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The chromium subgroup is an element of group VIB in the periodic table, including three elements: chromium (Cr), molybdenum (Mo), and tungsten (W)
.
The valence electron layer structure of the ground state atoms of chromium and molybdenum is (n-1)d 5n s l , while that of tungsten is 5d 5 6s 2 , and their s electrons and d electrons are involved in bonding, and the highest oxidation state is +6
Chromium The most important chromite mineral is a FeCr 2 O .
4 (i.
e.
, · of FeO of Cr 2 O .
3 ), the other has a chromium ore crocoite PbCrO .
4 , chromium ore salary of Cr 2 O .
3 and the like
.
The mass fraction of chromium in the earth's crust is 1×10%, ranking 21st
Simple substance of chromium subgroup elements
1.
The nature of the element
The melting point (1907°C) and boiling point (2671°C) of chromium elemental substance are both high, and chromium is the most hard metal
.
This is consistent with chromium's many single electrons and strong metal bonds
The passivation of chromium with dilute nitric acid , concentrated nitric acid or aqua regia is generally believed to be due to the formation of a dense oxide on the metal surface
.
If the purity of chromium is very high, it can resist the erosion of dilute sulfuric acid
Chromium is an active metal and is easily soluble in hydrochloric acid to produce a blue CrCl 2 solution.
Cr 2+ is unstable and quickly oxidized to Cr 3+
.
Molybdenum and tungsten are heavy metals with higher melting points and boiling points, among which tungsten is the metal with the highest melting point (which can be used as a filament)
.
Molybdenum and tungsten are eutecticized with KNO 2 , KCIO 3 or Na 2 O 2 and are oxidized to MoO 4 2- and wO 4 2-
2.
Refinement of simple substance
The chromite and sodium carbonate are strongly thermally converted into water-soluble chromate (where iron is converted into insoluble Fe 2 O 3 ), and further leached with water and acidified to precipitate dichromate
.
The dichromate is heated and reduced to convert to Cr 2 O 3 , and Cr 2 O 3 is reduced with aluminum or the like to obtain metallic chromium
The carbon used for chromite ore is reduced in an electric arc furnace to generate a carbon-containing ferrochrome alloy, which can be directly used to make alloy steel
.
FeCr 2 O 4 +4C=2Cr(s)+Fe+4CO
Molybdenite ore is burned in the air to form MoO 3 , and MoO 3 is converted to (NH 4 ) 2 MoO 4 and then refined to obtain pure MoO 3 , which is reduced with H 2 , A1 or C at high temperature to produce metal Molybdenum
.
Metal tungsten is prepared by high temperature reduction of pure WO 3 with H 2
.
WO 3 +3H=W+3H 2 O
The eutectic product of wolframite and sodium hydroxide obtains soluble N 2 WO 4 through water leaching
.
After acidification, tungstic acid H 2 WO 4 is precipitated
.
The action of scheelite and hydrochloric acid can also produce insoluble tungstic acid
.
Tungstic acid is sintered to obtain WO 3
.
Related Links: Compounds of Vanadium Subgroup Elements