What is the use of lime stone in the preparation of potassium dichromate from chromite ore ?

							Manufacturing of potassium dichromate from chrornate ore :-

Chromate ore i.e. FeO, Cr2O3is also called chrome iron ore and it is converted in potassium dichromate as follows. .

i)	Concentration of ore : By washing the powdered chromate ore with water lighter impurites are removed leaving behind the heaviour ore.

ii) Roasting : Conversion of chromite ore in sodium chromate :

Concentrated ore is heated with sodium carbonate and lime stone in reverberatory furnance in air to form yellow sodium chromate.

Lime stone is added to keep the reaction mixture porous.

4FeO Cr2O3+ 8Na2CO3+ 7O2-----> 2Fe2O3+ 8Na2CrO4+ 8CO2.

Sodium chromate is seperated by dissolving in water.

iii) Conversion of sodium chromate into sodium dichromate : Sodium chromate on reacting with H2SO4(cone.) produce sodium dichromate.

2Na2CrO4+ H2SO4-----» Na2Cr2O7+ Na2SO4+ H2O

Sodium sulphate is separated by concentration of solution on filteration aqueous solution of Na2Cr2O7is obtained.

iv) Conversion of sodium dichromate into potassium dichromate : Sodium dichromate on reacting with KC1 form K2Cr2O7. Na2Cr2O7+ 2KC1 -----> K2Cr2O7+ 2NaCl.

Less soluble K2Cr2O7is seperated on cooling and it is purified by crystalization.
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