Looking at the entire universe, helium can account for 23% of the mass of helium, but in the Earth's atmosphere, the concentration of helium is very low, only 5.2 parts per million (5.2 ppmv) by volume. This is because helium has a relatively small atomic weight, and its molecular velocity is faster than that of other gases in the atmosphere, which makes it easy for it to escape from the Earth's atmosphere.
In addition, helium is also a very stable gas, it is not easily adsorbed or converted by other gas chemical reactions in the Earth's atmosphere, which also causes it not to accumulate in the atmosphere. In contrast, although hydrogen is lighter, it can form compounds, such as combining with oxygen to form water, and combining with carbon to form various hydrocarbons, so the hydrogen content on Earth is higher.
There are 8 isotopes of helium, and only two of them are stable, which are helium-3 and helium-4. The main component of helium in the earth's atmosphere is helium-4, which accounts for about 99.99986% of helium, helium-3 is other gases in the atmosphere, making it easy on the earth, only about 0.00014%, and nuclear weapons tests still produce a small amount of helium-3. As one of the first elements to form in the universe, the main cause of helium-3 on Earth is the Big Bang and nuclear fusion reactions inside stars.
The other is radioactive helium-4, which is a α particle released by the decay of radioactive elements (such as thorium uranium) in the Earth's interior, and helium-4 is mainly found in the earth's crust, especially in sedimentary basins, and can be released to the surface through faults or geological fluids.
Why is helium expensive?
helium is widely used as a rare gas, but the helium resources on the earth are extremely limited.
helium is widely used in military industry, scientific research, petrochemical, refrigeration, medical, semiconductor, pipeline leak detection, superconductivity experiments, metal manufacturing, deep-sea diving, high-precision welding, optoelectronic product production, etc. The most well-known are helium inflatable balloons and helium ships carrying people. helium is in such a wide demand in all walks of life that naturally the price will not be too low.
In addition, it is difficult to obtain and recycle helium resources. The helium used as an inflatable ball cannot be recycled, and once used, it means that the helium resource is lost.
How do we get this precious gas?
Currently, there are four main ways to obtain helium.
The first is the natural gas separation method. This method uses natural gas containing helium as raw material, and finally obtains pure helium through steps such as liquefaction fractionation and activated carbon adsorption and purification.
The second is the ammonia synthesis method. In the industrial production process of synthetic ammonia, the tail gas contains helium, which can be obtained by separation and purification;
The third method is air fractionation. This method extracts helium from a neon-helium mixture by fractionation in liquid air;
Finally, there is a uranium ore method. In this method, helium-containing uranium ore is roasted to separate the gas, and then chemically removed impurities to obtain pure helium.
