Salt Production

 

Sources of salt           

There have been two main sources for salt: sea water and rock salt. Rock salt occurs in vast beds of sedimentary evaporite minerals that result from the drying up of enclosed lakes, playas, and seas. Salt beds may be up to 350 m thick and underlie broad areas. In the United States and Canada extensive underground beds extend from the Appalachian basin of western New York through parts of Ontario and under much of the Michigan basin. In the United Kingdom underground beds are found in Cheshire and around Droitwich. Salzburg was named "the city of salt" for its mines.

Salt is extracted from underground beds either by mining or by solution mining using water or brine. In solution mining the salt reaches the surface as brine, which is then turned into salt crystals by evaporation.

Salt production

On an industrial scale salt is produced in one of two principal ways: the evaporation of salt water (brine) or by mining. Evaporation can either be solar evaporation or using some heating device.

Solar evaporation of seawater    
In the correct climate (one for which the ratio of evaporation to rainfall is suitably high) it is possible to use solar evaporation of sea water to produce salt. Brine is evaporated in a linked set of ponds until the solution is sufficiently concentrated by the final pond that the salt crystallizes on the pond's floor.

Open pan production from brine
One of the traditional methods of salt production in more temperate climates is using open pans. In an open pan salt works brine is heated in large, shallow open pans. Earliest examples date back to prehistoric times and the pans were made of ceramics known as briquetage, or lead. Later examples were made from iron. This change coincided with a change from wood to coal for the purpose of heating the brine. Brine would be pumped into the pans, and concentrated by the heat of the fire burning underneath. As crystals of salt formed these would be raked out and more brine added.

Closed pan production under vacuum    
The open pan salt works has effectively been replaced with a closed pan system where the brine solution is evaporated under a partial vacuum.

Salt mines      

In the second half of the 19th century industrial mining and drilling techniques originating in China made the discovery of more and deeper deposits possible, increasing mine salt's share of the market. Although mining salt was generally more expensive than extracting it from brine via solar evaporation of seawater, the introduction of this new source reduced the price of salt due to a reduction of monopolization.