What is this "rock salt" everybody is talking about...
Rock Salt
Salt occurs naturally in underground deposits, and occasionally in surface deposits in arid areas, as the mineral halite. Ancient salt deposits are widespread. There are ten major salt basins in the western hemisphere, in both North America (for example, Michigan 1 2 3 4, Texas 1 2, Kansas and New York 1 2 and beneath the Gulf of Mexico), South America, Iran ( 1 2 3 )and Poland as well as deposits around the world. A number of these salt deposits are mined for halite, commonly known as rock salt. Salt deposits formed as horizontal salt beds in ancient oceans and were later buried deeply beneath sediments as mountains eroded. Later, some of these buried salt deposits were geologically deformed by tectonic forces ( 1 2 ) within the earth. Salt tectonics ( 1 2 ) help geologists understand other minerals too. Salt domes ( 1 2 (diapirs) are a major type of salt structure resulting from tectonic deformation, though off-shore salt basins can also be tectonically active, as off the coast of Brazil. Both bedded salt deposits and salt domes or diapirs are mined by drilling and blasting.
Bedded salt deposits are nearly horizontal, although some contain fault zones and other anomalies. Salt beds range in thickness from a few tens of feet to several thousands of feet. Salt's differential compaction compared to the sediment that covers it can often produce interesting geological results. Salt domes or diapirs formed as salt flowed plastically (because of pressure and heat) upward through overlying sediments. The result is a vertically elongated salt deposit of a mile or more in diameter and perhaps 15,000 to 20,000 feet in vertical length. The tops of some U.S. gulf coast salt domes are very near the surface.
The geology of rock salt mines means they are often ideal for storage of petroleum products, even archived documents, and special radioactive wastes storage facilities are located in salt formations. The primary use for rock salt excavated from North American salt mines is roadway deicing to preserve safe and passable highways during the winter, though, often, the public is unaware of the activity. In other cases, mines have been used as tourist attractions ( 1 2 and more )
Mining of both types of salt deposit is similar. The method of mining is called "room and pillar" because the salt is excavated by blasting and loading out a series of rectangular entries and cross cuts. Rectangular pillars in a checkerboard-like pattern, typically 35% to 50% of the original salt, remains to support the mine roof. Rooms in bedded salt mines are 10 ft to 45 ft high, while rooms in domal mines can exceed 100 ft in height. This method has been used for centuries.
When a new mine is constructed, two shafts are excavated down to the salt deposit, usually between 500 feet to more than 2,000 feet deep. Shafts are about 20 ft in diameter and lined with concrete. After the shaft is sunk to the salt, rooms are mined in a planned pattern by undercutting, drilling and blasting. An undercutter cuts a horizontal slot or kerf along the floor of the advancing room to provide a second free face for blasting. A drilling rig drills a series of holes into the face, and an ammonium nitrate-fuel oil mixture (ANFO) is pneumatically placed into the holes. The room is then blasted, creating a "muck" pile of salt ready to be transported to an underground crushing and screen station. After loose pieces of salt are removed from the roof and pillars for safety reasons, the blasted salt is loaded into large trucks by front end loaders, or loaded by load-haul-dump units (LHDs) of lesser capacity that pick up the salt and haul it to the crusher.
After crushing, the salt is transported by conveyor belts to salt "pockets" at the shaft bottom for loading and hoisting to the surface. Two counter balanced salt skips of up to twenty short tons each are loaded automatically for the 1,000 ft/min trip. At some mines, the skip loading, hoisting, and surface dumping operations are managed completely by computers.
On the surface the salt is rescreened to remove fines, bagged, palletized and prepared for shipment to the salt customer. Most rock salt from underground mines in North America is transported in bulk by ship, barge, truck, or rail. Some is packaged in ten to 80 lb bags or compressed into salt blocks for animal nutrition and water softening.
Want a virtual tour of a salt mine?
Rock salt mines often have rich histories. Typical are histories of the salt mines at Avery Island, Louisiana; Goderich, Ontario and Detroit, Michigan or read this account of an oil drilling accident ( 1 2 ) that destroyed a salt mine in Louisiana. Rock salt sales in the U.S. were 17.5 million tons in 2004.