How is sodium chromate converted into sodium dichromate in the manufacture of potassium dichromate from chromite ore?

When a chromite ore $(A)$ is fused with sodium carbonate in free excess of air and the product is dissolved in water,a yellow solution of compound $(B)$ is obtained. After treatment of this yellow solution with sulphuric acid,compound $(C)$ can be crystallised from the solution. When compound $(C)$ is treated with $KCl$,orange crystals of compound $(D)$ crystallise out. Identify $A$ to $D$ and also explain the reactions.

Preparation of potassium permanganate from $MnO_2$ involves a two-step process in which the $1^{st}$ step is a reaction with $KOH$ and $KNO_3$ to produce:

$FeSO_4 \cdot 7H_2O$ (Green Vitriol) salt on thermal decomposition does not produce

Assertion : Solution of $Na_2CrO_4$ in water is intensely coloured.
Reason : Oxidation state of $Cr$ in $Na_2CrO_4$ is $+VI$.

An acidified manganate solution undergoes disproportionation reaction. The spin-only magnetic moment value of the product having manganese in higher oxidation state is $....\, B.M.$ (Nearest integer)