Easily soluble impurities can be removed by washing;

Slightly soluble impurities can only be removed by filtration;

Because barium chloride and sodium hydroxide are added to coarse salt , filter sodium carbonate to obtain a filtrate containing a mixture of sodium chloride, sodium hydroxide, and sodium carbonate. Add hydrochloric acid to remove excess sodium hydroxide and sodium carbonate, and then evaporate to obtain sodium chloride crystals and remove excess hydrochloric acid solution, because hydrochloric acid The solute in the solution is hydrogen chloride gas.

Extended information:

Coarse salt contains insoluble impurities such as sediment, as well as soluble impurities such as: Ca2+, Mg2+, SO42-?, etc. Insoluble impurities can be removed by filtration. Ca2+, Mg2+, and SO42- in soluble impurities can be removed by adding BaCl2, NaOH and Na2CO3 solutions to form a precipitate, or they can be removed by adding BaCO3 solid and NaOH solution. The water is then evaporated to obtain purer refined salt.

Baidu Encyclopedia - Coarse Salt Purification