Salt producers use three basic technologies to create salt for its myriad uses. Now-buried dried-up oceans of geologic ages past have left many areas, under both land and sea, with concentrated salt sedimentary layers which can exceed fifty feet in thickness. Two technologies exploit these underground deposits: conventional shaft mining where miners go underground to remove solid rock salt and solution mining where water is pumped underground dissolving the solid salt and then pumping out the salty brine which is de-watered to crystallize the salt. The third method extracts salt from oceans and saline lakes, growing salt crystals much as a farmer grows crops of vegetables or grain. Respectively, the products of these technologies are known as rock salt, evaporated salt (or vacuum pan salt) and solar (or sea) salt.
Among the three technologies, most producers around the world are engaged in solar salt production, the least expensive technology available, when favored by a dry and windy climate. But vast quantities of rock salt are extracted in large commercial mines and chemical companies utilize an enormous amount of salt in the form of brine that never is crystallized into dry salt.
Logistical considerations heavily influence production facility site selection decisions and these, in turn, heavily influence the size of production units and the structure of the salt industry. “Countries,” after all, do not produce salt, salt producers do.
Rock salt is mined from underground deposits by drilling and blasting. Deposits are reached through a circular shaft, typically about 20 feet in diameter and as deep as 2,000 feet, depending on the depth and location of the salt deposit.
Shafts are lined with concrete, at least through the overburden and into the top of the salt deposit, and often all the way to the shaft bottom.
Mining methods depend on whether the salt is configured as a relatively horizontal sedimentary deposit or a more vertical salt dome. The differences in mining methods depend on the thickness and structure of the salt deposit. Bedded or layered deposits are mined using the room and pillar mining method, as horizontal rooms or entries of about 10-25 feet high and 50 feet wide. Openings or cross-cuts are created perpendicular to the length of the rooms to connect the rooms at planned intervals. Salt pillars are left in place to provide structural support for the overlying roof and other layers. Most room-and-pillar mines recover about 45-65% of the salt available, with the remainder left behind as pillar supports with margins both above and below the mined area. Each day, based on production needs, several rooms are blasted, each blast bringing down 350-900 tons.
In salt domes, after a level of room-and-pillar extraction is completed, the usual practice is to “bench” the mine by drilling and blasting the floor extending the excavation downward and removing vast quantities with each blast.
Typically salt is mined using large, diesel-powered equipment designed for undercutting, drilling, blasting, loading and transporting the blasted salt.
More recently, continuous mining machines have been more common; formerly they produced too many unusable fines.
Diesel-powered trucks take the salt freed by blasting to a system of crushers and conveyor belts and, ultimately, to the hoist or “skip.” Sometimes the salt is stockpiled in the mine awaiting hoisting; other operations maintain surface storage stockpiles. Each skip can lift 18-20 tons of salt and they move quickly – a large mine may be able to hoist up to 900 tons an hour.
Commercial solar salt is produced by natural evaporation of seawater or brine in large, diked, earthen concentration ponds called condensers. Seawater averages about 3.5% NaCl (salty lakes like the Dead Sea and the Great Salt Lake can be much higher) when it enters the condensors.
Climate is very important in solar salt production. The sun and wind provide the energy to evaporate the water and raise the salt concentration to the point of crystallization, 25.8% NaCl (25.4o Bé).
As the water concentrates, calcium carbonate is the first chemical to crystallize. By moving the increasingly-saline brine through a series of ponds, sometimes over a period that can be as long as two years, the calcium carbonate is thus removed from the final salt product. When the concentration has increased to the most favorable crystallizing level, 26o Bé, the brine is introduced into the crystallizing ponds.
As salt crystallization proceeds, the concentration continues to increase. At 29 or 30o Bé between 72% and 79% of the total salt has been crystallized. Proper brine control during concentration and crystallization results in salt of purity of >99.7% NaCl.
The crystallizing pond is then drained of the remaining highly concentrated magnesium brine (called “bitterns” because of its taste) which are either discharged or further processed for other minerals.
Mobile harvester equipment then strips the newly-deposited layer of salt crystals and they are washed (in clean brine to prevent loss), crushed and sometimes dried in kilns or fluidized-beds driers.
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