Question

Write reactions involved in preparation of potassium dichromate from chrome iron ore

Write the preparation of potassium dichromate from chromite ore. 

Answer 1

Conversion of chrome iron ore into sodium chromate (Roasting): In the industrial production, finely powdered chromite ore \[\ce{FeO.Cr2O3}\] is heated with anhydrous \[\ce{Na2CO3}\] (Sodium carbonate) and a flux of lime in air in a reverbatory furnace.

\[\ce{\underset{ore}{\underset{Chrome iron}{4FeO.Cr2O3}} + \underset{Carbonate}{\underset{Sodium}{8Na2CO3}} + \underset{Oxygen}{7O2} ->[\Delta] \underset{Oxide}{\underset{Ferric}{2Fe2O3}}+ \underset{Chromate}{\underset{Sodium}{8Na2CrO4}} + \underset{dioxide}{\underset{Carbon}{8CO2}}}\]

Conversion of sodium chromate into sodium dichromate: \[\ce{Na2CrO4}\] is extracted with water and treated with concentrated sulphuric acid to get \[\ce{Na2SO4.H2O}\].

\[\ce{\underset{Chromate}{\underset{Sodium}{2Na2CrO4}} + \underset{conc. Sulphuric acid}{H2SO4} -> \underset{dichromate}{\underset{Sodium}{Na2Cr2O7}} + \underset{sulphate}{\underset{Hydrated sodium}{Na2SO4.H2O}}}\]

Conversion of sodium dichromate into potassium dichromate: To the concentrated solution of \[\ce{Na2Cr2O7}\], \[\ce{KCl}\] is added then less soluble orange-red-coloured \[\ce{K2Cr2O7}\] is pricipitated.

\[\ce{\underset{dichromate}{\underset{Sodium}{Na2Cr2O7}} + \underset{chloride}{\underset{Potassium}{2KCl}} -> \underset{dichromate}{\underset{Potassium}{K2Cr2O7}} + 2NaCl}\]