Natural Occurrence and Production
The primary chromium ore is the mixed metal oxide chromite, FeCr2O4, found as brittle metallic black crystals or granules. The rare mineral crocoite, PbCrO4, occurs as spectacular long red crystals. Rare potassium chromate minerals and related compounds are found in the Atacama desert.
Chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms iron(III) oxide, Fe2O3.
- 4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2Fe2O3 + 8 CO2
The subsequent leaching at higher temperatures dissolves the chromates and leaves the insoluble iron oxide. Normally the chromate solution is further processed to make chromium metal, but a chromate salt may be obtained directly from the liquor.
Read more about this topic: Chromate And Dichromate
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