Preparation of Potassium Dichromate

The industrial preparation of potassium dichromate begins with chromite ore (chemically, FeCr₂O₄, essentially iron(II) chromite), which is first concentrated by gravity separation to remove lighter, unwanted gangue (rock) material, exploiting the ore's relatively high density. The concentrated ore is then intimately mixed with an excess of sodium carbonate and lime (calcium oxide/hydroxide), and this mixture is roasted at high temperature (roughly 900-1000°C) in a reverberatory furnace, in the presence of atmospheric oxygen. This roasting step is described by the balanced equation 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂↑, in which the chromium originally present as Cr³⁺ in the ore is oxidised all the way up to the +6 state, emerging as soluble, yellow sodium chromate (Na₂CrO₄), while the iron is left behind as insoluble iron(III) oxide (Fe₂O₃).

The roasted mass is next treated with water, which selectively dissolves the soluble sodium chromate away from the insoluble iron oxide residue, allowing the two to be physically separated by filtration. The resulting yellow aqueous solution of sodium chromate is then treated with concentrated sulphuric acid, which converts the yellow chromate ion into the orange-red dichromate ion: 2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O, giving a solution of sodium dichromate alongside sodium sulphate as a by-product.

This combined solution is then concentrated by evaporation to remove the comparatively less-soluble sodium sulphate, which crystallises out first, in its hydrated form Na₂SO₄·2H₂O, and is removed by filtration; the remaining filtrate is further concentrated to leave a saturated solution of sodium dichromate essentially free of sodium sulphate. …