The insoluble impurities in crude salt are mainly removed by filtration. The crude salt can be dissolved in water, filtered to remove impurities, and then evaporated to obtain salt crystals. For magnesium chloride and calcium chloride, the real impurities should be magnesium ion and calcium ion, which can be removed by combining with hydroxide ion and carbonate ion to form precipitation respectively. In order to remove impurities, the added reagent must be excessive, so hydrochloric acid is finally added to remove excess sodium hydroxide and sodium carbonate.