How do you make pure salt from rock salt?

Firstly you grind the rock salt with a pestle.

Then you start to dissolve the salt. After that you filter it.

Last but not least we allow the water to evapourate.

In the evapourating basin there should be pure salt left over.

Add. In rock salt, there will be other contaminants such as magnesium and calcium salts. These have different solubilities, and careful decanting is needed to decant the (say) Mg rich liquid, leaving the Na material behind.

But this is the way it is done at the salt works.