HELIUM 3 of 3   Leave a comment

Occurrence and production

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 nucleo-synthesis, 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. The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth’s atmosphere escapes into space by several processes. 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 and monazite, 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 kilometre 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.

The world’s helium supply may be in danger, according to Washington University in St. Louis chemist Lee Sobotka. The largest reserve is in Texas and would run out in eight years if consumed at the current pace. Helium is non-renewable and irreplaceable by conventional methods.

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. In a final production step, most of the helium that is produced is liquefied via a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers.

In 2005, approximately one hundred and sixty million cubic meters of helium were extracted from natural gas or withdrawn from helium reserves, with approximately 83% from the United States, 11% from Algeria, and most of the remainder from Russia and Poland. In the United States, most helium is extracted from natural gas 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.

Isotopes

Although there are eight known isotopes of helium, only helium-3 and helium-4 are stable. In the Earth’s atmosphere, there is one He-3 atom for every million He-4 atoms. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of He-3 is around a hundred times higher. Rocks from the Earth’s crust have isotope ratios varying by as much as a factor of ten; this is used in geology to study the origin of such rocks.

The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.

Evaporative cooling of liquid helium-4, in a so-called 1-K pot, cools the liquid to about 1 kelvin. In a helium-3 refrigerator, similar cooling of helium-3, which has a lower boiling point, reaches a temperature of about 0.2 kelvin. Equal mixtures of liquid helium-3 and helium-4 below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions). Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

There is only a trace amount of helium-3 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, helium-3 is more abundant, a product of nuclear fusion. Extra-planetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon’s surface contains helium-3 at concentrations on the order of 0.01 ppm. A number of people, starting with Gerald Kulcinski in 1986, have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion.

The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. These differing isotope abundances can be used to investigate the origin of rocks and the composition of the Earth’s mantle.

It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is helium-5 with a half-life of 7.6×10−22 second. Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are hyper-fragments that are created in certain nuclear reactions.

The exotics helium-6 and helium-8 are known to exhibit a nuclear halo.

Helium-2 (two protons, no neutrons) is a radioisotope of helium that decays by proton emission into protium (hydrogen) with a half-life of 3×10−27 second.

Biological effects

The voice of a person who has inhaled helium temporarily sounds high-pitched. This is because the speed of sound in helium is nearly three times the speed of sound in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. (The opposite effect, lowering frequencies, can be obtained by inhaling sulphur hexafluoride).

Inhaling helium, e.g. to produce the vocal effect, can be dangerous if done to excess since helium is a simple asphyxiant, thus it displaces oxygen needed for normal respiration. Death by asphyxiation will result within minutes if pure helium is breathed continuously. In mammals (with the notable exceptions of seals and many burrowing animals) the breathing reflex is triggered by excess of carbon dioxide rather than lack of oxygen, so asphyxiation by helium progresses without the victim experiencing air hunger. Inhaling helium directly from pressurized cylinders is extremely dangerous as the high flow rate can result in barotrauma, fatally rupturing lung tissue.

Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen ( heliox) can lead to high pressure nervous syndrome, a sort of reverse anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem.

Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature.

Compounds

Helium is chemically un-reactive under all normal conditions due to its valence of zero. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds with tungsten, iodine, fluorine, sulphur and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10, WHe2 and the molecular ions He2+, He22+, HeH+, and HeD+ have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces. Theoretically, other compounds may also be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000.

Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure of the gas. The neutral molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. If helium-3 is used, it can be readily observed by helium NMR spectroscopy. Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances fit the definition of compounds in the Handbook of Chemistry and Physics. They are the first stable neutral helium compounds to be formed.

Synopsis of: https://en.wikipedia.org/wiki/Helium

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Posted 2018/01/25 by Stelios in Education

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