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.