Chromium

From NOWA-CL
Jump to navigation Jump to search
 
Chromium
Chemical formula Cr
Atomic Number 24 
OTP appearance solid 
Melting Point(°C) 1907 
Boiling Point(°C) 2671 
Density(g/cc) 7.2 
Coefficient of Thermal Expansion(×10-6 °C-1) 4.9
NFPA 704
NFPA704.png
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 (FeCr2O4) occurrs as the mineral Chromite
  • Lead chromium oxide (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
    3 FeCr2O4 + 8 Al 3 FeCr2 + 2 Al2O3

III-oxide

via dichromate

  1. Melt sodium carbonate and combine with minerals including (Fe,Al,O) + CrO3
    4 FeCr2O4(s) + 8 Na2CO3(l) + 7 O2(g) 2 Fe2O3(s) + 8 CO2(g) + 8 Na2CrO4(s) or
    2 FeCr2O4 + 4 Na2CO3 + 3 O2 2 FeCO3 + 2 CO2 + 4 Na2CrO4 or
    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
    Na2Cr2O7 + 3 C 2 Cr2O3 + 2 Na2CO3 + CO2

VI-oxide

From crocoite

  • Crocoite is treated with hydrochloric acid producing chromium trioxide.
    PbCrO4 + 2 HCl PbCl2 + CrO3 + H2O

Elemental chromium

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

Purification

Testing

Storage

Disposal

See Also

Referencexs