 Natural Abundance
|
| Helium is the second most abundant element in the known Universe after hydrogen and constitutes 23% of the elemental mass of the universe. It is concentrated in stars, where it is formed from hydrogen by the nuclear fusion of the proton-proton chain reaction and CNO cycle. According to the Big Bang model of the early development
of the universe, the vast majority of helium was formed during Big Bang nucleosynthesis, from one to three minutes after the
Big Bang. As such, measurements of its abundance contribute to cosmological models.
|
| In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million, largely because most helium
in the Earth's atmosphere escapes into space due to its inertness and low mass. In the Earth's heterosphere, a part of the
upper atmosphere, helium and other lighter gases are the most abundant elements.
|
| Nearly all helium on Earth is a result of radioactive decay. The decay product is primarily found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite, monazite and beryl, because they emit alpha particles, which consist of helium
nuclei (He2+) to which electrons readily combine. In this way an estimated 3.4 litres of helium per year are generated per cubic kilometer of the Earth's
crust. In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4
parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. The greatest concentrations on the planet are in natural gas, from which most commercial helium is derived.
|
|
| Modern Extraction
|
| For large-scale use, helium is extracted by fractional distillation from natural gas, which contains up to 7% helium. Since
helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost
all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually
resulting in 99.995% pure, Grade-A, helium. The principal impurity in Grade-A helium is neon.
|
| As of 2004, over one hundred and fifty million cubic metres of helium were extracted from natural gas or withdrawn from helium
reserves, annually, with approximately 84% of production from the United States, 10% from Algeria, and most of the remainder from Canada, China, Poland, Qatar, and Russia. In the United States, most helium is produced in Kansas and Texas.
|
| Diffusion of crude natural gas through special semi-permeable membranes and other barriers is another method to recover and
purify helium. Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but this is not an economically viable method of production.
|
|