Chromium: Difference between revisions

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==Disposal==
==Disposal==
==See Also==
==See Also==
* [[chromium processing]]
* [[Chromium processing]]


==Referencexs==
==Referencexs==
<references/>
<references/>

Revision as of 21:37, 5 February 2025

 
Chromium
Chemical formula {{#Chem:Cr}}
Atomic Number 24 
OTP appearance solid 
Density(g/cc) 7.2 
Melting Point(°C) 1907 
Boiling Point(°C) 2671 
Coefficient of Thermal Expansion(×10-6 °C-1) 4.9
NFPA 704
1
2
1
 

Uses

Primary

  • Stainless steel
  • High speed steel
  • Electroplating

Secondary

  • Tanning
  • Cleaning glassware via chromic acid
  • Combined with Nickel for heating elements

Natural Occurrence

  • Elemental chromium does not occur naturally
  • Iron chromium oxide ({{#Chem: FeCr2O4}}) occurrs as the mineral Chromite
  • Lead chromium oxide ({{#Chem: PbCrO4}}) occurrs as the mineral Crocoite

Hazards

Chromium is toxic and carcinogenic in its hexavalent state. Production of chromium metal often takes a path through (VI) to separate other materials, only to reduce it to (III) later. This makes chromium production via this path very dangerous.

Production

Chromium can be produced in at least 9 different oxidation states. We will not attempt to show each one, but will note which is being discussed.

Ferrochromium

  • Ferrochromium is produced from chromite via aluminothermic reduction
    {{#Chem: 3FeCr2O4 + 8Al = 3FeCr2 + 2Al2O3}}

III-oxide

via dichromate

  1. Melt sodium carbonate and combine with minerals including {{#Chem: (Fe,Al,O) + CrO3}}
    {{#Chem: 4 FeCr2O4(s) + 8 Na2CO3(l) + 7 O2(g) = 2 Fe2O3(s) + 8 CO2(g) + 8 Na2CrO4(s)}} or
    {{#Chem: 2 FeCr2O4 + 4 Na2CO3 + 3 O2 = 2 FeCO3 + 2 CO2 + 4 Na2CrO4}} or
    {{#Chem: 3 FeCr2O4 + 6 Na2CO3 + 5 O2 = Fe3O4 + 6 CO2 + 6 Na2CrO4}}
    N.B. In each case, The dichromate is soluble and the other materials are not.
  2. Add water in excess
  3. Filter
  4. Residue is other rock forming materials (carbonates, oxides, silicon, calcium, iron, aluminum...) Discard or save and reuse
  5. Evaporate filtrate including sodium dichromate
  6. Recrystallize
  7. Carbothermically reduce the dichromate, giving III-oxide
    {{#Chem: Na2Cr2O7 + 3C = 2Cr2O3 + 2Na2CO3 + CO2}}

VI-oxide

From crocoite

  • Crocoite is treated with hydrochloric acid producing chromium trioxide.
    {{#Chem: PbCrO4 + 2HCl = PbCl2 + CrO3 + H2O}}

Elemental chromium

  • Carbothermic reduction is possible, but impractical, requiring > 2050°C. Instead, either {{#Chem: CrO3}} or {{#Chem: Cr2O3}} can be reduced via |aluminothermic reduction:
    {{#Chem: CrO3 + 2Al = Cr + Al2O3}}

Purification

Testing

Storage

Disposal

See Also

Referencexs