If you want to make more shells (CaCO3) then which direction should the reaction proceed?

The reaction will proceed in the reverse direction

Explanation:

Now consider the equation;

CaCO3 (s) ⇄CO2 (g) + CaO (s)

This is a reversible reaction. The reaction could proceed in either the forward or the reverse reaction depending on the conditions of the reaction. Under certain conditions, the forward reaction is favoured, under certain conditions, the reverse reaction is favoured.

The direction in which the reaction proceeds depends on the desired products as the equilibrium position could be manipulated at will by altering the reaction conditions. If the desire is to produce more CaCO3, then the equilibrium position is manipulated so that it shifts towards the left hand side. This implies that the reverse reaction is favoured.