section_path: Production
section_name: Production
paragraph_text: Production
  
  Food-grade salt accounts for only a small part of salt production in industrialized countries (7% in Europe), although worldwide, food uses account for 17.5% of total production.
  
  In 2018, total world production of salt was 300 million tonnes, the top six producers being China (68 million), the United States (42 million), India (29 million), Germany (13 million), Canada (13 million) and Australia (12 million).
  
  The manufacture of salt is one of the oldest chemical industries. A major source of salt is seawater, which has a salinity of approximately 3.5%. This means that there are about 35 g (1.2 oz) of dissolved salts, predominantly sodium (Na) and chloride (Cl) ions, per kilogram (2.2 lbs) of water. The world's oceans are a virtually inexhaustible source of salt, and this abundance of supply means that reserves have not been calculated. The evaporation of seawater is the production method of choice in marine countries with high evaporation and low precipitation rates. Salt evaporation ponds are filled from the ocean and salt crystals can be harvested as the water dries up. Sometimes these ponds have vivid colours, as some species of algae and other micro-organisms thrive in conditions of high salinity.
  
  Elsewhere, salt is extracted from the vast sedimentary deposits which have been laid down over the millennia from the evaporation of seas and lakes. These sources are either mined directly, producing rock salt, or are extracted by pumping water into the deposit. In either case, the salt may be purified by mechanical evaporation of brine. Traditionally, purification was achieved in shallow open pans that were heated to accelerate evaporation. Vacuum -based methods are also employed. The raw salt is refined by treatment with chemicals that precipitate most impurities (largely magnesium and calcium salts). Multiple stages of evaporation are then applied. Some salt is produced using the Alberger process, which involves vacuum pan evaporation combined with the seeding of the solution with cubic crystals, and produces a grainy-type flake. The Ayoreo, an indigenous group from the Paraguayan Chaco, obtain their salt from the ash produced by burning the timber of the Indian salt tree (Maytenus vitis-idaea) and other trees.
  
  The largest mine operated by underground workings in the world is the Sifto mine, located mostly 550 meters below Lake Huron, in Goderich, Ontario (Canada). About seven million tons of salt are extracted from it annually. The Khewra Salt Mine in Pakistan has nineteen storeys, eleven of which are underground, and 400 km (250 mi) of passages. The salt is dug out by the room and pillar method, where about half the material is left in place to support the upper levels. Extraction of Himalayan salt is expected to last 350 years at the present rate of extraction of around 385,000 tons per annum.The mine is also a major tourist attraction, receiving around a quarter of a million visitors a year.