Critical Elements Chemistry Tutorial

⚛ Critical elements are those chemical elements which, although in demand, will be available in limited supply in the near future.

⚛ 44 elements are, or are at risk of becoming, critical elements.

⚛ To insure that elements are available to meet society's needs in the future, critical and endangered elements should be

· re-used where possible

· recycled where possible

· replaced with a suitable, more sustainable alternative

⚛ Examples of critical elements include:

· helium (a non-metal)

· phosphorus (a non-metal)

· arsenic (a semi-metal or metalloid)

· indium (a post-transition metal)

· zinc (a transition metal)

· lithium (a Group 1 element)

· neodymium (a rare-earth element)

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Just 8 elements make up a little over 99% of the Earth's crust. These elements are shown in the table below:

About 90% of the Earth's crust is made up of silicate minerals which contain silicon and oxygen, some of which are aluminosilicate minerals containing silicon, oxygen and aluminium. The rest is made up largely of oxides which contain oxygen. Hence, the most abundant elements in the Earth's crust are oxygen, silicon and aluminium.

We mine, then refine, these minerals in the Earth's crust to obtain useful elements; iron for steel, copper for electrical wiring, silicon for silicon chips, lithium for batteries, and many, many others. Some of these elements, like iron, are not only a major component of the Earth's crust, but geological action has also concentrated them in certain areas which makes obtaining them easier than others. Some elements have not been concentrated by geological action, like rare-earth elements, making them harder to obtain.

None of these elements we obtain through mining can be considered truly "renewable", that is, we consume them faster than geological processes make them availble to us. Add to this the problem that modern society is becoming reliant on harder to obtain and less abundant elements, like helium and lithium, and you can see that our society is going to face a serious supply problem in the future. In response to this, attempts have been made to classify elements, in the same way as we classify living things, as endangered or critically endangered. One such list is shown in the table below:

The sustainable management of the extraction, use and re-use of these endangered and critical elements is essential.

Continuing research into the more efficient use, recycling and recovery of these elements is also essential, along with research into more sustainable alternatives.

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Non-metallic elements, except for hydrogen, occur on the extreme right hand side of the Periodic Table. There are 17 naturally occurring non-metallic elements. At room temperature and pressure, 5 of these non-metallic elements exist as solids, 1 is a liquid and 11 are gases. Some of these elements are found in nature as the element, for example the Group 18 elements (helium, neon, argon, krypton, xenon, radon), oxygen, nitrogen, some carbon (as coal or diamond) and some sulfur (yellow crystals in volcanic areas). All of these elements, except the Group 18 elements, are found naturally in the compounds that make up the Earth's crust.

While the non-metallic elements carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus are all essential to life, only phosphorus is considered to be an element at risk, or an endangered element. On the other hand, helium, which is not used to maintain life, is essential to the continuation of our technological society and is considered a critical element, an element that we are quite likely to run out of!

Helium is an odourless, colourless, gas at room temperature and pressure. It belongs to Group 18, the Noble Gas group, of the Periodic Table of the Elements and as such is very non-reactive. Hence, helium is found on Earth as a monoatomic gas, a gaseous molecule made up of just one atom of helium.

Helium gas turns into helium liquid at a very low temperature, about -269°C (about 4 K) which is almost as cold as outer space (about -270°C or 3 K). This means that you can use liquid helium to keep things very, very, cold. When some materials get this cold they become "superconductors" which means electricity can flow through these materials without experiencing any resistance, and this creates huge magnetic fields. We put these enormous magnetic fields to use in things like Magnetic Resonance Imaging (MRI), Nuclear Magnetic Resonance (NMR), and particle accelerators like the Large Hadron Collider (LHC). Helium that is used to cool instruments can be captured and re-used thereby minimising its loss.

Because helium is so unreactive it can be used to provide an inert protective atmosphere for making fibre optics and semiconductors, and for arc welding.

The Earth's atmosphere contains helium at a concentration of about 5 ppm which is too low for extraction from the atmosphere to be considered economically viable. This means that every time helium is vented from a machine, or a helium-filled balloon is released into the atmosphere, this helium is lost to us permanently. The greatest economically-viable source of helium on Earth is from natural gas. Natural gas that contains more than 0.3 % helium is considered an economically viable source. The USA, Qatar and Algeria have the world's largest helium reserves. First the helium is separated from the methane resulting in a crude helium product (50-70% helium). The crude helium is them purified resulting in concentrations of about 99.99 %

Although helium is the second most abundant element in the universe it is extremely scarce on Earth. Most of Earth's helium is a product of the nuclear decay of uranium and of thorium which means that helium is a truly non-renewable resource!

An alternative to liquid helium in some applications might be liquid hydrogen. Hydrogen is far more abundant that helium and also has a low melting point (about -259°C or 14 K), unfortunately hydrogen is extremely flammable which restricts its use.

Phosphorus is essential to life. Apart from being found in bones and teeth, it is found in cell membranes, and plays an essential role in the transfer of energy within living organsims. Humans obtain phosphorus from food, but farming crops leads to depletion of nutrients like phosphorus from the soil. Phosphate fertilizers, which contain phosphorus, are added to soils to increase the concentration of phosphorus and increase crop production.

Historically, guano (build up of bird droppings or bat excrement) has been used as a source of phosphate. By the mid-nineteenth century the demand for guano was so high that the USA passed the Guano Islands Act which gave any of its citizens who discovered a source of guano on an unclaimed island exclusive rights to the deposits. Other countries, like the UK, also used the need to find new guano deposits as a reason for extending their empires. World demand for guano was outstripping supply!

Today, phosphate fertilizers are produced by mining the phosphate rocks which were produced as a result of geological processes occurring over tens of millions of years. Morocco is thought to hold about 71% of the world's phosphate rock reserves. We are expected to reach peak production of phosphates within about 50 years, with declining production thereafter. Since phosphorus is an essential building block for life, there can be no substitution by another element.

Much of the phosphorus added to soil may be lost as agriculatural run-off as water from rainfall or irrigation transports the added phosphorus to natural water systems like rivers, lakes and oceans. Agricultural run-off can lead to a build up of phosphorus in natural waterways causing eutrophication, that is, a build up of nutrients in the water, leading to algal blooms.

In much the same way that birds and bats produce deposits rich in phosphorus, human excrement also contains phosphorus, and this phosphorus should be recoverable from waste treatment plants.
Encouraging individuals to compost food scraps also helps add phosphorus to soils, while reducing the farming of livestock would also reduce our reliance on phosphate fertilizers.
More research is also required to improve soil fertility and to improve fertilizers, including time-release fertilizers.

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The semi-metals, or metalloids, are a group of elements that occur between the non-metallic elements on the right of the Periodic Table of the Elements and the metallic elements on the left. Semi-metals are semi-conductors of electricity and are used in electronic devices which makes them very important to the continuance of our modern technological society. Fortunately, silicate minerals (a combination of silicon and oxygen) make up a large proportion of the Earth's crust so silicon is readily available for use in electronics, but the other semi-metals are much less abundant.

Boron is used to make borosilicate glass, ceramic glazes, as well as in electronics. Germanium is used in fiber optics, solar cells and electornics. Antimony is used in infrared sensors and electronics. Tellurium is used in solar panels. Polonium is highly radioactive and astatine has a short half-life of only about 8 hours so neither of these are expected to be commercially significant.

Although a small amount of arsenic is found in its native state, the vast majority of arsenic occurs naturally in sulfide deposits with other metals such as iron, nickel or cobalt. It is a toxic substance that has been implicated in the deaths of some famous people in history like Napoleon Bonaparte and King George III of Great Britain. Arsenic is not just toxic to humans, it is also toxic to many insects, bacteria and fungi so it has also been used as an insecticide and as a wood preservative in the past. While the use of arsenic for these purposes has generally been phased out, you might still find some arsenic compounds being used in agriculture, for example, poultry feed may contain organoarsenic compounds to help prevent disease and improve weight gain in commercial birds.
Today, China produces most of the world's arsenic while arsenic refining operations in many countries have closed down due to environmental concerns.

One of the most important uses of arsenic today is in combination with gallium (a semi-metal that is also a critical element) to produce gallium arsenide, GaAs. Gallium arsenide, GaAs, is used to make integrated circuits, infrared light-emitting diodes, and solar cells. While silicon is a cheap and effective semi-conductor for use in electronics, gallium arsenide has many advantages over silicon. One benefit to using gallium arsenide is that it is less likely to overheat than silicon and it creates less noise in electrical signals, but the main reason GaAs is preferred is because it has a direct band gap which means it can be used to absorb and emit light more efficiently than silicon.

It is important that electronic waste (e-waste) does NOT go to landfill where harmful substances like arsenic can leech into soils, nor should they be incinerated since that releases toxic elements into the atmosphere. For this reason, electronic devices should be separated out from other waste. It is possible to recycle GaAs containing components and although the processes can be efficient at separating out the gallium, they can also be expensive.
In many consumer products, silicon can be used in preference to GaAs. Research continues into finding more sustainable alternatives to high performing GaAS.

Post-transition metals occur between the semi-metals and the transition metals in the Periodic Table of the Elements. All of these post-transition elements are considered "at risk", except aluminium which makes up a little over 8% of the Earth's crust and is found in oxides (such as the mineral bauxite) and aluminosilicates (compounds of aluminium, silicon and oxygen).

Sometimes zinc, cadmium and mercury are also included in this group (in this discussion they will be treated more traditionally as transition metals).

The post-transition metals generally have lower melting points than transition metals and are usually softer, for example, lead is a bit harder than talc but not as hard as gypsum. Lead has about the same hardness as the graphite used in "lead" pencils.

Aluminium and tin are widely used to make utensils, lead can be used in petrol-car batteries as well as radiation protection, bismuth has been used in pharmaceuticals and in pigments, while gallium, thallium and indium are used in electronic devices.

Indium is a silvery-white metallic solid at room temperature and pressure, and is one of the least abundant elements in the Earth's crust. Although it is possible to find indium in its native state, most of it is found associated with zinc sulfide ores, as well as iron, lead and copper ores. Commercially, indium is produced as a by-product of zinc refining. China, Korea, Canada and Japan are the main producers of indium.

The most important use of indium is in the form of indium tin oxide (ITO) which conducts electricity, bonds to glass and is transparent, so it is used in the production of touch screens, flatscreen TVs and solar panels.
However, other compounds of indium are also important. Indium nitride (InN), indium phosphide (InP) and indium antimonide (InSb) are all semiconductors and are used in transistors and microchips.

Indium can be recovered from electrical appliances, but because there is such a small amount of it present the cost of recovery is generally considered prohibative. On the other hand, it is easier to recover elements such as indium if you are processing tons of the same equipment (instead a mix of different appliances), so thinking about ways to separate out appliances, such as company "take-back" schemes, would be beneficial.

Research is being conducted into replacing indium with other semi-conducting materials like carbon nanotubes, conductive polymers and antimony.

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27 naturally occurring elements are classed as transition metals, making this the largest group of elements on the Periodic Table. When we think of metals, we often think of transition metals such as chromium, iron, nickel, copper, zinc, silver, gold and platinum. In general, transition metals are hard with a high melting point (except for mercury which is a liquid at room temperature and pressure), they are malleable, ductile, and have good electrical and thermal conductivity. These metals are so useful to us that most of them have been classified as endangered.

It has been estimated that 80% of the world's mercury reserves, 75% of its silver, tin and lead, 70% of its gold and zinc, and 50% of its copper and manganese have already been processed. Silver is used as a catalyst in many commercial chemical reactions while zinc is used to protect iron and steel against corrosion. Tellurium and hafnium are both used in electrical devices and special alloys.
Technetium does not occur naturally on Earth but is produced by nuclear fission reactions and is used in nuclear medicine.

Zinc is a silvery-white, soft, metal that tarnishes in air. It is found in nature combined with sulfur in zinc sulfide ores, and in combination with silicon and oxygen in zinc silicate ores. China supplies about 35% of the world's zinc.

Zinc is a moderately active metal, not as chemically reactive as the Group 1 and 2 metals, but more reactive than many of the other harder, stronger, transition metals. Hence, zinc can be used to protect these less reactive metals like iron so that the zinc preferentially reacts with oxygen from the air instead of the iron. The zinc corrodes, but the structural iron or steel does not. When a coating of zinc is used to protect steel in this way, we refer to it as galvanising. About a third of all the zinc produced each year is used in galvanising. You will see galvanised steel in use on car bodies and the structural steel in buildings and bridges. There are alternative methods of protecting steel from rusting, but in many cases, such as structural steel, galvanising is preferred.

Zinc is also widely used in the form of zinc oxide, ZnO, a white solid at room temperature and pressure. Although zinc oxide occurs naturally in the mineral zincite, most of the zinc oxide used commercially is produced synthetically by the oxidation of zinc.

Most of the zinc oxide produced is used in the rubber industry but zinc oxide can also be found in many personal-care products such as mouthwash, toothpaste, antiseptic creams and lotions, because it acts as an anti-bacterial agent. It is found in paints, plastics and even food where it can be added to increase the amount of zinc in a foodstuff. Extremely small zinc oxide particles, nanoparticles of zinc oxide, do not scatter light and therefore do not appear to be white so they can be found in sunscreens.

It is likely that semi-conducting zinc oxides will find uses in the electronics in the future.

Zinc can be recycled from the waste generated during manufacturing and from sold scrap such as galvanised steel. However, because of the multitude of uses we have for zinc, we will never be able to recycle 100% of the zinc we use. The zinc from personal-care products and food for example, will be lost to waterways.

The zinc oxide is some applications, such as coatings and sunscreens, can be replaced by titanium oxide (found in sands).
Research continues on alternatives for antibacterial applications.

Group 1 elements, alkali metals, lie on the extreme left hand side of the Periodic Table of the Elements. They are all quite soft metals with relatively low melting points compared to the transition metals.
The Group 2 elements, alkaline earth metals, lie to the right of the Group 1 elements and have higher melting points and are harder.

Given that group 1 and 2 elements are very reactive, they are not found naturally in their native state, but are found in combination with other elements in compounds.

Calcium is an important for humans, being the element that helps make up teeth and bones, and it also makes up the shells of many marine creatures. Sodium, potassium and magnesium are also essential to human life. Barium compounds are toxic but because barium scatters X-rays it is sometimes given to patients before an X-ray in a the form of a barium "meal", insoluble barium sulfate, which will pass safely through the body without being absorbed. Strontium is used to make brilliant red fireworks and flares, and to make "glow-in-the-dark" paints. Beryllium is alloyed with other metals to improve their ability to conduct electricity and heat. The most important use for caesium is in the caesium clock, used to provide the standard measure of time. Rubidium, radium and francium have few uses outside of research. Radium is raraely used commercially because it is highly radioactive, although it had been used in the past to paint the luminous dials on clocks and watches. Francium is not used because, with a half-life of about 22 minutes, it is too short lived to make it useful.

Lithium is a soft, silvery metal. It has the lowest density of all the metals and is chemically very reactive. Because lithium is so reactive it is not found in nature in its native state, but can be found combined with other elements in compounds. Chile has the world's largest reserve of lithium although Australia produces more lithium.

Lithium is used in many alloys with aluminium and magnesium to make lighter, stronger materials which can be used in aircraft, bicycle frames and high-speed trains. While some lithium is used to make the non-rechargeable batteries used in heart pacemakers, by far the most important use for lithium is in rechargeable batteries such as in mobile phones, laptops, and electric vehicles.

As countries like Australia move towards more renewable energy sources such as solar and wind power, the need to store electricity and release it as required is becoming increasingly important. One solution is to build "big batteries", and the current preferred type of "big battery" is a lithium ion battery. As more and more of these "big batteries" are built, as more electric vehicles hit our roads, and as our fixation with portable digital devices increases, then the demand for lithium will continue to rise, making it a critically endangered element.

It is possible to recycle lithium batteries to extract the elements, including the lithium, as long as these batteries do not end up in land fill.
The ocean's also contain lithium at a concentration of about 0.2 ppm. Researchers are studying ways to selectively separate this lithium from seawater.
Given the abundance of sodium on Earth, in its crust and its waters, it is not surprising that a great deal of research is being conducted to see if sodium batteries could replace lithium batteries in the future.

The rare-earth elements includes the Group 3 elements scandium, yttrium, lanthanum as well as the lanthanoids. Despite the name of this group of elements, some of them are relatively abundant in the Earth's crust. Cerium, for example, is more abundant than copper, yet you've probably seen copper put to more uses than cerium.

The chemistry of the rare-earth elements is very similar and hence these elements often ocur together in nature. This similarity in chemistry makes them quite difficult to separate from their ores, and, they tend not be concentrated by geological processes so they are usually produced as a by-product of mining some other mineral.

Scandium has been used in the metal alloys used in the aerospace industry, yttrium is being used in phosphors and some fuel cells, while neodymium magnets have a number of different uses.

Lanthanoids are found in nature combined with phosphorus and oxygen in the phosphate mineral monazite, or combined with carbon, oxygen and fluorine in the carbonate mineral bastnaesite. China supplies about 70% of the world's rare-earth elements, but mines are opening in Australia and re-opening in the USA.

Researchers in 1983 found that when neodymium was combined with iron and boron, a very strong permanent magnet was produced. This discovery enabled the miniaturisation of electric motors and electronic components. Without this discovery there would be no pocket-size mobile phones, no electric vehicles and no wind turbines! And as demand for these products increases, the demand for neodymium increases.

Neodymium is also used to make the glass used in lasers. Apart from use in laser pointers, this glass is also a component in the lasers used for eye surgery and the treatment of skin cancers.

Neodymium can be recycled, but first we have to prevent electronic gadgets and components going into landfill. Even when these devices are recycled, the first step is often shredding, and because such small amounts of neodymium are involved in each device a large proportion of this can be lost as dust during this phase.

Further research is needed to find alternatives to neodymium. Improvements are also needed in the recycling process.

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