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  Extraction of chromium

Extraction of chromium metal from chromite ore:

The extraction of chromium metal from chromite ore comprises of the steps shown below:

1. Concentration:

The crushed ore is first concentrated by gravity separation technique.

2. Conversion of the concentrated chromite ore into Na2CrO4:

The concentrated ore is then mixed with excess of Na2CO3 and a small quantity of lime and roasted in reverberatory furnace at around 900-1000°C in the presence of free supply of air. During this procedure, chromite ore is transformed into soluble sodium chromate.

4(FeO. Cr2O3) + 8Na2CO3 + 7O2 (from air)
Chromite ore                         ↓ 900-1000°C
                                           8Na2CrO4 + 2Fe2O3 + 8CO2
                                           Soluble      Insoluble

Conversion of Na2CrO4 into Na2Cr2O7:

The solution containing Na2CrO4 is treated with a computed quantity of H2SO4; Na2CrO4 is then transformed into Na2Cr2O7.

2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

Conversion of Na2Cr2O7 into Cr2O3:

Na2Cr2O7 is heated with carbon to obtain sodium chromite, Na2Cr2O4 that on treatment with H2O provides Cr2O3 precipitate.

 Na2Cr2O7 + 3C → Na2Cr2O4 + 3CO↑
 Na2Cr2O4 + H2O → Cr2O3↓ + 2 NaOH

Reduction of Cr2O3 to chromium metal:

Aluminothermic process:

2245_Aluminothermic process.jpg

                                                            Figure: Aluminothermic process

1. BaO2 + Mg Powder
2. Fireclay crucible
3. Magnesium ribbon,
4. Sand
5. Cr2O3 + Al

Chromic oxide is mix with powdered Aluminium in the ratio 3:1 and is located in a large fire clay crucible. The mixture of barium peroxide and Mg powder is positioned over this. The crucible is enclosed by sand that prevents loss of heat by radiation. The mixture is then ignited by a piece of Mg ribbon. During this procedure a large quantity of heat is liberated, in which Cr2O3 is diminished to chromium.

The molten chromium is then collected in the crucible and aluminium oxide is eliminated as slag.

Cr2O3 + 2Al → 2Cr + Al2O3 + 468.6 kJ

 

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