Common salt, sodium chloride (NaCl), is widely distributed throughout Earth’s crust, and every gallon (4.5 l) of seawater contains about 4 oz. (113 g) dissolved in it. Huge deposits of rock salt, or halite, formed by the evaporation of seawater in geological periods exist in many parts of the world, notably in the United States, Canada, Mexico, Russia, and Europe.
Rock salt deposits are worked by well-used mining techniques, and if the salt is sufficiently pure, it can be used without further purification. Mining is usually done by the room and pillar method, which ensures that substantial rock salt pillars are left for support.
In a typical salt mine, the working height is approximately 25 ft. (7.6 m), with the working faces being around 50 ft. (15 m) wide according to the local conditions. The face is undercut to a depth of 12 ft. (3.7 m) using a heavy-duty rock-cutting machine and then drilled to a standard pattern with a mobile hydraulic rotary drilling carriage. The face is then charged with explosives and short-delay detonators. Firing the explosives produces a heap of over 1,000 tons (900 tonnes) of well-fragmented rock salt, which is then transported to an underground crushing plant. From the crusher, a further system of conveyor belts takes the processed rock salt to the base of the mine shaft, where it is hoisted to the surface.
When mined, the purity of rock salt is generally greater than 97 percent and for many uses requires only crushing and grading. If pure salt is required, the mined material may be dissolved in water to form brine, which is then evaporated by one of the methods described below. Alternatively, the salt may be extracted directly by drilling boreholes down into the salt deposits. Water is then pumped down the boreholes to dissolve the salt, forming brine, which is pumped back up to the surface for evaporation.
Evaporation techniques
Salt is produced from brine by evaporation in long, shallow pans. Heat is supplied by steam pipes near the bottom of the pan or directly to the bottom using hot gases from a fire. Salt crystals formed in the brine fall to the bottom of the pan, where they are removed by a scraper system. The salt produced by this process is in the form of characteristic flaky crystals and is known as flake, or grainer, salt.
Brine may also be evaporated in open ponds using the heat of the sun. The brine is usually run first into large concentration ponds with a surface area of up to about 50 acres (20 ha), where impurities such as clay and sand and the less soluble salts such as calcium carbonate and calcium sulfate, separate out. The concentrated brine is then run through a series of smaller crystallizing pans, where salt of varying grades is deposited.
Vacuum evaporation
A typical vacuum plant for salt making consists of a series of three or more closed vertical cylindrical vessels with conical bottoms. Each of these vessels, or effects as they are generally called, has a steam chamber, or calandria, either totally submerged in the brine to be evaporated or externally connected. The calandrias contain a large number of vertical tubes through which the brine can be circulated, an arrangement that exposes a very large surface area. This system is most efficient for transferring the heat of the steam to the brine.
The condensation of the steam from the boiling brine in each vessel produces a reduction in pressure, and as a result the brine boils at temperatures lower than would be the case at ordinary atmospheric pressures. The salt crystals that form are taken from the bottom of the vessels and pumped in the form of a slurry to filters, which separate the salt crystals from the brine.
