Chromium processing

Starting with mineral chromite, you get to sodium chromate, then sodium dichromate, then sodium chromite, then chromium (III) oxide then chromium (VI) oxide and lastly to chromium metal by aluminum thermite.

Cheat sheet

• FeO.Cr₂O₃ chromite ore
• Na₂CrO₄ (2-1-4) sodium chromate
• Na₂Cr₂O₇ (2-2-7) sodium dichromate
• Na₂Cr₂O₄ (2-2-4) sodium chromite

Core Reactions

1. Roast chromite ore with sodium carbonate at 1000°C in air, giving a mixture of sodium chromate and hematite

   4 FeO.Cr₂O₇₃ + 8 Na₂CO₃ + &O₂

   {

   →

   1000°C, air}

   8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂(g)↑

2. Wash solids in water and filter giving aqueous sodium chromate

   Na₂CrO₄(s) + Fe₂O₃(s) + H₂O → Na₂CrO₄(aq) + Fe₂O₃(s)↓

3. Filter
4. Discard residue (hematite).
5. Convert sodium chromate to sodium dichromate with sulfuric acid

   2 Na₂Cr₂O₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

6. Reduce sodium dichromate with carbon to give sodium chromite

   Na₂Cr₂O₇ + 3 C → Na₂Cr₂O₄ + 3 CO

7. Convert sodium chromite to chromium (III) oxide and sodium hydroxide with water

   Na₂Cr₂O₄ + H₂O → Cr₂O₃↓ + 2 NaOH