Chromium

 

Chromium

Chemical formula	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 (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