Strontium price, occurrence, extraction and use

Strontium is a chemical element with the element symbol Sr and the atomic number 38. In the periodic table it is in the 5. Period as well as the 2. Main group, or the 2. IUPAC group and thus belongs to the alkaline earth metals. It is a soft (Mohs hardness: 1,5) and very reactive metal.

The element was discovered by Adair Crawford 1790 and named after his first site Strontian in Scotland. Elemental, albeit contaminated by foreign admixtures, 1808 could be represented by electrolysis by Humphry Davy. Robert Bunsen succeeded 1855 also the representation of pure strontium. The element is used only in small quantities, especially for cathode ray tubes, pyrotechnics (red flame color), permanent magnets and in the aluminum smelting.

Strontium occurs in small amounts in the human body, but has no known biological significance and is not essential. Strontium ranelate is a drug for the treatment of osteoporosis.

History

Initial evidence of the existence of the element was found by Adair Crawford and William Cruickshank in the year 1790, when they examined in more detail a Strontian originating in Scotland mineral that was initially thought to be "air-borne barite" (barium carbonate, witherite). They prepared the chloride and compared several properties of the later strontium chloride with those of barium chloride. Among other things, they found different solubilities in water and other crystal forms. 1791 named Friedrich Gabriel Sulzer (1749-1830) the mineral after his locality Strontian Strontianite. He and Johann Friedrich Blumenbach studied the mineral more closely and thus found more differences to the Witherit, such as the different toxicity and flame color. In the following years, chemists such as Martin Heinrich Klaproth, Richard Kirwan, Thomas Charles Hope and Johann Tobias Lowitz further investigated the strontianite and obtained other strontium compounds from it.

1808 succeeded Humphry Davy by electrolytic reduction in the presence of red mercury oxide, the representation of strontium amalgam, which he then purified by distillation and thus received the - albeit contaminated - metal. He named it after the strontianite analogous to the other alkaline earth metals strontium. Pure strontium was won by Robert Bunsen 1855 by electrolysis of a strontium chloride melt. He also determined properties of the metal such as the specific gravity of the element.

occurrence

Strontium is relatively abundant with a share of 370 ppm on the continental crust on Earth, the abundance of elements in the Earth's crust is comparable to that of barium, sulfur or carbon. Also in seawater a larger amount of strontium is present. The element does not appear solid, but always in different connections. Due to the low solubilities, the most important strontium minerals are strontium sulfate or celestin with a strontium content of up to 47,7%. as well as the strontium carbonate or strontianite with a strontium content of up to 59,4% Altogether (as of: 2011) around 200 strontium-containing minerals are known.

The deposits of the most important strontium mineral, celestine, were formed by precipitation of the sparingly soluble strontium sulfate from seawater. Also a hydrothermal formation of the mineral is possible. Strontianite also forms hydrothermally or as a secondary mineral from celestine. The main strontium deposits and mining sites are in Spain, Mexico, Turkey, China and Iran. Great Britain was also an important producing state for a long time, but production ended with 1992. The extraction of strontium minerals in 2008 worldwide amounted to 496.000 tonnes.

Extraction and presentation

The starting material for the production of strontium and strontium compounds is usually celestin (strontium sulfate). Strontium carbonate is usually first obtained from this. This is the industrially most important strontium compound and raw material for the extraction of metal and other compounds.

To produce strontium carbonate, strontium sulfate is first reacted with carbon at 1100-1200 ° C. In this case, the sulfate is reduced to the sulfide and there are strontium sulfide and carbon dioxide. The strontium sulfide is purified by extraction with hot water.

Subsequently, either carbon dioxide is passed through the strontium sulphide solution or the strontium sulphide is reacted with sodium carbonate. In addition to strontium carbonate, hydrogen sulfide or sodium sulfide are formed. Which of the two variants is used depends on the availability of the raw materials and the possibility to sell the by-products.

Finely ground strontium sulfate can also be reacted directly with sodium or ammonium carbonate to give strontium carbonate. However, elaborate cleaning steps are necessary.

To obtain strontium metal, strontium oxide is reduced with aluminum (aluminothermie). In addition to elemental strontium, a mixture of aluminum and strontium oxide is formed. The reaction takes place in a vacuum, because under these conditions, the strontium is gaseous, can be easily separated and collected in a condenser.

Physical Properties

Strontium is a high-grade state light golden yellow, otherwise silver-white alkaline earth metal. With a melting point of 777 ° C and a boiling point of 1380 ° C, it stands at the boiling point between the lighter calcium and the heavier barium, with calcium having a higher melting point and barium having a lower melting point. Strontium has the lowest boiling point of all alkaline earth metals after magnesium and radium. With a density of 2,6 g / cm3, it is one of the light metals. Strontium is very soft with a Mohs hardness of 1,5 and can be easily bent or rolled.

Like calcium, strontium crystallizes at room temperature in a cubic-face-centered crystal structure in the space group Fm3m (space group no. 225) (copper type) with the lattice parameter a = 608,5 pm and four formula units per unit cell. In addition, two other high-temperature modifications are known. At temperatures greater than 215 ° C, the structure transforms into a hexagonal-dense sphere packing (magnesium type) with the lattice parameters a = 432 pm and c = 706 pm. Finally, above 605 ° C, a cubic-centered structure (tungsten type) is the most stable.

Chemical properties

Strontium is the most reactive alkaline earth metal after barium and radium. It reacts directly with halogens, oxygen, nitrogen and sulfur. It always forms compounds in which it is present as a divalent cation. When heated in air, the metal burns with the typical crimson flame coloration to strontium oxide and strontium nitride.

As a very base metal, strontium reacts with water to form hydrogen and hydroxide. Strontium hydroxide also forms on contact of the metal with moist air. Strontium is also soluble in ammonia, forming blue-black ammonates.

In groundwater, strontium is usually similar to calcium. Strontium compounds are insoluble under weakly acidic to basic conditions. Only at lower pH values ​​does strontium appear in dissolved form. If decomposition of carbon dioxide (CO2) occurs as a result of weathering processes or the like, the precipitation of strontium is intensified together with calcium (as strontium or calcium carbonate). In addition, a high cation exchange capacity of the soil can promote the binding of strontium.

isotope

There are a total of 34 isotopes and nine other nuclear isomers known. Of these, there are four, 84Sr, 86Sr, 87Sr and 88Sr, of course. In the natural isotopic composition, the isotope 88Sr predominates with a proportion of 82,58%. 86Sr with 9,86% and 87Sr with 7,0% and 84Sr with a share of 0,56% are rarer.

90Sr is a beta emitter with a decay energy of 0,546 MeV and decays with a half-life of 28,78 years to 90Y, which in turn decays rapidly (t1 / 2 = 64,1 h) with emission of high-energy beta radiation (ZE = 2,282 MeV) and of gamma radiation to the stable 90Zr , It occurs mostly as a secondary cleavage product. It is formed within a few minutes by multiple beta decay from primary fission products of the mass number 90, which occur in 5,7% of all nuclear fissions of 235U in nuclear power plants and atomic bomb explosions. This makes 90Sr one of the most common fission products ever.

Larger volumes of 90Sr are released into the environment in all nuclear disasters. Accidents involving 90Sr were the Windscale fire, which released 0,07 TBq 90Sr, and the Chernobyl disaster, where the released activity on 90Sr 800 TBq was. After the aboveground nuclear weapon tests, especially in the years 1955-58 and 1961-63, the load on the atmosphere increased strongly with 90Sr. This, along with the burden on 137C's 1963, led to the adoption of the Nuclear Test Ban Treaty in Space and Underwater, which banned such testing in the signatory states. As a result, in the following years the pollution of the atmosphere dropped significantly again. The total nuclear weapon-released activity on 90Sr was approximately 6 · 1017 Bq (600 PBq).

The intake of 90Sr, which can get into the body via contaminated milk, is dangerous. Due to the high-energy beta radiation of the isotope, cells in bone or bone marrow can be altered and thus bone tumors or leukemia can be triggered. It is impossible to decorate the bone strontium with chelating agents because they preferentially complex calcium and the strontium remains in the bone. Barium sulfate decorporation is only possible if done quickly after incorporation before installation in bone. The degradation by biological processes is also very slow, the biological half-life is in bone at 49 years, the effective half-life of 90Sr in 18,1 years. 90Sr may bind to parathyroid cells. This would explain the accumulation of cases of hyperparathyroidism among liquidators of the Chernobyl reactor.

The beta radiation of 90Sr and 90Y can be used in radionuclide batteries, such as remote lighthouses and beacons in the former Soviet Union, for long-lived isotope marking, for measuring the thickness of materials or for calibrating Geiger counters.

87Sr is the decay product of the 48Rb rubidium isotope with a half-life of 87 billion years. From the ratio of the different strontium isotopes, the age of rubidium- and strontium-containing rocks, such as granite, can therefore be determined within the framework of a strontium isotope analysis.

Strontium is stored under different conditions in varying amounts in bones and teeth. At the same time, the isotope ratio of 86Sr and 87Sr depends on the rocks of the environment. Therefore one can draw from the isotope ratios of strontium sometimes conclusions on migratory movements of prehistoric people.

According to the operator, the small German pebble bed reactor called AVR next to the site of the research center in Jülich is considered to be the nuclear facility most heavily contaminated with 90Sr worldwide. Also in the soil under the reactor is strontium. This is to be removed consuming when dismantling the reactor to 2017 [obsolete].

Usage

Strontium is produced and used only in small quantities. The majority of the strontium carbonate produced is used for cathode ray tubes, permanent magnets and pyrotechnics.

Metallic strontium is mainly used in the aluminum industry (aluminum primary and secondary smelters, as well as foundries) as well as sodium as an interfacial agent in aluminum-silicon alloys with 7-12% silicon. Small additions of strontium change the eutectic in silicon-aluminum alloys and thus improve the mechanical properties of the alloy. This is because aluminum-silicon alloys without strontium at the eutectic precipitate coarse, acicular, mechanically unstable grains, which is prevented by the strontium. Its "refining" effect lasts longer in pourable melts (foundry and holding furnaces) than that of sodium, as it is less easily oxidized. In the field of slowly solidifying melts (sand casting), it has already partially displaced sodium, which has been used for decades alone. At rapid solidification in metallic permanent mold, especially in die casting, the use of strontium is not always mandatory, the formation of the desired fine, "refined" structure is already favored by the rapid solidification.

Strontium is added to ferrosilicon, it regulates the structure of the carbon and prevents uneven solidification during casting.

Furthermore, strontium can be used as a getter material in electron tubes, for removing sulfur and phosphorus from steel, and for hardening lead acid battery plates.

Biological significance

Few creatures use strontium in biological processes. These include Acantharia, single-celled eukaryotic creatures that belong to the radiolarians and are a common component of zooplankton in the sea. These are the only protists to use strontium sulfate as a building material for the skeleton. Thus, they also cause changes in the strontium content in individual sea layers, first taking up strontium and, after dying, sinking into deeper layers where they dissolve.

Physiological and therapeutic significance

Strontium is not essential, only few biological effects of the element are known. Thus, it is possible that strontium has an inhibitory effect on tooth decay.

Animal studies in pigs showed symptoms such as lack of coordination, weakness and paralysis symptoms due to a diet rich in strontium and low in calcium.

Strontium is very similar in its properties to calcium. However, in contrast to calcium, it is only absorbed in small amounts via the intestine. This may be due to the larger ionic radius of the element. On average, the content of strontium in an 70 kilogram man is only 0,32 g, compared to about 1000 g calcium in the body. The absorbed strontium is - like calcium - stored mainly in the bones, which is a treatment option for osteoporosis. By salt formation with organic acids such as ranelic acid or malonic acid a correspondingly high bioavailability is achieved.

89Sr is used as chloride (under the trade name "Metastron") for radionuclide therapy of bone metastases.

safety instructions

Like other alkaline earth metals, strontium is flammable. It reacts with water or carbon dioxide so that it can not be used as extinguishing agent. For extinguishing, metal fire extinguishers (class D) should be used and the use of dry sand, salt and extinguishing powder is also possible. Furthermore, hydrogen forms on contact with water, which is explosive. For the elimination of small amounts strontium can be reacted with isopropanol, tert-butanol or octanol.

Connections

Like all alkaline earth metals, strontium occurs in stable compounds only in the oxidation state + 2. These are usually colorless, often readily water-soluble salts.

halides

With the halogens fluorine, chlorine, bromine and iodine strontium in each case forms a halide with the general formula SrX2. They are typical, colorless and, except for strontium fluoride, readily water-soluble salts. They can be prepared by reaction of strontium carbonate with hydrohalic acids such as hydrofluoric acid or hydrochloric acid. Strontium chloride is used as an intermediate for the production of other strontium compounds as well as in toothpaste, where it is supposed to act against pain sensitive teeth.

Salts of oxygen acids

Above all, the strontium salts of oxygen acids such as strontium carbonate, strontium nitrate, strontium sulfate or strontium chromate are industrially important. Strontium carbonate is the most important commercial form of strontium compounds, the majority of the degraded celestin is converted to strontium carbonate. It is used mainly for the production of X-ray absorbing glass for cathode ray tubes, but also for the production of strontium ferrite for permanent magnets or electroceramics. Strontium nitrate is mainly used in pyrotechnics for the strontium-typical red flame coloration, the yellow strontium chromate serves as a primer against corrosion of aluminum in aircraft or shipbuilding.

Further strontium compounds

Strontium (I) compounds were detected as unstable intermediates in hot flames. Here, strontium (I) hydroxide, SrOH, similar to strontium (I) chloride, SrCl, is a strong emitter in the red spectral region and acts as the sole colorant in high-intensity and deep-saturated red pyrotechnic flares.

Organic strontium compounds

Organic strontium compounds are only slightly known and studied because they are very reactive and can also react with many solvents such as ethers. In nonpolar solvents, however, they are insoluble. Among others, a metallocene with pentamethylcyclopentadienyl anions (Cp *) was shown, which is angled in the gas phase in contrast to other metallocenes such as ferrocene.

Strontium prices

Strontium price -> prices for strategic metals