Manganese is the third most abundant transition metal and ranks about 12th in abundanceamong elements in the earth's crust. The metal does not occur in the free state, except in meteors. The most common manganese minerals are: oxides, silicates, and carbonates. The most important ore is pyrolusite, manganese(IV) oxide. The minerals hausmannite Mn3O4 and brownite Mn2O3 are also quite important. The most abundant ores of manganese are pyrolusite (MnO2), psilomelane (BaMn8O16∙2H2O), rhodochrosite (MnCO3), manganite (MnOOH). Ukraine, Georgia, and South Africa are the main producers of manganese.
The discovery of large quantities of manganese nodules on the floor of the oceans may become a source of manganese in the near future. These nodules contain about 24% manganese together with many other elements in lesser abundance. It is an important trace element and may be essential for utilization of vitamin Bl. It is an essential element for all living organisms. The average human body contains about 12 milligrams and takes in about 4 milligrams per day from such foods as nuts, bran, wholegrain cereals, tea and parsley.
Handling. Exposure to manganese dusts, fume, and compounds should not exceed the ceiling value of 5 mg/m3 for even short periods because of the element's toxicity level.
Rhenium is one of the rarest elements in the Earth's crust (7 10-8%). Rhenium resembles manganese chemically and is obtained as a by-product of molybdenum and copper refinement because of its scattering in nature. For instance, the content of rhenium in molybdenite (MoS2) is about 0.05 - 21 g per 1 ton of the ore. It was only recently that the first rhenium mineral was found and described (in 1994), a rhenium sulfide mineral (ReS2) condensing from a fumaroles on Russia's Kudriavy volcano, in the Kurile Islands.
Technetium is a radioactive and unstable element. It is one of the two elements among the first 82 that have no stable isotopes. Only minute traces of the most stable 99Тс isotope (Т1/2 = 2.2.105 years) were detected in uranium ores (10-10 g per 1 ton of the ore) as a result of spontaneous fission of uranium. It was the first element to be artificially produced, hence its name derived from the Greek τεχνητός, meaning “artificial”.
Preparation
Manganese is widely used in the form of ferromanganese (70-80% Mn) that is added to produce alloyed steel or cast iron. For this purpose, the mixture of ores of iron and manganese is reduced by carbon (coke):
MnO2 + C ® Mn + 2CO;
Fe2O3 + 3C ® 2Fe + 3CO.
Pure manganese can be obtained by igniting pyrolusite (manganese dioxide) with aluminum, sodium, or magnesium powder. Manganese is produced also by reduction from its oxides with silicon in electric furnaces. Reduction of the pyrolusite ore by heating with aluminium gives an explosive reaction, and the oxide Mn3O4 must be used to obtain the metal.
3Mn3O4 + 8Al = 4Al2O3 + 9Mn.
The latter is purified by distillation in vacuo just above its melting point (1517 K); The silicothermic method is cheaper than electrolytic, but yields a less pure product. The pure metal can also be obtained by electrolysis of aqueous manganese(II) sulfate. In the electrolytic method, the ore is reduced to manganese compounds with the oxidation state +2. These compounds are next dissolved in a mixture of sulfuric acid and ammonium sulfate. The solution obtained is electrolyzed. The deposited metal is removed from the cathodes and remelted into ingots.
Technetium is usually obtained: (1) by electrolysis from its sulfate solutions; (2) by reduction of Tc2S7 (1100°C) and also as a result of nuclear fission reaction of uranium.
Rhenium is obtained by reduction of ammonium perrhenate, NH4ReO4, at the presence of hydrogen:
NH4ReO4 + 4H2 
2Re + N2 + 8H2O
At first, Re2О7 is concentrated at the annealing of molybdenum ores, thereafter NH4ReO4 is produced.
Uses
More than 95 percent of the manganese produced is used in the form of ferromanganese and silicomanganese alloys for iron and steel manufacture. Manganese ores containing iron oxides are first reduced in blasts furnaces or electric furnaces with carbon to yield ferromanganese, which in turn is used in steelmaking. Adding manganese, which has a greater affinity for sulfur than does iron, converts the low-melting iron sulfide in steel to high-melting manganese sulfide. Produced without manganese, steel breaks up when hot-rolled or forged. Steels generally contain less than 1 percent manganese. Manganese steel, also called Hadfield steel, is used for very rugged service; containing 12-14 percent manganese, it provides a hard, wear-resistant, and self-renewing surface over a tough unbreakable core. Pure manganese produced electrolytically is used mostly in the preparation of nonferrous alloys of copper, aluminum, magnesium, and nickel and in the production of high-purity chemicals. Practically all commercial alloys of aluminum and magnesium contain manganese to improve corrosion resistance and mechanical properties.
Ferromanganese contains about 78 percent manganese, and spiegeleisen (дзеркальний чавун) contains from 12 to 33 percent manganese. Nonferrous manganese alloys include manganese bronze (composed of manganese, copper, tin, and zinc), which resists corrosion from seawater and is used for propeller blades on boats and torpedoes, and manganin (containing manganese, copper, and nickel), used in the form of wire for accurate electrical measurements because its electrical conductivity does not vary appreciably with temperature.
Compounds found wide use. For, instance Dry cells, alkaline batteries – MnO2; the oxidant in organic chemistry – KMnO4. Manganese(IV) oxide is used as a depolariser in Leclanche cells (the cells used in ordinary batteries), as a glaze for pottery and as a decoloriser for glass. The decolorising action occurs because the manganese(IV) oxide oxidises green iron(II) silicates to the less colourful iron(III) compounds; hence the one-time name of "glassmaker's soap' and also "pyrolusite' (Greek pur and lusis, dissolution by fire).
Technetium is used in nuclear medicine, as a constructional material for black metals passivation, in rocket production and etc. Practical applications of rhenium are electrolytic coatings of metals and alloys with anticorrosion properties.
Electronic Configurations & Oxidation States
Manganese exhibits a wide range of oxidation states, from +7 (corresponding to formal loss of all the outer electrons, 3d54s2) to 0. Its +2 oxidation state differs from the preceding transition metals due to its stability indicative of the stability of the half-filled d sublevels.
The most stable oxidation states of manganese are +2, +4, +7. Technetium has most stable states +4, +7, and rhenium has +7. Compounds of Re(VII) and Tc(VII) are more stable than that of Mn.
Дата добавления: 2015-07-25; просмотров: 98 | Нарушение авторских прав
| <== предыдущая страница | | | следующая страница ==> |
| History Of Discovery | | | Chemical Properties. |