Mining and refining: helium | Hackade

With a seemingly endless list of deficiencies of basic items trotted across the newsfeed on a daily basis, you will be forgiven for not noticing any one deficiency in particular. But in the absence of everything from eggs to fertilizer to Sriracha sauce, there is a growing realization that we are actually moving beyond something basic that could have repercussions that would be felt throughout all aspects of our technological society: helium.

It is difficult to overstate the level at which helium is concentrated in almost every aspect of daily life. The unique properties of helium, such as the fact that it is only a few degrees above absolute zero, contribute to its use in countless industrial processes. From leak detection and casting to the production of silicon wafers and the cooling of superconducting magnets that enable magnetic resonance imaging, helium has entered technology in a way that denies its relative lack.

But where does helium come from? As we shall see, the second light element of the periodic table is not readily available, and sufficient effort is made to extract and refine it sufficiently for industrial use. Although great progress is being made towards improved methods of extraction and the discovery of new deposits, helium is a non-renewable resource for all practical purposes. So it makes sense to know a thing or two about how we can get our hands on it.

A product of decay

Despite being the second most abundant element in the visible universe, helium is surprisingly rare on Earth. Although it was first discovered in spectral graphs of the Sun and other stars in the 1860s, enough helium was found to be studied and it was decided that this element would wait another 30 years, when a gas with the same spectral signature was released by dissolving a sample. . Acid uranium ore.

Uranium decay series. When U-238 th-234 (upper left) decays, it releases an alpha particle, which is a helium nucleus. The particle rapidly forms a new atom of helium and receives two electrons. Source: Tosaka, CC BY 3.0, via Wikimedia Commons

The discovery of helium on Earth came at an opportune time in the history of chemistry. In the late 1800’s and early 1900’s, the focus of chemistry was on the atomic reaction of the atoms to expand in the subatomic state as a whole over the levels of electrons, protons and neutrons that make up the atom. Radioactivity has only just begun to be explored, and the existence of alpha, beta, and gamma rays was already known when helium was first isolated. And so when Rutherford and Boyd discovered that the alpha ray was actually a particle consisting of two protons and two neutrons, resembling the nucleus of a helium atom, it immediately suggested a process of how helium was able to get stuck in uranium ore.

Like all heavy radioactive elements, uranium is degraded with a certain series of elements. The uranium series begins with isotopes 238U, uranium is a naturally occurring and relatively abundant isotope. 238The half-life of U is about 4 billion years, and when it decays, it emits an alpha particle. A pair of protons and a pair of neutrons are lost 238You in 234M, or thorium-234. The free alpha particle, which is really a helium nucleus, easily absorbs two electrons when it is absorbed by any object, forming an atom of helium.

This nicely explains why helium was inside the uranium ore sample – over time, the uranium decay released alpha particles that were absorbed by the rock, gaining the electrons needed to make helium atoms. Helium accumulates over time, accumulates in the pores of the rock, and is released only when the minerals in the rock are finally dissolved. And this same process, albeit on a geological scale, is key to industrial helium production.

A gas is a gas

Unlike most industrial gases, helium is not present in the atmosphere at any significant concentration. Any helium that is not separated in any way after it is generated will find its way into the atmosphere and quickly disappear, quickly ascend into the upper atmosphere and eventually exit into space. So it is not practical to separate helium from air for oxygen, nitrogen, argon and other gases. Rather, we need to look down at the significant reservoirs of helium.

Fortunately, the same geological conditions that tend to trap natural gas in underground reservoirs also trap helium, and so natural gas wells are the largest source of helium. Historically, the United States has been a major supplier of helium to world markets, mostly coming from natural gas wells in Oklahoma, Kansas, and Texas. The gas emitted here is up to 7% helium, which is sufficient for profitable extraction.

Natural gas is a mixture of methane, nitrogen, carbon dioxide and higher gaseous alkene such as ethane and propane. Where sufficient helium is mixed – something more than 0.4% is considered profitable – helium extraction and purification is performed by fractional distillation. Helium has the lowest boiling point of any component, which means that every other gas can be separated by controlling the temperature drop and pressure.

The first step in helium production is to scrub any CO2 And hydrogen sulfide (H.2S) From natural gas. This is done in an amine tractor, where the chemical monoethanolamine (MEA) is sprayed into the gas flow inside a reaction vessel. MEA ionizes acidic compounds and makes them soluble in water, allowing them to be scrubbed from natural gas. The scrubbed gas is pretreated through a molecular sieve, such as zeolite and a bed of activated carbon, to remove water vapor and heavy hydrocarbons.

A general industrial distillation process is planned. Source: H Padlekas, CC-BY-3.0

What remains after this pretreatment step is mainly methane and nitrogen, but also some neon and helium. The gas is cooled through a heat exchanger, then passed through an expansion valve in a wide fraction column. A sudden drop in pressure lowers the temperature of the gas enough that methane, which boils down to -161.5 সেল C, condenses into a liquid and moves to the bottom of the column.

The remaining gas, now mostly nitrogen and helium, flows through a condenser that cools the current. When the temperature of the mixture drops below -195.8 স C, nitrogen condenses as a liquid. Along with liquid methane, liquid nitrogen is piped into heat exchangers that were initially used to cool the incoming pretreated process gas. Currently gaseous nitrogen and methane, both valuable products, are piped into storage tanks.

About half of the remaining process gas is helium, the rest is a mixture of contaminated methane and nitrogen, with little hydrogen and neon. This mixture is called cold unrefined helium, and must now go through further refining to reach the level of purity required for industrial use. Purification begins with another heat exchanger that drops the crude helium mixture below the boiling point of nitrogen to concentrate the remaining nitrogen and methane contaminants. This step brings the crude helium to about 90% purity.

Final refinement

To get rid of hydrogen, oxygen is introduced and the mixture is heated in the presence of a catalyst. Hydrogen and oxygen form water, which can be separated from the process gas flow before moving towards final purification by pressure swing absorption or PSA. Pressure-swing absorption is the same process used in oxygen concentration, including many DIY versions that we have seen as a reaction to COVID-19. PSA selectively uses the power of a substance known as molecular sieves to absorb a gas. In helium refining, 90% pure gas is pumped into a pressure vessel containing a molecular sieve, usually zeolite. Contaminated gases are preferentially absorbed into the zeolite, leaving the output stream almost pure helium. When the first column is filled with contaminants, the flow is switched to the second column, which was previously regenerated by backflushing with pure helium. The gas flow switches back and forth between the two columns, one purifying the helium and the other regenerating. The result is helium in gaseous grade at 99.995% purity.

The process described here is not the only way to extract helium from natural gas, but it represents the most common method of gas production, mainly because most pretreatment and initial refining steps are already used to process natural gas for fuel. And as a feedstock for the chemical industry. Other methods include a complete PSA process, which can use natural gas with only 0.06% helium concentration and can disintegrate membranes, depending on the fact that helium can enter a semipermeable membrane much easier than large methane and nitrogen molecules. Can Membrane separation technology can be much more energy-efficient than traditional fractional distillation, as it does not require phase change and the energy they need.

But do we run out?

It is possible to estimate the abundance of uranium-238 in the Earth’s lithosphere and its half-life, as well as the amount of helium produced by radiogenic processes. Turns out it’s not too much – about 3,000 metric tons a year. And almost everything escapes into the atmosphere and into space. So much like the natural gas in which it is commonly found, helium is effectively a non-renewable resource.

But does that mean we’re running out? Yes, like any other limited asset, we will eventually find what we need. But that doesn’t mean we’ve found helium. Exploration has led to new deposits in the United States, and huge helium was found in places like Algeria, which became the world’s second largest helium producer in the early 2000s. A huge helium was found in Qatar in 2013, making it the second largest in the world. With the recent discovery of natural gas wells containing up to 12% helium in South Africa, these findings promise to address some concerns about losing access to this unchanging gas.

But at the end of the day, these new discoveries simply push back the clock and inevitably stick to the day when helium eventually runs out. We can take a break if commercial-scale fusion ever becomes a thing, but that progress is “only twenty years away” from the last 80 years.

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