Evaporated salt (refined salt)
Salt purity is defined as the percentage of sodium chloride in the final crystal; a higher number means fewer impurities. The impurities are other minerals, not necessarily detrimental to the salt’s intended use, but not contributing to the benefit of salt. Solar salt and rock salt both can attain 99% salt purity, but often have far less. Sometimes, far, far less. The purest grades of salt are “evaporated salt.” Most of us use a table salt which is evaporated salt. It is manufactured using a system of “pans” which boil away the water from salt brine. The brine, which can itself be purified, is crystallized under controlled conditions, often in plants that resemble food processing plants where much of the evaporated salt is destined. The process has two steps: obtaining the brine, usually from a solution mine, and then thermally reducing it to crystallized salt.
Vacuum pan refining
While many places in the world use rock salt and solar salt for food, most of the “table salt” in Europe, Australia East Asia and the Americas is the product of vacuum pan refining.
Water is evaporated from brine using steam-powered multiple-effect or electric-powered vapor recompression evaporators. Multiple-effect systems typically contain three or four forced-circulation evaporating vessels connected together in a series.
The evaporators operate under a vacuum to reduce the energy requirements. Steam from boilers is recycled/fed from one evaporator to the next to increase energy efficiency in the multiple-effect system. Vapor recompression forced-circulation evaporators consist of a crystallizer, a compressor, and a vapor scrubber. Feed brine enters the crystallizer vessel, where salt is precipitated. Vapor is withdrawn, scrubbed, and compressed for reuse in the heater.
The crystallized salt is removed in a slurry, dewatered using a centrifuge, dried in a rotary kiln or fluidized bed dryer, treated with any additives (e.g. potassium iodide or iodate making iodized salt) and packaged.
A lesser-utilized method of producing evaporated salt is the less energy-efficient grainer method where the brine is crystallized in open pans. The process creates salt flakes instead of cubic crystals. The flakes form on the surface of the brine and sink to the bottom as they grow. The end result is a coarse salt desirable in some food applications. Cargill Salt operates a plant in St. Clair, MI using the Alberger process, a radically modified grainer method that produces a combination of salt flakes and cubic salt crystals.
The weak brine remaining after the salt is crystallized is usually recycled to the solution-mined cavern.