Question

Write the chemical equations involving the preparation of K₂Cr₂O₇ from chromite ore.

Answer 1

Potassium dichromate is often made from chromite ore (FeO · Cr₂O₃ or FeCr₂O₄). The process consists of the following steps.

Conversion of chromite ore to sodium chromate: The chromite ore is finely pulverized and combined with soda ash (Na₂CO₃) and quicklime. The mixture is roasted in a reverberatory furnace using extra air. Roasting yields a yellow material containing sodium chromate.

\[\ce{4FeCr2O4 + O2 -> 2Fe2O3 + 4Cr2O3}\]

\[\ce{4CrO3 + 8Na2CO3 + 6O2 -> 8Na2CrO4 + 8CO2}\]

\[\ce{\overline{4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \leftarrow 2\text{Fe}_2\text{O}_3 + 8\text{Na}_2\text{CrO}_4 + 8\text{CO}_2}}\]

Quicklime creates a porous mass, allowing air to oxidize the ore.

The reaction may also be carried out by using NaOH in place of Na₂CO₃.

\[\ce{4FeCr2O4 + 16NaOH + 7O2 -> 2Fe2O3 + 8Na2CrO4 + 8H2O}\]

Conversion of chromate into dichromate: The sodium chromate solution from the previous stage is treated with strong sulphuric acid to produce sodium dichromate.

\[\ce{2Na2CrO4 + H2SO4 -> Na2Cr2O7 + Na2SO4 + H2O}\]

The solution becomes concentrated as less soluble sodium sulphate (Na₂SO₄ . 10H₂O) crystallizes. This is filtered hot. Allow the solution to cool until sodium dichromate crystals (Na₂Cr₂O₇ · 2H₂O) separate upon standing.

Conversion of sodium dichromate into potassium dichromate: To create potassium dichromate, treat a solution of sodium dichromate with potassium chloride, as the latter is less stable and more soluble than the former. To produce orange crystals of potassium dichromate, a heated and concentrated sodium dichromate solution is mixed with a determined amount of potassium chloride and allowed to cool.

\[\ce{Na2Cr2O7 + 2KCl -> K2Cr2O7 + 2NaCl}\]