gallium

Gallium, Ga, atomic number 31

Gallium - Gallium price, occurrence, extraction, use

General

gallium is a rarely occurring chemical element with the element symbol Ga and the atomic number 31. In the periodic table it is in the 4th period and is the third element of the 3rd main group (group 13) or boron group. It is a silver-white metal that is easy to liquefy. Gallium does not crystallize in one of the crystal structures that are otherwise often found in metals, but in its most stable modification in an orthorhombic structure with gallium dimers. In addition, six other modifications are known which form under special crystallization conditions or under high pressure. In terms of its chemical properties, the metal is very similar to aluminum.

In nature, gallium occurs only to a small extent and mostly as an admixture in aluminum, zinc or germanium ores; Gallium minerals are very rare. Accordingly, it is also obtained as a by-product in the production of aluminum or zinc. Most of the gallium is processed into the semiconductor gallium arsenide, which is mainly used for light emitting diodes.

 

occurrence 

Gallium is a rare element on earth, with a content of 19 ppm in the continental crust, its abundance is comparable to that of lithium and lead. It does not occur in elemental form, but only in bound form, mainly in aluminum, zinc or germanium ores. The ores richest in Gallium include bauxite, zinc blende ores and germanite.

The gallium contents are mostly low, the bauxite found in Suriname with the highest known content only contains 0,008% gallium. The gallium reserves in bauxite worldwide are 1,6 · 106 Tons estimated. Higher contents of up to 1% gallium occur in germanite. Only the Apex mine in the US state of Utah has such high ore grades that an attempt was made to mine it to extract gallium. However, this failed after a short time for reasons of profitability.

Only a few gallium minerals are known, among these are the gallite (CuGaS) found mainly in Tsumeb in Namibia2), Söhngeit (Ga (OH)3) and tsumgallite (GaO (OH)).

 

Extraction and presentation 

Gallium is obtained as a by-product in the production of aluminum from bauxite in the Bayer process. The mixture of sodium aluminate and sodium gallate dissolved in sodium hydroxide is used as the starting product. Gallium can be separated from aluminum using various processes. One possibility is fractional crystallization with the aid of carbon dioxide, with aluminum hydroxide initially preferentially precipitating while the more soluble sodium gallate accumulates in the sodium hydroxide solution. Gallium hydroxide is only precipitated after further process steps, mixed with aluminum hydroxide. The mixture is then dissolved in sodium hydroxide solution and gallium is obtained by electrolysis. Since this process is energy and labor intensive, it is only used in countries with low costs, such as the People's Republic of China.

Gallium can also be obtained directly from the sodium hydroxide solution by electrolysis. Mercury cathodes are used for this purpose, a gallium amalgam being formed during electrolysis. It is also possible to add sodium amalgam to the solution.

With the help of special hydroxyquinolines as chelating ligands, it is possible to extract gallium from the caustic soda with kerosene and thus separate it from the aluminum. Other elements, which are also extracted, can be separated off with dilute acids. The remaining gallium compound is then dissolved in concentrated hydrochloric or sulfuric acid and reduced electrolytically to the metal.

Very pure gallium is required for many technical applications; for semiconductors, for example, it may only contain a hundred millionth of foreign substances. Possible cleaning methods are vacuum distillation, fractional crystallization or zone melting.

The amount of gallium produced is small, in 2008 the world primary production was 95 tons. Another important source is the recycling of waste containing gallium, from which a further 2008 tons of gallium were extracted in 135. The main producing countries are the People's Republic of China, Germany, Kazakhstan and the Ukraine, for gallium recycling also the United States, Japan and the United Kingdom.

On a laboratory scale, gallium can be produced by electrolysis of a solution of gallium hydroxide in sodium hydroxide solution on platinum or tungsten electrodes.

Features 

Physical Properties

Gallium is a silvery white, soft (Mohs hardness: 1,5) metal. It has an unusually low melting point for metals, which is 29,76 ° C. After mercury and cesium, it is the metal with the lowest melting point, which is also well below that of the neighboring elements aluminum and indium. This is probably due to the unusual crystal structure, which, in contrast to the structures of other metals, does not have a high degree of symmetry and is therefore not very stable. Since the boiling point is comparatively high at 2204 ° C, gallium has an unusually large area in which it is liquid. Due to the difficult crystallization, liquid gallium can easily be cooled below the melting point (subcooling) and crystallizes suddenly when crystallization nuclei form.

Like silicon, some other elements and water, gallium has a density anomaly; its density in the liquid state is around 3,2% higher than in the solid form. This is typical for substances that have molecular bonds in the solid state.

Gallium is diamagnetic in the solid state, but becomes paramagnetic in the liquid state (\ Chi_ {m} = 2,4 10−6 at 40 ° C)

The formation of gallium-gallium bonds is characteristic of its structures. Various modifications are known which form under different crystallization conditions (four known modifications, α- to δ-gallium, under normal pressure) and under pressure (a total of three further high-pressure modifications, Ga-II, Ga-III, Ga-IV). The most stable modification at room temperature is α-gallium, which crystallizes in an orthorhombic layer structure. Two atoms bound to one another via a covalent bond form a dimer. Each gallium atom is also adjacent to six other atoms of other dimers. Metallic bonds exist between the individual dimers. The gallium dimers are so stable that they are initially retained even when they melt and can also be detected in the gas phase.

Further modifications are formed during the crystallization of supercooled, liquid gallium. At −16,3 ° C, β-gallium forms, which has a monoclinic crystal structure. In the structure there are parallel zigzag chains of gallium atoms. If crystallization occurs at a temperature of −19,4 ° C, trigonal δ-gallium is formed, in which, comparable to α-boron, there are distorted icosahedra made up of twelve gallium atoms. These are connected to one another via individual gallium atoms. At −35,6 ° C, γ-gallium is finally formed. In this orthorhombic modification, tubes are formed from interconnected Ga7Rings in the middle of a linear chain of other gallium atoms.

If gallium is placed under high pressure at room temperature, various high-pressure modifications are formed one after the other when the pressure is increased. The cubic gallium-II modification is stable above 30 kbar, in which each atom is surrounded by eight more. If the pressure is increased to 140 kbar, the metal now crystallizes as tetragonal gallium-III in a structure that corresponds to that of indium. If the pressure is increased further to around 1200 kbar, the face-centered cubic structure of gallium IV is finally formed.

 

Chemical properties

The chemical properties of gallium are similar to those of aluminum. Like this, gallium is passivated by the formation of a dense oxide layer in the air and does not react. Only in pure oxygen at high pressure does the metal burn with a bright flame to form the oxide. Similarly, it does not react with water either, since the insoluble gallium hydroxide is formed here. If, on the other hand, gallium is alloyed with aluminum and is liquid at room temperature due to the lowering of the melting point, it reacts very violently with water. Gallium also reacts quickly with halogens to form the corresponding salts GaX3.

Gallium is amphoteric and soluble in both acids and bases with evolution of hydrogen. In acids, like aluminum, salts with Ga are formed3+Ions, in bases gallates of the form [Ga (OH)4]-. It dissolves slowly in dilute acids and quickly in aqua regia and concentrated caustic soda. Gallium is passivated by nitric acid.

\ mathrm {2 \ NaOH + 2 \ Ga + 6 \ H_2O \ rightarrow 2 \ Na [Ga (OH) _4] + 3 \ H_2 \ uparrow}

Reaction of gallium with caustic soda

Most metals are attacked by liquid gallium, so it can only be stored in containers made of quartz, glass, graphite, aluminum oxide, tungsten up to 800 ° C and tantalum up to 450 ° C.

isotope 

There are a total of 30 gallium isotopes between 56Ga and 86Ga and another seven core isomers known. Of these are two 69Ga and 71Ga stable and also occur in nature. In the natural isotopic composition predominates 69Ga with 60,12%, 39,88% are 71Ga. Of the unstable isotopes 67Ga has the longest half-life with 3,26 days, the other half-lives range from seconds to a maximum of 14,1 hours 72Ga.

Two gallium isotopes, 67Ga and the short-lived one with a half-life of 67,71 minutes 68Ga are used in nuclear medicine as tracers for positron emission tomography. 67Ga is produced in a cyclotron, while in the production of 68No cyclotron is needed. Instead, the longer-lived germanium isotope 68Ge by irradiation of 69Ga generated with protons. This falls apart 68Ga, the resulting 68Ga can be extracted in a gallium-68 generator. For studies, the gallium is usually bound in a complex with a strongly chelating ligand such as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA).

Usage 

Due to the rarity of the element, gallium is only used to a limited extent. Various gallium compounds are made from most of the gallium produced. The economically most important ones by far are those with elements of the 5th main group, above all gallium arsenide, which is required for solar cells and light emitting diodes, among other things. In 2003, 95% of the gallium produced was used for this purpose. It also serves as a material for doping silicon (p-doping).

The large temperature range in which the element is liquid and the low vapor pressure at the same time are used for the construction of thermometers. Gallium thermometers can be used up to temperatures of 1200 ° C. Liquid gallium can be used as a barrier liquid to measure the volume of gases at higher temperatures and as a liquid electrode material in the extraction of ultra-pure metals such as indium.

Gallium has a high wettability and good reflectivity and is therefore used as a coating for mirrors. It is also used in fused alloys, for heat exchangers in nuclear reactors and as a replacement for mercury in lamps.

Alloys of gallium with other metals have various uses. Magnetic materials are created by alloying with gadolinium, iron, yttrium, lithium and magnesium. The alloy with vanadium in the composition V3Ga is a superconductor with a comparatively high transition temperature of 16,8 K. In nuclear weapons, it is alloyed with plutonium to prevent phase changes. Many gallium alloys such as Galinstan are liquid at room temperature and can replace the toxic mercury or the very reactive sodium-potassium alloys.

proof 

Gallium can be detected qualitatively with various typical color reactions. These include the reaction with rhodamine B in benzene, which fluoresces orange-yellow to red-violet when gallium is added, morin, which shows green fluorescence as in the reaction with aluminum, and potassium hexacyanidoferrate (III), with the gallium a white precipitate of gallium hexacyanidoferrate (III ) forms. Furthermore, a spectroscopic detection is possible via the characteristic violet spectral lines at 417,1 and 403,1 nm.

Quantitative evidence can be provided via complexometric titrations, for example with ethylenediaminetetraacetic acid or via atomic absorption spectrometry.

Toxicology and biological significance 

No toxicological data exist for gallium metal; however, it is corrosive to the skin and mucous membranes. The compounds gallium (III) nitrate Ga (NO3)3 and gallium (III) oxide Ga2O3 possess oral LD50Values ​​in the gram range: 4,360 g / kg for the nitrate and 10 g / kg for the oxide. Gallium is therefore considered to be of low toxicity and, as far as is known, plays no role in humans as a trace element.

Connections 

In compounds, gallium occurs almost exclusively in the +3 oxidation state. In addition, rare and usually very unstable gallium (I) compounds are known as well as those that contain both mono- and trivalent gallium (formally gallium (II) compounds).

Compounds with elements of the nitrogen group

The technically most important compounds of gallium are those with the elements of the nitrogen group. Gallium nitride, gallium phosphide, gallium arsenide and gallium antimonide are typical semiconductors (III-V semiconductors) and are used for transistors, diodes and other components in electronics. In particular, light-emitting diodes of various colors are produced as compounds of gallium and nitrogen groups. The color, which depends on the band gap, can be adjusted by the different ratio of the anions or by replacing gallium with aluminum or indium. Gallium arsenide is also used for solar cells. These are used in satellites in particular, since gallium arsenide is more resistant to ionizing radiation than silicon.

halides

Gallium halides of the form GaX3 are similar in many properties to the corresponding aluminum compounds. With the exception of gallium (III) fluoride, they occur as dimers in an aluminum bromide structure. Gallium (III) chloride is the only halide of little economic importance. It is used as a Lewis acid in Friedel-Crafts reactions.

Further connections

Like aluminum oxide, gallium (III) oxide is a colorless, high-melting solid. It occurs in five different modifications, of which the cubic β modification is the most stable.

Organic gallium compounds exist as Gallane GaR3Gallylene GAR and as higher gallansthat contain gallium-gallium bonds. Like many other organometallic compounds, they are unstable to air and hydrolysis. One of the few organic gallium compounds of economic importance is trimethylgallium, which is used as a doping reagent and for the production of thin layers of gallium arsenide and gallium nitride in organometallic gas phase epitaxy.

General
Name, symbol, atomic number Gallium, Ga, 31
Series Metals
Group, period, block 13, 4, p
Appearance silvery white
CAS number 7440-55-3
Mass fraction of the earth shell 14 ppm
Atomic
atomic mass 69,723 u
Atomic radius (calculated) 130 (136) pm
Covalent radius 122 pm
Van der Waals radius 187 pm
electron configuration [Ar] 3d10 4s2 4p1
1. ionization 578,8 kJ / mol
2. ionization 1979,3 kJ / mol
3. ionization 2963 kJ / mol
Physically
Physical state fixed
modifications seven
density 5,904 g / cm3
Mohs hardness 1,5
magnetism diamagnetic (\ Chi_ {m} = -2,3 10−5)
melting point 302,91 K (29,76 ° C)
boiling point 2477 K (2204 ° C)
Molar volume 11,80 · 10−6 m3/ mol
Heat of vaporization 256 kJ / mol
heat of fusion 5,59 kJ / mol
speed of sound 2740 m / s at 293,15 K
Specific heat capacity 371 J / (kg K)
Electric conductivity about 7,14 · 106 A / (V · m)
thermal conductivity 29 W / (m K)
Chemical
oxidation states 3
normal potential −0,53 V (Ga3+ + 3 e- → Ga)
electronegativity 1,81 (Pauling scale)
isotope
isotope NH t1/2 ZA ZE (MeV) ZP
67Ga {Syn.} 3,2612 d ε 1,00 67Zn
68Ga {Syn.} 67,629 min β+, ε 2,921 68Zn
69Ga 60,1% Sturdy
70Ga {Syn.} 21,14 min β- 1,656 70Ge
71Ga 39,9% Sturdy
72Ga {Syn.} 14,10 p.m. β- 4,001 72Ge
73Ga {Syn.} 4,86 p.m. β- 1,593 73Ge
safety instructions
GHS hazardous substances labeling
05 - Corrosive

 

Gallium prices, charts, price

 

Chart Gallium 2001-2011

Chart Gallium 2001-2011

 

Gallium Prices -> Gallium Prices at prices for strategic metals

 

 

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