, and still others only a few parts per thousand.
The extraction method is also quite interesting:
Natural gas is primarily composed of hydrocarbons (such as methane). When these are liquefied and condensed, helium, due to its extremely low boiling point (only 4.2 K), does not condense into a liquid and can be directly separated from the gaseous portion.
In addition, some chemical plants that use natural gas as a feedstock can also recycle helium.
2. Other Noble Gases: All Found in the Air
Except for radon (a radioactive gas rarely used), the other noble gases—neon, argon, krypton, and xenon—are almost all derived from air.
The key technology lies in two words: cryogenics.
Step 1: Liquefaction of Air
First, cool the air to approximately -200 degrees Celsius, turning it into a liquid—this is "liquid air."
Liquid air primarily consists of:
Liquid nitrogen (boiling point 77 K)
Liquid oxygen (boiling point 90 K)
The noble gases are dissolved in it.
Step 2: Fractional Distillation
Different gases have different boiling points, allowing them to be "separated" layer by layer.
Neon has a lower boiling point than nitrogen, so it vaporizes along with the nitrogen. To separate it, the nitrogen must be re-liquefied, and then the remaining nitrogen must be removed using activated carbon adsorption. What remains is neon.
Argon's boiling point (87.3 K) is very close to that of oxygen, so it primarily remains in the liquid oxygen. Adding hydrogen to the mixed gas converts the oxygen into water, and then removing the excess hydrogen yields high-purity argon.
Krypton and xenon have higher boiling points than oxygen and tend to concentrate in liquid oxygen. Further cryogenic fractionation can separate them from oxygen, and then selective adsorption can be used to distinguish between krypton and xenon.
III. Production Process Schematic
Air → Liquefaction → Fractionation → Produces Nitrogen and Oxygen → Separates Neon from Nitrogen → Separates Argon, Krypton, and Xenon from Oxygen
Natural Gas → Liquefaction and Separation → Produces Helium
IV. What are these noble gases used for?
Because of their chemical stability and resistance to reaction, noble gases are ideal for use in electric light sources:
Neon (Ne): Made into red neon lights;
Argon (Ar): Commonly used as a filler gas in fluorescent lamps and energy-saving lamps;
Xenon (Xe): Used in car lights, flashlights, and movie projectors for strong light sources;
Krypton (Kr): Used in photographic lights and airport signal lights;
Helium (He): Used in cooling, high-voltage discharge tubes, and cryogenic experiments.
V. Conclusion
The world of rare gases, seemingly unpopular, illuminates every aspect of our lives.
From the flickering neon lights of the night sky to the xenon headlights of cars, to the cryogenic environments of scientific experiments, these invisible gases are quietly illuminating human civilization.
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