Based on the method of production, salt is categorised as four types viz., evaporated salt, rock salt, solar salt and salt in brine. Nearly all salt in brine and a small amount of dry salt is used to produce chemicals such as chlorine gas, caustic soda (sodium hydroxide) and soda ash. For the production of dry salt, four methods are largely used. These are:
Underground mining: Here, the underground halite deposits are conventionally mined by drilling and blasting to produce rock salt. Shafts of about 20ft diameter are sunk into the salt deposits -- which may be at depths of 500 ft to more than 2,000 ft below the earth surface.

The blasted or machine mined salt is then crushed and screened to the desired size and hoisted to the surface, packaged or shipped in bulk to the customers.

Solar evaporation: Solar salt is produced by using the solar power of wind and sunlight to evaporate the salt water got from the ocean or sea or from a saline lake in large open ponds.

The water evaporates in successive ponds until the brine is fully concentrated and salt crystallises on the floor of the ponds.

Solar salt plants must be located in areas of low rainfall and high evaporation rates. Seawater contains about 3.5 per cent by weight of dissolved minerals. Sodium chloride (NaCl) is 77 per cent of that amount, or about 2.7 per cent of seawater. The other 0.8 per cent consists chiefly of calcium, magnesium and sulphate ions.

As the seawater evaporates, its volume decreases and the concentration of sodium chloride in the resulting brine, increases. Often, the concentrating ponds will have distinct colouration depending on the salt concentration and what species of plants and animals find it habitable.

Salt crystals begin to form when the brine concentration reaches 25.8 per cent sodium chloride.

As evaporation proceeds, a layer of salt forms on the earthen crystalliser floors to a thickness of 10 to 25 cm. Sometimes, a layer of salt remains in the crystallisers as salt floors to provide support or harvesting equipment and, to lessen the chance of clay or soil contamination of the salt. A modern, properly operated solar salt plant can produce salt with purity levels of more than 99.7 per cent.

After the salt crop reaches appropriate thickness, the salt is harvested usually once a year with mobile equipment washed and is stockpiled to drain. The principal impurities in solar salt are small amounts of calcium and magnesium sulphate and magnesium chloride.

Clean brine, made by dissolving fine salt, is used to wash the salt and remove small amounts of impurities such as these. Seawater can also be used but, salt losses increase due to dissolution. Depending on the intended use, solar salt may be crushed, screened and dried in kiln or fluidised bed dryers.