An underground salt deposit may be solution-mined by drilling wells into an underground salt deposit, injecting fresh or recycled water through the well casings to dissolve the salt, and leaving a residence time long enough for the brine solution to reach saturation with sodium chloride.
Solution mines can vary in depth from 500 to 5000 feet deep. Sometimes they use a single well; more commonly several wells are linked within a brine field or “gallery.” Solution-mining technology provides control over the size and shape of the caverns and minimizes the potential for surface subsidence. State-of-the-art drilling and operating techniques, well and cavern logging instruments, and other devices provide precise control over salt cavern development and use.
Brine is produced from a single well by injecting water into the salt deposit through tubing, then extracting brine through a concentric annulus between the tubing and the well casing. When multiple wells are used, water injected into one well, dissolves the salt in the cavern and is extracted from a different well.
Insoluble impurities, such as anhydrite (calcium sulfate) settle out in the underground gallery, and the saturated sodium chloride brine is pumped to the surface for processing. The brine is sometimes then treated to reduce levels of dissolved calcium, magnesium, and sometimes sulfate. Chlor-alkali plants with their own brine wells treat the brine and treated brine is converted in vacuum pan salt refineries into exceptionally pure products for medicine and industry including salt for chlor-alkali producers without access to captive brine wells, the common situation in Europe.