ZIRCONIUM: HISTORY, PROPERTIES, STRUCTURE, RISKS, USES - CHEMISTRY - 2024

The zirconium is a metallic element that is located in group 4 of the periodic table and which is represented by the chemical symbol Zr. It belongs to the same group as titanium, being below this, and above hafnium.

Its name has nothing to do with the "circus", but with the golden or gold color of the minerals where it was recognized for the first time. In the earth's crust, and in the oceans, its atoms in the form of ions are associated with silicon and titanium, thus being a component of sand and gravel.

Metal zirconium bar. Source: Danny Peng

However, it can also be found in isolated minerals; including zircon, a zirconium orthosilicate. Likewise, we can mention baddeleyite, which corresponds to the mineralogical form of its oxide, ZrO ₂, called zirconia. It is natural for these names: 'zirconium', 'zircon' and 'zirconia' to intermingle and cause confusion.

Its discoverer was Martin Heinrich Klaproth, in 1789; While the first person to isolate it, in an impure and amorphous form, was Jöns Jakob Berzelius, in 1824. Years later, processes were improvised to obtain samples of zircon of higher purity, and its applications increased as its properties were deepened.

Zirconium is a silvery white metal (top image) that has a high resistance to corrosion, and a high stability against most acids; Except hydrofluoric and hot sulfuric acid. It is a non-toxic element, although it can easily catch fire due to its pyrophoricity, nor is it considered harmful to the environment.

Materials such as crucibles, foundry molds, knives, watches, pipes, reactors, fake diamonds, among others, have been manufactured from zirconium, its oxide, and its alloys. It is therefore, together with titanium, a special metal and a good candidate when designing materials that must withstand hostile conditions.

On the other hand, from zirconium it has also been possible to design materials for more refined applications; for example: organometallic frameworks or organic metal frameworks, which can serve as heterogeneous catalysts, absorbents, storage of molecules, permeable solids, among others.

History

Recognition

Ancient civilizations already knew about zirconium minerals, especially zircon, which appears as golden gems of a color similar to gold; From there it derived its name, from the word 'zargun' which means 'golden color', because from the mineral jergón, composed of zircon (a zirconium orthosilicate), its oxide was recognized for the first time.

This recognition was made by the German chemist Martin Klaproth in 1789, when he was studying a pallet sample taken from Sir Lanka (by then called the Island of Ceylon), and which he dissolved with alkali. He gave this oxide the name of zirconia, and found that it constituted 70% of the mineral. However, he failed in his attempts to reduce it to its metallic form.

Isolation

Sir Humphrey Davy also tried to reduce zirconia, without success, in 1808, using the same method by which he was able to isolate metallic potassium and sodium. It was not until 1824 that Swedish chemist Jacob Berzelius obtained impure and amorphous zirconium by heating a mixture of its potassium fluoride (K ₂ ZrF ₆) with metallic potassium.

However, Berzelius' zirconium was a poor conductor of electricity, as well as being an ineffective material for any use that could offer other metals in its place.

Crystal bar process

Zirconium remained forgotten for a century, until in 1925 the Dutch scientists Anton Eduard van Arkel and Jan Hendrik de Boer, devised the process of the crystalline bar to obtain a metallic zirconium of higher purity.

This process consisted of heating the zirconium tetraiodide, ZrI ₄, on an incandescent tungsten filament, so that the Zr ⁴⁺ ended up being reduced to Zr; and the result was that a crystalline bar of zirconium coated the tungsten (similar to the one in the first image).

Kroll process

Finally, the Kroll process was applied in 1945 to obtain metallic zirconium of even higher purity and at a lower cost, in which zirconium tetrachloride, ZrCl ₄, is used instead of tetraiodide.

Physical and chemical properties

Physical appearance

Metal with a lustrous surface and silver color. If it rusts, it turns dark grayish. Finely divided it is a grayish and amorphous powder (superficially speaking).

Atomic number

40

Molar mass

91.224 g / mol

Melting point

1855 ºC

Boiling point

4377 ºC

Autoignition temperature

330 ºC

Density

At room temperature: 6.52 g / cm ³

At melting point: 5.8 g / cm ³

Heat of fusion

14 kJ / mol

Heat of vaporization

591 kJ / mol

Molar heat capacity

25.36 J / (mol K)

Electronegativity

1.33 on the Pauling scale

Ionization energies

-First: 640.1 kJ / mol (Zr ⁺ gas)

-Second: 1270 kJ / mol (Zr ²⁺ gaseous)

-Third: 2218 kJ / mol (Zr ³⁺ gaseous)

Thermal conductivity

22.6 W / (m K)

Electrical resistivity

421 nΩ m at 20 ° C

Mohs hardness

5.0

Reactivity

Zirconium is insoluble in almost all strong acids and bases; diluted, concentrated, or hot. This is due to its protective oxide layer, which forms quickly when exposed to the atmosphere, coating the metal and preventing it from corroding. However, it is very soluble in hydrofluoric acid, and slightly soluble in hot sulfuric acid.

It does not react with water under normal conditions, but it does react with its vapors at high temperatures to release hydrogen:

Zr + 2 H ₂ O → ZrO ₂ + 2 H ₂

And it also reacts directly with halogens at high temperatures.

Structure and electronic configuration

Metallic bond

Zirconium atoms interact with each other thanks to their metallic bond, which is governed by their valence electrons, and according to their electronic configuration these are found in the 4d and 5s orbitals:

4d ² 5s ²

Therefore, zirconium has four electrons to form syd valence bands, the product of the overlapping of the 4d and 5s orbitals, respectively, of all the Zr atoms in the crystal. Note that this is consistent with the fact that zirconium is positioned in group 4 of the periodic table.

The result of this "sea of ​​electrons", propagated and delocalized in all directions of the crystal, is a cohesive force that is reflected in the relatively high melting point (1855ºC) of zirconium, compared to other metals.

Crystalline phases

Likewise, this force or metallic bond is responsible for ordering the Zr atoms to define a compact hexagonal structure (hcp); this is the first of its two crystalline phases, denoted as α-Zr.

Meanwhile, the second crystalline phase, β-Zr, with a cubic structure centered in the body (bcc), appears when the zirconium is heated to 863 ºC. If the pressure increases, the bcc structure of β-Zr will end up distorting; it deforms as the distance between the Zr atoms is compacted and shortened.

Oxidation numbers

The electron configuration of zirconium reveals at once that its atom is capable of losing up to four electrons if it combines with elements more electronegative than itself. Thus, if the existence of the Zr ⁴⁺ cation is assumed, whose ionic charge density is very high, then its number or oxidation state will be +4 or Zr (IV).

In fact, this is the main and most stable of its oxidation numbers. For example, the following series of compounds have zirconium as +4: ZrO ₂ (Zr ⁴⁺ O ₂ ^2-), Zr (WO ₄) ₂, ZrBr ₄ (Zr ⁴⁺ Br ₄ ^-) and ZrI ₄ (Zr ^{4 +} I ₄ ^-).

Zirconium can also have other positive oxidation numbers: +1 (Zr ⁺), +2 (Zr ²⁺) and +3 (Zr ³⁺); however, its compounds are very rare, so they are hardly considered when this point is discussed.

Much less are zirconium with negative oxidation numbers considered: -1 (Zr ^-) and -2 (Zr ^2-), assuming the existence of “zirconides” anions.

In order for conditions to be formed, they must be special, the element with which it is combined must have an electronegativity lower than that of zirconium, or it must bind to a molecule; as it happens with the anionic complex ^2-, in which six molecules of CO coordinate with a center Zr ^2-.

Where to find and obtaining

Zircon

Sturdy zircon crystals embedded in quartz. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

Zirconium is a considerably abundant element in the earth's crust and seas. Its main ore is the mineral zircon (upper image), whose chemical composition is ZrSiO ₄ or ZrO ₂ · SiO ₂; and to a lesser degree, due to its scarcity, the mineral baddeleyite, which is composed almost entirely of zirconia, ZrO ₂.

Zirconium shows a strong geochemical tendency to associate with silicon and titanium, which is why it is enriching the sands and gravels of ocean beaches, alluvial deposits and lake floors, as well as igneous rocks that have not been eroded.

Kroll treatment and process

Therefore, the zircon crystals have to be separated first from the rutile and ilmenite, TiO ₂, and also from the quartz, SiO ₂. For this, the sands are collected and placed in spiral concentrators, where their minerals end up separating depending on the differences in their densities.

The titanium oxides are then separated by applying a magnetic field, until the remaining solid consists of only zircon (no longer TiO ₂ or SiO ₂). Once this is done, chlorine gas is used as a reducing agent to transform ZrO ₂ to ZrCl ₄, as is done with titanium in the Kroll process:

ZrO ₂ + 2Cl ₂ + 2C (900 ° C) → ZrCl ₄ + 2CO

And finally, ZrCl ₄ is reduced with molten magnesium:

ZrCl ₄ + 2Mg (1100 ° C) → 2MgCl ₂ + Zr

The reason direct reduction from ZrO ₂ is not performed is because carbides can form, which are even more difficult to reduce. The zirconium sponge generated is washed with hydrochloric acid solution, and melted under an inert atmosphere of helium to create metal zirconium rods.

Separation of hafnium from zirconium

Zirconium has a low percentage (1 to 3%) of hafnium in its composition, due to the chemical similarity between its atoms.

This alone is not a problem for most of your applications; however, hafnium is not transparent to neutrons, while zirconium is. Therefore, metallic zirconium must be purified from hafnium impurities in order to be used in nuclear reactors.

To achieve this, mixture separation techniques are used, such as crystallization (of their fluoride salts) and fractionated distillation (of their tetrachlorides), and liquid-liquid extraction using the solvents methyl isobutyl ketone and water.

Isotopes

Zirconium is found on Earth as a mixture of four stable isotopes and one radioactive, but with such a long half-life (t _1/2 = 2.0 · 10 ¹⁹ years) that it is practically as stable as the others.

These five isotopes, with their respective abundances, are listed below:

- ⁹⁰ Zr (51.45%)

- ⁹¹ Zr (11.22%)

- ⁹² Zr (17.15%)

- ⁹⁴ Zr (17.38%)

- ⁹⁶ Zr (2.80%, the radioactive mentioned above)

Being the average atomic mass of 91,224 u, which is closer to ⁹⁰ Zr than ⁹¹ Zr. This shows the "weight" that its higher atomic mass isotopes have when they are taken into account in the weighted average calculation.

Besides ⁹⁶ Zr, there is another radioisotope in nature: ⁹³ Zr (t _1/2 = 1.53 · 10 ⁶ years). However, it is found in trace quantities, so its contribution to the average atomic mass, 91.224 u, is negligible. That is why zirconium is far from being classified as a radioactive metal.

In addition to the five natural isotopes of zirconium, and the radioisotope ⁹³ Zr, other artificial ones have been created (28 so far), of which ⁸⁸ Zr (t _1/2 = 83.4 days), ⁸⁹ Zr (t _1/2 = 78.4 hours) and ¹¹⁰ Zr (30 milliseconds).

Risks

Metal

Zirconium is a relatively stable metal, so none of its reactions are vigorous; unless it is found as a finely divided powder. When the surface of a zirconia sheet is scratched with sandpaper, it emits incandescent sparks due to its pyrophoricity; but these are immediately extinguished in the air.

However, what does represent a potential fire hazard is heating zirconium powder in the presence of oxygen: it burns with a flame that has a temperature of 4460 ° C; one of the hottest known for metals.

The radioactive isotopes of zirconium (⁹³ Zr and ⁹⁶ Zr) emit radiation of such low energy that they are harmless to living beings. Having said all of the above, it can be stated for the moment that metallic zirconium is a non-toxic element.

Ion

The zirconium ions, Zr ⁴⁺, can be found widely diffused in nature within certain foods (vegetables and whole wheat) and organisms. The human body has an average concentration of 250 mg of zirconium, and so far there are no studies that have linked it to symptoms or diseases due to a slight excess of its consumption.

Zr ⁴⁺ can be harmful depending on its accompanying anions. For example, ZrCl ₄ at high concentrations has been shown to be fatal to rats, also affecting dogs, as it reduces the number of their red blood cells.

Zirconium salts are irritating to the eyes and throat, and it is up to the individual whether or not they can irritate the skin. Regarding the lungs, there are few abnormalities reported in those who have inhaled them by accident. On the other hand, there are no medical studies that certify that zirconium is carcinogenic.

With this in mind, it can be said that metal zirconia, nor its ions, pose an alarming health risk. However, there are zirconium compounds that contain anions that can have negative impacts on health and the environment, especially if they are organic and aromatic anions.

Applications

- Metal

Zirconium, as a metal itself, finds various applications thanks to its properties. Its high resistance to corrosion, and to the attack of strong acids and bases, as well as other reactive substances, make it an ideal material for the manufacture of conventional reactors, pipes and heat exchangers.

Likewise, with zirconium and its alloys refractory materials are made that must withstand extreme or delicate conditions. For example, they are used to make casting molds, veneers and turbines for ships and space vehicles, or inert surgical devices so that they do not react with body tissues.

On the other hand, its pyrophoricity is used to create weapons and fireworks; since the very fine zirconium particles can burn very easily, emitting incandescent sparks. Its remarkable reactivity with oxygen at high temperatures is used to capture it inside vacuum seal tubes, and inside light bulbs.

However, its most important use above all is to serve as a material for nuclear reactors, since zirconium does not react with the neutrons released in radioactive decays.

- Zirconia

Cubic zirconia diamond. Source: Pixabay.

The high melting point (2715 ºC) of zirconia (ZrO ₂) makes it an even better alternative to zirconium for the manufacture of refractory materials; For example, crucibles that resist sudden changes in temperature, tough ceramics, knives sharper than steel ones, glass, among others.

A variety of zirconia called 'cubic zirconia' is used in jewelry as it can be used to make perfect replicas of sparkling faceted diamonds (picture above).

- Sales and others

Inorganic or organic zirconium salts, as well as other compounds, have countless applications, among which we can mention:

-Blue and yellow pigments to glaze ceramics and false gems (ZrSiO ₄)

-Carbon dioxide absorber (Li ₂ ZrO ₃)

-Coatings in the paper industry (zirconium acetates)

-Antiperspirants (ZrOCl ₂ and mixtures of complex salts of zirconium and aluminum)

-Paints and inks for printing

-Kidney dialysis treatment and for the removal of contaminants in the water (phosphates and zirconium hydroxide)

-Adhesives

-Catalysts for organic amination, oxidation and hydrogenation reactions (any zirconium compound that shows catalytic activity)

-Aditives to increase the fluidity of the cement

-Alkali ion permeable solids

- Organometallic frames

Zirconium atoms as Zr ⁴⁺ ions can form coordination bonds with oxygen, Zr ^IV -O, in such a way that it can interact without problems with oxygenated organic ligands; that is, zirconium is capable of forming various organometallic compounds.

These compounds, by controlling the synthesis parameters, can be used to create metal-organic frameworks, better known as metal organic frameworks (MOFs). These materials stand out for being highly porous and having attractive three-dimensional structures, just like zeolites.

Its applications depend greatly on which are the organic ligands selected to coordinate with the zirconium, as well as on the optimization of the synthesis conditions (temperature, pH, stirring and reaction time, molar ratios, solvent volumes, etc.).

UiO-66

For example, among the MOFs of zirconium we can mention UiO-66, which is based on Zr-terephthalate interactions (from terephthalic acid). This molecule, which acts as a ligand coordinated with the Zr ⁴⁺ by their -COO groups ^-, forming four bonds Zr-O.

Researchers from the University of Illinois, led by Kenneth Suslick, observed that UiO-66, under intense mechanical forces, undergoes structural deformation when two of the four Zr-O bonds are broken.

Consequently, UiO-66 could be used as a material destined to dissipate mechanical energy, even being capable of withstanding a pressure equivalent to the detonation of a TNT before suffering molecular fractures.

MOFs-808

By exchanging terephthalic acid for trimesic acid (a benzene ring with three -COOH groups in positions 2, 4, 6), a new organometallic scaffold for zirconium emerges: MOFs-808.

Its properties and ability to function as a hydrogen storage material have been studied; that is, the H ₂ molecules end up hosting the pores of MOFs-808, and then extract them when necessary.

MIP-202

And finally we have the MOFs MIP-202, from the Institute of Porous Materials in Paris. This time they used aspartic acid (an amino acid) as a binder. Again, the Zr-O bonds of Zr ⁴⁺ and the oxygens of aspartate (deprotonated -COOH groups), are the directional forces that shape the three-dimensional and porous structure of this material.

MIP-202 proved to be an excellent conductor of protons (H ⁺), which move through its pores, from one compartment to another. Therefore, it is a candidate for use as a manufacturing material for proton exchange membranes; which are essential for the development of future hydrogen batteries.

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