General

Zirconium, often too zirconium, is a chemical element with the element symbol Zr and the atomic number 40. Its name is derived from zircon, the most common zirconium mineral. In the periodic table it is in 5th Period; it is the second element of the 4th group (obsolete 4th subgroup) or titanium group. Zirconium is a very corrosion-resistant heavy metal. Biological functions are not known; it occurs in small amounts (4 mg / kg) in the human organism and is not toxic.

The important zirconium-containing mineral zircon (Zr [SiO₄]) has been known as a gemstone since ancient times. Zirconium as an element was discovered in 1789 by Martin Heinrich Klaproth in a sample of the mineral zircon from Ceylon and named after it. The metal was first presented in 1824 by Jöns Jakob Berzelius through a reduction of K.₂ZrF₆ with potassium. To do this, he heated "A mixture of hydrofluoric zirconium potash with potassium in an iron tube". After treatment with water, drying and prolonged heating with dilute hydrochloric acid, Berzelius received a "Lumpy powder that looks like coal black" was and only "By squeezing with the polishing steel a dark gray color and shine" received. The correct atomic mass, on the other hand, could not be determined until 1924 because - besides errors in the implementation of the experiments - it was not known that zirconium always contains small amounts of hafnium. Without this information, measurements always gave a slightly too high atomic mass. The first practical application of zirconium was as a smokeless flash powder.

occurrence 

Zirconium occurs in the earth's crust with a content of approx. 0,016%. In the list of elements, sorted by frequency, zirconium is ranked 18th and is more common than the better-known elements chlorine and copper. Although it is very widespread, it is usually only found in very small quantities and in very small crystals (typically around 0,1 mm). This is why zirconium was considered rare in the past. Zirconium is found primarily in silicate intrusive rocks such as granite. It does not come naturally, but only in some minerals, especially zircon (ZrSiO₄) and Baddeleyit (ZrO₂) and the rarer red eudialyte (Na₄(CaCeFeMn)₂ZrSi₆O₁₇(OHCl)₂) bound before. It is almost always associated with hafnium. Zircon is - because of its high melting point of 2550 ° C, its great hardness and low reactivity - the oldest mineral that can be found on earth and can be used for radiometric age determinations due to the stored uranium and thorium isotopes.

Secondary deposits, so-called soap deposits, are usually used as raw materials. These arise when the surrounding rock is weathered and only the particularly weather-resistant zircon remains. Other such deposits can arise from water currents that wash out the zirconium crystals and wash them up in other places. Primary deposits, on the other hand, usually have a zirconium content that is too low for profitable mining.

Temporal development of zircon promotion

The most important zirconium deposits are in Australia, the USA and Brazil. With minable reserves of 38 million tons, the world annual production of zirconium minerals in 2006 was 920.000 tons (calculated as zircon). Only about 5% of this is processed into metal and alloys. The most important producing countries in 2006 were by far Australia and South Africa.

Extraction and presentation

Zircon, the most common zirconium raw material, has to be converted into zirconium dioxide before further processing. To do this, the zircon is boiled in a sodium hydroxide melt (alkaline digestion). The zirconium dioxide is then reacted with coke in an electric arc to form zirconium carbonitride (carbon and nitrogen-containing zirconium) and then with chlorine to form zirconium tetrachloride.

A direct reduction of zirconium dioxide with carbon (as in the blast furnace process) is not possible, since the carbides formed are very difficult to separate from the metal. Instead, zirconium tetrachloride is reduced to zirconium metal in the so-called Kroll process with magnesium in a helium atmosphere.

The Van Arkel de Boer process is used to obtain purer zirconium. During heating under vacuum, the zirconium initially reacts with iodine to form zirconium (IV) iodide. This is broken down again to zirconium and iodine on a hot wire:

Zirconium tetraiodide is formed from zirconium and iodine at 200 ° C; it disintegrates again at 1300 ° C.

Zirconium and hafnium cannot be separated in a simple chemical way. This is why this high-purity zirconium still contains hafnium. Since it is important for many applications in reactor technology that the zirconium no longer contains hafnium, separation processes for these two metals play an important role. One possibility are extraction processes in which the different solubility of zirconium and hafnium compounds in special solvents is used. The thiocyanates and their different solubility in methyl isobutyl ketone are often used. Ion exchangers or the fractional distillation of suitable compounds offer further possibilities.

Features 

Physical Properties 

Crystal structure of α-zirconium

Zirconium is a silvery, shiny heavy metal (density 6,501 g / cm³ at 25 ° C), it looks like steel. The metal crystallizes in two different modifications, into which it can be converted by changing the temperature. Below 870 ° C, α-zirconium crystallizes in the hexagonal crystal system (hexagonal close packing of spheres, magnesium type) in the space group 6/ mmm with the lattice parameters a = 323 pm and c = 514 pm as well as two formula units per unit cell. At 870 ° C the crystal structure changes to the body-centered cubic β-structure (tungsten type) with the space group  and the lattice parameter a = 361 pm.

Zirconium is relatively soft and pliable. It can be easily processed by rolling, forging and hammering. Small amounts of hydrogen, carbon or nitrogen in the metal make it brittle and difficult to process. The electrical conductivity is not as good as that of other metals. It is only about 4% of that of copper. In contrast, zirconium is a good conductor of heat. Compared to the lighter homologue titanium, the melting and boiling points are slightly higher (melting point: titanium: 1667 ° C, zirconium: 1857 ° C). The electrical and thermal conductivity are also better. Below 0,55 K, zirconium becomes superconducting.

The properties of zirconium and the heavier homologue hafnium are very similar due to the lanthanide contraction. This requires similar atomic radii (Zr: 159 pm, Hf: 156 pm) and thus similar properties. The two metals differ considerably in their density (Zr: 6,5 g / cm³, Hf: 13,3 g / cm³).

An important property, because of which zirconium has gained great importance in reactor construction, is its small capture cross-section for neutrons. In this capacity, zirconium is also very different from hafnium. This makes the complex separation process necessary for these applications.

Chemical properties

Zirconium is a base metal that reacts with many non-metals, especially at high temperatures. Mainly as a powder, it burns with a white flame to form zirconium dioxide, in the presence of nitrogen also to zirconium nitride and zirconium oxynitride. Compact metal only reacts with oxygen and nitrogen when it is white heat. At increased pressure, zirconium reacts with oxygen even at room temperature, since the zirconium oxide formed is soluble in the molten metal.

Zirconium is passivated in the air by a thin, very dense layer of zirconium oxide and is therefore inert. It is therefore insoluble in almost all acids, only aqua regia and hydrofluoric acid attack zirconium at room temperature. Aqueous bases do not react with zirconium.

Isotope 

There are many isotopes of the zirconium between ⁷⁸Zr and ¹¹⁰Zr known. Natural zirconium is a mixed element that consists of a total of five isotopes. these are ⁹⁰Zr, which occurs most frequently with a share of 51,45% of natural zirconium, as well as the heavier isotopes ⁹¹Zr (11,32%), ⁹²Zr (17,19%), ⁹⁴Zr (17,28%) and ⁹⁶Zr with 2,76% share. ⁹⁶Zr is the only natural isotope that is weakly radioactive; it decays with a half-life of 24 · 10¹⁸ Years under double beta decay ⁹⁶Mo. The isotope ⁹¹Zr can be detected with the aid of NMR spectroscopy.

Usage

An important use for zirconium is the shells made from zircaloy for uranium fuel elements in nuclear power plants. This alloy consists of approx. 90% zirconium and small amounts of tin, iron, chromium or nickel, but must not contain any hafnium. The reason for the choice of this element is the small cross-section for thermal neutrons already described above and at the same time great corrosion resistance, which also makes it suitable as a building material for chemical plants, especially for special apparatus parts such as valves, pumps, pipes and heat exchangers. As an alloy additive to steel, it also increases corrosion resistance. Surgical instruments, among other things, are made from these alloys.

Since zirconium reacts with small amounts of oxygen and nitrogen, it can be used as a getter material in incandescent lamps and vacuum systems to maintain the vacuum. This property is also used in metallurgy to remove oxygen, nitrogen and sulfur from steel.

Because of its property of emitting a very bright light when burned, it was used as a flashlight powder in addition to magnesium. In contrast to magnesium, zirconium has the advantage of being smoke-free. This property is also used in fireworks and signal lights.

Zirconium emits a surge of sparks when it hits metal surfaces and is flammable. The military uses this in some types of ammunition such as the special shotgun ammunition Dragon's Breath and the American all-purpose glide bomb AGM-154 JSOW. In film technology, this effect is used for non-pyrotechnic impact effects of, for example, bullets on metal surfaces.

Zirconium-niobium alloys are superconducting and remain so when strong magnetic fields are applied. They are therefore used for superconducting magnets.

In addition to aluminum-containing alums, zirconium salts are used in the “white tanning” of skins.

safety instructions

There are no known toxic effects of zirconium and its compounds. Because of the dense oxide layer, compact zirconium is not flammable. In powder form, on the other hand, it can start to burn when heated in the air. Zirconium fires are very dangerous because neither water (violent reaction with formation of hydrogen), nor carbon dioxide or halon may be used to extinguish them. Zirconium fires must be extinguished with metal fire extinguishers (class D) or dry sand.

Proof 

With alizarin red-S, zirconium forms a characteristic red-violet compound (colored varnish) in acid, which disappears again when fluoride ions are added to form the zirconium fluorocomplex. This reaction can serve as qualitative evidence of both zirconium and fluorine. Since even small amounts of fluoride (and other anions) interfere, this detection is unsuitable for mineral analyzes. In addition, some other organic compounds, such as tannin, copperron, phenylarsonic acid, oxine or xylenol orange, are suitable as detection reagents. Another characteristic compound is zirconium oxychloride ZrOCl₂ · 8 H₂O that crystallizes in typical needles. In modern analysis, zirconium can be detected using atomic absorption spectrometry (AAS) or mass spectrometry (also using the isotope pattern).
One possibility for quantitative analysis is the precipitation of sparingly soluble zirconium hydroxide with ammonia and subsequent incineration to form zirconium dioxide.

Precipitation of the hydroxide

Transfer to the weighing mold

Connections

As a base metal, zirconium forms a multitude of compounds. Most zirconium compounds are salts. They are often very stable and have a high melting point. The + IV oxidation state is preferred and the most stable. But there are also compounds in the oxidation states + III to + I, and in complexes even in the states 0, −I and −II.

zirconia

The most important zirconium compound is zirconium dioxide ZrO₂, a very stable and high-melting oxide. Zirconium dioxide is used to make refractory linings in crucibles and furnaces. In order to use it for this purpose, it must be stabilized with calcium, yttrium oxide or magnesium oxide to stabilize the cubic high-temperature phase. It is also used as an abrasive and, because of its white color, as a white pigment for porcelain.

Zirconium dioxide crystals are colorless and have a high refractive index. That is why they are used under the name zirconia as an artificial gemstone and substitute for diamonds.

If zirconium oxide is mixed with yttrium oxide, further application possibilities arise. At three percent yttrium oxide content, the ZrO₂ stabilized in a distorted fluorite structure. As a result, it acts as a conductor for oxygen ions at temperatures of over 300 ° C. An important application for this is the lambda probe in cars, which is used to measure the oxygen content in exhaust gases for the catalytic converter. With 15% yttrium oxide content, zirconium oxide emits a very bright, white light at 1000 ° C. This is used in the so-called Nernst lamp. Since yttrium-zirconium ceramics have an extremely high fracture toughness, they are used, for example, in dental technology as a highly stable crown and bridge framework, in artificial hip joints and dental implants or as a connecting element in telescopes. They are increasingly replacing gold and other metals in their function.

Zirconia is also often used for ball bearings. Especially for the bearing races, ZrO₂ the great advantage that the coefficient of thermal expansion is close to that of steel. Other technical ceramics such as silicon nitride usually have a considerably lower coefficient of thermal expansion.

halides 

With the halogens fluorine, chlorine, bromine and iodine, zirconium forms several series of compounds. All halogens are compounds of the form ZrX₄, ZrX₃ and ZrX₂ known. In addition there are the chlorides, bromides and iodides of the form ZrX. The tetrahalides of the form ZrX are the most stable₄. No important areas of application are known for any of the zirconium halides, with zirconium chlorides being formed as intermediate products in the production of pure zirconium.

Further zirconium compounds

Zirconium silicate, ZrSiO₄, better known under the mineral name zircon, is the most common zirconium compound found in nature. It is the most important source of zirconium and its compounds. Zircon is also used as a gemstone.

Organic zirconium compounds are mostly unstable. Organic zirconium complexes, so-called. zirconocenes, with radicals such as cyclopentadienyl. They are technically important as catalysts in the polymerization of alkenes, in particular for the production of polypropylene. Another application of an organic zirconium compound is in hydrozirconation. Here, alkenes with the help of the Schwartz reagent Cp₂ZrHCl (Cp = cyclopentadienyl) converted into alcohols or halogenated hydrocarbons. In the reaction of terminal alkynes with the Schwartz reagent, trisubstituted double bonds are formed during hydrozirconation; further reaction with an electrophilic reagent leads to trans-functionalized alkenes in high stereochemical purity.

Aluminum-zirconium complexes can be used as an antiperspirant.

Potassium hexafluoridozirconate (IV) K₂ZrF₆ (CAS: 16923-95-8) can be used to separate zirconium from hafnium.

Zirconium carbonate exists as a basic complex. It is used, among other things, in the paper industry.

General
Name, symbol, atomic number	Zirconium, Zr, 40
Series	Transition metals
Group, period, block	4, 5, d
Appearance	silvery white
CAS number	7440-67-7
Mass fraction of the earth shell	0,021%
Atomic
atomic mass	91,224 u
Atomic radius (calculated)	155 (206) pm
Covalent radius	148 pm
electron configuration	[Kr] 4d2 5s2
1. ionization	640,1 kJ / mol
2. ionization	1270 kJ / mol
3. ionization	2218 kJ / mol
4. ionization	3313 kJ / mol
Physically
Physical state	fest
modifications	two (α- / β-Zr)
crystal structure	hexagonal; cubic> 1140 K (867 ° C)
density	6,501 g / cm3 (25 ° C)
Mohs hardness	5
magnetism	paramagnetic ( = 1,1 10−4)
melting point	2130 K (1857 ° C)
boiling point	4682 K (4409 ° C)
Molar volume	14,02 · 10−6 m3/ mol
Heat of vaporization	590,5 kJ / mol
heat of fusion	16,9 kJ / mol
vapor pressure	0,00168 Pa at 2125 K
speed of sound	4650 (long.), 2250 (trans.) M / s at 293,15 K
Specific heat capacity	270,0 J / (kg · K)
Electric conductivity	2,36 · 106 A / (V · m)
thermal conductivity	22,7 W / (m K)
Chemical
oxidation states	4, 2
normal potential	−1,553 V (ZrO2 + 4 H.+ + 4 e- → Zr + 2 H2O)
electronegativity	1,33 (Pauling scale)
Isotope
isotope NH t1/2 ZA ZE (MeV) ZP 89Zr {Syn.} 78,41 p.m. ε 2,832 89Y 90Zr 51,45 % Stabil 91Zr 11,22% Stabil 92Zr 17,15% Stabil 93Zr {Syn.} 1,53 · 106 a β- 0,091 93Nb 94Zr 17,38% Stabil 95Zr {Syn.} 64,02 d β- 1,125 95Nb 96Zr 2,8% 24 · 1018 a β-β- 3,350 96Mo
NMR properties
Spin γ in rad * T−1· s−1 Er(1H) fL at W = 4,7 T in MHz 91Zr -5 / 2 2,496 · 107 0,00948 18,7
safety instructions
GHS hazard labeling from EU regulation (EG) 1272/2008 (CLP) Danger H and P phrases H: 250-260 EUH: no EUH rates P: 222-​223-​231+232-​370+378-​422 Hazardous substance labeling from EU regulation (EG) 1272/2008 (CLP)powder Light- flammable (F) R and S phrases R: 15-17 (not stabilized) R: 15 (phlegmatized) S: (2) -7 / 8-43

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